Effects of Compositional Ratio of Ti-Al-C on Formation of Ti₂AlC by Self-Sustaining Combustion Synthesis

The formation of Ti₂AlC was investigated by self-propagating high-temperature synthesis (SHS) from the elemental Ti-Al-C powder compacts. The compositional ratios of Ti:Al:C varied from 2:1:1 to 2:1.2:0.8 to explore the effects of deficient carbon and excess Al on the combustion kinetics and product formation. For the Ti-Al-C powder compacts, self-sustaining combustion featuring a distinct combustion wave was readily achieved upon ignition. Excess Al caused a decrease in combustion temperature and flame-front velocity, while deficient carbon showed relatively little influence. The synthesized product from the sample with an exact stoichiometry of Ti:Al:C = 2:1:1 was composed of 79.5 wt.% Ti₂AlC, 9.8 wt. Ti₃AlC₂, 10.7 wt.% TiC, and a small amount of Ti₃AlC. The addition of excess Al by 20 at.% not only increased the yield of Ti₂AlC but avoided the formation of Ti₃AlC. A reduction of carbon further improved the evolution of Ti₂AlC. The sample with an off-stoichiometric proportion of Ti:Al:C = 2:1.2:0.9 yielded the optimum product composition of 91.9 wt.% Ti₂AlC, 4.2 wt.% Ti₃AlC₂, and 3.9 wt.% TiC. This was attributed to the fact that excess Al and deficient carbon facilitated the formation of TiAl and sub-stoichiometric TiC, both of which acted as the intermediate phases to combine into Ti₂AlC. The as-synthesized Ti₂AlC grains were in the shape of thin platelets with a size of 4-8 μm and a thickness of about 1.0 μm. A laminated microstructure formed by closely stacked platelets is typical of the MAX carbide.