Reverse transformation crystallography of deformed martensite in Ni-Mn-Ga shape memory alloys

Shape memory alloys exploit the crystallographically reversible martensitic transformation to achieve the significant shape memory effect. However, the crystallography of reverse transformation from deformed martensite to austenite that specifies the strain recovery remains less understood. In this work, based on the detailed microstructural and crystallographic examinations on the deformed seven-layered modulated (7M) martensite in a Ni50Mn30Ga20 alloy through uniaxial tension, it is revealed that the reverse transformation from deformed martensite to austenite may not conform to the transformation crystallography of self-accommodated martensite, but involve more crystallographic routes and higher lattice discontinuity to resume the crystallographic orientation of austenite, owing to the activation of new twinning systems in the deformed martensite. Comprehensive knowledge on the reverse transformation of deformed martensite is of practical importance for understanding the intrinsic characteristics of shape memory behavior and theoretical interest for insights into the crystallographic reversibility of martensitic transformation.