Observation Of An Orthorhombic Phase Formation In An Advanced Powder Processed High Niobium Containing $\gamma$-TiAl Alloy

In extruded conditions the advanced γ-TiAl alloy Ti-42Al-8.5Nb (at.%) exhibits an excellent combination of strength and ductility. In an attempt to transfer these excellent properties to powder metallurgical processed alloys, powder with an extra low level of impurities was produced by plasma arc melting and Electrode Induction Melting Gas Atomization technique. The spherical powder particles with a size of 0-180 µm were hot isostatically pressed in cylindrical titanium capsules at 1250°C and 200MPa for 2h. A characteristic microstructural feature which is believed to contribute to the balanced properties of these TiAl alloys is a nano-scale modulation of phases within a lamellar microstructure. As a result one set of lamellae appears with a tweed-like contrast when imaged in the TEM. Detailed TEM investigations and in-situ synchrotron measurements have shown that the tweed-like appearing lamellae consist of an arrangement of multiple stable and metastable phases with at least one orthorhombic constituent