Randomly packed Ni$_2$MnIn and NiMn structural units in off stoichiometric Ni$_2$Mn$_{2-y}$In$_y$ alloys

Ni$_2$Mn$_{2-y}$In$_y$ alloys transform from the martensitic $L1_0$ antiferromagnetic ground state near $y = 0$ to austenitic ferromagnetic $L2_1$ Heusler phase near $y = 1$ due to doping of In impurity for Mn. The off stoichiometric alloys prepared by rapid quenching are structurally metastable and dissociate into a mixture of $L2_1$ (Ni$_2$MnIn) and $L1_0$ (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$_2$MnIn and NiMn structural units in the temper annealed as well as the rapidly quenched off stoichiometric Ni$_2$Mn$_{2-y}$In$_y$ alloys irrespective of their crystal structure. This random packing of the $L2_1$ and $L1_0$ 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.