Atom probe characterization of Cu-rich precipitates in different phases of 15–5 PH stainless steel in over-aged condition

The 15–5 PH stainless steel was aged at 500 °C for 128 h after solution treatment at 1040 °C for 1 h. The microstructure and composition of each phase in the alloy were characterized by using transmission electron microscopy (TEM) and atom probe tomography (APT). It was found that martensite, retained austenite and reverted austenite contained Cu-rich precipitates and coexisted in the alloy. These precipitates possess face-centered cubic (fcc) structure and orientation relationships with the matrix as following: [113]M//[112]A//[011]Cu and (111¯)Cu//11¯0)M//(111¯)A. But the morphology, size and number density of the precipitates in the three phases are different. In the martensite, the Cu-rich precipitates are coarse, in rod-like shape, up to ~40 nm in length; while in the retained austenite, they are small spherical with an average radius of ~0.8 nm, and coherent with the matrix. In the reverted austenite, the precipitates are large and spheroidal with an average radius of ~2 nm, larger than the size in the martensite but smaller than that in the retained austenite, which could be due to the faster growing of precipitates in martensite than in austenite.