On the Impact of Additive Manufacturing Processes on the Microstructure and Magnetic Properties of Co-Ni-Ga Shape Memory Heusler Alloys

Microstructure design allows to prevent intergranular cracking and premature failure in Co-Ni-Ga shape memory alloys. Favorable grain boundary configurations are established using additive manufacturing techniques, namely, direct energy deposition (DED) and laser powder bed fusion (L-PBF). L-PBF allows to establish a columnar grain structure. In the Co-Ni-Ga alloy processed by DED, a microstructure with strong ⟨001⟩ texture is obtained. In line with optimized microstructures, the general transformation behavior is essential for performance. Transition parameters such as transition temperature and thermal hysteresis depend on chemical composition, homogeneity, and presence of precipitates. However, these parameters are highly dependent on the processing method used. Herein, the first-order magnetostructural transformation and magnetization properties of Co-Ni-Ga processed by DED and L-PBF are compared with single-crystalline and as-cast material. In the alloy processed by L-PBF, Ga evaporation and extensive formation of the ferromagnetic Co-rich gamma'-phase are observed, promoting a very wide transformation range and large thermal hysteresis. In comparison, following DED, the material is characterized by minor chemical inhomogeneity and transition and magnetization behavior being similar to that of a single crystal. This clearly renders DED-processed Co-Ni-Ga to become a promising candidate material for future shape memory applications.