On the Nature of the Growth of Bainitic α plates in Metastable β Ti Alloys

S.M.C. van Bohemen; Jilt Sietsma; Sybrand van der Zwaag

doi:10.4028/www.scientific.net/MSF.539-543.3684

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HomeMaterials Science ForumMaterials Science Forum Vols. 539-543On the Nature of the Growth of Bainitic α plates...

On the Nature of the Growth of Bainitic α plates in Metastable β Ti Alloys

Abstract:

The growth mechanism of bainitic α plates in Ti-4.5Fe-6.8Mo-1.5Al, 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 α 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 T0.