Cocrystallization of Max-Phases in the Ti–Al–C System

The structure and phase transformations in the Ti–Al–C system were studied by X-ray diffraction, differential thermal analysis, and scanning electron microscopy, including energy-dispersive X-ray spectroscopy and electron backscatter diffraction on samples obtained by arc melting and annealing at high temperatures. The ternary system has a cocrystallization region for the two MAX-phases, N and H. The Ti 41 . 5 Al 38 . 5 C 20 samples contain three phases at all experimental temperatures (from 650 to 1660°C): Ti 3 AlC 2 (N-phase of Ti 3 SiC 2 type), Ti 2 AlC (H, Cr 2 AlC type), and binary intermetallic TiAl 3 (ε, its own crystal type). The morphology of the as-cast alloy and annealed samples (at temperatures above and below the solidus temperature, 1660 and 1250°C, respectively) shows that invariant solidification at 1405°C (solidus temperature) precedes the univariant simultaneous solidification of N- and H-phases, i.e. both MAX-phases separating from the melt.