PHASE FORMATION IN TI–AL–C SYSTEM DURING SHS

Abstract

The phase formation of Ti–Al–C powder mixtures with compositions close to the composition of the MAX phases in self-propagating high-temperature synthesis (SHS) was investigated using time resolved X-ray diffraction. It is found that material formation during combustion in air under low heat removal rates is a staged process. At the first stage, the dominant is the reaction of titanium carbide formation providing major heat release and combustion front propagation. As a result, TiC crystals surrounded by the Ti–Al melt are formed. Behind the combustion front titanium carbide dissolves in the surrounding melt and then the Ti 2 AlC ternary compound is crystallized. TiC formation is not observed with the synthesis in helium providing high heat removal rates. The first phase emerging on the diffraction field is Ti 2 AlC. The TiC life cycle of 5–10 s for air-synthesized mixtures is significantly reduced for helium processes and does not exceed 1 s. SHS reaction in helium yielded a Ti 2 AlC-based composite containing less than 20 wt.% of TiAl, and 2 wt.% of TiC. The material structure is characterized by laminated Ti 2 AlC grains surrounded by the TiAl matrix. The microhardness of synthesized materials was 4,0–4,5 GPa that corresponds to that of the Ti2AlC phase. Ti 2 AlC grains synthesized in helium are smaller than in air. Laminated MAX-phase grain sizes grow up to 8–15 μm in length and 2–5 μm in width at slow air cooling. The Ti 2 AlC grain size in helium is lower – up to 8 μm in length and 1 μm in width.