First-principles Study Of The Effects Of Hydrogen In γ-TiAl Crystal

The crystal structure models of γ-TiAl and γ-TiAl(H)were constructed and then the effects of hydrogen on the binding energies,electronic structures,and elastic properties of γ-TiAl crystal were investigated by using the CASTEP program based on the first-principles with pseudopotentials method.The diffusion path and activation energy for H interstitial diffusion in γ-TiAl were calculated.The single vacancy formation energies and self-diffusion activation energies for Ti and Al vacancy diffusion in γ-TiAl were calculated and the effects of hydrogen on the properties of self-diffusion for Ti and Al atoms in γ-TiAl were studied.The generalized stacking fault energies of γ-TiAl and γ-TiAl(H)were calculated and the effects of hydrogen on the properties of generalized stacking fault energies for γ-TiAl were studied.In the Ti-rich octahedral interstice(O1),Al-rich octahedral interstice(O2)and tertrahedral interstice of γ-TiAl,hydrogen tended to occupy the O1 interstitial site.The densities of states(DOS),Mulliken population,electron density and electron density difference of γ-TiAl and γ-TiAl(H)were calculated and the results show that the existence of hydrogen weakens the bonding interaction of Ti-Ti and Ti-Al bonds and enhances the bonding interaction of Al-Al bond.The calculated results of elastic properties of γ-TiAl and γ-TiAl(H)indicate that hydrogen exaggerates the value of G/B of γ-TiAl and makes it more brittle.The interstitial diffusion path for hydrogen atom in γ-TiAl crystal is O1→T→O1.The interstitial diffusion activation energies are 0.5052 e V,0.4713 e V and 0.4628 eV for hydrogen diffuses in γ-TiAl when the hydrogen content is 0.917 at.%,1.54 at.% and 2.04 at.% for γ-TiAl(H)crystal,respectively.And the calculated interstitial diffusion coefficients of hydrogen atom in γ-TiAl are in good agreement with the experimental values.The Ti single vacancy formation energy,Al single vacancy formation energy,Ti antisite formation energy and Al antisite formation energy of γ-TiAl are 1.4018 eV,2.2226 e V,0.8629 e V and 0.1302 e V,respectively.And the nearest four point defect formation energies of hydrogen atom for γ-TiAl(H)crystal are 1.1931 e V,1.6598 e V,0.3092 eV and 0.2291 eV,respectively.The self-diffusion activation energy for Ti vacancy is 2.4899 e V and it shows good agreement with the experiment result(2.59 e V).The self-diffusion activation energy for Al vacancy is 3.3646 e V and it is a little less than the experimental one.The self-diffusion activation energies for the nearest Ti vacancy and Al vacnacy of hydrogen atom are 2.3793 e V and 2.9718 e V for γ-TiAl(H),respectively.The calculated results show that the addition of hydrogen atom could increase the self-diffusion coefficients of Ti and Al atoms and promote the plasticity of γ-TiAl crystal.With the increase of hydrogen content,the reduction of the self-diffusion activation energy of Ti atom has a little change,while which of the self-diffusion activation energy of Al atom increases.The calculated values of unstable stacking fault energies and stable stacking fault energies of γ-TiAl are 333.82 eV and 209.45 e V,respectively.And which are in good agreement with the values of other calculations.With the increase of hydrogen content,the unstable stacking fault energies and stable stacking fault energies of γ-TiAl(H)decrease gradually,which could increase the possibility of twinning appearance of γ-TiAl(H)crystals.The calculated results of electron density difference show that the addition of hydrogen atom leads to a decrease in the strength of Ti-Ti bond,and it causes a decrease in the stacking fault energy of γ-TiAl crystals.