On the nature of the growth of bainitic α plates in metastable β Ti alloys

摘要

The growth mechanism of bainitic α plates in Ti-4.5Fe-6.8Mo-l.5Al, a metastable β Ti alloy, has been investigated by optical microscopy, electron probe microanalysis (EPMA) and dilatometry. The observations are compared with the transformation characteristics of primary α plates, which form at relatively high temperatures. The primary α plates form predominantly on β grain boundaries, whereas the bainitic α plates nucleate both at grain boundaries and intragranularly. It is shown that the morphological transition with decreasing temperature is associated with a change in transformation mechanism. The EPMA results show that the primary α plates are formed by a partitioning transformation. In contrast, the growth of the bainitic a plates is partitionless, followed by a post-transformation redistribution of Fe. This mechanism is similar to bainite in steel. The Fe diffusion from the supersaturated bainitic α plates to the β matrix causes the observed dilatation signal. The results of dilatometry in conjunction with optical microscopy indicate that a low misfit between the lattice structures exists, which is favourable for a partitionless transformation to occur at a low undercooling below T_0.