Factors influencing the martensitic transformation in Ni-Mn-Z (Z = Ga, Sn) Heusler alloys

Abstract The martensitic transition temperature of the Ni-Mn-Z (Z = Ga, In, Sn or Sb) generally scales with the average valence electron per atom (e/a) ratio. The e/a ratio can be increased either by increasing Ni content in lieu of Mn or Z (Ni-rich) or by substituting Mn for Z (Mn-rich). While the Mn-rich alloys display the expected scaling behaviour of the martensitic transition temperature, the Ni-rich
alloys, with the exception of Z = Ga alloys, do not exhibit any dependency on the e/a ratio. By carefully studying and comparing the structure, local structure and the physical properties of the Ni-rich and Mn-rich Ni-Mn-Ga and Ni-Mn-Sn alloys, the cause of difference in behaviour is identified. The off-stoichiometry brought about by increasing the Ni or Mn content results in formation of new local structural environment in addition to the Heusler like environment. If either of the two local structural environments are martensitic then the resulting Ni-rich or Mn-rich alloy also exhibits martensitic transformation.