Orientation dependence of the high cycle fatigue properties in a hot-cross rolled Al-Li 8090 alloy plate

High cycle fatigue properties were studied in different directions within the rolling plane and in the short transverse direction of a hot-cross rolled Al-Li 8090 alloy plate (45 mm thick). The fatigue tests were conducted at constant stress amplitudes ranging from 30% to 100% of the yield strength σ_y in the rolling direction, R = 0.1, a frequency of 20 Hz, room temperature in air using a self-aligning four-point bend rig. It was found that the fatigue life of a sample loaded in the rolling direction was about ten times longer than that of a sample loaded in the short transverse direction at the same stress amplitude. The fatigue limit for the short transverse samples was measured to be about 30% σ_y, while it was about 40% σ_y for the longitudinal samples. Growth of fatigue cracks in the short transverse samples was predominantly intergranular, whereas it was markedly crystallographic (along a 111 plane in each grain) in the samples cut from the plate center when loading axis was parallel to the rolling plane. The inferior high cycle fatigue property of the short transverse samples is believed to be due to the combination of planar grain boundaries (parallel to {110} plane) being perpendicular to the load axis and the existence of trace elements, Na and K, in the alloy. Within the rolling plane, the fatigue behavior was relatively isotropic in the surface region of the plate, while in the plate center it was significantly inferior in the direction of about 35° relative to the rolling direction. Strong texture (mainly {110}〈111〉 and {110}〈112〉) in the center of this alloy was the main factor that contributed to the in-plane anisotropy.