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 cz? structure has been proposed: B2 (Pmjm)B19 (Pmma)-0’ (Cmcm) 0’ (Cmcm) a2 (P6,/mmc). Age-hardening was found to accompany the B2 + 0’ transitions. The hardness increased to a maximum value of about 5.4 GPa when the microstructure consisted of mostly 0’ laths. Further aging resulted in a softening correlated with 0’ coalescence and transformation to the a2 phase. The hardness decreased to a minimum value of about 2.6 GPa when the microstructure had evolved to an equiaxed a2 structure. The results reveal that the O’a2 reaction becomes extremely slow, i.e. the 0’ phase is kinetically stabilized, at temperatures below about 500 “C.