Abstract
Fatigue tests carried out over the frequency range 8–100 Hz and temperature range 293–373 K on a commercial, superplastic Zn-Al eutectoid alloy showed a marked frequency effect even at 293 K, which is almost certainly the result of residual superplasticity, for when the superplastic microstructure is destroyed, the effect disappeared. Following this previously reported mechanical study, microstructural features of the fatigue process have been investigated in an attempt to identify the metallurgical factors which control the fatigue behaviour. Microscopic studies revealed two significant features: β-phase segregation and large (∼50 μm) inclusions from which fatigue cracks were multi-initiated after a few hundred cycles. Stage I occupies ∼95% of the fatigue life and failure occurs soon after the change to Stage II, for now the crack travels rapidly along the interphase boundaries. It is postulated that the frequency effect arises from the frequency dependence of Stage II, for the transition to Stage II is a function of flow stress, which is in turn a function of strain rate, which is, in turn, related to frequency.
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