PRESSURELESS REACTIVE SINTERING OF TiAl-TiC AND Ti_3Al-TiC COMPOSITES

摘要

TiAl and Ti_3Al based composites reinforced with volume fractions 10-50 percent of TiC particles were successfully prepared by pressureless reaction sintering of reaction mixtures consisting of commercial titanium aluminide powders (TiAl with traces of Ti_3Al and the single phased Ti_3Al) blended with the appropriate amount of ceramic reinforcement, 5-10 percent of Al powder and, in some cases, also 5 percent of Ti powder added as sintering agents. The green compacts made from the blended powder mixture were reaction sintered at 1300 deg C for 2 h in an Ar + 4 percent H_2-rich environment using a vacuum furnace. The morphology of the commercial powders and the microstructure of the as-sintered composites were studied by scanning electron microscopy and X-ray diffraction analysis. The pressureless sintering of as-received TiAl and Ti_3Al powders resulted in samples with 10-15 percent of the retained porosity. On the other side, the addition of 10 percent of TiC particles to the sintering mixture improved pressureless densification enabling fabrication of composite samples with >95 percent of theoretical density without addition of free aluminium. In these particular cases, densification was promoted by chemical reactions between TiAl or Ti_3Al and TiC leading to the formation of Al_2Ti_4C_2 and Ti_3AlC secondary bonding phases, respectively. However, as it was confirmed by sintering experiments, for successful (>95 percent of theoretical density) pressureless densification of composite samples with more than 10 vol. of TiC, the addition of 5-10 percent of free aluminium and 5 percent of titanium, depending on the actual amount of TiC reinforcement, was necessary. The addition of Al and Ti promotes liquid reaction sintering and the formation of secondary Ti-Al-C bonding phases in an Al-Ti co-continuous network. The tensile properties and Vickers hardness of composite samples were measured at room temperature. The improvement in tensile properties (except elongation) and Vickers hardness was found to correlate with the amount of TiC reinforcement in the matrix.