Comparative study on microstructures and mechanical properties of ultra ductility single-phase Nb40Ti40Ta20 refractory medium entropy alloy by selective laser melting and vacuum arc melting

Single-phase BCC Nb40Ti40Ta20 (NTT) refractory medium entropy alloy (MEA) was prepared by selective laser melting (SLM) and vacuum arc melting (VAM). Compared with the VAMed sample, the SLMed samples has smaller grain size (Only 1/22 of VAMed alloy). Attributing to the difference in cooling rate of molten pool, the typical dendrite is formed in VAMed NTT, while the microstructure in the SLMed NTT exhibits a mixed morphology of columnar and cellular grains. The component microscopic segregation occurs in the SLMed NTT due to the poor element diffusion caused by the insufficient powder mixing and short laser acting time. The hardness and room temperature strength of SLMed NTT (278.6HV, 1030 MPa) are significantly higher than that of VAMed NTT (219.18HV, 860 MPa), while the yield strength exhibits no significant difference at high temperature. The results indicate that the grain refinement does not significantly improve the high temperature strength of the NTT alloy. This is attributed to the fact that finer grains can easily slip at high temperatures, which leads to the weakening of the grain boundary strengthening effect. The high ductility (e>80 %) of NTT not only expands the application field of refractory alloys, but also the novel MEA is a promising candidate material for AM.