Microstructural evolution and mechanical properties of the AA2219/TiC nanocomposite manufactured by ultrasonic solidification

Abstract

The mechanical properties of metallic matrix composites can be tailored through the microstructural modification between metal matrix and reinforcing particles. In this work, the AA2219/TiC nanocomposite containing 0, 0.5, 0.9, 1.3 and 1.7 wt.% TiC nanoparticles were individually manufactured using the ultrasound-assisted solidification technique. There existed an optimal addition level of TiC nanoparticles (0.9 wt.%) beyond which grain coarsening occurred. TiC nanoparticles were observed to reduce the average grain sizes of α-Al matrix alloy by 61%. Meanwhile, 0.9 wt.% TiC nanoparticles corresponded to the maximum tensile strength and hardness. When the addition level reached over 0.9 wt.% in the 2219 Al alloy matrix melt, the TiC nanoparticles tended to agglomerate in a form of condensed clusters, which cannot be well dispersed by ultrasonication due to the increasing viscosity. Such agglomerating TiC nanoparticles deteriorated the tensile strain and hardness of AA2219/TiC nanocomposite. Then, five major potential strengthening mechanisms in AA2219/TiC nanocomposite was quantitatively analysed, including grain refinement, Orowan strengthening, load transfer effect, mismatch of thermal expansion, and mismatch of elastic modulus. Analysis shows that Orowan strengthening acts as the most dominant strengthening mechanism, followed by another comparable strengthening mechanism from the mismatch between the thermal expansion of α-Al matrix and TiC nanoparticles. The other three strengthening mechanisms induced very limited improvement in mechanical properties. Besides acting as reinforcing agents, the TiC nanoparticles also possess very high potency as heterogeneous nucleation sites. The relevant nucleation potency was validated by crystallographic investigation using the edge-to-edge matching model. Finally, the grain refining mechanism of α-Al matrix alloy was unraveled in terms of the nucleation crystallography and the ultrasonic cavitation.