Dislocations in Deformed Cu–Al Single Crystals

Abstract

The distributions, arrangement and density of dislocations in single crystals of Cu–Al alloys deformed in tension are studied by means of the etch pit technique. The observations of etch pits are made on the surfaces parallel to the (1\bar1\bar1) plane or the surfaces cut out parallel to the (11\bar1) and (111) planes of each specimen where the (11\bar1) plane is the primary slip plane.
As the Al concentration increases the multiplied dislocations tend to arrange along the traces of the intersections between the observed surface and the slip planes. The distribution of the primary dislocations observed on the {111} plane other than the (11\bar1) plane changes from the Cu type to the Lüder’s band propagation type as the Al concentration is increased.
On the (11\bar1) plane the dislocations lying on the (1\bar1\bar1) plane are observed early in stage I, but the dislocations lying on the (1\bar11) and (111) planes are multiplied inhomogeneously in the transition region from stage I to stage II . These results may suggest the formation of the Lomer-Cottrell dislocations at this strain.
The density of the dislocations observed on the (111) plane is higher than that on the (11\bar1) plane, but the difference is lessened with increasing strain. In the Cu-2.5 at% Al composition, the density of dislocations observed on these two planes is of the same order at the beginning of stage II, while in the Cu-15 at% Al composition there is a larger difference between them even at the largest stress applied.
In stage I the dislocation density is proportional to the shear stress. In stage II the square root of the dislocation density is proportional to the shear stress. This relation holds well not only for the primary dislocation density but also for the secondary one. And in the equation τ=αGbN^1⁄2, the numerical constant, α, is found to be approximately 0.55 for the Cu–Al alloys of various Al concentrations.