Effect of rapid solidification and post-processing on microstructure, magnetic and structural transition temperatures and magnetic properties in Ni50Mn29Ga21 magnetic shape-memory alloy

This study investigated the effect of rapid solidification and post-processing on microstructure, composition and magneto-structural properties of Ni50Mn29Ga21 (at.%) magnetic shape memory alloy (MSMA). Samples with varying cooling rates (1.1 ∙ 102 to 3.2 ∙ 103K/s) were prepared using arc melting for a comparative study in as-solidified condition and after two different heat treatments. A notable change from columnar-dendritic to cellular-dendritic solidification and a decrease in solidification length scale was observed at more rapid solidification. All samples showed Mn enrichment and corresponding Ga depletion at the sub-grain boundaries. The fine-scale solidification structure, chemical heterogeneity and lack of ordering resulted in broad martensitic transformations. At more rapid solidification, a decrease in saturation magnetization and abnormal slope changes in the hysteresis loop were observed. Heat-treatment resulted in grain growth, significant reduction in transformation breadth, and increase in magnetization saturation. Rapidly solidified samples with an estimated characteristic cooling rate of 1.1 ∙ 103K/s most closely resembled microstructures and magneto-structural properties from laser directed energy deposition (L-DED) processing. The results of this study help to understand cooling rate effects in L-DED additive manufactured Ni-Mn-Ga and provide post-processing routes that will result in microstructures and properties that resemble those of functional MSMA.