FATIGUE DAMAGE AT ROOM-TEMPERATURE IN ALUMINUM SINGLE-CRYSTALS .4. SECONDARY SLIP

Two Al single crystals were fatigued in push-pull in air at room tempe rature, at a constant shear stress amplitude of 4 MPa, zero mean stres s and frequency of 20 Hz. Secondary slip lines were observed in the ma trix, 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 x 10(6) cycles), in addition to extrusions and intrusio ns which were probably formed by local persistent secondary slip in PS Bs, short cracks were also usually observed on the side-surfaces mainl y along the interface between the matrix and the extrusions in the PSB s. 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 cr ack initiation in the extrusions. Secondary slip takes place commonly to assist lattice rotation during fatigue, and plays a significant rol e in fatigue damage at room temperature, especially in the later stage of fatigue, in Al single crystals. Copyright (C) 1996 Acta Metallurgi ca Inc.