A metallography solution for grain boundary diffusion estimation in polycrystals

The diffusion rate in polycrystals is higher than in single crystals due to the fact that diffusion in solids is known to occur along grain boundaries and over free surfaces more rapidly than through the lattice. The diffusio n mechanism in a polycrystalline medium has been discussed and analyzed ext ensively since Fisher first studied this subject in 1951, by assuming high diffusivity along the grain boundaries [1-5]. Most of the experimental data was produced by thin layers and free surface bars with a finite source of the element to be diffused. Whipple modified the Fisher model and Levine-Ma cCallum based their theories upon the constant source in which the concentr ation did not change during the diffusion time [3-4]. Since the experimenta l sources usually consist of a finite amount, the validity of the models is confined to the low temperature range where the diffusion is slow such tha t the source is practically invariant. The suggested technique adopts the instantaneous source model developed by Suzuoka [2]. In addition, the concentration of the source at the medium sur face does not change during the diffusion time, due to the sample design ha ving a constant source.