Massive-parent interphase boundaries and their implications on the mechanisms of the α → γ _M massive transformation in Ti-Al alloys

Abstract

The massive-parent interphase boundaries associated with the α → γ _M massive transformation in a Ti-46.5 at. pct Al alloy were studied. Special experiments were performed to arrest the transformation at an early stage. Orientation relationships (ORs) between the γ _M and parent α (retained as α ₂) phases were determined using electron backscattered diffraction (EBSD) in a scanning electron microscope and by electron diffraction, and the interphase boundaries were characterized by two-beam bright-field/weak-beam dark-field (WBDF) transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The results reveal that the γ _m nucleates at grain boundaries generally with a low-index Burgers orientation relation and a coherent interface with one parent grain, but grows into the adjacent grain with a high-index/irrational orientation relation. The growth interfaces between the two phases are generally free of misfit dislocations or other defects and consist of curved parts as well as planar facets, whose macroscopic habit planes are of generally high-index/irrational orientation and deviate substantially from the close-packed planes. On an atomic scale, the growth interfaces are sometimes found to be faceted along {111} planes, as well as along other planes, with closely spaced steps, but are concluded to be incoherent with respect to the parent grain into which growth occurs. The implications of these results on the nucleation and growth mechanisms associated with the α-to-γ _M massive transformation are discussed. In particular, the nature of the interphase boundaries and their relation to whether growth occurs by a ledgewise motion of the interfaces or by continuous growth are addressed.