Phase diagram of CaO–Al2O3-CeOx slag system at 1600 °C in reducing atmosphere and air atmosphere

CaO–Al₂O₃-CeO_x slag system is of great significance for designing a new type of metallurgical slag system suitable for rare earth steel. In this paper, the phase equilibria of CaO–Al₂O₃-CeO_x system were experimentally studied. Isothermal sections of CaO–Al₂O₃–Ce₂O₃ system and CaO–Al₂O₃–CeO₂ system at 1600 °C were constructed, respectively. 4 three-phase fields, 5 two-phase fields and a single liquid phase field in CaO–Al₂O₃–Ce₂O₃ system were determined; 2 three-phase fields, 3 two-phase fields and a single liquid phase field in CaO–Al₂O₃–CeO₂ system were determined. On this basis, the dissolution process of three typical inclusions (viz. Al₂O₃, CeAl₁₁O₁₈ and CeAlO₃) in CaO–Al₂O₃–Ce₂O₃ slag system was discussed.     

Enhanced oxidation properties of ZrB2–SiC composite with short carbon fibers at 1600 °C

This study investigated the effect of short carbon fiber (C_f) on the oxidation behavior of ZrB₂–SiC composites with fiber volume fractions in the range of 0–20%. Precisely, highly dense composite compacts were manufactured by hot-press process at 2000 °C and 30 MPa for 60min. The addition of C_f increased the relative density of sintered composite The oxidation treatment at 1600 °C in air tube furnace for 0.5 h revealed that oxidation rate of ZrB₂–SiC-C_f composites decreased from 292.4 μm/h to 77.6 μm/h (almost 73.5% decline), when the content of C_f changed from 0 to 20%. Moreover, C_f played important roles in blocking and deflecting oxygen diffusion during the oxidation process, which provided a local reduction environment of oxidation products.     

Microstructural evolution of HfB2 based ceramics during oxidation at 1600–2000°C

HfB₂–2?wt-%-La₂O₃, HfB₂–20?vol.-%SiC and HfB₂–20?vol.-%SiC–2?wt-%La₂O₃ were sintered by spark plasma sintering for 5?min between 1900 and 2000°C. Oxidation studies were carried out in static air on sintered ultra high temperature ceramics at 1600, 1800 and 2000°C, and the weight change and microstructure changes after oxidation were examined. LaBO₃ formed on the surface after oxidation for 1?h above 1600°C in all La₂O₃ containing materials, and the cross-section microstructure shows growth of LaBO₃ fibres from pores due to vapour phase reaction. Addition of La₂O₃ altered the oxidation kinetics of HfB₂, significantly increasing the oxidation layer thickness above 1600°C. However, above 1800°C reaction has occurred between the sample and the zirconia crucible.     

Enhanced high-temperature strength of HfB2–SiC composite up to 1600°C

Using B₄C and C additives, a HfB₂–SiC composite with an enhanced strength up to 1600?°C was prepared using high-energy ball milling followed by hot pressing. The composite microstructure comprised equiaxed large HfB₂ and fine SiC grains and an intergranular amorphous phase. The mechanical behaviour of the composite was evaluated up to 1600?°C via a four-point bending test. At or below 1500?°C, only a linear stress–strain response was observed. At 1600?°C, however, the initial linear response was followed by nonlinear deformation behaviour. The flexural strength was constant between room temperature and 1400?°C; subsequently, the flexural strength significantly increased with increasing temperature up to 1600?°C, with strengths in the range of 650–750?MPa.     

Microstructure and oxidation resistant of Si–NbSi2 coating on Nb substrate at 800°C and 1000°C

The Si–NbSi₂ composite coating with a smooth surface was successfully prepared on Nb substrate by hot dip silicon-plating (HDS) technology. The composite coating is composed of Si outer layer, NbSi₂ interlayer and Nb₅Si₃ interfacial layer. And the average surface roughness (RSa) and specific surface area growth rate (Sdr) are only 0.275 μm and 2.85%, respectively. The cyclic oxidation test shows that the Si–NbSi₂ composite coating has a very excellent oxidative resistance after oxidation at 800 °C for different times. After oxidation for 40 h, the Δm/S and oxide layer thickness of the coating are only 3.72 mg/cm² and 8 μm, respectively. After oxidation at 1000 °C for 20 h, the coating surface is almost completely covered by a dense SiO₂ layer, the Δm/S and oxide layer thickness of the coating are 7.28 mg/cm² and 15 μm, respectively. The Si–NbSi₂ composite coating presents good self-healing ability and excellent oxidation resistance, which can significantly prolong the service life of bare Nb in oxidation environment.     

Interaction of molten Armco iron with various ceramic substrates at 1600 °C

Ceramic substrates (Al₂O₃·MgO, CA6, CaZrO₃, AZT (Al₂O₃ based substrates containing ZrO₂ and TiO₂)) were interacted with liquid Armco iron under a low oxygen partial pressure in the heating microscope. No noticeable dissolution of the ceramic substrates and reaction was observed for most of the substrates. Furthermore, deposition of SiO₂ and Fe was found on the cross section between iron and these substrates. However, for the substrate AZT, a reaction layer of complex composition was found on the interface between the iron sample and the AZT substrate. After all experiments, the total number of inclusions in the iron samples greatly decreased. The deposition, agglomeration of inclusions, and the evaporation of some elements were the factors for the inclusions decreasing. Furthermore, the stability of oxides consisted of substrates under low oxygen partial pressure was confirmed by the thermodynamic calculation.     

Enhancement of oxidation resistance at 1000–1400 °C of low carbon Al2O3–C refractories with pre-synthesized SiCnw/Al2O3

The oxidation resistance of low carbon Al₂O₃–C refractories with the addition of SiC_nw/Al₂O₃ composite powders and the enhancement mechanisms were investigated. The oxidation resistance was evaluated by oxidation index (O.I.) and oxidation rate constant (k). The enhancement mechanisms of SiC_nw/Al₂O₃ on oxidation resistance were analyzed based on the phases and microstructures. The results showed that the SiC_nw/Al₂O₃ can improve the oxidation resistance of Al₂O₃–C refractories, the O.I. and k of A6 (6 wt% SiC_nw/Al₂O₃ addition) were 26.0% and 34.5% lower than those of reference sample A0, respectively. The oxidation resistance of refractories was improved in a range of 1000–1400 °C due to the introduction of SiC_nw/Al₂O₃. The enhancement mechanisms can be explained that SiC_nw is more susceptible to be oxidized due to its high specific surface area, which expanded the action temperature range of other antioxidants and itself. The mullite and dense protective layer generated during oxidation is also beneficial to impede the diffusion of O₂.     

Phase relations of the MgO–SiO2–CrOx system at 1600°C in air and reducing atmospheres

The equilibrium phase relations of the MgO–SiO₂–CrO_x system were investigated at 1600°C in air and at pO₂ of 10^–10 to 10^–11 atm using a high-temperature isothermal equilibration technique followed by rapid quenching and direct phase composition analyses with electron probe X-ray microanalysis. Two-phase equilibria (liquid–cristobalite, liquid–spinel, liquid–corundum, and liquid–olivine) and three-phase equilibria (liquid–cristobalite–spinel, liquid–olivine–spinel, liquid–spinel–corundum, and cristobalite–spinel–corundum) were observed. The 1600°C isothermal sections at various oxygen partial pressures were constructed for the MgO–SiO₂–CrO_x system based on the experimentally determined liquid and solid compositions. Data from the literature and the predictions by FactSage and MTDATA software were compared with the present experimental results.     

Corrosion behaviour of carbon materials exposed to molten lithium chloride–potassium chloride salt

The corrosion behaviour of four carbon materials namely low density graphite, high density graphite, glassy carbon and pyrolytic graphite were investigated in molten LiCl–KCl electrolyte medium at 600 °C for 2000 h under high pure argon atmosphere. Structural and microstructural changes in the carbon materials after exposure to molten chloride salt were investigated from the weight change and using scanning electron microscopy, atomic force microscopy, X-ray diffraction and laser Raman spectroscopic techniques. Microstructural analysis of the samples revealed the poor corrosion resistance of high density and low density graphite and severe attack was observed at several places on the surface. On the other hand, glassy carbon and pyrolytic graphite were relatively inert, while pyrolytic graphite showed the best corrosion resistance to molten salt attack. In the order of increasing corrosion resistance to molten salt, the carbon materials were found to follow the sequence: low density graphite < high density graphite < glassy carbon < pyrolytic graphite.     

Microstructure evolution and mechanical behavior of a mullite fiber heat-treated from 1100 to 1300°C

In this paper, the effect of phase transformation on microstructure evolution and mechanical behaviors of mullite fibers was well investigated from 1100 to 1300°C. In such a narrow temperature range, the microstructure and mechanical properties showed great changes, which were significant to be studied. The temperature of the alumina phase transformation started at below 1100°C. The main phases in fibers were γ-Al₂O₃ and δ-Al₂O₃ with amorphous SiO₂ at 1150°C. The stable α-Al₂O₃ formed at 1200°C. Then the mullite phase reaction occurred. As the alumina phase reaction took place, the tensile strength increased with the increasing temperature. In particular, the filaments achieved the highest strength at 1150°C with 1.98 ± 0.17 GPa, and the Young's modulus was 163.08 ± 4.69 GPa, showing excellent mechanical performance. After 1200°C, the mullite phase reaction went on with the crystallization of orthorhombic mullite. The density of surface defects increased rapidly due to thermal grooving, which led to mechanical properties degrade sharply. The strength at 1200°C was 1.01 ± 0.15 GPa with a strength retention of 63.13%, and the Young's modulus was 184.14 ± 10.36 GPa. While at 1300°C, the tensile strength was 0.64 ± 0.14 GPa with a strength retention of only 40.00%.     

Elastic properties of high alumina cement castables from room temperature to 1600°C

High-alumina refractory castables with compositions in the systems CaO–Al₂O₃ and CaO–Al₂O₃–SiO₂ were studied using an ultrasonic technique. The technique allows in-situ, non-destructive measurement of Young's modulus from room temperature to 1600°C. Elastic and dilatometric properties were investigated in relation to phase changes (followed by XRD) and sintering phenomena. The conversion of CAH₁₀, the hydration of still-anhydrous cement phases, and the dehydration of C₃AH₆ and AH₃ are related with events in Young's modulus evolution. Addition of 1 wt% of silica fume strongly decreases the high-temperature mechanical properties.     

Phase equilibria in TiO2-rich part of the MgO–CaO–TiO2 system at 1500–1600 °C

The equilibrium phase relations of the MgO–CaO–TiO₂ system were investigated at 1500–1600 °C in air using the high-temperature isothermal equilibration of samples in platinum crucibles followed by the rapid quenching technique. The phase compositions were analyzed using an electron probe X-ray microanalyzer (EPMA). The isothermal sections at 1500 °C and 1600 °C were constructed based on the experimentally determined liquid phase compositions. The present results deviate significantly from previous observations as well as the predictions by MTDATA and FactSage using their oxide databases, in particular in the perovskite primary phase field.     

Interaction of carbon monoxide with Cu–Pd and Cu–Ni bimetallic clusters

Interaction of CO with Cu–Pd and Cu–Ni bimetallic clusters deposited on a ZnO substrate has been investigated by core-level spectroscopy. The surface reactivity of both these alloy clusters increases with the decrease in cluster size, giving rise to dissociative adsorption at small cluster size. The surface reactivity also increases with the increase in Pd or Ni content and the reactivity of the alloy clusters is unlike that of either component metal. Thus, dissociative adsorption occurs on small Cu–Pd clusters unlike on either Cu or Pd clusters of comparable size. The reactivity of the Cu–Ni clusters, on the other hand, falls somewhere between those of Cu and Ni clusters. This revised version was published online in August 2006 with corrections to the Cover Date.