Phase transformation behavior and microstructures of rapidly solidified Co-Ni-Ga alloys

Martensitic transformation behavior, magnetic properties and microstructures of Co-Ni-Ga alloy ribbons are investigated and contrasted with those of Co-Ni-Ga bulk alloy. The rapidly solidified 40-60 μm-thickness ribbons of the alloy were prepared by an electro-magnetically controlled single-rolled melt-spinning apparatus. Martensitic transformation characteristics of the Co-Ni-Ga ribbons resemble those of bulk samples. However, transformation temperatures in the ribbons are higher than those of the bulk, suggesting the formation of stress-induced martensites. Coercive force of the Co-Ni-Ga ribbons strongly depend on the direction of the applied magnetic field. Maximum coercive force is obtained at θ = 70° (θ is the angle between the magnetic field and the ribbon plane). This anisotropy of the magnetic properties is considered to be characteristics of the rapidly solidified ribbons. Microstructures of the martensitic phase in the ribbons are similar to those in the bulk samples, that is, optical and scanning electron microscopies reveal a typical lenticular shape and transmission electron microscopy shows internal twins and specific streaks on selected area electron diffraction patterns.