Intermetallic TiAl-alloys combine excellent high-temperature strength with low density and good corrosion resistance. These properties make them attractive candidates for structural applications in aero-engines. The addition of 5-10 at.-% of Nb to alloys of the general composition Ti-(40-44)Al further enhances the alloys' mechanical properties, whereby the improvement was attributed to the presence of an orthorhombic phase in the alloy's microstructure .
For comparison, the orthorhombic O-phase Ti2AlNb exists in alpha2-Ti3Al based alloys in the composition range Ti-(12-31)Al-(12.5-37)Nb . In gamma-TiAl based alloys the formation of orthorhombic phases can be promoted by an excess amount of Al in the hexagonal alpha2 phase or by alloying with Nb in combination with appropriate heat treatments [3, 4]
We produced the alloy Ti-42Al-8.5Nb by powder metallurgy and optimized the heat treatment for the presence of orthorhombic phases. The alloy investigated here was compacted by hot iso-static pressing (HIP) at 1250°C and 200 MPa and annealed at 650°C for 2 hours, followed by oil quench.
On the micrometer scale the alloy's microstructure consists of lamellar (alpha2 + gamma) colonies with small amounts of omega_o phase at grain boundaries, no change is observed after the heat-treatment. Using single crystal electron diffraction and high-energy x-ray diffraction (HEXRD) we identified the orthorhombic O-phase within alpha2 laths of the lamellar colonies. The nominal composition of the O-phase is Ti2AlNb, therefore a Nb depleted form of the O-phase is present in our alloy. Imaging in the TEM reveals facets within the former alpha2 laths as shown in Fig. 1. The presence of facets suggests that the hexagonal alpha2 phase decomposes into several orientation domains of product phase. STEM-HAADF imaging confirms that the contrast variation between adjacent facets results from small changes in crystal orientation and not from variations in z-contrast. In high resolution imaging in Fig. 2 lattice distortions in the alpha2 basal plane are visible between regions of different atomic ordering.
In this presentation, the spatial arrangement of phases within the lamellar (alpha2 + gamma) colonies will be discussed.
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