By means of surface mechanical attrition treatment to a pure Fe plate, a nm-grained surface layer without porosity and contamination was fabricated. The average grain size in the top surface layer (5μm thick) was some 10 to 25nm, and the grain size stability could be maintained up to 653K. The Cr diffusion kinetics in the nanocrystalline Fe phase were measured by using secondary ion mass spectrometry at 573 to 653K. The experimental results showed that the diffusivity of Cr in the nanocrystalline Fe was 7 to 9 orders of magnitude higher than that in the Fe lattice and 4 to 5 orders of magnitude higher than that in the grain boundaries of α-Fe. The activation energy for Cr diffusion in the Fe nano-phase was comparable to that for grain boundary diffusion, but the pre-exponential factor was much higher. The enhanced diffusivity of Cr was suggested to originate from a large volume fraction of non-equilibrium grain boundaries and a considerable number of triple junctions in the present nanocrystalline Fe sample processed by means of the surface mechanical attrition treatment technique.

Diffusion of Chromium in Nanocrystalline Iron Produced by Means of Surface Mechanical Attrition Treatment. Z.B.Wang, N.R.Tao, W.P.Tong, J.Lu, K.Lu: Acta Materialia, 2003, 51[14], 4319-29

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