Grain size and temperature dependence of superplastic deformation in an Al-Mg alloy under isostructural condition

The mechanical behavior of a superplastic Al-4.7% Mg-0.8% Mn-0.4% Cu alloy has been characterized by a new type of step strain-rate test which preserves the initial microstructure of the alloy (i.e. an isostructural test). Four different grain sizes of the alloy (8-30 |jm), prepared by variations in thermomechanical processing practice, were examined. A sigmoidal relationship between log σ and log ε̇ is observed for each isostructural condition. The value of maximum m (= dlog σ/dlog ε̇) increased with increasing temperature and with decreasing grain size. The isostructural log σ vs log ε̇ data are evaluated using the grain mantle based quantitative model proposed by Ghosh. In the dislocation creep region (ε̇ > 10^-1/sK the stress exponent is 4.55 and activation energy is close to that for lattice self-diffusion, but the grain size exponent is non-zero (∼0.37). In the grain mantle deformation region (ε̇ < 10^-3/s), the value of the stress exponent based on effective stress (σ - σ₀, where σ₀ is threshold stress) is ∼ 1.7, and the grain size exponent is 2.3; but interestingly activation energy is the same as that for dislocation creep. Grain mantle creep is now also believed to be controlled by dislocation glide and climb processes, but its rate is enhanced many times due to a high concentration of vacancies near grain boundaries. σ₀ computed based on the model shows that it increases with increasing grain size and with decreasing temperature.