FATIGUE DAMAGE IN ALUMINUM SINGLE-CRYSTALS .1. ON THE SURFACE CONTAINING THE SLIP BURGERS VECTOR

An aluminium single crystal with the axial direction of [25 (1) over b ar] was fatigued in push-pull at the constant resolved shear stress am plitude 4 MPa, frequency 20 Hz and room temperature. Microcracks, micr ovoids, macrobands, extrusions and intrusions were observed on the sid e-surface containing the Burgers vector b. Most microcracks were opene d, and were within, but approximately perpendicular to, PSBs. Slip ste ps were found in the extrusions and intrusions. There was net irrevers ible slip in one direction in most PSBs. Some short cracks along the P SBs on the side-surface were also observed at 5 x 10(6) cycles. These observations indicate that, without the aid of the surface roughness o f PSBs, cracks can still be nucleated, and that, apart from the notch effect of a PSB, there are other factors controlling crack initiation in single crystal aluminium. There may be an internal tensile stress e xisting in a PSB in the direction of b, and a shear stress applied by the specimen grips in the specimen due to the irreversible dip in one direction in PSBs. These stresses and the applied stress are responsib le for the formation of microcracks, microvoids, extrusions, intrusion s and macrobands on the side-surface.