Long term and in situ measurements of nanostructure evolution of Al-3.9Cu-1.5Mg alloys by laboratory high energy small-angle X-ray scattering

The effect of nanostructures on the hardness of Al-3.9Cu-1.5Mg alloys with and without cold rolling (CR) during natural and isothermal aging at 190°C for up to 48 h has been investigated by in situ high energy laboratory small-angle X-ray scattering combined with micro-Vickers and calorimetry measurements. In the naturally aged samples, the formation of Cu-Mg co-clusters of 1 nm in diameter was observed. In the specimens with CR, the formation of clusters due to CR was observed at the beginning, while little change were observed during aging. Consequently, the total amount of clusters was less than in the specimens without CR in the later stage of natural aging. The change in the amount of age hardening corresponded to the change in the amount of Cu-Mg co-cluster formation, indicating that the Cu-Mg co-cluster was responsible for the precipitation strengthening at room temperature. At 190°C for the artificial aging process, precipitate scattering of three different sizes and shapes was observed, i.e., the intermediate and S phases were precipitated in addition to the Cu-Mg co-cluster. The continuous change in volume fraction and size with time suggests that the Cu-Mg co-cluster has grown into an intermediate phase. The time evolution of hardness at 190°C artificial aging without CR was simply explained by Orowan's equation. These results show that the dispersion state of precipitates mainly affects the hardness in Al-Cu-Mg alloys with low Cu/Mg ratios rather than the phases themselves.