Discontinuous precipitation enables an exceptional cryogenic strength-strain hardening synergy in a heterostructured medium entropy alloy

Abstract

Precipitation strengthening through coherent nanoprecipitates emerges as a desirable strategy to design high-performance materials for cryogenic applications. Generally, discontinuous precipitation (DP) is regarded as a detrimental factor to the strength and toughness of materials, and numerous methods are aimed at suppressing DP behavior. However, in this work, we utilized DP to develop a heterostructure in a fully recrystallized (CoCrNi)₉₄Al₃Ti₃ medium-entropy alloy, resulting in an ultrahigh tensile strength of 1750 MPa and remarkable ductility of 34% at -173 °C. This exceptional mechanical property was attributed to the presence of fine and dense shearable nanoprecipitates and a high fraction of fine grains induced by optimized and pronounced DP behavior, respectively. In-situ high-temperature electron back-scatter diffraction (EBSD) and element distribution analysis revealed that the formation of heterogeneous grains was ascribed to the driving force provided by the chemical diffusion at DP reaction fronts across grain boundaries, leading to a diffusion-induced recrystallization. Further, the impressive strain-hardening rate at cryogenic temperature was attributed to three key factors: First, DP-induced heterogenous grains resulted in strong strain partitioning behavior, leading to a strong heterogeneous deformation induced (HDI) stress. Second, a strong dynamic slip refinement mechanism induced by shearable nanoprecipitate, contributed to the persistent generation of new slip bands and dislocation accumulation. Third, high flow stress, HDI effect and shearing mechanism jointly lead to unusual stacking faults and nanotwins, which impedes dislocation motion by reducing their mean free path. Overall, reasonable DP behavior provides a novel route for the design of precipitation-strengthening alloys for harsh environment applications.