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Graphical abstract, Highlights • High-speed imaging captured de-agglomeration and dispersion of oxide aggregates. • De-agglomeration occurred from both bulk and surface of the aggregate. • Floating microbubble cluster drones produced fine suspension of loose agglomerates. • Ultrasonic capillary effect promoted de-agglomeration of particles from within. • Subharmonic and low order ultra-harmonic bubble emissions induced oxide breakage., Ultrasonic de-agglomeration and dispersion of oxides is important for a range of applications. In particular, in liquid metal, this is one of the ways to produce metal-matrix composites reinforced with micron and nano sized particles. The associated mechanism through which the de-agglomeration occurs has, however, only been conceptualized theoretically and not yet been validated with experimental observations. In this paper, the influence of ultrasonic cavitation on SiO2 and MgO agglomerates (commonly found in lightweight alloys as reinforcements) with individual particle sizes ranging between 0.5 and 10 μm was observed for the first time in-situ using high-speed imaging. Owing to the opacity of liquid metals, a de-agglomeration imaging experiment was carried out in de-ionised water with sequences captured at frame rates up to 50 kfps. In-situ observations were further accompanied by synchronised acoustic measurements using an advanced calibrated cavitometer, to reveal the effect of pressure amplitude arising from oscillating microbubbles on oxide de-agglomeration. Results showed that ultrasound-induced microbubble clusters pulsating chaotically, were predominantly responsible for the breakage and dispersion of oxide agglomerates. Such oscillating cavitation clusters were seen to capture the floating agglomerates resulting in their immediate disintegration. De-agglomeration of oxides occurred from both the surface and within the bulk of the aggregate. Microbubble clusters oscillating with associated emission frequencies at the subharmonic, 1st harmonic and low order ultra-harmonics of the driving frequency were deemed responsible for the breakage of the agglomerates.

Graphical abstract, Highlights • Mechanical properties of extracted intermetallics determined using a nano-indenter. • High speed imaging captures shock waves breaking intermetallics. • Shock waves seen to promote crack growth leading to fragmentation. • In-situ shock wave pressure measurements obtained using a fibre optic hydrophone. • Subharmonic emissions generate repetitive bending and fracture of intermetallics., One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al3Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20–40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation.

Continuous fiber reinforced metal matrix composites (MMCs) presents the architecture or arrangement of fiber in matrix materials that offer realization of the highest possible mechanical and thermal management properties from the combination. The highest forms of specific stiffness, strength, fatigue and creep resistance are realized in this form of composite compared to particulate and short fiber reinforced MMCs. This chapter presents the types of matrix and fiber materials that are commonly used for the fabrication of long fiber reinforced MMCS. It presents the factors that influence the choice of fiber and matrix for the fabrication and the type of reaction problems that can occur during the high temperature processing that is often required. The types of fabrication methods such as liquid, physical and chemical vapor deposition methods, matrix coated fiber, foil–fiber–foil, and more recently ultrasonic processes are highlighted. The initiatives that have been taken to mitigate deleterious interfacial reaction are discussed. The effects of fiber volume fraction and mismatch of the fiber and the matrix properties on residual stress generated during fabrication and its consequent effects on fatigue and creep are underscored. The potentials of the composite have not been fully realized due to high processing cost. This has reflected in the unstable interests and involvement of manufacturing companies in its production over the years. The chapter lists outstanding suppliers of fibers and long fiber MMCs. Demonstrator components mainly for aerospace and electric power distribution industries are highlighted. Electric power distribution Al/Alumina fiber MMC cables that has become a commercial success case in diverse parts of the world is also presented.