SURFACE HARDENING BY DIFFUSION IN COPPER SINGLE CRYSTALS

The effect of composition changes below the surface of copper single crystals on the critical-resolved-shear stress and on dislocation configuration near the surface was studied. Various gradients with a solute concentration decreasing from the surface were developed by diffusing platinum from an electroplated layer into copper crystals. Concomitant with the resulting concentration gradients are gradients of elastic constants and lattice parameters. For all surface treatments used in these experiments the critical-resolved-shear stress was increased; the highest value was found to be 360 g/mm2, which is about four times that measured for pure untreated crystals. The steeper the composition gradient near the surface the greater was the measured critical-resolved-shear stress. For most treatments the structural changes that accompanied the strengthening were identified as networks of accommodation dislocations in the subsurface region. The mesh size of these networks increased when the composition gradient was decreased. In evaluating the surface strengthening mechanism the effects of shear modulus, orientation of the crystal, and ordering were considered. From these experiments the prominent strengthening mechanism appears to be due to the impedance of glide dislocation movement by the acaommodation-dislocation network in the subsurface layer. © 1969.