Mechanical High-Temperature Properties and Damage Behavior of Coarse-Grained Alumina Refractory Metal Composites

The present study provides the mechanical properties of a new generation of refractory composites based on coarse-grained Al2O3 ceramic and the refractory metals Nb and Ta. The materials were manufactured by refractory castable technology and subsequently sintered at 1600 °C for 4 h. The mechanical properties and the damage behavior of the coarse-grained refractory composites were investigated at high temperatures between 1300 and 1500 °C. The compressive strength is given as a function of temperature for materials with two different volume fractions of the refractory metals Ta and Nb. It is demonstrated that these refractory composites do not fail in a completely brittle manner in the studied temperature range. The compressive strength for all materials significantly decreases with increasing temperature. Failure occurred due to the formation of cracks along the ceramic/metal interfaces of the coarse-grained Al2O3 particles. In microstructural observations of sintered specimens, the formation of tantalates, as well as niobium oxides, were observed. The lower compressive strength of coarse-grained Nb-Al2O3 refractory composites compared to Ta-Al2O3 is probably attributed to the formation of niobium oxides. The formation of tantalates, however, seems to have no detrimental effect on compressive strength.


Introduction
Ceramic-metal composites benefit from the combination of a high melting point, hardness and the chemical stability of ceramics, with the high toughness and ductility of metals. In general, the upper limit of the application temperature of such composites is restricted by the melting point of the metal, the reaction between the metal and ceramic particles, the chemical interaction with the environment, or the thermal mismatch between the ceramic and the metallic phases. Therefore, the use of refractory metals with a high melting point could make such composites applicable at even higher temperatures [1].

Materials and Preparation
The materials under investigation were coarse-grained refractory composites based on coarse-grained alumina and fine-grained refractory metals niobium and tantalum. The following raw materials were used: tabular alumina T60/64 (Almatis GmbH, Frankfurt, Germany), reactive alumina CL370 (Almatis GmbH), alumina Martoxid MR-70 (Martinswerk GmbH, Bergheim, Germany), re-hydratable alumina binder Alphabond 300 (Almatis GmbH), dispersing aluminas ADS-1 and ADW-1 (Almatis GmbH), Nb powder (EWG E. Wagener GmbH, Heimsheim, Germany), Ta powder (haines & maassen Metallhandelsgesellschaft mbH, Bonn, Germany). The refractory metals were used as powders with a particle classification d ≤ 75 µm, whereas the alumina used had different particle classifications (0-20 µm: 10 vol. %, 0-45 µm: 11 vol. %, 0-0.5 mm: 21 vol. %, 0.5-1 mm: 10 vol. %, 1-3 mm: 13 vol. %, and 2-5 mm: 24 vol. %). The two fractions with the smallest alumina particles (0-20 µm: 10 vol. %, 0-45 µm: 11 vol. %) were replaced by fine powder fractions (0-20 µm) of Ta and Nb, respectively, both with a purity of 99.95%. If both fractions of fine alumina particles were replaced by refractory metals composites with 21 vol. % of refractory metal resulted, whereas composites with 11 vol. % were obtained by replacing only one fraction of alumina (0-45 µm). Thus, Ta-Al 2 O 3 composites and Nb-Al 2 O 3 composites were manufactured with both 11 vol. % and 21 vol. % of refractory metals. The coarse-grained refractory composites were fabricated using a refractory castable technology consisting of a mixing procedure of powder mixtures with a volume of ≈350 cm 3 in a concrete mixer (ToniMAX, Toni Baustoffprüfsysteme GmbH, Berlin, Germany) using 5 mass% water and 1 mass% of dispersing alumina as additives. Produced castable blends were cast into prismatic (150 × 25 × 25 mm 3 ) sample molds and remained there for 1 to 3 days. Hereafter, the samples were taken out and dried in air at 120 • C for 24 h, followed by a pressureless sintering regime under argon atmosphere at 1600 • C for 4 h. Heating rates of 3 K/min up to a temperature of 500 • C, and 10 K/min up to the sintering temperature of 1600 • C, were applied. Details of the chemical composition of the raw materials, the castable recipe and the castable technology were reported recently in [39]. Cylinders with a diameter of 15 mm and a height of 25 mm were prepared for the high-temperature mechanical tests from the sintered prism by hollow drilling. Since the mechanical tests (compression, stress-relaxation) require plane-parallel front faces, these faces were mechanically grinded using a special tool designed for this purpose (see Figure 1a). Therefore, the specimens were fixed by a semi-circular clamping part with well-defined geometry according to the diameter of the specimens. Subsequently, the upper and lower front faces (after turning the tool by 180 • around the axis perpendicular to the cylinder axis) were ground. The porosity was determined before and after mechanical tests using both mercury porosimetry and the Archimedes' method.

Mechanical Testing
To investigate the mechanical behavior of the coarse-grained refractory composites, quasi-static compression and stress-relaxation tests were carried out on an electro-mechanical high-temperature 20 kN testing machine (Zwick/Roell, Neu-Ulm, Germany), as shown in Figure 1b. The compression tests were performed with a crosshead displacement rate of 15 µm/s which results in an initial strain rate of about 7.5 × 10 −4 ·s −1 . The tests were carried out under argon atmosphere (gas-chamber: Maytec, Singen, Germany) after two evacuation cycles, in order to avoid oxidation of the metallic phases of the specimens. A detailed view of the specimen set-up and the heating system is shown in Figure 1c. The cylindrical specimen (1) is placed in a susceptor cage (6) between two Mo-based susceptors (5) (TZM alloy) to set a homogenous temperature distribution. The heating was performed by a water-cooled copper induction coil (2) controlled by a middle-frequency generator (Hüttinger HF 5010, Freiburg, Germany) with heating rates up to 20 K/s. Strain was measured with a high-temperature extensometer system with alumina rods (3). Si 3 N 4 -pistons (4) transfer the load from the pressure tubes to the shown setup. The specimens were tested at 1300, 1400, and 1500 • C. For the metallic components, this corresponds to a homologous temperature of 0.43 ≤ T hom ≤ 0.49 for Ta, and 0.52 ≤ T hom ≤ 0.60 for Nb. The temperature was measured on the surface of the specimens using a pyrometer Metis MS09 (Sensortherm, Sulzbach, Germany). A dwell time of 5 min was allowed prior to the tests in order to guarantee a homogenous temperature distribution and to minimize the effects of thermal expansion of the susceptors on the strain measurement. One specimen was tested for each material composition (11 vol. % Ta or Nb and 21 vol. % Ta or Nb, respectively) and each temperature. It should be noted here that the authors are aware of scatter in mechanical data of ceramic materials. However, it is known from other ceramic materials, such as carbon-bonded magnesia (MgO-C), that the scatter of data decreases with an increase in the "pseudo ductility" of these materials at high temperatures, depending on the content of graphite. However, for these new coarse-grained Ta-Al 2 O 3 and Nb-Al 2 O 3, the Weibull distribution of the mechanical data cannot be provided due to limited material. composition (11 vol. % Ta or Nb and 21 vol. % Ta or Nb, respectively) and each temperature. It should be noted here that the authors are aware of scatter in mechanical data of ceramic materials. However, it is known from other ceramic materials, such as carbon-bonded magnesia (MgO-C), that the scatter of data decreases with an increase in the "pseudo ductility" of these materials at high temperatures, depending on the content of graphite. However, for these new coarse-grained Ta-Al2O3 and Nb-Al2O3, the Weibull distribution of the mechanical data cannot be provided due to limited material. Stress relaxation tests were performed under compressive load at the same temperatures. The compressive load for the relaxation tests was chosen according to 80% of the maximum load achieved during the high-temperature compression tests.

Microstructural Investigations
The microstructure of the refractory composites was investigated before and after the compression tests, using a Scanning Electron Microscope (SEM), (Mira 3 XMU, Tescan, Brno, Czech Republic). SEM micrographs were taken in Secondary Electron (SE) or Back-Scattered Electron (BSE) contrast. In addition, Energy-Dispersive Spectroscopy (EDS) was performed in terms of point and line analysis, in order to study possible reactions of the refractory metals with alumina using EDS detector (Apollo 10 mm²) and acquisition software from EDAX.

Initial Microstructure of Refractory Metal-Alumina Composites
The microstructure of the specimens after the sinter process was investigated on cylindrical specimens cut parallel to the cylinder axis. Figure 2 shows exemplarily SEM micrographs of specimens, with 21 vol. % Ta and Nb, respectively. In Figure 2a, the coarse-grained fraction of alumina particles (>1 mm) is clearly visible, with pores remaining from the manufacturing process. Both the Ta and the Nb particles exhibit a good bonding with the alumina particles. However, in both materials, a reaction between alumina and refractory metals seems to occur during the sintering process. Clear indicators for this assumption are the different grey levels found both beside Ta particles (Figure 2b), as well as Nb particles (Figure 2c). Stress relaxation tests were performed under compressive load at the same temperatures. The compressive load for the relaxation tests was chosen according to 80% of the maximum load achieved during the high-temperature compression tests.

Microstructural Investigations
The microstructure of the refractory composites was investigated before and after the compression tests, using a Scanning Electron Microscope (SEM), (Mira 3 XMU, Tescan, Brno, Czech Republic). SEM micrographs were taken in Secondary Electron (SE) or Back-Scattered Electron (BSE) contrast. In addition, Energy-Dispersive Spectroscopy (EDS) was performed in terms of point and line analysis, in order to study possible reactions of the refractory metals with alumina using EDS detector (Apollo 10 mm 2 ) and acquisition software from EDAX.

Initial Microstructure of Refractory Metal-Alumina Composites
The microstructure of the specimens after the sinter process was investigated on cylindrical specimens cut parallel to the cylinder axis. Figure 2 shows exemplarily SEM micrographs of specimens, with 21 vol. % Ta and Nb, respectively. In Figure 2a, the coarse-grained fraction of alumina particles (>1 mm) is clearly visible, with pores remaining from the manufacturing process. Both the Ta and the Nb particles exhibit a good bonding with the alumina particles. However, in both materials, a reaction between alumina and refractory metals seems to occur during the sintering process. Clear indicators for this assumption are the different grey levels found both beside Ta particles (Figure 2b), as well as Nb particles (Figure 2c). To study the reaction of Ta and Nb with Al2O3, EDS line measurements were performed on both microstructures. The results are summarized in Figure 3 and Figure 4, for Nb and Ta refractory composites, respectively. It is clearly visible that, in the dark-grey area of Nb particles (cf. Figure 3a), an enrichment of oxygen was measured by EDS, whereas, in the brighter-grey area, nearly 100% of Nb was measured (cf. Figure 3b). This enrichment of oxygen in Nb was observed frequently in the microstructure and is most probably the result of the sintering process. Similar behavior was found for Ta particles. Here, also different grey levels in the SEM micrograph (see Figure 4a) serve as an indicator for the formation of a new phase beside pure Ta and alumina. In contrast to Nb refractory composites, here, an enrichment not only of oxygen but also of aluminum was detected, as the EDS line profile shows in Figure 4b. Thus, a sequence from pure tantalum (bright-grey; 1) over pure alumina (dark-grey) to an area composed of O, Ta and Al (lightgrey) to pure Ta again (bright-grey), ending in pure alumina (2), was detected. This frequently observed enrichment of oxygen and aluminum in Ta-particles is mostly attributed, as mentioned above, to the consequence of reactions during the sintering process. To study the reaction of Ta and Nb with Al 2 O 3 , EDS line measurements were performed on both microstructures. The results are summarized in Figures 3 and 4, for Nb and Ta refractory composites, respectively. It is clearly visible that, in the dark-grey area of Nb particles (cf. Figure 3a), an enrichment of oxygen was measured by EDS, whereas, in the brighter-grey area, nearly 100% of Nb was measured (cf. Figure 3b). This enrichment of oxygen in Nb was observed frequently in the microstructure and is most probably the result of the sintering process. To study the reaction of Ta and Nb with Al2O3, EDS line measurements were performed on both microstructures. The results are summarized in Figure 3 and Figure 4, for Nb and Ta refractory composites, respectively. It is clearly visible that, in the dark-grey area of Nb particles (cf. Figure 3a), an enrichment of oxygen was measured by EDS, whereas, in the brighter-grey area, nearly 100% of Nb was measured (cf. Figure 3b). This enrichment of oxygen in Nb was observed frequently in the microstructure and is most probably the result of the sintering process. Similar behavior was found for Ta particles. Here, also different grey levels in the SEM micrograph (see Figure 4a) serve as an indicator for the formation of a new phase beside pure Ta and alumina. In contrast to Nb refractory composites, here, an enrichment not only of oxygen but also of aluminum was detected, as the EDS line profile shows in Figure 4b. Thus, a sequence from pure tantalum (bright-grey; 1) over pure alumina (dark-grey) to an area composed of O, Ta and Al (lightgrey) to pure Ta again (bright-grey), ending in pure alumina (2), was detected. This frequently observed enrichment of oxygen and aluminum in Ta-particles is mostly attributed, as mentioned above, to the consequence of reactions during the sintering process. Similar behavior was found for Ta particles. Here, also different grey levels in the SEM micrograph (see Figure 4a) serve as an indicator for the formation of a new phase beside pure Ta and alumina. In contrast to Nb refractory composites, here, an enrichment not only of oxygen but also of aluminum was detected, as the EDS line profile shows in Figure 4b. Thus, a sequence from pure tantalum (bright-grey; 1) over pure alumina (dark-grey) to an area composed of O, Ta and Al (light-grey) to pure Ta again (bright-grey), ending in pure alumina (2), was detected. This frequently observed enrichment of oxygen and aluminum in Ta-particles is mostly attributed, as mentioned above, to the consequence of reactions during the sintering process. It is assumed that this phase could be representative of the group of aluminum tantalates [40,41]. Tantalum has a high affinity for oxygen [42,43]. Tantalum powders, therefore, form a tantalum-(V)oxide (Ta2O5) [42,43] at the surface. At higher temperatures (approx. 1300 °C), this oxide can react with Al2O3 to aluminum tantalate AlTaO4 [40,41,44] (light-grey in Figure 4a). However, the ternary system Ta-Al-O is not yet well investigated regarding both the occurring phase equilibria as well as the thermodynamics. King et al. [45] and Roth and Waring [46] showed, for the subsystem Ta2O5-Al2O3, a solubility of about 10 mol. % Al2O3 in Ta2O5, and the formation of an intermediate phase -TaAlO4-with an extended range of solid solution, as reported by Yamaguchi et al. [47]. In addition, Huang et al. [24] showed for hot-pressed specimens (1450 °C, 1650 °C) that, for these temperatures, a diffusion of oxygen in Ta as well as, at 1650 °C, a diffusion of Al in tantalum, occurred, since the formation of an interphase layer was observed. However, a diffusion of Ta in Al2O3 was not observed [24]. In similar way, the formation of aluminum niobates of the form AlNbO4 was expected [44] for the system Al2O3-Nb. However, here the formation of niobium-oxide was observed, as reported earlier by other groups [19,29]. A maximum solubility of oxygen in niobium of 0.9 at. %, and the formation of different types of stable niobium oxides (NbO, NbO2, Nb2O5 and Nb12O29 [48]) were reported. The formation of different intermetallic phases like niobium-aluminides of types AlNb2 and Al3Nb, as mentioned in [19,29], was not observed in our SEM investigations. In all cases, the stress-strain curves exhibit the highest strength and elongation for 1300 °C. The maximum strength decreases significantly with an increase in temperature. Furthermore, the increase in volume fraction of the refractory metals results in a significant decrease in strength. However, the ductility seems to be comparable. Figure 6 shows the dependence of maximum compression strength on temperature for all tested materials. In general, the compressive strength of the materials with 11 vol. % of Ta or Nb is higher compared to the materials with higher volume fraction of metals. Furthermore, the compressive strength decreases for all materials with an increase in temperature. For the lower volume fraction, the Ta-composite exhibits a higher strength than that with Nb. For the higher volume fraction, both composite materials show comparable strength values. However, the decrease in compressive strength with an increase in temperature is less pronounced for materials with a higher volume fraction of refractory metals. The elongation to failure decreases with an increase in temperature. Furthermore, it should be noted that the porosity of all materials is It is assumed that this phase could be representative of the group of aluminum tantalates [40,41]. Tantalum has a high affinity for oxygen [42,43]. Tantalum powders, therefore, form a tantalum-(V)-oxide (Ta 2 O 5 ) [42,43] at the surface. At higher temperatures (approx. 1300 • C), this oxide can react with Al 2 O 3 to aluminum tantalate AlTaO 4 [40,41,44] (light-grey in Figure 4a). However, the ternary system Ta-Al-O is not yet well investigated regarding both the occurring phase equilibria as well as the thermodynamics. King et al. [45] and Roth and Waring [46] showed, for the subsystem Ta [47]. In addition, Huang et al. [24] showed for hot-pressed specimens (1450 • C, 1650 • C) that, for these temperatures, a diffusion of oxygen in Ta as well as, at 1650 • C, a diffusion of Al in tantalum, occurred, since the formation of an interphase layer was observed. However, a diffusion of Ta in Al 2 O 3 was not observed [24]. In similar way, the formation of aluminum niobates of the form AlNbO 4 was expected [44] for the system Al 2 O 3 -Nb. However, here the formation of niobium-oxide was observed, as reported earlier by other groups [19,29]. A maximum solubility of oxygen in niobium of 0.9 at. %, and the formation of different types of stable niobium oxides (NbO, NbO 2 , Nb 2 O 5 and Nb 12 O 29 [48]) were reported. The formation of different intermetallic phases like niobium-aluminides of types AlNb 2 and Al 3 Nb, as mentioned in [19,29], was not observed in our SEM investigations. Figure 5 shows the results of the high-temperature compression tests on the composites. Figure 5a,b show the stress vs. strain curves for 11 vol. % (a) and 21 vol. % (b) niobium, and Figure 5c,d for tantalum. It is noteworthy that the materials show at least some limited ductility and do not fail in a brittle manner after maximum strength, despite the high-volume fraction of coarse-grained alumina. In all cases, the stress-strain curves exhibit the highest strength and elongation for 1300 • C. The maximum strength decreases significantly with an increase in temperature. Furthermore, the increase in volume fraction of the refractory metals results in a significant decrease in strength. However, the ductility seems to be comparable. Figure 6 shows the dependence of maximum compression strength on temperature for all tested materials. In general, the compressive strength of the materials with 11 vol. % of Ta or Nb is higher compared to the materials with higher volume fraction of metals. Furthermore, the compressive strength decreases for all materials with an increase in temperature. For the lower volume fraction, the Ta-composite exhibits a higher strength than that with Nb. For the higher volume fraction, both composite materials show comparable strength values. However, the decrease in compressive strength with an increase in temperature is less pronounced for materials with a higher volume fraction of refractory metals. The elongation to failure decreases with an increase in temperature. Furthermore, it should be noted that the porosity of all materials is comparable before and after mechanical testing, and lies in the order of about 17%, which means that no consolidation occurred during the high-temperature compression tests. comparable before and after mechanical testing, and lies in the order of about 17%, which means that no consolidation occurred during the high-temperature compression tests.   comparable before and after mechanical testing, and lies in the order of about 17%, which means that no consolidation occurred during the high-temperature compression tests.   The macroscopic damage patterns of coarse-grained Ta-Al 2 O 3 and Nb-Al 2 O 3 specimens deformed at 1300 • C are summarized in Figure 7. The upper row of Figure 7 shows one of the two front faces, while the lower row shows side views of the cylindrical specimens. The coarse-grained Al 2 O 3 with grain sizes up to 5 mm is clearly visible on the end faces. In addition, macro-porosity is visible. From the side views of the specimens it can be seen that the cracks run predominantly along the ceramic/metal interfaces. Furthermore, the specimens did not fail completely in a brittle manner; instead, some remaining ductility occurred. However, in all cases mechanical spallation of outer areas was observed. The macroscopic damage patterns of coarse-grained Ta-Al2O3 and Nb-Al2O3 specimens deformed at 1300 °C are summarized in Figure 7. The upper row of Figure 7 shows one of the two front faces, while the lower row shows side views of the cylindrical specimens. The coarse-grained Al2O3 with grain sizes up to 5 mm is clearly visible on the end faces. In addition, macro-porosity is visible. From the side views of the specimens it can be seen that the cracks run predominantly along the ceramic/metal interfaces. Furthermore, the specimens did not fail completely in a brittle manner; instead, some remaining ductility occurred. However, in all cases mechanical spallation of outer areas was observed. The damage behavior was also investigated by SEM after the compression tests. Figures 8a,b show specimens with 11 vol. % Nb (a) and Ta (b), respectively, after compression tests at 1500 °C. The pronounced porosity and the severe damage of the microstructure with cracks running from pores along the interfaces between the large Al2O3 grains and the grains of the refractory metal can be clearly seen in both specimens. In addition, Figures 8d-f also reveal the damage behavior of the Nb particles. In particular, Nb particles with enrichment of oxygen seem to be favored for crack initiation. Thus, many tiny cracks (marked by black arrows) were observed in these areas indicating an embrittlement of the Nb particles by the formation of niobium oxides, which can be the reason for the lower strength of coarse-grained Nb-Al2O3 refractory composites compared to Ta-Al2O3 composites. The formation of fine Ta fringes along the grain boundaries of Al2O3 was observed (Figure 8c), indicating the diffusion of the refractory metals along the grain boundaries during the sintering process. The damage behavior was also investigated by SEM after the compression tests. Figure 8a,b show specimens with 11 vol. % Nb (a) and Ta (b), respectively, after compression tests at 1500 • C. The pronounced porosity and the severe damage of the microstructure with cracks running from pores along the interfaces between the large Al 2 O 3 grains and the grains of the refractory metal can be clearly seen in both specimens. In addition, Figure 8d-f also reveal the damage behavior of the Nb particles. In particular, Nb particles with enrichment of oxygen seem to be favored for crack initiation. Thus, many tiny cracks (marked by black arrows) were observed in these areas indicating an embrittlement of the Nb particles by the formation of niobium oxides, which can be the reason for the lower strength of coarse-grained Nb-Al 2 O 3 refractory composites compared to Ta-Al 2 O 3 composites. The formation of fine Ta fringes along the grain boundaries of Al 2 O 3 was observed (Figure 8c), indicating the diffusion of the refractory metals along the grain boundaries during the sintering process. Thus, the chemical bonding between alumina and refractory metals contributes to good mechanical properties in the Ta-Al2O3 and Nb-Al2O3 refractory composites compared to carbonbonded oxides (MgO-C or Al2O3-C), where only a mechanical clamping between square-edged oxide particles contributes to the strength of these materials, since no chemical reaction between oxides and graphite occurs [49].

Stress-Relaxation Tests
In addition to the high-temperature compression tests, stress relaxation experiments were carried out at 1300 °C and 1500 °C. The initial compressive stresses were set to 80% of the maximum compressive stresses determined from the respective quasi-static compression tests (cf. Figure 5). The results of the relaxation tests are summarized in Figure 9. The samples with 11 vol. % Ta show the highest resistance to stress relaxation at 1300 °C, which corresponds to a homologous temperature of the metallic phase of Thom = 0.43. Within the first 120 s, a stress drop of approx. 5-6 MPa occurs (approx. 10% of the stress). After that, only a small decrease in stress is observed, due to creep processes. For the higher testing temperature (i.e., Thom = 0.49) a much stronger relaxation is observed (approx. 10 MPa stress drop, i.e., up to 30%). For the composite materials with 11 vol. % and 21 vol. % Nb, respectively, at 1300 °C and 1500 °C (equivalent to the homologous temperatures of Nb of Thom = 0.52 and 0.6), a more pronounced relaxation behavior was observed within the first 3-4 min. Thus, the chemical bonding between alumina and refractory metals contributes to good mechanical properties in the Ta-Al 2 O 3 and Nb-Al 2 O 3 refractory composites compared to carbon-bonded oxides (MgO-C or Al 2 O 3 -C), where only a mechanical clamping between square-edged oxide particles contributes to the strength of these materials, since no chemical reaction between oxides and graphite occurs [49].

Stress-Relaxation Tests
In addition to the high-temperature compression tests, stress relaxation experiments were carried out at 1300 • C and 1500 • C. The initial compressive stresses were set to 80% of the maximum compressive stresses determined from the respective quasi-static compression tests (cf. Figure 5). The results of the relaxation tests are summarized in Figure 9. The samples with 11 vol. % Ta show the highest resistance to stress relaxation at 1300 • C, which corresponds to a homologous temperature of the metallic phase of T hom = 0.43. Within the first 120 s, a stress drop of approx. 5-6 MPa occurs (approx. 10% of the stress). After that, only a small decrease in stress is observed, due to creep processes. For the higher testing temperature (i.e., T hom = 0.49) a much stronger relaxation is observed (approx. 10 MPa stress drop, i.e., up to 30%). For the composite materials with 11 vol. % and 21 vol. % Nb, respectively, at 1300 • C and 1500 • C (equivalent to the homologous temperatures of Nb of T hom = 0.52 and 0.6), a more pronounced relaxation behavior was observed within the first 3-4 min.
Specimens with 21 vol. % Nb show a stress relaxation only within the first few seconds, and a horizontal curve develops in the further course of the test, i.e., there are almost no further relaxation phenomena. The reason for the significant stress relaxation in all specimens within the first seconds or minutes is probably due to plastic deformation/dislocation rearrangement within the metallic components of the composite materials. It is highly probable that there are changes in the dislocation arrangement introduced by the initial compressive loading. Specimens with 21 vol. % Nb show a stress relaxation only within the first few seconds, and a horizontal curve develops in the further course of the test, i.e., there are almost no further relaxation phenomena. The reason for the significant stress relaxation in all specimens within the first seconds or minutes is probably due to plastic deformation/dislocation rearrangement within the metallic components of the composite materials. It is highly probable that there are changes in the dislocation arrangement introduced by the initial compressive loading.

Summary
High-temperature mechanical properties were determined for the new class of coarse-grained refractory composites, based on Al2O3 and 11 or 21 vol. % of refractory metals Ta and Nb, respectively. It was demonstrated that these coarse-grained refractory composites do not fail in a completely brittle manner during compression tests between 1300 and 1500 °C. The compressive strength for all materials significantly decreases with an increase in temperature. Severe damage due to cracks running along the ceramic/metal interfaces of the coarse-grained Al2O3 particles was observed. The lower compressive strength of coarse-grained Nb-Al2O3 refractory composites compared to Ta-Al2O3 ones is probably attributed to the formation of niobium-oxides during the 4 h sinter process at 1600 °C. In contrast, the formation of tantalates seems not to have a detrimental effect on the compressive strength.