**3. Radiation modificationof the titanium alloys properties**

#### **3.1. Problem statement**

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 *ε* = 80 % was implemented by rolling

radiation using the DRON-2 diffractometer. The packing defects formation on prismatic plane α(1010) probability calculation results along with the annihilation parameters are summarized

> 0.30 0.44

> 0.28 0.42

> 0.29 0.39

> 0.23 0.34

9.0 10-3 0.24 0.35 -

10.2 10-3 0.28 0.35 -

Accuracy ± 0.001 0.01 2.0 0.05 0.1

One can see that for all investigated alloys the plastic deformation leads to an increase of the parameter *F* by 25-50% with a simultaneous decrease of the Fermi angle *θ*F by 5-10%. The changes of the annihilation parameters depending on the composition are of monotonous nature. The packing defects probability on the basal plane both for Ti and alloys remains practically without changes and equals α(0001) = 2 10-3, whereas on the prismatic plane (1010) the probability monotonically increases from 4.2 10-3 up to 10.2 10-3 depending on V concen‐ tration. Thus, the packing defects formation under plastic deformation in Ti – V alloys became

The results of the isochronal annealing for deformed Ti, V and Ti – V alloys are presented in

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 *Е*а2 = 2.35eV. For all other alloys this

0) lines profiles were taken in the filtered CuK<sup>α</sup>

**θF mrad**

> 6.33 5.83

> 6.36 5.75

> 6.40 5.83

> 6.40 6.00

> 6.42 6.08

> 6.29 5.79

**ΔθF %**







**ΔF %**





44

25

\_\_

at room temperature. (101

130 Titanium Alloys - Advances in Properties Control

in Table 2.

**Materials at. %**

Ti

Ti-0.5 V

Ti-1.5 V

Ti-2 V

Ti-4.6 V

Ti-5.8 V

Fig. 10.

\_\_

annealed ε=80%

annealed ε=80%

annealed ε=80%

annealed ε=80%

annealed ε=80%

annealed ε=80%

**Table 2.** The packing defects probability and Ti-V alloys annihilation parameters

an established fact and alloying with V only facilitates this.

0), (0002) and (101

**State** *F*







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.

at a temperature lower than 70ºС with the following ADAP spectra measurement and structure sensitive annihilation parameters determination. The results of these investigations are shown in Table 3. One can see that the positrons and conduction electrons annihilation relative probability practically grows in linear fashion with the fluence increase and rate of this growth decreases only at the last two fluence values: 1019 and 3.7 1019 cm-2. At the same time the Fermi

> 0.21 0.36 0.40 0.45 0.49 0.53

**Table 3.** The annihilation parameters dose dependence for titanium irradiated by electrons

**θ***F,* **mrad.**

> 6.33 5.85 5.75 5.70 5.72 5.69

Accuracy ± 0.01 0.02 0.05 0.05

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: *r* = 0.81Å. This means that the positrons

**Figure 11.** The structure damages annealing in deformed and electron irradiated titanium 1- deformed on ε =50%; 2-

*R***<sup>С</sup>** *Ea, eV*

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133

1.23 1.28


Physicochemical and Radiation Modification of Titanium Alloys Structure

momentum angle *θ*F has a trend to stabilization at 5.70 mrad.

**Fluence, cm-2** *F*

capture centers in this case are really the vacancies.

electron irradiated Ф1 = 1018 cm-2; 3 - electron irradiated Ф2= 1019cm -2.

0 3,7·1017 1018 3,7·1018 1019 3,7·1019

### **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 condi‐ tions 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 investi‐ gated 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 *(*1.5 − 2*)* × 1016 m−2 s−1, the sample's temperature did not exceed 60–700 C.

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 primarycoolant 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 (*Е*<sup>P</sup> >*E*d), then atom will leave its place in the lattice, which leads to formation of the elemental Frenkel pair, i.e. interstitial atom and vacancy. When *Е*р values are high the displacement cascades can appear and they consist of two or three vacancies and a certain number of interstitial atoms. The latter move towards the sinks or recombine with vacancies at the room temperature. Therefore, as a consequence of high energy electrons irradiation vacancy defect structure is generally formed in the crystal, which can be effectively studied by positrons diagnostics methods.

To this end, iodide titanium samples of high purity were irradiated by *Е* = 4MeV electrons with intensity of about 5 10<sup>12</sup> cm-2s-1 and up to 3.7 1017; 1018; 3.7 1018; 1019 and 3.7 1019 cm-2 fluencies at a temperature lower than 70ºС with the following ADAP spectra measurement and structure sensitive annihilation parameters determination. The results of these investigations are shown in Table 3. One can see that the positrons and conduction electrons annihilation relative probability practically grows in linear fashion with the fluence increase and rate of this growth decreases only at the last two fluence values: 1019 and 3.7 1019 cm-2. At the same time the Fermi momentum angle *θ*F has a trend to stabilization at 5.70 mrad.


**Table 3.** The annihilation parameters dose dependence for titanium irradiated by electrons

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

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 condi‐

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 investi‐ gated 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 *(*1.5 − 2*)* × 1016 m−2 s−1, the sample's temperature

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-

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 (*Е*<sup>P</sup> >*E*d), then atom will leave its place in the lattice, which leads to formation of the elemental Frenkel pair, i.e. interstitial atom and vacancy. When *Е*р values are high the displacement cascades can appear and they consist of two or three vacancies and a certain number of interstitial atoms. The latter move towards the sinks or recombine with vacancies at the room temperature. Therefore, as a consequence of high energy electrons irradiation vacancy defect structure is generally formed in the crystal, which can be effectively studied by positrons

To this end, iodide titanium samples of high purity were irradiated by *Е* = 4MeV electrons with intensity of about 5 10<sup>12</sup> cm-2s-1 and up to 3.7 1017; 1018; 3.7 1018; 1019 and 3.7 1019 cm-2 fluencies

the results of authors' own investigations in this field are thoroughly described.

**3.2. The methodology of materials irradiation on accelerator and reactors**

**3.3. Titanium structure modification as a result of electrons irradiation**

tions of irradiation.

132 Titanium Alloys - Advances in Properties Control

did not exceed 60–700

diagnostics methods.

C.

coolant system heat carrier (+80ºС).

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: *r* = 0.81Å. This means that the positrons capture centers in this case are really the vacancies.

**Figure 11.** The structure damages annealing in deformed and electron irradiated titanium 1- deformed on ε =50%; 2 electron irradiated Ф1 = 1018 cm-2; 3 - electron irradiated Ф2= 1019cm -2.

This statement is also confirmed by calculation results of the configuration parameter *R*с, which is determined according to the positrons capture model (Table 2). It can be seen that within the calculation error *R*с remains constant (*R*c = 1.55±0.05), that is regardless of electrons fluence the radiation defects configuration remains invariant. Consequently, the observed increase of the probability of positrons annihilation with the conduction electrons *F* is caused only by a respective increase of radiation defects in Ti. These data are verified by the isochronal annealing results performed for three cases (Fig.11). One can see that in the temperature interval 170-240ºС only one return stage for irradiated materials is observed, which is related to removal of radiation defects regardless of electrons fluence. The higher return effect value for Ф2=1019 cm-2 fluence in comparison with Ф1=1018 cm-2 fluence also confirms enhanced concentration of vacancy defects formation. The defect migration activation energy value did not exceed *Е*а = 1.22±0.05 eV and according to the data of [11, 12] it enabled us to identify them as point defects. Thus, the titanium structure modification by high energy electrons irradiation leads to formation of point defects of vacancy type, the concentration of which depends on the irradiation fluence.

**Figure 12.** The alloys structure modification: Ti-Ge (a), Ti-Sn (b) and Ti-In (c) in different states: 1 – annealed; 2 – de‐

Physicochemical and Radiation Modification of Titanium Alloys Structure

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135

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: *r* = 16 Å for Ti–1.4 Ge at.% alloy; *r* = 15Å for Ti-1.4 at.% In alloy and *r=* 20 Å for Ti –

**Figure 13.** Annealing kinetics of titanium alloys, irradiated by α - particles with Е = 50MeV: 1 - 1.2 at.% Ge 2 - 1.2 at.%

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 *Е*а=1.50-1.55eV and corresponds to vacancy complexes that occur in the Ti alloys' α−phase.

7.6 аt.% Sn alloy. These data are confirmed by the isochronous annealing results.

formed by ε=50 %; 3 – irradiated by α-particles with Е=50MeV

Sn 3 - 2.9 at.% In

#### **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 *Е* = 50 MeV with the flux density 1.5 1012 cm-2 с-1 and the temperature not higher than 70ºС.

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 *ε* =50% (Fig.11). The probability of positron annihilation with con‐ duction electrons as a result of irradiation by α-particles in all cases significantly exceeds its values for alloys both in initial state and also after strong plastic deformation with simultane‐ ous and significant decrease of the Fermi angle *θ*F. It is important that the nature of change of the concentration dependence of these parameters is preserved from one alloy to another. Meanwhile, those alloys, in which abnormally high increase of the annihilation parameter after plastic deformation is observed, also show this tendency after α-particles irradiation. For some alloys these changes exceed by more than twice the corresponding indicators for the deformed state. This indicates that the α-particles irradiation appears to be more effective on structure changes in metals than plastic deformation.

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).

This statement is also confirmed by calculation results of the configuration parameter *R*с, which is determined according to the positrons capture model (Table 2). It can be seen that within the calculation error *R*с remains constant (*R*c = 1.55±0.05), that is regardless of electrons fluence the radiation defects configuration remains invariant. Consequently, the observed increase of the probability of positrons annihilation with the conduction electrons *F* is caused only by a respective increase of radiation defects in Ti. These data are verified by the isochronal annealing results performed for three cases (Fig.11). One can see that in the temperature interval 170-240ºС only one return stage for irradiated materials is observed, which is related to removal of radiation defects regardless of electrons fluence. The higher return effect value for Ф2=1019 cm-2 fluence in comparison with Ф1=1018 cm-2 fluence also confirms enhanced concentration of vacancy defects formation. The defect migration activation energy value did not exceed *Е*а = 1.22±0.05 eV and according to the data of [11, 12] it enabled us to identify them as point defects. Thus, the titanium structure modification by high energy electrons irradiation leads to formation of point defects of vacancy type, the concentration of which depends on the

**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 *Е* = 50 MeV with the flux density 1.5 1012 cm-2 с-1

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 *ε* =50% (Fig.11). The probability of positron annihilation with con‐ duction electrons as a result of irradiation by α-particles in all cases significantly exceeds its values for alloys both in initial state and also after strong plastic deformation with simultane‐ ous and significant decrease of the Fermi angle *θ*F. It is important that the nature of change of the concentration dependence of these parameters is preserved from one alloy to another. Meanwhile, those alloys, in which abnormally high increase of the annihilation parameter after plastic deformation is observed, also show this tendency after α-particles irradiation. For some alloys these changes exceed by more than twice the corresponding indicators for the deformed state. This indicates that the α-particles irradiation appears to be more effective on structure

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).

irradiation fluence.

134 Titanium Alloys - Advances in Properties Control

and the temperature not higher than 70ºС.

changes in metals than plastic deformation.

**Figure 12.** The alloys structure modification: Ti-Ge (a), Ti-Sn (b) and Ti-In (c) in different states: 1 – annealed; 2 – de‐ formed by ε=50 %; 3 – irradiated by α-particles with Е=50MeV

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: *r* = 16 Å for Ti–1.4 Ge at.% alloy; *r* = 15Å for Ti-1.4 at.% In alloy and *r=* 20 Å for Ti – 7.6 аt.% Sn alloy. These data are confirmed by the isochronous annealing results.

**Figure 13.** Annealing kinetics of titanium alloys, irradiated by α - particles with Е = 50MeV: 1 - 1.2 at.% Ge 2 - 1.2 at.% Sn 3 - 2.9 at.% In

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 *Е*а=1.50-1.55eV and corresponds to vacancy complexes that occur in the Ti alloys' α−phase.

#### **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.

weak influence on positrons annihilation process nature both in respect of the probability *W*<sup>P</sup>

Accuracy ± 0.02 0.02 0.02 0.02 0.02 0.02

**Materials state**

**After annealing**

Ti 0.22 0.38 0.30 0.41 0.42 0.45

0.36 0.29 0.28 0.34 0.30

**Protons fluence, Е=30MeV 5·1015 cm –2 2.5·1016 cm –2**

Physicochemical and Radiation Modification of Titanium Alloys Structure

**After annealing**

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0.38 0.31 0.38 0.37 0.41

**After deformation ε=50%**

137

0.36 0.38 0.40 0.46 0.41

**After deformation ε=50%**

> 0.32 0.34 0.37 0.33 0.36

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 *ε* = 50% as saturating, then the results for alloys irradiated by protons from an annealed state confirms the absence of tendency of the annihilation parameter to saturate within the range of reached fluence level. The annihilation parameter increase with the fluence characterizes the respective increase of radiation defects concentration in the materials' structure. The largest increase of positron traps as a result of proton irradiation is observed in the alloys containing 1.2 and 7.6 at.% Sn, i.e. these alloys, as in the case of α−irradiation

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 *W*P caused by proton irradiation of deformed materials is probably related to the appropriate decrease in the efficiency of structure damages towards positron trapping. The following increase of *W*<sup>P</sup> is obviously caused by probable radiation-stimulated reconstruction of alloys dislocation structure as a

influence, manifest certain instability to proton irradiation influence.

= *S*p/*S*o and the Fermi angle *θ*F.

**Alloys composition, at.%.**

> Ti-1.2 Sn Ti-2.5 Sn Ti-4.3 Sn Ti-6.2 Sn Ti-7.6 Sn

**Annealed**

0.25 0.22 0.26 0.27 0.26

**Plastic deformed ε=50%**

> 0.44 0.39 0.41 0.41 0.43

**Table 4.** The positrons annihilation probability in titanium alloys irradiated by protons

result of proton irradiation.

**Figure 14.** Positrons capture efficiency dosage dependencies for Ti (1) and Ti–3.1 at.% Ge (2)

#### **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 (*ε* = 50%) states underwent irradiation by protons with *E* = 30 MeV up to two-value (5 1015 and 2.5 1016 cm−2) fluence for the purpose of elucidating not only the dopant agent role but also the material's history in the formation of the final defective structure of alloys. It is necessary to point that for the protons with *E* = 30 MeV, the thickness of the samples used (1 mm) was absolutely insufficient for providing their complete deceleration. The calculated value of protons energy on the backside of the samples differed from protons initial energy only by 5–6MeV [15, 16]. Therefore, all investigated samples were irradiated actually by shooting. The results of this investigation are presented in Table 4. One can see that for the materials' initial state the alloying elements concentration increase has a


**Table 4.** The positrons annihilation probability in titanium alloys irradiated by protons

**3.5. The dose dependence of titanium alloys structure modification under α−particles**

**Figure 14.** Positrons capture efficiency dosage dependencies for Ti (1) and Ti–3.1 at.% Ge (2)

**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 (*ε* = 50%) states underwent irradiation by protons with *E* = 30 MeV up to two-value (5 1015 and 2.5 1016 cm−2) fluence for the purpose of elucidating not only the dopant agent role but also the material's history in the formation of the final defective structure of alloys. It is necessary to point that for the protons with *E* = 30 MeV, the thickness of the samples used (1 mm) was absolutely insufficient for providing their complete deceleration. The calculated value of protons energy on the backside of the samples differed from protons initial energy only by 5–6MeV [15, 16]. Therefore, all investigated samples were irradiated actually by shooting. The results of this investigation are presented in Table 4. One can see that for the materials' initial state the alloying elements concentration increase has a

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

**irradiation**

136 Titanium Alloys - Advances in Properties Control

principle.

weak influence on positrons annihilation process nature both in respect of the probability *W*<sup>P</sup> = *S*p/*S*o and the Fermi angle *θ*F.

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 *ε* = 50% as saturating, then the results for alloys irradiated by protons from an annealed state confirms the absence of tendency of the annihilation parameter to saturate within the range of reached fluence level. The annihilation parameter increase with the fluence characterizes the respective increase of radiation defects concentration in the materials' structure. The largest increase of positron traps as a result of proton irradiation is observed in the alloys containing 1.2 and 7.6 at.% Sn, i.e. these alloys, as in the case of α−irradiation influence, manifest certain instability to proton irradiation influence.

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 *W*P caused by proton irradiation of deformed materials is probably related to the appropriate decrease in the efficiency of structure damages towards positron trapping. The following increase of *W*<sup>P</sup> is obviously caused by probable radiation-stimulated reconstruction of alloys dislocation structure as a result of proton irradiation.

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 *E* = 30 MeV protons up to 2.5 1016 cm−2 fluence gives *R*V = 10Å, i.e. in the investigated materials one can assume a generation of vacancy clusters.

**3.7. The metals and their alloys structure modification under neutron irradiation**

investigations are presented in Fig.16.

deformation (1) and neutron irradiation (2)

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 [16]. For tackling this problem alloys of this system of compositions investigated earlier underwent irradiation by fission neutrons.

Physicochemical and Radiation Modification of Titanium Alloys Structure

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139

Irradiation of the annealed materials by fission neutrons with *Е*>0.1 MeV was conducted at the fluence up to 2 1022 cm–2 according to the method described above. The results of the realized

**Figure 16.** The concentration dependencies of annihilation relative probability for Ti-Al alloys, are subjected to plastic

In the titanium alloys that are inclined to phase transformations, as a result of plastic defor‐ mation one should expect formation of defects related to the α→β transformation. The vacancydislocation 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*F* increases under the neutron irradiation by more than three times, while after plastic deformation by *ε* = 50% it was only 64 and 127 %, respectively. Such behavior of the annihilation parameter indicates that with an increase of Al content in

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 equiv‐ alent efficiency of positrons capture potentials. Materials, which manifest this regularity,

the alloy considerable changes in the defects electron density distribution occur.

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 – 2200 C (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.

**Figure 15.** Kinetics of Ti (a) and Ti–7.6 at% Sn alloy (b) annealing under different types of exposure. 1 – deformed by ε = 50%; 2 – irradiated by protons in a deformed state; and 3 – the same in an annealed state.

In addition, one can observe a second high-temperature stage with Δ*N*<sup>2</sup> = 2.5% in the temper‐ ature range of 330–3600 C. The defect migration activation energies by stages were equal to *E*a1 = 1.21 eV and *E*a2 = 1.93 eV, respectively. This fact confirms an assumption about dislocation structure evolution and its partial transformation into the vacancy structure.

#### **3.7. The metals and their alloys structure modification under neutron irradiation**

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 *E* = 30 MeV protons up to 2.5 1016 cm−2 fluence gives *R*V = 10Å, i.e. in the investigated materials one can assume a generation of vacancy

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

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

(a) (b)

**Figure 15.** Kinetics of Ti (a) and Ti–7.6 at% Sn alloy (b) annealing under different types of exposure. 1 – deformed by ε

In addition, one can observe a second high-temperature stage with Δ*N*<sup>2</sup> = 2.5% in the temper‐

= 1.21 eV and *E*a2 = 1.93 eV, respectively. This fact confirms an assumption about dislocation

C. The defect migration activation energies by stages were equal to *E*a1

= 50%; 2 – irradiated by protons in a deformed state; and 3 – the same in an annealed state.

structure evolution and its partial transformation into the vacancy structure.

ature range of 330–3600

C (Fig. 15(a), curve

cm−2 fluence from a deformed state within the temperature range of 60 – 2200

by form and by temperature region of manifestation.

clusters.

138 Titanium Alloys - Advances in Properties Control

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 [16]. 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 *Е*>0.1 MeV was conducted at the fluence up to 2 1022 cm–2 according to the method described above. The results of the realized investigations are presented in Fig.16.

**Figure 16.** The concentration dependencies of annihilation relative probability for Ti-Al alloys, are subjected to plastic deformation (1) and neutron irradiation (2)

In the titanium alloys that are inclined to phase transformations, as a result of plastic defor‐ mation one should expect formation of defects related to the α→β transformation. The vacancydislocation 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*F* increases under the neutron irradiation by more than three times, while after plastic deformation by *ε* = 50% it was only 64 and 127 %, respectively. Such behavior of the annihilation parameter indicates that with an increase of Al content in the alloy considerable changes in the defects electron density distribution occur.

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 equiv‐ alent 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.

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 *Е*а1 = 1.27 eV. This

**Figure 18.** Dosage (a) and concentration (b) dependencies of annealing kinetics for Ti-Al alloys irradiated by neutrons

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

A considerable interest is presented by the annealed defects spectra against the alloys com‐ position 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 substan‐ tially 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,

1.Ti; 2. Ti-5.2 at.% Al;

3.Ti-10.2 at.% Al; 4. Ti-16.5 at.% Al

Physicochemical and Radiation Modification of Titanium Alloys Structure

http://dx.doi.org/10.5772/55485

141

stage evidently corresponds to the return of small vacancy clusters.

1. = 1017 cm-2; 2. = 1018 cm-2; 3. = 2.1019 cm-2

annealing stages (curve 3).

protons, and electrons.

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*F* parameter for Ti and its three alloys with Al. The sharply distinct nature of change of this parameter related to the alloys composition should be noted. Given these conditions one can assume that in the alloys, which are different by composition, the changes of the defects concentrations and their configuration can also be different. The strongly concentrated alloys are more stable to neutron exposure and for this reason the fluence increase of the latter is not accompanied by sharp changes of the annihilation parameters and this is probably stimulated by an interatomic bond energy increase in these alloys comparing with Ti and Ti–5.2 at.% Al alloy.

**Figure 17.** The dosage dependence of annihilation parameters change kinetics in Ti-Al alloys irradiated by fission neu‐ trons (a) and Fermi momentum (b).

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 *Е*а1 = 1.27 eV. This stage evidently corresponds to the return of small vacancy clusters.

possess enhanced stability to external influence, including a neutron irradiation. The Ti alloys

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*F* parameter for Ti and its three alloys with Al. The sharply distinct nature of change of this parameter related to the alloys composition should be noted. Given these conditions one can assume that in the alloys, which are different by composition, the changes of the defects concentrations and their configuration can also be different. The strongly concentrated alloys are more stable to neutron exposure and for this reason the fluence increase of the latter is not accompanied by sharp changes of the annihilation parameters and this is probably stimulated by an interatomic bond energy increase in these

**Figure 17.** The dosage dependence of annihilation parameters change kinetics in Ti-Al alloys irradiated by fission neu‐

containing more than 10 at.% of Al probably belong to this category of materials.

alloys comparing with Ti and Ti–5.2 at.% Al alloy.

140 Titanium Alloys - Advances in Properties Control

trons (a) and Fermi momentum (b).

**Figure 18.** Dosage (a) and concentration (b) dependencies of annealing kinetics for Ti-Al alloys irradiated by neutrons

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 com‐ position 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 substan‐ tially 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.
