Microstructure and cavitation in the superplastic Zn-22 wt% Al alloy: Effect of solution heat treatment

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Abstract
The effect of post-solution treatment on the microstructure and cavitation behaviour of the superplastic Zn-22 wt% Al was investigated by imposing different periods of solution heat treatment above the eutectic temperature with otherwise identical experimental conditions. Final microstructures show that groups of fine α and β-phases, which form by spontaneous decomposition, are surrounded by traces of former a boundaries which consist of fine elongated α grains. Microstructural observations indicate that these groups behave as independent domains during superplastic deformation. Cavity stringers parallel to the stress axis are typically found to exist along the traces of former α boundaries. This observation is a result of preferred cavitation at the former a boundaries and the tendency of these boundaries to become aligned with the stress axis during superplastic deformation.