Liquid metal route for creation of Metal Matrix Composites (MMC)
It is possible to change and improve the mechanical, physical and chemical properties of a metal by adding nonmetallic nano-sized particles and thus creating a metal matrix composite (MMC). Currently the focus is mostly on light metals, i.e. aluminum (Al), magnesium (Mg) and titanium (Ti) and their alloys which are produced as composite materials with the inclusions ranging from different sized particles to fibers and applied practically. In the meantime, development of steel – the most consumed metal in the world – MMCs is limited due to higher working temperatures and harsher working environment which the particles have to sustain, however improving the properties of this alloy would be of the highest interest for both - industry and consumers. Already two important composites have been identified - the oxide dispersion strengthened (ODS) steels, which have been proven of great value in nuclear fusion and fission systems because of higher tolerance against radiation, higher operational temperature and creep resistance than standard steels, and high modulus steels (HMS) which aim in reducing the weight while increasing the stiffness, ductility and strength of the material.
Currently it is only possible to create steel composites using the expensive powder metallurgy route which limits the production to very small volumes. A simpler solution how to disperse micro sized particles is by using mechanical stirring, but this method doesn’t apply for nanoparticles because the specific surface area is much higher. Problems can also arise from weak particle wetting which supports the creation of particle clusters. A rather inexpensive solution is ultrasonic treatment of the melt – cavitation created by ultrasound can break particle clusters, clean the particle surface and support the dispersion It has been shown that the collapse of bubbles can create “hot spots” with effective temperatures of 5000 K, pressures of 1000 atm, and heating and cooling rates above 1010 K/s.
Due to the high melting temperature of the steel direct introduction of the ultrasonic probe into the melt is not possible. Therefore, for industrial applications a contactless ultrasonic treatment of the melt is needed which can be achieved by electromagnetic vibration – a steady axial magnetic field is applied to a liquid metal heated by electromagnetic induction. Superimposing stationary axial and alternating magnetic field
where the alternating component induces currents
leads to a creation of radial force (as shown in the Video 1.) and successive vibration of the melt
Observation of cavitation
The acoustic spectrum of a cavitation is characterized by sub-harmonics of the drive frequency fn = f0/n, n = 2, 3… as well as ultra-harmonics mf0 ± fn, m = 1, 2… emitted as a sound signal by the collapsing bubbles. To be sure that cavitation conditions are reached, this signal is recorded using 4 piezo elements. By applying the radial force, cavitation phenomena has been observed in different materials, both with particles and without. The recorded sound spectra of different metals are shown in Figure 1, while results with mixture of steel and different particles can be seen in Figure 2. However, the occurrence of cavitation cannot be defined by a precise threshold as it depends on uncontrollable factor, i.e. amount of nuclei sites – impurities and dissolved gas in the melt. This makes it difficult to “control” as shown by the intermittence of the cavitation signal in the graphs.
Figure 1. Temporal dependency of the sound power spectrum in case of (a) tin; (b) zinc; and (c) aluminum. Depth of the logarithmic color palette is 40 dB. Instants of AC and steady magnetic field variation are marked by black and white vertical lines with labels, respectively. First signs of cavitation are marked by circles. Insets show the corresponding instant spectrum. The beginning of a continuous cavitation signal is pointed to by an arrow. 
Figure 2. Temporal dependency of the sound power spectrum in case of steel and particles – CeO2 (a), TiB2 (b) and TiN (c) aluminum. Depth of the logarithmic color palette is 80 dB. Insets show the corresponding instant spectrum.
Effects of cavitation
It is not yet possible to assess the feasibility of the proposed method as the particle dispersion is a highly complex problem which is dependent not only of the strength of applied forces (e.g. higher DC field doesn’t yield better separation of particles), but also on the physical and chemical compatibility between the selected particles and metal matrix. For example, Ti-compounds (nitrides, carbides, borides) show similar characteristics by transforming to eutectic form when exposed to the liquid steel environment for extended periods as well as similar dispersion properties as shown in Figure 3 and Figure 4, while oxides (CeO2 for example) tend to agglomerate with MnS phase (Figure 5). Additionally, more cavities can be found inside the matrix with different shapes as seen in Figure 5 (d) as bright spots. While the smallest, because of being spherical, can be related to gas inclusions and provide hints of the nuclei size for production of cavitation, the larger ones have particle like shapes. If the particles are removed during sample preparation it leads to believe that the bounding between matrix and particle is not strong enough.
Figure 3. Overview of TiC (left) and TiB2 (right) microstructure. Micrographs reveal that the inclusion sizes vary between edge (a,b) and matrix (c,d). TiC (a) experiences much larger structures on the edge of the sample, however, the difference diminishes in the matrix. Arrows in (d) indicate the TiB2 phases which can be found both attached to MgS phase (colored black) or separately in the matrix (dark gray).
Figure 4. EDS analysis of a TiN inclusions (a) shows that TiN (b) compounds in the matrix can be found either separately (denoted with black arrows in (a)) or together with MgS (c,d) phase (white arrow)
Figure 5. EDS analysis of a CeO2 particle cluster (a) showing that Ce phase (b) is forming in clusters with MgS (c) phase. Micrograph (d) allows to see the amount of cavities (bright phase) inside the matrix
Currently the focus of the project is to find the right steel/particle pairs which would have a physical and chemical compatibility. Additionally, an experiment in a superconducting magnet in the Dresden High Magnetic Field Laboratory is under construction. This will allow to do experiments in a DC field up to 18T for extended period. The aim is to find the threshold value when a continuous excitation of cavitation can be reached in various melts. Additionally it will allow to determine how increased strength of melt vibration will influence the particle separation.
The financial support of the Helmholtz Alliance “Liquid Metal Technologies – LIMTECH” is gratefully acknowledged.
 Sarma, M., Grants, I., Kaldre, I., Bojarevics, A., Gerbeth, G.
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