The effects of hydrogen and segregation on fatigue crack nucleation at defined grain boundaries in nickel bicrystals

Abstract

In order to explore the role of impurity segregation in intergranular fatigue crack initiation and propagation, tests have been made on nickel bicrystals, variously heat-treated to induce increasingly severe degrees of sulfur segregation, and using air, vacuum and hydrogen as the environment. The grains of the bicrystals were oriented to yield two types of boundaries: type I, with strong elastic-plastic incompatibility and type II, a compatible tilt boundary. If the boundaries were clean, persistent slip band cracking in the grains occurred in preference to intergranular cracking. Although equal degrees of sulfur segregation, as measured by Auger-spectroscopy, could be produced at the two types of boundaries by the heat treatments, the incompatible one was much more susceptible to intergranular cracking than the other, which could be made to crack intergranularly only by high partial pressures of hydrogen. The results show that high stresses associated with incompatibility coupled with a lowering of cohesive forces at the boundary produced by the segregant are the main factors controlling intergranular fracture.