Microstructure, shape memory effect and mechanical properties of rapidly solidified Co-Ni-Al magnetic shape memory alloys

The present work proposed a rapid solidification process based on suction casting method to fabricate bulk Co-Ni-Al alloys. The main purpose is to seek a balance between functional characteristics (pseudoelasticity and shape memory effect) and mechanical properties (strength and ductility) in this system. The results revealed that two grain refinement transitions occurred in the quenched alloys with the variation of the cooling rate. Under the cooling rate of about 24-50 K/s, rapidly solidified Co-Ni-Al alloys exhibited the divorced β + γ eutectic structure. It is confirmed by magnetic force microscopy observation that the γ phase surrounded by the β grains showed a strong stripe-like magnetic contrast whereas the intracrystalline β phase possessed weak magnetic anisotropy. The divorced eutectic structure weakened the hindrance of the γ phase to the reversible motion of martensitic variants of the matrix, and thus offered good shape memory recovery and pseudoelasticity. At the same time, the ductility was significantly improved owing to the presence of the γ phase distributed at the grain boundaries. It is believed that the rapidly solidified Co-Ni-Al alloys are promising smart materials for practical application.