Hydrogen-assisted cracking and stress-corrosion cracking of Al-Li and Al-Zn-Mg alloys

Abstract

The characteristics of hydrogen-assisted cracking (HAC) in high-strength aluminum alloys have been studied as a function of microstructure, strain rate, and hydrogen distribution by using cathodically hydrogen charging. The mechanisms of stress-corrosion cracking (SCC) in high-strength aluminum alloys were also investigated in aqueous solutions. Moreover, in order to understand the role of oxide film in the SCC process, a brief examination was made on the electrochemical properties of thin barrier-type oxide films formed in aqueous solution. In chapter III, the effects of microstructure, strain rate on the HAC behavior of Al-Li alloy and the effects of hydrogen redistribution on the HAC behaviors of Al-Li and Al-Zn-Mg alloys have been investigated. It is concluded that, of three different microstructures defined in the Al-Li alloy system, the peak-aged specimen is most susceptible to HAC and the overaged specimen is least susceptible. The difference in the dependence of HAC susceptibility on microstructure is explained in terms of the combined effect of slip planarity and yield strength. It is known that the HAC susceptibility experiences two-staged variation with strain rate irrespective of heat treatment. The susceptibility increases with decreasing strain rate up to $3\times10^{-6}s^{-1}$ and then decreases. The decreased susceptibility with decreasing strain rate in the lower range is based upon the fact that absorbed hydrogen originating from moist air markedly affects the uncharged specimen more than the hydrogen-charged specimen. This is substantiated by fractographic observations. In tests performed immediately after hydrogen changing, the Al-Li alloy is shown to be less susceptible to HAC than the Al-Zn-Mg alloy. By storing after hydrogen charging, it is reveabled that both the Al-Li and Al-Zn-Mg alloys appear to lose some charged hydrogen, but the charged hydrogen mainly diffuses into the bulk and redistributes within the alloys, and it is known that ...