R. Le Gall

In-situ SEM studies of grain boundary migration during recrystallization of cold-rolled nickel

The grain boundary migration (GBM) rate during primary recrystallization depends on the driving force P exerted on the boundary and on the boundary mobility M. M is composed of two terms: an intrinsic mobility of the boundary depending on its structure and a chemical mobility due to the action exerted on the boundary by impurities. The purpose of the present work is to investigate quantitatively the law V = f(P) during primary recrystallization of nickel.

Diffusion of sulfur from the grain boundaries to the surface in polycrystalline nickel

Segregation of an impurity on the free surface of a metal is a commonly used phenomenon to obtain bulk diffusion data from surface coverage kinetic measurements. During annealing of nickel, sulfur segregates to grain boundaries as well as onto the free surface as the two phenomena act simultaneously. Yet a driving force exists to convert segregation into surface segregation since the free enthalpy of surface segregation ({minus}140 kJ mol{sup {minus}1}) is more negative than the free enthalpy grain boundary segregation ({minus}98 kJ mol{sup {minus}1}). As a result, as long as the equilibrium value of surface coverage is not reached, the intergranular sulfur can diffuse to the surface along the grain boundaries path. In this work the authors use this property to deduce grain boundary diffusion constants of sulfur from surface enrichment kinetics.

Application of percolation theory to surface segregation during recovery

Abstract 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.

Use of AES to determine low solubilities of impurities: Case of MnS in austenitic stainless steel

The solubility of manganese sulfide in a commercial 304 austenitic stainless steel was studied using AES to determine surface sulfur segregation. The segregation mechanism at stainless steel surfaces were extensively studied, but thermodynamic data such as segregation free energy, or manganese sulfide dissolution heat have not been established, although these data are of great interest because the MnS dissolution parameters give the amount of free sulfur in solid solution. The only solubility data for manganese sulfide in austenitic steel was given to Turkdogan in 1955 and concerned MnS in iron-manganese alloys. In the present study, the Auger spectra are used to calculate the free energy of dissolution of manganese sulfide as well as the sulfur segregation parameters in austenitic stainless steel.

Influence of recovery on dynamic segregation of sulphur to surface of cold rolled nickel

Abstract It is now well established that the kinetics of impurity segregation is greatly increased by the defect annihilation process occurring during the return to equilibrium of a material initially in a metastable state (quenched, irradiated, cold worked). During the return to equilibrium of a cold worked material, i.e. during a recrystallisation anneal, both the recovery and recrystallisation stages contribute to the acceleration of the segregation kinetics. In this work, an attempt is made to study the contribution of each stage separately, with particular emphasis on the recovery mechanisms of cold worked pure nickel and the way these mechanisms affect sulphur segregation taking place in this material. For this purpose, recrystallisation and segregation anneals at 455°C were carried out on lightly (0.25 true strain) and heavily (0.8 true strain) cold rolled nickel. It was found that sulphur segregation in the lightly deformed material took place entirely during the incubation time preceding recrystallisation, i.e. during the recovery stage. The process is thought to involve dislocation pipe diffusion for which a diffusion coefficient of 6.4×10-9 m2 s-1 was estimated. In the heavily deformed material, segregation was concomitant with recrystallisation and the mechanism thought to be most probable is that involving impurity drag.

Quantification of Grain Boundary Segregation Monolayers by X-ray Spectroscopy in a Scanning Electron Microscope

P. Nowakowski*, F. Christien*, M. Allart*, Y. Borjon-Piron*, R. Le Gall*, J.C. Menard**, H. Mantz*** * Laboratoire Genie des Materiaux et Procedes Associes (LGMPA), Universite de Nantes, Polytech’Nantes, Rue Christian Pauc, BP 50609, 44306 Nantes Cedex 3, France ** Carl Zeiss NTS sas, 27 rue des peupliers, 92752 Nanterre Cedex, France *** Carl Zeiss NTS GmbH, Research & Development, Carl-Zeiss-Strase 56, D-73447 Oberkochen, Germany