Response to isochronal annealing up to 440 °C of squeeze cast Mg–Y–Zn alloy and of the same alloy prepared by powder metallurgy (PM) and extruded at 280 °C was studied by resistivity and microhardness measurement, differential scanning calorimetry (DSC) and microstructure investigation. Electrical resistivity was measured at 77 K and microhardness was measured at room temperature after each annealing step. DSC measurement was performed at various heating rates. Transmission and scanning electron microscopy and optical microscopy revealed ribbons of long-period ordered structure (LPSO) 18R and planar defects within grain boundaries. Relatively high density of planar defects was found in grain interiors of the cast alloy with the grain size approximately 50 µm. Well pronounced subgrains were observed in the PM prepared alloy. Secondary phase particles decorate grain boundaries in this alloy. Three precipitation processes were detected in the cast alloy during repeated isochronal annealing up to 440 °C, whereas only one significant process was revealed in the PM alloy. These processes were identified as embedding of stacking faults by solutes, development and rearrangement (18R → 14H) of LPSO phase and development of grain boundary particles. A coarsening of grain boundary particles rich in Y and Zn only proceeds in the PM alloy. Activation energies of the precipitation processes were determined. Microhardness exhibits good thermal stability against annealing up to 360 °C in the PM alloy

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 λ-Al(Mn,Fe,Si) phase. Strengthening during non-isothermal (isochronal) annealing is caused by a combination of formation of the GP zones, η’-phase, T-phase and co-presence of the primary and secondary Al₃(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 η’-phase, stable T-phase and stable η-phase were calculated.

The microstructure, electrical properties and microhardness of as-cast and cold rolled AlYb and AlMnYbZr alloys were investigated. The addition of Mn, Yb and Zr has a positive influence on grain size. A deformed structure of the grains with no changes of their size was observed after cold rolling. The Al₃Yb particles coherent with the matrix were observed in the AlYb alloys. The size of the particles was about 20 nm in the initial state; after isochronal treatment up to 540 °C the particles coarsen, and their number density was lower. The deformation has a massive effect on the microhardness behavior until treatment at 390 °C, after which the difference in microhardness changes between as-cast and cold rolled alloys disappeared. Relative resistivity changes show a large decrease in the temperature interval of 330–540 °C which is probably caused by a combination of recovery of dislocations and precipitation of the Al₃(Yb,Zr) particles. Precipitation hardening was observed between 100 and 450 °C in the AlYb alloy after ageing at 625 °C/24 h and between 330 and 570 °C in the AlMnYbZr alloy after ageing at 625 °C/24 h.