Solid-liquid interface morphology and crystal growth of NiMnGa magnetic shape memory alloys

Zone-melting directional solidification was applied to prepare oriented or single crystals of NiMnGa magnetic shape memory alloys. The concave height of the quenched solid-liquid interface of the NiMnGa crystals was investigated. It was found that by increasing the temperature gradient, the crystal growth velocity, and the zone-melting length, the concave height of the solid-liquid interface could be reduced. Flat and convex interfaces were obtained by raising the temperature gradient. The stoichiometric Ni₂MnGa, Ni-rich NiMnGa single crystals and Mn-rich NiMnGa oriented crystals were successfully prepared with flat or convex solid-liquid interfaces. 〈100〉 of the high temperature austenite was detected to be the preferred orientation of the NiMnGa crystal growth. The composition distribution along the axis of the NiMnGa oriented or single crystals was uniform. The martensitic transformation temperature variation along the axis of the NiMnGa single crystals or oriented crystals is less than 10 °C, much better compared with those prepared by the Bridgman method. The function of the concave height of the solid-liquid interface is given as a function of the temperature gradient, the crystal growth velocity, the diameter of the crystals and the thermal physical parameters. The theoretical analysis results of the concave height are consistent with the experimental ones.