The Phase Evolution And Microstructure Of TiAl/Ti₂AlC Composites

TiAl-matrix intermetallic compounds have become one of the hottest high-temperature structural materials recently, due to its excellent high-temperature properties and lower density. However, the room-temperature brittleness is the main obstacle of its application. Fabrication of titanium aluminide composites with ceramic particles is one way to improve their mechanical properties, and their merits of low dendity etc can still be kept.Layered ternary TiiAlC ceramic, as the typical representation of H-phase, combines unusual properties of both metals and ceramics. Like metals, it is a good thermal and electrical conductor, relatively soft and can be easily machined with traditional drill without lubrication or cooling water. Like ceramics, it is elastically stiff; exhibits excellent high temperature mechanical properties. It is resistant to thermal shock and unusually damage tolerant, and exhibits excellent corrosion resistance. Therefore, introduction of small amount of Ti2AlC particles into the TiAl matrix to form TiAl/TiaAlC composites may help to get the merits of both metals and ceramics.In the present research, three series of raw materials have been adopted: (a) the elemental Ti, Al to investigate the reaction of Ti-Al system; (b) Ti,Al and TiC powders to investigate the reaction of Ti-Al-TiC system; (c) the elemental Ti, Al and active C powders to investigate the reaction of Ti-Al-C system, and was compared with Ti-Al-TiC system. Differential thermal analysis (DTA) of starting powers and X-ray diffraction (XRD) of samples sintered at different temperatures from 600 C to 1300 C by hot pressing were utilized to analyse the phase composition. Phase evolution and the mechanics of synthesis were the focal points. Scanning electron microscope (SEM)coupled with energy-dispersive spectroscopy (EDS) were utilized to investigate the morphology characteristics of the products. The portion mechanical properties, denstity and Vickers microhardness of products were tested in the present paper.Results indicated that the reaction of Ti-Al-TiC system could be divided into two stages. Before 900 C, Ti reacts with Al to form TiAl intermetallics; above 900 C, TiAl reacts with TiC to produce dense TiAl/Ti2AlC composites. The reaction procedure of Ti-Al-C system could be divided into three stages. Before 900 C, Ti reacts with Al to form TiAl intermetallics; above 900 C, C reacts with remain Ti to form TiC triggered by the exothermal reaction of Ti and Al; TiAl reacts with TiC to produce dense TiAl/Ti2AlC composites. In the holding stage of Ti-Al-TiC system and Ti-Al-C system, ternary Ti2AlC develops to layered polycrystal and composites pyknosis at the meanwhile. The mechanics of synthesis were investigated.The microstructure of the products sintered at 1100 C~1300 C were discussed by SEM coupled with EDS. Combined with XRD patterns, the products sintered at 1200 C have the finest crystals and the densest fabric. At this temperature, the bending strength, fracture toughness and Vickers hardness of the composites produced by raw materials mixed with 7vol.% and 15vol.%TiC were tested. Results of mechanical properties indicated that the introduction of small quantities of Ti2AlC can improve the mechanical property, due to the absorption of energy of particles flexibility. However, further increasing of Ti2AlC is disadvantage to the mechanical property.