# Advanced Metal Matrix Nanocomposites
## 4 Dispersion
### Cavitation in Composite Systems
#### 4.2.2 Cavitation in Molten Metal-Nanoparticle Composite Systems
This work demonstrated that the number density of cavitation bubbles was increased two-fold when the nanoparticles were presented in the molten metal [120], implying that the introduction of nanoparticles provided a large number of heterogeneous nuclei for cavitation bubbles. It is then reasonable to hypothesize that the collapses of those cavitation bubbles may strongly impact the nearby clustered nanoparticles. Consequently, deagglomeration of those nanoparticle clusters may be achieved through ultrasonic transient cavitation. In addition, approximately 20% increased acoustic streaming or flow rate was observed in the Al-10 wt%Cu melt with an addition of 0.05 wt% Al\({}_{2}\)O\({}_{3}\) nanoparticles [120]. The higher flow velocity caused by the greater ultrasonic field may increase the turbulent nature of the molten nanocomposite system [120]. The increased agitation in the melt may contribute to the deagglomeration of nanoparticles though re-agglomeration might also happen depending on the local flow patterns [120]. Figure 15: Main mechanisms of ultrasonic deagglomeration of nanoparticle clusters: (**a**) collapse of cavitation bubble; and (**b**) acoustic streaming. Despite very few experiments having been reported on the in situ imaging of ultrasonic deagglomeration process, a great number of ex situ trials have been performed to study the effect of ultrasonic melt treatment on the deagglomeration of nanoparticle clustering. This is usually to compare the microstructure and/or mechanical properties of the solid nanocomposite at ambient temperature before and after ultrasonic melt treatment [116]. Microscopically, the reduced size of nanoparticle clusters or more homogenous distribution of nanoparticles in the resultant nanocomposite represents the effectiveness of the ultrasonic melt treatment. As a result, mechanical properties of the metal matrix nanocomposite are enhanced [8, 21]. In the nanocomposites processed by ultrasonic melt treatment, metal matrices are largely focused on aluminum [116, 117, 123, 124, 125, 126, 127, 128], magnesium alloys [129, 130, 6, 131], and transition metal alloys [8]. Typical nanoparticle reinforcers [132, 8, 133, 134] include SiC, Al\({}_{2}\)O\({}_{3}\), and various carbon polymorphs (e.g., graphene, carbon nanotubes, and diamond nanoparticles). During ultrasonic melt processing, nanoparticles, on the one hand, act as heterogeneous nucleation sites for cavitation bubbles and, on the other hand, alter the physical properties of the metal melt. The viscosity of the molten metal nanocomposite system increases significantly with the increase of the volume fraction of nanoparticles [135, 136], thus strongly impacting the ultrasonic melt processing as well as the dynamics of cavitation bubbles in molten metal nanocomposites. Typically, a limited volume fraction of nanoparticles (up to \(\sim\)5 vol% [21, 137]) in the molten metals can be effectively dispersed by the ultrasonic melt treatment [136, 137]. Prior studies also reported that the ultrasonically deagglomerated nanoparticles may re-agglomerate and then segregate after ultrasonic processing stops, mostly due to the attractive van der Waals forces between nanoparticles [137, 138]. In view of the aforementioned experimental challenges, it is difficult to achieve direct imaging about the dynamic impact of ultrasonic cavitation bubbles on the nanoparticle clusters in molten metals. Modeling is, thus, often utilized to provide in-depth understanding of this process. The effect of ultrasonic cavitation in the melt on deagglomeration and the rate of deagglomeration can then be studied symmetrically.