The effects of macro- and micro-texture on the fatigue properties of Al-Li 8090 alloys

This paper reviews the recent series of research work conducted at Oxford on the effects of macro- and micro-texture on the fatigue properties of two Al-Li 8090 alloys. One alloy was a direct-chill cast and hot-cross rolled plate, and the other was a spray-cast and extruded bar. Both were solution heat-treated, water quenched, stretched and peak aged. Samples cut from different positions through the thickness of the plate and the bar were fatigued at a stress amplitude of 50% to 80% σ_y, frequency of 20 Hz, R=0.1 and -1 and room temperature, in air. The texture profile through the plate thickness and the local texture ahead of the tip of a short fatigue crack in the sample were characterized using EBSD. Both the type and intensity of the texture varied sharply through the plate thickness. It was found that the global and local textures have significant effects on fatigue crack initiation, short crack growth and fatigue lives of the alloys. In the spray-cast alloy cracks were often formed in grains with orientations close to Goss orientations ({110}<001>), while trace elements (Na and K) could be responsible for crack initiation on {001} planes which were perpendicular to the stress axis in the direct chill cast alloy. It was found that the twist angle on the grain boundary plane between the crack planes across the grain boundary was the key factor that retarded crack growth through the boundary, and that a sample with more random texture had higher resistance to short crack growth.