Enhanced strength but deteriorated fatigue crack propagation behaviors of an aviation Al-Li alloy sheet caused by solid-solution time extension

Abstract

The fatigue crack propagation (FCP) property and strength in T8-aged Al-Li alloy sheets with varying solid-solution time were investigated and the influence mechanism of the secondary phase on fatigue crack initiation was revealed. The results demonstrate that both grain dimension and recrystallization texture of the alloys with different solid-solution time are similar, but the average diameter of secondary phase particles decreases nearly monotonically with solid-solution time extension. The prolongation of solid-solution time leads to higher number density and size of T₁ precipitates during T8 aging due to the increased solute concentration, which enhances the strength and exacerbates the stress concentration in the plastic zone ahead of fatigue crack tip. While the homogeneously distributed fine dispersed phase particles separate from the matrix and form the site for crack initiation when the stress in the plastic zone reaches a relatively high level. The crack initiation sites provide highways for main crack propagation which accelerates the FCP. Consequently, prolonging the solid-solution time further facilitates the crack initiation process caused by the dispersed phase particles, and renders the FCP rate much higher at the rapid propagation stage.