Twin relationships between nanotwins inside A–C type variant pair in Ni–Mn–Ga alloy

The twin relationships of the A–C type non-modulated martensitic variant pair in Ni54Mn25Ga21 ferromagnetic shape memory alloy were investigated in detail through experimental observations and theoretical calculations. Variants A and C are microtwins connected by intervariant interface, while the lamellae in each variant are nanotwins separated by twin boundaries. The nanotwins with different thicknesses in each variant have two types: the major lamella and the minor lamella. The orientation relationships between the lamellae across an intervariant interface vary with the volume fraction of the minor lamella. The twin relationships between the nanotwins were identified experimentally and theoretically. Specifically, the lamellae in a variant have a {1 1 2} compound twin relationship, the major lamellae (or minor lamellae) are twin related with a twinning plane of high indices, but the major and minor lamellae across the A–C interface are not twin related. The mobility of various interfaces, such as the twin boundary in a variant and intervariant interface is discussed, based on the identified twin relationships.