INTERFACIAL DAMAGES ASSOCIATED WITH DYNAMIC SEGREGATION OF SULFUR IN NICKEL

Sulfur is well known to segregate onto nickel surface as well as at gr ain boundaries. These interfacial segregations induce intergranular em brittlement, intergranular and uniform corrosion, welding defects, etc . This paper deals with segregations which take place in a cold-worked metal e.g. nickel containing an excess of dislocations and vacancies. Because the kinetics of this kind of segregation are under the depend ence of the fluxes of defects we propose to name it ''dynamic segregat ion''. Superficial segregation is studied by Auger electron spectrosco py and intergranular segregation by both tensile tests and grain bound ary electrochemical dissolution. The recrystallization of the cold-wor ked metal is studied by Calvet differential microcalorimetry. We show that each stage of recovery (vacancy annihilation and recrystallizatio n) is associated with a particular mechanism of segregation. In both c ases the kinetics of segregation are strongly enhanced compared to equ ilibrium ones. Before recrystallization the segregation kinetics are u nder the dependence of both dislocation density and vacancy concentrat ion. When segregation takes place during recrystallization, the kineti cs are at maximum for the threshold strain for recrystallization, then decrease when the deformation ratio increases. This behaviour can be explained by the impurity drag theory.