Time-temperature transformation behavior of Ti-24Al-11Nb

The time-temperature transformation behavior of a rapidly solidified (RS) Ti-24Al-11Nb alloy has been studied between 450 and 850 °C using X-ray diffraction, electron diffraction, transmission electron microscopy and microindentation methods. A revised isothermal transformation path from the initial B2 to the final α2 structure has been proposed: B2 (Pm3m) → B19 (Pmma) → O′ (Cmcm) → O′ (Cmcm) → α2 (P63/mmc). Age-hardening was found to accompany the B2 → O′ transitions. The hardness increased to a maximum value of about 5.4 GPa when the microstructure consisted of mostly O′ laths. Further aging resulted in a softening correlated with O′ coalescence and transformation to the α2 phase. The hardness decreased to a minimum value of about 2.6 GPa when the microstructure had evolved to an equiaxed α2 structure. The results reveal that the O′ → α2 reaction becomes extremely slow, i.e. the O′ phase is kinetically stabilized, at temperatures below about 500 °C.