Effect of anisotropy, SIA orientation, and one-dimensional migration mechanisms on dislocation bias calculations in metals

Abstract

Swelling in metals exposed to neutron irradiation has long been known to be enhanced by the preferential absorption of interstitials rather than vacancies, by dislocations. A common measure of this preference is called the dislocation bias factor and is computed from the ratio of the capture efficiency of interstitials to that of vacancies, by dislocations. Bias factors calculated through analytical and numerical means have been known to be approximately an order of magnitude larger than those expected by empirical swelling data. While this discrepancy has been justified by some in the past, it has remained an issue of debate among many. A major issue that has not been explored, however, is the effect of the numerous assumptions and approximations in previous studies. In this study, we explore the effect of three major assumptions of past studies. First, anisotropy in the displacement fields of both point-defects and dislocations is accounted for in our calculations. This relieves the assumption that self-interstitial atoms (SIAs) are centers of dilatation in isotropic media. Secondly, since SIAs have energetically-preferred orientations in dislocation stress fields, we use a spatially dependent elastic dipole tensor that accounts for preferred orientations. Lastly, since SIAs are known to undergo fast one-dimensional (1-D) migration in the Burgers direction in the vicinity of dislocations, we specify a fraction of SIAs to migrate one-dimensionally using a modified SIA diffusion tensor. These attributes are implemented into a combined finite-element method (FEM) rate-theory (RT) approach to calculate dislocation bias factors in body-centered cubic iron and face-centered cubic copper. In copper, we observe a significant increase in the bias when anisotropy and preferred SIA orientations are considered. In iron, our results show a drastic decrease in the bias when anisotropy and 1-D SIA motion are implemented, but these effects are countered when preferred SIA orientations are considered.