Insight into variant selection of seven-layer modulated martensite in Ni–Mn–Ga thin films grown on MgO(0 0 1) substrate

Abstract

The microstructural and crystallographic features of seven-layer modulated (7M) martensite in Ni50Mn30Ga20 thin films grown on MgO(001) substrate were revealed by electron backscatter diffraction (EBSD) and secondary electron imaging (SEI). Locally, each group of martensite plates consisted of four orientation variants that are twin-related to one another. The Type-I and Type-II twin relationships were most prevalent in individual plate groups with low or high relative SEI contrast, respectively. A general procedure was developed to quantitatively assess the accommodation capacities of twin variant pairs constrained by the rigid substrate. It is understood that the configuration of either Type-I twin variant pairs in low relative contrast zones or Type-II twin variant pairs in high relative contrast zones can effectively accommodate the shear deformation in the film normal direction – a deformation that tends to “peel” the film surface off the substrate. As a consequence, it brings about the preferential formation of Type-I and Type-II twins during the martensitic transformation. This finding is of significance as it highlights the role of external constraint on microstructure control toward property modification.