Abstract: The precipitation characteristics of the secondary phases in Custom 455 stainless steel aged at 540 °C for 4 h were investigated using Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT). The results revealed that lamellar austenite existed between martensite laths. In the martensite, both rod-shaped η (Ni₃Ti) phase and spherical G-phase precipitated adjacent to the Cu-rich precipitates, having the following orientation relationship: (111)_Cu//(0004)_η//(02$\overline{2}$)_G//(01$\overline{1}$)_M and [11$\overline{2}$]_Cu//[2$\overline{11}$0]_η//[111]_G//[111]_M. The nucleation of both the Ni₃Ti phase and G-phase was promoted by the Cu-rich precipitates. The Ni₃Ti phase is incoherent with the matrix, prone to coarsening and thus larger, while the G-phase is coherent with the matrix, making it less likely to grow and consequently smaller. In the austenite, both the rod-shaped Ni₃Ti phase and the ellipsoidal G-phase precipitated out separately near the austenite/martensite interface, with the orientation relationship (0004)_η//(02\overline{2})_G//(01\overline{1})_M//(1\overline{1}1)_γ and [21\overline{11}0]_η//[111]_G//[111]_M//[011]_γ. The sizes of the Ni₃Ti phase and the G-phase in the martensite were different from those in the austenite, and the reason for the difference was discussed.

Key words: Cu-rich precipitates, Ni₃Ti phase, G-phase, Austenite, Atom probe tomography