Unraveling microstructure and mechanical response of an additively manufactured refractory TiVHfNbMo high-entropy alloy

Abstract

Refractory high-entropy alloys (RHEAs) have attracted considerable interest due to their elevated melting points and exceptional softening resistance. Nevertheless, the ambient-temperature brittleness and inadequate high-temperature oxidation resistance commonly restrict the processability of RHEAs. Direct energy deposition (DED) additive manufacturing technology is ideal for fabricating refractory alloys due to design flexibility and oxygen-free environment. In this work, a novel Ti41V27Hf13Nb13Mo6 RHEA was successfully manufactured by DED, and a comprehensive investigation was conducted to explore the microstructure evolution and mechanical response during tension. The as-deposited RHEAs exhibit a grain-size graded microstructure with a body-centred-cubic (BCC) matrix and precipitates. Increasing laser energy density suppressed the grain boundary precipitation, effectively enhancing the ambient-temperature tensile ductility to ∼11.3%. The optimized specimens achieved an unprecedented yield strength of ∼1.2 GPa among the DEDed RHEAs, which can be attributed to a significant solid solution strengthening from the volume misfit of 5.03%. We revealed that dislocation interactions maintained the working hardening capacity. Moreover, in-situ characterization indicated that slip transfer, grain rotation, and kinking accommodated the plastic deformation. Crack nucleation was caused by slip inhibition at grain boundaries and dislocation pile-ups at intragranular precipitates. The kink band formation relieved stress concentration induced by intragranular precipitates and promoted a ductile fracture. These exceptional outcomes provide opportunities for additive manufacturing RHEAs and greatly advance understanding of their strengthening and deformation mechanisms.