Mechanical properties of aged AI-5.8% Mg alloy at 423 and 473 K, over the ageing time range 1-35 h, have been investigated to assess the effect of ageing temperature on deformation in the presence of precipitation. The results indicate that the 0.2% yield strength, ultimate tensile strength, flow stress, work hardening exponent and ductility increase with ageing time reaching a maximum value, and then decrease to minimum value, followed by an increase at longer ageing times. The variation in yield, ultimate tensile strength, work hardening exponent of the flow, as well as the hardness at different degrees of deformation, were recorded as functions of experimental variables. Electron microscopic investigations revealed that the strengthening and loss in ductility of the alloy may be attributed to the precipitation of different shapes of MnAI 6, Mg2AI 3 and ~-Mg23AI3o, whose size, quantity and morphology depend on the experimental conditions. An attempt has been made to correlate strength, ductility and structural changes at different ageing times. The Brinell hardness increases and the recrystallization temperature decreases with deformation. From parabolic stress-strain relation, the ~_~1/2 curves could be divided into two linear parts.

The microstructural evolution during semi-solid processing and thixoformability of a 7075 alloy prepared by conventional and new modified strain induced melt activation (SIMA) processes were comparatively investigated in this paper. The semi-solid slurries were thixoformed at 600 °C, at which temperature the solid fraction was estimated to be 0.8. The coarsening process of the semi-solid samples was described using Lifshitz–Slyozov–Wagner theory and the effect of pre-deformation on the coarsening kinetics of the solid particles was discussed. The coarsening rate constant of the new modified SIMA sample showed a remarkable decrease compared to that of the conventional SIMA sample. Microstructural and mechanical investigations indicated that the sample with a near-equiaxed microstructure deforms through the plastic deformation of solid grains mechanism. However, the sliding of solid grains and flow of liquid incorporating solid grains mechanisms were dominant in the sample with a globular microstructure. Also, it was observed that the yield and ultimate strengths and hardness of the sample prepared by the new modified SIMA process after thixoforming and T6 heat treatment increased by about 15 %, 10 % and 25 % respectively, compared to those of the conventional SIMA sample.