The aging behavior of precipitation hardenable 17-4 PH stainless steel is studied by analyzing the changes in microstrain, crystallite size, and dislocation density derived from the modified Williamson–Hall (mWH) method and the Fourier analysis of XRD profiles. Aging treatment of this steel at 380, 430, and 480 °C for 0.5, 1, and 3 h durations leads to changes in the microstrain due to precipitation and substructural changes caused by dislocation annihilation. The microstrain estimated from the mWH method is dominated by the precipitate-induced effects. The influence of precipitates and dislocations on the mean squared strain 〈ε2(L)〉 are separated by fitting the variation of 〈ε2(L)〉 with an expression P0 + P1/L + P2/L2, where the parameter (P0)0.5 and P1 are shown to be related to the precipitate-induced and dislocation density-induced microstrain, respectively. The study shows that the XRD profile analysis can be used to separate the combined effects of precipitation and dislocation annihilation.

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.