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

含少量Ti_3Al的双相TiAl合金作为一类新型的高温材料引起了人们极大的兴趣。在Ti-56 at. % Al合金中观察到的扩散强度不能被认为是来自于具有Ll_0结构的TiAl相。这种漫射强度在沿沿着<001]、<100]、<111]和<211]的衍射图样中观察到，但在沿沿着<110]、<011]和<112]的衍射图样中没有观察到。虽然我们首次推测这种扩散强度的发生与TiAl相中氧杂质的存在密切相关，但在Ti-56 at. %的Al合金，分别含有300、500和800 wt. ppm氧的扩散强度不依赖于这些合金中的氧含量。然而，扩散强度显然取决于Al含量。当原子含量小于53 at时，没有观察到扩散强度。%Al时，扩散强度随Al含量的增加而增加。对弥散强度的仔细观察表明，弥散强度来自Al_2Ti相。高分辨电子显微镜（HREM）观察显示，沿沿着<100]方向，Al_2Ti相具有细小的畴结构。Al_2Ti相的体积分数随Al含量的增加而增加，与衍射实验结果一致。这些结果清楚地表明Ti-54 at. %Al处于TiAl/Al_2Ti两相区，获得TiAl单相合金的成分范围应比以往考虑的要小得多。

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.