Crystallographic features of variant pair A-B type in Ni-Mn-Ga shape memory alloy

The crystallographic features of the non-modulated martensitic variant pair A-B in Ni-Mn-Ga shape memory alloy were studied by transmission electron microscopy. The results show that the martensitic variant is a microscale lath, which consists of two kinds of nanoscale lamellae, the major lamella and the minor lamella, and they are (112) compound twins relationship. In different variants, the orientation relationships between the lamellae change with the volume fraction of minor lamella(λ). For high λ, the lamellar pairs in different variants (i.e. major-major, major-minor, minor-minor lamellae) are not (112) twins. As λ approaches zero, the minor lamellae disappears, and the major-major lamellae becomes (112) twins. With the decreasing of λ, the A-B intervariant interface changes from wavy interface into straight and coherent twin boundary.