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

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. (0 0 1)7M//(1 1 2)NM and [1 0 0]7M//[1 1 1¯]NM as well as (0 0 1)7M//(1 1 2)NM and [1¯ 0 0]7M//[1 1 1¯]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.