Soft/hard phase heterostructures in low-alloy steels for improved low-temperature toughness: a review

Low-alloy steels are widely used in cold-region load-bearing structures, but low temperature restricts dislocation motion and promotes cleavage or quasi-cleavage fracture, resulting in decreased impact toughness and an increased ductile-to-brittle transition temperature. This review summarizes soft/hard phase heterostructures as an effective microstructural strategy for improving low-temperature toughness in low-alloy steels. The toughening mechanisms are discussed from the perspectives of crack initiation, crack propagation, and crack-tip plastic energy dissipation. Available studies indicate that the beneficial effect of heterogeneity does not originate from phase coexistence alone, but depends on the coordinated optimization of structural scale, phase topology, interfacial stability, and metastable-phase behavior. Continuous or semi-continuous soft-phase pathways can accommodate local plastic strain and promote crack-tip blunting, while dispersed non-percolating hard phases provide load-bearing capacity without forming continuous brittle channels. Stable hierarchical interfaces can further deflect, branch, or arrest cracks and support hetero-deformation-induced hardening. However, excessive soft/hard strength mismatch, hard-phase percolation, coarse brittle constituents, weak interfaces, or unstable retained austenite may accelerate strain localization and cleavage initiation. The design window is also sensitive to loading rate and crack-tip constraint; under impact loading or high-constraint conditions, the three-stage hetero-deformation response may be qualitatively retained but quantitatively compressed. These considerations provide practical guidelines for designing low-temperature-tough low-alloy steels with improved fracture resistance in engineering applications.