Abstract
Microstructure and precipitation processes in an Al-Si-Mg-Mn-Sc-Zr alloy were studied. The as-cast alloy contains fine Al-3(Sc,Zr) particles with L1(2) structure, rods and/or needles of the Mn,Fe-containing phase and Si,Mg co-clusters.
These co-clusters have a detrimental effect on precipitation hardening which is caused exclusively by fine needle-shaped beta" phase particles. The pronounced precipitation of the Al-6(Mn,Fe)-phase was observed during isochronal annealing in the as-cast alloy.
Nevertheless, the precipitation has a negligible effect on hardness. The apparent activation energy for the Al-6(Mn,Fe)-phase precipitation was calculated as (170 +/- 20) kJ.mol(-1).
Heat treatment at 530 degrees C for 45 min dissolves Si,Mg co-clusters but causes a precipitation of the Al-12(Mn,Fe)(3)Si phase particles while the Al-3(Sc,Zr) particles remain unaffected. The Sc and Zr addition to the Al-Si-Mg-Mn alloy influences precipitation hardening in the course of isochronal annealing neither in the as-cast nor in the solution-treated alloy.
The age hardening is very poor in the as-cast alloy (similar to 4%) and it is more pronounced in the solution-treated alloy (similar to 60%) due to more convenient size, aspect ratio and number density of the beta" and beta' phases. The apparent activation energy values of precipitation of the transient beta" and beta' phase were determined as similar to 90 kJ.mol(-1) and similar to 130 kJ.mol(-1), respectively.