AGEING EFFECTS ON FATIGUE CRACK CLOSURE OF AN Al–Li–Zr ALLOY IN Na2SO4, SOLUTION

Abstract

Abstract— Fatigue of an Al–Li–Zr alloy has been studied as a function of ageing stage by measuring crack closure in Na2SO4 solution with an unloading elastic compliance technique and comparing results in dry air and oxygenated solution. Anodic behaviour of peak‐aged and overaged alloy specimens in the Na2SO4 solution has been investigated by potentiodynamic polarisation and potentiostatic current transient experiments. The Na2SO4 solution increased the intrinsic fatigue crack propagation (FCP) rate for the overaged specimen in dry air compared to that for the peak‐aged sample. The Na2SO4 solution inhibited the development of crack closure for the peak‐aged specimen in dry air, but aided it in the overaged condition. The result of the environmental crack closure study is discussed in terms of the more enhanced through‐thickness tortuosity of the overaged specimen in the Na2SO4 solution when compared to that of the peak‐aged specimen. The oxygen dissolved in the Na2SO4 solution slightly increased the environmental intrinsic FCP rate, which seems to be due to the reduced repassivation rate as compared to that in the N2‐purged solution. The anodic dissolution rate from the bare surface of the overaged specimen in the Na2SO4 solution was higher than that from the peak‐aged sample. The difference between environmental FCP rates and crack closures for the peak‐aged and overaged specimens is discussed in terms of environment‐assisted crack‐tip damage processes involving anodic dissolution.