EFFECT OF OXYGEN ON MICROSTRUCTURE AND PHASE TRANSFORMATION OF HIGH Nb CONTAINING TiAl ALLOYS

Due to the low density, high specific strength, elastic modulus and oxidation resistance at high temperature, TiAl-based alloys have attracted much attention as a candidate of the next generation high temperature materials in aerospace and automobile application. Meanwhile, the excellent properties oxidation resistance, creep strength and tensile strength at the elevated temperature make the high Nb containing TiAl alloys be one of the promising development directions of future TiAl alloys. During the studies about alloying which is an efficient way to improve the performance of TiAl alloys, researchers have found that interstitial atoms B, C and N notably refine the grains and then improve mechanical properties including yield strength, micro-hardness, and tensile ductility of TiAl alloys. During the melting, casting, forging and the application environment, the TiAl alloys also are always inevitable to be contaminated by the O. In this work, the high Nb containing Ti-46Al-8Nb-xO alloys (atomic fraction) were prepared by non-consumable vacuum arc remelting under the protection of Ar atmosphere. The aim of the present work is to study the influence and the corresponding mechanism of oxygen atoms on the microstructure evolution and phase transformation of high Nb-TiAl alloys. The results indicate that oxygen atoms in Ti-46Al-8Nb-xO alloys remarkably increase the amount of alpha(2) phase. The increasing oxygen content leads to the grain refinement. Meanwhile, the duplex microstructures translate into fully lamellar. It indicates that the interstitial oxygen essentially reduces the kinetics of alpha -> gamma. Consequently, the fully lamellar is easier formation than the duplex microstructures. It is found that interstitial oxygen atoms preserve significantly influence on the microstructure of Ti-46Al-8Nb-xO alloys. With the increase of oxygen content, the beta solidification translates into alpha solidification and the peritectic reaction alpha+L -> gamma moves to a lower Al content. At the same time, the eutectoid reaction alpha -> alpha(2)+gamma shifts to a higher Al content which extends the area of alpha phase. The DSC results show the effect of oxygen on the phase transformation of Ti-46Al-8Nb. The DSC curves indicate that the addition of oxygen increases the eutectoid reaction temperature of Ti-46Al-8Nb-xO alloys, but this effect can be gradually reduced with the further increase of oxygen content.