FATIGUE DAMAGE AT ROOM-TEMPERATURE IN ALUMINUM SINGLE-CRYSTALS .2. TEM

Dislocation structures in aluminium single crystals fatigued at room t emperature have been studied by TEM and compared with those reported f or copper crystals. The fatigue tests were performed in push-pull at a stress amplitude 4 MPa, R = -1 and frequency of 20 Hz. A band structu re was observed in thin foils with planes parallel to the stress axis and the Burgers vector. Some parts of the bands were similar in appear ance and dimensions to the ladder-like pattern formed in the same samp le by several microcracks in a PSB on the surface containing b. This s uggests that the microcracks are associated with dislocation walls in the PSBs. In the matrix, the dislocation arrangement was a cell struct ure, but areas of a high dislocation density which might be a vein str ucture were also seen. Dislocation walls in PSBs might be described as two sets of entangled edge dislocations, one set being the primaries and the other being large jogs in primary screw dislocations lying on the cross-slip planes. A large number of dislocation loops were observ ed, suggesting that their formation was a significant form of irrevers ible deformation in single crystal Al fatigue. Three types of dislocat ion loops were identified.