Ni-Mn-Ga Single Crystal Casting

Ni-Mn-Ga is a magnetic shape-memory alloy (MSMA). When in single crystal form, such an alloy exhibits a plastic and reversible strain up to 10% through twin boundary motion. The change in shape is induced mechanically and magnetically. Growing an oriented Ni-Mn-Ga single crystal utilizing the Bridgmen-Stockbarger method with an oriented seed results in a crystal with substantial chemical segregation due to non-congruent solidification. This segregation affects the crystallographic structure and the twinning stress. To minimize segregation, we developed a method which increases the thermal gradient and allows rapid solidification with minimal segregation. The new method consist of melting Ni-Mn-Ga in a crucible using a 3.5 kW induction furnace, then casting the molten material into a heated alumina mold. The mold contains an oriented seed crystal interfaced with a water-cooled copper base. After casting, the resulting ingots were polished to remove surface stress and etched to reveal the microstructure. Optical images for the cross-section area of the etched surface showed elongated grains with an average length of 1 mm. The substantial large grain size and the directional solidification are a step towards growing segregation-free single crystals