The martensitic microstructure of 5M and NM martensites in off-stoichiometric Ni2MnGa ferromagnetic shape memory alloys

Abstract

In recent years, extensive studies have been performed on many aspects of the NiMnGa alloys, such as crystal structure, phase transformation, magnetic field induced strain, magnetic properties, mechanical behaviour and the effects of magnetic fields on martensitic transformation. Depending upon the composition, the most important martensites that exist in off-stoichiometric Ni2MnGa alloys are the modulated 5M and 7M, and, the non-modulated tetragonal NM ones. In this study, a systematic investigation was carried out on the 5M and NM martensite in well characterised Ni50Mn28.9Ga21.1 and Ni53Mn25Ga22 alloys, respectively using transmission electron microscopy (TEM) and X-ray diffractometry for determining crystal structure, substructure, intervariant interfaces and self-accommodating microstructure. It was established that both NM and 5M are internally twinned martensites with tetragonal and monoclinic crystal structures, respectively, Figures 1–4. The crystallographic features of the martensitic microstructures as observed by TEM are found to be in agreement with those predicted by computations based on the phenomenological theory of martensite crystallography.