A NEW MECHANISM OF WORK-HARDENING IN THE LATE STAGES OF LARGE-STRAIN PLASTIC-FLOW IN FCC AND DIAMOND CUBIC-CRYSTALS

A new mechanism of work hardening is proposed to explain the athermal hardening in Stage IV of f.c.c. and diamond cubic crystals. The mechanism is related to a cellular dislocation microstructure in which during Stage III, hardening by dislocation accumulation and recovery by various mechanisms occurs primarily in the cell walls. Hardening of the cells is through the build-up of long range misfit stresses that result when the primary dislocation flux cuts through the geometrically required dislocation density of the cell walls that is associated with the lattice misorientations between cells. Experiments show that these misorientations increase monotonically with increasing strain. There is no recovery in the cells. At the end of Stage III, hardening in the cell walls saturates, but the hardening due to misfit stresses in the cells continues unabated, giving rise to the rate independent hardening of Stage IV. Eventually this hardening is also terminated in Stage V when the misfit stresses inside cells reach a critical level that triggers rate dependent stress relaxation in the cells by secondary glide processes. The new mechanism makes successful predictions for Stage IV processes, including: hardening rate, plastic resistance levels, the gradual increase in hardening rate with plastic resistance, the residual lattice strains on unloading that can be measured with X-ray peak distortions and broadening, and for the Bauschinger effect.