Effect of eutectic Si on surface nanocrystallization of Al-Si alloys by surface mechanical attrition treatment

Pure Al (AA196) and Al–Si (A356) alloy were subjected to surface nanocrystallization through surface mechanical attrition treatment (SMAT). Strain induced microstructure evolution, including grain refinement of Al matrix and Si particles, was examined using transmission electron microscopy. Nanocrystallization of Al matrix in both pure Al and A356 alloy occurs through formation of dislocation cells separated by dense dislocation walls and dislocation tangle within the original coarse grains or subdivided subgrains. During SMAT, the brittle Si phase is preferentially refined through direct breakage. The Al matrix refinement process is greatly facilitated by the dispersed small Si particles, because such particles can not only act as obstacles for dislocation movement, but also promote dislocation generation and multiplication. Comparing to pure Al, thicker nanocrystalline zone can be obtained with much smaller stabilized nano-metered grains in the Al–Si alloy. Due to the low diffusion rate of Si in Al matrix at low temperature, there is no obvious re-dissolve of Si particles into Al during SMAT process