Studies on the corrosion and corrosion fatigue crack propagation processes in Al-based alloys and Cu-based alloys

Abstract

The present work is concerned with the corrosion and corrosion fatigue crack propagation processes in Al-based alloys and Cu-based alloys. In chapter II, fatigue crack propagation (FCP) behaviours of Al-Zn-Mg alloy have been investigated in 3.5 wt% NaCl solution as a function of loading frequency at stress ratios of 0.1 and 0.5. The FCP rates showed a positive frequency dependency at low ΔK and a negative frequency dependency at high ΔK. The decrease of the stress ratio increased the transition cyclic stress intensity necessary for the occurrence of the positive frequency dependence of the FCP rate. The corrosion-enhanced crack closure has been suggested to account for the reversal of frequency effect on the FCP rate at low ΔK. Various corrosion-enhanced crack closure sources were substantiated by scanning electron micrography. In chapter III, fatigue cracking of peak-aged and overaged Al-Li-Zr alloys has been investigated by measuring crack closure in deaerated and oxygenated 0.5 M $Na_2SO_4$ solutions with an unloading elastic compliance technique and by comparing that in dry air. Anodic behaviour of both specimens in both 0.5 M $Na_2SO_4$ solutions has been investigated by the potentiodynamic polarization and potentiostatic current transient experiments. The anodic dissolution rate on the bare surface in the deaerated $Na_2SO_4$ solution was higher for the overaged specimen than for the peak-aged one. The intrinsic fatigue crack propagation (FCP) rate for the overaged specimen was more affected by the deaerated $Na_2SO_4$ solution as compared to that for the peak-aged one. The crack closure in the deaerated $Na_2SO_4$ solution decreased for peak-aged specimen but increased for overaged one as compared to that in dry air. The intrinsic FCP rate in the oxygenated $Na_2SO_4$ solution was slightly higher than that in the deaerated solution. The difference between environmental FCP rates and crack closures for both specimens has been discussed in terms of envir...