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 ran ge of maximum stress levels from 60 to 120 pet of the 0.2 pet proof st ress. The alloy, in the form of 12.5-mm rolled plate, was investigated in underaged (UA), peak-aged (PA), and overaged (OA) conditions, corr esponding 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 pla te were investigated. Orientation and aging condition influenced the d egree of surface topographical development but not fatigue life. Detai led transmission electron microscopy (TEM) of the fatigued surface ind icated that deformation in all aging conditions occurred by planar sli p. Slip was generally restricted to a single slip system within each g rain, 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 i n the OA condition is attributed to shearing of large strengthening pr ecipitates, which is promoted by long slip line lengths. No evidence o f surface specific changes in slip character was observed.