High Temperature X-Ray Diffractometry of Ti-Al Alloys

Abstract

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 situX-ray diffraction data for a titanium-45 atomic percent aluminum alloy clearly showed the disappearance of the ordered Ti₃A1 structure on heating to 1300°C, but with the fundamental a-Ti diffraction peaks remaining. All diffraction peaks are indexed and prove the existence of the previously proposed Ti₃A1 + TiAI → α Ti eutectoid reaction near 1125°C in this alloy. No BCC I3-Ti phase was detected for this alloy up to 1400°C. In addition, two sets of hexagonal a-Ti diffraction peaks in Ti₅₅A1₄₅were detected during both heating and cooling between 1250-1400°C, suggesting the formation of a new high temperature disordered hexagonal a’ phase. This conclusion is supported by the discovery of a discontinuity in the volume expansion coefficient for a-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 13-Ti phase up to 1350°C. Debye Waller factor analysis of the γ-TiA1 phase diffraction peaks for Ti₄₈A1₅₂also indicated the absence of any phase changes between 850-1250°C. The modified Ti-Al phase diagram presented here includes a shift in the y-phase transus lines to higher aluminum contents, the addition of a new α’ phase region, and the elimination of the ß + γ phase field.

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