RECOVERY REVISITED

Recovery of mechanical properties during annealing of deformed metals has been modelled based on a microstructural representation comprising two elements, (i) the cell/subgrain structure (size delta) and (ii) the dislocation density (rho) within the subgrains. These two microstructural elements are treated as independent internal state variables, and the recovery of flow stress obtained by adding the time dependent contributions due to subgrain growth [sigma proportional to 1/delta(t)] and dislocation network growth [sigma proportional to root rho(t)]. The growth of a dislocation network has been treated in terms of thermally activated glide, thermally activated cross-slip, climb and solute drag as rate-controlling mechanisms. Subgrain growth has been analysed in a manner analogous to normal grain growth, with climb of the boundary dislocations being the rate controlling mechanism. The model has successfully been applied in the interpretations of recovery observations in iron, aluminium and AlMg alloys. It follows from the theoretical treatment as well as from the analysis of experimental data that the characteristic logarithmic time dependence of low temperature recovery is the result of a reaction controlled either by thermally activated glide of jogged screw dislocations or by solute drag. It has been demonstrated that a mechanism based on thermally activated cross-slip does not apply in this context.