Compositional inhomogenuity and mechanical properties of gamma/gamma interfaces in two-phase TiAl alloys

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Two-phase TiAl (gamma) alloys containing a small fraction of Ti_3Al (alpha_2) have attracted a great deal of interest as a new class of high-temperature materials. Such TiAl alloys exhibit a lamellar microstructure when produced by ingot metallurgy methods. There are six types of differently oriented TiAl variants in the lamellar structure because of the orientation relationship of {111} _<gamma>// (0001) _<alpha2>, [110] _<gamma>// [1120] _<alpha2>. This leads to the occurrence of three different types of gamma/gamma lamellar interfaces, in addition to alpha_2/gamma lamellar interfaces. We investigated compositional variation in the close vicinity of three different types of gamma/gamma lamellar interfaces for binary and some ternary TiAl alloys with a transmission electron microscope equiped with a field-emission gun where an electron prove of as small as 0.5 nm is used. At the same time, we investigated mechanical properties of these alloys in order to see whether the occurrence of … More compositional variation at gamma/gamma lamellar interfaces affects the interfacial strength or not. No evidence for compositional variation is detected at ordered twin gamma/gamma interfaces for any of the alloys investigated. On the other hand, at high-energy boundaries (120ﾟ -rotational and pseudotwin gamma/gamma interfaces), we observed compositional variation which varies with alloying elements. Segregation of Ti atoms onto these two-types of interfaces is observed for binary, Mn-and Nb-doped alloys whereas not only Ti atoms but also alloying element are observed to segregate there for Cr-, Mo- and Ta-doped alloys. We have observed that failure tends to occur across lamellar boundaries for alloys with compositional variation at the boundaries when tensile tests are made with the lamellar boundaries inclined from the loading axis, indicating the boundaries are strengthened with segregation of alloying elements. Tensile testing made perpendicular to the lamellar boundaries, however, reveals that failure occurs parallel to the boundaries with no tensile elongation. This indicates that the extent of strengthening with segregation of alloying elements is not large enough for the boundaries not to be cleaved against stresses normal to the boundaries. Less