Effect of solution treatment on precipitation behaviors and age hardening response of Al–Cu alloys with Sc addition

Influences of solution treatment on precipitation behaviors and age hardening response of Al–2.5 wt% Cu–0.3 wt% Sc alloys were investigated, in comparison with Sc-free one. The Al3Sc dispersoids, formed during homogenization, were either survived or dissolved to become Sc solute atoms in solution treatment, depending on the solution temperature. When the temperature for solution treatment is 873 K, most of the Al3Sc dispersoids were dissolved and a significant enhancement in the uniform precipitation of finer θ′-Al2Cu particles was achieved in following aging treatment, causing a noticeable increase in peak-aging hardness by about 90% compared to Sc-free alloys. The enhanced age hardening effect was quantitatively related to the remarkable reduction in effective inter-particle spacing of the plate-shaped θ′-Al2Cu precipitates. When the temperature for solution treatment is 793 K, however, most of the Al3Sc dispersoids were survived after solution treatment and facilitated the heterogeneous precipitation of θ′-Al2Cu plates directly on the {1 0 0} facets of dispersoids in following aging treatment. Concomitantly, the uniform precipitation of θ′-Al2Cu plates was greatly suppressed, resulting in a reduced age hardening response. The age hardening responses were quantitatively assessed by using a modified strengthening model that is applicable to the plate-shaped precipitates. The calculations were in good agreement with experimental results. The present results show the importance of controlling solution treatments to achieve significant promotion effect of Sc addition on the precipitation hardening in heat-treatable aluminum alloys.