Project C4: Casting technology for ODS steelsPI: Gunter Gerbeth (HZDR)
Partner: HZDR, KIT, IPUL
1. Scientific case for the project
1.1 BackgroundOxide dispersion strengthening (ODS) of high-Cr steels is a promising way to get materials for high-temperature applications. The mechanical and corrosive properties are significantly improved compared to present-day steels. ODS steels are a hot-topic for new energy-related materials, both for conventional as well as nuclear reactor structure materials. ODS steels contain homogeneously dispersed Yttria nanoparticles, typically up to 1 wt% of Y2O3 is distributed in the steel. The problem is: this material can up to now only be produced via a powder metallurgical route, which drastically limits the amount of material and makes it very expensive. Highly desirable would be a casting route for the manufacture of ODS steels.
The project will address basic problems of such a casting process. One approach might be a tailored electromagnetic stirring of a steel melt in order to submerge the nanoparticles. With the available knowledge on electromagnetically driven tornado-like flows  the aspect of submergence shall be no principle problem. However, it is very likely that the nanoparticles will agglomerate during this submergence process. It is difficult to disperse ceramic particles uniformly in liquid metals because of their poor wettability and their large surface-to-volume ratio, which easily induces agglomeration and clustering . Strong forces are needed to avoid this clustering. These forces have to overcome both the van-der Waals forces between the nanoparticles and rather strong capillary effects due to their small curvature radius. Very likely, a fluid mechanic melt mixing alone will be not sufficient for that purpose.
In case of nano-dispersed aluminum alloys the homogeneous distribution of nano-particles has been obtained by ultrasonic vibration of the melt. This approach has successfully been applied to metal matrix nanocomposite materials [2-4]. Ultrasonic excitation proved to be very efficient in producing fine-dispersed distributions of nano-particles in aluminum or magnesium based alloys. The interpretation is that the ultrasonic vibration leads to cavitation which involves formation, growth, pulsation and collapse of micro-bubbles. Thus, the high-intensity ultrasonic waves are assumed to generate transient cavitation and acoustic streaming which locally leads to high pressures and high temperatures during short pulses in the order of a few microseconds. Note that a deeper understanding of cavitation and collapsing bubble phenomena is subject of intense recent investigations [5-7].
Due to the much higher temperatures in case of steel, an ultrasonic approach seems not feasible. For the steel case a contactless quasi-ultrasonic treatment of the melt would be needed. This can, in principle, be obtained by electromagnetic vibration. This technique, first suggested by Vives [8,9] was till now mainly used for grain refinement during solidification [10,11]. The paper  reported on the development of a casting route for ODS steels, unfortunately without going into details how the dispersion of nano-particles has in detail been obtained.
1.2 Most important goals of the planned workThe far-reaching goal of the project consists in the development of a casting technology for ODS steels based on melt stirring and electromagnetic vibration. This implies to get a better understanding of cavitation phenomena in liquid metals and the contact-less electromagnetic generation of strong vibrational forces in metal melts. Therefore, the project will address basic problems of melt flow stirring and melt vibration. The works will involve studies on macroscopic melt stirring for the purpose of particle submergence and homogeneous distribution in the melt. The most important part consists in the problem if alternating electromagnetic forces can be designed in such a way that an intense vibration of the melt can be achieved, similar to the case of ultrasonic vibration. The possibility of creating electromagnetically induced pressure waves in a metal melt shall be investigated and developed in frame of the project.
In parallel, thermodynamic and kinetic calculations on ODS steels will be performed, and an experimental facility will be prepared allowing to test the electromagnetic vibration and stirring under controlled gas atmospheres for various types of ODS steels. The samples processed with and without electromagnetic treatment will then be characterized.
The project will demonstrate this electromagnetic vibration in a first step at copper melts since the electromagnetic forces are much stronger there and the necessary combination of magnetic fields is almost available. For the steel case higher field strengths will be needed, which shall be built up and tested after analyzing the copper results.
2. Existing competencies and infrastructureThe possibility for an intense stirring of metal melts, causing so-called tornado-like flows, has been developed at HZDR . A suitable ratio of azimuthal to radial-meriodional flow driving can result in a very intense vortex, which demonstrated a strong capability for the submergence of lighter particles. This flow driving can be obtained by a combination of rotating and travelling magnetic fields of rather low field strengths of a few milli-Tesla.
Combinations of magnetic AC and DC fields have already been studied [14,15] by HZDR and IPUL, but mainly for the purpose of a magnetic DC damping of AC-field driven flows. The idea of an AC-DC field combination for the generation of intense waves in a liquid metal has not yet been realized at the project partners.
KIT will contribute to the project since a lot of material related experience with ODS steels is available there . Prof. Möslang and Prof. Seifert are well-known experts on the research and development of ODS steels.
For the characterization of solidified samples modern techniques such as SEM, TEM, EDX, WDX are available both at KIT  and partly also at HZDR.
3. Resource planning and Budget JustificationDue to the complexity of the experiments an experienced researcher is needed at HZDR. One PhD student will be at IPUL in order to perform the described experiments. Another PhD student will be at KIT.
Links: There are close relations to projects A4, A5, C1 and C3.
References X. Li, Y. Yang, X. Cheng, 2004, Ultrasonic-assisted fabrication of metal matrix nanocomposites. J. Materials Science, Vol. 39, 3211-3212.
 Y. Yang, J. Lan, X. Li, 2004, Study on bulk aluminum matrix nano-composite fabricated by ultrasonic dispersion of nano-sized SiC particles in molten aluminum alloy. Materials Sci. Eng. A, Vol. 380, 378-383.
 M. De Cicco, L.S. Turng, X. Li, J.H. Perepezko, 2006, Semi-solid casting of metal matrix nanocomposites. Solid State Phenomena, Vol. 116-117, 478-483.
 X. Li, Y. Yang, D. Weiss, 2008, Theoretical and experimental study on ultrasonic dispersion of nanoparticles for strengthening cast Aluminum alloy A356. Met. Science and Technology, Vol. 26-2, 12-20.
 D. Obreschkow, M. Tinguely, N. Dorsaz, P. Kobel, A. De Bosset, M. Farhat, 2011, Universal scaling law for jets of collapsing bubbles. Phys. Rev. Lett., Vol. 107, 204501.
 Yu An, 2012, Nonlinear bubble dynamics of cavitation. Phys. Rev. E, Vol. 85, 016305.
 B. Karri, S.R.G. Avila, Y.C.Loke, S.J. O’Shea, E. Klaseboer, B.C. Khoo, C.D. Ohl, 2012, High-speed jetting and spray formation from bubble collapse. Phys. Rev. E, Vol. 85, 015303.
 C. Vives, 1998, Grain refinement in aluminum alloys by means of electromagnetic vibrations including cavitation phenomena. Journ. of Metals, Vol. 50, No. 2, 1-8.
 C. Vives, 1996, Effects of forced electromagnetic vibrations during the solidification of aluminum alloys. Metall. Mat. Trans., Vol. 27B, 457-464.
 Q. Wang, T. Momiyama, K. Iwai, S. Asai, 2000, Non-contact generation of intense compression waves in a molten metal by using a high magnetic field. Mat. Transactions, Vol. 41, 1034-1039.
 B. Wang, Y. Yang, X. Ma, W. Tong, 2010, Simulation of electromagnetic-flow fields in Mg melt under pulsed magnetic field. Trans. Nonferrous Met. Soc. China, Vol. 20, 283-288.
 K. Verhiest, A. Almaouzi, N. de Wispelaere, R. Petrov, S. Claessens, 2009, Development of oxides dispersion strengthened steels for high temperature nuclear reactor applications. J. Nucl. Materials, Vol. 385, 308-311.
 I. Grants, C. Zhang, S. Eckert, G. Gerbeth, 2008, Experimental observation of swirl accumulation in a magnetically driven flow. J. Fluid Mech., Vol. 616, 135-152.
 A. Bojarevics, A. Cramer, Yu. Gelfgat, G. Gerbeth, 2006, Experiments on the magnetic damping of an inductively stirred liquid metal flow. Experiments in Fluids, Vol. 40, 257-266.
 I. Grants, A. Pedchenko, G. Gerbeth, 2006, Experimental study of the suppression of Rayleigh-Benard instability in a cylinder by combined rotating and steady magnetic fields. Phys. Fluids, Vol. 18, 124104.
 A. Möslang, Th. Wiss, 2006, Materials for Energy – From Fission towards Fusion. Nature Materials, Vol. 5, 679-680.
 A. Schlieter, U. Kühn, J. Eckert, H. J. Seifert, 2010, Microstructure, thermal, and mechanical characterization of rapidly solidified high strength Fe84.3Cr4.3Mo4.6V2.2C4.6. , J. Mater. Res., Vol. 25, 1164-1171.