A graded microstructure was produced in the surface layer of a pure Cu sample by means of surface mechanical attrition treatment (SMAT). The diffusion behavior of 63Ni in such a surface layer was investigated by the radiotracer technique at temperatures <438K. It was shown that the effective diffusivity in the top 10 μm surface layer was more than 2 orders of magnitude higher than that along conventional high-angle grain boundaries in Cu of similar purity. The diffusion rate increases gradually with increasing depth up to 30–50μm, and then decreases with further increasing depth. The enhanced diffusivities reveal higher-energy states of various interfaces in the SMAT surface layer. The excess free energy of high-angle grain boundaries in this layer was estimated to be some 30% higher than that of conventional grain boundaries. An apparent retardation of the effective diffusion rate in the top 25 μm surface layer was induced by tracer leakage into numerous twin-boundary-like interfaces, while the gradual decrease in interface excess free energy correlates with the observed decrease in diffusivity in the subsurface layer at depths exceeding 50μm.

Interfacial Diffusion in Cu with a Gradient Nanostructured Surface Layer. Z.B.Wang, K.Lu, G.Wilde, S.V.Divinski: Acta Materialia, 2010, 58[7], 2376-86