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We examined the influence of the boundary plane on grain-boundary diffusion in Ni through a series of molecular dynamics simulations. A series of 〈010〉 ∑5 tilt boundaries, including several high symmetry and low symmetry boundary planes, were considered. The self-diffusion coefficient is a strong function of boundary inclination at low temperature but is almost independent of inclination at high temperature. At all temperatures, the self-diffusion coefficients are low when at least one of the two grains has a normal with low Miller indices. The grain boundary self-diffusion coefficient is an Arrhenius function of temperature. The logarithm of the pre-exponential factor in the Arrhenius expression was shown to be nearly proportional to the activation energy for diffusion. The activation energy for self-diffusion in a (103) symmetric tilt boundary is much higher than in boundaries with other inclinations. We discuss the origin of the boundary plane density–diffusion coefficient correlation.

I. INTRODUCTION

A complete crystallographic description of a flat grain boundary requires the specification of five independent variables: three associated with the misorientation of the two grains with respect to one another and two to describe the orientation of the boundary plane. Many grain boundary properties (e.g., grain boundary energy, selfdiffusion coefficients, mobility, and electrical resistance) are sensitive to these five crystallographic parameters. Since preparation of well-controlled bicrystals is significantly more difficult than that for polycrystalline samples, most such properties are reported as polycrystalline averages. Further, many bicrystal studies (e.g., of grain boundary migration) are performed on curved boundaries, where only three of these five parameters are fixed.1,2 An additional difficulty in describing the variation of grain boundary properties with these crystallographic and the remaining physical parameters (e.g., temperature) is associated with the large dimensionality of the parameter space. As a result, most systematic studies

a)

Present address: Ames Laboratory, 207 Metals Development, Ames, Iowa, 50011 b) Address all correspondence to this author. e-mail: [email protected] c) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/publications/jmr/policy.html. DOI: 10.1557/JMR.2005.0177 1146

http://journals.cambridge.org

J. Mater. Res., Vol. 20, No. 5, May 2005 Downloaded: 11 Mar 2015

of boundary properties focus only on either misorientation or boundary plane. Of these, almost all have focused on the misorientation variables. In the present study, we fix the misorientation and examine the effect of varying the grain boundary plane on grain boundary self-diffusion in Ni. Our focus on grain boundary diffusivity was motivated by the fact that the self-diffusion coefficient is sensitive to grain boundary structure. Many earlier studies have exa