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

The microstructural and crystallographic features of seven-layer modulated (7M) martensite in Ni50Mn30Ga20 thin films grown on MgO(0 0 1) 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.