Experimental observations elucidating the mechanisms of structural bcc-hcp transformations in β-Ti alloys

The formation mechanisms of two hcp alpha phase morphologies in Ti-4.5Fe-6.8Mo-1.5Al have been investigated by optical microscopy (OM), atomic force microscopy (AFM), electron probe microanalysis (EPMA) and dilatometry. At relatively high temperatures primary alpha forms predominantly on prior bcc beta grain boundaries, whereas at lower temperatures so-called bainitic alpha plates nucleate both at grain boundaries and intragranularly. This morphological transition with decreasing temperature is associated with a change in transformation mechanism. The combined results of EPMA, OM and dilatometry show that the growth of these bainitic alpha plates is partitionless, and not accompanied by a volume change. Subsequently, a post-transformation redistribution of Fe takes place, which causes a dilatation that can be modelled based on the diffusion of Fe and the interface-area density. This mechanism as well as the formed microstructure are similar to bainite in steel, and therefore we chose to denote this transformation product as bainitic alpha. In addition, the AFM observations on bainitic alpha plates show an invariant plane strain surface relief with tilt angles that are consistent with the Burgers' transformation model based on shear. In contrast, the AFM results show that the formation of primary alpha is accompanied by an irregular dip on a free surface, which is in agreement with the volume decrease measured using dilatometry. Furthermore, the EPMA results show that primary alpha is formed by a partitioning transformation. The change in transformation mechanism with decreasing temperature is supported by the observed trend in both the dilatation and the volume fraction alpha as a function of temperature.