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 °C. 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 Ti1−xAl1+x is ruled out, at least in the investigated temperature range. The phase h-Al2Ti is a metastable phase which forms instantly from TiAl supersaturated with aluminium during cooling but transforms only very slowly into the equilibrium phase r-Al2Ti. h-Al2Ti is frequently observed because the solubility of aluminium in TiAl increases markedly with increasing temperature. Above 1215 °C 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 Al5Ti2-structure. The 1d-APSs/Al5Ti2 are stable down to a temperature of about 980 °C where they disintegrate at a final composition between 70 and 71 at.% Al by eutectoid decomposition: 1d-APSs/Al5Ti2 ↔ r-Al2Ti + Al3Ti.