Influence of recovery on dynamic segregation of sulphur to surface of coldrolled nickel

It is now well established that the kinetics of impurity segregation is gre atly increased by the defect annihilation process occurring during the retu rn 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 re crystallisation stages contribute to the acceleration of the segregation ki netics. In this work, an attempt is made to study the contribution of each stage separately, with particular emphasis on the recovery mechanisms of co ld worked pure nickel and the way these mechanisms affect sulphur segregati on taking place in this material. For this purpose, recrystallisation and s egregation anneals at 455 degreesC were carried out on lightly (0.25 true s train) and heavily (0.8 true strain) cold rolled nickel. It was found that sulphur segregation in the lightly deformed material took place entirely du ring the incubation time preceding recrystallisation, i.e. during the recov ery stage. The process is thought to involve dislocation pipe diffusion for which a diffusion coefficient of 6.4 x 10(-9) m(2) s(-1) was estimated. In the heavily deformed material, segregation was concomitant with recrystall isation and the mechanism thought to be most probable is that involving imp urity drag.