Phases and phase equilibria in the Al-rich part of the Al-Ti system above 900°C

The Al-rich part of the Al-Ti system in the composition range 55-75 at.% Al has been investigated in the temperature range 900-1300 degreesC. By combining results obtained by metallography, X-ray diffraction (XRD), electron probe microanalysis (EPMA), differential thermo analysis (DTA) and transmission electron microscopy (TEM), the phase equilibria in the respective part of the Al-Ti system have been established. According to these results the existence of the phase Ti(1-x)Al(1+x) is ruled out, at least in the investigated temperature range. The phase h-Al(2)Ti is a metastable phase which forms instantly from TiAl supersaturated with aluminium during cooling but transforms only very slowly into the equilibrium phase r-Al(2)Ti. h-Al(2)Ti is frequently observed because the solubility of aluminium in TiAl increases markedly with increasing temperature. Above 1215 degreesC a continuous homogeneity range exists which extends up to about 71 at.% Al. It is not yet clear whether TiAl is stable up to this Al-content or if there is a second-order transition to the so-called one-dimensional antiphase domain structures (1d-APSs) which are observed in all alloys with at least 68 at.% Al by TEM after cooling. The XRD patterns of the 1d-APSs could be completely indexed by the Al(5)Ti(2)-structure. The 1d-APSs/Al(5)Ti(2) are stable down to a temperature of about 980 degreesC where they disintegrate at a final composition between 70 and 71 at.% Al by eutectoid decomposition: 1d-APSs/Al(5)Ti(2) <----> r-Al(2)Ti + Al(3)Ti. (C) 2002 Elsevier Science Ltd. All rights reserved.