Martensitic Transformation And Microstructure In Ni-Mn-Ga Alloy Ribbons

The microstructure, martensitic transformation and magnetic properties of Ni-Mn-Ga alloy ribbons have been investigated by means of DSC, XRD, SEM, TEM and PPMS. The effects of the addition of rare earth element Y on the martensitic transformation of Ni-Mn-Ga ribbons have also been revealed.The results show that Ni50Mn28+xGa22-x ribbons exhibit a one-step thermalelastic martensitic transformation. The martensitic transformation temperatures increase remarkably with the increase of Mn content. Compared with the martensitic transformation temperatures of as-cast alloys, with the same composition, those of ribbons decrease slightly, whereas those increase clearly after anneal treatment.Heat-treatment has notable effect on the microstructure of Ni50Mn28+xGa22-x ribbons. With the increase of annealing temperature, the size of grains increased, and columnar crystals become equiaxed crystals. After planar-flow casting, the structure of room-temperature martensites is seven-layered modulated, which is different from the five-layered modulated structure of the as-cast alloys. After magnetic heat treatment the ribbon samples have a preferentially orientation in [400].TEM observations show that martensite variants exhibit self-accommodating morphology and the substructure inside martensite variants are regularly arranged martensite bands. The ( 121 ) type twin relationship has been detected between the martensite variants.Compared to 600℃annealed ribbons, the ribbon after magnetic heat treatment is easy to be magnetized at room temperature. Furthermore the saturation magnetization and magnetocrystalline anisotropy constant have been increased by magnetic heat treatment.Ni-Mn-Ga-Y ribbons have also exhibited a one-step thermalelastic martensitic transformation. The martensitic transformation temperatures increase remarkably with the increase of Y content. The structure of room-temperature martensites is seven-layered modulated. The indope of Y refined the grain, and caused the formation of Y-riched phase. The Y-riched phase increasd with the increase of Y content, and distributed along the crystal boundaries.