| dc.description.abstract | Ti2AlC and Ti3AlC2 belong to the family of MAX phase and they are considered to be good candidates for high-temperature structural materials as they show excellent oxidation resistance up to 1450oC, good damage tolerance and pseudo-ductile behavior. This Ph.D. dissertation reports on the reaction synthesis of Ti2AlC and Ti3AlC2 and improvements on their mechanical properties by either solid solution strengthening due to substitution of Al with Si, or reinforcement with alumina ceramic fibers.
The reaction synthesis of Ti2AlC from Ti-Al-TiC powder mixture was investigated in the temperature range of 700oC and 1500oC using pulsed electric current sintering (PECS). A bulk high-purity Ti2AlC was successfully fabricated with fine grain microstructure through PECS in one step, possessing the highest fracture strength ever reported. However, no Ti2(Al1-xSix)C solid solution could be sintered from Ti-Al-Si-TiC using pressureless sintering as they are thermodynamically unstable.
Because it is unpractical to improve the mechanical properties of Ti2AlC by substitution of Al with Si, they were reinforced by addition of Nextel^TM 720 and Nextel^TM 610 alumina fibers. 20vol.% of Nextel^TM 720-Ti2AlC and 20vol.% of Nextel^TM 610-Ti2AlC composites with uniform distribution of short fibers were successfully fabricated using colloidal processing and densification through PECS. With addition of alumina fibers, the elastic modulus of Ti2AlC was improved only slightly (by ~4.5%), while Vickers hardness and fracture toughness were enhanced significantly, i.e. for more than 35.8% and 15% in alumina fiber reinforced composites, respectively. In addition, results of mechanical testing in compression show that fabricated composites have higher compressive strength than the pure Ti2AlC, for more than 16% at both room temperature and 1100oC. Therefore, result of this study implies strongly that alumina fibers can be used to improve mechanical properties of the Ti2AlC at both room and high temperatures.
In order to expand the application of Ti3AlC2 MAX phase at high temperatures, a series of high-purity Ti3(Al1-xSix)C2 solid solutions with 0<x<1 and fine and coarse grained structure were reaction sintered PECS. It was found that c-lattice parameter and elastic moduli (both Young's and shear modulus) increase linearly with increasing amount of Si in those solid solutions. Regardless of grain size, Vickers hardness results demonstrate significant hardening effect in Ti3(Al1-xSix)C2 solid solutions with a maximum around Ti3(Al0.5Si0.5)C2. At room temperature, the strengthening effect was found to be marginal for fine grained (FG) structure with improvement of only 7.3%, but significant strengthening effect was observed in coarse grained (CG) one of ~25.8%. Above brittle-to-plastic transition (BPT) temperature, the solid solution strengthening effect diminishes and the strength of solid solutions is still higher than that of Ti3AlC2 but less than Ti3SiC2. Meanwhile, Ti3Al0.6Si0.4C2 forms a protective alumina oxide layer at 1200oC with comparable oxidation resistance to Ti3AlC2. | en |
