Characterization and mechanism of repeated precipitation on dislocations in an Al-Cu alloy

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Abstract
The process of repeated precipitation on dislocations in Al-3-85 wt.% Cu has been examined by transmission electron microscopy. The θ phase is found to precipitate repeatedly on climbing dislocations during quenching from the solid-solution range directly to the ageing temperature, occurring on both dislocation-climb sources and glide dislocations which climb off their slip planes. The resultant colonies of stable 0’ precipitates are unresolved immediately after quenching but become visible upon sufficient ageing. The mechanism for repeated precipitation of 0’ in Al-Cu differs from mechanisms proposed for other alloy systems since it does not require a cooperative vacancy flux between growing precipitates and climbing dislocations. Rather, the dislocations climb by annihilation of quenchedin vacancies, while 0’ precipitates simultaneously by simple heterogeneous nucleation in the stress fields of the moving dislocations. Methods for enhancing the local solute concentration for repeated nucleation are considered, and a model is proposed which accounts for the spatial distribution and orientations of precipitates in the colonies.