Interfacial segregation in metals during hot deformation : case of the nickel - sulphur system
Equilibrium grain boundary segregation in metals has been widely studied, but not the non-equilibrium one. In this study, we aimed to characterize the segregation during annealing and during hot deformation in the nickel - sulphur system. Two different methods of quantification were used: Auger electron spectroscopy and wavelength dispersive X-ray spectroscopy (WDS). The latter was originally adapted by our research group to the quantification of monolayers on a substrate. The diffusion coefficient of sulphur in nickel was measured at 550°C and 750°C and we obtained respectively 2.9 10-14 cm2.s-1 and 2.6 10-12 cm2.s-1. Measurements on samples annealed at different temperatures led to a free energy of segregation of 102 kJ.mol-1 and a concentration of sulphur in the grain boundary of 63.5 ng.cm-2 when saturated. We also investigated the influence of the temperature (450 and 550°C) and the deformation rate (3.9 10-5 s-1 and 3.8 10-4 s-1) on the interface segregation. During hot compression, we obtained a segregation up to 150 000 times faster compared to simple annealing at the same temperature. Multiplying the deformation rate by ten leads to a similar deformation-dependence of the segregation but a time-dependence 6,5 times as fast. In the investigated ranges of temperature and deformation rate, the segregation rate is nearly independent of the temperature. The segregation kinetics during hot deformation was satisfactorily modelled by taking account of the effect of the deformation-induced vacancies on the solute diffusion coefficient. The phenomenon of accelerated sulphur grain boundary segregation in nickel can then be attributed to the excess vacancies.