Investigación - Departamento de Ciencias Naturales

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Browsing Investigación - Departamento de Ciencias Naturales by Subject "Creep"

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    Compressive creep behavior of zirconia/nickel oxide composites
    (International Committee on Composite Materials, 2017) Huamán Mamani, Fredy; Jiménez Melendo, Manuel
    Nickel oxide/zirconia composites with different NiO contensts have been fabricated by mechanical mixing of nickel oxide and zirconia powders and sintering at 1500 ºC for 10 h in air. The resulting microstructures have been characterized by scanning and transmission electron microscopy. The mechanical behaviour of the composites has been characterized by compressive tests at high temperatures between 1100 and 1350 ºC. For NiO contents above the percolation limit, the softer phase was found to be rate-controlling in oxidizing atmospheres, achieving extended steady states of deformation without macroscopic failure. By contrast, the overall creep strength of the composites decreased severely in reducing conditions. © 2017 International Committee on Composite Materials. All rights reserved.
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    Creep properties of polycrystalline Al2O3 – Based ceramic composites
    (International Committee on Composite Materials, 2017) Huamán Mamani, Fredy; Jiménez Melendo, Manuel
    Three-phase Al2O3 - ZrO2 - Er2O3 composites with the ternary eutectic composition were fabricated by a conventional solid-state reaction route starting from commercial powders of alumina, erbia and monoclinic zirconia. Bulk composites with relative densities higher than 98% were obtained after calcining at 1400 ºC for 10 h in air and sintering at 1500 °C in air for 10 h. X-ray diffraction and energy-dispersive X-ray spectroscopy showed the presence of the three expected phases: alumina, EAG and zirconia. The composites exhibit a homogeneous microstructure with equiaxed grains of about 1 µm in size. Mechanical tests were carried out in compression at temperatures between 1300 – 1450 °C in air. Extended steady states of deformation were attained without macroscopic damage. The stress exponent in such experimental conditions was close to 2, suggesting that grain boundary sliding is the main deformation mechanism, as found in other fine-grained ceramics. © 2017 International Committee on Composite Materials. All rights reserved.
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    Fabrication and creep properties of eutectic-composition Al2O3/YAG/YSZ sintered composites
    (Elsevier Ltd, 2021) Huamán-Mamani, F.A.; Jiménez-Holgado, C.; Jiménez-Melendo, M.
    "Three-phase alumina/YAG/yttria-stabilized cubic zirconia (YSZ) composites were fabricated by a solid-state reaction route starting from commercial powders of Al2O3, Y2O3 and monoclinic ZrO2. The final phases Al2O3, YAG and YSZ were obtained after calcination of the powder mixtures at 1400 °C. Dense bulk composites were obtained after sintering, with a homogeneous microstructure of fine and equiaxed grains with sizes of 1 μm. Compressive mechanical tests were performed at 1300–1450 °C in air at constant load and at constant initial strain rate. A brittle-to-ductile transition was found with increasing temperature. Grain boundary sliding is the main deformation mechanism in the ductile regime, characterized by a stress exponent of 2 and by the absence of dislocation activity and changes in grain morphology. Alumina seems to be the rate-controlling phase owing to the improvement in creep resistance by the presence of yttrium and zirconium of the other two phases."
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    Fabrication, microstructure and high-temperature plastic deformation of three-phase Al2O3/Er3Al5O12/ZrO2 sintered ceramics
    (Elsevier Ltd, 2021) Huamán-Mamani, F.A.; Jiménez-Holgado, C.; Jiménez-Melendo, M.
    "The fabrication, microstructure and high-temperature creep behavior of chemically compatible, three-phase alumina/erbium aluminum garnet (Er3Al5O12, EAG)/erbia fully-stabilized cubic ZrO2 (ESZ) particulate composites with the ternary eutectic composition is investigated. The composites were fabricated by a solid-state reaction route of α-Al2O3, Er2O3 and monoclinic ZrO2 powders. The final phases α-Al2O3, EAG and ESZ were obtained after calcination of the powder mixtures at 1400 °C. High dense bulk composites were obtained after sintering at 1500 °C in air for 10 h, with a homogeneous microstructure formed by fine and equiaxed grains of the three phases with average sizes of 1 μm. The composites were tested in compression at temperatures between 1250 and 1450 °C in air at constant load and at constant strain rate. As the temperature increases, a gradual brittle-to-ductile transition was found. Extended steady states of deformation were attained without signs of creep damage in the ductile region, characterized by a stress exponent of nearly 2 and by the lack of dislocation activity and modifications in grain size and shape. The main deformation mechanism in steady state is grain boundary sliding, as found in superplastic metals and ceramics. In the semibrittle region, microcavities developed along grain boundaries; these flaws, however, did not grow and coalescence into macrocracks, resulting in a flaw-tolerant material. Alumina is the creep-controlling phase in the composite because of the grain boundary strengthening caused by the (unavoidable) Er3+- and Zr4+-doping provided by the other two phases. © 2021"