Plasticity in β phase Cu-Zn-Al alloys

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Abstract
The β Cu-Zn-Al alloys undergo pseudo-elastic martensitic transformations or plastic deformations under an applied stress depending on the temperature and alloy composition. It is of particular interest to study the interactions between both deformation modes. In this paper a detailed study of the plastic deformation in the β phase of Cu-Zn-Al alloys is carried out around room temperature. The stress-strain behaviour shows that the addition of Al increases the critical resolved shear stress for plastic deformation compared with that of binary Cu-Zn. Optical and electron microscopy observations show that the slip direction is [111] and the glide planes are (112) in the twinning sense or a combination of {123}-type planes when the (112) is no longer operative. In no case is slip observed on the (101) or on the (211) antitwinning plane. The effect of the L21 long-range ordering on the dislocation characteristics is also investigated. In particular, it is found that the superlattice dislocations split into partials. A pair of dislocations is observed separated by a distance of about 3 nm and joined by a ribbon of nearest-neighbour antiphase boundary (APB). The dislocation pairs move as in a B2-type superlattice dragging next-nearest-neighbour (NNN) APBs because of the low value of the NNN APB energy compared with the applied stress multiplied by the Burgers vector.