A transmission electron microscope (TEM) study was made of the interphase boundary structure of delta plates precipitated from the gamma phase in alloy 718. A variety of interfacial defects were examined and identified. These results, together with available data obtained from bcc laths in fcc Ni-Cr alloys, were used to develop a method for predicting precipitate orientation relationships and boundary orientations. The method employs a geometric matching approach in three dimensions based upon the concept of near-coincidence sites. It is suggested that precipitates in a given system select an orientation relationship which produces the greatest areal density of near-coincidence sites and that the habit plane adopts an orientation that yields the greatest area of boundary containing contiguous near-coincidence sites.
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 ␥ M grains often have an irrational orientation relationship (OR) and structurally incommensurate (incoherent) interface with respect to the parent ␣ (retained ␣ 2 ) phase, although the degree of commensurability varies, depending on the orientation and planarity of a particular ␣ 2 /␥ M interface. In-situ HRTEM observations of several ␣ 2 /␥ M 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 context of the massive transformation and the motion of interphase boundaries in solids.
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