Dislocation-mediated grain boundary evolution behaviors and surface nanocrystallization mechanisms of 42CrMo steel modified by ultrasonic surface rolling

The ultrasonic surface rolling process is a highly efficient method for inducing surface nanomodification in metallic components, significantly enhancing mechanical properties such as wear and fatigue resistance. Despite its advantages, the underlying mechanisms of microstructural evolution remain insufficiently understood. This study investigates the surface nanocrystallization mechanisms of 42CrMo high-strength steel subjected to ultrasonic surface rolling treatment. The microstructural characteristics are systematically characterized using electron backscatter diffraction and transmission electron microscopy. The experimental results demonstrate that the ultrasonic surface rolling process generates a gradient nanostructure approximately 815 nm in depth. Specifically, the initial lath-like martensite (average size ∼582 nm) is transformed into equiaxed nanocrystalline grains with a minimum grain size of approximately 50 nm. From the topmost surface to the matrix, a distinct morphological transition is observed: equiaxed nanocrystalline grains, followed by short rod-shaped fractured ultrafine grains, elongated segmented ultrafine grains, deformed bent grains, and finally lath vertical grains. The surface nanocrystallization evolution is primarily attributed to the vertical division of grain boundaries and the lateral shearing of dislocations. Furthermore, the dislocation morphology exhibits a clear gradient evolution from the interior to the topmost surface, progressing through high-density dislocations, dislocation tangles, dislocation networks, and eventually dislocation cells. The synergistic interaction between dislocation movement and grain boundaries is identified as the dominant factor driving the surface nanocrystallization of 42CrMo steel. This study can provide a fundamental insight into the nanoscale structural evolution of metallic surfaces during the ultrasonic surface rolling process.