Factors determining solid solution phase formation and stability in CoCrFeNiX0.4 (X=Al, Nb, Ta) high entropy alloys fabricated by powder plasma arc additive manufacturing

In high entropy alloys (HEAs), the formation of solid solution phase is governed by main factors such as mixing entropy, mixing enthalpy, atomic radii, and atomic size difference. However, factors such as electronegativity, valence electron concentration, and melting point, also significantly influence the formation of the solid solution phase, individually or in combination in specific alloys, and are often less studied and reported. In this work, CoCrFeNiX0.4 (X = Al, Nb, Ta, elements with equi-atomic radii) high entropy alloys have been prepared by powder plasma arc additive manufacturing (PPA-AM). The effect of equi-atomic radii element addition on the microstructural evolution was studied. The results showed that although Al, Nb, and Ta were equi-atomic radii element additions, the resulting HEAs had variations in their phase formation and mechanical properties. Following observations were made: (i) Al addition: FCC + BCC +Sigma phase formation with lowest hardness and (ii) Nb and Ta addition: FCC + Laves phase + Eutectic phases formation with relatively high hardness (>85%–～110% increase). Considering other factors that determine the formation of solid solution phase, it has been identified that: (i) high melting point of the metal has a superior influence on the formation of topologically closed packed phases (TCP) solid solution phase and (ii) elements with large electronegativity differences tends to be rich in the second solid solution phase. The HEAs deposited by PPA-AM have similar/better mechanical stability when compared to the as-cast alloys. Using an innovative AM technology to fabricate HEAs, this work emphasizes the importance of the control of multiple variables in manipulating the solid solution phase formation and mechanical properties of HEAs.