Preparation of melt textured Ni–Mn–Ga

Abstract

Ni–Mn–Ga alloys are the best-known group of magnetically active alloys, which show magnetic field-induced strain (MFIS) via twin boundary motion. This phenomenon is often referred to as the magnetic shape memory effect. In single crystals, strains of up to 10% have been achieved that way. Up to now, no systematic study of this effect in polycrystals is available. In order to obtain this effect also in polycrystals, stationary casting is employed to achieve coarse-grained, textured samples. Therefore, a hot ceramic mold is mounted on a cold copper plate. Thus, a unidirectional heat flow along the sample axis is realized. The direction of heat flow coincides with the growth direction, which is preferentially [1 0 0]. After solidification, the samples were annealed at high temperature (1000 °C) to ensure chemical homogeneity on a local scale. The samples were cut by spark erosion. In order to allow for a direct investigation of the resulting microstructure by electron backscatter diffraction (EBSD) measurements, an alloy composition with a martensitic transformation below room temperature was chosen (Ni 48Mn 30Ga 22). The transformation temperature was checked by differential scanning calometry (DSC). The orientation of the grains and the preferred growth direction was analyzed using EBSD.