Effect of directional solidification rate on the solidified morphologies and phase transformations of Ni _50.5Mn ₂₅Ga _24.5 alloy

The effects of directional solidification rate on the solidified morphologies and phase transformations were studied in polycrystalline Ni _50.5Mn ₂₅Ga _24.5 alloy prepared by induction zone melting method. With the directional solidification rate increasing from 10 mm/h to 800 mm/h, the solidified morphologies of the austenite phase evolved from planar, to cellular, and then to dendritic. The premartensitic transformation was only observed in the planar austenite, and was suppressed in the cellular and dendritic morphologies. The martensitic transformation was observed in all austenitic morphologies. Increasing the directional solidification rate, the martensitic transformation temperatures were slightly decreased, and the transformation hysteresis was expanded. The local energy barrier related to the compositional segregation from the cellular and dendritic austenitic morphologies suppressed the premartensitic transformation and had little influence on martensitic transformation.