Adiabatic heating and the saturation of grain refinement during SPD of metals and alloys: experimental assessment and computer modeling

Severe plastic deformation methods include equal-channel angular pressing/extrusion, high-pressure torsion, and plane strain machining. These methods are extremely effective in producing bulk microstructure refinement and are generally initiated at a low homologous temperature. The resulting deformation-induced microstructures exhibit progressively refined cellular dislocation structures during the initial stages of straining that give way to refined, equiaxed grain structures at larger strains. Often, grain refinement appears to saturate but frequently coarsening is observed at the largest strains after a minimum in grain size is attained during SPD. Here, we summarize results on grain refinement by these processing methods and provide an analysis that incorporates adiabatic heating to explain the progressive refinement to intermediate strains and that may be followed either by an apparent saturation in grain refinement or by grain coarsening at the largest strains. This analysis is consistent with continuous dynamic recrystallization in the absence of the formation and long-range migration of high-angle boundaries.