High-Entropy Alloys for Bone Tissue Engineering: Recent Developments in New Methods of Manufacture

Abstract

The demand for bone implants with superior biocompatibility and mechanical properties in bone tissue engineering is increasing due to rising demand for artificial bones and bone implant to replace degraded bones in human bodies. The causes of bone degradation in human bodies are not just due to rising number of road traffic accidents but are also due to disease burdens and injuries due to war and game activities. As a result, there is an urgent need to develop modern methods of manufacturing materials for implantable bone substitutes required in defective skeletal structures that cannot grow or heal on their own. It is believed that high-entropy alloys (HEAs) are best alternative materials for bone implants and development of modern methods for processing such materials could lead to manufacturing bone implants with the superior biocompatibility and mechanical properties. Therefore, this chapter examines the recent advances made in developing new methods for manufacturing bone implants using HEAs as raw materials. The chapter finally recommends the most appropriate methods for this purpose.

1. Introduction

The demand for bone implants with superior biomechanical properties is increasing due to rising number of causes for degraded bones in human bodies. These may include road traffic accidents, disease burdens, and injuries arising from wars and game activities [1]. As a result, there is an urgent need to develop modern materials for implantable bone substitutes required in defective skeletal structures that cannot grow or heal on their own. Generally, these biomaterials need to be biocompatible; that is, they must be bio-inert, with tensile strength of 3.7–140 MPa, Young’s modulus of 0.16–18.1 GPa, high corrosion resistance, and porosity of 30–60% for easy osseointegration [2, 3, 4, 5]. These properties have contradicting existence and therefore require careful preparation methods.

In the recent past, several research activities have been carried out to develop biomaterials with most favorable properties such as good biocompatibility, appropriate corrosion resistance, low elastic modulus, and comparable scratch hardness [6, 7, 8, 9, 10, 11, 12, 13]. According to literature, titanium (Ti) alloys, especially with nickel (Ni) as an alloying element, emerged to be the most suitable biomaterials and these have been extensively investigated for possible application in biomedical implants. In addition, TiNi alloys are preferred due to their super elastic ability and shape memory properties [14]. Further, TiNi displays excellent biocompatibility due to the formation of a thin titanium oxide surface [15]. Moreover, the addition of extra elements to Ti such as copper resulted in an alloy with the improved properties. Therefore, TiNiCu alloy has attracted interest for biomedical and other applications due to superelastic behavior, better fatigue, and improved shape memory properties [16, 17]. From the aforementioned, it can be seen that alloying is among the most appropriate methods for improving properties of metal materials. Moreover, studies have shown that when there is an increase in the number of alloying materials, a significant improvement in the properties of the alloy occurs. Consequently, high alloy materials also called high-entropy alloys (HEAs) are emerging to be among the best alternative materials with the favorable properties for newer applications [18, 19]. However, further research suggests that the medical application of HEAs especially those that are Ti-based is limited as these materials are still being developed. This implies that there is not yet any comprehensive in vivo evaluation of the Ti-based HEAs as implants to assess aspects such as biomechanics, biocompatibility, histology, and osseointegration [13, 20].

On the whole, studies have proved that during examining the traditional alloys, it was established that there was one principal metallic element that was seldom mixed with more than three principal metallic elements. Such a practice was common in alloys such as steels that are usually based on iron, and are sometimes made as super alloys that include nickel and cobalt as alloying elements, and intermetallics that had two metallic elements such as nickel-aluminum as compounds and metal-matrix composites that were based on three elements such as nickel, titanium, and aluminum. As such, under these traditional arrangements, metallurgists could make and process the alloys, and study their microstructure and properties for targeted applications. Obviously, the degrees of freedom in the alloy development are confined by the alloying concept. However, the development of new metal alloys took center stage in the recent past in order to improve the properties of the materials to meet the modern demands. As a consequence, alloys composed of multiple elements having higher mixing entropy than conventional alloys are being proposed with a view to improve their properties mostly due to mixing enthalpy that allows the addition of suitable alloying elements to improve their properties [20].

As such, several research works have been carried out to develop metallic biomaterials with the highest biocompatibility and least toxicity properties. As a result, more complex compositions with higher mixing entropies have been introduced. However, such complex compositions do not necessarily guarantee a complex structure and microstructure due to accompanied brittleness. Conversely, significantly higher mixing entropy from complex compositions could simplify the structure and microstructure and impart attractive properties to the alloys [20]. Accordingly, high-entropy alloys (HEAs) are emerging to be among the best alternative materials with favorable biocompatibility properties. In this respect, this chapter examines the HEAs and, based on their structure and properties, suggests them as alternative materials for biomedical implants. The chapter also examines the traditional and concurrent methods of manufacturing to guide processing engineers during selection of the most suitable method.

2. High-entropy alloy (HEA)

2.1 Definition of HEAs

High-entropy alloys (HEAs) are defined as those alloys with at least five alloying elements, each of which has an atomic concentration between 5 and 35% and are mixed in equal or relatively large proportions. Prior to the synthesis of these alloys, typical base metal alloys comprise one or two major components with smaller amounts of other elements. The alloying elements can be added to base alloys to improve their properties, thereby creating a complex alloy, but typically in fairly small proportions. The alloys are created by a typical solid solution process. This makes the high-entropy alloys to be among the most novel class of materials. The term “high-entropy alloys” was coined because the large number of alloying elements increases the mixing proportions that are also more nearly equal. This mixing strategy significantly distorts the crystallographic structure of the HEAs with a high tendency to improve their properties thereby making these materials to be currently the focus of significant attention in materials science and engineering because they can be made with potentially desirable properties. Furthermore, literature indicates that some HEAs have considerably better strength-to-weight ratios, with a higher degree of fracture resistance, tensile strength, and corrosion and oxidation resistance than conventional alloys [20, 21, 22].

2.2 Lattice structures of HEAs

It is well known that the introduction of several alloying elements through substitution solute atoms into a solvent parent matrix causes the displacement of neighboring atoms from their ideal lattice positions. These displacements do not only generate significant lattice distortions but also generate a strain energy field that also induces changes in bulk lattice parameter, as illustrated in Figure 1. The introduction of strain energies induces new properties to the metal crystals. Also, these localized distortions around the solute atom will interact elastically with dislocations moving through the material, resulting in solid solution with enhanced altered properties [23]. A schematic representation of localized lattice distortion effect in HEAs is illustrated in Figure 1. As shown in this Figure 1, the four colors (green, orange, red, and blue) represent the four elements in the HEA alloyed by substitution to create the distinctive distortion in the lattice structures. These distortions induce distinctive properties to the alloy. What is also vivid from this structure is that the geometric orientation, and the extension and location of the lattice distortions are influenced by the type, size, and location of the alloying element. This effect is very vital in choosing the type of alloying elements, sequencing of atoms, and method(s) of alloying.

Figure 1.

Schematic representation of strained lattices in HEA [14].

Well-established models for solution strengthening have been produced for both dilute and concentrated alloys, and their modification for HEAs is discussed in the literature [14]. A number of studies have suggested that severe lattice distortion contributes significantly to new HEA properties, most notably with respect to increasing alloy strength. Importantly, however, it is apparent that the strengthening effect of precipitates may have been overlooked in some cases. It has been suggested that these distortions arise not only from atomic size misfit, but also differences in the crystal structure and bonding preferences of alloying elements before [14].

2.3 Properties of HEAs

Several studies that have been carried out and are reported in the literature demonstrate that the HEAs could be made, processed, and analyzed just like conventional alloys. Moreover, these alloys exhibit several interesting features that are also reported in the literature. These alloys can be formed by simple solid solution phases such as FCC and BCC with nanostructures or even amorphous structures. The following are selected properties possible in HEAs materials:

• Elevated temperature strength and oxidation resistance.

• Improved resistance to (stress) corrosion.

• Low density.

• Super plasticity and high-strain-rate.

• Surface hardness with Hv100 to Hv 1100.

• Good thermal stability in microstructure [20, 24]

2.4 Applications of HEAs

Due to the above special properties, HEAs have many potential traditional and nontraditional engineering applications: firstly, traditional applications such as aerospace, marine, furnaces, and several other parts requiring the properties of high strength, thermal stability, and wear and oxidation resistances, anticorrosive high-strength materials in chemical plants, and foundries. Secondly, nontraditional applications such as biomedical implants, special game tools, super elastic alloys, ultra-large-scale integrated circuits, and soft magnetic films for ultra-high-frequency communication [13, 24].

3. Methods for manufacturing HEAs

The methods for manufacturing HEA can generally be classified as traditional bulk material consolidation methods and nontraditional additive manufacturing. Bulk consolidation methods include casting and sintering or powder metallurgy, while additive manufacturing methods include surface coating technologies such as thermal spraying, laser cladding, magnetron sputtering, selective laser melting, Among these methods, powder metallurgy and selective laser melting are the most used techniques to prepare the HEAs, while laser cladding and magnetron sputtering methods are commonly used in order to prepare HEA thin films or coatings. The advantages and limitations of selected methods of preparation for HEAs are listed in Table 1.

Preparation methods	Advantages	Limitations
Powder metallurgy	Near-net shape forming Higher utilization of material Uniform composition Save metals and reduce costs	Poor toughness Higher die cost
Magnetron sputtering	Film thickness can be controlled by adjusting sputtering parameters. Low requirements on target composition High sputtering rate, and low base temperature can obtain a dense film surface	Higher preparation cost. Complex equipment Lower target utilization
Laser cladding	High melting and cooling rate Small heat-affected zone Metallurgical combination of coating and substrate	The cladding layer is easy to crack Uneven distribution of composition

Table 1.

Advantages and limitations of methods for preparations of HEAs [25].

These methods are viable and advanced means of production and also the use of technological method to improve the quality of the products such as metal alloys and/or high-entropy-alloying processes, with the relevant technology being described as “advanced,” “innovative,” or “cutting edge.” These technologies evolved from conventional processes some of which have been developed to achieve various components of alloys [15]. The following are the selected methods used to produce HEAs.

3.1 Powder metallurgy

Powder metallurgy (PM) also called bulk sintering, illustrated in Figure 2, is one of the widely applied methods for manufacturing titanium alloys. This method can also be applied to make HEAs. In this method, the feedstock powder elements are mixed thoroughly using a suitable powder blender, followed by compaction of the mixture under high pressure and by sintering at appropriate temperature for suitable duration. In this manner, the powder particles bond to each other with minor change in their shapes. The porosity of the alloy can be regulated by controlling the sintering temperature and time. When properly implemented, this method can produce accurate parts with near-net shapes. The major advantages derived from this methods include good mechanical properties of the products, near-net shape, lower cost, full dense materials, minimal inner defect, nearly homogenous microstructure, good particle-to-particle bonding, and low internal stress [26].

Figure 2.

Powder metallurgy process [26].

3.2 Magnetron sputtering

Magnetron sputtering is one of the additive manufacturers, which uses certain high-energy particles to bombard the surface of a specific material. This process is shown in Figure 3, in which argon gas is placed in a magnetron sputtering drum, in which due to the action of strong electric field, argon particles are initially ionized into argon ion and electrons. Then, these ions are accelerated toward the cathode in the electric field to bombard target surface with high energy. Thus, the impact of the ions causes the sputtering of the target. As a result, the target material will emit secondary electrons and ionizes due to continued bombard of the target causing the target to sputter deposition on the substrate surface to form a thin film [27].

Figure 3.

(A) The schematic of magnetron sputtering and (B) balanced and unbalanced types of magnetron configurations [27].

3.3 Laser cladding

Laser cladding, illustrated in Figure 4, is one of the additive methods applicable in manufacturing of HEAs. This method is sometimes called surface modification process because it is predominantly a surface technology. As such, this process can improve the surface hardness, wear resistance, and corrosion resistance of the surface by cladding the alloy powder on the substrate. Laser cladding can also be carried out using either a wire (including hot or cold wire) or powder feedstock. The laser developed provides the energy to melt the pool on the surface of the work piece into which the wire or powder is simultaneously added. The result of the melt is that a metallurgically bonded layer is created and usually, this is tougher than the layer that can be achieved with thermal spray and less dangerous to health than the process of hard chromium plating. This process is flexible because the operator can easily mix many powders and he can control the feed rate for both separately and independently thus making this process suitable for fabricating heterogeneous components on functionally graded materials. In addition, this technology allows the material gradient to be altered at the microstructural level due to the localized fusion and mixing in the melt pool. This means that the clad materials can be designed to meet the performance requirement of the added layer [18].

Figure 4.

Schematic diagram of the laser cladding [18].

4. Conclusion

In conclusion, the properties and the need for HEAs have been reviewed, in which it was established that HEAs are materials of the choice for future generation alloys. Selected methods of manufacture for HEAs were briefly discussed, from which it was evident that these methods range from traditional bulk forming to newer additive manufacturing technologies. With the advances in technologies, this trend in manufacturing technologies for HEAs offer a new and exciting approach for alloy design and manufacturing to meet the complex demand in bone tissue engineering. In this respect, it is recommended that the research activities should move away from trying to obtain single-phase HEAs, but instead develop alloys that possess the correct balance of desired properties for biomedical implants.

Conflict of interest

The author declares no conflict of interest.