The effect of the melt cooling rate on the atomic ordering of austenite and, as a consequence, on the
martensitic transformation of a nonstoichiometric alloy of the Ni-Mn-In system has been studied. In-situ TEM
observations revealed differences in the mechanism of phase transformations of the alloy subjected to different
cooling conditions. It is shown that during quenching a high density of antiphase boundaries (APB) is formed
and the alloy is in the austenite-martensitic (10M and 14M) state up to a temperature of 120K. In a slowly cooled
alloy, a lower APB density is observed, and a two-stage transformation L21/B2 → 10M → 14M occurs in the
range of 150–120 K.