CRYSTALLOGRAPHY AND INTERFACIAL STRUCTURE OF PROEUTECTOID ALPHA-GRAIN BOUNDARY ALLOTRIOMORPHS IN A HYPOEUTECTOID TI-CR ALLOY

The interphase boundary structure of grain boundary allotriomorphs of proeutectoid alpha-formed at beta-grain faces in a Ti-6.62 at. % Cr alloy was studied with TEM. Allotriomorphs were shown to have a near-Burgers orientation relationship with respect to one beta-grain and usually an irrational orientation relationship with respect to the "opposite" beta-grain. Two types of ledges were observed on both interfaces of alpha-allotriomorphs. One type is growth ledges, with irregular paths and interledge spacings; the other is misfit compensating ledges, lying along a well-defined direction with a uniform spacing. The latter ledge has an effective Burgers vector of either a c-type or (c + a)-type near-edge dislocation. These results provide direct support for the proposal that the interfaces of face-nucleated grain boundary allotriomorphs should be partially coherent during growth with respect to both bounding matrix grains.