Crystallographic and morphological relationships between β phase and the Widmanstatten and allotriomorphic α phase at special β grain boundaries in an α/β titanium alloy

In the present study, the relationship between the crystallographic orientations and growth directions of grain boundary-allotriomorphic-alpha (GB alpha) and secondary Widmanstatten alpha laths growing from the GB alpha at grain boundaries separating beta grains with specific misorientations has been examined. These relationships have been determined using a variety of characterization techniques, including scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy (TEM) and a dual-beam focused ion beam instrument to provide site-selected TEM foils. Two very interesting cases, one in which the two adjacent beta grains are rotated mutually by approximately 10.5 degrees about a common < 110 > direction and the other in which the two beta grains are in a twin relationship, i.e. a 60 degrees rotation about a common < 111 > direction, have been studied. It was discovered that the alpha laths growing into two adjacent beta grains from the common grain boundary may have the same orientation in both grains, while they may have either large (similar to 88.8 degrees) or small (28.8 degrees) angular differences in growth directions in the two adjacent beta grains, depending on the relative misorientation of the beta grains. The growth directions of the alpha laths growing from such boundaries are explained on the basis of the Burgers orientation relationship between the Widmanstatten alpha and the beta phases and the interfacial structure proposed previously by various workers. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.