PRECIPITATION CHARACTERIZATION OF NiAl AND Cu-RICH PHASES IN DUAL-PHASE REGION OF PRECIPITATION STRENGTHENING STEEL

Precipitation strengthening plays an important role on improving the mechanical properties of steels, Ni Al and Cu-rich phases are two kinds of common precipitates. This work aims to reveal the precipitation characteristics of these two phases in martensite and retained austenite in precipitation strengthening steel by atom probe tomography(APT). The hot rolled samples were aged at 500 ℃ for 1 h after solution treatment at 900 ℃for 2 h, followed by microstructure analysis. The results show that Ni Al and Cu- rich phases form in martensite phase as well as at martensite/austenite phase boundaries, while no precipitate develops in retained austenite. Precipitation was not observed near the phase boundaries in martensite. Equivalent radius, spacing and concentration of the strengthening phases at phase boundary are larger than that inside martensite. In addition, Ni Al phase tend to separate from Cu-rich phase, and the separated tendency becomes stronger at phase boundaries than in martensite.Besides, the growth of Ni Al and Cu- rich phases at phase boundary differs from that within martensite, which should be induced by the defect density difference between them.