Engineering melt pool boundaries for strength ductility Enhancement in additive manufactured CoCrFeNiWx high entropy alloys

• Melt pool boundary engineering enables nano-twin formation in printed alloys. • Tungsten addition lowers stacking fault energy, promoting twin formation. • Nano twins (∼5 nm thickness) enhance strain hardening and tensile deformation. • Achieved 906 MPa yield strength and 7 % elongation without complex post processing. Melt pool boundaries (MPBs) in additively manufactured (AM) alloys are often weakened by the presence of brittle phases, residual stresses, and coarsened microstructures, severely limiting tensile ductility. In this study, we propose MPB engineering as an effective strategy to mitigate these limitations by introducing nano twins at MPBs. This approach leverages enhanced W solubility in the metastable FCC matrix and localized thermal stresses during rapid solidification. CoCrFeNiWx alloys were selected and fabricated via Laser Powder Bed Fusion (LPBF) using in-situ alloying with W powder. CALPHAD simulations reveal that the stacking fault energy (SFE) of the FCC phase in W0.6 is approximately 7mJ/m 2 , favourable for twin formation. High-resolution microscopy confirms the presence of nano twins (∼5 nm thick) at MPBs in W0.6, attributed to the combined effects of low SFE and stress localization. These twins significantly enhance strain hardening and uniform tensile deformation, resulting in a yield strength of ∼ 906 MPa and elongation of 7 %. These properties are comparable to those of W 0.3 Cr 0.7 CoNi, one of the strongest FCC-based alloys strengthened by brittle topologically closed packed (TCP) phases but achieved without complex thermomechanical treatments. This work highlights MPB engineering as a promising route for tailoring mechanical properties in high entropy alloys through microstructural control during additive manufacturing.