Characterizing Changes in Grain Growth, Mechanical Properties, and Transformation Properties in Differently Sintered and Annealed Binder-Jet 3D Printed 14M Ni-Mn-Ga Magnetic Shape Memory Alloys

Ni-Mn-Ga Heusler alloys are multifunctional materials that demonstrate macroscopic strain under an externally applied magnetic field through the motion of martensite twin boundaries within the microstructure. This study sought to comprehensively characterize the microstructural, mechanical, thermal, and magnetic properties near the solidus in binder-jet 3D printed 14M Ni50Mn30Ga20. Neutron diffraction data were analyzed to identify the martensite modulation and observe the grain size evolution in samples sintered at temperatures of 1080 degrees C and 1090 degrees C. Large clusters of high neutron-count pixels in samples sintered at 1090 degrees C were identified, suggesting Bragg diffraction of large grains (near doubling in size) compared to 1080 degrees C sintered samples. The grain size was confirmed through quantitative stereology of polished surfaces for differently sintered and heat-treated samples. Nanoindentation testing revealed a greater resistance to plasticity and a larger elastic modulus in 1090 degrees C sintered samples (relative density similar to 95%) compared to the samples sintered at 1080 degrees C (relative density similar to 80%). Martensitic transformation temperatures were lower for samples sintered at 1090 degrees C than 1080 degrees C, though a further heat treatment step could be added to tailor the transformation temperature. Microstructurally, twin variants <= 10 mu m in width were observed and the presence of magnetic anisotropy was confirmed through magnetic force microscopy. This study indicates that a 10 degrees C sintering temperature difference can largely affect the microstructure and mechanical properties (including elastic modulus and hardness) while still allowing for the presence of magnetic twin variants in the resulting modulated martensite.