Processing and Mechanical Properties of Ti2AlC Reinforced with Alumina Fibers

Abstract

The fabrication and mechanical properties of Ti2AlC composites reinforced with the alumina oxide fibers, such as NextelTM 720 and ALBF1, were described in this thesis. Alumina fibers and Ti2AlC powders were dispersed in the water and slip cast in the molds to form green bodies. Sedimentation test were carried out to optimize pH of the slurry. It was found that suspensions prepared with PAA as a dispersant and has an excellent stability in the pH range of 4 ~ 5. Composite green bodies were densified by pressureless sintering or hot isotatic pressing (HIP) at different temperatures. The microstructure of fabricated samples was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), and porosimetry. It was found out that HIPing at 1300 oC for 4 hrs at 100 MPa results in almost fully dense composites with majority phases being alumina fibers and Ti2AlC. However, fully dense Ti2AlC composites could not be obtained by the pressurless sintering, even at temperature as high as 1400 oC at which reaction between Ti2AlC and NextelTM 720 was observed. The double torsion (DT) tests were carried out at room temperature to measure the fracture toughness of the HIPed pure and 5vol% alumina fiber reinforced Ti2AlC. DT results showed increase in the fracture toughness of Ti2AlC reinforcing with NextelTM 720 alumina fibers. However, fracture toughness of the samples reinforced with ALBF1 was lower than that of pure Ti2AlC because of the low relative densities of those composites. SEM study of the fracture surfaces after DT tests showed that toughening mechanisms by crack bridging and fiber pull outs at the crack tip are operative in all reinforced samples. In addition, elastic moduli of HIPed Ti2AlC measured by Resonant Ultrasound Spectroscopy (RUS) do not show significant change due to reinforcement with alumina fibers, while the Vickers hardness of composites was found to be larger for Ti2AlC reinforced with NextelTM 720 and lower for the samples reinforced with ALBF1.