Interstitial Atomic Diffusion Coefficients

An attempt is made to interpret the temperature independent factor $D_0$ of the previously determined diffusion coefficients of interstitial solute atoms in metals. The primary uncertainty in the value of $D_0$ given by the standard reaction rate theory resides in an entropy factor $\exp \left(\frac{\Delta S}{R}\right)$. 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 $\alpha -\mathrm{Fe}$ are consistent with this theoretical range. The $D_0\text{'}\mathrm{s}$ 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.