Grain boundary segregation of impurity elements in reactor pressure vessel steels
The segregation of a number of impurity elements to grain boundaries in reactor pressure vessel steels, under both thermal and irradiation conditions, have been observed to cause embrittlement. In low alloy steels, the embrittlement has been associated with small additions of phosphorus to alloys, an impurity element that lowers the cohesive strength of grain boundaries, thereby permitting brittle, intergranular fracture to occur more easily. Conversely, carbon additions to the same steel alloys have been shown to increase the grain boundary cohesiveness, thereby reducing the propensity for the alloy to fail in an intergranular manner. An increased understanding of the behaviour of these alloys under typical reactor service conditions is therefore sought after. Experimental grain boundary segregation data is available for long-term thermally aged material, and theoretical models exist which can reasonably predict the magnitude and temperature dependence of impurity element segregation. Isothermal ageing-induced segregation, known as equilibrium segregation, has been predicted using a variety of analytical models, that can predict the effect of alloying elements that both interact during segregation and that segregate competitively. However, grain boundary segregation data for irradiated material is scarcer, primarily owing to the difficulty of dealing with radioactive samples, but also due to the relative scarcity of material itself. Theoretical models, based on thermal non-equilibrium types of segregation, currently exist but are somewhat limited in their approach, since they only predict segregation in binary alloys. These models have been extended in this Thesis to predict the behaviour of ternary alloy systems. Comparison with currently available experimental results has shown that these modifications have resulted in a more accurate prediction of the segregation behaviour of these impurity elements. In addition, the effect of thermally induced segregation has been incorporated into theoretical models to predict the behaviour of M234 type precipitates under long term thermal ageing conditions in austenitic stainless steels. These predictions have also been compared to experimentally observed precipitation behaviour in a number of alloys and have been found to show close agreement.