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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2011-12, Vol.530, p.304-314
• Main Authors: Chang, H.-W., Kelly, P.M., Shi, Y.-N., Zhang, M.-X.
• Format: Article
• Language: English
• Subjects: Alloys; Aluminum; Aluminum base alloys; Al–Si cast alloy; Applied sciences; Cross-disciplinary physics: materials science; rheology; Dislocation density; Dislocation gliding; Dislocations; Exact sciences and technology; Grain refinement; Growth from solid phases (including multiphase diffusion and recrystallization); Intermetallic compounds; Materials science; Metals. Metallurgy; Methods of crystal growth; physics of crystal growth; Nanocrystalline material; Nanocrystalline materials; Nanocrystals; Nanoscale materials and structures: fabrication and characterization; Nanostructure; Physics; Production techniques; Silicon; SMAT; Surface treatment
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2011.09.090

► During surface mechanical attrition (SMAT) process of Al–Si alloy, unlike Al matrix, the eutectic Si is refined through direct breakage. ► The refined Si particles, in turn, promote grain refining of the Al matrix. Thus, the nanocrystalline grains in the A356 alloy are much smaller. ► Under the present SMAT conditions, for a particular alloy the grains within the top nanocrystalline region cannot be refined indefinitely. ► The minimum grain size not only depends on the nature of the alloys, but also relies on the strain rate applied to the alloy. 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.