Enhancement of siliconizing behaviors in pure iron induced by surface mechanical attrition treatment

A gradient nanostructured surface layer of about 100 μm in thickness was synthesized on the pure iron induced by surface mechanical attrition treatment (SMAT). The average grain size in the top surface is about 20 nm. The formation behaviors of Fe-Si compound layer were investigated in nanostructured Fe during the electroless siliconizing process. In comparison with the coarse-grained (CG) sample, the thickness of compound layer on the SMAT sample is about twice larger than the CG sample and the diffusion activation energy decreased from 231 kJ/mol to 165 kJ/mol for the sample after SMAT 20 min. Evident inter-diffusion of elements such as Fe and Si could be found under the EDS analysis, suggested the achievement of metallurgical bonding. The porosity appeared at the interface of matrix and Fe3Si layer in the CG sample due to Kirkendall effect, however, the bonding of the compound layer and matrix was tighter due to the promotion of the elements diffusion in the SMAT sample. The enhanced siliconizing kinetics is attributed to numerous grain boundaries (GBs) in the nanostructured surface and deformed layer.