Dislocation behaviour and the formation of persistent slip bands in fatigued copper single crystals observed by high-voltage electron microscopy

Dislocation behaviour in fatigued copper single crystals with [123] and [111] fatigue axes has been investigated dynamically by in situ experiments with a high-voltage electron microscope to clarify the origin of persistent slip bands (PSBs) and how the ladder structure is formed. From observations of the fatigued [123] specimen, the following points have been clarified concerning the development of the PSBs and the ladder structure. (1) Only primary dislocations are activated in the ladder structure whose dislocation walls are perpendicular to the primary Burgers vector. (2) The dislocation walls of the ladder structure are easily cut by the glide of dislocations on the cross-slip plane. (3) The ladder structure disappears more quickly than the vein structure when the fatigued specimens are heated inside the electron microscope. (4) The vein structures observed from the direction perpendicular to the cross-slip plane tend to arrange themselves parallel to the traces of the conjugate and critical slip planes. (5) Many secondary dislocations belonging to the conjugate and critical slip systems are generated from the dislocation tangles located between the well-developed ladder wall and the vein structure. (6) The vein structure is gradually changed to the ladder structure by a loosening of the dislocation tangles. On the contrary, in the [111] specimen the vein structures are stable against the motion of mobile dislocations moving on all slip planes, and the ladder structure is never formed. On the basis of the experimental results, the origin of PSBs and the mechanism of the formation of the ladder structure are discussed.