| Thermal exposure at700℃in air was carried out on isothermally forged alloy Ti-45Al-8.5Nb-(WBY) for up to10000hours. The changes in microstructure and phase structure were characterized using optical microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The corresponding changes in mechanical properties were examined by tensile testing at room temperature. It was found that the a2lamellae in the duplex microstructure decomposed to some degree after1000h exposure, which is assumed to be responsible for the increase of tensile properties. After1700h exposure, equiaxed B2(ω) grains formed by consuming a2+γ lamellar colonies, which can be represented by a2+γâ†'B2(ω) phase transformation. And after exposure for3000h, the volume fraction of the B2(ω) grains formed approach to18%. Moreover, need-shaped phase precipitated extensively in the B2(ω) grains. It was important to note that the amount of the B2(ω) grains formed after10000h exposure is quite similar to that after3000h exposure. As a result, the strength and ductility were reduced slightly after10000h thermal exposure, no more than10%for both.Oxidation resistance of the alloy was examined at700℃and800℃for up to500hours, respectively. It shows that a relatively fast oxidation occurred at the initial stage for the two temperatures but a stabilized condition was reached roughly after100hours. There is no typical rutile structure in the700℃-oxidized surface. An uneven multilayer oxidation layer with A12O3and TiO2(-20μm in thickness) formed as the outer layer in stead. For800℃-oxidized surface, however, typical rutile structure was produced and outer oxidation layer was formed with a higher percentage of TiO2. The weight gain at800℃is25%more than that at700℃oxidation but both remained at a relatively low level so that the weight gain is less than0.4mg/cm3throughout the500hour-long oxidation. This alloy therefore demonstrates a relatively good oxidation resistance both at700℃and at800℃. |
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