Achieving strength-ductility synergy of an additively manufactured metastable high-entropy alloy via deep cryogenic treatment followed by laser shock peening

Laser powder bed fusion (LPBF) is an attractive additive manufacturing technology for preparing high-performance high-entropy alloys (HEAs) engineering components. Unfortunately, the existence of inherent thermal residual stress and non-equilibrium microstructures in the additively manufactured components results in unsatisfactory mechanical properties. Herein, we propose a novel strengthening strategy, namely deep cryogenic treatment (DCT) followed by laser shock peening (LSP), to tailor the microstructures and enhance performances of an LPBF additively manufactured metastable HEA. The post-treatment effects of DCT + LSP on the LPBF-fabricated Fe₅₀Mn₃₀Co₁₀Cr₁₀ HEA are evaluated in terms of microstructural modifications, residual stress, and microhardness redistribution, as well as tensile properties. Results indicate that a gradient heterogeneous structure is formed on the as-built sample surface, featuring gradient variations in grain size, martensitic phase content, and dislocation density, due to the grain refinement and martensitic phase transformation under DCT + LSP. The initial tensile residual stress on the surface is fully transformed into compressive stress, achieving a peak of −289 MPa, and the surface microhardness attains a maximum of 380.8 HV. The various strengthening mechanisms of gradient heterogeneous structures, as well as the multiple effects of heterodeformation-induced (HDI) hardening, transformation-induced plasticity (TRIP), and twinning-induced plasticity (TWIP), are responsible for achieving strength-ductility synergy. This work provides a practical pathway and valuable scientific insights for enhancing the mechanical behaviors of additively manufactured metastable HEAs via microstructural engineering.