GRAIN BOUNDARY EMBRITTLEMENT STUDY ON NICKEL 270 BY EQUILIBRIUM AND DYNAMIC INTERFACE SULFUR SEGREGATION

Overview & Synopsis

The purpose of this paper is to present some information obtained about sulfur segregation at surfaces using A.E.S. to explain the results of mechanical testing on nickel 270 (99,98%, 1 wt ppmS). The kinetic segregation measurements by A.E.S. were carried out on samples quenched after annealing 4 hours at 1150°C. The analyses were performed in situ at 950, 900 and 800°C. The bulk S diffusion coefficients were obtained using McLean's model. FOWLER's formalism (regular solution) was used to determine surface segregation energy ƊGs : - 140 - 3Ys kJ.mol-1 (Y : rate of segregation).The defects in supersaturation, such as quenched-in vacancies and those produced by plastic strain (cold worked metals) increase considerably the surface kinetics of sulfur segregation (in comparison with McLEAN's kinetics). The sulfur surface segregation kinetics, on cold worked structures presented two mechanisms : elimination of over saturated vacancies and sulfur atoms dragging by grain boundaries during recrystallization. The results of mechanical testing after specific annealings can be explained by dynamic segregation mechanisms.