\r\n\tIn this book the authors will provide complete introduction of Polymers chemistry. The book is mainly divided into three parts. The readers will learn about the basic introduction of general polymer chemistry in the first part of the book.
\r\n\tThe second part of the book starts with a chapter which includes kinetics of polymerization. Polymer weight determination, molecular weight distribution curve and determination of glass transition temperature. The final part of the book deals polymer degradation which includes types of degradation. The chapters of the present book consist of both tutorial and highly advanced material.
Thymus gland is a primary lymphoid organ, situated in thoracic cavity, ascends from the endodermal layer of the third pharyngeal pouch of the embryo. Based on same origin, thymus can be linked with that of the parathyroid, but during embryogenesis it is separated from that endocrine gland [1]. Thymus faced the process of evolutionary atrophy with age in almost all the animals which leads to the architectural alterations [2]. Its anatomy is variable among species. In new-born fowl, its color is greyish pink and has two left and right lobes. It is ventral to the trachea and the large vessels, but its lobules may prolong up toward the thyroid gland.Thymus gland in dog is a compressed bilobed structure located in the cranial mediastinum that is laying cranial to the heart and behind the sternum. Its size is largest in young which is followed by atrophy with the age progression until only a trace remains [3]. When it is fully developed, its caudal part is melded on the cranial surface of pericardium. Divisions of the inferior thyroid, internal thoracic arteries, and superior thyroid artery supply blood to the thymus. These arteries travel along the connective tissue septa, which is extended from the covering capsule into the thymic parenchyma [2]. Histologically, however, it is an old technique, but it is still used excessively in the medical field for the understanding of the organ’s microarchitecture [4]. The septa divide the parenchyma into small incomplete microscopic lobules, Where they entered the thymus gland. Veins, inferior thyroid, internal thoracic and left brachiocephalic vein takes blood away from thymus gland. Nerve supply of the thymus gland arises from the sympathetic nerves of the cervical chain and the vagus nerve. Extended divisions of the phrenic nerves stretch up to the covering capsule of the thymus but are not arrive to the gland parenchyma [3]. The function of such enervations to the thymus gland is not well comprehended. Lymphatic vessels drain into the lymph nodes viz. parasternal, tracheobronchial, and brachiocephalic. Histologically, the thymus gland appears as a lobulated lymphoid organ, enclosed with a capsule, made up of a fibrous connective tissue (FCT). Capsule surrounding the organ have blood vessels which supply blood to the thymus gland parenchyma. The CT-composed trabeculae descended downward from the capsule, splits the thymus parenchyma into many incomplete lobules by extending into the interior of the organ [5]. These lobules consist of the following two parts: the cortex is a dark staining outer region just beneath the FCT capsule. It contains densely packed lymphocyte that is not involved in the formation of lymphatic nodules. This portion support the early thymocyte development also positively selects the major self-histocompatibility complex. This portion is very thick at the earlier age. The junctional point between the two compartments is called as corticomedullary junction. This is the specific area where the thymic precursor cells enter in the adult age and few of them differentiate into NK cells and the dendritic cells later few reached to the subcapsular sinuses. This corticomedullary area is also very clear and become blurting and even more fuzzy with the progression of age [6]. The medulla is a light staining inner/central region. Medulla contains later thymocyte differentiation to subpopulation like CD-4 and CD-8, also have fewer lymphocytes than cortex but have more epithelial reticular cells. It also has many thymic (Hassall’s) corpuscles which differentiate it from other lymphoid tissues/glands [3]. The Hassall’s corpuscles are variable sized ovoid structures composed of granule cells, epithelioid cells, and concentric layer of reticular cells containing keratohyalin and eosinophilic fibers. Under microscope the Hassall’s corpuscles and the bubble-shaped adipose tissue appears in the area and their number increases with the age progression [7]. Medulla also shows the continuity between the lobules, because the lobules are incomplete. Thymocytes mature, downregulate, and reach the medullary regions.
Cellular components of the thymus glands comprise of emerging thymus-derived T cells (later population reached to 95%), the stromal cellular system including the microvasculature, the mesenchymal cells, the dendritic cells, and the very important thymic epithelial cells (TEC) [8]. Few macrophages are present in almost all parts of the gland but in medulla it plays important role in the apoptosis. The TEC are categorized into three key classes including cortical, medullar and subcapsular/perivascular based on localization in the thymic parenchyma. The dendritic cells are mostly found in the corticomedullary junction and in the medulla. All the aforementioned cells participated in the thymocyte function started from the receiving of the progenitor cells till its final training and maturation. During the period of advance gestation, the thymus in the fetus has unclear cortex and medullary regions, contains differentiating T cells, macrophages along with B cells and a developed CT capsule with the vasculature connection [9]. Soon after birth, the thymus develops altogether along with the cellular compartments. In the aged individuals, the involution of the thymus is initiated, which is easily seen in the histological sections in the form of thinning of the cortex as well as the haziness of the corticomedullar junctions. Thymic epithelial cell proliferation is a key player in the development of the thymus in the infant [5]. Recently, the hyperplastic proliferation of the thymic epithelial cells was observed in the transgenic lab animals. Thymic fragments of the neonates and that of the bone marrow transplant to the adult individual is also observed experimentally. It has been suggested that stem cells have the capacity to differentiate and develop the organ system of the same kind cells [10]. The progeny of the stem cells may develop the tissue directly or may differentiate into a new stem cell. It is possible to grow the stem cell in vitro, and it is needed to support these cells in the living individual. It would be a big achievement in the science, if the stem cells could possibly grow and could differentiate into the thymic cells in the thymus parenchyma like those present in the intestinal crypts, skin, and liver. In this chapter, we will focus on our current understanding about thymus architectural modulations in health and disease and its possible physiological improvement.
Major role of the thymus gland is the training of variety of T cells that respond to the antigens. Its function is mainly regulating by the response of the cytokines and for this the equilibrium among anti-inflammatory and proinflammatory cytokines of the body is crucial. It has been observed that thymic atrophy is associated with age linked with diminished interleukin-7 expression [6]. Thymic epithelial cells are originated from a mutual bipotent ancestor and are also the main constituents in the growth of T cells in the thymic microstructure. It comprised of the two regions including the cortex TECs which is positioned in the cortical regions and the medulla TECs which is in the internal medulla. They experience a sequential progress which is organized by various signals, which later leads to support in physiological maturation and development of the thymocyte. The TECs playing a role in the selection of T- cells in the thymus parenchyma [5]. Both cortical and medullar TECs play distinctive responsibilities in the positive and negative selections of the thymocyte.
The undifferentiated lymphocytes are migrated from the reservoir, that is, bone marrow to the thymus gland by means of blood stream. The thymic cortex contains the reticular cells, also known as thymic nurse cells. These cells surround the lymphocytes and enhance the differentiation, proliferation, and maturation of the cells [11]. The lymphocytes get matured and get transformed into immunocompetent cytotoxic T cells, helper T cells, and the T cell. At this stage, the receptor is being attached at surface of the lymphocytes for the recognition of antigens. This process starts just before the birth and continues till some month after the birth. Almost 1% of the mature lymphocytes are getting out of thymus toward the margin on daily basis. The differentiation and further activations of T cells to CD-4 and CD-8, and then established T cells travels from thymus to the marginal blood vessels and secondary immune organs [9, 11]. Thus, the size and mass of thymus reflect the maturation of the immune system.
It is a physical barrier formed by endothelial cells, epithelial reticular cells, and macrophages. Its function is to prevent developing lymphocytes from the exposure of blood borne antigen [8]. This barrier provides tremendous environment for the substance exchange between vasculature and the thymus also help maturation of the immature thymocytes. Macrophages present outside the capillaries prevent the interaction of the substances that are transported in the blood vessels with the developing T cells in the cortex. Matured T cells leave the thymus gland through the blood vessels and colonize in the lymph node, spleen, and lymphatic tissue of the organism [11].
Maturation is the condition of developing progenitor within the thymus parenchyma, where the cells known as thymocytes, undergoes various developmental processes to perform exclusively. These cells can be recognized based on manifestation of various markers on the cell surface and the antigen presenting cells present the T cells with self and foreign antigen [11]. It usually consists of positive selection in which the lymphocytes that recognize the foreign antigens survived and reached to the maturity then enter the medulla through the cortex. Later, goes to the other sites in the body via blood [9]. Maturation is a very complicated process and only a small number of lymphocytes reach to the stage of maturity in the thymus. The negative selection in which the lymphocytes which are incapable to distinguish the self-antigens are eliminated by the macrophages. This is approximately 95% of the total cells.
Epithelial reticular cells, also called as TECs, are present both in the cortex and medulla; however, it can be easily recognizable in the thymic medulla through histology. These cells contained the thymic granules which is assumed to be the called as the thymic hormone [12]. This structure has the following functions;
It formed the blood-thymus barrier.
Secrete hormone which are required for proliferation, differentiation, and maturation of T cells. Also, for the expression of their surface markers. The hormones including thymulin, thymopoietin, thymosin, thymic numeral factor, interleukin, and interferon are secreted.
It forms thymic (Hassall’s) corpuscles, distinctive whorls, in the medulla of the thymus gland. The thymus gland is identified by this thymic corpuscle.
The pluripotent progenitor cells migrated from the bone marrow to the thymus parenchyma, where the maturation of the unexperienced T cells occurs in the complex microarchitecture. However, this structure changes with the age.
Aging is an irreversible, on-going, and inevitable progression that is correlated through manifold organ dysfunction. The key organ of immune system and primary organ of T cell production is the thymus gland which is endodermal in nature. Involution of the thymus with progressing age is into the consequences of a decreased T cell production primarily and leads toward a long list of the following diseases and even a mortality of the individual [6]. The corticomedullary junction is disrupted and the number of medullary epithelial cells are also decrease. This age-related cellular apoptosis and atrophy is still un-answered. There are several reasons to be considered for this process, but the main cause known is reactive oxygen species (ROS). The entity that are assumed to be responsible for expressing the age-related changes in the thymus is due to the discrepancy amongst the free oxygen-derived radicles and that of the antioxidants. Mitochondria is the main site were such reactive species are produced. Inside mitochondria, the oxidative stress produce ROS which results into mitochondrial damage within the cells and leads to liberate more ROS. In fact, aging is a physiological multifactorial process accompanied by decline of organ function. Histologically, the thymus gland of mammals divided into three consecutive morphological stages; the epithelial, the lymphopoietic, and the differentiated cellular microenvironment [7]. The progenitor cells are synthesizing in the thymus which later differentiated into mature T cells. Thymus also comprises the main stromal niche termed as thymic epithelial space. It supports T cell development and maturation [6]. The thymus is greater in size and is very dynamic in the neonates and pre-adolescents. With the progression in age, the involution starts and ultimately disappears and are then replaced by rudiments and fat. The process of involution started just after 1 year of birth [2]. If in case the thymus is absent in individual congenitally, then there would be a probable chance of deficiency of T cells. The main components of the thymus which undergoes involution during the aging include the T cells of hematopoietic origin and the TECs of non-hematopoietic origin. During the process of involution, disruption of the thymic epithelial/endothelial ratio happened and results into gradual loss of pro-T cells. Primarily just after the start of involution, the thymic epithelium mass is decreased in the parenchyma. This decrease in epithelium leads toward the disorganization of corticomedullary junction and results into loss of demarcation between the thymic cortex and medulla. This process where a continuous loss of cells and their functions is called aging [6]. Histology of thymus gland varies with the individual’s age. It is observed that this gland is extremely delicate to stand against the biological abnormalities, for example, autoimmune diseases, infection, and age progression. It attains its maximum development shortly after birth. After attaining the age of puberty, the thymus gland regress and degenerate. Due to this effect the lymphocyte production decreases and the reticuloepithelial cells (thymic corpuscles) increases. Cellular portion, especially of T cell of thymus gland, decreases and are being replaced by connective tissue and adipose cells. Parenchyma of the thymus gland at and after puberty is filled with adipose tissue. Immunity, however, in this stage it is not compromised because progeny of the T lymphocytes has already been established. Thymus gland is well developed only in late fetal life and persists for a few months after birth [6]. Subsequent to this period, it undergoes rapid atrophy, fatty infiltration, and the amyloid degeneration [13]. Increase in the amount of adipose tissue and fat-bearing cells in the thymus parenchyma indicates that the body is now vulnerable to the infection and autoimmune diseases. So, in adult, only a thin remnant appeared in the anterior mediastinum or has entirely disappeared. Thymus gland size is also affected by the sex-steroid hormone and hypothalamic-pituitary-adrenal axes hormones [14]. It has been found that during the thymus involution, the CT which is present in the capsule, septa, perivascular tissue, and in the stroma of the cortex and medulla is getting enriched with the fibronectin contents. Later, most of the thymic parenchymal areas are being replaced by the stromal cells.
Thymus gland is an important component of immune system. This system defends the host from several infections [11]. It’s inappropriate performance may produce embarrassment or even fatality. If the thymus gland is removed from the new borne individual, then there will be no possibility of the T cell production [6]. Other, lymphoid organs will also lack or decrease the number of T cell to fight against the pathogens. Consequently, death of the individual may occur, predominantly due to complications with infection and lack of immunity.
Thymus experiences atrophy with time which is triggered by numerous factors viz. growth and aging, infections and endocrine instabilities which give rise to a nonstandard liberation of T cells and consequently weakened the immunity [6]. The main organic task of this gland is to spawn a distinct T cells range to establish a crucial portion in host immunity counter to external pathogens, whereas the thymus correspondingly theaters a serious part in self-tolerance through negative assortment and the T reg cells formerly known as suppresser T cells production. Some of the thymic-derived malfunctionings are categorized bellow.
When the immune system of body trigger against its own body cells and show the excessive action then the allergic condition developed [15]. Allergens are the substances which initiate the allergic response. IgE antibodies are more common in this condition. Histamine is also present in allergic condition which are released from the Mast cells. Anaphylaxis is a condition developed when these allergens cause an acute to severe reaction and that sometimes proves fatal condition. In this case, the immune system becomes hyperactive and cause the death of own host cells instead of killing foreign pathogens. Thymus parenchymal cells have the ability to recognition the body own cells and prevent them from killing central tolerance process that is immunologic tolerance to self-antigens. During such diseases, the demarcation between the cortex and the medulla deteriorated and the medullar epithelial cells are also dispersed [16]. A rare autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy is an inherited syndrome and is because of alterations in the specific gene [14]. Such gene permits the illustration of tissue related particular proteins present in the thymic medulla. This disease also affects multiple endocrine tissues. As T cell production take place in the thymus gland and if there is any defect in thymocyte development, it leads to profound decrease in the production of T cell, which may result into immunodeficiency disease. Abnormality to the thymic epithelial cells leads to the T cells dysfunctions, which may result in chronic inflammatory disease in the host [11, 12]. The tolerance is failed due to available antigens in the tissue is because of the autoimmune illnesses. If there is a defect in the production of both T cell and B cell, it results into severe combined immunodeficiency (SCID). When the combine deficiency of T lymphocytes and B lymphocytes occurs together, then the condition developed is known as SCID. This is a rare congenital disease, which is initiated by the non-functional hematopoietic progenitor cells that act as precursor of the B lymphocytes and T lymphocytes. When this condition develops, there is decrease in the lymphocytes production which results into the thymus atrophy [16]. There are also some other factors which are responsible for the causation of this disease such as IL-7 deficiency, recombination activating gene deficiency and common gamma chain deficiency. Another autoimmune disease in which the antibodies blocks the acetylcholine receptors at neuromuscular junction. This autoimmune disease is known as myasthenia gravis. This illness is categorized as softness and fatigability of the muscle and is triggered by autoantibodies depending T cell counter to the neuromuscular junction. Its exact cause is unknown; however, it is assumed that changes in the thymus and the thymic epithelial cells is one of the main causes of its pathogenesis [14, 15]. Sometimes, this disease is associated with thymic hyperplasia. This disease is commonly cured by the surgical amputation of the thymus which is known as thymectomy. The conditions like thymic hyperplasia or malignancy are nearly 70% more common in the patients suffering with such autoimmune disease. Type-1 diabetes is also an autoimmune-based disease, which results from the obliteration of 𝛽 cells in the islets of Langerhans. The pancreas infiltrating T lymphocytes cause the destruction of the insulin producing cells in the islets of Langerhans. The body, in this case, does not produce insulin. It is seemed to be more commonly present in adults. It has been observed that lack of immune tolerance to the 𝛽 cells present in the pancreas is the initial cause of such diabetes development. The Human immunodeficiency virus (HIV virus) effect the developing lymphocytes and results into killing of these cells [18]. When the lymphocytes decrease in the body, then the T cell immunodeficiency syndrome occurs, which are acquired in nature. The HIV virus usually cause the killing the CD4 T cells and it also mainly effect the mature T lymphocytes which are present at the periphery. In this case, a rapid atrophy of the thymus takes place in the infected individual.
This inherited ailment is because of the obliteration of a minor piece of chromosome-22, which is one of 23 pair of human chromosomes [17]. The syndrome affects the individual through various means like cleft palate, facial defects, delay development, learning problems, and promote infections. As far as the immunity is concerned, this syndrome leads to inherited shortcomings containing thymic atrophy and aplasia. Such affected persons may have intense deficiency of T cell lineage. In this disease, the thymic parenchymal cells are lost and the area is then replaced by the stromal CT.
Thymomas is a scarce neoplasia (benign in nature) arise from the thymic epithelial cells. In the thymoma patients, there are chances for the occurrence of a disease known as thymoma-associated multiorgan autoimmunity (TAMA). In these patients, the donor does not act like a basis of pathogenic T cells instead, the individual particular thymus gland yields the infected T cells which is directed toward its own body cells [19]. If the thymoma indicates the malignancy of the thymus gland, then its product is the defective T cells which are unable to recognize their own body cells as self-antigens. So that is why we can hardly distinguish this disease from GVHD. These types of tumors, usually 10–15%, are presented in the patients who are already suffering from myasthenia gravis. The symptoms include strong cough which sometimes get confused with bronchitis because the laryngeal nerve is compressed by the tumor. All thymomas are cancerous, but they are varied from each other in different aspects like some develop slowly and some tumors grow with the rapid rate and infect the surrounding tissue [20]. The treatment of this condition is the surgical removal of the infected part or whole gland. Thymic lymphomas are tumors which originate from the thymocytes of the thymus gland. These lymphomas or leukemia acts like the precursors of the origin of the thymocyte are often classified as T acute lymphoblastic leukemia/lymphoma. Before 1950, the radiation was used to cure the people who are suffering from the enlarged thymus gland particularly the children. The post-operative complication in the treated people includes an elevated incidence of thyroid cancer and leukemia. The rare malformation of thymus gland includes the cervical thymus cyst which is something confusing with the tumors. In the literature, there is no as such detail study of the thymic cyst prevalence. There is no such common lesion appeared which are present in this condition. In the childhood, there are more chances of the thymic cyst and the ectopic cervical thymus than the adults.
Thymic involution with age has negative impacts on the immune system. Proper performance of the thymus and integral immune system are required to protect against disorders. With the advancements of the modern sciences, a technique named as photobiomodulation is newly employed to slower the process of thymic involution. This delayed process improves immunity and may add in extension of the individual lifespan. Another new method which reduce the thymus atrophy and boost the thymus functioning is the sex-steroid ablation therapies [14]. Genetics has also an important role for determining the initial thymus size and rate of involution. Many hormonal medications, surgical procedures, and applications of antioxidants are testified as replacements for the reversal of aging in atrophy of the thymus. Males display a trend of lower grade thymopoiesis in comparison to females. There are several procedures that can improve the normal physiology of the thymus gland which ultimately leads to strengthen the immune system active for prolong time [20]. Continuous use of dietary supplements like rosehips, echinacea, olive leaf, and cruciferous vegetables are tested to be connected to establishment of the thymus health. These supplements are comprised of glucosinolates. This substance is famous for fighting contrary to the tumorous cells and other malformations in the tissues. Vegetables especially cruciferous vegetables like cauliflower, Bok Choy, broccoli, and cabbage are good source of nutrients that trigger antioxidants and anti-inflammatory responses. This can also assist thymus function. Proper timely intake of antioxidants and various vitamins like E and C in the diet is mostly necessary. These are needed for proper functioning of the thymus cells [21]. Zinc is also a very useful micronutrient required for the growth and growth of the vertebrate. The immune system is very sensitive to zinc deficiency and produces the multiple disturbances like atrophy of the thymus parenchyma and the increased chances for a disease to occur [22]. Many genes which regulate thymus gland activity are under the response of metalloenzymes in which zinc act as metal. So, dietary supplementation is very necessary for the proper functioning of the thymus gland otherwise, there may be a serious consequence related to the synthesis of the T lymphocytes. Exercise on daily basis is additional tool to ensure blood circulation throughout the body and the thymus gland. In this way increased blood flow will ensure that thymus waste products are removed promptly and do not cause damage to the thymus. Furthermore, through the circulation the key nutrients reach around the body more rapidly which allow quicker recovery to take place. Thymus gland stimulate the production of white blood cells that help against infection by thymosin hormone. This thymus glandular extract regulates many other immune functions. The extract called as thymomodulin, obtained from bovine species and act as immune boosting activities in immunodeficient individuals. Such extract act as a substitute and work against respiratory infections and many other infections like asthma, food allergies and hay fever. Olive leaf extract has a significant effect on thymocyte apoptosis and cell cycle progression to protect the thymus gland from toxicity. Tapping sternum is a practice which may also stimulate thymus gland. But this practice is a bit laborious. Organic acid has a positive effect on the immune system. It has been reported that sodium butyrate improves the thymus histology and ultimately improves the immune system of a body [23]. Probiotics are called as the group of beneficial bacteria. Probiotics are generally defined as the friendly living micro-organ¬isms, when taken orally in an appropriate dose, exhibit a beneficial effect on the host health. When the probiotics is taken orally, they go along side of the lumen of the gastrointestinal tract and interact with the mucosal immune system. Here, the whole bacteria or its cell wall mediate a network of signal production and activate the immune system. They produce different kinds of cytokines and chemokines and result into the activation of T lymphocytes. Another model of mechanism of action of probiotics is that they can suppress the growth of pathogenic bacteria and help in the balancing of microflora environment in the gastrointestinal tract. Probiotics protect the body against the pathogens by the induction of direct killing, nutritional competency of pathogens, and meanwhile by triggering the gut-associated immune repertoire. Thymus gland is known as the “Master gland of immunity” [2]. It regulates the immune system by producing different types of chemical known as cytokines that enhance the migration of T lymphocytes and meantime enhances the immunity [20]. The probiotic fermented milk (PFM) is a nutritional supplement which is used to improve the histology of the thymus gland mean to say it will cause a decrease in cellular apoptosis and enhance the percentage of CD4/CD8 cells. The PFM enhance the production of different kinds of cytokines in the thymus gland. This type of milk is usually used in case of protein-energy malnutrition; which result into malfunctioning of the immune system. This milk or bacterial-free supernatant helps to improve the immune system by activating thymus gland activity. Commonly used probiotics are specific strains of the lactic acid bacteria. The main genus includes in the probiotics are the
The thymus is a part of primary immune organs, having excellent example of connection between the cellular organization and function. Not like other well-organized organs, the microstructure of the thymus parenchyma has the very complex meshwork, where T cells differentiate, proliferate, and die. Disorganized thymic architecture of the elderly and disease thymus added cavitation and FCT proliferation and atrophy. Moreover, defects in the thymus caused to lesser the production of T cells and the interruption of self-tolerance. This may result in worsening the development of disease. Consequently, the thymus is declared as one of the most significant organs in maintaining immunity and safeguard the host against progression of age and development of ailments. Subsequently, this gland acts a crucial part in health and disease. The size, architecture, and function of this gland decreases with progression of age. There are some possible pathways to modify the thymus microarchitecture and function, in order to progress the physiology during autoimmune diseases, infections, and aging.
I have written about casting processes extensively, some would say, over-extensively, but useful new concepts enshrining the new insights into this important subject are being only slowly realized and understood. Their awful importance to the reliability of engineering is yet to be fully adopted. It is useful therefore to offer a short summary in this appropriate volume. A personal reading list is appended.
Space unfortunately forbids any lengthy discussion of the influence of the melting processes on the quality of the resulting castings. However, of course, the quality of the metal prior to casting is of critical importance; a liquid metal, especially if a secondary metal (such as recycled metal, as is common in the aluminum industry) can be literally crammed with defects, so that despite the excellence of the casting process and the heat treatment process, the resulting casting predictably fails all mechanical property requirements and is scrapped. We shall have time to give only a brief mention of this important problem. Our subject is the casting process.
Castings have had a poor reputation as a result of their poor and variable properties. For many years this was thought to be somehow associated with the turbulence of the pouring process, but the details were not understood, and efforts to control turbulence, despite all claims to the contrary, were failures. No-one was aware of the degree of failure to control turbulence during mold filling because, of course, molds made or sand or steel were opaque: the awful internal damaging mechanisms were unseen and unsuspected.
The breakthrough in understanding came from X-ray video studies of mold filling. Although occasional demonstrations of this technique had been made a number of times over the years, it was only in the 1990s that intensive and systematic studies were carried out at Birmingham University, UK [1]. It was quickly realized that because the liquid metal practically always exhibited a surface oxide film, the mutual impingement of drops and splashes, or the folding of the liquid surface, occurred as oxide film to oxide film. The liquid metal, in general, never made contact with itself. Furthermore, the upper surface of the film in contact with the air was dry. Thus, the mutual impingement processes occurring during turbulence of the surface occurred as dry-film-to-dry-film (Figure 1). No bonding occurred between these two ceramic films which for many metals and alloys, including steels, consisted of alumina (Al2O3) and similar very stable high melting point oxides.
The impingement and folding processes to produce bifilms as cracks in the liquid metal.
The practical result of this impingement of two unbonded ceramic films, is the effective creation of a crack in the liquid. This defect is called a bifilm. The turbulent pouring of a liquid into a mold can fill a liquid with cracks. The properties of the subsequent casting are, of course, significantly impaired. This is the fundamental problem of all casting processes. It affects nearly all processes in a major way. It is an issue which cannot be ignored.
Throughout this chapter, it should be kept in mind that if oxides in metals are mentioned, it necessarily means double oxides, in other words, bifilms, which implies cracks. Careful consideration of the entrainment mechanism will convince the reader that the surface oxide cannot be entrained and submerged without it occurring as a doubled oxide to create a bifilm crack; all oxides indicate the presence of cracks in the metal. As will be discussed in detail, the bifilm cracks survive plastic working, and so enter the world of the metallurgist and engineer. Because nearly all our engineering metals are intrinsically ductile, all cracks observed in metals almost certainly originate from the turbulence of the casting process.
The presence of bifilms in most metals comes to the rescue of the reasons why metals fail by cracking. After extensively surveying the metallurgical and fracture literature it was a tremendous surprise to this author arrive at the realization that there was no metallurgical mechanism to explain fracture. The lattice mechanisms such as the dislocation pile-up leading to the initiation of a crack were widely believed but have over recent years seen to be in error. Thousands of pile-ups have been observed by electron microscopy and studied in detail by computer simulation, but a crack from a pile-up has never been reported. Other theories such as the condensation of vacancies has been known for many years to result not in cracks but in totally collapsed lattice features such as dislocation rings and stacking fault tetrahedra, depending on the stacking fault energy. In brief, the bonds between atoms are simply too strong. Atoms cannot be separated mechanically by any normal forces; pores and cracks cannot be opened up by atomic or lattice mechanisms [2].
The fact that fracture occurs in so many ways and often at modest stresses cannot be explained by conventional metallurgy. This amazing fact is, however, obvious when it is realized that bifilms are present in most metals, usually as a result of poor casting techniques. It follows that if bifilms could be eliminated from metals, there would be no residual mechanism for fracture. Failure by fracture could not occur. This was a sobering realization to this author which it is hoped the reader will be convinced by this short account. If the short account fails to convince, the references at the end of this chapter are recommended.
Before moving on to the discussion of the techniques of casting processes, in addition to the bifilm, a further serious entrainment defect must be described.
In the maelstrom of pouring processes, in addition to the entrainment of oxide films as bifilm cracks, bubbles of air can also be entrained. The bubbles are serious defects in themselves, but their buoyant flotation makes a bad situation worse. Their buoyancy force causes the oxide film at the crown of the bubble to tear, so that it moves to one side, but is immediately replaced by fresh oxide film (Figure 2). It can be seen therefore that the skin of the bubble effectively slides around the bubble, coming together underneath to form a kind of collapsed tube, which extends back to where the bubble was effectively tethered, the point where it first entered the liquid; probably some early location in the channels of the filling system. This bubble trail is a kind of long bifilm. It can be metres long. Thus, bubbles can create macroscopic crack-like defects out of all proportion to the original size of the bubble. Furthermore, it is common for hundreds or thousands of bubbles to make their way up through the metal, creating masses of tangled defects [1, 3].
Bubbles and bubble trails as collapsed oxide tubes.
The reader may by now be already appalled, realizing the reality of grossly poor metallurgical processing which still bedevils our casting world today. The fact is that as a result of these fundamental entrainment mechanisms, most casting processes are bad. Books are full of the descriptions of casting processes, but none state that nearly all of them usually are capable of delivering only badly defective products.
This short summary will attempt to redress this key issue, illustrating how engineering and the world copes at this time simply by accepting the mediocre properties of metals, often by building in substantial safety factors. For the future, impressive improvements in properties and reliability are forecast for fundamentally improved casting technology.
If liquid metal is allowed to fall under gravity, after a fall of about 10 mm the metal has accelerated to near 0.5 m/s. This is the critical velocity at which the liquid now has sufficient energy to jump or splash up to about 10 mm high, and so be in danger of entraining its own oxide skin during its fall back under gravity. Thus, fall heights and speeds less than these values are safe from the introduction of damage due to surface turbulence. Above these heights and speeds, entrainment of air and oxides becomes increasingly severe [1]. Therefore, when pouring an average sand casting, which might be 500 mm tall, the falling stream reaches speeds of over 3 m/s, far higher than is wanted, so that, in general, copious amounts of defects are entrained. The situation is worse still for the pouring of steel ingots where a fall of 3 or more meters creates speeds of near 8 m/s, generating conditions similar to emulsification with air and oxides.
The skill in the filling of shaped castings by gravity pouring is to limit air ingress into the filling system and limit the velocity at which the metal enters the mold cavity. Only in the last few years have these problems been solved for the first time [3].
The sand casting process (of which there are very many variants) and investment casting processes similarly require these new solutions for design of filling system if, as is usual, filling is by pouring under gravity. Interestingly, these processes both exhibit rather low properties compared to castings poured in metal molds. The improved properties of faster cooled metals are traditionally attributed to a refinement of the dendrite arm spacing (DAS). In steels and Mg alloys there is some truth in this as a result of their limited number of slip planes. However, for Al alloys, with its extremely ductile face centered cubic (FCC) structure, the benefit from DAS is negligible.
The benefit to the faster freezing of Al alloys is a bifilm mechanism. Bifilms arrive in the mold in a compact raveled state because of the dramatically vicious bulk turbulence (high Reynolds number) in the filling system, so that their crack-like morphology is initially suppressed to some extent. Metal molds solidify quickly and freeze in these favorably compact and convoluted defects. In comparison, slow solidification in sand and investment molds allows more time for the bifilms to unfurl. This opening-out process, in which the crumpled bifilms unfold and straighten, resembling the opening of a flower, in which the petals adopt the morphology of planar engineering cracks. The unfurling process generally takes several minutes, and is driven by a number of mechanisms, including gas in solution which precipitates into the ‘air-gap’ inside the double film, or because of dendrite pushing and other factors [1, 3]. When all the bifilms have straightened out to resemble engineering cracks, the metal properties are at an all-time low. The metal now contains a snow-storm of cracks.
Turning to steelmaking, the technology of casting includes some astonishingly retrograde techniques. In an electric arc furnace, the steel quality is probably quite good as a result of the length of time available for the flotation of oxides. However, the metal quality is ruined by the tilting of the furnace and the fall of metal by several meters into a ladle. The turbulent churning of the steel has to be seen to be believed. However, it takes several minutes for the ladle to be lifted from the pit and taken to the casting station, during which time its quality recovers somewhat because of the very different density of the oxides compared to the dense liquid metal. But this improvement is destroyed a second time by ingot casting. Although some of the damage during casting floats out, not all escapes. The ingot is permanently degraded.
The move to ladle metallurgy is a valuable modern step in steelmaking, but the final pour into the ingot mold is unchanged and undoes much of the good achieved in the ladle.
This problem is especially acute for the casting of special steels, in which the tonnage is often too low to consider the use of the rather superior continuous casting process. Special steels are therefore mainly cast as ingots. At the time of writing, this is a poor process, in which steels which may be required to be especially good for a special purpose are actually made especially badly.
All castings which are top poured under gravity, including many sand castings, nearly all investment castings, and nearly all ingots, suffer the maximum damage from entrainment of air and oxides (Figure 3). All top pouring is bad.
Illustrating top pouring; uphill teeming; and contact pouring of steels.
In an effort to upgrade the ingot casting process, a bottom gating (sometimes known as uphill teeming) is carried out (Figure 3). The reduced splashing by uphill teeming improves the surface finish of the ingot. However, unfortunately, the interior quality of the steel is little improved. The falling stream jetting from the base of the ladle enters the start of the filling system at conical intake (often known as the trumpet). The trumpet and following channels need to be oversized with respect to the falling jet to avoid back-filling and over-flowing. This geometry results in at least 50 per cent of the fluid entering the conical basin as air. In the filling system pipe-work, the 50/50 air/steel mix is substantially thrashed together at speeds of up to 10 m/s, ensuring that the bifilm mix will never properly de-segregate, and the bubble trails will further contribute to the copious residual inclusion population, each trail contributing an impressively long crack.
At the high temperatures of some steels, and because of the compositions of some oxides, the crack can evolve to reduce its surface energy. The double film coarsens by diffusion, finally forming sheets of granular solid particles of oxide. The final product is therefore sometimes oxide fragments attached to a void, or gas-filled cavity such as an argon bubble, the residue of the bifilm ‘air gap’. The argon bubble remains after the oxygen and nitrogen have been taken into solution in this energetic mixing, leaving the 1 per cent argon in the air as the insoluble residue.
The overall result is that the internal quality of the bottom gated ingot is hardly any better than the top poured ingot.
A dramatic improvement to gravity pouring is achieved by contact pouring (Figure 3). The author now insists on contact pouring for all his shaped castings of any metal. The foundries which use this technique find that their cast products are transformed, including cast steels, Ni alloys, Al alloys and bronzes.
Returning to the casting of bulk steels, the continuous casting process certainly delivers a superior product to those steels cast as ingots. This is partly because the ladle take time to be delivered to the top of the casting machine, and then only slowly releases its melt from the base of the ladle – the steel at the base of the ladle having the best quality as a result of the melt cleaning automatically by flotation, and the extended time which is available for flotation, which can easily be 10 times longer than the time required to cast an ingot.
The continuous casting process could probably be much improved by paying attention to important details. The use of tapered nozzles for ladles and launders (the tapering avoids air entrainment into the nozzle which is probably the reason that nozzles block by oxide accumulation [1, 2]). Any fall exceeding 10 mm has to be controlled, so as not to occur in air but submerged under metal or slag. There is a huge amount of research concentrating on the
All the difficulties of mold filling by pouring under gravity, at which metals are accelerated to unwanted high speeds, and so creating masses of unwanted defects, are avoided by not employing gravity.
If now, by some means, the metal can be pumped uphill into a mold, its velocity can be controlled at every point, and need never exceed the critical velocity 0.5 m/s at which entrainment becomes possible. Furthermore, air need never be entrained, so that bubble damage from bubble trails cannot occur. The contrast between conventional gravity pouring and counter-gravity filling is seen in Figure 4. In the counter-gravity process the surface oxide film is never entrained; as the metal rises, the surface film simply splits and moves to one side, but instantly reforms and splits, moving aside etc. The surface film becomes the skin of the casting. It is never entrained. In principle, the counter-gravity casting of metals promises perfection.
Conventional gravity casting and counter-gravity casting.
However, attempts to achieve this perfection are, unfortunately, often not conspicuously successful.
The most disappointing process which nominally adopts counter-gravity filling is the low pressure permanent mold process for the casting of automotive castings, particularly wheels. Most embodiments of this process currently employ a large melting furnace to tip metal into a ladle, in which it falls at least a meter. This damaged metal is then driven by forklift truck to a treatment station, then to the furnace of the casting unit, into which it is tipped again, falling another meter and suffering more damage. The consequence is a really poor quality of metal, full of bifilm cracks, giving poor strength and toughness. If this were not bad enough, there is worse to come! The furnace is pressurized to displace the metal up the riser tube and into the mold (Figure 5(i)). After solidification of the casting the release of the pressure causes the melt to fall down the riser tube, thereby displacing all the oxide sediment, which has taken its time to settle at the bottom of the furnace, back into suspension, just in time for the next casting to be made. In addition, the depressurizing action causes bubbles to expand from pressurized gas trapped in crevices in the refractory walls, and the creating of generous quantities of bubble trails. Sufficient bubble trails can sometimes be created to make the metal uncastable; the furnace becomes filled with a slurry of metal and oxide films resembling concrete. Crucible furnaces (Figure 5(ii)) appear to be somewhat more resistant to the worst excesses of this problem because of the finer pore sizes from use of isostatic consolidation during their manufacture.
Low-pressure casting in (i) a refractory lined pressurized furnace, compared to (ii) a pressurized crucible furnace.
A more recent development is the application of pressure to the mold, pressurizing the incoming metal, and therefore acting to keep bifilms closed, with a benefit to properties. Naturally, this pressure effectively acts to counter the pressure used to pressurize the metal up the rise tube, hence the name ‘Counter-Pressure Casting.’ However, if counter-gravity is employed to cast good quality metal, in which the bifilm population has been reduced or eliminated prior to casting, the counter-pressure becomes redundant. The counter-gravity counter-pressure process seems to this author to be a step too far. Liquid metals, like all liquids, is effectively incompressible, and cannot be improved by pressure.
When counter-gravity casting is carried out well, with cleaned metal free from dense populations of bifilms, and when transferred uphill, against gravity, carefully controlled by a pump, the resulting castings can be spectacularly excellent.
In his early days in the casting industry, when the author first set up the Cosworth counter-gravity process, the castings requiring aerospace quality were cast in the half of the foundry containing the counter-gravity system using an electromagnetic pump for the liquid aluminum alloy. The other half of the foundry was retained for less important gravity cast products. Eventually however, it was found that with counter-gravity it was difficult to make a bad casting, whereas with gravity casting it was difficult to make a good casting. After 6 months, the gravity area was closed, and all castings were made on the pump.
The production of castings by high pressure die casting (HPDC) are generally limited to the low melting point metals Al, Mg, Zn and Pb. Some brasses are cast by this technique but attempts to cast stainless steels seem to have been abandoned. This brief description will concentrate only on the casting of Al alloys.
Although the term ‘high pressure’ seems to offer reassurance of a well-consolidated pore-free product, as most readers will be aware, this can be far from the truth and should never be forgotten by potential users. In general, the HPDC process can never guarantee freedom from porosity and leakage. Nevertheless, the process has valuable features and capabilities which distinguish it widely from other casting methods.
The process is often described as high productivity. It is true it benefits enormously from its ability to cast thin sections which can freeze quickly. But in common with all metal mold casting processes, the metal mold cannot be opened until the casting has frozen, or nearly frozen. This waiting time for the casting to solidify is a major contribution to the production cycle. High production sand casting systems can be much faster for any thickness of casting section, because after pouring, the mold can be moved away, allowing the immediate pouring of a second mold, and so on. Both sand and die systems can benefit from multiple impressions, giving multiple castings per filling.
For most HPDC machines, metal is spooned from an open holding furnace, and poured into a shot sleeve, from where it is rammed into a steel die by a piston. The steel die is sunk into a massive steel bolster, which is kept closed during the shock of the filling process by hydraulic rams developing hundreds or thousands of tons of force. This brutal description is not too far from reality, although the injection stroke and filling pattern is now often optimized by computer simulation to reduce air entrainment, which has resulted in significant improvements to the reduction in porosity in castings.
The turbulence during the injection process, in which the metal velocity usually exceeds 50 to 100 m/s, is so great that defects are necessarily created but are accepted as a feature of the process. Interestingly, the high density of bifilms is not necessarily the disadvantage that might be imagined; the long oxide flow tubes (the oxide tubes which surrounded the jets of metal entering the mold cavity) and other bifilms are aligned along the flow direction, giving a fibrous microstructure whose properties somewhat resemble the directional features of wood. The rapidity of the filling process, being completed within milliseconds, probably also suppresses the degradation of the casting by bifilms, whose constituent films have so little time to grow and are necessarily extremely thin. Their limited thickness may permit some bonding between the two films as a result of atomic rearrangements during their transformation from pure alumina to spinel as Mg in the alloy diffuses into the bifilm. The high pressure, keeping the two sides of the bifilm closely in contact is a further aid to bonding and, in any case, provides strength by the bifilm being enabled to resist shear force, because of jogs and wrinkles, if not direct tensile force. Even so, the HPDC castings can never be relied on not to leak, and sometimes, not to fail unexpectedly. Their use for safety critical purposes should therefore only be accepted with very great caution. (In contrast, gravity sand and gravity die castings [permanent mold castings] are typically favored for safety critical components).
Traditionally, small HPDC machines provide high productivity for small thin-walled products. The accuracy and surface finish are good, often eliminating machining, making the process favored by engineers. Recently, extremely large HPDC machines have been built to produce castings of several square meters area with walls only a millimeter or two in thickness, creating large pieces of automobiles in one shot.
There are some genuine reasons why vacuum is needed for the melting and casting of certain alloys and certain products. Sometimes, a limit on the oxidation of reactive metals or alloying elements is required. At other times the vacuum is needed to ensure the filling of extremely narrow and tapering sections as in turbine blades.
Alternatively, vacuum casting is used, imagining that this will prevent the formation of defects during a top pour. This appears to be a widespread but dangerously incorrect assumption. The entrainment defects resulting in bifilm creation appear to be the same no matter what environment is used, whether this is air, inert gas or vacuum. The reason is that both the inert gas and the vacuum environments always contain sufficient oxygen and/or nitrogen to create oxide or nitride films on the surface of the pouring liquid, so that defects of identical size and geometry are formed if entrainment of the surface occurs – the only difference being the thickness of the resulting bifilms. Bifilms are generally so thin that they are not easily seen when cast in air, but are, of course, far more difficult to detect in vacuum castings. The vacuum casting has a lower oxygen content, and is assumed to be cleaner, which in a way it is. But the distribution and sizes of its population of cracks appears to be unchanged [2].
The formation of bifilms in vacuum casting is practically universal, because ingots and castings poured in a vacuum furnace are nearly always top poured. In huge industrial VIM installations, the fall can be many meters, creating much damage to the metal. For instance, all the metal used by the aerospace industry for remelting for the casting of turbine blades is damaged during VIM preparation of the Ni-base alloys; the metal is top poured, falling many meters, down long vertical steel tubes; the larger the diameter of the tubes the worse the damage to the alloy by splashing and entrainment.
In probably all the leading R&D institutions in the world, metals and alloys for research are melted and poured in laboratory VIM furnaces, the top pouring, with the metal falling by a meter or more, fundamentally undermining or complicating nearly all metallurgical R&D worldwide (Figure 6). It has greatly contributed to the lack of understanding of more complex failure forms of metals such as fatigue, stress corrosion cracking and hydrogen embrittlement among others as a result of all researchers being unaware that their research materials were densely pre-cracked [2].
A simple laboratory vacuum induction furnace illustrating the awful top pouring, creating damaged products.
It is with great regret therefore that we have to conclude that the preparation of most metals and alloys by vacuum casting is a snare and delusion. It would be easily possible to make castings in air of far greater perfection by simply avoiding surface turbulence during the casting process. This is most effectively achieved by abandoning gravity pouring and adopting counter-gravity filling of the mold. The world needs to convert its casting operations to counter-gravity casting. The suffering of the casting world from the ubiquity of casting defects will then be of interest only for historians.
The secondary remelting processes for steels and Ni alloys are designed to deliver a premium quality of metal in the form of an ingot. Their starting material is a reasonably good metal in the form of a consumable electrode which is slowly and progressively remelted by arc, plasma, electron beam, or joule heating in a liquid slag layer etc. As the tip of the electrode melts, a new ingot is then slowly built up drop by drop within its protective environment of vacuum or slag. The ingot solidifies tolerably rapidly because of the use of a water-cooled mold.
At the time of writing, it requires to be noted, with regret, that none of the secondary remelting processes are totally reliable. All can have serious crack defects which can survive the subsequent forging or rolling, and the heat treatment, making these products unreliable in service. Some, as we shall see, can be seriously unreliable.
VAR is probably the most widely used of all the secondary remelting processes (Figure 7). The marketing of VAR benefits from its name: engineers are attracted to the concept of ‘vacuum’ suggesting cleanness.
VAR and ESR secondary remelting processes.
However, the VAR process is particularly susceptible to its slightly oxidizing vacuum conditions, growing an oxide skin on the horizontal ledges formed by the slow layer-by-layer advance of the solidifying liquid. This variety of advance occurs because of the strength of the oxide on the advancing meniscus as it rolls over the solidified or solidifying metal around the edge of the ingot. The vertical advance occurs by the horizontal flow of the liquid front, gradually spiraling upwards, advancing vertically by the 8 mm high steps corresponding to the height of the meniscus. This is the height which surface tension can support against the hydrostatic pressure due to this depth [1]. As the meniscus rolls over the oxide film on the freezing ingot, the meniscus lays down its own oxide film on top of the surface oxide film, creating a bifilm. It is a substantial crack, possibly extending up to 50 mm deep [2].
The presence of cracks around the circumference of VAR ingots is widely known. It is proven by the cracking of the ingots in forging (in contrast to ESR ingots which forge like butter). The manufacturers machine off around 5 mm depth of the outer surface as a token gesture to remove cracks. Because ingots forge better after the removal of the 5 mm it is certain that most cracks are removed. However, of course, it is unlikely, given the variability of conditions during arc melting, that all will have been removed.
The falling-in of the ‘crown’ of spatter and evaporated metal (Figure 7) into the forming ingot may introduce additional macroscopic bifilms. A further source of major bifilms is the electrode. The electrode is typically made by top pouring into an ingot mold, sometimes in air and sometimes in vacuum (the VIM/VAR process combination) but as we have seen, whether air or vacuum, the seriously deleterious defect distribution will be essentially the same. A bifilm taking up a substantial area of the cross section of the electrode may cause a large piece of the electrode to detach and fall into the melt. This unmelted fragment will be effectively surrounded by a bifilm (its own oxide surface collecting a covering of the oxide on the liquid as it plunged through the surface) together with its own internal oxide bifilms.
Nearly all producers of VAR material also produce ESR. The ESR process is probably the next most popular secondary remelting process. The tip of the electrode is heated by the passage of an electrical current through a slag layer (Figure 7). The thin film of melted metal, gathering over the base of the electrode, and finally detaching and falling through the slag droplets of metal, ensures that metal arriving in the melted pool contains only rather small bifilms. Any bifilm which happens to touch the slag will be sucked out of the liquid metal and into the liquid slag by capillary attraction: the solid oxide will be wetted by the slag, a mainly oxide liquid. It will then be dissolved in the slag and disappear. This sets a limit of around 1 mm for the maximum size of bifilm defect which could be present in an ESR ingot (contrasting interestingly with the potential for 50 mm defects in VAR). This ability of ESR to actively extract oxides and dissolve them is a fundamental and unique benefit of the ESR system. (In the early days of the process, no-one could understand how the ESR process improved the properties because no significant changes to the metallurgical structure could be seen!).
There remains a threat to the integrity of ESR material through no fault of the ESR process itself. The threat lies once again with the desire to provide only the cheapest electrode, and so, once again, electrodes are usually cast by top pouring. An electrode top-poured in air contains bifilm defects as surface-appearing laps which can be seen by the unaided eye from 100 m distance. It is no wonder therefore that, once again, large fragments can detach from the electrode during melting and can fall into the melt. These defects contain unmelted and unrefined material. The author has personally seen such a defect the size of his hand on the section of a 600 mm diameter ingot.
For the future, if completely reliable metal is required, the ESR process is the only currently available source, but requires the provision of an electrode cast by a reliable process. Such a process includes low-cost ingots cast by contact pouring (especially if enhanced by flush filters and spin traps), or perhaps an improved continuously cast material, or, ultimately, counter-gravity casting of some kind. The world would then have, for the first time ever, a totally reliable metal process free from macroscopic cracks, but containing only microscopic cracks of maximum size perhaps 1 mm.
The result of entrainment of bifilms during casting production results in huge losses in metals processing such as forging, rolling and extrusion. All these processes suffer from cracking of the processed metal, sometimes to the extent that the metal cannot be processed. Many steel ingots suffer from cracking during cooling. For this reason, fluted molds assist to disperse stresses across the faces of the ingot, although the technique is not especially successful for some steels. The break-outs of liquid steel from the cast strand during continuous-casting are almost certainly bifilm problems which is the reason this rather common disaster has remained unsolved. A number of Ni-alloy ingots are known for cracking at the first stroke of the forge, and rolled steels suffer edge cracking, longitudinal cracks, transverse cracks, internal cracks. Many metals suffer edge cracking during extrusion and rolling. Aluminum alloy semi-continuously cast slabs suffer cracks of all sorts, some measuring their length in meters across the slab face.
For those working in metal processing, valuable R&D has often been carried out to provide process ‘windows’ defining the limits of successful processing. Many such limits have been set by the onset of cracking. Processors would be delighted to see these limits eliminated. The processing of metals remains to be revolutionized.
If the metal is successful to survive processing, it then can suffer from its internal bifilm population during its service life. All its mechanical properties and failure modes are affected by its bifilms. Some of these aspects are discussed below.
Practically all of our engineering metals are intrinsically ductile. Basic dislocation theory predicts that if a stress is applied to a crack in most engineering metals, dislocations are emitted prior to the advance of the crack tip. The result is that the crack blunts, and crack propagation cannot occur.
(This behavior contrasts with the rather few intrinsically brittle metals, including W, Cr and Be, for which the imposition of a tensile stress causes the crack to propagate first, without the emission of dislocations. Fracture by cleavage is a variety of brittle failure but is only known for certain to exist in zinc).
In theory, therefore, tensile overload in the majority of our metals should result in plastic necking down to 100% reduction in area (RA) despite the metal possibly having high strength, resulting in high stress supported during the plastic failure.
Cast aluminum alloys fail this expectation lamentably, having typical elongations to failure in single figures, typically 3 ± 3%. Al alloys generally contain a dense populations of bifilm cracks because the alumina bifilms are slightly denser than the liquid, but contain some entrained air lending some buoyancy, causing bifilms to be close to neutral buoyancy, and thus remaining in suspension for hours or days. Conversely, steels typically reach 50% elongation because the rapid flotation of bifilms within minutes results in much cleaner metal. Other factors leading to some bonding across the central interfaces of bifilms in some steels further contribute to improvement [2].
In contrast to steels, the lack of a definitive yield point in Al alloys is probably due to the presence of bifilms, raising the stress around the bifilm because of the loss of load supporting area and the sharpness of the bifilm crack. Thus, plastic flow occurs early, spreading from scattered locations throughout the matrix of an Al alloy before the macroscopic yield point is reached. Similarly, the lack of a fatigue limit in Al alloys compared to steels can be similarly explained.
In the experience of the author, much of the area of many fatigue fracture surfaces is comprised of bifilms. The genuine fatigue areas characterized by ‘beach marks’ appear to be generally confined to a few regions which happen to be devoid of bifilms. The remainder of the surface is often described as quasi-cleavage failure, which is simply a polite admission of ignorance – no-one seems to know what quasi-cleavage is, except that it is definitely not cleavage. These regions appear to be bifilms, hiding in plain sight. The regions often outline grains because the bifilms tend to be trapped intergranularly between grains or are straightened by dendrite growth transversely across grains. When the advancing fracture reaches the limit of one bifilm and has to migrate out of its plane to continue its advance by opening the next bifilm, the plastic shearing process between bifilms outlines the grains.
A typical well-known example is the fatigue failure of the main bearings of wind turbines. These huge steel rings are forged from a single large ingot. The interior surface of the ring is naturally composed of the center of the ingot. Bifilms will have been segregated here by dendrite pushing. Because of the huge size of the ingot, the plastic deformation involved in forming this into a ring is modest; the bifilm cracks are merely pushed around a little but are by no means ‘welded’ closed. Large tangled masses of bifilms are therefore present on the inner surface of the bearing ring. These masses of pre-cracked regions are likely to be millimeters or even centimeters across. They experience the high (2000 MPa) rolling stresses, with the result that minute connections inside these regions, or linkages holding the masses to the matrix, will suffer even higher concentrations of stress, resulting in genuine fatigue failure of the tiny isolated connections holding the pre-fractured regions together. Ultimately, whole, macroscopic blocks of material break away among the rollers because of the minute, almost negligible amounts of fatigue, signaling the imminent death of the bearing.
There is excellent evidence for creep being significantly controlled by the presence of bifilms. In the comparison between polycrystal and single crystal turbine blades, it was traditionally explained that the overwhelming benefit to resistance to creep failure was the elimination of the transverse grain boundaries. It was assumed that the boundaries were weak. However, as much recent research has now demonstrated, grain boundaries are immensely strong. The traditional explanation is clearly unsatisfactory.
The realization that bifilms are present in the liquid alloy leads to a logical explanation. In the conventional polycrystalline casting grains nucleate and grow randomly throughout the cooling liquid. Bifilms in suspension therefore become trapped as grains collide, the bifilm effectively becoming coincident with the newly formed grain boundary. The boundary is therefore weak, effectively pre-cracked, and the polycrystalline casting is observed to have poor creep properties as a result of a high proportion of its boundaries harboring cracks.
In contrast, in the conditions for growth of the single crystal, the slow vertical advance of the freezing front will push bifilms ahead. Those that are not pushed may float. The result is a casting relatively free from bifilms, and displaying astonishingly good creep life.
Bifilms can act as invasive pathways for corrodents into the interior of metals. The outside surface of a metal may be tolerably resistant to corrosion, but at the location at which a bifilm emerges, breaking the surface, the ingress of rain or salt water is likely to form an etch pit. The localized corrosion around the bifilm may be enhanced by precipitates of second phases and intermetallics which favor the wetted exterior surface of the bifilm (its wetted exterior surface contrasts with its dry, unbonded inner interfaces). These different compounds with different electrochemical potentials attached to the exterior surface of the bifilm can provide vigorous corrosion couples.
Figure 8 shows a typical etch pit. Although the conventional explanation of the image would be that the etch pit has initiated the formation of cracks, the reverse is true. The cracks are bifilms, as can be identified from their morphology and precipitates. They have initiated the etch pit.
Etch pit in a steel turbine blade.
In the past decade there have been at least three, perhaps four or more, helicopter crashes, some extremely tragic, in which items of the drive train appear to have failed by fatigue initiated from an etch pit. Experts from around the world have been puzzled because an etch pit was far too small to have initiated the fatigue crack. In the case of one main rotor shaft, which appeared to have failed in this way, the shaft was designed with a safety factor of five. It is not conceivable that such a robust shaft could be threatened by an etch pit.
It is easily appreciated, that the etch pit is merely the witness to the presence of a bifilm crack. Furthermore, the bifilm could have been extensive, such as possibly extending over a major portion of the shaft. The shaft was formed, of course, from VAR steel, so that the probability of its being pre-cracked is virtually certain. The crack would have evaded detection because, being formed by oxidation in vacuum, its oxide films would have been extremely thin. Also, as a universal feature of castings, and heat-treated products, especially if quenched, the interior is in tension, but the exterior surfaces are in compression. The crack on the outside of the shaft would therefore have been tightly closed.
Attempts to find bifilms by nondestructive testing (NDT) has proven to be tragically unreliable. As always in such difficulties, the clear way forward is to use only those processes which do not generate bifilms and which are therefore intrinsically reliable.
This dangerous failure mode involves almost no loss of metal by corrosion but can generate deep cracks by a time-dependent advance, often under only low stress. A metal can be sensitized to SCC by heat treatments.
There seems to be good evidence that SCC is a bifilm phenomenon, whereby the corrodent is simply moving through the ‘air gaps’ of the bifilms, linking bifilms by corrosion, driven by the stress concentration at the bifilm linkages [2].
The action of certain heat treatments to enhance SCC susceptibility is here proposed to arise from the precipitation of second phases on the bifilm. The favored formation of precipitates on bifilms seems to be the result of the reduction in the strain energy of formation, because the volume change and shape change of the new arrival can be more easily accommodated by the ‘air gap’ of the bifilm. The movement of part of the new phase into the air gap is likely to assist the forcing open of the air gap, so that percolation of the corrodent is facilitated [2].
There are numerous theories which have attempted to explain HE, but the phenomenon cannot yet be claimed to be clearly understood. In practice, the ingress of hydrogen into a stressed steel can result in gradual loss of ductility, and final fracture. The process has been identified as the slow progress of a crack until the final fracture when there is insufficient area to support the load. Hydrogen enters the steel as a proton released from certain corrosion mechanisms. For research purposes, hydrogen is introduced by electrochemical processes. Significantly, researchers complain about the interference of blistering which upsets their experiments during the charging processes, and report they are at a loss to know how the blisters can nucleate [2].
Once again, the bifilm seems more than adequate to explain all these observed characteristics of HE. The blisters are the observation of bifilms, inflated by hydrogen, near to the surface of the metal. Clearly, bifilms in the interior of the metal will also be experiencing the pressurization of hydrogen gas. Bifilms will almost certainly aid the progress of the gas into the interior of the metal, greatly accelerating the apparent rate of diffusion. However, it is probably mainly those isolated bifilms whose internal pressurization is leading to internal stress build up which is countering the ability of the metal to withstand tension.
There has been much interest in the attempts to desensitize a metal to HE by providing sinks for hydrogen. The sinks have been generally thought to be dislocations, and stress fields around carbide precipitates. This author has proposed that bifilms are probably significantly more capacious sinks, and the action of carbides is to precipitate onto bifilms and to prize them open, enabling them to accommodate even more hydrogen. He suggests that in an increasing supply of hydrogen, the bifilm would act as a temporary reduction in the deleterious effect of hydrogen, but this benefit would only exist at low hydrogen levels. When the hydrogen pressure in the bifilms equalled and the exceeded the yield stress, the damaging effects of HE would resume unchanged [2].
The entrainment of the oxide film on the liquid metal during casting processes leads to widespread damage to metals. Pre-cracking by a poor casting technique is central to the loss of properties, and to numerous failure modes, including those during solidification, during cooling to room temperature, during metal processing, and during service conditions.
Casting processes involving top pouring are especially damaging.
Casting processes using gravity pouring can be designed to yield significant benefits in bifilm reduction and are recommended if counter-gravity cannot be provided [3].
Ultimately, counter-gravity casting is strongly recommended to be the new casting norm, capable of delivering defect-free cast products.
The current use of VAR steels in all critical applications (especially such applications as helicopter drive chains) appears to be dangerously unreliable.
The reliable secondary remelting process could be ESR if combined with a reliable electrode. The implementation of this process combination would be greatly valued by the engineering world.
Both primary and secondary casting processes can now be made to deliver economic metals which cannot fail; metals we can trust.
Ove Odredbe i uvjeti ističu pravila i regulacije u svezi korištenja IntechOpenove stranice www.intechopen.com i svih poddomena u vlasništvu IntechOpena, tvrtke sa sjedištem u 5 Princes Gate Court, London, SW7 2QJ, Ujedinjeno Kraljevstvo.
',metaTitle:"Odredbe i uvjeti",metaDescription:"Ove Odredbe i uvjeti ističu pravila i regulacije u svezi korištenja IntechOpenove stranice www.intechopen.com i svih poddomena u vlasništvu IntechOpena, tvrtke sa sjedištem u 5 Princes Gate Court, London, SW7 2QJ, Ujedinjeno Kraljevstvo.",metaKeywords:null,canonicalURL:"/page/cro-terms-and-conditions",contentRaw:'[{"type":"htmlEditorComponent","content":"Pristupom na stranicu www.intechopen.com slažete se s ovim odredbama, sa svim primjenjivim zakonskim odredbama, te se slažete s poštovanjem svih lokalnih zakona. Korištenje i/ili pristup ovoj stranici temelji se na potpunom prihvaćanju ovih odredbi. Svi materijali na ovoj stranici zaštićeni su primjenjivim zakonima o autorskim pravima i žigu.
\\n\\nSljedeća terminologija odnosi se na Odredbe i uvjete, te na sve naše ugovore:
\\n\\nKlijent, stranka, vi, vaš odnosi se na vas, osobu koja pristupa ovoj stranici i prihvaća IntechOpenove Odredbe i uvjete;
\\n\\nKompanija, tvrtka, mi, naše odnosi se na tvrtku IntechOpen;
\\n\\nStranke, strane odnosi se na klijenta i na nas, ili samo na klijenta ili nas.
\\n\\nSve odredbe koje se odnose na ponudu, prihvat ili razmatranje plaćanja, a za koja mi pružamo asistenciju klijentu, bilo na ugovoreni ili fiksni način, a s ciljem da se ostvare potrebe i želje klijenta u svezi s našim uslugama, su podložne zakonskim odredbama Ujedinjenog Kraljevstva.
\\n\\nOsim ako nije suprotno navedeno, IntechOpen i/ili svi davatelji licence vlasnici su intelektualnog vlasništva nad svim materijalima na www.intechopen.com. Sva prava intelektualnog vlasništva su pridržana. Stranice sa www.intechopen.com možete gledati, preuzimati, dijeliti, dijeliti poveznice i printati za osobnu uporabu, a temeljem pravila sadržanih u ovim Odredbama i uvjetima.
\\n\\nMi koristimo kolačiće. Korištenjem IntechOpenove stranice slažete se s korištenjem kolačića u skladu s IntechOpenovom Politikom privatnosti. Većina modernih, interaktivnih stranica koristi kolačiće kako bi omogućila ponovno pronalaženje korisničkih detalja kod svakog posjeta. Na našoj stranici kolačići se uglavnom koriste kako bi omogućili funkcionalnost i olakšali posjetiteljima korištenje stranice.
\\n\\nIntechOpen ili njegovi suradnici niti u jednom slučaju neće biti odgovorni za štete (štete uključuju gubitak podataka ili profita, druge poslovne prekide, te sve ostale štete) koje nastanu zbog korištenja materijala na IntechOpenovoj stranici ili nemogućnosti da se iste koriste, čak i ako je IntechOpen ili njegov predstavnik o takvoj šteti obaviješten pismenim ili usmenim putem. Neke jurisdikcije ne dozvoljavaju ograničenja garancija ili ograničenja obveza za posljedične ili slučajne štete pa se u tom slučaju ova ograničenja možda ne odnose na vas.
\\n\\nMaterijali koji se pojavljuju na IntechOpenovoj stranici mogu sadržavati manje greške, tipfelere ili fotografske greške. IntechOpen može napraviti promjene na bilo kojem materijalu koji se nalazi na stranici u bilo koje vrijeme.
\\n\\nIntechOpen nije formalno povezan niti s jednom vanjskom stranicom čije poveznice vode na www.intechopen.com, osim ako to nije izravno navedeno. Iz tog razloga IntechOpen nije odgovoran za sadržaj koji se pojavljuje na takvim stranicama. Poveznica na IntechOpenovu stranicu ne implicira povezanost sa IntechOpenom. Korištenje takvih poveznica isključiva je odgovornost korisnika.
\\n\\nZadržavamo pravo vlasništva nad cjelokupnom stranicom www.intechopen.com i nad svim materijalom na toj stranici. Koristeći se našim uslugama, slažete se da maknete sve poveznice na našu stranicu odmah nakon što to od vas zatražimo. Također, zadržavamo pravo da ove Odredbe i uvjete, i politiku o poveznicama izmjenimo u bilo koje vrijeme. Koristeći se poveznicama na naše stranice slažete se s ovim Odredbama i uvjetima.
\\n\\nAko smatrate da je bilo koja poveznica na našoj stranici sumnjiva iz bilo kojeg razloga, molimo vas da nas kontaktirate. U tom slučaju razmotrit ćemo micanje poveznice s naše stranice, iako nismo obvezni to napraviti.
\\n\\nBez prethodne privole i izričite pisane dozvole, ne možete stvarati okvire oko naših stranica ili koristiti druge tehnike koje na bilo koji način mogu promijeniti prezentaciju ili izgled naše stranice.
\\n\\nIntechOpen može ove Odredbe izmijeniti u bilo koje vrijeme i bez prethodne obavijesti. Koristeći ovu stranicu vi se slažete s trenutnim Odredbama i uvjetima koje su na snazi.
\\n\\nOve Odredbe i uvjeti su sastavljeni u skladu s odredbama prava Ujedinjenog Kraljevstva, a za sve sporove nadležan je sud u Londonu, Ujedinjeno Kraljevstvo.
\\n"}]'},components:[{type:"htmlEditorComponent",content:"Pristupom na stranicu www.intechopen.com slažete se s ovim odredbama, sa svim primjenjivim zakonskim odredbama, te se slažete s poštovanjem svih lokalnih zakona. Korištenje i/ili pristup ovoj stranici temelji se na potpunom prihvaćanju ovih odredbi. Svi materijali na ovoj stranici zaštićeni su primjenjivim zakonima o autorskim pravima i žigu.
\n\nSljedeća terminologija odnosi se na Odredbe i uvjete, te na sve naše ugovore:
\n\nKlijent, stranka, vi, vaš odnosi se na vas, osobu koja pristupa ovoj stranici i prihvaća IntechOpenove Odredbe i uvjete;
\n\nKompanija, tvrtka, mi, naše odnosi se na tvrtku IntechOpen;
\n\nStranke, strane odnosi se na klijenta i na nas, ili samo na klijenta ili nas.
\n\nSve odredbe koje se odnose na ponudu, prihvat ili razmatranje plaćanja, a za koja mi pružamo asistenciju klijentu, bilo na ugovoreni ili fiksni način, a s ciljem da se ostvare potrebe i želje klijenta u svezi s našim uslugama, su podložne zakonskim odredbama Ujedinjenog Kraljevstva.
\n\nOsim ako nije suprotno navedeno, IntechOpen i/ili svi davatelji licence vlasnici su intelektualnog vlasništva nad svim materijalima na www.intechopen.com. Sva prava intelektualnog vlasništva su pridržana. Stranice sa www.intechopen.com možete gledati, preuzimati, dijeliti, dijeliti poveznice i printati za osobnu uporabu, a temeljem pravila sadržanih u ovim Odredbama i uvjetima.
\n\nMi koristimo kolačiće. Korištenjem IntechOpenove stranice slažete se s korištenjem kolačića u skladu s IntechOpenovom Politikom privatnosti. Većina modernih, interaktivnih stranica koristi kolačiće kako bi omogućila ponovno pronalaženje korisničkih detalja kod svakog posjeta. Na našoj stranici kolačići se uglavnom koriste kako bi omogućili funkcionalnost i olakšali posjetiteljima korištenje stranice.
\n\nIntechOpen ili njegovi suradnici niti u jednom slučaju neće biti odgovorni za štete (štete uključuju gubitak podataka ili profita, druge poslovne prekide, te sve ostale štete) koje nastanu zbog korištenja materijala na IntechOpenovoj stranici ili nemogućnosti da se iste koriste, čak i ako je IntechOpen ili njegov predstavnik o takvoj šteti obaviješten pismenim ili usmenim putem. Neke jurisdikcije ne dozvoljavaju ograničenja garancija ili ograničenja obveza za posljedične ili slučajne štete pa se u tom slučaju ova ograničenja možda ne odnose na vas.
\n\nMaterijali koji se pojavljuju na IntechOpenovoj stranici mogu sadržavati manje greške, tipfelere ili fotografske greške. IntechOpen može napraviti promjene na bilo kojem materijalu koji se nalazi na stranici u bilo koje vrijeme.
\n\nIntechOpen nije formalno povezan niti s jednom vanjskom stranicom čije poveznice vode na www.intechopen.com, osim ako to nije izravno navedeno. Iz tog razloga IntechOpen nije odgovoran za sadržaj koji se pojavljuje na takvim stranicama. Poveznica na IntechOpenovu stranicu ne implicira povezanost sa IntechOpenom. Korištenje takvih poveznica isključiva je odgovornost korisnika.
\n\nZadržavamo pravo vlasništva nad cjelokupnom stranicom www.intechopen.com i nad svim materijalom na toj stranici. Koristeći se našim uslugama, slažete se da maknete sve poveznice na našu stranicu odmah nakon što to od vas zatražimo. Također, zadržavamo pravo da ove Odredbe i uvjete, i politiku o poveznicama izmjenimo u bilo koje vrijeme. Koristeći se poveznicama na naše stranice slažete se s ovim Odredbama i uvjetima.
\n\nAko smatrate da je bilo koja poveznica na našoj stranici sumnjiva iz bilo kojeg razloga, molimo vas da nas kontaktirate. U tom slučaju razmotrit ćemo micanje poveznice s naše stranice, iako nismo obvezni to napraviti.
\n\nBez prethodne privole i izričite pisane dozvole, ne možete stvarati okvire oko naših stranica ili koristiti druge tehnike koje na bilo koji način mogu promijeniti prezentaciju ili izgled naše stranice.
\n\nIntechOpen može ove Odredbe izmijeniti u bilo koje vrijeme i bez prethodne obavijesti. Koristeći ovu stranicu vi se slažete s trenutnim Odredbama i uvjetima koje su na snazi.
\n\nOve Odredbe i uvjeti su sastavljeni u skladu s odredbama prava Ujedinjenog Kraljevstva, a za sve sporove nadležan je sud u Londonu, Ujedinjeno Kraljevstvo.
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