Abstract
Composites of 1 to 20 vol % sapphire whiskers contained in a nickel matrix were annealed in vacuum in the temperature range 1100 to 1400° C for times up to 1000 h. After annealing, whiskers and alumina particles were extracted from composites and examined by optical and electron microscopy. The change of the aspect ratio distribution of whiskers during annealing was determined and related to the theory of tip ovulation. Pronounced spheroidization of whiskers occurred. This was only partly due to ovulation from the tips of whiskers by interfacial diffusion. Ostwald ripening made a significant contribution to the extent of spheroidization. For sapphire whiskers in nickel, ovulation times and interfacial diffusion constants were determined. The activation energy for the growth of ovoidal particles increased from 35 to 110 kcal during spheroidization. This is believed to indicate that the rate controlling process changes from interfacial diffusion to bulk matrix diffusion as the rate of tip ovulation decreases during annealing.
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Stapley, A.J., Beevers, C.J. The stability of sapphire whiskers in nickel at elevated temperatures. J Mater Sci 8, 1296–1306 (1973). https://doi.org/10.1007/BF00549345
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DOI: https://doi.org/10.1007/BF00549345