Application of percolation theory to surface segregation during recovery

Surface segregation of sulphur on heavily cold rolled nickel is examined during recovery using in-situ Auger Electron Spectroscopy. An unexpected incubation period immediately followed by a very quick segregation step is observed. This unexpected behaviour is discussed using a model, which takes the different stages of recovery and especially the role of dislocation walls into account. In as-cold rolled nickel, dislocations appear as cells with thick walls. During recovery, dislocation wall refinement makes the diffusion of sulphur along dislocation walls much faster. The percolation of this structure is responsible for sudden acceleration of surface segregation. The main conclusion of this study is that superficial segregation kinetics remains very slow until a network of subgrain boundaries acting as diffusion short passes is established and used by the sulphur to reach the surface quickly. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.