MECHANISMS OF CREEP FRACTURE AND PREDICTION OF CREEP DUCTILITY (CAVITATION, CRACK GROWTH)

At high temperatures and relatively low stresses creep fracture occurs intergranularly by cavitation and this leads to a low creep ductility. The purposes of the present investigation have been to obtain a better understanding of mechanisms of intergranular cavitation, to find the relation between cavitation processes and creep behavior, to disclose the nature of creep ductility, to explore the controlling factors for creep crack propagation and to predict the time to failure and the creep ductility.;It is found that a threshold shear stress is needed for cavity nucleation to occur. When the resolved shear stress on a grain boundary segment reaches the threshold stress, a critical normal stress for cavity nucleation is produced on the particle-matrix interface by grain boundary sliding and cavity nucleation occurs. Cavity growth is controlled by the coupling of diffusion and power law creep over a range of stresses and temperatures. The stress and temperature dependence of rupture can be described using an analysis of the type suggested by Chen and Argon, provided that the diffusional length is based on the ligament stress rather than the applied stress.;It is found that the creep ductility of a class I solid solution alloy, Cu-2.7 at .% Sn, is closely related to the creep mechanism. It reaches a maximum in class I regime and decreases in both the power law breakdown and class II creep regimes. The higher creep ductility is explained in terms of the difficulty of intergranular cavity nucleation. Based on the Kachanov damage law and the cavity growth law, the time to fracture and the strain to failure can be calculated. The model predictions for the Cu-Sn alloy compare favorably with experimental data.;Creep crack growth tests show that at a constant displacement rate, steady state crack growth can be well characterized by the net section stress and the power rate C* measured using a technique developed by Landes and Begley. The results suggest that the HRR stress field does not exist at the crack tip under creep conditions and that steady state creep crack growth is not due to a stress singularity. Rather, crack growth appears to occur in a progressive manner because of a damage gradient which develops during the transient stage.;Cavity nucleation on grain boundary particles during creep has been analysed statistically using a classical thermodynamic method. A technique based on pre-creeping and sintering to create large, widely spaced cavities at grain boundaries has been used to study the mechanisms of cavity growth. High temperature creep, fracture and crack growth properties of a class I solid solution alloy, Cu-2.7 at .% Sn, have been investigated. A numerical model for the prediction of tertiary creep and creep ductility has been proposed.