High temperature X-ray diffractometry of Ti-Al alloys

High temperature X-ray diffraction, an established technique for high temperature materials characterization, has been applied to the titanium-aluminum system in order to obtain structural information on the material at elevated temperatures. In situ X-ray diffraction data for a titanium-45 atomic percent aluminum alloy clearly showed the disappearance of the ordered Ti3Al structure on heating to 1300°C, but with the fundamental α-Ti diffraction peaks remaining. All diffraction peaks are indexed and prove the existence of the previously proposed Ti3Al + TiAl &rarr α Ti eutectoid reaction near 1125°C in this alloy. No BCC B-Ti phase was detected for this alloy up to 1400°C. In addition, two sets of hexagonal α-Ti diffraction peaks in Ti55Al45 were detected during both heating and cooling between 1250-1400°C, suggesting the formation of a new high temperature disordered hexagonal α′ phase. This conclusion is supported by the discovery of a discontinuity in the volume expansion coefficient for α-Ti at the low end of this temperature range. Only slight modifications to the existing Ti-Al phase diagrams are required to account for the present results. High temperature X-ray diffraction measurements on a titanium-52 atomic percent aluminum alloy also showed no B-Ti phase up to 1350°C. Debye Waller factor analysis of the γ-TiAl phase diffraction peaks for Ti48Al52 also indicated the absence of any phase changes between 850-1250°C. The modified Ti-Al phase diagram presented here includes a shift in the γ-phase transus lines to higher aluminum contents, the addition of a new α′ phase region, and the elimination of the B + γ phase field.