In order to improve the creep fracture properties of the material it had to contain 30 to 50ppm of phosphorus, as opposed to O-free copper without P. It was suggested that the phosphorus impedes grain boundary sliding in copper and recently a quantitative theory based on this idea had shown that there was no risk for creep-brittle fracture of O-free copper with P under nuclear-waste storage conditions. In order to verify the basis of this theory grain boundary sliding was investigated in copper with and without P additions. The method was to examine intentionally scratched surfaces of tensile specimens tension tested to plastic strains of 1%, 2% and 4% at 150 and 200C. After testing specimen surfaces were examined in SEM and sliding distances were measured as in-surface displacement of scratches. The results were plotted as distribution functions where the fraction of slides smaller than a given value was plotted versus sliding distance. The result was that in most cases the distribution functions for O-free, and O-free copper with P, overlapped. In a small number of cases there was a tendency that less sliding had occurred in O-free copper with P. The overall conclusion was however that although there may be a slight difference between the materials with regard to grain boundary sliding it was not large enough to explain the observed difference in creep brittleness. Tension tests to fracture at 100 to 200C showed that the tensile properties of the two copper qualities were more or less identical until intergranular cracking starts in the O-free copper. Then the flow stress decreased in comparison with O-free copper with P. It was suggested that at least part of the observed differences in creep strength between the two coppers may be due to the effect of intergranular cracking.

A Study of Grain Boundary Sliding in Copper with and without an Addition of Phosphorus. K.Pettersson: Journal of Nuclear Materials, 2010, 405[2], 131-7