Nano-precipitates evolution and their effects on mechanical properties of 17-4 precipitation-hardening stainless steel

Precipitates play significant roles in materials design but in-depth understanding of the evolution of nano-scale precipitates in complex martensitic steels is still limited. In this study, the evolution of nano precipitates and their effects on the mechanical properties of 17-4 precipitation hardening stainless steel (17-4 PH SS) aged at 450 degrees C were investigated by high-resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). The results from APT revealed that the abundant nucleation of Cu-rich clusters was responsible for the initial significant hardening effect during aging. Core-shell structured Cu-rich precipitates (CRPs) were formed at peak aging condition. As aging was prolonged, co-precipitation of Ni, Mn, Si and Nb-rich precipitates (NMSN) and CRPs occurred at the expense of core shell CRPs precursors with untwinned 9R structure. Ni, Si and (Mn, Nb) atoms were preferentially segregated at the (009)9R plane of twinned or W-shaped twinned CRPs, with atom ratio of 16:7:6. Furthermore, as CRPs became coarser, Cr atoms were rejected from CRPs into the matrix enabling nucleation of Cr-rich alpha'domains. The density and size of Cr-rich alpha' domains increased as aging was prolonged to 200 h. Finally, the individual contributions of CRPs and Cr-rich alpha' domains were calculated. Overall, these findings indicated the evolution of multiple precipitates and their interactive effects, which can be helpful for the design and fabrication of next generation PH steels for wide applications. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.