Strengthening and precipitation hardening mechanisms of surface-mechanically treated 17-4PH stainless steel

Achieving ultra-high strength without sacrificing too much ductility is the focus of attention in nanostructured materials. Here, the strengthening mechanism and property enhancement of surface-mechanically treated 17-4PH stainless steel (SS17-4PH) were investigated. Our findings show that a grain refinement and elongated lath-like martensitic grain (~ 50 nm thick) could be produced after surface treatment. The grain size remains in the nanoscale, and random crystallographic orientations with the presence of nanocrystallites characterize the nanocrystalline grains formed on the treated sample. This contributes to the property enhancement with a yield strength of about 901 MPa and a reduced elongation to failure of about 17%. The atom probe tomography (APT) characterization unveiled the emergence of high-density precipitate (Cu-rich) at the material surface, with a number density of about 2.6255 × 10²⁴ m⁻³ and an average radius of 2.22 nm. Besides, the dislocation activities caused by SMAT result in the gradual breakdown of precipitates into smaller sizes and final dissolution in the matrix, increasing the number of nucleation sites and leading to more grain refinement processes. The grain boundary, dislocation densities, and the Cu-rich precipitate greatly influence the strengthening mechanism of surface-treated SS17-4PH.