(1) Interfacial diffusion behaviors in nanostructured metallic materials

Within the temperature range of 300-380°C, diffusivities of Cr in the nanostructured Fe produced by the Surface mechanical attrition treatment (SMAT) were measured to be 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 a-Fe (Acta Materialia 2003;51:4319).

Interfacial diffusion behaviors of ⁶³Ni in the SMAT Cu with a gradient nanostructured surface layer have been studied from room temperature to 165°C. The results also showed much enhanced diffusivities in the nanostructured Cu. The diffusivities along GBs and twin boundaries (TBs) were determined and GB energy of in the nanostructured Cu was derived to be ~30% higher than that of the conventional GB energy, as shown in Fig. 1. The consistent analysis of microstrain indicates that the significantly enhanced diffusivity is induced by a pronounced non-equilibrium state of the interfaces in the SMAT surface layer. The excess free energy value of high-angle GBs (HAGBs) in the SMAT sample is estimated to be about 30 % higher than that of the conventional (relaxed) HAGBs (Acta Materialia 2010;58:2376; Applied Physics Letters 2008;93:131904; Scripta Materialia 2011;64:1055).

GB and TB diffusion in a nanostructured Cu produced by means of dynamic plastic deformation at liquid nitrogen temperature (LNT-DPD) was also studied. As shown in Fig. 2, The results showed that the diffusivities along them were much lower than those in the SMAT Cu and comparable to those of conventional values (Acta Materialia 2011;59:1818).

(2) Austenitization process in a nanostructured ferritic steel

The austenitization processes of ferrite and carbides were investigated in the surface layers of a SMAT ferritic steel sample. Experimental results showed that the onset temperature of the austenitization is ~120°C lower in the top SMAT surface layer compared with that in the original sample. In addition, the two-step austenitization process in the surface layers became a one-step one when the mean size of carbide particles was smaller than 20 nm. The refined microstructure accelerated the austenitization process in the SMAT sample from two aspects: to promote nucleation and growth rates of austenite grains by providing more nucleation sites and fast-diffusion channels, and to increase the driving force for the transformation from ferrite to austenite with a higher stored energy. The latter might be more significant in the top surface layer with extremely fine carbide particles. Upon annealing, ferrite grains coarsened with increasing temperature below 700°C. However, further increasing temperature led to an obvious reduction of ferrite grain sizes, as shown in Fig. 3. The refining process of ferrite grains within 700-900 °C was believed to be related with the accelerated phase transformations in the nanostructured steel, i.e. the formation of nano-grained austenite from ferrite and carbides during heating and the decomposition of austenite grains into refined ferrite and carbide grains during cooling (Acta Materialia 2011;59:3710; Journal of Materials Science & Technology, 2012;28:41).

(3) Lower-temperature nitriding, chromizing, aluminizing, and galvannealing behaviors of nanostructured steels

With the help of SMAT, the Cr-penetration depth and compounds formation abilities were increased markedly in the surface layer of the SMAT low carbon steel and alloyed steel, due to numerous grain boundaries with a high excess stored energy in the nanostructured surface layer. The SMAT samples chromized at lower temperatures possess evidently enhanced corrosion and wear resistance (Acta Materialia 2005;53:2081; Surface & Coatings Technology 2006;201:2796; Material Science & Engineering A 2010;527:995).

In comparison with the coarse-grained (CG) Fe, the Fe-Zn reaction in the nanostructured Fe showed a decreased onset temperature by ~21°C and an increased enthalpy change by ~70 %. The activation energy for the growth of Fe-Zn compound layer was decreased from ~167.1 kJ/mol in the CG sample to ~108.0 kJ/mol in the SMAT sample. Fig. 4 shows a much thicker Fe-Zn compound layer formed on the SMAT sample than on the CG sample after a same diffusion treatment (Acta Materialia, 2012).