Quantitative analyses of dissolution and precipitation kinetics of θ-Al₂Cu phase in an Al-6.2Si-2.9Cu alloy using electrical resistivity

The dissolution kinetics of the θ phase during solution treatment and isothermal precipitation kinetics during aging in an Al-6.2Si-2.9Cu alloy were quantitatively examined through electrical resistivity, hardness, and transmission electron microscopy (TEM). The electrical resistivity successfully predicted that pre-existing θ particles in the as-cast Al-Si-Cu alloy were fully dissolved after 4.5 h at 500 C, which was in a good agreement with that directly observed by TEM. The electrical resistivity immediately decreased during aging at 250 C due to the precipitation of θ′ phase and was saturated within 2 h. A maximum hardness peak appeared in 1 h at 250 C due to the formation of a metastable θ′ phase, and then gradually decreased due to the coarsening of the θ′ phase as well as the annihilation of the θ″ phase. The θ′ phase was finally transformed to θ phase after 3.5 h at 250 C.