Reinforced Cu precipitation strengthening by matrix transformation from martensite to austenite in high-strength low-alloy steel

We demonstrate here that a new type of Cu precipitation was observed in metastable austenite (γ′). The Cu precipitates had twin boundaries, array of atoms, dislocations with a large lattice misfit (~3.4 %) and close packed plane misfit (~5.4°) with matrix γ'. This was accomplished by partitioning and tempering heat treatment in a 0.117C-2.88Mn-1.13Ni-0.89Cu steel. Cu precipitation and austenitization occurred together during tempering, and Cu precipitates were nucleated in martensite, which subsequently migrated to austenite during austenitization. In view of the closed crystal relationship of martensite/Cu precipitate and martensite/austenite, crystal relationships between Cu, austenite and martensite were: {111}_Cu//{111}_γ'//{011}_α, _Cu//_γ'//_α. Misfit between one pair close-packed plane (1–11)_Cu and (1–11)_γ' was inherent from the misfit between martensite and austenite. Twin boundaries (twin spacing ~2.5 nm) were inherent from twin boundary of 9R–Cu in martensite. The matrix transformation from martensite to austenite compressed the Cu precipitates, and contributed to lattice misfit and defects in Cu precipitate. The misfit and defects in Cu precipitates provided significant strengthening effect for γ′ and contributed to the excellent mechanical properties of the experimental steel. The mechanism of Cu precipitation was studied using 3DAP, HRTEM and thermodynamic simulation.