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YSTALLINE metals have been recognized as an important class of materials with the potential of achieving properties that coarse-grained metals usually do not possess. One major difference between nanocrystalline materials and the conventional polycrystalline materials is the increased volume fraction of grain boundaries, which can act as fast atom diffusion channels. Therefore, the atomic diffusion in nanocrystalline materials is expected to be much faster relative to their coarse-grained polycrystalline counterparts. This hypothesis has been supported by the experimental results reported in the literature.[1–11] Kolobov et al.[4] reported that grain boundary diffusivities of Cu in a nanocrystalline Ni (with a gain size of about 300 nm, synthesized by means of severe plastic deformation) are about four to five orders of magnitude higher than those in the coarse-grained Ni. Wang et al.[9] proved that the diffusivity of Cr in a nanocrystalline Fe (with a gain size of about 10 nm, produced by surface mechanical attrition treatment (SMAT)) is seven H.-W. CHANG, Researcher, P.M. KELLY, and M.-X. ZHANG, Professors, are with the Division of Materials, School of Mechanical and Mining Engineering, University of Queensland, St Lucia, Brisbane, QLD 4072, Australia. Contact e-mail: mingxing.zhang@ uq.edu.au Y.-N. SHI, Scientist, is with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Manuscript submitted May 29, 2011. Article published online April 14, 2012 2378—VOLUME 43A, JULY 2012

to nine orders of magnitude higher than that in Fe lattice and four to five orders of magnitude higher than that in the grain boundaries of a-Fe. For the case of interstitial atom diffusion in nanocrystalline materials, the diffusivity of H along the grain boundaries in nanostructured Ni was significantly increased[6] and the diffusivity of N was strongly enhanced in steel and greatly reduced the iron nitriding temperature to 573 K (300 C).[10,12] However, to date, the reported experimental data on atomic diffusion in nanocrystalline materials are still very limited and scattering. Furthermore, most of the previous experimental studies were performed on consolidated nanocrystalline samples from ultrafine powders, in which nanometer-sized pores as well as gaseous contaminations could not be avoided. This type of defects can also promote atomic diffusion. This increases the complexity in understanding the diffusive performance of atoms in nanocrystalline materials. Relatively, surface mechanical attrition treatment (SMAT) is regarded as an advanced technique to produce nanocrystalline layers on metals without introducing high density pores and contamination.[13,14] Thus, it is more suitable to investigate the effect of nanostructure on atomic diffusion kinetics.[9–11] In the present work, Zn powder will be deposited on the SMATed pure Al and A356 substrates using kinetic metallization (KM) technology, which is a special type of cold spray operating at sonic spray velocity