Crossing twin of Ni–Mn–Ga 7M martensite induced by thermo-mechanical treatment

Abstract Thermo-mechanical treatment is an effective way to tune the microstructure and enhance the output strain of NiMn-based magnetic shape memory alloys through field driven variant reorientation or structural transformation. Comprehensive knowledge on the microstructural feature and related transformation crystallography is of great importance for the microstructure control and property optimization. In this work, we demonstrate the crossing twin of seven-layered modulated (7 M) martensite assembled in herringbone shape induced by thermo-mechanical treatment in a directionally solidified Ni50Mn30Ga20 alloy. Based on the electron backscatter diffraction (EBSD) measurements, it is found that the crossing twin is composed of four 7 M martensite variants with two mutually crossing systems of twin, i.e., type I twin and compound twin. The type I twin in the crossing twin, being with {1 –2 10}7 M as the twinning plane, is quite different from that of the self-accommodated martensite (i.e., {1 –2 –10}7 M as the twinning plane). A novel transformation orientation relationship between austenite and 7 M martensite in crossing twin is resolved to be {1 0 1}A//{1 –2 10}7 M and A// 7 M, in contrast to that of self-accommodated case, i.e., {1 0 1}A//{1 –2 –10}7 M and A// 7 M. By using the experimentally determined orientation relationship between two phases to construct the deformation gradient matrix, the underlying mechanism for the selection of twinning system is further discussed.