Solidification Control And Solid Phase Transformation Characterization Of Co-base Ferromagnetic Shape Memory Alloys

Recently, ferromagnetic shape memory alloys have been developed as the actuators and sensors, which exhibit the magnetic field controlled shape memory effect as well as the magnetic field induced strain. Co-based ferromagnetic shape memory alloys, including Co-Ni-Ga and Co-Ni-Al, with the high magnetocrystalline anisotropy energy and good hot-working ability become one of the important materials by investigators all over the world.The dissertation systematically investigates structure and transformation of martensite, morphologies and precipitation behaviors of second phases (γandγ′), shape memory effect, superelasticity and compressive fracture properties of Co-Ni-Ga and Co-Ni-Al magnetic shape memory alloys. The microstructural evolution and mechanical properties were studied based on various processing, such as conventional solidification, high undercooling solidification, directional solidification, rapid solidification and heat treatment. It might be one way to overcome the great brittleness for polycrystalline B2 alloys by controlling the text of crystal growth and the desired microstructure. An effort by introducing and adjusting a ductile phase has been attempted to improve the combination of good superelasiticity and shape memory ability with high strength and ductility. The objective of the present work aims to the application of these alloys.The results reveal that the effect of composition on microstructure of as-cast or heat-treated Co-Ni-Ga alloys is significant. The volume fraction ofγphase decreases gradually as increasing Ga. The orderedγ′occurs in composition far from the Co corner from phase diagram upon annealing or ageing. The precipitates exhibit rod-like, Widmanst?tten and blocky structures, which were controlled by the diffusion solid transformation and the degree of mismatch with matrix. TEM with SEAD shows the interface of martensitic variants is straight and sharp. The substructure of martensitic lath contains high-density microtwins and was believed to be the tetragonal L10 with c/a = 0.86. The thermal-elastic martensite exhibits the excellent shape memory recovery. Waved...