Abstract
The effect of cold rolling (6% and 21%) on electrical, mechanical and thermal properties and microstructure of the classical mould-cast Al-Zn-Mg alloy with and without Sc,Zr-addition were studied. Samples were isochronally annealed up to 460 degrees C.
Electrical resistivity and microhardness together with differential scanning calorimetry measurements were compared to microstructure development. The initial microhardness values of the alloys reflect the deformation degree.
Distinct changes in electrical resistivity as well as in heat flow of the alloys studied are mainly caused by dissolution of Zn, Mg-rich Guinier-Preston zones and subsequent precipitation of the metastable eta' phase. Precipitation of this phase is responsible for a slight hardening effect.
Probably an easier diffusion of Zn and Mg atoms along dislocations in the cold-rolled alloys is responsible for the dissolution of the Guinier-Preston zones and precipitation of the Zn, Mg-containing particles at the relatively lower temperatures in comparison to classical mould-cast alloys. Microstructure observation proved that the Sc, Zr-content is mainly concentrated in randomly-localized regions in the Al matrix and together with Zn and Mg in the particles at grain boundaries.
The hardening effect after annealing above 280 degrees C was observed in the AlZnMgScZr alloy. Microhardness decrease at temperatures above approximate to 350 degrees C in the alloys without Sc and Zr is probably connected with faster recovery processes.
Cold rolling has no significant effect on the apparent activation energy values of the Guinier-Preston zones dissolution and formation of the Zn, Mg-containing particles.