Fatigue damage at room temperature in aluminium single crystals - IV. Secondary slip

Two Al single crystals were fatigued in push-pull in air at room temperature, at a constant shear stress amplitude of 4 MPa, zero mean stress and frequency of 20 Hz. Secondary slip lines were observed in the matrix, macrobands and PSBs (mainly in extrusions and intrusions) on the side-surfaces of the two specimens. At the later stage of the fatigue test (after 5 × 10⁶ cycles), in addition to extrusions and intrusions which were probably formed by local persistent secondary slip in PSBs, short cracks were also usually observed on the side-surfaces mainly along the interface between the matrix and the extrusions in the PSBs. It is suggested that an internal stress at each slip offset (caused by net irreversible secondary slip in the extrusions within PSBs due to lattice rotation on the side-surface), is mainly responsible for crack initiation in the extrusions. Secondary slip takes place commonly to assist lattice rotation during fatigue, and plays a significant role in fatigue damage at room temperature, especially in the later stage of fatigue, in A1 single crystals.