Anomalous martensitic transformation with diffusion in the all-d-metal Heusler alloys of Ni2MnTi and Ni2MnV from first-principles investigation

Using the first -principles exact muffin -tin orbitals method in combination with the coherent potential approximation, the Ti- and V -alloying effects on the crystal structure and lattice parameter, phase stability, magnetic and elastic properties of Ni2MnGa have been systematically investigated with Ni2MnGa1-xZ' x(Z'x, Z' = Ti and V, 0 <= x <= 1) Heusler alloys. It is shown that the two all -d -metal alloys of Ni2MnZ' are both stabilized by the simple -tetragonal (ST) structure with c/a being very close to 1. The Ti- and especially V -doping can increase both the tetragonal lattice deformation (|1 - c/a|) of the body -centered -tetragonal (BCT) martensite and the energy barrier between the phase and the parent L21 one, and then favor improving the martensitic transformation (MT) in Ni2MnGa. However, since the relative energy of the ST structure decreases more rapidly with increasing x, these Tix (x > 0.3) and Vx (x > 0.2) alloys are supposed to occur an anomalous MT with diffusion instead, L21-BCT-ST, where the ST structure is transformed only by Mn and Ga/Z ' atoms on their second layer of the BCT one swapping the positions, because of c/a of the latter phase being right almost 2c/a of the former one. The strong anti -ferromagnetic coupling between Mn and Z' atoms is further proved to be responsible for energetically promoting the anomalous MT with Z'-doping. During these phase transitions, the shear elastic constant C ' = (C11 -C12)/2 of Ni2MnV softens to be negative more easily, and then the Ti -doping is more beneficial to enhance the ductility of Ni2MnGa. The anomalous MT is finally confirmed also originating from the Jahn-Teller instability, dominated by 3d electrons of Ni and Ti in the Tix but Ni, Mn, and V in the Vx alloys.