Some observations on cyclic deformation structures in the high-strength commercial aluminum alloy AA 7150

Load-controlled fatigue testing of the aluminum alloy AA 7150 has been conducted using four-point bending with an R ratio of +0.1 over a range of maximum stress levels from 60 to 120 pct of the 0.2 pct proof stress. The alloy, in the form of 12.5-mm rolled plate, was investigated in underaged (UA), peak-aged (PA), and overaged (OA) conditions, corresponding to a change in average precipitate sizes from 5 nm in the UA condition to 21 nm in the OA condition. Three orientations of the plate were investigated. Orientation and aging condition influenced the degree of surface topographical development but not fatigue life. Detailed transmission electron microscopy (TEM) of the fatigued surface indicated that deformation in all aging conditions occurred by planar slip. Slip was generally restricted to a single slip system within each grain, and subgrain boundaries offered little resistance to dislocation movement facilitating long slip line lengths (measured up to 310 mu m) between adjacent high-angle grain boundaries. Planar slip observed in the OA condition is attributed to shearing of large strengthening precipitates, which is promoted by long slip line lengths. No evidence of surface specific changes in slip character was observed.