Effect of Gradient Energy on Diffusion in Gold-Silver Alloys

Abstract

Interdiffusion coefficients have been measured in Au–Ag alloys containing 18–50 at.% Au at temperatures from 200° to 260°C and over very short penetration distances (7–35 Å). The diffusivities were determined from the rate of decay of composition modulations in vapor‐deposited films. The wavelength and amplitude of the modulations were measured by x‐ray diffraction. From the dependence of the diffusion coefficient on the wavelength of the composition modulation, it was established that it is necessary (as proposed by Hillert and by Cahn) to allow for the effect of the ``gradient energy'' on the driving force when diffusion occurs in the presence of very sharp composition gradients. The observed gradient‐energy coefficient was K=−(2.6±0.5)×10⁻⁶ erg·cm⁻¹ in agreement with a value calculated on the basis of nearest‐neighbor interactions. The interdiffusion coefficients (corrected to an infinite penetration distance) were in good agreement with an extrapolation of existing high‐temperature data obtained with conventional diffusion specimens, thus demonstrating that the vapor‐deposited film technique is capable of determining, with good precision, diffusivities of the order of 10⁻²⁰ cm² sec⁻¹. This is some eight orders of magnitude smaller than those that can be measured conventionally.