An attempt is made to interpret the temperature independent factor D Q of the previously determined diffusion coefficients of interstitial solute atoms in metals. The primary uncertainty in the value of DQ given by the standard reaction rate theory resides in an entropy factor exp(&S/R). When cognizance is taken of an additional strain in the lattice surrounding a solute atom as it passes over a potential energy divide, and of the increase in entropy associated with an increase in lattice strain energy, one can estimate a "theoretical" range within which these entropy factors should lie. All past observations except for C and N in a-Fe are consistent with this theoretical range. The Do's for these two systems were, therefore, redetermined by more precise measurements, and are found to be an order of magnitude higher than the original values. The associated entropy factors are consistent with the theoretical range.
The exact theory of the rate of growth of spherical precipitate particles has previously been developed for the initial stage where the individual precipitate particles do not interfere with one another. In the present paper this theory is carried one stage further to include their mutual interference. It is found that in the particular case of a very dilute solution this interference may be accounted for in a relatively simple manner. The applicability of the theory has been tested by new observations upon the precipitation of carbon from α-iron. Good agreement is found up to 95 percent completion of precipitation.
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