Processing defects and damage mechanisms in refractory high-entropy alloys additively manufactured via directed energy deposition

Refractory high-entropy alloys (RHEAs) are promising candidates for high-temperature applications due to their intrinsic resistance to plastic flow softening at elevated temperatures. However, their brittleness makes it difficult to manufacture engineering components with complex geometries. Additive manufacturing via direct energy deposition (DED) technique offers flexibility in design and forming, yet processing defects caused by marked differences in the physical properties of the constituent multi-principal elements and the rapid solidification conditions associated with DED limit RHEAs' practical application. This study elucidates the formation of inherent defects, strategies for their suppression, and their influence on the mechanical response of a DED Ti₄₁V₂₇Hf₁₃Nb₁₃Mo₆ RHEA prepared by mixed powders. Correlation of the molten pool characteristics to processing parameters reveals that laser power and scanning speed are pivotal in regulating defect formation. Insufficient energy input induces unmelted defects, rendering as-printed specimens brittle during tensile tests. Detailed microstructural characterization shows that the unmelted defects act as crack nucleation sites (through micropore coalescence), promoting premature failure. To address this, remelting (Strategy Ⅰ) and high-energy density processing (Strategy Ⅱ) were implemented via temperature field simulations and proved to be effective. The damage mechanism of the RHEA with moderate defects fabricated via Strategy Ⅰ is primarily governed by cracking, whereas that of the low-defect-content RHEA produced via Strategy Ⅱ is dominated by void nucleation. In the latter, reduced cracking effectively suppresses strain localization during deformation. The optimized RHEA exhibits a high tensile elongation of 17.9% and a yield strength exceeding 1 GPa. These findings offer a framework to design ductile DED RHEAs by tailoring processing parameters to avoid defect-induced brittleness.