Abstract
The plate-to-needle morphological transition was investigated by means of a stability analysis of a growing platelet. Conditions were found for the evolution of a parallel array of needles from a sinusoidal perturbation of the plate tip. The theory was applied to the Fe-C system using recently obtained kinetic data and the transition region was shown on the Fe-C phase diagram. On the basis of the maximum growth rate postulate the evolving needle spacing was determined by the fastest growing perturbation wavelength. This spacing was plotted vs the dimensionless supersaturation for transformation temperatures of 700°, 650°, and 550°C. A multiple-sectioning technique was used to experimentally determine the morphology of Widmanstatten precipitates in four samples and good agreement was found with theory. Blade or lath-like precipitates, rather than needles, were found to be the result of platelet instability.
Published Version
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