Abstract
The replacement of lead by tin in Al-Cu-Bi alloy leads to differences in the early stages of decomposition processes. While Al-Cu-Bi-Pb decomposes similarly to Al-Cu alloy, the presence of Sn suppresses the formation of Cu-rich GP zones. Instead, a modulated structure develops and transforms directly into α-matrix and fine dispersed θ' (metastable, Al₂Cu) plates oriented along {100} planes of α-matrix. We attribute it to the high tin-vacancy binding energy and the formation of tin-vacancy agglomerates. The influence of small tensile plastic deformation at room temperature preceding the isochronal annealing of solution treated alloys on precipitation is more pronounced in Al-Cu-Bi-Pb than in Al-Cu-Bi-Sn. The fine dispersed rationally oriented θ' plates in Al-Cu-Bi-Sn cause a high degree of hardening. The hardness values and the increments of flow stress due to dislocation looping of precipitates, calculated from observed precipitate volume fraction and their aspect ratio, are in a good agreement.

Key words
Al-Cu-Bi-Sn, Al-Cu-Bi-Pb, precipitation processes, hardness, modified Orowan equation

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