Effect of Ta element on microstructure formation and mechanical properties of high-Nb TiAl alloys

In order to improve mechanical properties, different Ta contents were added into Ti42A16Nb2.6C alloys, which were prepared by vacuum arc melting. Microstructure evolution and mechanical properties of the alloys were experimentally and theoretically studied, especially the B2 phase disappearing mechanism. Results show that the B2 phase is disappeared when Ta content is more than 0.2at.%. With increasing Ta content, the length-width ratio of Ti(2)AIC decreases, but the lamellar colony size increases, even up to 2.2 times. EDS results show that Ta distributes uniformly. Owe to its higher melting point and bigger atom radius, Ta has lower diffusion rate and impedes diffusion of Nb during solidification, which results in disappearance of B2 phase. Because Ta can expand solid-liquid temperature range, and this provides more time for grain growing, which accounts for the increasing lamellar colony size. Compressive testing shows that the strength increases 1.2 times and the strain increases 3.2 times when Ta content is 0.2 at.%. Tensile strength at 750 degrees C is improved by adding Ta. These improvements are mainly resulted from the solid-solution strengthening of Ta and Nb, and the elimination of B2 phase, although the lamellar colony size is increased.