Deformation of Ni-Mn-Ga 7M Modulated Martensite Through Twinning/Detwinning and Forward/Reverse Intermartensitic Transformation Studied by In-Situ Neutron Diffraction and Interrupted In-Situ EBSD

Abstract

Shape memory alloys, especially the newly developed Ni-Mn-based heusler-type intermetallic compounds, exhibit specific mechanical responses to mechanical loading. Although the deformation behaviors have been studied for reducing the number of martensite variants, the mechanisms are not fully revealed. Thus in this work the compression process of twin-related 7M modulated martensite of Ni-Mn-Ga intermetallic compound was studied by in-situ neutron diffraction at macroscopic scale and by interrupted in-situ EBSD at microscopic scale. It is revealed that the mechanical response of the 7M martensite is featured by three states: a linear elastic-plastic state, a steady plastic state, and a second linear plastic state. The plastic deformation is initiated by the detwinning of the existing variants (TrF-twins) in the first linear state. It proceeds to the steady state by intensive detwinning of the TrF-twins and by twinning of the remaining variants to form DeF-twins that result in the disappearance of the existing variants and the appearance of new variants, then by intermartensitic transformation to form non-modulated martensite (NM). The three shear processes are highly coordinated and compatible with the annihilation of the local incompatible strains by reverse intermartensitic transformation, which allows a steady progress of deformation and a continuous reorientation of the variants. The reorientation produces new twins with unfavorable orientations and limited deformation capacity, leading to a stress increase for further deformation. The present work provides comprehensive information on deformation mechanisms of Ni-Mn-Ga 7M martensite at each characterized deformation step that is useful for mechanical simulation of deformation behaviors of intermetallic compounds.