Randomly packed Ni sub(2) MnIn and NiMn structural units in off-stoichiometric Ni sub(2) Mn sub(2-y) In sub(y) alloys

Abstract

Ni sub(2) Mn sub(2-y) In sub(y) alloys transform from the martensitic L1 sub(0) antiferromagnetic ground state near y = 0 to the austenitic ferromagnetic L2 sub(1) Heusler phase near y = 1 due to doping of In impurity for Mn. The offstoichiometric alloys prepared by rapid quenching are structurally metastable and dissociate into a mixture of L2 sub(1) (Ni sub(2) MnIn) and L1 sub(0) (NiMn) phases upon temper annealing. Despite this structural disintegration, the martensitic transformation temperature remains invariant in the temper annealed alloys. Investigations of the local structure of the constituent atoms reveal the presence of strongly coupled Ni sub(2) MnIn and NiMn structural units in the temper annealed as well as the rapidly quenched off-stoichiometric Ni sub(2) Mn sub(2-y) In sub(y) alloys irrespective of their crystal structure. This random packing of the L2 sub(1) and L1 sub(0) structural units seems to be responsible for invariance of martensitic transition temperature in the temper annealed alloys as well as the absence of strain glass transition in rapidly quenched alloys.