Nonequilibrium Precipitation Pathway Enabling Direct δ‐phase Formation and Enhanced Cryogenic Strength–Ductility in an Additively Manufactured Multi‐Principal‐Element Alloy

Abstract

Overcoming the strength–ductility trade‐off in face‐centered cubic (FCC) multi‐principal element alloys (MPEAs), particularly at cryogenic temperatures, remains a significant challenge. Here, a laser powder bed fusion (LPBF)–enabled direct aging strategy is employed to modify the precipitation behavior in a Ni 53 Fe 20 Cr 23 Nb 4 (at%) alloy. The rapid solidification associated with LPBF produces a metastable, chemically heterogeneous cellular microstructure, which, upon aging at 650°C, promotes the formation of an interconnected Widmanstätten δ‐phase network. Such a nonequilibrium precipitation pathway leads to a hierarchical microstructure that combines a strong δ‐phase network and a soft FCC matrix. As a result, the alloy achieves a superior strength–ductility combination at 77 K, with a yield strength of ∼1403 MPa, an ultimate tensile strength of ∼1768 MPa, and a uniform elongation of ∼17.5%. Loading–unloading–reloading experiments indicate a significant contribution of hetero‐deformation‐induced (HDI) stress to the overall flow stress, arising from strain partitioning between the δ network and the matrix. In addition, cryogenic deformation activates planar slip, stacking fault formation, and deformation twinning, which contribute to sustained strain hardening and delayed strain localization. These results highlight the critical role of nonequilibrium microstructural heterogeneity in governing precipitation pathways and mechanical behavior and provide insights into designing alloys with enhanced strength–ductility balance under cryogenic conditions.