Effects of micro-texture and β′ particle distribution on short fatigue crack growth in an Al-Li 8090 alloy

Samples cut from an Al-Li 8090 alloy plate were fatigued in four-point bend at a stress amplitude of 50% σ_y, R=0.1, a frequency of 20 Hz and room temperature in air. The plate had been hot cross-rolled, solution heat-treated, water-quenched, stretched by 6% and peak-aged. Crack initiation and propagation were monitored in detail using an optical microscope and SEM. The growth rate of short cracks was found to fluctuate greatly mainly due to the variation of crystallographic texture of the neighbouring grain ahead of the crack tip. In most grains the crack propagated along a {111} plane, but in some grains the crack became non-crystallographic and grew roughly along the direction perpendicular to the stress axis. Measurement of the crack growth rate revealed that the non-crystallographic part of the crack grew more slowly than the crystallographic part. TEM studies of the material indicated that β′ particle distribution was markedly non-uniform. In most of grains only a small number of β′ particles were observed, but in some grains they were densely distributed showing the 'doughnut-shape' contrast in TEM due to their acting as a substrate for δ′ nucleation. Because the β′ particles cannot be sheared by dislocations the slip behaviour in the grains with high concentration of β′ particles is expected to show non-planarity. As a result, the crack growth became non-crystallographic in these grains.