Compositional modification for process improvement of laser powder bed fusion of Ni–Mn–Ga-based magnetic shape memory alloys

Abstract

Due to twin boundary (TB) motion, magnetic shape memory (MSM) alloys based on Ni-Mn-Ga have the potential to generate reversible magnetic field-induced strains (MFIS) of up to 11%. Grain boundary defects in polycrystalline Ni-Mn-Ga, with small grains, often prevent significant MFIS due to the geometrical restrictions of nearby grains.

Efforts to increase grain size in polycrystalline alloys have been made through directional solidification, texturing, high-level porosity, and sintering, aimed at transitioning the polycrystalline structure towards an oligocrystalline state. Recent advancements in laser powder bed fusion (L-PBF) have shown promise in producing Ni-Mn-Ga polycrystals with complex geometries previously unattainable with conventional methods.

This research explores the oligocrystalline structure in Ni-Mn-Ga alloys by doping them with a fourth element. Small additions of Co, Cu, and Fe (1 at.%) have been found to enhance grain boundary mobility after heat-treatment at high temperatures, resulting in larger grain sizes. Co-doping at 1-1.5 at.% concentration produces a modulated five-layered martensite (10M) crystalline structure and significantly increases the grain size, leading to enhanced MFIS in a separated grain. Atomized Ni-Mn-Ga powders containing Co-dopant have been developed for use in L-PBF to promote grain growth in additively manufactured devices. Additionally, Ni-Mn-Ga alloy doped with 1 at.% Fe has been selected for producing melt-spinning ribbons, which have shown an increase in average grain size after heat-treatment. Results show that precise elemental doping significantly increases the grain size and with crystal structure of 10M martensite across various fabrication methods such as casting, melt-spun ribbon, and L-PBF in the production of Ni-Mn-Ga alloys. This improvement is essential for the development of MSM-based devices, particularly those requiring complex geometries, as well as nano-scale and micro-scale devices customized for a wide range of applications.