XLVI. Anomaly in the rigidity modulus of copper alloys for small concentrations

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Abstract
In his recent theory of work hardening, Mott assumes that face-centred cubic crystals usually contain, prior to deformation, a network of dislocations, as suggested first by Frank; and he computes the decrease in the modulus of rigidity due to the existence of such a network. His computation may be extended to solid solutions, if one assumes the dislocations to be ‘pinned down’ by the solute atoms. Thus the addition of solute atoms will always at first increase the shear modulus, though larger concentrations may lead to a decrease. An anomaly of the type predicted has actually been observed by Bradfield and Pursey (1953) for small concentrations in copper alloys. One deduces from Bradfield's recent measurements an average length of about 10-5 cm for the dislocations of the network; and the small value of the anomaly for pure copper perhaps indicates that a large fraction of the dislocations are sessile. The anomaly has also been observed for silver alloys. It disappears after long annealing at 650°c, probably by formation of loose Cottrell clouds.

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