Characterization of as-built and heat-treated Ni-Mn-Ga magnetic shape memory alloy manufactured via laser powder bed fusion

Abstract

Magnetic shape memory (MSM) alloys have a high potential as an emerging class of actuator materials for a new generation of fast and simple digital components. In this study, the MSM alloy Ni50.5Mn27.5Ga22.0 was built via laser powder bed fusion (L-PBF) using gas atomized powder doped with excess Mn to compensate for the expected evaporation of Mn during L-PBF. The built samples were subjected to stepwise chemical homogenization and atomic ordering heat treatments. The experiments followed a systematic experimental design, using temperature and the duration of the homogenization treatment as the varied parameters. Overall, the produced samples showed only a minor variation in relative density (average density ~98.4%) and chemical composition from sample to sample. The as-built material showed broad austenite-martensite transformation and low saturation magnetization. The crystal structure of the as-built material at ambient temperature was a mixture of seven-layered modulated orthorhombic (14 M) and five-layered modulated tetragonal (10 M) martensites. Notably, ordering heat treatment at 800 °C for 4 h without homogenization at a higher temperature was enough to obtain narrow austenite-14 M martensite transformation, Curie temperature, and saturation magnetization typical for bulk samples of the same composition. Additionally, homogenization at 1080 °C stabilized the single-phase 14 M martensite structure at ambient temperature and resulted in considerable grain growth for homogenization times above 12 h. The results show that post-process heat treatment can considerably improve the magneto-structural properties of Ni-Mn-Ga built via L-PBF.