Atomic Structure, Growth Mechanisms and Kinetics of Precipitate Interfaces in Metal Alloys

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9630092 Howe This research effort employs high-resolution analytical electron microscopy to address several fundamental questions concerning the interfacial structure and growth kinetics of precipitate interfaces in metal alloys. Specifically, the diffusion fields (solute concentration profiles) surrounding growth ledges on the faces of gamma precipitate plates and the plate edges in an Al-Ag alloy are determined using the high-resolution (0.14 nm image resolution), small-probe (0.5 nm diameter) capabilities of a field-emission transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectrometer (EDS). These data are compared with concentration profiles obtained from theoretical models to provide the first tests of these models. The Gatan imaging filter (GIF) on the same microscope is used in conjunction with the 0.5 nm probe to obtain electron energy-loss spectra (EELS) across the faces, edges and ledges of gamma hydride plates in Ti-H alloys. These data make it possible to determine whether the titanium hydride structure is fully formed at the transformation interface or whether the compositional and structural changes may be decoupled in this mixed-mode transformation. Such analyses should clarify key fundamental questions related to the growth of precipitate plates in metal alloys. %%% This experimental research provides theorists important information on the concentration profiles in the vicinity of ledges on the faces of growing precipitate plates during early stages of growth. Ti-H alloy research explores the question of whether shear of Ti permits the titanium hydride, TiH, to grow. The mechanism of growth is obscure at the present time, but the hydride is an undesirable phase in titanium alloys. ***