Static and in-situ high-resolution transmission electron microscopy investigations of the atomic structure and dynamics of massive transformation interfaces in a Ti-Al alloy

Static and in-situ high-resolution transmission electron microscopy (HRTEM) and three-dimensional near-coincident-site (NCS) atomic modeling were used to determine the atomic structure, growth mechanisms, and dynamics of massive transformation interfaces in a Ti-Al alloy. Results from these experiments show that massive γM grains often have an irrational orientation relationship (OR) and structurally incommensurate (incoherent) interface with respect to the parent α (retained α2) phase, although the degree of commensurability varies, depending on the orientation and planarity of a particular α2/γM interface. In-situ HRTEM observations of several α2/γM interfaces during growth revealed evidence of both continuous and stepwise mechanisms of interface motion, again, depending on the orientation of the interface plane for a given OR between the two phases. These findings are discussed within the contex of the massive transformation and the motion of interphase boundaries in solids.