Anomalous increase of fracture toughness of TiAl-based alloys at high temperature

Abstract

The anomalous increase of mechanical property of TiAl-based alloys at high temperature caters to the property requirements as the high-temperature structural materials, which is thus important and worth studying. However, people currently are not clear whether the fracture toughness of TiAl-based alloys has an anomalous increase at high temperature, and if so, how much it increases, and why it increases. In order to answer these questions, the forged Ti-44Al-8Nb model alloy with fine and uniform microstructure was used. The fracture toughness of the alloy at different temperatures was tested. The results indicated that the alloy did show an anomalous increase of fracture toughness at 700 °C–800 °C. The main crack deflected a large angle at 700 °C, and terminated at the upper part at 750 °C and 800 °C. The fracture morphology was fine, and the cleavage plane became smaller and less at high temperature. These characteristics corresponded to the anomalous increase of fracture toughness at high temperature. Through reverse thinking, we found that the high dislocation density was the root causing the anomalous increase of fracture toughness at high temperature. The forging-induced texture, such as B2 〈100〉//RD and γ-TiAl 〈011]//RD could also promote the dislocations slip on the main slip surfaces during the high temperature fracture toughness test, contributing to the final high dislocation density. The critical crack size at 700 °C–800 °C was about 4 times that of room temperature, corresponding to the higher fracture toughness of the alloy at high temperature.