Estimation of average size of convex particles
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the reciprocal of the nucleation rate as a function of the same variable. The latter provides the time required to form the first nucleus. Except at initial relative supersaturations so low that nucleation kinetics should be negligible, the supersaturation-dependencies of these two quantities are drastically different. As Langer and Schwartz make clear, the level of agreement (or disagreement) achieved between their detailed treatment of the half-completion time and experiment is thus a very poor test of nucleation theory. Hence including "Ostwald ripening from the very beginning" does not constitute a suitable "absolute check of nucleation theory".
Author's Reply P. HAASEN The discussers' point about the different particle densities used in FIM and TEM experiments is well known and well taken. One should, however, also compare the minimum observed particle sizes in the TEM experiments on C___u_uCo Is and the FIM experiments 16 in relation to the calculated size of real nuclei. They are particularly small in C___uuCoas is described in more detail in a recent publication (H. Wendt and P. Haasen, Scripta Met., 1985, vol. 19, p. 1053). In this paper it is also pointed out that our as-quenched specimens were homogeneous on an almost atomic scale, so no "anisothermal transformation" occurred. Still, there is a striking discrepancy in the size of observed and calculated nuclei for our Cu 2.7 at. pct Co alloy. On the other hand, a study by the same technique of precipitation in Ni 14 pct A121'22 gave perfect agreement with classical nucleation theory. There must be other factors than Ostwald ripening in the nucleation in C__p_uCowhich are still unsolved.
Estimation of Average Size of Convex Particles A.M. GOKHALE Average particle size is an important stereological parameter for characterization of microstructures containing particulate dispersoids. For an ensemble of polydispersed convex particles of various shapes, the average size is conventionally defined in terms of an average value of the caliper diameter (i.e., distance between two parallel tangent planes; see Figure 1). Let D be the local caliper diameter. The average or mean caliper diameter Do is conventionally defined as follows: ~'2
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A. M. GOKHALE, formerly with Hindustan Brown Boveri Ltd., Baroda, India, is now with the Department of Metallurgical Engineering, Indian Institute of Technology, KANPUR-208016, India. Manuscript submitted November 13, 1984. METALLURGICAL TRANSACTIONS A
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example, see Figure 1) and the tangent planes of surface elements in different pairs are not parallel. The distance between the parallel tangent planes is the local caliper diameter. Thus, for convex particles, one local caliper diameter is uniquely associated with each surface element. Consider a microstructure containing an ensemble of convex particles of various shapes and sizes uniformly distributed in a matrix of another phase. Define the surface area averaged caliper diameter Ds as follows:
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