A unified description of solid solution creep strengthening in Al–Mg alloys

► Mobile dislocation density explains apparent n variation. ► Strain rate for dislocation climb is five times slower than for dislocation glide. ► Self-diffusion of aluminum is the controlling process for creep of Al–1%Mg. It is proposed that the creep strengthening of aluminum due to the addition of Mg atoms in solid solution and the variation of the stress exponent, n, with the stress (from n≈5 to n≈3) is due to a unique microstructural feature, that is, the stress variation of the total to mobile dislocation density ratio. To support this idea, creep data recorded from the literature of pure Al–Mg alloys and of pure aluminum have been analyzed in the frame of the strength difference method, SDM. A strengthening proportional to the applied stress is found. On this basis, a model which considers a change of the dislocation density/velocity due to the presence of the Mg atoms in solid solution and the solute drag and climb forces for dislocation motion was assumed. The new model, which also takes into account published data of the dislocation density measured at different applied stress, describes naturally the curvature of experimental Al–Mg creep data, associated traditionally with the change in deformation mechanism from dislocation glide controlled (n=3) to dislocation climb controlled (n=3) mechanism. The model does not undermine the relevance of aluminum self diffusion for dislocation climb process (vacancy diffusion) as the creep controlling mechanism in this solid solution alloy.