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

Abstract

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 back-scatter diffraction on samples obtained by arc melting and annealing at high temperatures. It is shown that the ternary system has a cocrystallization region for both MAX-phases, N and H. The Ti_41.5Al_38.5C₂₀ samples contain three phases at all experimental temperatures (from 650 to 1660 °C): Ti₃AlC₂ (N-phase of Ti₃SiC₂ crystal structure type), Ti₂AlC (H, Cr₂AlC type), and binary intermetallic TiAl₃ (ε, its own crystal structure type). The morphology in 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, both MAX-phases separating from the melt.