# Advanced Metal Matrix Nanocomposites
## 4 Dispersion
### Cavitation in Composite Systems
#### 4.2.2 Cavitation in Molten Metal-Nanoparticle Composite Systems  
One of the greatest challenges in manufacturing metal matrix nanocomposites is to disperse nano-sized ceramic particles uniformly in molten metals. This is usually problematic because nanoparticles tend to agglomerate as clusters due to the high surface energy that leads to high van der Waals attractive forces between nanoparticles. Thus, deagglomerating nanoparticle clusters are of major significance to achieve homogeneous distribution of nanoparticles in the metal matrix. Liquid metal processing assisted by external ultrasound field has been demonstrated to be an efficient way to homogenize the nanoparticle distribution in molten metal [114]. This processing is followed by appropriate solidification, e.g., a sufficiently slow cooling rate [21], which can give rise to homogeneous microstructure in the nanocomposite. Thus, the optimum enhancement effect of nanoparticles on the mechanical properties of nanocomposite can be achieved [115]. The fundamental mechanism and process of deagglomeration of nanoparticle clusters in molten metal alloys via ultrasonic treatment has been studied extensively in both experiments and modeling. The two main hypotheses of the deagglomeration mechanisms are: high-intensity ultrasonic waves generate strong cavitation [116] and acoustic streaming effects in liquid metal [117]. Transient cavitation can produce an implosive impact strong enough to break up the nearby clustered nanoparticles and disperse them uniformly in the liquid metal [116] (see Figure 15a). For example, in an aluminum melt, such implosive impact can generate pressure (depending on the proximity of the imploding bubble to the nanoparticle cluster) ranging from 1 MPa to over 4 GPa; on average, 10\(\sim\)400 MPa [116; 118]. Moreover, acoustic streaming (a liquid flow due to an acoustic pressure gradient) is also very effective for stirring, which may contribute to the deagglomeration process (see Figure 15b). Thus far, there is no direct in situ imaging of the rupturing process of nanoparticle clusters in liquid metals during the ultrasonic cavitation process. There are many challenges; one is that liquid metals are usually optically opaque and the majority of metal alloys require relatively high temperature (above a few hundred degrees Celsius) to maintain liquidus structure; special techniques, such as X-ray imaging, are needed to \"see\" through the liquid metals. In addition, in situ observation requires imaging with nanoscale resolution and extremely high speed (up to on the order of \(10^{5}\) frames per second [119]) to be able to capture both activities of nanoparticle clusters and cavitation bubbles (with radii from a few to a few hundreds of micrometers [114; 120]). According to the Minnaert equation [121], the critical radius of a cavitation bubble in liquid aluminum is around 60-70 \(\upmu\)m when the acoustic pressure is sufficiently high [114]. Nevertheless, there are some in situ imaging experiments focused on the observation of cavitation bubble dynamics in molten metal with and without the presence of nanoparticles [120]. The influence of nanoparticles on the molten metal and cavitation bubbles can then be revealed through comparisons. In a recent experiment, the in situ synchrotron X-ray radiographic imaging of ultrasound cavitation in a molten Al-10 wt% Cu with and without the addition of Al\({}_{2}\)O\({}_{3}\) nanoparticles (0.05 wt%, mean size about 80 nm) was carried out [120]. As shown in Figure 16, references [114; 118; 120] show the experimental setup of ultrasonic processing of molten nanocomposite system mainly consists of a furnace, crucible, and an ultrasonic probe (integrated with an ultrasound processer) generating adequate processing power inside the melt [122].