On stress corrosion cracking in aluminum-lithium alloys

The stress corrosion cracking (SCC) susceptibility of two aluminum-lithium alloys, a binary AlLi and a ternary AlLiCu alloy, in 0.5 M NaCl solution was investigated using the constant elongation rate technique (CERT). Susceptibility increased with decreasing strain rate and with aging. The alloys were susceptible under both anodic and cathodic applied potentials. The susceptibility dependence of the alloys as a function of applied potential correlates well with published hydrogen permeability data. The susceptibility increased dramatically when hydrogen was charged into the specimen using a hydrogen re-combination "poison" during CERT testing. These experiments suggest that hydrogen plays a major role in the SCC of these alloys. A brittle hydride having the composition LiAlH₄ forms in the AlLi system under conditions of severe SCC susceptibility. The brittleness of the hydride is explained. The formation of the hydride is a sufficient condition for SCC of AlLi alloys. A process of SCC in AlLi alloys is proposed wherein hydrogen causes damage by the formation of a hydride.