Precipitation processes in Al-Cu-MS alloys microalloyed with Si

Microalloying additions of Si are known to increase significantly the response to age hardening of 2xxx series Al-Cu-Mg alloys, and commercial alloys such as 2618 are based on this effect. Previous work has attributed this effect to a refined dispersion of S' or S phase (Al2CuMg) precipitates. This work reports the results of a detailed microstructural characterization, employing transmission electron microscopy-based techniques, on the effects of Si additions to a base Al-2.5Cu-1.5Mg (wt pet) alloy. It was found that the peak hardness microstructure contains a fine and uniform dispersion of Si-modified Guinier-Preston-Bagaratsky (GPB) zones. These zones are lath shaped, possessing {100}(alpha) facets, elongated along [100](alpha) directions and contain Si. The S phase was also observed at peak hardness, although it is concluded that these precipitates do not contribute significantly to hardening due to their coarse dispersion, which arises from their heterogeneous nucleation on the quenched-in defect structure. Overaging was associated with the replacement of the zones by the S phase through a process involving dissolution and reprecipitation together with heterogeneous nucleation of S at the zones. The precipitation of theta' (Al2Cu) and sigma (Al5Cu6Mg2) phase was also observed in alloys containing greater than or equal to0.5 wt pet Si. It is demonstrated that the total solute content of the alloy has a major influence on the precipitation reactions during aging.