eusterholz

The combination of conventional advanced ceramics with refractory metals permits the electrification of metallurgical processes. Alumina and niobium are well suited for thermal cycling with large gradients due to their similar thermal expansion behavior. In oxidizing atmospheres, special attention is required to the possible oxidative attack of the ceramic-metal interface. For this article, we prepared a high-purity bilayer composite by physical vapor deposition. After a heat treatment with low O exposure at 1600 °C, a combined approach of transmission electron microscopy and atom probe tomography showcases the intial stages of oxidation at Nb grain boundaries and the Al₂O₃-Nb interface.

dyck

Analytical descriptions of materials response are important for experimental materials science as they allow quick and efficient assessment of apparent fundamental mechanisms. However, these analytical models are frequently used beyond their limits. In our latest publication with the colleagues of Institute of Engineering Mechanics at Karlsruher Institut für Technologie (KIT), we propose modifications of the commonly applied Kelly-Street model to describe the stationary creep response of fibrous composites. By a modification of the inclusion spacing formulation and introduction of stress compatibility across the fiber-matrix interfaces along the loading direction, reproduction of experimental and simulated data in an extended parameter range is obtained. ITM's modern FFT-based micromechanical simulation schemes were used to validate the novel model description.

laube

Compositionally complex alloys (CCA) based on refractory metals are promising candidates as structural materials for high-temperature application. However, many alloys investigated so far suffer from low plastic deformability at room temperature, which limits a possible application. This was attributed to a combination of undesirable (intermetallic) phases with an ordered crystal structure of the matrix (Laube et al. in Acta Materialia 218 (2021) 117217).

In our recent study, the following objectives were addressed in more detail: (i) phase separation, (ii) precipitate growth and coarsening, as well as (iii) the influence of precipitates on the mechanical properties. In collaboration with the research groups of Prof. H.-J. Christ at the University of Siegen as well as of Prof. Y. Eggeler and Prof. C. Greiner, the phase separation was investigated on multiple time and length scales. The B2 phase formation is a diffusion-controlled, discontinuous phase transformation with sharp, moving interfaces. Thus, spinodal decomposition was invalidated for the present alloy as oppposed to the suspects in other CCA.

In the investigated temperature range of 800 to 1000 °C, the microstructure showed little tendency to grow or coarsen, which is positive considering a possible application under creep conditions. A correlation of hardness to inter-particle distance and volume fraction was established and revealed that the highest hardness can be obtained at 800 °C.

tirunilai

Mo–Si–Ti alloys, like eutectic Mo–20Si–52.8Ti (at%), have previously shown excellent oxidation resistance at high temperatures. The present article investigates high temperature mechanical properties, to confirm the suitability of Mo–20Si–52.8Ti for high temperature structural applications. This investigation consists of (i) detailed microstructural analysis via 3D focused ion beam (FIB) based tomography, in collaboration with colleagues from Saarland University, (ii) brittle to ductile transition temperature (BDTT), determined through bending tests, and (iii) high temperature creep performance comparing tensile and compressive creep results. 3D microstructure reveals that both phases in the alloy, the Mo solid solution (Mo_SS) as well as the (Ti,Mo)₅Si₃, are similar in feature size, distribution and volume fraction. Both phases are intensely interconnected indicating no distinct matrix phase. The bending tests indicate a BDTT of 1100-1150 °C as a result of the severely interconnected ductile Mo_SS and brittle (Ti,Mo)₅Si₃ phases. The ductile phase also results in crack bridging and trapping above the BDTT, showing that it plays a crucial role in increasing the energy necessary for crack propagation. The tensile creep results are consistent with the previously obtained compressive creep results (Schliephake et al. Intermetallics 104 (2019) 133). The consistency was observed both in terms of creep rates as well as microstructural features in the deformed specimens. This indicates that regardless of loading state creep performance is consistent in Mo–20Si–52.8Ti.

hinrichs

Improvement of oxidation resistance and understanding of oxidation protection mechanisms in refractory metal based alloys is still focus of our research. In our recent publication with Dechema FI and LEM in the framework of the Research Training Group 2561, we present a novel alloy from the Cr-Si-Mo system with respect to its oxidation resistance between 800 and 1200 °C. Cr promises the formation of a passivating Cr₂O₃ layer. Si might further increase the oxidation resistance of the Cr-based alloy, while Mo increases the solidus temperature of the alloy and thus potentially improves creep resistance.

The alloy was prepared by a standard arc melting process and solidified to a monolithic intermetallic compound. A heat treatment was used to induce a solid state decomposition reaction into a fine structured, two-phase microstructure. Cyclic oxidation tests showed that the alloy has exceptionally high oxidation resistance at 800 °C due to the formation of a dense Cr₂O₃. Spallation, nitridation or the evaporation of volatile oxides, as often observed in Cr- and Mo-based alloys, do not occur. Even at 1100 °C, parabolic scale growth is observed without weight and scale losses due to evaporation. Significant internal oxidation only occurs at 1200 °C. The good oxidation properties are mainly attributed to the homogeneous distribution of Cr between the constituent phases. The resistance to nitridation is explained by the formation of a Mo-rich region below the Cr₂O₃ layer.

Most ultra-fine grained Al alloys show very limited work hardening capability due to their very high stacking fault energy. This is also the case with additively manufactured precipitation strengthened Al-Mn-Mg-Sc-Zr alloys, which we are investigating together with our colleagues at Monash University. Over the course of our investigations into the precipitation behavior of this alloy after additional rotary swaging at IFW Dresden, we noticed not only the accelerated precipitation nucleation and growth due to the high defect density, but also the increase in work hardening during tensile tests. The latter becomes apparent after a suitable heat treatment, which, in contrast to the additively manufactured alloy, additionally leads to a significant increase in uniform elongation at almost unchanged strength. The change in the interaction between precipitate particles and dislocations from cutting the precipitates to bypassing them was identified as the cause and will be verified by appropriate characterization methods in our ongoing research.

eusterholz

Components in high-temperature applications such as melt-casting processes are subjected to large thermal gradients. A significant increase in the thermal shock resistance of the refractory ceramics used can be achieved by introducing a metallic component and resistive pre-heating. Due to their similar thermal expansion behavior, aluminum oxide together with the refractory metals Nb or Ta are promising combinations. During the sintering of the composites, the possible formation of further phases through a reaction of the powders with each other or with the environment has a decisive influence on the material properties. X-ray diffractometry and scanning electron microscopy reveal that samples of Al₂O₃ and Nb form the binary oxide NbO, while in Al₂O₃-Ta the ternary compound AlTaO₄ (aluminum tantalate) in its tetragonal high-temperature modification is present. Thermodynamic calculations also show that the changing oxygen solubility in the Nb or Ta solid solutions is responsible for the formation of NbO or AlTaO₄ and explain the lack of a ternary phase (AlNbO₄) corresponding to aluminum tantalate.

tirunilai

(Fe_40.4Ni_11.3Mn_34.8Al_7.5Cr₆)C_1.1 is an interstitial high entropy alloy (HEA) that shows excellent mechanical behavior at room temperature (RT), attributed to the Taylor lattice and microband formation during deformation (Wang et al., Acta Materialia 120 (2016) 228 - 239). The present collaborative study with colleagues from Dartmouth College and ITEP investigates mechanical behavior at low temperatures through cryogenic tensile testing, verifying if these deformation mechanisms extend to or change at such temperatures. Taylor lattices and microbands, seen during RT deformation, are characteristic of intermediate to high stacking fault energy (SFE) metals/alloys, but decreasing temperatures have been associated with lowering the SFE in FCC metals and alloys. This article highlights that (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 surprisingly retains microband formation during cryogenic deformation (down to 4 K), while also exhibiting deformation twinning, generally associated with intermediate to low SFE metals and alloys. The latter point is an extension of the intense solid solution strengthening from interstitial C, increasing the yield stress intensely with decreasing temperature, allowing stress during deformation to exceed twin stress. (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 thus benefits from both microband formation and deformation twinning at cryogenic temperatures, even though these deformation mechanisms are not generally observed simultaneously.

schulz

Our preliminary work already showed that for the resulting microstructure in NiAl-(Cr,Mo) alloys,nucleation selection is decisive rather than the growth of the constituting phases (Gombola et al. in Metals 10 (2020) 961). The fundamental reason for the formation of lamellar or fibrous morphology remained open and was now addressed in our recent contribution to Acta Materialia. The article resolves the ambiguities in literature regarding the formation of preferred crystallographic orientations during directional solidification and discusses their dependence on composition and growth rate. Moreover, an independency of the morphology and preferred orientation from each another was demonstrated. However, the morphology is controlled by the orientation relationship as demonstrated by the simultaneous appearance of colonies with two different orientation relationships and different morphologies.