Investigation On The Effect Of Alloying Elements On Martensitic Transformation Of TiNi Based Shape Memory Alloys By First-principles Theory

Doping of alloying elements has been proved to be an effective way to improve the properties of TiNishape memory alloys,such as martensitic transformation temperature and hysteresis et al.Among the alloying elements,the doping of Cu and Pd can not only change the transformation temperature,but also effectively reduce the hysteresis.Therefore,in present work,the martensitic transformation type,transformation temperature and hysteresis of Ti₅₀Ni_50-xPd_x(x=0,3.125,6.25,9.375,12.5),Ti₅₀Ni_50-xCu_x(x=0,7.5,10,12.5,15),Ti_53.125Ni_34.375Cu_12.5and Ti₅₀Ni_37.5-xCu_12.5Pd_x(x=3.125,6.25)alloys were studied by the first-principle calculation based on the density functional theory and the influencing mechanism was discussed.Furthermore,TiNiCu alloys were prepared and characterized for the purpose of experimental verification.The results show that the type of martensitic transformation changes from B2?B19?to B2?B19 with the increase of Pd or Cu content.With the increase of Pd or Cu content,the martensitic transformation temperature of B2?B19?decreases,while that of B2?B19increases.In addition,the doping of Pd or Cu changes the electronic structure of Ti Ni-based alloys,which reduces the stability of parent phase,and leads to the increase of martensitic transformation temperature.With the increase of Ti content,the electronic density of state at Fermi level of Ti_53.125Ni_34.375Cu_12.5 alloy increases,and the minimum value near the pseudogap disappears,which leads to the decrease of the stability of parent phase and the increase of martensitic transformation temperature.The calculation results of ?₂ of TiNiPd and TiNiCu alloys show that the compatibility between B19 phase and parent phase is generally higher than that between B19?phase and parent phase.It is found that the energy difference between the parent phase and the stable martensite gradually decreases with the increase of Pd,Cu or Ti content.Meanwhile,?₂gradually approaches to 1,which indicates that the compatibility between parent phase and martensite increases gradually,thus reduces the hysteresis.The simulation study of TiNiCu Pd quaternary alloys shows that the Pd mainly replaces the position of Ni.The transformation type is B2?B19,and the transformation temperature increases with increase of Pd content.The Ti₅₀Ni_34.375Cu_12.5Pd_3.125 alloy has a?₂closer to 1 than any other alloys.The experimental study of TiNiCu alloys shows that microstructure of Ti₅₀Ni_32.5Cu_7.5 alloy consists of B19,B19?and Ti₂(Ni,Cu)phase at room temperature,while B19 and Ti₂(Ni,Cu)phase are the main phases in the other four alloys.The lattice parameters of parent phase and martensite are close to the simulated results,and all of them are within the allowable error range.As the Cu or Ti content is increasing,the?₂ of Ti Ni-based alloys approaches to 1.Moreover,the results show that Ti_53.125Ni_34.375Cu_12.5 alloy has the best thermal cycle stability,which is also consistent with the simulated results.