Characteristics of diffusion in Al-Mg alloys with ultrafine grain sizes

Equal-channel angular pressing (ECAP) was used to refine the grain sizes of pure Al and an Al-3 wt% Mg alloy containing minor additions of either Sc or Zr. Following ECAP, the grain sizes were in the ultrafine submicrometre range. Diffusion couples were prepared from the fine-grained material produced by ECAP and from the coarse-grained material without ECAP and these couples were used to measure the interdiffusion coefficients for Mg in an Al lattice. The results show the interdiffusion coefficient is larger in the fine-grained material and the experimental data from this couple agree with predictions from molecular dynamic simulations using the embedded-atom method. An activation energy of about 87 kJ mol(-1) is estimated for grain-boundary diffusion of Mg in Al where this value is consistent with the expectations from measurements of the activation energy for lattice diffusion of Mg in Al. There is no evidence for enhanced diffusion in the boundaries produced by ECAP due, it is suggested, to the rapid equilibration of these non-equilibrium boundaries at elevated temperatures.