Microstructure and evolution of mechanically-induced ultrafine grain in surface layer of AL-alloy subjected to USSP

Experiments were conducted to investigate the ultrafine-grained (UFG) microstructures in the surface layer of an aluminum alloy 7075 heavily worked by ultrasonic shot peening. Conventional and high-resolution electron microscopy was per-formed at various depth, of the deformed layer. Results showed that UFG structures were introdued into the surface layer of 62 mum thick. With increasing strain. the various microstructural features, e.g., the dislocation emission source, elongated microbands, dislocation cells, dislocation cell blocks, equiaxed submicro-, and nano-crystal grains etc.. were successively produced. The grain subdivision into the subgrains was found to be the main mechanism responsible for grain refinement. The simultaneous evolution of high boundary misorientations was ascribed to the subgrain boundary rotation for accommodating further strains. Formed inicrostructures were highly nonequilibratory. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.