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 degrees C using X-ray diffraction, electron diffraction, transmission electron m icroscopy and microindentation methods. A revised isothermal transform ation path from the initial B2 to the final alpha(2) structure has bee n proposed: B2 (Pm $$($) over bar Pmma)-->O'(Cmcm)-->O'(Cmcm)-->alpha( 2)(P6(3)/mmc). Age-hardening was found to accompany the B2-->O' transi tions. The hardness increased to a maximum value of about 5.4 GPa when the microstructure consisted of mostly O' laths. Further aging result ed in a softening correlated with O' coalescence and transformation to the a, phase. The hardness decreased to a minimum value of about 2.6 GPa when the microstructure had evolved to an equiaxed a, structure. T he results reveal that the O'-->-->alpha(2) reaction becomes extremely slow, i.e. the O' phase is kinetically stabilized, at temperatures be low about 500 degrees C.