Transformation behavior of nearly stoichiometric Ni-Mn alloys

Abstract

The transformation behavior of Ni-Mn alloys in the vicinity of the stoichiometric composition has been studied using transmission electron microscopy, X-ray diffraction, optical microscopy, and electrical resistivity measurements. The transformation behavior was found to be markedly different in Mn-rich alloys and Ni-rich alloys. In Mn-rich alloys a martensitic transformation between L2₀ (B2) and L1₀ structures takes place, which possesses many features common to alloys exhibiting a thermoelastic martensitic transformation. On the other hand, in Ni-rich alloys an order-disorder transformation between A2 and L1₀ structures occurs. The martensitic transformation features {111} transformation twins as the transformation substructure while the ordering reaction involves {101} order twins. In the Mn-rich alloys, the martensitic phase, if either slowly cooled or annealed at intermediate temperatures, becomes “tempered”, resulting in a noncrystallographic, essentially featureless microstructure apart from the presence of occasional {111} twins.