The formation of copper-rich precipitates of 17-4 precipitate hardened stainless steel has been investigated, after tempering at 350–570°C for 4 h, by atom probe tomography (APT). The results reveal that the clusters, enriched only with Cu, were observed after tempering at 420°C. Segregation of Ni, Mn to the Cu-rich clusters took place at 450°C, contributing to the increased hardening. After tempering at 510°C, Ni and Mn were rejected from Cu-rich precipitates and accumulated at the precipitate/matrix interfaces. Al and Si were present and uniformly distributed in the precipitates that were <1.5 nm in radius, but Ni, Mn, Al, and Si were enriched at the interfaces of larger precipitates/matrix. The proxigram profiles of the Cu-rich precipitates formed at 570°C indicated that Ni, Mn, Al, and Si segregated to the precipitate/matrix interfaces to form a Ni(Fe, Mn, Si, Al) shell, which significantly reduced the interfacial energy as the precipitates grew into an elongated shape. In addition, the number density of Cu-rich precipitates was increased with the temperature elevated from 350 up to 450°C and subsequently decreased at higher temperatures. Also, the composition of the matrix and the precipitates were measured and found to vary with temperature.

The microstructural evolution of precipitation in two model alloys, Fe–NiAl and Fe–NiAl–Cu, was investigated during aging at 500°C for different times using atom probe tomography (APT). The APT results reveal that the addition of Cu effectively increases the number density of NiAl precipitates. This is attributed to Cu promoting the nucleation of NiAl particles by increasing the chemical driving force and decreasing the interfacial energy. The NiAl precipitates of the Fe–NiAl–Cu alloy grow and coarsen at a slower rate than that of the Fe–NiAl alloy, mainly due to the slower diffusion rate of the Cu atoms. The mechanical properties of the two alloys were characterized by Vickers hardness and tension tests. It was found that the addition of Cu results in the formation of core–shell precipitates with a Cu-rich core and a NiAl shell, leading to a dramatic improvement of peak hardness and strength. The effect of Cu on precipitation strengthening is discussed in terms of chemical strength and coherency strength.