Role of local structural distortions in the variation of martensitic transformation temperature with e/a ratio in Ni2Mn1+xZ1−x (Z = In, Sn or Sb) alloys†

Ni₂Mn_1+xZ_1−x (Z = In, Sn or Sb) undergo martensitic transformation with transformation temperature (T_M) scaling with the average valence electron per atom (e/a) ratio. However, the rate of increase of T_M depends on the type of Z atom, with the slope of T_Mvs. e/a curve increasing from Z = In to Z = Sb. Local structural distortions are believed to be the leading cause of martensitic transformation in these alloys. A careful study of the Ni and Mn local structures in several Ni₂Mn_1+xZ_1−x alloys with varying e/a ratio and the same Z atom, with the same e/a ratio but different Z atoms and with the same T_M but with different Z atoms and different e/a ratio, revealed that the difference between Ni–Mn and Ni–Z nearest neighbor distances decreases as the Z atom changes from In to Sb. This decrease in the local structural distortion accommodates a higher content of Mn until the L2₁ structure becomes unstable and the alloy undergoes a martensitic transformation.