beta-solidifying Ti-43.5Al-4Nb-1Mo-0.5B has attracted considerable attention owing to its higher strength and excellent creep resistance at elevated temperature. Indeed, its application temperature is much higher than that of Ti-48Al-2Cr-2Nb. Because gamma-TiAl alloys are exposed to air at elevated temperatures for a long time during application, an oxidation layer is formed in the surface. The oxidation layer, which is potentially harmful to the mechanical properties of the crack nucleation sites, was observed near the surface. Concerning the beta-solidifying Ti-43.5Al-4Nb-1Mo-0.5B, it has median Nb content and low Al content. Additionally, a considerable beta(0)-phase with lower Al content is retained. To better understand the influence of the composition and microstructure on the oxidation behavior of gamma-TiAl alloys, it is necessary to investigate the oxidation behavior and microstructure evolution in the surface of beta solidifying gamma-TiAl alloys during thermal exposure. In this study, samples of beta-solidifying Ti-43.5Al-4Nb1Mo-0.5B were obtained by investment casting and thermal exposure at 700 degrees C for different times, and the oxidation behavior and microstructure of different phases in the surface were compared. The results showed that the constituents of the oxidation layer on the surface varied with the exposure time. The volume fractions of TiO2-R, alpha-Al2O3, Ti2AlN, and Nb2Al increased by increasing the exposure time. Metastable gamma-Al2O3 was detected in the sample exposed for a short time, but it was transformed into alpha-Al2O3 after exposure for 200 h. Moreover, metastable Ti4O7 and TiAl2O5 were detected in samples exposed for 200 and 500 h. The microstructures, morphologies, and heights of oxidations in the surface of a specific phase are different, varying by increasing the exposure time. These variations are related to the different oxidation behaviors during thermal exposure, i. e., the gamma-phase experienced selective oxidation after a short time exposure, a 2-phase changed from internal oxidation to selective oxidation when the exposure time reached 200 h, while the beta(0)-phase suffered internal oxidation during the entire exposure. The different oxidation behaviors of each specific phase contributed to the different Al contents. Dispersed TiO2 was formed during internal oxidation, and it kept growing during thermal exposure, forming a continual layer at the end. The continual Al2O3 layer was formed during selective oxidation, in which the Ti element was rejected in the reaction interface. When the content produced during the internal oxidation of the Ti element reached a critical value, dispersed TiO2 was formed and kept growing to form the continual layer. The alternating formation of continual Al2O3 and TiO2 layers resulted in the layer structure observed in the surface.
