Effect of directional heat treatment on microstructure evolution for TiAl alloys

Abstract

TiAl alloy is a new lightweight structural material developed for high-temperature applications. Acquiring directional structure with columnar crystals is an effective way to improve its service temperature. We report here a low-cost directional heat treatment (DHT) method to prepare columnar grain structure for TiAl alloy. To reveal the dynamic process of grain boundary migration during DHT, influence of heating temperature and withdraw rate on the microstructure evolution were investigated. The results showed that the columnar crystals can be acquired under heating temperature of 1250°C and 1300°C. The aspect ratio of the columnar crystals changed both with heating temperature and directional withdraw rate, and there existed an optimal withdrawal rate under which the maximum aspect ratio of columnar crystals can be obtained. This optimal withdrawal rate was found to be in accordance with the grain boundary migration velocity at specific temperature. Therefore, if the withdrawal rate was too fast or too slow, the grain boundary migration cannot match with it, under which condition only equiaxed grains can be formed. In addition, the lamellar spacing of TiAl alloys was found to be inversely proportional to the withdrawal rate, which provided the basis for controlling lamellar structure through DHT. Our results can be valuable guide for the development of high-temperature TiAl alloys, therefore benefiting the design of lightweight mechanical systems.