A Study on the Microstructure, Rate-Dependent Mechanical Responses, and Failure Mechanisms of a Novel TiAl/Ti3Al-Al2O3 Cermet

  • Author / Creator
    Haoyang Li
  • Detailed characterization and experimental studies have been carried out on a novel self-propagating high-temperature synthesized (γ + α2) – TiAl/Ti3Al-Al2O3 cermet to investigate the microstructure, rate-dependent mechanical response, and rate-dependent failure mechanisms under uniaxial compressive loading. Cermet materials have gained significant research attentions recently in structural applications because they process greater ductility and toughness over most advanced ceramics while maintaining moderate strength and hardness. To apply a material in full-scale industrial applications, a comprehensive understanding of the material microstructure and its rate-dependent responses is critical. For the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet studied in this thesis, limited information is provided in the literature regarding the mechanical properties, stress-strain response, failure mechanisms, and their rate-dependency. This thesis seeks to address this gap and provide an in-depth investigation of the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet. This thesis consists mainly of two parts, which are adapted from the two published journal articles written by the author. A progressive study has been carried out in this thesis by first examining the as-received material microstructure, followed by a series of mechanical testing and post-mortem analysis.
    In the first part of this thesis, the micro/nanostructural features and mechanical responses of the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet were explored. The material composition, phase distribution, and elemental concentration were characterized. Three phases were identified in the cermet, including γ-TiAl, α2-TiAl, and Al2O3. The material exhibited ultrafine microstructure, with the size of alumina particles between 0.5 microns to 1.5 microns and occupying 65 ± 1% areal fraction of the material. Some alumina particles are connected to form clusters in the material, where a heterogeneous and complex microstructure is observed. Transmission electron microscopy investigation found iron/nickel-based nano-precipitates, and these were believed to be contributing to the mechanical properties of the material. The rate-dependency on the compressive strength, stress-time history profile, and failure mechanisms were studied by quasi-static and dynamic uniaxial compression. Surface texturing behavior was observed under dynamic loading condition using high-speed imaging, which was further explored in the second part of this thesis.
    In the second part of this thesis, the rate-dependent mechanical properties and failure of the cermet were investigated using mechanical testing and advanced characterization tools. Quasi-static and dynamic uniaxial compression tests coupled with high-speed imaging and digital image correlation were used to determine the rate-dependency of stress-strain behavior, compressive strength, and failure strain. The stress-strain curves in the dynamic experiments exhibited a series of alternating stress relaxation and strain hardening cycles, where a 1.3 times increase in compressive strength from 2780 ± 60 MPa to 3410 ± 247 MPa, and a 1.4 times increase in failure strain from 0.0166 ± 0.0017 to 0.0264 ± 0.0032 were determined with a seven order increase in strain rates from ~ 10-4 s-1 to ~ 103 s-1. The scatter in strength and failure strain measurements indicate large variability in the material, especially for such composites with complex microstructures. Advanced characterization tools, including scanning electron microscopy, high-resolution (scanning) transmission electron microscopy, and two-dimensional x-ray diffraction were used to map out the failure mechanisms activated under the different loading rates, specifically focused on identifying causalities of the texturing and soften/hardening cycling. Globally distributed dislocations and twinning were observed as a consequence of dynamic loading, and extensive cleavage in the titanium aluminide phase, void growth, transgranular cracking, and particle fracture were identified as dominant failure mechanisms activated under high strain rate loading. Crystalline texturing with profound microstructural evolution in the titanium aluminide phase was also found under dynamic loading, and this was correlated with the macroscopic surface texturing observed using high-speed imaging and the cyclical stress relaxation and strain hardening behavior in the stress-strain curves. The crystalline texturing, which manifested globally as the surface texturing behavior, is thought to be the consequence of dynamic recrystallization and grain reorientation, although additional studies are needed.
    Overall, this thesis presents: 1. A preliminary data set of the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet for material design, manufacturing, and modeling of advanced cermets; and 2. A thorough understanding of the rate-dependency of the (γ + α2) – TiAl/Ti3Al-Al2O3 cermet and provides insights in cermet material micromechanical modeling and improvement.

  • Subjects / Keywords
  • Graduation date
    Spring 2020
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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