Nanoscale Cu particle evolution and its impact on the mechanical properties and strengthening mechanism in precipitation-hardening stainless steel

Copper-rich precipitation can effectively strengthen stainless steel; however, the underlying mechanism is still unclear due to an insufficient understanding of the transformation of Cu-rich precipitates. Herein, using a high-resolution transmission electron microscope, we characterized the evolution of nanoscale Cu-rich precipitates during the aging of precipitation-hardening stainless steel and determined their effects on mechanical properties. Results indicate that the structure of the Cu-rich nanoprecipitates consecutively evolved from B2 (ordered Fe-Cu) to BCC (body-centered cubic) to 9R (the stacking structure of Cu-atom layers is ABC/BCA/CAB/A) and eventually to FCC (face-centered cubic) with aging. Consequently, the dislocation-precipitation interaction changes from a cutting to a looping mechanism. The BCC-structured Cu-rich nanoprecipitates formed at 480 °C after 1 h of aging exhibit excellent strength and ductility combination properties of 412 MPa and 18%, respectively.