Recrystallization behavior of a Ni–20%Cr alloy subjected to severe plastic deformation

Abstract

The microstructure evolution in a Ni–20%Cr alloy was examined during deformation at large strains by high-pressure torsion (HPT) at ambient temperature with subsequent annealing. The structural changes during cold working to a strain of 2 were characterized by the development of a lamellar-type microstructure consisting of deformation twins and microbands. Further increase in the cold strain resulted in the evolution of nanoscale grains with a size of ∼50 nm, and their volume fraction increased with strain, approaching 100% at ɛ ∼ 6. The recrystallization processes under subsequent annealing were strongly dependent on the deformation structures. Discontinuous static recrystallization (DSRX) occurred in the alloy that was strained to ɛ ≤ 2. This process occurred because of nucleation and the growth of new grains in the cold worked lamellar structure when the driving pressures for grain growth associated with stored dislocations and the grain boundary surface energy were approximately equal. The increasing fraction of nanoscale grains in the strain ranging from 3 to 4 led to the transition from DSRX to discontinuous grain coarsening (GC). Continuous GC occurred at ɛ > 4 because of the slow growth of the nanoscale grains. The transition from DSRX to GC took place when the driving pressure for grain growth associated with the grain boundary surface energy significantly exceeded the driving pressure attributed to stored dislocations.