The role of crystallographic and geometrical relationships between α and β phases in an α/β titanium alloy

The present study has examined for α/β-Ti alloys the relationship between the morphology and crystallography of Widmanstätten plates of α-Ti in colonies within a prior grain of β-Ti. Thus, optical metallography, scanning electron microscopy and transmission electron microscopy have been used to characterize the morphological features of the microstructure, whereas orientation-imaging microscopy (OM) and transmission electron microscopy (TEM) have been employed to reveal crystallographic information. It has been discovered that within a prior β-Ti grain, although the growth direction of the Widmanstätten plates in given colonies may differ by large angles from α-plates in other colonies, they may exhibit very close crystallographic relationships. For example, inclined α-plates may share common basal planes and be related by a rotation of ~10.5° about the c-axis of the crystals. This phenomenon has been interpreted on the basis of variant selection of the Burgers orientation relationship commonly adopted between the α and β phases in these alloys. Similar relationships have been observed in α-colonies growing either side of a given prior β grain boundary. These latter observations have been used to draw conclusions concerning the precipitation of α on prior β grain boundaries.