An Investigation of Interstitial Sites in the bcc Lattice

Abstract

The evidence for Snoek's model of interstitial position and diffusion in bcc metals is reviewed in the light of recent calculations. Other models are equally tenable in many instances, on the available evidence, if the internal friction relaxation strength associated with the tetrahedral sites is appreciable. To investigate the matter further, a systematic comparison of the models involving octahedral and tetrahedral sites is carried out using an elementary method for estimating strain energies about stable sites and about saddle points for diffusion. These estimates indicate that the tetrahedral sites are the more favored positions in many instances and that diffusion paths involving the tetrahedral sites may be important even when the octahedral sites are more favored. Next, a model of an interstitial in a tetrahedral site is developed for use in continuum mechanics, using the same forces as were hypothesized for the strain‐energy calculation. It is deduced from this model that the relaxation strength associated with a tetrahedral site depends sensitively on an unknown angle, but that some values within a reasonable range for this angle do lead to significant relaxation strengths. It is also deduced that an impurity in a tetrahedral site interacts with strain gradients, as well as strains. A comparison of predicted patterns with measured relaxation strengths does not bear out Snoek's model but instead supports the hypothesis that in most systems based on the group VA metals the interstitial impurities occupy the tetrahedral sites.