Molecular Dynamics Study of Solute Pinning Effects on Grain Boundary Migration in the Aluminum Magnesium Alloy System
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RAIN boundary motion controls the rate of the important metallurgical processes of grain growth and recrystallization. The kinetics of boundary motion has been examined in a number of studies and considerable attention has been paid to the solutes and impurities on the boundary mobility.[1–12] In particular, all of these studies have concluded that the presence of such defects, even in very small amounts, provides significant resistance to the boundary movement and this phenomenon is known as solute drag. As an example of a more recent investigation, in a study of static recrystallization, Hutchinson et al.[13] have described the importance of the solute drag effect explicitly. The authors reported that 0.05 at. pct Nb solute impedes the grain boundary motion very strongly in the ferrite phase in Nb-steel. A classic model of impurity drag on the boundary migration was developed by Cahn,[7] where two types of force–velocity relationships were suggested. At low driving forces the impurities segregated to the boundary MD. JAHIDUR RAHMAN, formerly Ph.D. Student, Junior Research Engineer with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada, is now Postdoctoral Fellow with Canadian Nuclear Laboratories, Chalk River, ON, Canada. Contact e-mail: [email protected] HATEM S. ZUROB, Associate Professor, and JEFFREY J. HOYT, Professor and Chair, are with the Department of Materials Science and Engineering, McMaster University. Manuscript submitted September 22, 2014. Article published online January 21, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
are able to diffuse along with the moving boundary, producing a large drag. At higher driving forces the velocity can exhibit a discontinuous jump where the boundary has broken free of the solute atmosphere. In addition, another pioneering solute drag analysis was published by Hillert and Sundman[8,14] that focused on the dissipation of free energy due to the diffusional processes that take place during interface migration. Hillert’s and Cahn’s models are considered to be two leading theoretical approaches in the literature of solute drag and both models have been employed in a number of studies.[15–24] Lately the effect of impurity drag was studied in detail,[25–28] both experimentally and theoretically, in boundary migration, grain growth, and phase transformation. In an experimental investigation, Molodov and Gottstein[29,30] have shown noticeable impact of solutes on the grain boundary motion. The authors illustrated a gradual decrease in boundary migration rate with the increase in impurity content in aluminum. It is apparent from all the previous studies that the motion of the impurity-induced grain boundary motion always depends on the concentration of those impurities around the interfaces through their interactions with the boundaries. In relevance to this study, the effect of solute atoms on migrating boundaries has also been studied in terms of discrete pinning points along the boundary.[31,32] In a relevant study, Machlin[31] des
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