Effects of growth rate and composition on the microstructure of directionally solidified NiMnGa alloys

Magnetic shape memory (MSM) alloys give recoverable strain when subjected to an applied magnetic field. The strongest MSM effect has been observed in single crystals. The magnitude of the effect and the consistency of behavior over the entire volume of a sample is strongly dependent on the solute and phase distributions in crystals. Samples of stoichiometric and off-stoichiometric Ni2MnGa magnetic shape memory alloys were directionally solidified by a seedless Bridgman method using different rates of growth. The growth conditions used resulted in oriented polycrystals exhibiting a coarse cellular structure. Significant macro-segregation was observed, with the top of the ingot enriched in Mn and the bottom enriched in Ga. Micro-segregation also occurred, resulting in Mn-rich intercellular eutectic or eutectoid structures, and coarse intra- and inter-cellular Mn-rich particles. An increase in the pulling rate during the directional solidification process resulted in finer cellular and eutectic / eutectoid structures, as well as finer particles.