3-D atomic scale characterisation of growing precipitates

Recent advances in atom probe microscopy have made it possible for the first time to three-dimensionally reconstruct, with near-atomic accuracy, the positions of atoms in and around precipitates. This technique has been utilised to examine the structure of precipitate interfaces and their surrounding matrix for the Cu- 1.0at%Co and Ni-11at%AI systems. The Cu-1.0at%Co system decomposes to form nearly pure, spherical, coherent cobalt precipitates within a nearly pure copper matrix. The Ni-11at%Al system decomposes, in early stages, to form spherical Ni_3Al precipitates that have asymmetric strain fields in a matrix containing about 10% aluminium. In order to make an accurate characterisation of the fine scale three-dimensional structure of a precipitate/matrix interface calibration procedures were developed to increase the accuracy of the atomic positions within three-dimensional atom probe reconstructions. Aberrations caused by variations in the local radius of curvature of the specimen needle were examined using analytical models, finite element methods, and experimental measurement of the local density within a reconstruction. Errors caused by the standard calibration technique were corrected for based upon the precipitate shape and the known density of the reconstruction. The interface thickness of precipitates in the Cu-Co system was found to be between 0.4 and 2 nm for precipitates less than 5 nm in radius and less than 0.5 nm for precipitates greater than 5 nm in radius. The interface thickness of precipitates simulated in Monte Carlo simulations was found to be between 0.3 and 0.6 nm for precipitates of varying sizes. The interface thickness of #gamma# precipitates in the Ni-Al system of about 1.0 nm in radius was measured to be between 1.1 nm to 3.5 nm. Applying the non-classical model to the Cu-Co system predicts an interface thickness of about 0.25 nm and therefore experimental measurements are much larger than expected. This could be a consequence of the nonclassical model not considering kinetic effects. Because the interface thickness measurements are on the order of the precipitate radius in many cases, the basic assumption of the classical model that the precipitate interface is perfectly sharp is not accurate for very small precipitates in the Cu-Co and Ni-Al systems. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:D205972 / BLDSC - British Library Document Supply CentreGBUnited Kingdo