Etch-Pit Observations of Dislocation Arrangements under Reverse Stress in Copper 9at. % Aluminum Alloy Single Crystals

With a view of solving the cause and essence of the Bauschinger effect, dislocation behaviour under reverse stress has been investigated in Cu-9at% Al alloy single crystals, using an etch-pitting technique. From the direct measurements of the beginnings of backward movements of dislocations, the frictional resistance force to moving dislocations due to solid solution hardening is estimated to be approximately 0.8kg/㎟, which is ≃4/5 of easy glide stress. It is found that pile-up dislocations against a barrier move well under the reverse stress range from 0.6 to 0.7 to the pre-stress level, but then hardly move more than the reverse stress ratio of 0.8. Evidence of almost complete annihilation of double ended pile-ups which are generated by the same source is presented. Another striking evidence of radical annihilation of dislocations within uniformly aligned dislocation groups of the same sign is also discovered. Mechanisms acceptable for explaining such results are proposed respectively, i.e., the mutual annihilation of dislocations of opposite signs, and the double cross-slip mechanism. It is suggested that the characteristics of rearrangements of dislocations against stress reversal are probably connected with the latter mechanism, which would be responsible for cyclic strain hardening

Dislocation Arrays and the Bauschinger Effect in Copper 9 at. % Aluminum Bicrystals

Dislocation arrays in lightly deformed Cu-9 at.% Al bicrystals, whose component crystals have a crystallographic mirror symmetry with respect to the grain boundary, are observed by the etch-pitting technique. Then, the influence of the grain boundary on the Bauschinger effect is discussed. It is shown that the induced secondary slips near the boundary can be explained quantitatively in terms of stress concentration due to the piled-up dislocations of adjacent crystal. Some of them can be also explained by the microscopic incompatibility owing to the mismatch of primary slip bands in each component crystal at the boundary. From the observations of reversed bicrystals, it becomes evident that the induced secondary disloca tions near the boundary, which exist mostly by forming double ended pile-ups between the primary slip bands, are unstable against reverse stress. Namely, most of them are annihilated in the first stage of reverse stressing, and are subsequently recovered by a further reverse stress. It is pointed out that although primary dislocations near the boundary can move even a small reverse stress backwards by the aid of high back stress on dislocations because of the latent hardening by multiple slips and pile-up dislocations, the mean free path of the primary dislocations is thought to be small due to the interference of secondary dislocations. Consequently, the Bauschinger effect in a multiple slip layer is presumably smaller than that in the center of each component crystal