Atom probe field ion microscopy of polysynthetically twinned titanium aluminide

Two phase {gamma}-based TiAl alloys are attractive for structural applications at high temperatures because they possess good elevated-temperature mechanical properties, low density, and good creep and oxidation resistance. The microstructures of these alloys consist of plates of the near equiatomic {gamma} phase (L1{sub 0}-ordered structure) and the Ti{sub 3}Al {alpha}{sub 2} phase (D0{sub 19}-ordered structure). It is of great interest to study the details of the lamellar {alpha}{sub 2} + {gamma} microstructure because the interface stability is the key to providing a usable high temperature material. Polysynthetically twinned (PST) TiAl crystals have been developed in order to systematically study the lamellar microstructure. These PST materials contain no high angle grain boundaries and have an single set of aligned lamellar of {alpha}{sub 2} and {gamma} phases. Therefore, PST samples facilitate the study of the dependence of mechanical properties on lamellar structure by providing a known, consistent set of aligned lamellar. Previous transmission electron microscopy studies of PST TiAl have shown that Cr and Mo segregation occurs at certain non-coherent {gamma}/{gamma} twin boundaries. These studies also found a depletion of aluminum at certain {gamma}/{gamma} interfaces, showing ``{alpha}{sub 2}-like`` compositions. However, no evidence was found of even a few unit cells of the D0{sub 19}-ordered structure at the interface by microdiffraction. This paper investigates the feasibility of applying atom probe field ion microscopy to PST TiAl samples and presents some preliminary characterization results from {gamma}/{gamma} interfaces in PST TiAl.