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The effect of the annealing twin boundary on plastic deformation of fee metals was investigated by comparing the deformation behaviour of copper crystals containing a twin band with that of single crystals. As deformation proceeds, dislocations pile-up against the twin boundary and the slip mode near the boundary is changed from that in the centre of the grain. Therefore, the flow stress of a crystal containing a twin band is larger than that of a single crystal. The twin boundary acts as a barrier against the dislocation motion and the effect of the annealing twin boundary on plastic deformation of copper crystals is thought to resemble that of the grain boundary. Especially, it is pointed out that the annealing twin boundary must be treated in the same way as the grain boundary with respect to the Hall-Petch relationship.

initially rapid hardening and subsequently, after about several per cent elongation, the tensile stressstrain curve becomes very flat until failure. It was reported by the present authors that propagation of clustered slip accompanied by prominent cross slip was observed in this flat region of the curve. frequently with increasing deformation temperature. However, it was found that the frequency of prominent cross slipping in the copper crystal was much fewer than that in aluminum crystal, and the flat region of the stress-strain curve was not observed. The difference in deformation behavior between copper and aluminum crystals is considered to be due to the difference in stacking fault energy between these two metals.

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