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

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 μm 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 α-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.