Valence Electron Concentration and Phase Transformations of Shape Memory Alloys Ni-Mn-Ga-X

Abstract

In the Ni₂MnGa based alloys with additions of transition element Ni–Mn–Ga–X, the martensitic transformation temperature T_M was observed as a function of the valence electron concentration per atom e⁄a. The T_M(e⁄a) strongly depends on e⁄a and increases with increasing e⁄a. In this paper, to examine the effect of X atom on the phase transformation in Ni–Mn–Ga–X alloys, the electronic structures for six systems were calculated for four phases, that is, the paramagnetic cubic, the paramagnetic monoclinic, the ferromagnetic cubic and the ferromagnetic monoclinic phases. Moreover, the total energy differences ΔE(e⁄a) between two phases among four phases were calculated as a function of e⁄a. The variations of T_M(e⁄a) were predicted by the difference ΔE(e⁄a) between the cubic and monoclinic structures in a ferromagnetic state. It was found that their correspondence is good for some systems and that the features of T_M(e⁄a) reflect the changes of the density of states of X atoms.