Orientation relationships between nanotwins inside A-D type variant pair in Ni-Mn-Ga magnetic memory alloy

The crystallographic features of the A-D type non-modulated martensitic variant pair in Ni₅₄Mn₂₅Ga₂₁ magnetic memory alloy were investigated by transmission electron microscopy. The results show that the microscale martensitic variant is composed of two types of nanoscale lamellae, which have different thicknesses, the major lamella and the minor lamella, and the nanoscale lamellae are (1 1 2) compound twins. The orientation relationships between the lamellae in different variants vary with the volume fraction of the minor lamella. For the major (or minor) lamellae, they have nearly identical orientation relationship; for the major-minor lamellae, they are nearly (1 1 2) twin relation. Viewed along the <11 0> direction of the lamellae, the interface between variants is at edge-on position, and is curved. The lamellae in one variant cross to those in the other variant at the interface, and the width of the crossing region is less than 10 nm.