Phase transformations in commercial cold-rolled Al–Zn–Mg–Cu alloys with Sc and Zr addition

Abstract

The influence of cold rolling on thermal and mechanical properties together with microstructure observation of cast AlZnMgCu(ScZr) alloys has been investigated. Differential scanning calorimetry measurements and microhardness were compared to microstructure that was observed by microscopy (scanning electron and transmission), electron backscatter and X-ray diffractions. Microstructure observation of all studied alloys proved eutectic phase at (sub)grain boundaries. The eutectic phase at grain boundary in the AlZnMgCuScZr has a disordered quasicrystalline structure (known as the T phase or Mg32(Al,Cu,Zn)49). In the AlZnMgCu alloy, the eutectic phase consists of two phases—predominant MgZn2 phase and minor quasicrystalline T phase. During casting and subsequent cooling, multilayer primary Al3(Sc,Zr) particles also precipitated in the alloy with Sc,Zr addition. Solute clusters and/or Guinier–Preston zones were dissolved during the annealing up to ~ 170 °C in the alloys. The highest hardening is caused by particle formation of metastable η′ and stable η phase in AlZnMgCu system observed at ~ 200 °C. Precipitation of the secondary Al3(Sc,Zr) particles is probably the reason of hardening after annealing above 300 °C in the Sc,Zr-containing alloys. Melting of eutectic phases was observed in DSC curves at temperatures ~ 481 and ~ 493 °C in the studied alloys. Activation energies of the Guinier–Preston zones dissolution and/or solute clusters were calculated using Kissinger and Starink method as QA ≈ 100 kJ mol−1 and the formation of the particles of Al–Zn–Mg–Cu system as QB ≈ 150 kJ mol−1. No significant effect on the calculation of activation energy values of thermal processes was observed in deformed alloys. Sc,Zr addition in the alloys stabilizes grains, and there is no recrystallization in the AlZnMgCuScZr alloy at temperature 450 °C/10 h.