<bold>Ti6Al4V</bold>合金相变及吸氢热力学研究

张小雪; 杜江飞; 彭菲菲; 肖兴茂; 袁宝国; Zhang Xiaoxue; Du Jiangfei; Peng Feifei; Xiao Xingmao; Yuan Baoguo

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Phase Transformations and Thermodynamics Analysis of Hydrogen Absorption of Ti6Al4V Alloy PDF

- ORCID：
Zhang Xiaoxue ¹
✉
- ORCID：
Du Jiangfei ²
- ORCID：
Peng Feifei ³
- ORCID：
Xiao Xingmao ³
- ORCID：
Yuan Baoguo ²
✉

1. Anhui Sanlian University, Hefei 230601, China； 2. Hefei University of Technology, Hefei 230009, China； 3. Southwest Technology and Engineering Research Institute, Chongqing 400039, China

Updated：2021-09-26

Abstract

Thermodynamics of hydrogen absorption and phase transformations in Ti6Al4V alloy were investigated by pressure-composition (P-C) isotherm measurement at hydrogenation temperatures in the range of 823~1023 K. Results show that the hydrogen pressure is increased with increasing the hydrogen content when Ti6Al4V alloy is hydrogenated at different temperatures. Only one sloped pressure plateau occurs in each P-C isotherm during the hydrogenation treatment because of the existence of original β phase in Ti6Al4V alloy. According to Vant's Hoff law, the values of enthalpy and entropy of the pressure plateau region are -50.7±0.26 kJ/mol and -138.4±0.69 J·K^-1·mol^-1, respectively. The Sieverts constant increases firstly and then decreases gradually with increasing the hydrogenation temperature. The phase composition and phase transformation of Ti6Al4V alloy during the hydrogenation treatment were analyzed.

Keywords

titanium alloy; hydrogen absorption; thermodynamics; phase transformation

Science Press

Recently, thermo-hydrogen processing (THP) has been widely investigated and applied to titanium and related alloys. THP is a technique using hydrogen as a temporary alloying element in titanium alloys to modify the microstructures and improve the mechanical properties of titanium alloys^[

1-5]. THP can improve the room-temperature plasticity^{[Reference 6-8}6-8], decrease the flow stress in hot forging^{[Reference 9-11}9-11], and enhance the superplastic forming behavior^{[Reference 12-14}12-14] of titanium alloys. Yuan et al^{[Reference 6

Baidu Scholar}6,8] investigated the room-temperature compressive properties of titanium alloys and found that hydrogen can improve the room-temperature plasticity of Ti6Al4V and TC21 alloys. THP is regarded as a gas-solid reaction, and its thermo-dynamics and parameters of hydrogenation reaction need to be studied further to optimize the hydrogenation reaction. The thermodynamic parameters of hydrogenation reaction can be determined through the relationship among hydrogen pressure (P), hydrogenation temperature (T), and hydrogen content (C). Shen et al^{[Reference 15

Baidu Scholar}15] measured the hydrogenation P-C isotherms of pure titanium and Ti6Al4V alloy at 823~973 K and calculated the thermodynamic parameters of hydrogenation reaction. Wang et al^{[Reference 16

Baidu Scholar}16] measured the hydrogenation P-C-T curves of TC21 alloy at 898~973 K and calculated thermodynamic parameters of hydrogenation reaction. However, the micro-structure evolutions of titanium alloy after measurement of P-C isotherms at different hydrogenation temperatures were seldom studied.

In this research, P-C isotherms of Ti6Al4V alloy hydrogenated at 823~1023 K were investigated. Thermo-dynamic parameters of hydrogenation reaction were calculated. Microstructure evolutions of Ti6Al4V alloy after measurement of P-C isotherms at different hydrogenation temperatures were analyzed. The effects of hydrogenation temperature on phase composition and phase transformation of Ti6Al4V alloy were discussed.

1 Experiment

The material was Ti6Al4V alloy bars with a diameter of 6 mm and a height of 9 mm. Thermodynamic experiments of hydrogen absorption of Ti6Al4V alloy were conducted using continuously multistep method in a tube-type furnace. Ti6Al4V alloy specimens were hydrogenated in hydrogen atmosphere at different hydrogenation temperatures of 823~1023 K with an interval of 50 K. The initial hydrogen pressure was 20.325 kPa lasting for 1 h, and then hydrogen of 8 kPa was inflated with an interval of an hour. When the difference between the initial hydrogen pressure and the hydrogen pressure after heat preservation for 1 h during the hydrogenation treatment was lower than 0.5 kPa, the hydrogenation treatment was finished. Finally, the Ti6Al4V alloy specimens were air-cooled to room temperature. The actual hydrogen content was determined by weighing the specimen before and after hydrogenation treatment using an electronic analytical balance (SHIMADZU AUW220D). Hydrogen content in the hydrogenated Ti6Al4V alloy specimens was expressed as H/M (ratio of hydrogen to metal atoms).

Microstructures were observed by an optical microscope (OM, Carl Zeiss Lab.A1). A mixed solution (1 mL hydrofluoric acid, 1 mL nitric acid, and 8 mL water) was used to etch the OM specimens. Phase analysis was identified by X-ray diffraction (XRD, X'Pert PRO MPD) with Cu Kα radiation under 40 kV and 40 mA and a scanning rate of 3°/min. Foils for transmission electron microscopy (TEM, TECNAI G2 F20) analysis were mechanically thinned to about 120 μm and then milled by an ion milling equipment (MODEL-691) with voltage of 5 kV and incidence angle of 4°.

2 Results and Discussion

2.1 P-C isotherms

The P-C isotherms of Ti6Al4V alloys hydrogenated at different temperatures are shown in Fig.1. Hydrogen pressure is increased with increasing the hydrogen content when Ti6Al4V alloy is hydrogenated at different temperatures. The P-C isotherms of Ti6Al4V alloys hydrogenated at tempe-ratures of 823~973 K are divided into three repions, which agrees with the results in Ref.[

16]. The solubility of hydrogen in α phase and β phase gradually reaches saturation with increasing the hydrogen content after hydrogen atoms reach the surface of Ti6Al4V alloy, i.e., the first region. With increasing the hydrogen content, the phase transformation from α_H into β_H and hydride occur with a sloped pressure plateau, which is the second region. There is only one pressure plateau during the hydrogenation treatment for each P-C isotherm of Ti6Al4V alloys hydrogenated at temperatures of 823~973 K, because of the existence of original β phase in Ti6Al4V alloy^{[Reference 15

Baidu Scholar}15]. The pressure plateau becomes more sloped and the length of the second region becomes shorter with increasing the hydrogenation temperature. The pressure plateau is not obvious when Ti6Al4V alloy was hydrogenated at 1023 K, indicating that the phase transformation from α_H into β_H is finished quickly and the P-C isotherm enters into the third region^{[Reference 16

Baidu Scholar}16]. The reaction of titanium and hydrogen atoms to generate hydride (Ti+xH⇌TiH_x) is an exothermic reaction (ΔH<0)^{[Reference 17

Baidu Scholar}17]. Therefore, the formation of hydride is hindered at high hydrogenation temperature. The third region is dominated by the solubility of hydrogen in β_H phase and hydride. When hydrogen pressure reaches the similar status, the hydrogen content increases firstly and then decreases with increasing the hydrogenation temperature during the hydro-genation treatment of Ti6Al4V alloy. When hydrogen content reaches the similar value, the hydrogen pressure decreases firstly and then increases with increasing the hydrogenation temperature during the hydrogenation treatment of Ti6Al4V alloy.

2.2 Thermodynamic parameters

The enthalpy and entropy of the pressure plateau region during the hydrogenation treatment of Ti6Al4V alloy are calculated by Vant's Hoff law, as expressed by Eq.(1)^[

15,16].

l n P_{H_{2}} = \frac{Δ H}{R T} - \frac{Δ S}{R}

(1)

where $P_{H_{2}}$ is the equilibrium hydrogen pressure; T is the hydrogenation temperature; R is the ideal gas constant; ∆H is the enthalpy of formation; ∆S is the entropy of formation. The relationship between hydrogen pressure and hydrogenation temperature is shown in Fig.2, and the corresponding enthalpy and entropy of the pressure plateau region in Ti6Al4V alloy are listed in Table 1. Error exists because of the sloped pressure plateau after analysis of the results of enthalpy and entropy. Therefore, the average values of hydrogen pressure in the pressure plateau region during the hydrogenation treatment

Table 1 Enthalpy and entropy of pressure plateau region of Ti6Al4V alloy

Temperature/ K	Plateau pressure/Pa	∆H/ kJ·mol^-1	∆S/ J·K^-1·mol^-1
823	10 825	-50.7±0.26	-138.4±0.69
873	15 137.5
923	21 825
973	34 325

of Ti6Al4V alloy are used to calculate the values of enthalpy and entropy for reducing the error. The average values of enthalpy and entropy of the pressure plateau region are -50.7±0.26 kJ/mol and -138.4±0.69 J/K·mol, respectively.

According to the Sieverts law, the relationship between hydrogen pressure $P_{H_{2}}$ and hydrogen content C (H/M) can be expressed by Eq.(2)^[

18] as follows:

K_{S} = \frac{C}{(P_{H_{2}})^{1 / 2}}

(2)

where K_S is the corresponding Sieverts constant. The relationship between H/M and $(P_{H_{2}})^{1 / 2}$ during the hydro-genation treatment of Ti6Al4V alloy is presented in Fig.3. Values of K_S for Ti6Al4V alloys hydrogenated at different temperatures were obtained by Eq.(2) and are listed in Table 2. From Table 2, it can be seen that the Sieverts constant increases firstly and then decreases gradually with increasing the hydrogenation temperature. The chemical activity and diffusion coefficient of titanium alloys are lower when the hydrogenation temperature is 823 K. The increase of Sieverts constant with increasing the hydrogenation temperature is caused by the increase of chemical activity and diffusion coefficient of the alloys. The Sieverts constant reaches its maximum value when the hydrogenation temperature is 923 K, indicating that the titanium atoms have the maximal affinity with hydrogen atoms at 923 K. When the hydro-genation temperature exceeds 923 K, the Sieverts constant is decreased with increasing the hydrogenation temperature, because the hydride becomes unstable and even decomposes at higher temperatures, and the solubility of hydrogen in the

Table 2 Sieverts constant K_S of Ti6Al4V alloys hydrogenated at different temperatures

Temperature/K	K_S
823	0.446
873	0.697
923	0.723
973	0.584
1023	0.560

alloys is decreased with increasing the hydrogenation temperature.

2.3 Microstructure evolution

2.3.1 OM observation

OM images of the as-received Ti6Al4V alloy and alloys hydrogenated at different temperatures are shown in Fig.4. The lamellar microstructure of the as-received Ti6Al4V alloy is shown in Fig.4a, in which α phase is light and β phase is dark. Microstructures of Ti6Al4V alloys after measurement of P-C isotherms at different hydrogenation temperatures change obviously. As shown in Fig.4b and 4c, the microstructure is still lamellar when Ti6Al4V alloy is hydrogenated at 823 and 923 K, but the content of α phase and β phase changes reversely, compared with that in as-received Ti6Al4V alloy. This is because the addition of hydrogen in Ti6Al4V alloy changes the relative chemical potential of α phase and β phase^[

19,20]. The chemical potential of α phase becomes weak because of the precipitation of hydride in α phase, which causes the elastic or plastic strains, internal stresses, disloca-tion, and crystal defects^{[Reference 20

Baidu Scholar}20]. In addition, cracks can be found in Ti6Al4V alloy hydrogenated at 823 K (Fig.4b), indicating that many hydrides are precipitated. After Ti6Al4V alloy is hydrogenated at 1023 K, the lamellar microstructure cannot be observed, but tiny acicular α' martensite and wide acicular α'' martensite can be found in the alloy, as shown in Fig.4d.

2.3.2 XRD analysis

XRD patterns of the as-received Ti6Al4V alloy and alloys hydrogenated at different temperatures are shown in Fig.5. As shown in Fig.5a, the as-received Ti6Al4V alloy contains a large amount of α phase and a small amount of β phase. XRD patterns of Ti6Al4V alloy change obviously after hydro-genation at different temperatures. The relative intensities of diffraction peaks of β phase are increased with increasing the hydrogenation temperature, as shown in Fig.5b~5d, sug-gesting that the amount of β phase is increased with increasing the hydrogenation temperature. The diffraction peaks of δ hydride appear in the XRD patterns of hydrogenated Ti6Al4V alloys. XRD patterns of Ti6Al4V alloys hydrogenated at 823 and 923 K are similar, but show significant difference from the case at 1023 K. Some diffraction peaks of α₂ phase (Ti₃Al) with hexagonal close packed (hcp) structure can be observed when the hydrogenation temperatures are 823 and 923 K, as shown in Fig.5b and 5c. When the hydrogenation temperature is 1023 K, the diffraction peaks of α₂ phase disappear, and the diffraction peaks of hcp α' martensite and orthorhombic α'' martensite appear, as shown in Fig.5d.

2.3.3 TEM analysis

TEM images of Ti6Al4V alloys hydrogenated at different temperatures are shown in Fig.6 and Fig.7. When the hydrogenation temperature is 923 K, lamellar δ hydrides can be observed in the alloy, as shown in Fig.6a. The corresponding selected-area electron diffraction (SAED) patterns of δ hydride with [011]_δ zone axis suggest that the crystal structure of δ hydride is face-centered cubic (fcc). Fig.6b shows the precipitation of δ hydride in β phase. When hydrogen content exceeds the hydrogen saturation threshold in β phase, the reaction of β_H+H→δ occurs. Platelet α₂ phase can be observed in α matrix, as shown in Fig.6c. The corres-ponding SAED patterns of α phase with [ $\bar{1}$ 101]_α zone axis and α₂ phase with [ $\bar{1}$ 102]_α₂ zone axis suggest that the crystal structure of α₂ phase is hcp. When the hydrogenation temperature increases to 1023 K, the observed α' martensite and α'' martensite are parallel, as shown in Fig.7a, which also shows the corresponding SAED patterns of α' martensite with [1 $\bar{2}$ 1 $\bar{3}$ ]_α_' zone axis and α'' martensite with [001]_α_'' zone axis. In addition, δ hydride twins can be observed in Ti6Al4V alloy hydrogenated at 1023 K, as shown in Fig.7b. The lamellar δ hydrides precipitated in Ti6Al4V alloy hydrogenated at 1023 K are thinner than those lamellar δ hydrides precipitated in Ti6Al4V alloy hydrogenated at 923 K (Fig.6a). The amount of δ hydride regions decreases when hydrogenation temperature increases from 923 K to 1023 K, indicating that δ hydrides become more unstable with increasing the hydrogenation temperature.

Fig.7 TEM images and SAED patterns of Ti6Al4V alloy hydrogenated at 1023 K: (a) α' and α'' martensite; (b) twins in δ hydrides

2.4 Discussion

According to the experiment results, it can be concluded that the Ti6Al4V alloys have different Sieverts constants and phase composition after hydrogenation at different tempe-ratures. The hydrogen saturation in β phase (42.5at% H) is higher than that in α phase (4.7at% H), as shown in the phase diagram of Ti-H^[

21]. The mass percent of α phase in the as-received Ti6Al4V alloy is about 83.8wt%^{[Reference 22

Baidu Scholar}22]. Therefore, most hydrogen dissolves in α phase during the hydrogenation treatment of Ti6Al4V alloy. When hydrogen content exceeds the hydrogen saturation threshold in α phase, the transfor-mation of α_H→α_H+β_H occurs. With increasing the hydrogen content, the reaction of β_H+H→δ occurs when hydrogen content exceeds the hydrogen saturation threshold in β phase. Therefore, the phase transformation of Ti6Al4V alloy during hydrogenation treatment can be expressed as follows: α+β→α_H+β_H→α_H+β_H+δ→β_H+δ. The phase transformations of α_H→α_H+δ and β_H→β_H+δ happen during the cooling process of the hydrogenated alloys^{[Reference 23

Baidu Scholar}23]. The phase composition of Ti6Al4V alloys hydrogenated at 823 and 923 K is similar but different from the case at 1023 K. When the hydrogenation temperatures are 823 and 923 K, α₂ phase can be observed in the hydrogenated Ti6Al4V alloys. The formation of α₂ phase is attributed to the decrease of β transus temperature after hydrogenation and the enrichment of Al in α phase^{[Reference 20

Baidu Scholar}20,23]. Based on the phase diagram of Ti-H^{[Reference 21

Baidu Scholar}21], δ hydride cannot be precipitated in α phase at high temperature. However, when the hydrogenated Ti6Al4V alloy is cooled to room temperature, δ hydride can be precipitated in α_H phase due to the reduction of hydrogen solubility in α phase. In addition, the Sieverts constant is increased with increasing the hydrogenation temperature from 823 K to 923 K. When the hydrogenation temperature is 923 K, the amount of β phase and δ hydride is larger than that of the Ti6Al4V alloy hydrogenated at 823 K, and a large amount of hydrogen dissolves in β phase and δ hydride. α₂ phase disappears when the hydrogenation temperature increases to 1023 K, because α₂ phase is completely transformed into α phase at higher temperatures^{[Reference 22

Baidu Scholar}22]. Tiny acicular α' martensite and wide acicular α'' martensite appear in Ti6Al4V alloy when the hydrogenation temperature is 1023 K. β phase is transformed into α' martensite and α'' martensite during the cooling process of Ti6Al4V alloy hydrogenated 1023 K. Because part of β phase does not have enough β-stabilizing elements, the eutec-toid reaction of β→α' occurs during the cooling process. Part of β phase is transformed into α'' martensite due to the redistribution of β-stabilizing elements in the alloy during the hydrogenation treatment^{[Reference 24

Baidu Scholar}24,25]. The Sieverts constant is decreased with increasing the hydrogenation temperature from 923 K to 1023 K. β phase becomes the main phase which dissolves plenty of hydrogen, but δ hydride becomes unstable and even decomposes. In general, the Sieverts constant is related to the amount of different phases and hydrogenation temperature.

3 Conclusions

1) Hydrogen pressure is increased with increasing the hydrogen content when Ti6Al4V alloy was hydrogenated at different temperatures. The pressure-composition (P-C) isotherms of Ti6Al4V alloy are divided into three regions. Only one pressure plateau exists during the hydrogenation treatment for each P-C isotherm of Ti6Al4V alloys hydrogenated at 823~973 K.

2) The values of enthalpy and entropy of the pressure pla-teau region are -50.7±0.26 kJ/mol and -138.4±0.69 J·K^-1·mol^-1, respectively.

3) The Sieverts constant increases firstly and then decreases gradually with increasing the hydrogenation temperature. The Sieverts constant reaches its maximum value when the hydrogenation temperature is 923 K.

4) The phase transformation of Ti6Al4V alloy during the hydrogenation treatment can be expressed as follows: α+β→α_H+β_H→α_H+β_H+δ→β_H+δ. The phase composition of Ti6Al4V alloys hydrogenated at 823 and 923 K is similar but different from that of Ti6Al4V alloy hydrogenated at 1023 K.

5) The Sieverts constant is related to the amount of different phases and hydrogenation temperature.

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English

Phase Transformations and Thermodynamics Analysis of Hydrogen Absorption of Ti6Al4V Alloy PDF

Abstract

Keywords

1 Experiment