Isotropy of precipitate distribution in pre-stretched Al-Cu-(Sc)-(Zr) alloys

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Pre-stretching prior to ageing in Al-Cu alloys leads to preferential formation of precipitates in certain habit planes and produces anisotropic mechanical properties. In this work, we attempt to eliminate this problematic anisotropy by adding Sc and Zr to an Al-Cu alloy. When pre-stretching the binary Al-Cu alloy along the [001]Al zone axis, transmission electron microscopy (TEM) reveals that precipitation of θ″ occurs mostly on the (001)Al plane (and not on the (100)Al and (010)Al planes). This “stress orienting effect” is caused by the local change in elastic strain energy induced by the pre-stretching. In the pre-stretched Al-Cu-Sc-Zr alloy, the presence of coherent Al3(Sc, Zr) dispersoids is found to promote equivalent nucleation of θ′ precipitates in all three habit planes (at the expense of θ″), resulting an isotropic precipitate distribution. The strengthening potential of these different precipitate distributions is analytically modelled and discussed.

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Scripta Materialia