Abstract: A combination of high strength and large ductility is desirable for alloys employed in cryogenic engineering applications. In this study, a (CoNiCr)₈₁Fe₉Ti₅Mo₄Al₁ multi-principal element alloy (MPEA) was designed using the alloy design principles and fabricated. A heterogeneous grain structure, comprising coarse retained deformed grains and fine recrystallized grains, was imparted through cold drawing followed by annealing and subsequent aging. The alloy, which also consists of dual-sized (supranano and large-sized) L1₂-γ' precipitates, exhibits an exceptional strength-ductility synergy at 77 K, achieving an ultimate tensile strength of 1980 MPa and a total elongation of 37.5 %. During tensile testing, in the retained deformed grains, high-density dislocations and deformation nanotwins effectively hinder the dislocation motion, contributing significantly to the strength enhancement. At the early plastic stage (ε = 8 %), geometrically necessary dislocations (GNDs) accumulate at the grain boundaries, contributing to hetero-deformation induced (HDI) hardening. As strain increased (ε = 20 %), formation of the stacking fault (SF) networks further impedes the dislocation motion, promoting dislocation entanglement and enhancing the work-hardening capacity. This, in turn, promotes deformation twinning, which further contributes to strength and ductility. Quantitative assessments of various strengthening mechanism contributions reveal that the HDI, precipitation and dislocation strengthening mechanisms are predominant in contributing to the high strength of the hetero-structured alloy. These findings offer valuable insights into the design of next-generation cryogenic structural materials for industrial applications.

Key words: Multi-principal element alloys, Heat treatment, Heterogeneous structure, Nanoprecipitates, Cryogenic mechanical properties