Electron microscopic study of the microstructure of Ni50Mn50 alloy after natural aging

Abstract

Recent studies have examined the aging of martensite in shape memory alloys. Due to the widespread use of these compounds, it is necessary to ensure their strength and long-term reliability. Studies of alloys based on the Ni–Mn system are insufficient. In the study of the microstructure of the binary equiatomic alloy of nominal composition Ni50Mn50 after homogenization annealing, quenching and natural aging was studied using scanning and transmission microscopy methods (SEM and TEM). It has been established that as a result of long-term keeping at room temperature, the lamellar twinned martensitic structure dissipates, apparently due to diffusion. Images of the microstructures of the alloy under study are presented: the martensitic matrix and the regions that have experienced phase separation. It is shown that the post-dissipation regions are characterized by their own substructure with individual grains. In this case, some orientations are inherited from the previous martensite. According to TEM studies, their fine structure also differs from the martensite one. Traces of the boundaries of martensite plates are observed, oriented relative to each other at an angle of approximately 70°. In the electron diffraction patterns, both martensite and non-twinned regions show additional reflections and diffuse scattering, probably due to the still unstable state.