Phase Formation in the Ti–Al–C System during SHS

Abstract

The phase formation of Ti–Al–C powder mixtures with compositions close to MAX phases during self-propagating high-temperature synthesis (SHS) is investigated using time-resolved X-ray diffraction. It is found that the formation of the material during combustion in air under low heat-removal rates is a staged process. At the first stage, the dominant reaction is titanium carbide formation, providing major heat release and combustion front propagation. This reaction leads to the formation of TiC crystals surrounded by the Ti–Al melt. Titanium carbide is dissolved in the surrounding melt behind the combustion front with the subsequent crystallization of the Ti2AlC ternary compound. No TiC formation is observed during synthesis in helium, which provides rapid heat removal. The first phase appearing in the diffraction field is Ti2AlC. The TiC life cycle of 5–10 s for the mixtures synthesized in air substantially decreases when performing the process in helium and does not exceed 1 s. SHS leads to the formation of the composite material based on Ti2AlC phase containing less than 20 wt % TiAl and 2 wt % TiC. The material structure is characterized by lamellar Ti2AlC grains surrounded by the TiAl matrix. The microhardness of synthesized materials is 4.0–4.5 GPa and corresponds to the microhardness of the Ti2AlC phase. The dispersity of Ti2AlC grains during the synthesis in helium is lower than during the synthesis in air. Lamellar MAX-phase grains grow to 8–15 μm in length and 2–5 μm in width during slow cooling in air. The dispersity of Ti2AlC grains grown in helium is lower, being no larger than 8 and 1 μm, respectively.