PNNL

Abstract

Tensile specimens of an Al-4Mg-0.3Sc alloy in the peak-aged (8 hours at 553K) and the overaged (96 hours at 623 K) conditions, cold-rolled to a 70% reduction, exhibited dynamic recrystallization during superplastic testing at 733 K and a strain rate of 10-3s-1. Although the removal of subboundaries and the development of an equiaxed grain structure occurred more rapidly in the overaged alloy, there was no discernible difference in the microstructures as observed in the transmission electron microscope (TEM). The fundamental mechanisms controlling dynamic recrystallization have been studied by a combination of post-mortem examinations of the developing microstructure frozen at several points during the forming process and dynamic, high-temperature, deformation experiments performed in the TEM. The latter experiments provide direct observation at high spatial resolution of the operating mechanisms and have shown migration, pinning, disintegration and annihilation of subboundaries as well as the incorporation of dislocations into grain boundaries as they occur in real time.