Theoretical Study Of The Martensitic Phase Transformation For TiNi And Ti₅₀Ni₂₅Cu₂₅Shape Memory Alloys

Recent years,as the most popular candidates in shape memory alloys,TiNi-based alloys have been widely applied in many technological materials owing to its remarkable behaviors,such as the shape memory effect,superelasticity,etc.For the binary TiNi alloys,structures except the B19 phase and the phase transformation behaviors of each phase have been clearly understood.The high temperature phase B2,metastable phase B19 and low temperature phase B19' of TiNi alloy have been confirmed to be cubic(space group Pm(?)m),orthorhombic(Pmcm)and monoclinic(P21/m)respectively.The experiments indicate that with the Cu element increasing,the B19 phase of TiNi would appear and the martensitic transformation behavior would be transformed from B2-B19'to B2-B19 when the Cu element increase to 25%.No one has ever studied the transformation mechanism of TiNi after Cu doping into it so far.In this paper,we address the above unresolved issues of calculations to acquire the crystal structures and transition pathway of TiNi and Ti50Ni25Cu25.For TiNi,we find the phase transformation direction of B2-319 and B19-B19'phase transition through the phonon-dispersion relations and the comparisons of each structures.The B2-B19 process is driven by the softening of phonon branches in the M point(0.5,0.5,0)of Brillouin zone,and the B19-B19' is driven by a shear and a in-plane shuffle.The transition pathway of Ti50Ni25Cu25 is the same as TiNi.In this way,the transition pathway of TiNi and Ti50Ni25Cu25 is studied,as a consequence,no energy barrier between B2 and B19,a energy barrier of 1.6 meV is calculated between B19 and B19'.A energy barrier of 10.3 meV at least is too high between B19 and B19' to climb the maximum energy for Ti50Ni25Cu25.