材料期刊网

A graded microstructure was produced in the surface layer of a pure Cu sample by means of surface mechanical attrition treatment (SMAT) [Wang K, Tao NR, Liu G, Lu J, Lu K. Acta Mater 2006;54:5281.]. The diffusion behavior of (63)Ni in such a surface layer was investigated by the radiotracer technique at temperatures <438 K. It is shown that the effective diffusivity in the top 10 mu m surface layer is more than 2 orders of magnitude higher than that along conventional high-angle grain boundaries (HAGB) in Cu of similar purity. The diffusion rate increases gradually with increasing depth up to 30-50 mu m, 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 HAGB in this layer is estimated to be similar to 30% higher than that of conventional grain boundaries. An apparent retardation of the effective diffusion rate in the top 25 mu m surface layer is 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 50 mu m. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.