EFFECTS OF THERMAL EXPOSURE ON THE TENSILE AND FATIGUE PROPERTIES OF CAST Ti-47Al-2Cr-2Nb-0.15B ALLOY

The elevated temperature fatigue strength and room temperature tensile properties and fatigue strength of cast Ti-47Al-2Cr-2Nb-0.15B(atomic fraction, %) alloy after thermal exposure were investigated. The microstructural change of the bulk alloy and the microstructure of surface layer after 650 ℃/100 h and 800 ℃/100 h exposures were analyzed by means of XRD and SEM. The room temperature tensile ductility and fatigue strength decreased slowly with increasing exposure temperature when it is below 650 ℃, and then rapidly decreased above this temperature. Dependence of fatigue strength for the unexposed alloy with test temperature exhibits the same trend. The surface layer formed after 650 ℃/100 h exposure is an oxygen-enriched layer, while that after 800 ℃/100 h exposure is an oxide layer consisting of TiO2 and Al2O3. Such a change in the nature of the surface layer corresponds to the onset of rapid drop in mechanical proerties and fatigue strength of the alloy with increasing exposure temperature.