Abstract
Precipitation in the AI-rich phase of the AI-Al2Cu eutectic has been studied by transmission electron microscopy and microhardness measurements. An increased ageing rate in comparison with bulk AI(Cu) alloys was confirmed and was caused by an increase in both θ″ nucleation and growth rate and θ′ nucleation frequency. The high density of helical dislocations and dislocation loops which formed during the quench was responsible for the much higher nucleation frequency of θ′ in the aged eutectic alloy. The density of helical dislocations, and hence that of θ′, decreased as interlamellar spacing decreased. The widths of the precipitate free zones adjacent to the α–θ interphase boundaries decreased with increasing quenching rate but they were only one-fifth as wide as those observed in bulk Al-Cu alloys quenched under similar conditions.
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