Role of local structural distortions in the variation of martensitic transformation temperature with e/a ratio in Ni sub(2)Mn sub(1+x)Z sub(1-x) (Z = In, Sn or Sb) alloys

Abstract

Ni sub(2)Mn sub(1+x)Z sub(1-x) (Z = In, Sn or Sb) undergo martensitic transformation with transformation temperature (T sub(M)) scaling with the average valence electron per atom (e/a) ratio. However, the rate of increase of T sub(M) depends on the type of Z atom, with the slope of T sub(M)vs. 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 Ni sub(2)Mn sub(1+x)Z sub(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 sub(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 L2 sub(1) structure becomes unstable and the alloy undergoes a martensitic transformation.