Chair of

High Temperature Materials

The aim of the project is to develop and qualify dispersoid-reinforced high-temperature resistant alloys for use as potential structural materials in the aerospace industry. Increased efficiency through increased operating temperatures and reduced weight lead to improved efficiency of turbines.
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V-Si-B alloys have been the focus of scientific material development for several years. Due to their outstanding specific mechanical properties, these alloys represent a promising alternative to Ni and Mo-based materials in the field of high-temperature alloys. In terms of its microstructure, the V-Si-B alloy system has some interesting similarities with the well-studied Mo-Si-B sister system. Both alloy systems form a ternary eutectic in the metal-rich region (e.g. vanadium) from a solid solution, V(Mk), and the two intermetallic phases V3Si and V5SiB2. Directional solidification allows the eutectic to be "grown" along the direction of solidification, which results in a strong directional dependence of the resulting mechanical properties (strength, creep resistance). Similar to Ni-based superalloys, these could be specifically adjusted for an application-relevant load case. The proposed project investigates the microstructure formation and the resulting properties (direction-dependent strength and creep properties) of directionally solidified, novel eutectic V-Si-B alloys. For this purpose, the zone melting process will be investigated and analyzed both ex-situ and the direct transition from the liquid to the solid phase at the moment of directional solidification in-situ.
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Metallic materials for applications as structural materials, e.g. in corrosive environments at different temperatures, must have a wide range of properties. By adding alloying elements, the properties can be influenced over a wide range. For example, the strength of molybdenum materials can be significantly increased by adding even small amounts of silicon. Other properties, such as tribological abrasion, the oxidation or corrosion rate and cyclic strength, are also heavily dependent on the selection, concentration and combination of alloying elements. In addition, the heat treatment condition of the alloys plays a major role in adjusting the range of properties to suit the application. For materials in the medical sector, e.g. implant materials, properties under varying stress conditions (cyclic loading) also play a decisive role. The aim of this project is to modify materials in such a way that hardness and wear resistance are increased and static and cyclic stress resistance is improved without reducing oxidation and corrosion resistance. The microstructure-property relationships are specifically influenced in order to create optimum conditions for the subsequent application.
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Vanadium silicide materials represent a potential alternative to currently used high-temperature materials, in particular due to their excellent specific mechanical properties. For example, V-Si-B alloys from the vanadium-rich region of the three-material system consist of a ductile vanadium solid solution (V-Mk) and the two intermetallic phases V₃Si and V₅SiB₂. However, this alloy system, which has been little researched to date, has some surprising similarities to its well-studied neighboring system Mo-Si-B in terms of microstructure. In initial preliminary tests on V-Si-B alloys, significantly better specific compressive strengths were achieved in the temperature range from 600 °C to 900 °C compared to Ni-based alloys. However, the mechanism of phase formation and the correlation of the microstructure-property relationships are still completely unexplored. The primary objective of this project is the development of novel V-Si-B alloys for high temperature applications. The aim is to develop ternary-eutectic alloys. In a series of V-rich binary and ternary test alloys, the phase formation and stability from the melt to the homogenized microstructure will be investigated. In the 2nd funding phase, research will focus on the significance of the newly discovered phase V8SiB4.
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The focus of the project is to gain a comprehensive understanding of refractory metal-based RM-Si-B systems. This includes phase formation and transformation during solidification, as well as phase stability and transformations in the equilibrium state. Research is being carried out specifically into ternary eutectics in the metal-rich part of the RM-Si-B system. For this purpose, the chemical compositions of the phases involved are identified by means of thermodynamic calculations and validated experimentally (e.g. by means of WDX or microprobe measurements). Ternary eutectics are considered advantageous with regard to their lowest melting point for the alloy range and the special mechanical properties associated with the microstructure. Furthermore, the eutectic microstructure can be easily controlled via the (process-dependent) cooling conditions and thus have a targeted influence on the mechanical properties of such alloys. This can be achieved, for example, by directional solidification of such RM-based eutectic systems. The aim is to develop RM-Si-B alloys with improved specific strength properties at temperatures between 600 °C and 1500 °C (possible application window for eutectic RM-Si-B systems) compared to Ni-based alloys. The scientific work focuses in particular on Mo- and V-based alloy systems.

Similar to Mo-Si-B materials, a technical application of, for example, vanadium silicide alloys with around 30 to 70% V(MK) phase and complementary silicide phases is the most promising and likely. A precise understanding of the microstructure-property relationships in combination with the thermodynamics of RM-rich RM-Si-B systems is therefore essential and a holistic approach to material development is being pursued. This includes alloy selection and material synthesis (electric arc furnace, directional solidification, heat treatments), characterization of microstructural evolution and mechanical properties (temperature-dependent compression and creep tests), and the development of effective oxidation mechanisms (via preceramic polymers and pack cementation) for the RM-Si-V alloy systems.
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The efficiency of gas and aircraft turbines could be increased considerably by a slightly higher gas inlet temperature, which would result in a significant improvement in the environmental balance and use of resources. The nickel-based superalloys currently in use are very limited in this context due to their comparatively low melting temperature, which is why hardly any improvements can be achieved with this class of materials. The most promising candidates for replacing nickel-based superalloys are the refractory metal-based Mo-Si-B alloys that have been under discussion for some time and whose range of properties is most balanced both at room temperature and at higher temperatures. In addition, earlier studies have shown that the addition of vanadium within these high-temperature alloys leads to a significant reduction in density, which would predestine them for possible use in aerospace technology.
The biggest challenge for these alloys is still their oxidation resistance, which needs to be improved in this respect. In particular, the range between 600 °C and 800 °C must be regarded as extremely critical, as this is where so-called "pesting", a catastrophic oxidation failure, occurs. From a temperature of 1000 °C, however, a protective borosilicate layer begins to form on the surface after a certain time, which protects the material from further oxidation.
The main focus of this project is the development and optimization of Mo-40V-9Si-8B materials, which are additionally provided with a coating [MoSi₂/RHEA Mo-Ta-Ti- (Cr, Al)] in order to meet the requirements of the aerospace industry in terms of mechanical properties and oxidation resistance. To this end, a suitable alloying strategy must first be developed for both the substrate and the coating material. Subsequently, a corresponding powder metallurgical production route is to be established via mechanical alloying. The base material is to be produced via a corresponding sintering process, while the oxidation protection layer is to be applied by means of high-performance cathode sputtering or pack cementing. In the final step, various tests (microstructure analysis, mechanical properties, oxidation resistance, etc.) will be carried out on both the uncoated and coated material in order to test the applicability of the developed material system as a structural material.
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Since the CCAs are being actively explored for next-generation structural materials for high-temperature applications and therefore, they should have a high creep resistance besides that a comprehensive understanding of their creep and fracture behaviors is also indispensable.
Among the several anomalies existing in the creep behavior of HEAs, the foremost important is the stress exponent, n, calculated from the Berkovich nanoindentation creep tests turns out to be much larger than that calculated based on the uniaxial stress relaxation and spherical nanoindentation creep tests, and this could not be explained using classical creep theory for crystalline metals. It is still uncertain whether the classical creep theory for conventional metals are applicable for the HEAs.
The Fe32.3Al29.3Cu11.7Ni10.8Ti15.9 CCA, developed by OVGU-HT Materials group – whose compression behavior was studied under a constant displacement test with quasi static strain rate between room temperature (RT) and 1100°C revealed a stable single phase bcc microstructure with precipitates at the grain boundaries. The high temperature deformation and creep behavior of this material will be studied during a 3 months visit of Prof. Puspendu Sahu, Professor of Physics), Jadavpur University, Kolkata, India. In addition, TEM analyses are planned to perform at Jadavpur University with the deformed materials to get insights into the deformation mechanisms.

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The International Graduate Program at Tohoku University in Sendai, Japan, was opened in 2018 with the participation of numerous colleagues from Asia, Europe and the USA. Prof. Manja Krüger and Dr. Georg Hasemann from Otto von Guericke University Magdeburg are involved in the program (see photo). Together with our Japanese colleagues Prof. Kyosuke Yoshimi and Ass. Prof. Shuntaro Ida and use the unique equipment in the laboratories of Tohoku University in Sendai and Otto von Guericke University Magdeburg.
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In cooperation with Prof. K. Yoshimi from Tohoku University in Sendai, Japan, vanadium-based high-temperature materials are being produced and investigated. The materials are selected on the basis of thermodynamic phase equilibria. They are produced using a melting metallurgical process with subsequent heat treatment. Results are discussed and alloy development is further optimized during reciprocal visits.
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Nickel-based superalloys are currently the material class of choice for high-temperature applications in turbine construction. Vanadium silicide materials represent a potential alternative, particularly due to their excellent specific mechanical properties. For example, V-Si-B alloys from the vanadium-rich region of the three-material system consist of a ductile vanadium solid solution (V-Mk) and the two intermetallic phases V₃Si and V₅SiB₂. However, this alloy system, which has been little researched to date, has some surprising similarities to its well-studied neighboring system Mo-Si-B in terms of microstructure. In initial preliminary tests on V-Si-B alloys, significantly better specific compressive strengths were achieved in the temperature range from 600 °C to 900 °C compared to Ni-based alloys. However, the mechanism of phase formation and the correlation of the microstructure-property relationships are still completely unexplored. The primary objective of this project is the development of novel V-Si-B alloys for high temperature applications. The aim is to develop ternary eutectic alloys. In a series of V-rich binary and ternary test alloys, the phase formation and stability from the melt to the homogenized microstructure will be investigated.
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Due to their excellent mechanical properties, vanadium alloys have the potential to be used as future structural materials. However, vanadium materials oxidize strongly at elevated temperatures, which means that their range of application is currently limited to typical ambient temperatures.

In the DFG-funded cooperation project "Oxidation protection coatings for vanadium materials" (project number 39807701, duration until 02/2019), suitable oxidation protection coatings were developed. These were applied in the laboratory of Prof. J. Perepezko (University of Wisconsin-Madison, USA) using the special powder packaging process. At the IWF, these layer-substrate composites are examined in detail using various microscopic methods and their protective effect is tested.
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In cooperation with the Karlsruhe Institute of Technology (KIT), the FlexiDS project aims to investigate the process of directional solidification in various high-temperature materials using in-situ X-ray diffraction. Within this framework, an innovative in-situ sample environment for directional solidification is to be developed and set up at the HEMS (High Energy Material Science) beamline at DESY (Deutsches Elektronen Synchrotron, Hamburg) . This will offer the partners involved completely new research and characterization possibilities through direct observation of the directional solidification process. The Helmholtz-Zentrum-Geesthacht (HZG), which is in charge of this beamline, will support the design, construction and operation of the sample environment.
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Molybdenum-silicon-boron alloys are regarded as attractive high-temperature materials with properties that go beyond those of conventional nickel-based superalloys. Eutectic Mo-Si-B alloys are particularly interesting for processes such as directional solidification by zone melting or selective laser melting because they are suitable for producing fiber-matrix structures.
To determine the eutectic point in the three-phase region Mo-Mo₃Si-Mo₅SiB₂, the cooling paths of different alloys are investigated and compared with thermodynamic simulations. The microstructural characteristics of these materials are correlated with the mechanical properties up to 1400°C.
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The project relates to the application of an energy-efficient method for the production of novel refractory metal alloys with melting temperatures above 2000°C. These materials have the potential to substitute the currently used nickel-based superalloys with maximum application temperatures of around 1100°C. With the help of such a material substitution, a contribution can be made to increasing thermodynamic efficiency, for example in power generation units.
In our approach, alloys with intermetallic structures are produced directly from a mixture of elemental powders in just one process step, whereby the size and distribution of the microstructure components are specifically influenced by the production parameters. The Ukrainian partner is providing a zone melting furnace specially designed for such high-melting-point materials. The expertise of the German partner will be used to characterize the relationships between the microstructure and the properties of the new materials. The new materials and the unique production technology can be used in the energy supply sector, the aircraft industry and other areas where extreme conditions are required in terms of thermal and mechanical stress.
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The materials sub-project will contribute to the characterization of the microstructure and properties of structured and coated cylinder running surfaces with regard to the reduction of friction losses.
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