| Theoretical and experimental studies have been done to understand the underlying micromechanisms of creep-fatigue interactive failure.;In the second stage, MERL-76, a newly developed powder metallurgical nickel-base superalloy, is tested and found that characteristics of creep-fatigue failure is quite similar to those of copper alloy. Especially, strong effects of grain boundary microstructure controlled by heat treatment have been identified. Large grain boundary particle size and spacing were found to be beneficial to creep-fatigue as well as to stress-rupture behaviors.;Finally, a mechanistic model is developed based on the understanding of micromechanical process of w-type creep damage in creep-fatigue and applied to the experimental fatigue life data with great success.;In the first stage, two types of model materials, Al-5%Mg commercial alloy and OFHC-Cu, were studied. By performing fatigue life tests in ultra-high vacuum of 10('-8) torr, it was found that, above a certain transition temperature, creep-fatigue failure is controlled by general creep damage rather than by a process of fatigue crack initiation and propagation. The transition temperature was found to be related with onset of grain boundary sliding and could be predicted by the technique of internal friction measurement. In particular, as in the case of stress-rupture, two types of creep damages, w-type nd r-type cavitation, have been also found in creep-fatigue depending on material, temperature and cyclic loading parameters. Generally, w-type cavities could be found in the, so called, nucleation-controlled materials such as aluminum alloys, at intermediate temperature ((TURN) 0.4 T(,M)) and for unbalanced triangular wave shape, in particular, slow-fast cycle. On the other hand, r-type cavities could be readily found in the growth-controlled materials such as copper and nickel alloys, at higher temperature (> 0.5 T(,M)) and for unbalanced hold cycle, especially, tension-hold cycle. |
