Dislocation behavior in fatigue V: Breakdown of loop patches and formation of persistent slip bands and of dislocation cells

Abstract

The loop patch and channel structure in fatigued cooper becomes unstable at a resolved sgear stress of about 30 MPa at room temperature and is replaced by persistent slip bands (PSBs). This instability depends little, if at all, on frequency of cycling and strain amplitude. It appears to be a function only of the dislocation density in the loop patches but includes a temperature dependence (i.e. the critical stress rises to about 70 MPa at 4 K) which suggests that cross-slip, defect dragging and/or “forest cutting” are involved. In view of the available evidence it is suggested that the discussed instability is due to secondary glide on systems intersecting the primary slip plane which, through moving primary edge dislocations normal to their slip planes, permits their mutual annihilation. It is envisaged that such secondary glide takes place on a small scale in a very large number of places in the loop patches once the critical dislocation density is reached. A semiquantitative analysis of the stresses involved in conjunction with micrographical evidence indicates that the intersecting glide occurs at least partly on cube planes. If this interpretation is correct, the removal of the primary loops leaves behind dislocations which are faulted but are fairly mobile and are liable to annihilate each other mutually to a large extent and to leave “debris” which is then swept into the dipolar walls in the PSBs, all in agreement with micrographical evidence. Cell formation in the PSBs is believed to result at stresses high enough to introduce secondary glide in the PSBs. Loop patches, consisting as they do of mobile dislocations, must be attracted to the surfaces on account of the (mild) dislocation surpluses at their boundaries with the channels and the image forces arising therefrom. Such image forces are virtually independent of the presence of thin surface films, but such surface films can inhibit the escape of the prismatic loops out of the metal. Correspondingly, loop patches will be either enriched or depleted at surfaces, to a depth comparable with the average loop patch diameter, depending on whether the surface films are or are not so thick as to bar the egress of the loops. One probable example of each case is discussed.