Microstructures and properties of a hot-extruded TiAl containing no Cr

Microstructures and tensile properties of the TiAl alloy CTI-8, with the composition of Ti–47Al–(2–4)Nb–(0.5–1.0)W–(0.08–0.18)B (at.%) were studied by hot extrusion at various temperatures below and above Tα (=1315 °C). Both extrusion temperature and boron concentration are important in controlling lamellar structures in the alloy. Uniform grain structures were formed in the alloy with >0.1% B, whereas abnormal grain growth was observed in the alloy containing less B and hot extruded at temperatures above 1350 °C. Electron microprobe studies revealed a non-uniform distribution of W and Nb in the extruded alloy, presumably due to the initiation of phase transformations involving the redistribution of these alloying elements in the α and γ phases during cooling from the extrusion temperatures. Tensile properties at room and elevated temperatures are sensitive to microstructure and boron addition. The post-extrusion heat treatment at 1315 °C moderately lowers the yield strength but substantially increases the tensile ductility of CTI-8. Grain size is the key parameter in controlling the room-temperature ductility of TiAl alloys with and without Cr additions. Boron does not affect the yield strength but lowers the ductility of CTI-8 containing >0.10% B. In comparison, hot-extruded CTI-8 is substantially stronger and much more ductile than other advanced TiAl alloys with lamellar and duplex structures.