Deformation behaviour of copper based solid solutions at elevated temperatures

The transition from discontinuous, thermally activated dislocation glide to viscous solute dragging at elevated temperatures (500 K less than or equal to T less than or equal to 1000 K) for polycrystals of Cu-10 at% Al and Cu-4 at% Mn has been studied by measurements of the critical flow stress, by changes from constant strain rate to deformation at constant stress (creep conditions) and by stress relaxation tests. The transition to viscous glide is found to depend on strain rate as well as on temperature and is shifted to increasing stress during deformation along the stress-strain curve. The activation energy of viscous glide determined from a joint analysis of the measurements at different strain rates and temperatures matches with the activation energy of bulk diffusion. The mobile dislocation density under conditions of viscous glide turns out to depend solely on the effective stress whereas an 'athermal' stress component accounts for hard solute obstacles. This appears to depend on temperature due to the diffusion controlled dissolution or rearrangement of such obstacle configurations. With further deformation the hardening by the total dislocation density only gives rise to an additional athermal stress contribution but leaves the effective stress unchanged. Accounting for the 'athermal' stress components reduces high stress exponents to the well known 'natural creep law'.