Crystallographic insights into the intermartensitic transformation in Ni–Mn–Ga alloys

Abstract

Ferromagnetic shape memory alloys such as Ni–Mn–Ga usually undergo intermartensitic transformation upon cooling or heating. Here, an attempt is made to explore the microstructural features and orientation relationships associated with the intermartensitic transformation from seven-layered modulated (7M) martensite to non-modulated (NM) martensite in a polycrystalline Ni53Mn22Ga25 alloy. Based on electron backscatter diffraction analysis, it is demonstrated that the intermartensitic transformation proceeds in an in-plate manner (through atomic reshuffling and lattice distortion) with specific orientation relationships between the two martensitic phases, i.e. (001)7M//(112)NM and [100]7M//[111¯]NM as well as (001)7M//(112)NM and [1¯00]7M//[111¯]NM, accompanied by the thickening of martensite plates and the surface relief effect. Consequently, one 7M variant (plate) evolves into one NM plate consisting of two twin-related fine variants, and four differently oriented 7M variants in one variant group result in a total of eight NM variants.