  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.
