Densification kinetics and microstructural evolution of binder jet printed and sintered porous Ni-Mn-Ga magnetic shape-memory alloys

Abstract

Binder Jet Printing (BJP) of polycrystalline magnetic shape-memory alloys (MSMAs) allows to create complex geometries from pre-alloyed powders, while avoiding elemental evaporation and internal stresses. However, to increase functionality, polycrystalline MSMAs require high porosity and large grains to decrease internal constraints to twin boundary motion. This study focuses on the microstructural evolution of BJPed porous Ni-Mn-Ga during isothermal sintering to understand the densification mechanisms taking place at different temperatures (1070, 1080 and 1090 ℃) over time (0–8 h) in order to tailor the microstructure to regimes of large, interconnected porosity and large grains. Stereology was used to determine the sintering mechanisms as defined by Coble's model for intermediate stage sintering. Densification and grain size increased over time for all temperatures at different rates, while pore size decreased to a minimum near 1 h sintering. Sintering at 1070 ℃ resulted in an interplay of densifying and coarsening mechanisms, while at 1080 ℃ the dominating mechanism was volume diffusion and at 1090 ℃ grain boundary diffusion. No changes in electron concentration occurred during sintering, and thus, transformation temperatures and magnetization remained nearly constant. Transformation enthalpy and Gibbs free energy showed an inverse behavior to pore size, indicating larger pores reduce transformation energy.