| Ti 3 AlC2 is a typical member of MAX phase materials, which has a lot of potential applications in many fields because it possesses a low density, high electric and thermal conductivities, good thermal shock resistance and ready machinability.In the present study, highly-pure Ti 3 AlC2 was prepared by combustion synthesis. The synthsis mechanism, the thermalphysical properties, the electrical properties and the cyclic-oxidation resistance were studied systematically.The maximum temperature of Ti-Al-C system during combustion syntheis was measured up to 1780°C, which was higher than the melting points of Al and Ti, respectively. The thermodynamic analysis indicated that the formation tendency of Ti3Al and TiC was the maximum at the low and high temperature range, respectively. The combustion synthesis mechanism of Ti 3 AlC2 was proposed based on the thermodynamic calculation of Ti-Al-C system and XRD, SEM, EDS and TEM results of the quenched samples. Al would first melt at 660°C and spread on Ti particles. Then Ti began to dissolve into liquid Al and form TiAlx intermetallics at the interface, which released large amounts of heat. As a result of the released heat, combustion synthesis was initiated and the temperature was elevated higher. Ti particles and previously formed TiAlx completely melted subsequently and formed Ti-Al melts which could wet and spread on C particles. Then C began to dissolve into Ti-Al melts and form TiC0.67, which again released large amount of heat and brought the system temperature to the maximum. After that the temperature began to decline and TiC 0.67 reacted with liquid Al to form Ti 3 AlC2 when the temperature was below 1370°C.Efforts were made on optimizing the relative density of the green compact and the initial molar ratio of Ti, Al and C powders. XRD results indicated that the former had little effect on the resulting phase, while the latter had greater effect on it. Heat treatment at 1100°C for 2 h was performed on the as-synthesized product and it was found the composing phases remained unchanged.The electrical resistivity of Ti3 AlC2 was measured by four-point probes at 51~900°C. The results indicated that the electrical resistivity was similar with a metal, increasing linearly with increasing temperature.The coefficient of thermal expansion of Ti 3 AlC 2 was examined at 200~1200 °C and yielded a mean value of 9.3×10 -6 K-1. The thermal conductivity and the molar heat capacity of Ti 3 AlC2 increased with increasing temperature at 200~1200°C and the results proved that both electron and phonon contributed to the thermal conductivity of Ti 3 AlC 2.A 40-cycle oxidation was performed on Ti 3 AlC2 at 550~1300°C and the results showed that abnormal oxidation occurred at 550 and 650°C. The cyclic-oxidation kinetics approximately obeyed an accelerated linear law at 550 and 650°C, while that at 750~1300°C basically followed a parabolic law. XRD and XPS results showed the scales on Ti 3 AlC 2 containedα-Al 2 O 3 and rutile TiO2 at 750~950°C, while at 550 and 650°C they contained anatase and rulite TiO2 and amorphous Al 2 O3. Surface and cross-section morphologies showed the scales formed at 750~950°C were dense, free of microcrack and well adhesive to Ti 3 AlC2 substrate. However, the scales formed at 550 and 650°C were loose and full of cavities, and cracks were found at the scale/Ti 3 AlC2 interface. At 1000 and 1100°C, the scales were composed of an outer and discontinuous layer of rutile TiO2, and an inner and continuous layer ofα-Al 2 O3. The scale formed at 1200°C contained an outmost layer of discontinuous Al 2 TiO 5 , an intermediate mixed layer of TiO 2 andα-Al 2 O3, and an inner layer ofα-Al 2 O3. At 1300°C, the scale contained an outmost layer of discontinuous Al 2 TiO 5 , an intermediate mixed layer of Al 2 TiO 5 andα-Al 2 O3, and an inner layer of continuousα-Al 2 O 3 which deeply intruded into the Ti 3 AlC2 substrate and greatly improved the interface strength.XRD results showed that the compressive stress within the scale formed on Ti 3 AlC2 was measured to be 0.82,0.65 and 0.49 GPa after 5, 20 and 40 thermal cycles at 1000°C. However, the compressive stress within the scale formed on Fe-Cr-Al superalloy was 4~5 GPa after 40-h oxidation at 1100~1300°C. It was obvious that the compressive stress within the scale formed on Ti 3 AlC2 was much smaller than that within the scale on Fe-Cr-Al superalloy.In order to improve the cyclic-oxidation resistance of Ti 3 AlC2 at low temperatures, preoxidation of Ti 3 AlC2 was employed. The results showed that the cyclic-oxidation resistance of Ti 3 AlC2 at 550 and 650°C could be greatly improved by preoxidation at 1100°C for 2 h.The oxidation mechanism of Ti 3 AlC2 was studied at room temperature. The oxidation process contained two stages. During the first stage, the adsorption and absorption of oxygen occurred and there was no evidence that titanium, aluminum and carbon were oxidized. During the second stage, the selective oxidation of aluminum occurred, whereas the binding energy of titanium and carbon remained unchanged after 3600 s exposure in air which meant they were not oxidized.
|
PDF Full Text Request