Formation of a-b twin induced by tension in Ni-Mn-Ga magnetic shape memory alloys

Deformation of twinned martensite can effectively reformulate the variant configuration as well as the intervariant correlation. In this work, the deformed microstructure of 7M martensite induced by tension were examined in a polycrystalline Ni50Mn30Ga20 alloy prepared by means of directional solidification. The uniaxial tensile loading applied along the solidification direction triggers the collective variant reorientation in the horizontal variant colonies, transforming four initial variants of a-c twin with self-accommodation into four differently oriented variants in b-c twin relation. Moreover, another four new variants can be induced by the variant penetration from an inclined variant colony into the reoriented variant region. Notably, the newly formed variants develop the a-c twin and a-b twin relation with the adjacent reoriented variants, thereby constituting a complicated microstructure covering a-c twin, a-b twin and b-c twin. Through further detwinning to eliminate a-c twin and b-c twin, the mixed three types of twins finally evolve into the simplified variant configuration including only a-b twin in the local region. This work advances the knowledge of variant configuration induced by deformation and contributes to the insights into martensite deformation behavior as well as twinning system evolution in Ni-Mn-Ga based magnetic shape memory alloys.