Creep behavior of rolled and extruded copper-chromium-niobium (GRCop-84)

Constant stress creep experiments were performed on rolled Cu-8Cr-4Nb (GRCop-84) from 673 to 1073K. These results were compared to previous constant stress creep data for the extruded form. The data indicate that the creep strength of the extruded alloy is significantly greater than that of the rolled form at intermediate creep temperatures. Microstructural characterization of both the extruded and the rolled materials was undertaken. The analysis suggests the grains have the (110) planes parallel to the plane of the sheet indicating the loading axis of each grain is along a random axis in the <110> sheet normal. Review of the fracture surfaces could find no correlation with the mechanical creep properties.;A review of creep theories involving dispersion strengthening and the effects of composite particulates is presented. A non-linear regression analysis was conducted using the Rosler-Arzt and Rosler-Baker creep models. These analyses were undertaken to determine whether dislocation detachment from particles is rate-controlling and if load shedding had occurred. The relaxation parameter (k) is a measure of the degree to which a dislocation's line energy is reduced at the particle-matrix interface. This parameter was calculated from the results. The significantly large values of k indicate the Rosler-Arzt and Rosler-Baker mechanisms were not active for creep deformation. A threshold stress analysis indicated the apparent stress exponent to be equal to 5, suggesting the mechanism to be dislocation-climb of dislocations over particles with a threshold stress. Finally, it is shown that the differences in texture between the two forms quantitatively accounts for the difference in creep strength.