Catalytic Effect of Nanoparticles on Primary and Secondary Phase Nucleation

Abstract

Nanoparticles were shown to catalyze nucleation of primary and secondary phases in metal matrix nanocomposites (MMNCs). This catalysis is important as it contributes to the mechanical property enhancement in the MMNCs. Primary aluminium grain refinement was demonstrated in A356 matrix nanocomposites. Various types and sizes of nanoparticles (SiC, TiC, γ-Al2O3; 10-96 nm) were used to make these MMNCs and in all cases the MMNCs had smaller, more equiaxed grains compared to the reference A356. Using the droplet emulsion technique, undercoolings were shown to be significantly reduced. Undercoolings in the MMNCs were in good general agreement with the undercooling necessary for free growth, suggesting the applicability of this model to nucleation on nanoscale catalysts. Secondary phase nucleation catalysis was demonstrated in a zinc alloy AC43A MMNC and a binary Mg-4Zn MMNC. In AC43A, secondary phase nucleation was catalyzed with the addition of various nanoparticles (TiC, SiC, γ-Al2O3). The secondary phase nucleation catalysis in AC43A coincided with ductility enhancement. In Mg-4Zn, SiC nanoparticle addition changed the secondary phases that formed. MgZn2 was formed in the MMNC at relatively high temperatures consuming the Zn and reducing the amount of the low temperature Mg2Zn3 phase that formed in the reference alloy. The change in secondary phase formation coincided with significant enhancement in strength and ductility.