Fatigue damage in aluminum single crystals-I. On the surface containing the slip burgers vector

An aluminum single crystal with the axial direction of [211̄0 was fatigued in push-pull at the constant resolved shear stress amplitude 4 MPa, frequency 20 Hz room temperature. Microcracks, microvoids, macrobands, extrusions and intrusions were observed on the side-surface containing the Burgers vector b. Most microcracks were opened, and were within, but approximately perpendicular to, PSBs. Slip steps were found in the extrusions and intrusions. There was net irreversible slip in one direction in most PSBs. Some short cracks along the PSBs on the side-surface were also observed at 5 × 10⁶ cycles. These observations indicate that, without the aid of the surface roughness of 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 existing in a PSB in the direction of b, and a shear stress applied by the specimen grips in the specimen due to the irreversible slip in one direction in PSBs. These stresses and the applied stress are responsible for the formation of microcracks, microvoids, extrusions, intrusions and macrobands on the side-surface.