Accelerated diffusion and phase transformations in Co-Cu alloys driven by the severe plastic deformation

The cast Co5.6 mass% Cu and Co13.6 mass% Cu alloys were subjected to severe plastic deformation (SPD) by the high-pressure torsion (HPT). The HPT treatment drastically decreases the size of the Co grains (from 20μm to 100 nm) and the Cu precipitates (from 2 μm to 10 nm). The metastable fcc-Co disappeared, and supersaturated Co-based solid solution present in the as-cast alloys completely decomposed after HPT. Only the phases stable below 400°C remained after severe plastic deformation (i.e. almost pure hcp-Co and fcc-Cu grains). The applicability of the concept of effective temperature originally developed for materials under irradiation for the SPD-accelerated diffusion is discussed.