Effects of double-aging heat-treatments on the microstructure and mechanical behavior of an Nb-modified Ti-5553 alloy

Ti-5553 and related alloys are known as important structural alloys in the aircraft industry. In this work, the effect of double aging heat-treatments on the phase transformations and mechanical behavior of an Nb-modified Ti-5553 alloy (Ti-12Nb-5Al-5V-3Cr-0.5Fe wt%) was investigated by X-ray diffraction, scanning, and transmission electron microscopy. Results show the phase transformation sequence is altered with the time spent at the first aging stage. After 3 h at 350 degrees C, isothermal omega-phase is extensively formed throughout the beta-matrix, while after 8 h, alpha-phase embryos were detected. The first stages of alpha-phase formation in the modified alloy seem to be influenced both by the presence of omega particles and low-angle grain boundaries. Microstructures resultant of the first stages of aging change the response to the second stage, performed at 525 degrees C and 600 degrees C, for 2 h. The final microstructure obtained for the modified alloy (Ti-12Nb) after the second stage of aging diverge from the original Ti-5553 alloy subjected to the same heat-treatment. Finally, tensile and compression tests indicate Ti-12Nb is a viable substitute for Ti-5553, with comparable strength and slightly lower ductility after double aging. (AU)