Microstructure control of TiAl alloys containing beta stabilizers by directional solidification

Abstract

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 beta stabilizers, such as Mo, Re and W, shifted the beta phase field at liquid/solid temperatures to the high Al composition side. So, the beta 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 beta (bcc) stabilizer among Mo, Re, W, and the Ti-47Al-2W composition was selected for beta 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 45degrees to the growth direction. This means that the beta 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. (C) 2002 Elsevier Science B.V. All rights reserved.