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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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Transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM) observations of pearlitic steel wire show that drawing to a true strain of 4.22 causes fragmentation of cementite lamellae into nanoscale grains. The drawing strain amorphizes some portions of the cementite lamellae in regions where the interlamellar spacing is very small, but most of the cementite lamellae are polycrystalline with nanoscale grains. The carbon concentration in the ferrite is inhomogeneous and varies from 0.2 to 3 at. pct; the carbon concentration in nanocrystalline cementite is less than 18 at. pct, significantly lower than that in stoichiometric Fe 3 C. Silicon is segregated to ferrite/cementite boundaries, but, in regions with a small interlamellar spacing, the silicon concentration is uniform across the lamellae. Annealing at 200 ЊC for 1 hour does not cause noticeable changes in the microstructure. Annealing at 400 ЊC or above for 1 hour causes spherodization of the cementite lamellae, and the carbon concentrations in ferrite and in cementite return to the predeformation values.

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