Microstructure and porosity evolution during sintering of Ni-Mn-Ga wires printed from inks containing elemental powders

Ni-29Mn-21.5Ga (at. %) wires are fabricated via a combination of (i) extrusion of liquid inks containing a binder, solvents, and elemental Ni, Mn, and Ga powders and (ii) heat treatments to remove the polymer binder and to interdiffuse and sinter the powders. To study the microstructural evolution, sintering mechanisms, and grain growth in these wires, both ex situ metallography and in situ X-Ray tomography were conducted while sintering at 800–1050 °C for up to 4 h. After debinding, Ga-rich regions melt and induce transient liquid phase sintering of the surrounding Ni and Mn powders, resulting in localized swelling of the wires and an increase in the wire porosity. After solidification of the melt and diffusion of the Ga into the Ni and Mn powders, solid-state sintering occurs. The interdiffusion of Ni, Mn, and Ga during solid-state sintering improves sintering compared to fully pre-alloyed powders. At the end of the 4 h sintering period, chemically homogenized, oligocrystalline wires with bamboo-like grains are observed with porosities ranging from 30 to 57%. Furthermore, significant grain growth occurs in wires sintered at 1000 and 1050 °C (11–35 μm vs. 1–10 μm initial powder size). The results from this study enable tailoring the porosity and grain size of printed Ni-Mn-Ga wires and 3D-printed micro-architectures and may be used to enhance their magnetic shape-memory and magnetocaloric effects in future work.