Experimental investigation of phase equilibria in the Ti-Al-Mo ternary system

Phase equilibria in Ti-Al-Mo ternary system are of fundamental importance to better acquire the phase-forming regulation of TiAl-based alloys. In this paper, phase relations in this system have been investigated with Ti-TiAl-Mo diffusion triples combined with key equilibrated alloys. Five isothermal sections of the Ti-Al-Mo ternary system at 1473, 1373, 1273, 1173, and 1073 K were experimentally determined by means of electron probe microanalysis, scanning electron microscope, and X-ray diffraction. The TiAl and TiAl3 phases show wide ternary composition ranges, while the solubility of Mo in the Ti3Al and alpha Ti phases is relatively limited. The solubility of Al in the beta(Ti, Mo) phase decreases from 49.0 at.% at 1473 K to 39.9 at.% at 1073 K while that of Ti in the AlMo3 phase decreases from 25.4 at.% at 1473 K to 20.8 at.% at 1073 K. Moreover, the ternary compound sigma forms at a temperature between 1423 and 1373 K through a peritectoid reaction beta(Ti, Mo) + AlMo3 -> sigma. A peri-eutectoid invariant reaction beta(Ti, Mo) + AlMo3 -> sigma + TiAl3 at 1373-1273 K was further deduced and the existence of another invariant reaction beta(Ti, Mo) + TiAl3 -> sigma + TiAl at a temperature between 1273 and 1173 K was also confirmed. The established phase equilibria of the Ti-Al-Mo ternary system may provide essential data required for the construction of reliable thermodynamic assessment of the Ti-Al-Mo system.