Comparative study of the microstructures and mechanical properties of laser metal deposited and vacuum arc melted refractory NbMoTa medium-entropy alloy

NbMoTa refractory medium-entropy alloy (MEA) was fabricated by laser metal deposition (LMD) and vacuum arc melting (VAM) respectively. The crystal structures of NbMoTa MEA under two processes are all single-phase solid solution of BCC structure. Compared with the MEA formed by VAM, the NbMoTa MEA formed by LMD has smaller grain size and component microscopic segregation. Due to the difference in cooling rate, cellular and columnar substructures are demonstrated within the grain of LMDed MEA while the substructure within the grain of VAMed alloy is typical dendritic. However, the refinement of the grains in LMDed MEA does not lead to improvements in mechanical properties. In this study, the theoretical yield strength of NbMoTa MEA is calculated through solid solution strengthening (SSS) theory. The theoretical value is consistent with the experimental value of VAMed MEA, which is higher than the experimental value of LMDed MEA. The main reason for this result is that there are some metallurgical defects like porosities and intergranular cracks appeared in the LMDed MEA. The EDS test showed there is no elemental segregation seen at both sides of the crack. The reason for the intergranular cracks can be attributed to the high residual thermal stress caused by the rapid solidification characteristic of LMD process.