Evidence for austenite to non-modulated martensite transformation crystallography and variant organization in Ni-Mn-Ga-Co ferromagnetic shape memory alloys

Abstract

Knowledge of transformation crystallography and variant organization of product phase was a prerequisite for tuning microstructure and enhancing functionalities in materials with displacive structure-transformation. However, the transformation orientation relationship (OR) between parent austenite and non-modulated (NM) martensite in traditional NiMnGa ferromagnetic shape memory alloy was not yet well-determined via direct experimental results, thus leading to ambiguity in understanding the self-accommodated configuration. In this work, by generating a microstructure with coexisting austenite and NM martensite through minor Co substitution and proper heat treatment, the transformation OR was unambiguously determined to be the N-W relation with {111}A//{101}NM and <2¯11>A//<101¯>NM on the basis of accurate EBSD orientation measurements. Analysis on deformation gradient matrix constructed from the N-W OR revealed that, the internal nano-twined structure allowed a maximum profit for eliminating the overall latticed deformation caused by transformation, overweighing the “sandwich” micro-variant pair structure. Such strain accommodation mechanisms consequently contributed to the resultant self-accommodated hierarchically twinned structure of martensite. Moreover, it was found that the prior austenite grain boundaries (PAGBs) provided dominated nucleation sites for NM martensite variants, where the preferential variant selection at PAGBs mainly depended on the inclination angle between the PAGB and matching close-packed direction (<2¯11>A//<101¯>NM) of variants. The present study provided comprehensive information on transformation crystallography and displacive characters of austenite to NM martensite transformation, which was useful for the efforts in property optimization and theoretical simulation.