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Crack initiation mechanisms in low cycle fatigue of aluminium alloy 7075 T6

Materials Science and Engineering A
Publication Date
DOI: 10.1016/0921-5093(89)90522-4


Abstract Fully reversed strain-controlled push-pull tests were performed on polycrystalline specimens of aluminium alloy 7075 T6 to investigate the influence of temperature and strain amplitude on the micromechanisms of cyclic plastic deformation and crack initiation. The tests were carried out between 20 and 260°C in the strain range 0.8%–2.4%. Gold-plated replicas and scanning electron microscopy were used to detect crack initiation (a 0 ≈ 10 μm) and microcrack growth. The results showed the crack initiation life (N i) to be a fraction of the total life (N f). The ratios N i/N f were found to be independent of the strain amplitude in the investigated strain range and to vary with temperature only. Transmission electron microscopy results indicated that the cyclic behaviour is controlled either by the evolution of the dislocation substructures at low temperature (with cycling) or by the changing morphology and distribution of the precipitates at elevated temperatures. The kinetics of dissolution and precipitation was shown to be strongly influenced by cyclic plastic strain. This entails that the results of precipitation kinetics studies cannot be used to rationalize the changes in precipitation morphology and distribution occuring in high temperature, low cycle fatigue testing. Finally, the occurrence of any of the three favoured sites for crack initiation is shown to correlate with the plastic-to-total strain ratio (Δε p/Δε t).

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