Southwest Jiaotong University Master Degree Thesis

The surface oxidation behaviors of a fully lamellar Ti-44Al-5Nb-0.85W-0.85B alloy subjected to exposure at 700℃in air for 0-1000h were studied in the master degree research project. The effects of the oxidation on tensile properties and the related mechanism were revealed for the whole oxidation process (200h,500h and 1000h). In the meantime, the oxidation mass gain curves were established using thermo gravimetric analysis equipment.It has been found that compared to manual polishing samples, the manual+electrolytic polishing samples exhibit thinner and denser oxide layer on the surface, which is able to improve the oxidation resistance of the alloy to some degree, including the room temperature tensile strength. However, no matter which polishing technique used, the room temperature ductility was reduced significantly for the thermal-exposed alloy. The oxidation layer structure depends upon the exposure temperature applied. For the alloy exposed at 700℃, the oxidized layer is composed of both Al2O3 and TiO2, of which the Al2O3 is the major constituent, whereas for the alloy exposed at 900℃the layer is mainly the TiO2, below which there is a mixed layer consisting of TiO2 and Al2O3. The oxide layer of electrolytic polishing Samples is split into more parts, each layer is evidence, and in contrast, the manual polishing samples contain fewer parts and are inconspicuous. Du to the different activity of alloying constituent during oxidation process, different elements are beneficiating in particular sector, finally appear stratified phenomenon. Also found is that the borides on the surface is oxidized faster than the lamellae of the alloy. Research shows the microstructures and Variation of phase structures of y-TiAl have no notable changes after thermal expose at 700℃for1000h.