Crossing and detwinning of fully twinned martensites in rapidly solidified CoNiGa alloy ribbons

Microstructures and martensitic transformation behavior have been examined as for CoNiGa ferromagnetic shape memory alloy (FMSMA) ribbons rapidly solidified by a single-roll melt spinning. Transmission electron microscopy (TEM) bright-field images revealed dense striations in the martensite variants. These striations are identified to be thin twins, as in other FMSMAs. The analysis of a pair of Kikuchi lines in a selected-area electron diffraction pattern indicates that the b/a and c/a axial ratios of the orthorhombic martensite are 1.01 and 0.961, respectively. Crossings of fully twinned martensite variants are frequently observed in the TEM bright-field images, and the internal twins disappear in the crossing regions, that is, the regions are subjected to detwinning. In addition to the crossing of martensite variants, checkers and straight bands of martensite variants are observed in the crystal grains of the ribbon surface. These typical morphologies are supposed to be caused by some self-accommodation of martensitic transformation stress.