TEM OBSERVATION OF PHASE TRANSFORMATION AND MAGNETIC STRUCTURE IN FERROMAGNETIC SHAPE MEMORY ALLOYS

摘要

Ferromagnetic shape memory alloys have been attracting much attention as a new class of magnetic actuator materials. It was reported that strain up to 5% can be obtained by magnetically-induced twinning/detwinning in the martensite phase of Ni-Mn-Ga. This report describes the results of in situ transmission electron microscopic observations of martensilic transformation and preceding intermediate phase transformation. It was revealed that the intermediate phase is characterized by the satellite spots at l/6g_(220) positions around the fundamental reflections, which suggest the formation of domains with the {110)<1-10> type modulation with 6 layers period. The position of the satellite is independent of the structure of martensite that forms by further cooling. Lorentz microscopy was used to investigate the magnetic domain structures in the martensite and its relation to the microstructures such as the variant and microtwins.