Precipitation Effects in Cast, Heat-Treated and Cold-Rolled Aluminium AA7075 Alloy with Sc,Zr-Addition
Abstrakt
The aluminium 7xxx series (Al-Zn-Mg-Cu-based) alloys are widely used in metalworking, transport industries as well as in space applications. The precipitation of the Al3(Sc,Zr) phase is known for its positive effect on a number of important utility properties, e.g. mechanical properties, recrystallization, temperature stability etc.
Precipitation processes in the cast, cold-rolled and heat-treated aluminium 7xxx series materials with and without addition of Sc an Zr were investigated. Eutectic phase at grain boundaries with Zn, Mg and Cu content by microstructure observations was proved.
Dissolution of the Zn, Mg (and Cu) containing (co-)clusters and Guinier-Preston (GP) zones and subsequent precipitation of the metastable phases from the Al-Zn-Mg-Cu system result the significant changes in hardening as well as in thermal response of the studied materials. Due to the presence of the high content of dislocations in materials after cold deformation and associated easier movement of atoms (Zn, Mg (and Cu)) the precipitation effects are shifted to lower temperature intervals compared to materials without rolling.
Hardening values of the materials after high temperature heat treatment increase immediately from the beginning of natural ageing thanks to the irregular Zn, Mg (and Cu) containing formations, very probably (co-)clusters, GP zones and their precursors. Sc and Zr elements added to the aluminium 7xxx series alloy caused a higher microhardness values after annealing above ~ 270 °C.
The nanosized Al3(Sc,Zr) phase has only a small influence on the development of the (co-)clusters, GP zones and their precursors. Coincidence Doppler broadening of the positron annihilation investigations confirmed that the early precipitation stages consist of Mg and Zn (and Cu) elements.
Concentration of the last named element (Cu) in the early precipitation stages is likely to increase with increasing natural annealing length. Results of positron lifetime measurements of the cast as well as heat-treated materials after natural ageing showed that the thermal vacancy component is not negligible.