A retrogression and re-ageing (RRA) treatment is a three-step heat treatment that can improve both the mechanical strength and the corrosion resistance in aluminum alloys. In this work, the mechanical and stress corrosion properties under various ageing treatment conditions were investigated in an Al-8.5Zn-2.1Mg-2Cu-0.2Ag (w/%) alloy. The treatments were the T6 conventional method followed by a retrogression and re-ageing (RRA) treatment. Tensile test, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and differential scanning calorimetry (DSC) were used to investigate the mechanical and stress corrosion cracking (SCC) properties. The results showed that both, the strength and the corrosion resistance criteria (SCR), which is defined as the ratio between the remaining strength percent in stressed and un-stressed conditions, improve after the RRA treatment. The tensile strength and SCR criteria for the T6 heat treatment were 547 MPa and 71 % initially and then increased to 612 MPa and 95 % for the RRA treatment, respectively. Moreover, the EDS results showed that in grain-boundary precipitates the Cu concentration is much higher for the RRA in comparison with the T6 treatment, but it is lower for the matrix precipitates in the RRA treatment. Keywords: aluminium alloys, stress corrosion resistance, precipitation, retrogression and re-ageing Obdelava z retrogresijo in ponovno o`ivitvijo (angl. RRA) je tristopenjska toplotna obdelava, s katero lahko izbolj{amo tako mehansko trdnost kot odpornosti proti koroziji pri aluminijevih zlitinah. V pri~ujo~em delu so bile raziskovane mehanske lastnosti in lastnosti odpornosti proti koroziji pri zlitini Al-8.5Zn-2.1Mg-2Cu-0.2Ag (w/%) pod razli~nimi pogoji obdelave staranja. Izvedeni so bili testi po konvencionalni T6 metodi, z upo{tevanjem RRA obdelave. Natezni preiskus, vrsti~na elektronska mikroskopija (SEM), energijsko disperzijska rentgenska spektroskopija (EDS) in DSC-testiranja so bili izvedeni, da bi raziskali mehanske in napetostno-korozijske lomne lastnosti (angl. SCC). Rezultati so pokazali, da se oba kriterija tako trdnost kot odpornost proti koroziji, ki definirata razmerje med procentom zaostale trdnosti pri napetostnih in nenapetostnih pogojih, po RRA-obdelavi izbolj{ata. Klju~ne besede: aluminijeve zlitine, odpornost proti koroziji, oborine, retrogresija in ponovno staranje

The effect of silver on mechanical properties and stress corrosion cracking (SCC) resistance in a predeformed and overaged 7055 aluminum alloy was investigated. The treatment consisted of solution treating the specimens and then applying 5 % plastic deformation (compression) before aging. The next operation was aging the specimens at 121°C for 24 hours and overaging them at 175°C for 10 hours. Properties of the alloys were investigated by mechanical testing, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and differential scanning calorimetry. Results show that the ultimate tensile strength (UTS) and yield tensile strength (YTS) for the silver-free alloy were 580 and 520 MPa, respectively, and increased to 594 and 567 MPa after the addition of 0.39 % silver. Moreover, the stress corrosion resistance increased ≥ 23 % after adding 0.2 % silver. It can be seen that the addition of silver to predeformed and overaged 7055 aluminum alloy slightly improves the UTS and YTS and increases resistance to SCC significantly. These improvements are due to the interrupting effect of silver on the continuity of magnesium-rich precipitates and the increasing copper absorption in the precipitates.