Elevated-temperature deformation at forming rates of 10-2 to 102 s-1

In the hot working at constant strain rate (epsilon(over dot)) of Al and alpha Fe alloys at 0.5 to 0.9 T-M (absolute melting temperature), steady-state deformation is achieved in similarity to creep, which is usually at constant stress. After an initial strain-hardening transient, the flow stress becomes constant in association with a substructure which remains equiaxed and constant in the spacing of sub-boundaries and of dislocations in both walls and subgrains. All these spacings become larger at higher temperature (T) and lower values as well as with lower stress, being fully consistent with the relationships established in creep. Because hot working can proceed to a much higher true strain in torsion (similar to100) and compression (similar to2) as well as in extrusion (similar to20) and rolling (similar to5), it is possible to confirm that grains continue to elongate while the subgrains within them remain equiaxed and constant in size. When the thickness of grains reaches about 2 subgrain diameters (d(s)), the grain boundaries with serrations (similar tod(s)) begin to impinge and the grains pinch off, becoming somewhat indistinguishable from the subgrains; this has been called geometric dynamic recrystallization (DRX). In polycrystals as at 20 degreesC, deformation bands form and rotate during hot working according to the Taylor theory, developing textures very similar to those in cold working. In metals of lower dynamic recoverability such as Cu, Ni, and gamma Fe, new grains nucleate and grow (discontinuous DRX), leading to a steady state related to frequently renewed equiaxed grains, containing an equiaxed substructure that develops to a constant character and defines the flow stress.