For Ni–Mn–Ga ferromagnetic shape memory alloys, the characteristic features of modulated martensite (including the number/shape of constituent variants, the inter-variant orientation relationship and the geometrical distribution of variant interfaces) determine the attainability of the shape memory effect. In the present work, a comprehensive microstructural and crystallographic investigation was conducted on a bulk polycrystalline Ni50Mn28Ga22 alloy. As a first attempt, the orientation measurements by electron back-scattering diffraction, using the precise information on the commensurate 5M modulated monoclinic superstructure (instead of the conventionally simplified non-modulated tetragonal structure) – were successfully performed to identify the crystallographic orientations on an individual basis. Consequently, the morphology of modulated martensite, the orientation relationships between adjacent variants and the characters of twin interfaces were unambiguously determined. With the thus-obtained full-featured image on the configuration of martensitic variants, the possibility of microstructural modification by proper so-called mechanical training was further considered. This new effort made it feasible to explore the crystallographic/microstructural correlations in modulated martensite with high statistical reliability, which in turn provided useful guidance for optimizing the microstructure and shape-memory performance.

Twin Relationships of 5M Modulated Martensite in Ni–Mn–Ga Alloy. Z.Li, Y.Zhang, C.Esling, X.Zhao, L.Zuo: Acta Materialia, 2011, 59[9], 3390-7