Abstract
Impurity segregation to grain boundaries and free surfaces of pure metals is thought to take place according to two mechanisms. One involves the thermodynamic and kinetic laws of equilibrium, as is the case, for example, in annealing for sufficiently long periods at sufficiently high temperatures (~O.5T M,TM being the melting temperature). The other takes place while a metal, initially in a non equilibrium state is returning to equilibrium, as is the case, for example, during the annealing of a quenched or cold worked metal.In this work, a kinetic study of sulphur segregation taking place during the annealing of both quenched and cold-rolled nickel is carried out.It is found that the sulphur segregation kinetics measured during the elimination of vacancies (quenched nickel) are two-three orders of magnitude higher than the ones predicted by McLean for equilibrium segregation. In the case of the cold worked material, it is found that the kinetics, compared to equilibrium ones, are accelerated by both the vacancy annihilation and the recrystallization stages of the heating process. A considerable reduction in strength and ductility, coupled with severe intergranular corrosion as shown by scanning electron micrographs, may be taken as clear evidence of intergranular segregation taking place during the recovery by the material of its equilibrium state. © 1998 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.RésuméLa segregation des impuretés aux interfaces (joints de grains, surfaces libres) des metaux purs a lieu selon deux mecanismes. Un des mecanismes a lieu se10n les lois thermodynamiques et cinetiques de l'equilibre comme dans le cas, par exemple, de longs recuits a des temperatures suffisamment élevées (≍0.5 Tf, Tf etant la temperature de fusion). L'autre a lieu durant le retour àl'equilibre d'un métal initialement dans un état hors d'equilibre comme dans le cas du recuit d'un metal trempe ou deforme à froid. Dans ce travail, une etude cinetiquede la segregation du soufre ayant lieu durant le recuit du nickel 270 trempe ou lamine à froid est effectuée.Les r ésultats obtenus montrent que la cinétique de la ségrégation du soufre mesurée durant l'eIimination des lacunes (matériau trempé) est deux à trois ordres de grandeur plus grande que celle prédite par McLean pour une ségrégation d'equilibre. Dans le cas du mat ériau lamine a froid, la cinetique est accélerée par les deux etapes du recuit, à savoir l'etape d'elimination des lacunes et l'étape de recristallisation. Une importante réduction de la résistance et de la ductilité, accompagnee d'une corrosion intergranulaire manifeste, peut etre per ue comme une indication d'une segregation intergranulaire ayant eu lieu lors de la restauration du materiau de son état d'équilibre. © 1998 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.
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