Sub-critical intergranular crack growth of iron and nickel

Abstract

The sub-critical intergranular crack growth behavior of iron and nickel showed several similarities and a few differences. The fracture thresholds and intergranular crack growth rates of iron and nickel were very similar which is consistent with the previously observed similarity of the effect of segregated sulfur and cathodic hydrogen on the fracture mode transition of these materials. The differences in their sub-critical crack growth behavior included the presence of a stage III crack growth regime in iron and its absence in nickel and the decrease in the fracture threshold of iron, K /SUB TH/, with increasing cathodic potential and the absence of a dependence of the K /SUB TH/ of nickel on cathodic potential. The present results were obtained with the same material used for the fracture mode transition tests but with compact tension (CT) samples in which the fracture mode was 100% intergranular for all cathodic potentials. Therefore, the present results represent the effect of cathodic potential on the intergranular crack growth rates of iron and nickel with large grain boundary sulfur concentrations (about 0.3 monolayers). The sub-critical crack growth behavior of iron and nickel with 0.3 monolayers of sulfur in their grain boundaries is similar. The fracture thresholdsmore » and Stage II crack growth rates are comparable; however, Stage III crack growth was observed in iron and not in nickel. The absence of Stage II crack growth in nickel was related to the large plastic zone size in nickel (about 47 mm) as compared to iron (about 2.6 mm).« less