Effects of surface films on the deformation of metals

Abstract

The effect of a thin coating on the mechanical deformation of a single crystal wire is found to be an increase in the stress required for the initiation of easy glide; the development of pseudo-elastic defor mation prior to easy glide, during which the sample deforms as an hollow elastic cylinder with a plastic core; and the concentration of slip at relatively few slip lines. A dislocation model used to explain these observations involves the generation of dislocations by the action of sources of various initial lengths, l, with a distribution of lengths, N( l) dl, assumed to be inversely proportional to their lengths. The increase in yield strength is due to the barrier effect of the coating and the axial strain of the sample, ε x , is shown to be approximately proportional to the applied stress, expressed as resolved shear stress, τ r , in the slip direction on the slip plane. A principal feature of the dislocation model is that the critical shear stress, τ c , required for piled-up dislocations to penetrate the interfacial barrier is proportional to nτ r , where n is the number of dislocations in the pile-up. This accounts for the observation that under the level of stress where dislocation break through begins, only those few sources for which nτ r , > τ c will be able to penetrate, and consequently slip lines are reduced in number and slip is concentrated. It is also pointed out that the initial distribution of source lengths is a factor in the effect of a coating on the mechanical properties of a metallic crystal.