Comparison of strengthening in wire-drawn or rolled Cu-20% Nb with a dislocation accumulation model

Strengthening after large deformations by wire-drawing or rolling of Cu, Nb and Cu-20% Nb was compared with the predictions of a proposed modified substructural strengthening model for ductile two-phase alloys. The comparisons indicate that the more extensive and refined model of Funkenbusch and Courtney offers no improvement over the original model of Ashby in predicting the strengthening with increased deformation processing or the dislocation densities necessary to produce the observed strengthening in Cu-20% Nb. Both models can predict the strengthening behaviour of Cu-20% Nb. However, neither model is in accord with the observations that the dislocation density in the Cu matrix is essentially independent of the degree of deformation processing, and that the magnitudes of the dislocation density are much the same in the Cu in Cu-20% Nb and pure Cu identically deformation-processed. In addition, there is no experimental support for the Funkenbusch and Courtney model prediction of an order of magnitude greater dislocation density in the Nb filaments than in the Cu matrix in Cu-20% Nb. It appears that a mechanism that does not require an accumulation of dislocations for strengthening, such as the difficulty in propagating dislocations between closely spaced barriers, is more likely to be responsible for strengthening in Cu-Nb-type deformation-processed composites.