Coarsening of Ni3Si precipitates in binary Ni-Si alloys at intermediate to large volume fractions

Four Ni-rich binary Ni-Si alloys containing 5.98. 6.50, 6.88, and 7.42 wt% Si were aged at 650 degrees C to produce dispersions of coherent Ni3Si particles with volume fractions, f, of 0.092, 0.171, 0.228, and 0.306, respectively. The alloys were aged for limes as long as 2760 h, and the kinetics of particle growth and the particle size distributions were measured using transmission electron microscopy. As has been observed in previous work, the particles are spherical in shape when small and evolve to a cuboidal shape, with flat faces parallel to [100], as they grow. Mo evidence was found For bifurcation into twofold symmetrical shapes; instead, the precipitates remain cuboidal for ''diameters'' as large as 400 nm. The gamma' precipitates in all four alloys tend to become spatially correlated parallel to the cube directions, but they are extremely resistant to coalescence or rafting. The coarsening kinetics are for the most part consistent with data from previous investigations. The rate constant for coarsening, it, remains essentially constant as f increases, though it can be argued that there is a tendency for k to increase weakly as f increases. The particle size distributions become slightly broader as f increases. This is manifested most obviously in the standard deviation of the distribution, which increases systematically with increasing f in a manner that is in fair agreement with the predictions of the theory of Tokuyama and Kawasaki. It is argued, however, that the observed agreement might be fortuitous because k is independent of f. (C) 1997 Acta Metallurgica Inc.