简介概要

Thermodynamics of hydrogen absorption and desorption in TC21 alloy

来源期刊：Rare Metals2020年第12期

论文作者：Xiao-Li Wang Yong-Qing Zhao

摘要：In this paper,it was addressed a hydrogen absorbing and desorbing thermodynamics in α+β type TC21 titanium alloy with high strength and toughness based on thermodynamic experiments and calculation.The relationship between concentration（C）,temperature（T）,and pressure（P） of TC21 alloy is shown by P-C-T curves during hydrogen absorption and desorption process,which were measured by multistep hydrogenation/dehydrogenation methods from 625 to 750℃.The P-C-T isotherms at a given temperature were separated into three regions.The partial thermodynamic functions of hydrogen reaction were evaluated by a modified form of Sievert’s law and P-CoTrelation of different regions was expressed by the modified Sievert’s law.The results show that the enthalpy of hydrogen reaction in the first and third region relies on hydrogen content.According to Vant’ s Hoff law,enthalpy and entropy of hydrogenation platform in TC21 alloys are-53.58 kJ·mol^-1 and-127.41 J·K·mol^-1,respectively.Compared with P-C-T curves of hydrogen absorption,that of hydrogen desorption exists hysteresis.

稀有金属(英文版) 2020,39(12),1413-1418

Thermodynamics of hydrogen absorption and desorption in TC21 alloy

Xiao-Li Wang Yong-Qing Zhao

School of Material Science and Engineering,Xihua University

Northwest Institute for Nonferrous Metal Research

作者简介：*Xiao-Li Wang,e-mail:yuqing820704@163.com;

收稿日期：4 September 2013

基金：financially supported by the Key Program in Xihua University (No.Z1120117);Department of Education Research Fund in China,Sichuan Province (No.1220145 3);the Open Research Fund of Key Laboratory of Special Materials and Preparation Technology,Xihua University (No.S2jj2012-019);

Thermodynamics of hydrogen absorption and desorption in TC21 alloy

Xiao-Li Wang Yong-Qing Zhao

School of Material Science and Engineering,Xihua University

Northwest Institute for Nonferrous Metal Research

Abstract：

In this paper,it was addressed a hydrogen absorbing and desorbing thermodynamics in α+β type TC21 titanium alloy with high strength and toughness based on thermodynamic experiments and calculation.The relationship between concentration(C),temperature(T),and pressure(P) of TC21 alloy is shown by P-C-T curves during hydrogen absorption and desorption process,which were measured by multistep hydrogenation/dehydrogenation methods from 625 to 750℃.The P-C-T isotherms at a given temperature were separated into three regions.The partial thermodynamic functions of hydrogen reaction were evaluated by a modified form of Sievert's law and P-CoTrelation of different regions was expressed by the modified Sievert's law.The results show that the enthalpy of hydrogen reaction in the first and third region relies on hydrogen content.According to Vant' s Hoff law,enthalpy and entropy of hydrogenation platform in TC21 alloys are-53.58 kJ·mol^-1 and-127.41 J·K·mol^-1,respectively.Compared with P-C-T curves of hydrogen absorption,that of hydrogen desorption exists hysteresis.

Keyword：

Thermodynamic functions; Enthalpy; Entropy; Hydrogen absorption/desorption;

Received： 4 September 2013

1 Introduction

In recent years,thermohydrogen treatment of titanium alloys has been investigated widely,which could lower the deforming temperature and resistance,improve cutting performance,and so on [ 1, 2, 3, 4, 5, 6] .Zong et al. [ 7] investigated the high temperature deformation behavior of TC21 alloy with 0.7 wt%hydrogen.The results showed that the suitable hydrogen addition could significantly decrease the flow stress and improve the hot workability of TC21 titanium alloy.In order to formulate hydrogenated and dehydrogenated technology,thermodynamics of hydrogenation and dehydrogenation was investigated.

The relationship between metal hydride concentration(C),temperature (T),and pressure (P) defines the solubility of hydrogen in titanium alloys,thermodynamic parameters(enthalpy,entropy,etc.) of hydrogenation reaction,and so on.Therefore,an accurate modeling of P-C-T correlation is the key factor in any metal hydride system [ 8] .

Using experiential P-C-T curves,thermodynamics of hydrogenation reaction in titanium alloys was studied by Van't Hoff equation in two-phase region and polynomial in single-phase region.Huang et al. [ 9] measured the hydrogenation P-C-T curves of titanium in 550-600℃and calculated hydrogenated enthalpy of the first platform(-(82.5±0.7) kJ·mol^-1) and the second platform((-120.9±4.8) kJ·mol^-1) by Van't Hoff equation.Dutta et al. [ 10] investigated the effects of the addition of Al,Ni elements on hydrogenation of Ti-45Nb alloy.Solid-solution enthalpy and entropy of hydrogen were calculated by equation of lnP=α+x+δx²+ln[x/(6-x)],where P is the partial pressure of hydrogen,x is the concentration of hydrogen in metal expressed as hydrogen to metal atomic ratio,andαandδare the temperature dependent constants.The results showed that the enthalpy first decreased with the increase of hydrogen content and then increased as a minimum existing.Using equation ln P=f (C)+2 lng(C)+h(C)/T (where P is the hydrogen pressure;C is the hydrogen content;and T is the temperature) reported by Wang [ 11] ,Gao and Huang [ 12] calculated hydrogenated enthalpy of Ti (-71.0 kJ.mol^-1),Ti-5Cr (-68.3 kJ·mol^-1),and Ti-5Ni (-59.7 kJ·mol^-1) inβ-phase region.

Therefore,hydrogen diffusion and phase transformation in titanium particles were studied by Gao et al. [ 13] based on thermodynamic calculation,and the mechanisms of hydrogen diffusion in different phases (α-Ti,β-Ti,and TiHx) were analyzed.The calculation results showed that the formation of a TiH_x layer resulted in a slow hydrogenization rate of titanium and the phase transformation rate ofβ-Ti fromα-Ti above 573 K increased with an increase in hydrogen content and reached a maximum value.

For dehydrogenation,Takeda et al. [ 14] studied hydrogen outgassing mechanism in titanium materials.It was found that hydrogen atoms diffusion rate at the surface layer-bulk boundary in titanium material was slow.However,hydrogen atom diffusion at the surface oxide layer was quick,and the desorption of adsorbed hydrogen atom at the surface was rapid.

The mechanism of hydrogenation and dehydrogenation in titanium alloys was investigated by different methods,and different results were obtained.In this paper,the thermodynamics of hydrogenation and dehydrogenation in TC21 alloy were investigated by experiments.

2 Experimental

Thermodynamic experiments of hydrogen absorption and desorption in TC21 alloy were carried out in a tubular vacuum furnace (L2210II/ZM),as shown schematically in Fig.1.The experimental procedure of hydrogenated thermodynamics is shown in Fig.2.

Using continuous multistep method,the thermodynamic experiment was carried out during different temperatures.P-C-T curve data were measured under different temperatures of hydrogen absorption.Hydrogen pressure changes with time as shown in Fig.3.

The initial hydrogen pressure and equilibrium hydrogen pressure of the i-th step were P_1i and P_2i,separately.According to POMA's law,the molar amount of hydrogen absorption could be represented by Eq.(1):

whereΔP_i(ΔP_i=P_1i-P_2i) is the change of hydrogen pressure;V (m³) is the volume of tubular vacuum furnace;and T(K) is the hydrogen absorption temperature.

Fig.1 Simple scheme of tubular furnace for thermohydrogen treatment (L2210II/ZM)

The total amount of hydrogen absorption from the first step to the i-th step is shown in Eq.(2).

The total amount of hydrogen absorption is shown in Eq.(3).

so.

where m is the sample mass and C_n is the hydrogen content of sample.

The i-th step hydrogen content is represented by Eq.(6).

The P-C-T curves were established by hydrogen content of every step calculated by Eq.(6).

The measurement of dehydrogenation P-C-T curves was similar to that of hydrogenation.Therefore,hydrogen content of dehydrogenation is calculated by Eq.(7).

Fig.2 Thermodynamic experimental process of hydrogen absorption in titanium alloys

Fig.3 Change of hydrogen pressure with time during hydrogen absorption process by continuous multistep method

3 Results and discussion

3.1 P-C-T curves of hydrogen absorption in TC21alloy

According to equilibrium hydrogen pressure and Eq.(6),P-C-T curves of TC21 alloy in different temperatures are shown in Fig.4.As seen from Fig.4,the P-C-Tisotherms at a given temperature can be separated into three regions.The first region is controlled by the solubility of hydrogen atom inαandβphase,and the third region is controlled by the solubility of hydrogen atom inβ_H phase and its hydride,in which equilibrium hydrogen pressure increases with the increase of hydrogen content and temperature.The second region is controlled by the phase transition from phase toβH phase andδhydride,that is plateau region.When the temperature of hydrogen absorption reaches to critical temperature,such as 973 K in Fig.4,plateau region does not exist,which means the transition fromα_H phase toβH phase quickly finishes and directly enters into the third region.

Fig.4 P-C-T curves of hydrogen absorption in TC21 alloy

3.2 Computation of thermodynamic parameters

3.2.1 A modified form of Siverts'law for P-C-T curves

In order to study thermodynamic reaction of hydrogen absorption in titanium alloys,a general P-C-T relation of a gas in equilibrium with hydrogen solution,suggested by Wang [ 11] ,can be expressed by Eq.(8).

where P (1×10⁵ Pa) is the equilibrium hydrogen pressure and C_H/Ti is the ratio of H/Ti;h(C_H/Ti) is the linear function of C_H/Ti;f(C_H/Ti) is polynomial function of C;g(C_H/Ti)=C/(ξ-C_H/Ti);ξis related to stoichiometry of hydride.

According to the data of P and C in Fig.4 and Eq.(8),the experimentally measured values of ln P_(α+β) and ln P_(β+hydride)(P_(α+β) and P_(β+hydride) are the equilibrium hydrogen pressure of two-phase region (α+β) and(β+hydride)) can be approximated with Eqs.(9) and (10).

Ordering

where a₁,b₁,a₂,b₂,Z₁,and Z₂ are constants which are related to materials and hydrogen contents,and C_H is hydrogen content.

(1) P-C-T relation of the first region can be expressed by Eq.(13).

According to the relation of ln P-1/T (In =ln(c/a)²+△H/RT [ 15] ,where c is equilibrium hydrogen pressure;a is material constant.),hydrogen solution enthalpy inαandβphase is-60.11 kJ·mol^-1.

(2) P-C-T relation of the third region can be expressed by Eq.(14).

Fig.5 Curve of

-1/T for TC21 alloy

In the third region,ln P-1/T appears linear relation.Hydrogen solution enthalpy in hydride is-98.27 kJ·mol^-1.For pure titanium,hydrogen solution enthalpy in hydride was suggested by Wang [ 11] through P-C-T curves.

Hydrogen solution enthalpy in hydride 2.7C kJ·mol^-1 if obtained from Eq.(15).The results quoted by Wang [ 11] from Dantzer's results showed the enthalpy at737 K (C_H/Ti=1.42 kJ.mol^-1and C_H/Ti=1.85 kJ·mol^-1).All the results show that the enthalpy relies on hydrogen content in a certain region of hydrogen content,which may be caused by the temperature dependence of .

Our results have discrepancy with other researchers',which are caused by different materials,experimental apparatus,and parameters.

3.2.2 Calculation of thermodynamic parameters in plateau region

According to Van't Hoff law ( =ΔH/RT-△S/T),enthalpy and entropy of plateau region can be obtained by plotting equilibrium hydrogen pressure and temperature,as shown in Fig.5 and Table 1.However,experiment results exist discrepancy.So the average value of equilibrium hydrogen pressure in plateau region was used.From Fig.5and Table 1,enthalpy and entropy of plateau region are-53.58 kJ·mol^-1 and-127.41 J·K^-1·mol^-1,respectively.

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Table 1 Thermodynamic parameters of hydrogen absorption in titanium alloys

Fig.6 P-C-T curves of dehydrogenation in TC21 alloy (H hydro-genation,D dehydrogenation)

3.2.3 P-C-T curves of hydrogen desorption in TC21 alloys

P-C-T curves of hydrogen desorption at different temperatures are shown in Fig.6.As seen from Fig.6,the variation tendency of hydrogen pressure with hydrogen content is similar to hydrogen absorption.The plateau region exists at 650℃,and the equilibrium hydrogen pressure is about 0.038×10⁵ Pa.Compared with P-C-T curves of hydrogen absorption,isothermal curves exist hysteresis.

Hydrogen desorption is the inverse process of hydrogen absorption,which exists solid-solution region and twophase regions.At the first stage of dehydrogenation,the relationship of equilibrium pressure of gas and hydrogen content expresses quadratic function ( (H/M)²)(Fig.7).According to Sievert's law,Sievert's constant S of hydrogen desorption inβ_H phase and hydride is obtained,as shown in Table 2.

4 Conclusion

Thermodynamic constraints and a modified form of Sievert's law were used to investigate the P-C-T relationship of hydrogen absorption/desorption in TC21 alloys.The results show that P-C-T isotherms at a given temperature could be separated into three regions.The first region and the third region are controlled by the solubility of hydrogen atoms in single phase.The second region is controlled by the phase transition fromα_H phase toβ_H phase andδhydride,that is plateau region.In the first and third region,hydrogen solution enthalpy in single phase and hydride is relied on hydrogen content in a certain region of hydrogen content.According to Van't Hoff law,the enthalpy and entropy of plateau region are-53.58 kJ·mol^-1 and-127.41 J·K^-1·mol^-1,respectively.

Fig.7 Relationship between equilibrium hydrogen pressure and hydrogen concentration in solution region during dehydrogenation process

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Table 2 Sievert's law constants of hydrogen soluted inβ_H and hydrides during dehydrogenation process

A error value,S Sievert's constant,R correlation coefficient

For hydrogen desorption,the variation tendency of hydrogen pressure with hydrogen content is similar to hydrogen absorption.Compared with hydrogen absorption,P-C-T curves of hydrogen desorption exist hysteresis.