On the Two-Stage Aging of Al-Mg-Si Alloys

Abstract

The two-stage aging effect in three Al-Mg-Si alloys containing 0.5, 1.0 and 1.5% Mg₂Si respectively is investigated by electrical resistivity measurements, tensile tests and electron microscopy. The critical temperature for homogeneous nucleation of precipitates exists between 160 and 200°C for an 1.0% Mg₂Si alloy, and for an 1.5% Mg₂Si alloy between 200 and 260°C. The Temperatures are also correspoding to the highest temperatures for the resistivity increase respectively. The slight decrease in resistivity perhaps due to redissolution of clusters is observed for 0.5% Mg₂Si alloy in the initial stage of isothermal aging at a low temperature. The activation energy Eₛ for migration of solute atoms in the fast reaction is estimated as 0.77 eV for all three alloys in good agreement with the values reported for a 1.4% Mg2Si alloy. The deleterious effect of two-stage aging is observed in only limited cases, though it appearts unfortunately often in practice. The whole effect of two-stage aging appears to be explained only by the stability of clusters or zones formed during preaging. The conservation in zone number is not observed. The nucleation or stabilization of clusters which takes place during heating to aging temperature must be considered not only for quenching effect but also for two-stage aging effect.