Electron concentration and structural transformation of Ni2MnGa-based shape memory alloys

Abstract

In shape memory Ni2MnGa based alloys, the structural transition temperature (Tt increases with increasing valence electron concentration per atom (e/a). That is, when Ni or Mn atoms in Ni2MnGa are replaced with a fourth element (X atom), the Tt increases with increasing atomic number of the X atom. To examine the experimental results, the electronic structures of these alloys were calculated for the cubic and monoclinic structures. The difference ΔE of total energies between the two structures was also calculated as a function of e/a. It was found that the features of Tt (e/a) are similar to those of ΔE (e/a). Their features are different in the lower and higher ranges than the boundary e/a = 7.625. The two features are characterized by two cases: a case that X atoms occupies Ni sites and the other case that X atoms occupy Mn sites. The characteristics mainly come from the difference of the density of states of X atoms at Ni and Mn sites.