Structural Analysis of diffuse scattering appeared in high-purity TiAl alloys

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Two-phase TiAl alloys containing a small fraction of Ti_3Al have attracted a great deal of interest as a new class of high-temperature materials. In electron diffraction patterns of Ti-56at. % Al alloys which have been considered to be well within the TiAl single-phase field, we have observed diffuse intensity which can not be considered to come from the TiAl phase with the Ll_0 structure. Such diffuse intensity iis observed in diffraction patterns taken along <001], <100], <111] and <211] but not in those taken along <110], <011] and <112]. Ahthough we firstly inferred that the occurrence of such diffuse intensity is closcly related to the existence of oxygen impurity in the TiAl phase, experiments conducted on Ti-56at. %Al alloys containing 300,500 and 800 wt. ppm oxygen have revealed that the diffuse intensity does not depend on oxygen content in these alloys. However, the diffuse intensity clearly depends on Al content. While no diffuse intensity is observcd for alloys containing less than 53at. %Al, the diffuse intensity increases with increasing Al content for alloys containing more than 54at. %Al. Close examination of the diffuse intensity has in dicated that the intensity is from the Al_2Ti phase. HREM observations along <100] have revealcd a fine domain structure of the Al_2Ti phase. The volume fraction of the Al_2Ti domains increases with increasing Al content, consistent with the results obtained from diffraction experiments. These results clearly indicate that a composition of Ti-54at. %Al is in the TiAl/Al_2Ti two-phase field and that the composition range where TiAl single-phased alloys are obtained should be much smaller than that ever considered.