Orientation and structure of planar facets on the massive phase γ_m in a near-TiAl alloy

The orientation and structure of planar facets on the γ_m massive phase formed in a Ti-46.5 at. pct Al alloy have been characterized using a combination of transmission electron microscopy (TEM) and electron diffraction. The planar γ_m/α₂ interfaces are irrational with respect to both the α₂ matrix phase and the γ_m phase, and there is neither evidence of a rational orientation relationship across such facets, nor resolution of a linear defect structure within the interface planes in conventional electron diffraction patterns and amplitude-contrast images, respectively. However, when imaged parallel to a particular direction in the interface, these irrationally oriented interfaces are invariably parallel to the Moiré plane, which is defined by the intersection between two sets of closely-packed planes in the γ_m and α₂ phases. The relationship is such that there is an effective continuity of these lattice planes across the interface and a one-dimensional coherency within the planar interface. This is interpreted to imply that such interface facets adopt a configuration of reduced energy and that they are not random and fully incoherent, as often described. It is suggested that these planar facets may migrate in a glissile manner normal to the interface plane by nucleation and rapid lateral movement within the interface plane of interracial defects that have the form of Moiré ledges, which are defined by the spacing of the Moiré pattern that may be formed by overlap of the relevant crystal planes across the interface.