Abstract
A detailed characterization of phase transformations in the heat-treated commercial 7075 aluminum alloys without/with low Sc-Zr addition was carried out. Mechanical and electrical properties, thermal and corrosion behavior were compared to the microstructure development.
The eutectic phase consists of four parts: MgZn2 phase, Al2CuMg phase (S-phase), Al2Zn3Mg3 phase (T-phase), and primary lambda-Al(Mn,Fe,Si) phase. Strengthening during non-isothermal (isochronal) annealing is caused by a combination of formation of the GP zones, eta'-phase, T-phase and co-presence of the primary and secondary Al-3(Sc,Zr)-phase particles.
Positive influence on corrosion properties is owing to the addition of Sc-Zr. Positron annihilation showed an evolution of solute Zn,Mg-(co-)clusters into (precursors of) the GP zones in the course of natural ageing.
The concentration of the (co-)clusters is slightly negatively affected by the low Sc-Zr addition. A combination of both precipitation sequences of the Al-Zn-Mg-Cu-based system was observed.
The apparent activation energy values for dissolution/formation of the clusters/GP zones and for formation of the metastable eta'-phase, stable T-phase and stable eta-phase were calculated.