The influence of test frequency on the fatigue resistance of a Ni-base eutectic composite

The high cycle fatigue life of aged Nitac 14B at 825°C in vacuum is strongly dependent upon frequency and, at high frequencies, surface finish. Crack initiation controls fatigue life at all frequencies, although the mechanism changes with frequency. Fiber cracking is especially prominent at low frequencies, as is dislocation penetration of γ′ rosettes. The results are consistent with a creep-fatigue interaction at low frequencies, such that cracks are initiated at the interior, and with surface-dominated crack initiation at high frequency. It is shown that fatigue lives at very low frequencies are consistent with creep-rupture data for Nitac 14B. Theoretical analysis of the cyclic deformation behaviour shows that decrease of frequency causes internal matrix stresses to relax and to increase stresses in the renforcing phase. This situation corresponds to transition from fatigue to creep loading conditions.