Grain-boundary cavity growth during high-temperature creep and fatigue

The influence of prior high-temperature fatigue on the elevated-temperature creep behaviour of Cu (450–550°C) and of Zr (700°C) was determined in conjunction with fractional density ∆/D measurements which were used to monitor the progress of cavitation. Prior fatigue increased the creep strength of Cu at low stress and had less effect on the highstress strength. In contrast, prior fatigue tended to decrease the creep strength of Zr at high stress and had little influence on the low-stress strength. The changes of ∆/D exhibited by Cu and Zr during creep differed markedly. For Cu, -∆/D increased with increasing time, strain, temperature, and stress, and with decreasing grain size. In contrast, Zr showed neither changes in ∆/D nor metallographic evidence for cavities during creep. For Cu, prior fatigue led to changes in ∆/D during creep 1-2 orders of magnitude greater than that expected of specimens with the same grain size but without prior fatigue. This effect, and the influence of prior fatigue on creep rate, were associated with particular structural modifications induced by high-temperature fatigue. Prior fatigue produced a small density change during the creep of Zr which was stress-dependent, indicating that, under creep conditions, cavity nucleation in Zr is more difficult than cavity growth. The differences in creep behaviour between Cu and Zr were examined in terms of the Beere-Speight and Dyson models for cavity growth.