Microstructure and hardness of Al2O3 nanoparticle reinforced Al-Mg composites fabricated by reactive wetting and stir mixing

The use of reactive wetting and stir mixing to disperse nano-sized ceramic particles in molten metal was studied using Al2O3 nanoparticle reinforced Al-Cu-Mg composites that were synthesized using gravity casting in a permanent mold and a single sample synthesized by squeeze casting. These experiments have shown that reactive wetting of nanoparticles of Al2O3 in Al-Cu-Mg alloys, combined with stir mixing, can result in significant improvements in the hardness of gravity cast specimens provided that a reaction between the reinforcement and the matrix was either on-going or recently completed at the time of solidification. The results indicate that microstructure and strengthening mechanisms depend on the degree of clustering after reaction completion. It is hypothesized that a small amount of clustering resulted in particle engulfment and Orowan strengthening and increased clustering resulted in particle pushing and grain refinement strengthening described by the Hall-Petch relation. TEM analysis of a squeeze cast sample showed that individual particles and small flocculi were present within the grains rather than at grain boundaries, suggesting that the higher solidification rate of squeeze casting should follow particle pushing models and result in engulfment of larger particles than gravity casting. A comparison of yield stress or hardness variation with the inverse of the square root of grain size or dendrite arm spacing clearly shows the dominant strengthening mechanism as a result of the different process parameters. This method is also used to compare the results of this study to those of other studies. (C) 2011 Elsevier B.V. All rights reserved.