Cinetica di invecchiamento di leghe di alluminio ultrafini

Equal channel angular pressing (ECAP) was performed on solution annealed samples of Al-Mg-Si-Zr alloys, both in reference composition and with Sc addition, in order to achieve substantial grain refinement. DSC analyses, carried out with heating rates ranging from 5 to 30°C/min, and TEM observations were performed to investigate the effect of Sc-addition and of severe plastic deformation on precipitation kinetics and on microstructure evolution. Analyses of the peak positions, activation energies and precipitate morphology as a function of ECAP passes revealed large modifications of precipitation kinetics in the ultrafine-grained alloys with respect to the coarse-grained materials. The precipitation of ?? and ?? phases occurred at significantly lower temperatures with increasing ECAP strain. The formation of ?? precipitates is supposed to be suppressed or at least strongly reduced due to the expected precipitation of competing Si-rich phases in the heavily deformed alloy structure. Formation of the stable ? precipitates (Mg2Si) during continuous heating in DSC tests revealed to be markedly anticipated and of decreasing intensity with increasing number of ECAP passes, owing to Si depletion from the matrix. Sc did not significantly affect the precipitation behaviour in the transformation range concerned with peak aging treatments. On the other hand, Sc promoted slight modifications in the precipitation of equilibrium ? phase, which corresponds to extensive averaging of the alloy. The reduction of the D peak temperatures and of the related activation energies were tentatively accounted for by Increased nucleation favoured by the Al3(ScxZr1-x) precipitates. The Sc-free alloy showed a recrystallization peak at temperatures ranging from 310°C to 340°C, depending on the strain accumulated by ECAP, while the Sc-containing alloy showed a greater grain stability.