The β−phase content in Ti–2.7at.% Zr alloy
Intense development of science and technology with ever-increasing needs in new materials with the unique properties requires implementation of careful research in this area. Development and production of new types of materials is always related to the enormous costs and solving new technical problems of analytical and experimental nature. Recently a large class of the model alloys on metallic base, which meets various demands, has been created. A special place among them is occupied by the metals and alloys that undergo phase transformations. The requirements for the radiation resistance of these materials are of extraordinary importance. Investigation of the fundamental properties of materials that determine their physical, chemical, mechanical, technological, operational and other characteristics enables one to establish a field of their rational application with maximal efficiency.
The attention of the researchers should be drawn to investigation of the structure transformations in crystals, especially, their electronic and defect structure as well as their role in the process of formation of the material’s final physical properties. Almost all of the material’s properties are related to its electronic structure, and the constancy of the structure under external exposure determines stability of the main characteristics and can serve as a principal indicator of materials radiation resistance.
The problems of nuclear and thermonuclear power pose an urgent demand on continuous and wide range investigation of interaction processes of nuclear radiation with metallic materials, along with subsequent modification of their structure. For authentic establishing of common regularities of the observed phenomena, deep understanding of the processes of nucleation, formation and subsequent evolution and modification of the metals and alloys defect structure is crucial.
In spite of considerable amount of realized investigations, the analysis of the obtained results justifies the following findings: by the time of preparation of this work, the lack of the systematic information was experienced about the character of the radiation damageability of some perspective constructional refractory metals and their alloys, which firstly undergo polymorphous or phase transformations; influence of the type and concentration of alloying elements on the character of the structural disturbances at plastic deformation and radiation exposure in conditions of vacancy and vacancy-impurity complexes formation, packing defects, dislocation loops subject to material history, fluence, energy, flux, nature of ionizing radiation, temperature of irradiation and postradiational annealing.
There was no a thorough research of the influence of preliminary thermochemical treatment, including hydrogen and other atomic gases saturation and cyclic thermal shocks with an account of reconstruction of electron structure and density of pulse distribution of electrons in the field of defect production on the materials’ final properties. Availability of such data would complete a full picture of purposeful properties changes and make possible working the materials with predetermined properties. This problem definition caused by demands of the state-of-the-art science and technology appears to be strategically important area of research in the fields of physics of metals, physics of radiation damage and radiative study of materials.
Therefore, the main goal of the present work, which is based on the authors’ own research, is investigation and establishment of regularities of the electron structure alteration and its correlation with different titanium alloys crystal lattice defects created as a result of deformation radiation and complex thermochemical treatment.
2. Experimental and software-supported investigations
Since science of metals is generally experimental science, the depth, objectivity and reliability of our understanding of investigated phenomena related to materials electronic and defect structure are determined by capacity of the technical means and methods of investigation used in order to solve the problem.
The method of positron spectroscopy is the most important instrument of investigation in this case. Not only did the relativistic quantum mechanical theory developed by Dirac (1928) explain the main properties of electron and obtain the right values of its spin and magnetic moment, but also it determined the positron existence probability. Positron is the antiparticle of electron with the mass
Natural positron sources normally do not exist. Therefore positrons are usually obtained from nuclear reactions in different nuclear power plants. The principal criteria for choosing positron sources are the cost and half-life period. The most widespread is the sodium isotope 22Na obtained from 25Mg by reaction (
The essence of using positrons for solid structure probing is explained in the following. A positron emitted by a source while penetrating in solid to a certain depth subject to energy, experiences numerous collisions with the atoms of the solid, and consequently this positron gradually loses its velocity and at the end gains energy that corresponds to environment’s absolute temperature:
Positron thermolysis process occurs during the time, which is considerably shorter than its life time before annihilation. This circumstance serves as grounds for using positrons in order to study the properties of condensed matters, because the conduction electrons, with which positron interacts, occupy the energetic band of the range of several electron-volt and more importantly positron does not contribute to the total pulse and energy of the pair and hence can be neglected. Therefore, the information, which is carried by the positron annihilation photons, corresponds to solid electrons state, in which positron’s thermolysis and interaction and annihilation processes have occurred.
Annihilation is the act of mutual destruction of a particle and its appropriate antiparticle. While no absolute destruction of either matter or energy occurs, instead there is a mutual transformation of particles and energy transitions from one form to another.
Due to the law of charge parity a positron in singlet state (1S0) decays with emission of even number (usually two) gamma-ray quanta. A positron in triplet state annihilates with emission of odd number (usually three) photons. The probability of the 3γ - process is lower by more than two orders than the probability of the 2γ - process. Therefore, all basic research that is oriented towards studying properties of condensed state properties is performed around this phenomenon.
If an annihilation pair is found in the state of rest in center-of-mass system (v=0), then in laboratory system of coordinates two photons would be emitted strictly in opposite directions at sin
This circumstance is initiating development of the method of measuring angular distribution of annihilated photons (ADAP). The purpose of the method is to obtain information about electrons distribution function in momentum space. To this effect, it is supposed that length of the detector , which registers the annihilated photons, must be much longer than its width (
At rest positron the impulse of annihilated photons is defined by electron impulse. The latter is uniformly distributed on whole Fermi sphere for ideal gas of electrons [2,3]. Therefore, ADAP measuring boils down to choosing thin sphere layer on distance
This distribution vanishes outside
The normalizing factor
The constant factors
The positrons annihilation process in solids can be described by a set of parameters. But, the most informative for material properties characteristics are those, which successfully fit into different physical regularities, i.e. those that carry in themselves one or another physical meaning. One of these parameters can be the values of probability of positrons annihilation with free and bound electrons. These parameters are derived from processing of experimental angular distributions spectra of annihilation emission (4). The area under each component () is usually defined by integration. Knowing the total area under the whole curve , we can calculate the positron annihilation probability with free electrons and electrons of ion core, respectively:
as well as the redistribution of positron annihilation probability between the free electrons and the electrons of ion core:
The example of decomposition of experimental spectra to components is shown in Fig. 3. Due to parabolic component spreading in the
The changes in the investigated material structure are by all means reflected on the spectra form and lead to redistribution of positron annihilation probabilities. In this case after normalization to a single area, they can be built on one axis for comparison purposes (Fig. 4).
While comparing results of one set of measurements, which are related to thermal, deformative or radiative influences on the investigated materials, one can use a special configuration parameter sensible to the presence of only one kind of defect in the crystal :
The basic specifications of the experimental spectrometer of annihilated photons angular distributions with line and slot geometry are the following:
Angular resolution of the setup changes within the range of 0.5–1.5 mrad.
The time resolution on the fast channel equals to 100 ns and on the slow channel ranges within the interval of 0.3–1.0μs.
The movable detector step width is set stepwise by 0.25, 0.5 and 1.0 mrad; in all the setup permits to measure up to 50 values of coinciding gamma photons intensity in one direction from the spectrum maximum position.
The counting rate instability in the course of three days of continuous work does not exceed two standard deviations.
The allowed maximum intensity of incoming information on the slow channel is no worse than 3 × 105 s−1.
The maximal vacuum in the measuring chamber is no worse than 10−4 Pa at the temperature of 300 – 1100 К.
The positrons source activity of 22Na is 3.7 × 108 Bk (10 mKi).
The reliability of positron investigations results depends on a number of reasons which are as far as possible taken into account during the process of experimental investigations.
The preparation of investigated objects of different composition was realized in the high temperature electroarc furnace on a copper bottom with nonexpendable electrodes. After batch charging and before alloying, a vacuum of ~10−2 Pa was created in the furnace. After that a high purity argon was introduced into the furnace, in which all the processes of melting were conducted in this atmosphere. For homogeneity, ingots were repeatedly melted (up to 5–6 times).The finished ingots were rolled at a temperature of 9000C up to 1–2mm strips and then annealed at 10−5 Pa vacuum at 900 0C during 2 hours. The annealed samples were prepared from 1mm strips, which repeatedly underwent plastic deformation (
2. Modification of titanium alloys defect structure by plastic deformation method
The progress of modern engineering and technology is closely related to the achievements in science of metals, which before taking a specified form and properties usually undergo plastic deformation. Not only is the deformation process one of the effective methods for giving the required form to a material but it is also an important means for modifying its structure and properties. Yet defect formation and defects influence on metals physico-mechanical properties is one of the important problems in metal physics. For investigation of modification processes in metals structures the titanium binary alloys, alloyed with Zr, Al, Sn, V, Ge and In within the range of solid solution, were prepared. The elements content in alloys was defined more precisely by chemical and spectral analysis.
Zirconium Zr is an analog of Ti and forms with it a substitutional solid solution of complete solubility. As far as concentration of Zr is increased the temperature of the allotropic transformation of Ti slightly drops and reaches the minimum at equiatomic ratio (50 at.% and 545ºС). Thus, Zr is a weak β–stabilizer for Ti. Usually β–phase is not preserved in this system at the room temperature. Al is a substitutional element for Ti with limited solubility in α- and β-phase at presence of peritectoid breakup of the β–solid solution. The Ti-Al system plays the same role as the Fe-C system for steels in physical metallurgy.
While alloying Ti by Sn, the eutectic systems are formed. Sn forms the system with limited solubility of alloying elements at presence of the eutectic breakup of β–solid solution. Sn considerably differs from Ti by its properties and it is restrictedly soluble in both Ti modifications. Ti-In is one of those systems that are most insufficiently explored due to considerable difficulties related to preparation of alloys. In slightly reduces the temperature of alloy polymorphic transformation and is therefore a weak β–stabilizer.
When alloying titanium with vanadium a solid solution in β-Ti is formed, with complete solubility. Ti-V phase diagram strongly depends on the method of obtaining Ti (iodide, hydride-calcic or magnesium-thermic). The V solubility in
2.1. Structural transformations in plastically deformed alloys of the Ti-Zr system
Influence of plastic deformation on the structural damages formation was investigated on the alloys that contained 0; 2.7; 8.3; 17.0; 22.0 and 39.0 at.% Zr. Plastic deformation
In order to establish regularities of changes of annihilation parameters depending on the structure damages level, Ti and its alloy Ti – 2.7 at.% Zr underwent plastic deformation by different degrees in the range from
In order to determine the phase composition, titanium and Ti–2.7at.% Zr alloy underwent X-ray analysis using the DRON-2 diffractometer in the filtered CuKα radiation with the help of the special methods of accuracy enhancement. Then the β−phase concentration in the lattice structure can be determined by the equation :
Hence, one can do the following conclusion. For those alloys that undergo transformation, plastic deformation at room temperature initiates phase transformation since energy rise in the crystal introduced by the defects may thereby decrease. In this case, the boundary with matrix regions of new phase nucleus should be assumed as the most likely positrons capture centers.
In such metals as Ti and Zr, in which the phase transformations from HCP to BCC-structure occur at relatively low temperatures, the packing defect formation energy in prismatic plane must be small . With this in mind one cannot help noticing the nature of dependence of the relative change Δ
The maximum changes Δ
Experimentally obtained value of Δ
Therefore, one should consider two main factors that are responsible for initiation of the polymorphic transformation in the Ti-Zr alloys: plastic deformation, which leads to the formation of packing defect with the BCC-phase structure in the HCP-phase matrix, and positron interaction with lattice instability. Separation of contribution pertaining to each factor to annihilation parameters change is as yet an impracticable problem for employed experiment conditions.
2.2. Structure modification in the Ti-Al and Ti-In alloys
In order to obtain additional information about the nature of positrons interaction with the structure damages in the plastically deformed metals the second group of binary titanium alloys was prepared. These alloys contain alloying elements from the III group of the periodic system, namely Al in concentrations 0; 5.2; 10.2; 12.5 and 16.5 at.%, and also In with concentrations 0; 1.4; 2.9; 5.1; 8.5 and 10.3 at.%. The maximum concentration of alloying element in each of these systems meets the requirements of mandatory occurrence of alloys in the solid solution region, where the chemical compounds formation is ruled out in advance.
All alloy samples with the specified concentrations of alloying elements were deformed by the cold rolling method by
If a strongly deformed alloy is to be considered as a two-phase system, then on the basis of positrons capture models one can define approximate bulk defect size therein:
2.3. Peculiarities of positrons annihilation in the deformed Ti-Sn alloys
These alloys contained the following concentrations of alloying element: 0; 1.2; 2.5; 4.3; 6.2 and 7.6 at.% Sn. All alloys were deformed by cold rolling by
One can make yet another observation which is typical for these alloys: the nature of change of the alloying elements parameters at small concentrations roughly coincide, whereas maximum decrease of the Fermi angle (Δ
Based on abnormally large changes of
On the basis of the findings related to studying Ti alloys one can state the following. For the deformed Ti alloys the sharply expressed anomalies are typical, which become even stronger with certain concentrations of the alloying elements and when the initial lattice is fully reconstructed as a result of considerable shortening of material interatomic distance. Consequently it is not possible for crystal lattice to preserve initial electron structure. Therefore, probably for the investigated Ti binary alloys we should adopt a concept of autonomy of d-electron matrix subsystem relative to the alloying elements interstitial atoms conduction band, when the wave functions of the matrix atoms d-electrons are overlapped with the wave functions of impurity atoms conduction electrons. The latter is probably correlated somehow with the lattice instability. With this in mind the largest lattice instability are displayed by the investigated Ti alloys systems that contain 1.2 at.% Sn, due to which the maximum change of Δ
2.4. Restoration of structure damages in plastically deformed titanium alloys
In many metals, the structure damages generated at low temperatures are usually “frozen”, which enables investigation of their spectrum by measuring some macroscopic properties of the crystal with its subsequent heating. Among the most applicable are the methods of residual electrical resistance measurement, crystalline lattice period measurement, X-ray line profile measurement, etc. For dislocation structure investigation, electron microscopic and neutron diffraction methods were most effective [7, 8].
The main task of investigating metals modified by cold deformation is the differentiation between effects that are related to the presence of the structure defects ensemble in them. These effects can be more or less successfully determined while investigating the processes of recovery and recrystalization of deformed metals by annealing. The most important structure imperfection annealing mechanisms are absorption of point defects by dislocations, mutual destruction of vacancy and interstitial atoms, breakup of point defects aggregations into individual one, etc. As a result of these mechanisms, while heating, the structure damages are partially or fully annealed in different temperature ranges. Eventually the annealing process can be interrupted after recrystallization, due to which full removal of the defect structure and the initial structure restoration is observed.
Of course, in the given temperature range more than one independent or concurrent processes can occur. Then the overall picture of annealing kinetics can be presented as superposition of separate individual processes each of which is responsible for a certain return mechanism. In addition, it appears to be impossible to avoid formation of the complexes, mobility of which can significantly differ from that of the single defects. The annealing kinetics of structure damages in metals, provided that there is one type of defects, can be described by following equation [9, 10]:
The essence of conducting the isochronal annealing by EPA method is based on the ADAP curve return for a defect material up to annealed state. As a result, the defects annealing states are usually established and the respective activation energy is defined by the following equation :
where is a Debye frequency; eV/K is the Boltzmann constant;
The results of annealing investigations for titanium deformed by
The results of annealing of Ti–2.7 at. % Zr alloy, which was exposed to different degrees of deformation, seems more interesting. It is easy to see that the annealing curves shape for alloy considerably differs from those of Ti that verifies the appropriate role of the alloying element Zr. With an increase of the plastic deformation degree the temperature threshold of recrystallization gradually shifts towards low temperature: from 200ºС at
2.5. The structure modification of Ti-V alloys system as a source of packing defects
The positrons interaction with packing defects is of certain interest because some researchers tend to doubt that positrons can be captured by defects of this type . Therefore, arrangement of special experiments with more precise methods for detecting the latter is the task of high importance. However, in order to ensure with the highest probability packing defects occurrence as a result of plastic deformation, the investigated titanium alloys have to be accordingly chosen. To this effect, the alloying elements must form a complete solubility of solid solution with titanium without eutectic and peritectic in the investigated concentration range of the second component with the successive decrease of the phase transformation temperature. In other words, the alloying element must be β−stabilizer.
Packing defects are a voluminous lesion. The objective of this research was to study packing defects by virtue of comparing X-ray structural analysis results with positron annihilation data. Based on the aforementioned, vanadium (V) was chosen as an alloying element, which is located in the Y-group of the periodic system. The energy of V packing defects formation is ν = 0.1 J/m2, that is five times greater than for Ti but considerably smaller than for other metals of the transition group. V is a β-stabilizing substitutional element for Ti with the atomic diameter of 2,72Å. The V alloys with 0; 0.5; 1.5; 2.0; 4.0 and 5.8 at. % content were prepared by the technique described above. The plastic deformation by
One can see that for all investigated alloys the plastic deformation leads to an increase of the parameter
The results of the isochronal annealing for deformed Ti, V and Ti – V alloys are presented in Fig. 10.
On the basis of stage II annealing results analysis one can notice that among the investigated materials the packing defects are most pronounced only in the alloy with 2 at.% V, which corresponds to the temperature range 350 - 720ºС with
3. Radiation modificationof the titanium alloys properties
3.1. Problem statement
The study of positrons behavior in the plastically deformed metals showed high sensitivity and selectivity of the EPA method to the structure damages in these materials. Therefore it is natural that investigators tend to use this method to learn about radiation effects in solids as a result of nuclear irradiations of a material. This irradiation is accompanied by a number of new phenomena. The most important among them are nuclear reactions and related to them change in the elemental composition, point defects formation and crystal integrity disturbance, point defects aggregations occurrence and matrix disturbance caused by atomic collisions cascades, etc.
It is clear that without careful and detailed study of all aspects of nuclear radiation interaction with material and its consequences it is impossible to predict behavior of the materials in the field of strong ionizing radiation. The positron annihilation methods are promising and sufficiently informative for investigations of this kind.
As known, interaction of nuclear radiation with a material occurs by elastic and inelastic collisions channels. It is impossible to trace the process of radiation damage, which happens during 10-13–10-11s. Therefore, using different experimental methods the final structure of radiation damaged material is usually studied, which is in the state of equilibrium with an environment. Consequently, investigation and control of construction materials radiation damageability is the task of primary importance and, undoubtedly, attracts a considerable scientific and practical interest. Though, as of today there are practically no systematic, detailed and purposeful investigations of titanium and its alloys radiation properties. In this chapter the results of authors’ own investigations in this field are thoroughly described.
3.2. The methodology of materials irradiation on accelerator and reactors
Reliable and reproducible results of the investigation of ionizing radiation influence on the solid can be obtained only under conditions of guaranteed high accuracy of measurement of irradiated target temperature. The final structure of the material is determined by the conditions of irradiation.
Since positron annihilation methods are mainly sensible to vacancy defects, in this case the problem was to preserve just this type of defects during the irradiation process of the investigated material. Taking into account these circumstances, the irradiation of the samples on the electron accelerator ELU-6 and isochronous accelerator U-150 was conducted in the air atmosphere with the water-cooled base and forced blow-off of the sample with liquid nitrogen vapor. With the charged particles intensity of
The major portion of the reactor irradiation, related to investigation of neutron flux influence on metals, was implemented using the nuclear reactor VVR-K at the National Nuclear Centre of the Institute of Nuclear Physics of the Republic of Kazakhstan. The reactor’s nominal power is 10MW. Energy distributions of the thermal and fast neutrons fluxes for irradiated channels were determined by the activation analysis method. For thermal neutrons cutoff the method of samples screening by cadmium was applied. After irradiation the materials were exposed to the chain of “hot chambers” where they undergo cutting and dosimetry control. The samples temperature in the irradiation process was taken to be equal to the temperature of the primary-coolant system heat carrier (+80ºС).
3.3. Titanium structure modification as a result of electrons irradiation
As any other charged particles, while interacting with the crystalline lattice, the high energy electrons experience losses of energy on excitation, ionization and atoms displacement. For metals the first two electron interaction processes usually end without consequences. The consequences of elastic interactions depend on electron and recoil atom mass ratio as well as recoil energy Ер. If the recoil energy is greater than the defect formation threshold energy (
To this end, iodide titanium samples of high purity were irradiated by
If the positrons and conduction electrons annihilation probability growth with a fluence increase can indicate an increase in the respective point defects concentration, then the Fermi momentum practical constancy indicates lack of changes in the electron structure of the latter. In other words, the vacancy defects configuration on the reached level of fluence remains the same. On the basis of positrons capture model one can calculate the average size of the defect region created in Ti as a result of electron irradiation:
This statement is also confirmed by calculation results of the configuration parameter
3.4. Radiation induced modification of titanium structure at the helium ions irradiation
Use of accelerators of high energy charged particles plays quite an important role in studying radiation modification fundamental problems. This is first of all important for prediction of the construction materials behavior and change of their properties. To this end, the structure modification process of titanium binary alloys that contain 0; 1.2; 2.5; 3.3; and 4.1 at. % Ge; 1.2; 2.5; 4.3; 6.2 and 7.6 at.% Sn and 1.4; 2.9; 5.1 and 10.3 at.% In was performed. The modification was realized by α-particles irradiation with
It should be noted that irradiation by α-particles with Е=50 MeV causes significant deformation of the spectra shape that considerably exceeds the influence of the plastic deformation of sufficiently high degree
Stability of the each alloys system towards the α-particles exposure depends on the nature and concentration of the alloying element. The smallest stability towards the α-particles exposure was observed in the alloys containing 0.8 at.% Ge; 1.2 and 7.6 at.% Sn, as well as 1.4 and 7.4 at.% In. Therefore, the damageability of alloys of the indicated compositions in relation with ? Ti under α-particles irradiation is higher than for alloys of other compositions (Fig.12).
On the basis of positrons capture model and assuming formation of new allocations in crystal structure due to α - particles irradiation we can estimate the average size of these regions. The sizes are:
The results of isochronal annealing of the Ti irradiated alloys presented in Fig.13 reveal only one stage of return regardless of the composition of the alloy. This annealing is completed at the temperature of 300-320ºС with radiation defects migration activation energy of
3.5. The dose dependence of titanium alloys structure modification under α−particles irradiation
This characteristics is estimated by radiation defects accumulation kinetics at α−particles irradiation with 1014; 3.2 1014; 3.2 1015 and 1016 cm-2 fluences on the example of the Ti-Ge alloys system. The α−particles energy was Е=29 MeV with the beam intensity 1.5 1012 cm-2с-1 (Fig.14). One can see that the accumulation curve character is practically not dependent on the Ge concentration. The 1014÷5 1015 cm-2 fluence is correspond to the incubation period of radiation defects accumulation. Further, the defects accumulation obeys the point defects clusterization principle.
3.6. The peculiarities of structure modification of titanium alloys irradiated by protons
By the time of setting the experiment for the purpose of studying radiation modifications of Ti and its binary alloys structure under irradiation by high energy protons, there was no a single research work with published results that was devoted to this problem. Therefore, investigation of radiation damageability caused by strong protons beam was realized on the example of Ti binary alloys, alloyed by Sn in aforementioned concentration. The Ti–Sn alloy samples in the initial annealed and deformed (
At the same time the proton irradiation of annealed alloys leads to a significant increase of positrons annihilation probability at a fluence of 5 1015 cm-2: by 36% for Ti and by 50% - on an average for alloys containing 0.8 and 1.5 at.% of Ge. If we take the WP value for the alloys deformed by
Analysis of the results of alloys irradiation to up to 5×1014 cm−2 fluence in the previously deformed state testifies a completely opposite picture. In this case the positron annihilation probability takes substantially smaller values than before irradiation practically for all investigated alloys of this system. This tendency remains even after re-irradiation of up to 2.5×1016 cm−2 fluence, which indicates the significant role of the material’s history and the dopant agent nature in the formation of the structure damages caused by proton irradiation. The primary decrease in the positron annihilation probability
The calculation of the average size of these centers on the basis of a one-trap model positron capture for Ti–2.5 at% Sn alloy irradiated by
These very peculiarities of the concerned problem are well confirmed by the results of isochronal annealing of structure defects. As an example, Ti and Ti–7.6 at% Sn alloy were chosen for studying annealing. These samples underwent all of the three abovementioned types of external exposure. The results of these investigations are reflected in Fig. 15(a) and (b) as curves of isochronal annealing. In each case, in order to establish the nature of the structural transformations, the curves of annealing for the deformed materials obtained earlier are provided. Comparing the annealing results for all states of the materials is useful as it enables formulation of quite important conclusions about some redistribution of defects in the crystal structure of materials that underwent a combined treatment. One can observe a pronounced low-temperature stage for Ti caused by an irradiation by protons up to 2.5 1016 cm−2 fluence from a deformed state within the temperature range of 60 – 2200C (Fig. 15(a), curve 2). It occurred as a result of the transformation, evolution and redistribution of the initial defect structure generated by an intense plastic deformation under a heavy proton radiation. This stage significantly differs from that of annealing curve for deformed titanium (curve 1) both by form and by temperature region of manifestation.
In addition, one can observe a second high-temperature stage with Δ
3.7. The metals and their alloys structure modification under neutron irradiation
Relatively high stability of some Ti–Al system alloys to fast α−particle influence was shown above experimentally. Though, in the literature one can encounter contradictory assertions about radiation characteristics of these alloys . For tackling this problem alloys of this system of compositions investigated earlier underwent irradiation by fission neutrons.
Irradiation of the annealed materials by fission neutrons with
In the titanium alloys that are inclined to phase transformations, as a result of plastic deformation one should expect formation of defects related to the α→β transformation. The vacancy-dislocation structure, which appears after strong neutron flux, can stimulate formation of certain allocations in the crystal, an integral part of which is the lattice instability related to the energy excess introduced by irradiation. One can see from the data that for Ti and 5.2 at.% of Al alloy the annihilation parameter Δ
At the same time sufficiently close values of the annihilation parameter for heavily doped alloys with 10.2 and 16.5 at.% of Al as a result of deformation and neutron irradiation can validate the possibility of structure damages formation in these alloys that assures an equivalent efficiency of positrons capture potentials. Materials, which manifest this regularity, possess enhanced stability to external influence, including a neutron irradiation. The Ti alloys containing more than 10 at.% of Al probably belong to this category of materials.
For investigation of the radiation defects accumulation kinetics, alloys of Ti-Al system were irradiated by fission neutrons in the wide range of fluencies: 1015; 1016; 1017; 1018; 1019 and 2 1019 cm-2. Fig 17a depicts the dose dependence of Δ
The certain clarity about the processes can be obtained from the annealing data. Firstly, let us consider the radiation defects annealing spectra in Ti–5.2 at.% of Al alloy, irradiated by neutrons at different fluencies (Fig.18a). As a result of neutron irradiation to up to 1017 cm-2 fluence, the generated radiation defects in the crystal structure are annealed in one stage in temperature range of 110 - 210ºС (curve 1) with migration energy activation
The fluence increase by single-order (to up to 1018 cm-2) leads to the radiation defects occurrence in the structure and these defects are annealed in two stages (curve 2) and with the fluence of 2 1019 cm-2 the full healing of the structure damages is performed in three evidently expressed annealing stages (curve 3).
A considerable interest is presented by the annealed defects spectra against the alloys composition irradiated at the same neutron dose 2 1019 cm-2. It is easy to determine that in the different alloys the radiation defects accumulation process occurs differently (Fig.18b).
In conclusion, a comparative analysis of titanium radiation damageability under the four types of particle radiation influence can be performed: electrons, protons, α-particles and fission neutrons. This comparison can only be approximate, since it is practically impossible to ensure same conditions for all cases. The titanium and its alloys radiation damageability is substantially higher at α-particles irradiation. Taking into account radiation defects generation rate the most damaging are (detrimental) α-particles, which are then followed by fission neutrons, protons, and electrons.
4. Titanium alloys defect structure modification by hydrogen saturation method
It is generally acknowledged that alterations in the metal electron structure caused by hydrogen should be taken into account when interpreting physical properties of the metal-hydrogen system. This is especially important for the Ti–H system, because of the high absorption capacity of titanium with respect to hydrogen, which to a significant degree have an influence on its technological properties. According to the well-known proton model, the proton gas penetrates into the electron shells and changes their energy state. At the same time the intensity of the force fields grows with an increase of the temperature of the system, which is accompanied by intensification of interaction between proton gas and electron shells.
In order to tackle this task, as an objects of the experimental study iodide titanium and a system of Ti–Al alloys (up to 50 at.% Al) were taken [13,17]. For hydrogenation, the original materials were annealed at 9000 C, deformed by rolling by
|Annealed (before hydrogenation)|
|Ti pressed powder||0.40||5.25||54||Error ±||0.01||0.05||1.0|
In ideal defect-free single crystals hydrogen dissolves in the metal, occupying the lattice interstitial positions and causing displacement of the metal atoms from their equilibrium positions. In the real crystals, though hydrogen segregates in various lattice defects, which leads to reduction of probability of positrons capture. But hydrogen saturation of materials in annealed state at
Plastically deformed metals facilitate accumulation of a considerable amount of hydrogen. The hydrogen saturation at comparatively low temperature (200ºС) is obviously accompanied by atomic hydrogen capture by structural damages. The new complexes appear in crystalline lattice which decrease the capture efficiency of positrons localization centers, previously introduced by the plastic deformation. The hydrogen saturation temperature increase up to 500ºС converts materials into a hydride state, which caused the cast, compact metals to crumble into powder. The reason for destruction of the compact metal after hydrogen saturation under 500ºС can be formation of cracks of deformative nature related to the hydrides formation. The penetrated hydrogen is dissociated in the internal surface of these defects but with formation of less mobile molecules which with more intensive arrival from outside gradually become bigger by volume and eventually cause sample’s destruction.
In order to establish the nature of the observed phenomena, the hydrogen accumulation nature in Ti and its alloys with Al in annealed state was investigated. In clean Ti, there are practically no reasons that could prevent hydrogen from accumulation to significant concentrations (Fig.19a). At the same time, intensity of hydrogen absorption by annealed alloys of Ti-Al system sharply drops with an increase of Al concentration in the alloys (Fig.19b). Therefore, the capabilities of Al, as an absorber of hydrogen, are rather limited comparing to those of Ti. This is reflected in the annihilation parameter changes.
Thus, the observed changes in the electron structure of the defect material indicate, on the one hand, on hydrogen’s significant role in its formation and, on the other hand, on enhancement of the interaction of hydrogen with the material due to the presence of damages of deformative and radiative character.
Isochronal annealing behavior of these materials is presented in Fig.19. For irradiated by electrons and not saturated by hydrogen Ti one stage of restoration is allocated under annealing in the temperature range 170-240ºС with
Fig. 19 Hydrogen accumulation kinetics (a), Al influence on hydrogen accumulation rate in Ti (b) and defects annealing kinetics in titanium (с)
Complex and systematic investigations of the electron structure of the titanium binary alloys depending on the type and concentration of the alloying elements and the modification degree by plastic deformation method enabled formulation of the following conclusions:
for the initial state the alloys electron structure reveals a weak dependence on type and concentration of an alloying element, whereas the structure modification by plastic deformation causes nonmonotonic changes to the annihilation parameters. At the same time, the main changes in the character of the annihilation parameters are observed at deformations of up to ε≤30 %, above which saturation is observed;
it was shown by X-ray investigations that in the metals modified by deformation one can find a certain amount of new phase precipitates which are interpreted as the β - phase of base metal in the matrix of α-phase, with vacancy-dislocation structure and electron density substantially smaller than in the initial phase;
it was shown that after electron irradiations, mono vacancies emerge in the titanium crystalline lattice and they remain at the room temperature; these mono vacancies ensure capture of positrons, electron structure and configuration of which do not depend on electrons fluence and significantly differ from the corresponding characteristics of vacancy formations generated by plastic deformation;
it was established that the electron structure and stability of each alloys system to the influence of high energy α-particles are the function of alloying elements nature and their concentration;
the possibility of the irradiation-induced swelling suppression in the titanium alloys by selective alloying and preliminary structure defects introduction was experimentally demonstrated;
the role of the initial structure defects in titanium alloys under the high energy proton irradiation is manifested in their transformation, evolution and redistribution with sharply distinct electron structure;
it was shown that in the deformed state hydrogen accumulation in titanium occurs in the defect regions with hydride formation which afterwards leads to sample destruction;
for hydrogen corrosion reduction it is necessary to use titanium alloyed by Al (over 5.2 at. %);
the radiation effects in preliminary hydrogen saturated titanium manifested themselves in emergence of hydrogen atom-vacancy coupled state.