Thermodynamics of hydrogen absorption and desorption in TC21 alloy
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
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
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.
Therefore,hydrogen diffusion and phase transformation in titanium particles were studied by Gao et al.
For dehydrogenation,Takeda et al.
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 P1i and P2i,separately.According to POMA's law,the molar amount of hydrogen absorption could be represented by Eq.(1):
whereΔPi(ΔPi=P1i-P2i) 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 Cn 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 inβH 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 fromαH 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
where P (1×105 Pa) is the equilibrium hydrogen pressure and CH/Ti is the ratio of H/Ti;h(CH/Ti) is the linear function of CH/Ti;f(CH/Ti) is polynomial function of C;g(CH/Ti)=C/(ξ-CH/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 CH 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
(2) P-C-T relation of the third region can be expressed by Eq.(14).
Fig.5 Curve of
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
Hydrogen solution enthalpy in hydride
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 (
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 (
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 fromαH phase toβH 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
Table 2 Sievert's law constants of hydrogen soluted inβH 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.
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