Microstructure control of TiAl alloys containing β stabilizers by directional solidification

Phase transformation at elevated temperatures in Ti-Al-X(X=Mo, Re, W) systems has been investigated by analyzing the dendrite morphology of arc-melted buttons. It was found that the addition of β stabilizers, such as Mo, Re and W, shifted the β phase field at liquid/solid temperatures to the high Al composition side. So, the β phase formed as a primary crystal even in higher Al compositions in Ti-Al-X ternary systems compared with Ti-Al binary. The addition of W was found to be the most effective β (bcc) stabilizer among Mo, Re, W, and the Ti-47Al-2W composition was selected for β solidification. In the directional solidification of Ti-47Al-2W alloy at the growth rate of 90 mm/h using the Bridgman type DS apparatus, it was found that the lamellar orientation in the columnar grains was parallel or inclined with the angle of 45° to the growth direction. This means that the β phase forms at the solid/liquid interface in the Ti-47Al-2W alloy. The tensile elongation of DS alloys was clearly improved compared with that of the polycrystalline alloy with a fully lamellar microstructure, while maintaining the same yield stress level.