Serrated Flow and Enhanced Ductility in Coarse-Grained Al-Mg Alloys

Conventionally, superplasticity requires the presence of a fine-grained microstructure to enable grain-boundary sliding to take place during deformation. However, coarse-grained materials have also been shown to exhibit higher than normal amounts of ductility, provided they possess a high-enough strain rate sensitivity. In this work, coarse-grained Al-3 pct Mg, Al-5 pct Mg, and AA 5056 alloys were tested for enhanced ductility. The dependence of flow stress on temperature was found to display some unusual characteristics; these were interpreted as resulting from the occurrence of dynamic strain aging (DSA). In these materials, a local peak in elongation coincided with the presence of an unusual peak in rate sensitivity. This region of higher than normal rate sensitivity was coupled with the usual region of negative rate sensitivity found in DSA-prone materials, such as the Al-Mg alloys. A maximum ductility of 170 pct was recorded at 723 K (450 °C) and a strain rate of 5 × 10−2 seconds−1 was found in the vicinity of the rate sensitivity peak. This was found to increase to nearly 300 pct when the gage length was shortened. These peaks in elongation occurred below the maximum test temperatures.