Saturation of Fragmentation During Severe Plastic Deformation

In this review, we focus on the saturation microstructure that evolves during severe plastic deformation (SPD). These nanocrystalline or ultrafine-grained microstructures consist predominantly of high-angle boundaries, although low-angle boundaries are also present. Deformation temperature, alloying, and strain path are the dominant factors controlling the saturation grain size in single-phase materials. The saturation grain size decreases significantly with decreasing deformation temperature, although the dependency is stronger at medium homologous temperatures and less in the low-temperature regime. The saturation microstructure is sensitive to strain rate at medium temperatures and less so at low temperatures. The addition of alloying elements to pure metals also reduces the saturation grain size. The results indicate that grain boundary migration is the dominant process responsible for the limitation in refinement by SPD. Therefore, second-phase particles of the nanometer scale can stabilize even finer microstructures. This mechanism of stabilization of the microstructure is an effective tool for overcoming the limit in refinement of single-phase materials by SPD. The improved thermal stability of the obtained nanostructures is another benefit of the introduction of second-phase particles.