Superplastic deformation of a fine-grained Zn-0.3wt.%Al alloy at room temperature

The aims of this study are firstly to obtain a fine and stable microstructure in a quasi-single phase Zn-Al alloy and secondly to characterize the superplastic deformation behavior of this alloy performing load relaxation and tensile tests, as well as a microstructural study using a scanning and a transmission electron microscopy. A very fine and stable microstructure with an average grain size of about 1 mum was produced by modifying the conventional thermomechanical treatment process for commercial Al alloys to obtain a remarkable elongation of 1400%: at room temperature, which is the largest ever reported for this alloy. The flow curves of log sigma versus log (epsilon) over dot constructed from load relaxation test and a series of tensile tests conducted under various initial strain rates appeared to have a sigmoidal shape typical for superplastic materials. Even after the specimen was elongated up to 100% at room temperature, no evidence for grain growth was noticed. From the observations of microstructural changes and the scratch offsets on the surface of a deformed specimen, the dominant deformation mechanism could be identified as the grain boundary sliding (GBS), possibly accommodated by plastic flow due to dislocation glide near grain boundaries. At a still higher strain rate, the accommodation mechanism of GBS appears to change from dislocation activity near grain boundaries to deformation twinning at stress concentrations such as triple junctions and grain boundary ledges. When the grain size was increased by an order, predominant deformation mechanism appeared to be dislocation activity evidenced by the cell structure reducing the tensile elongation substantially to around 100%. (C) 2001 Elsevier Science B.V. All rights reserved.