PRECIPITATION HARDENING IN AS-CAST AA6082 AND AA5754 COMMERCIAL ALLOYS WITH Sc, Zr-ADDITION

The as-cast AA5754 and AA6082 alloys with Sc,Zr-addition were investigated during isochronal annealing from room temperature up to 510 degrees C. Precipitation reactions were studied by electrical resistivity, hardness measurements and differential scanning calorimetry. The measurements were compared to microstructure development that was observed by transmission electron microscopy. Higher initial resistivity as well as hardness values are probably caused by a higher content of the solutes in the AA5754-ScZr alloy in comparison with the latter alloy. Both alloys contains spherical particles of the Al-3(Sc,Zr) phase in the as-cast state. Moreover, the AA6082-ScZr alloy contains Mg,Si,Sc-containing phase at grain boundaries and rods/needles of the Mn,(Fe)-containing phase. The transient Al-Mg-Si-phase (beta '' and beta') particles formed during isochronal annealing in the AA6082-ScZr alloy cause a poor age hardening. The additional Al-3(Sc,Zr) particles precipitation causes a pronounced hardening in the AA5754-ScZr alloy. The possible (weak) additional precipitation of the Al-3(Sc,Zr) particles in the AA6082-ScZr alloy has a lower effect on hardness than the precipitation of beta '' and beta' phases or Sc and Zr solutes are probably bound in the Al-3(Sc,Zr) particles and do not influence precipitation hardening in the course of isochronal annealing in the AA6082 alloy. The precipitation of the Al-6(Mn,Fe)-phase was observed in the alloys during the isochronal heat treatment. The apparent activation energy values of precipitation of the transient beta '' and beta' phases, Al-6(Mn,Fe)-phase and A(3)(Sc,Zr) phase were determined.