Crystal structure and macrotwin interface of five-layered martensite in Ni–Mn–Ga magnetic shape memory alloy

The crystal structure of the five-layered (5M) martensite and the martensite to martensite interface in the alloys Ni48.9Mn30.8Ga20.3 and Ni49.5Mn28.6Ga21.9 (numbers indicate at.%) were investigated by the X-ray diffraction as well as by the conventional and by the high resolution transmission electron microscopy (HRTEM). The martensite to martensite interface arises from meeting of growth fronts of two microtwin sequences with the twin planes (1 1 0) and ( 1   1 ¯   0 ) , respectively. The interface was found to consist of two different configurations, the crossing and the step type. The HRTEM images reveal that the martensitic structure is not a perfect five-layered structure, but there are also other periodic structures (seven-layered and ten-layered), together with aperiodic structure and the irregular plane faults.