Enhanced strength-ductility synergy in brittle high borated steel by tailoring strain hardening behaviour

High borated steels (HBSs) exhibit poor mechanical properties because brittle and coarse borides served as crack source during deformation, causing a premature failure. Here we proposed a novel strategy of tailoring strain hardening behaviour of metal matrix to enhance performance. Metastable austenite was introduced as matrix, where various deformation mechanism was obtained by manipulating deform temperature to change stacking fault energy (SFE). Unexpectedly, extraordinary strength-ductility synergy was achieved in HBSs with a moderate SFE at 100 °C. During hetero-deformation, hard borides caused serious stress gradients in austenite, triggering discontinuous transformation-induced plasticity effect. The cavities originating from fractured borides and interface debonding were arrested effectively by transformed martensite, which highly retarded the coalescence to form large cracks, allowing residual strain hardening to extend much longer to induce additional ductility. This work provides a new pathway to enhance strength-ductility synergy of materials containing many coarse and brittle secondary phases.