Diffusion of chromium in nanocrystalline iron produced by means of surface mechanical attrition treatment

By means of surface mechanical attrition treatment (SMAT) to a pure iron plate, a nanometer-grained surface layer without porosity and contamination was fabricated. The average grain size in the top surface layer (of 5 mum thick) is about 10-25 nm, and the grain size stability can be maintained up to 653 K. Cr diffusion kinetics in the nanocrystalline Fe phase was measured by using second ion mass spectrometry within a temperature range of 573-653 K. Experimental results showed that diffusivity of Cr in the nanocrystalline Fe is 7-9 orders of magnitude higher than that in Fe lattice and 4-5 orders of magnitude higher than that in the grain boundaries (GBs) of alpha-Fe. The activation energy for Cr diffusion in the Fe nanophase is comparable to that of the GB diffusion, but the pre-exponential factor is much higher. The enhanced diffusivity of Cr may originate from a large volume fraction of non-equilibrium GBs and a considerable amount of triple junctions in the present nanocrystalline Fe sample processed by means of the SMAT technique. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.