The effect of surface alloying on the plastic deformation behavior of copper

Abstract

The effect of surface-alloying on the mechanical properties and dislocation arrangements in high-purity copper single crystals was investigated. An alloyed region to 0.4–2 μ from the surface was created by limited diffusion of a given thickness of either nickel or gold surface films. Various surface concentration gradients resulted, with a corresponding gradient in elastic constants and lattice parameter. The critical resolved shear stress was increased for all the surface treatments. The maximum strength increase was about three times that of a pure copper crystal. Corresponding changes were also observed for the rate of work-hardening and extent of stage I deformation. In all cases, the surface-strengthening effects could be related to the surface concentration gradient. Several factors influencing the mechanical properties are operative, which include image forces, solid-solution hardening effects and interfacial misfit dislocation networks. An assessment of these contributors to surface-strengthening effects revealed solid-solution hardening as the most likely source, for the magnitude of the compositional gradients obtained in these experiments. Misfit dislocation networks were observed in the case of a gold solute, but were not associated with appreciable strengthening effects. It is expected that, for more severe surface concentration gradients than obtained in the present work, misfit dislocation networks would assume an importance equivalent to solid-solution hardening, at least for a similar range of lattice parameter mismatch.