Ni-Mn-Ga Micro-trusses via Sintering of 3D-printed Inks Containing Elemental Powders

Acta Materialia 2017 Volume 143, Pages 20-29

Ni-Mn-Ga magnetic shape memory alloy (SMA) micro-trusses, suitable for high magnetic field induced strains and/or a large magnetocaloric effect, are created via a new additive manufacturing method combining (i) 3D-printing ∼400 μm struts with an ink containing a polymer binder and elemental Ni, Mn, and Ga powders, (ii) binder burn-out and metallic powder interdiffusion and homogenization to create the final alloy, and (iii) further sintering to increase strut density. Controlled amounts of hierarchical porosity, desirable to enable twinning in this polycrystalline alloy, are achieved: (i) continuous ∼450 μm channels between the printed Ni-Mn-Ga ∼300 μm diameter struts (after sintering) and (ii) microporosity within the struts, from 50-200 μm voids where the largest Ga powders initially resided, and from residual 5-25 μm pores between powders due to incomplete sintering. The micro-trusses, sintered at 1000 °C for 12 h, and chemically ordered for 10 h at 700 °C, with overall porosities of 73-76%, have uniform compositions near Ni-32Mn-18Ga (at.%) and are comprised of a non-modulated martensite phase. Reversible martensite/austenite transformations between 45 and 90 °C, Curie temperatures of 85-90 °C, and saturation magnetizations of up to 58 Am2/kg are achieved.