THE EFFECT OF DEFECT ANNIHILATION ON THE SEGREGATION KINETICS OF SULFUR IN BOTH QUENCHED AND COLD-WORKED NICKEL

Impurity segregation to grain boundaries and free surfaces of pure met als is thought to take place according to two mechanisms. One involves the thermodynamic and kinetic laws of equilibrium, as is the case, fo r example, in annealing for sufficiently long periods at sufficiently high temperatures (approximate to 0.5T(M), T-M being the melting tempe rature). The other takes place while a metal, initially in a non equil ibrium 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 annea ling of both quenched and cold-rolled nickel is carried out. It is fou nd that the sulphur segregation kinetics measured during the eliminati on of vacancies (quenched nickel) are two-three orders of magnitude hi gher 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 anni hilation and the recrystallization stages of the heating process. A co nsiderable reduction in strength and ductility, coupled with severe in tergranular corrosion as shown by scanning electron micrographs, may b e taken as clear evidence of intergranular segregation taking place du ring the recovery by the material of its equilibrium state. (C) 1998 C anadian Institute of. Mining and Metallurgy. Published by Elsevier Sci ence Ltd. All rights reserved.