Creep during power-law breakdown in phosphorus alloyed copper

Copper alloyed with 50 ppm phosphorus (Cu-OFP) is selected for canisters for nuclear waste packages to avoid a low creep ductility, which is sometimes present in pure copper. The operating temperatures of these canisters are in the range from 0 to 100 degrees C. Creep readily takes place in copper even at room temperature. At temperatures below 100 degrees C, creep is well inside the power-law breakdown regime. The creep exponent is in the range from 30 to 100. Since creep models for this situation are missing in the literature, a new model for the minimum creep rate based on fundamental principles for climb and glide has been derived. This model gives the correct order of magnitude for the creep rate in the temperature range from 20 to 400 degrees C without the use of fitted parameters. Design against creep can either be based on the total applied stress or the effective stress. In the first case the constitutive equations can be directly obtained from the minimum experimental creep rates. A new approach is proposed to handle the effective stress case, which is based on the initial creep rates. The phi-model is used to relate the initial creep rate to the minimum one. It is shown how the constitutive equations for the creep rate and the back stress can be transferred to multiaxial stress states for use in FE-modelling.