Molecular Dynamics Study of Grain Growth in Nanocrystalline Materials in the Presence of Dopants
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Molecular Dynamics Study of Grain Growth in Nanocrystalline Materials in the Presence of Dopants Paul C. Millett, R. Panneer Selvam, Ashok Saxena Computational Mechanics Laboratory, BELL 4190 University of Arkansas, Fayetteville, AR 72701 Email: {pmillet,rps,asaxena}@uark.edu ABSTRACT Molecular dynamics simulations of bulk nanocrystalline Cu with dopant atoms segregated in the grain boundary regions were performed to investigate the impediment of grain growth during annealing at constant temperature of 800K. In this parametric study, the concentration and atomic radii mismatch between the dopants and the host atoms were systematically varied to determine how to most effectively retard grain growth. It is found that samples with positive excess enthalpy (∆H) underwent various degrees of grain growth; however, when ∆H was negative, no coarsening occurred. Also, ∆H varied linearly with dopant concentration with the slope equal to the enthalpy of segregation, in agreement with previous theoretical work. INTRODUCTION Nanocrystalline (nc) materials are polycrystalline solids with either single or multi-phase microstructure with an average grain size (d) of less than 100 nm. The surface-to-volume ratio of the constituent crystallites rapidly increases with decreasing d, therefore resulting in a large percentage of atoms located in interfacial regions or grain boundaries as opposed to regular lattice sites. In coarse-grained materials this percentage is negligible, and it is this marked difference that is chiefly responsible for the significant disparity and often superior performance of nc materials. However, the high fraction of interface area to volume implies that the excess enthalpy of the system (and the thermodynamic driving force for grain growth) is large. Due to the combination of increased curvature-driven grain boundary migration and grain coalescence events [1], the microstructure of nc-materials is exceptionally unstable and grain growth is rapid. Indeed, experimental results have shown that many materials, including Sn, Pb, Al, Mg, Cu, and Pd, with nano-sized grains will undergo significant grain growth at even room temperatures [24]. The superior mechanical, electrical, and magnetic properties of bulk nanocrystals, which are a unique result of the nanometer grain size, are lost when the microstructural features evolve back to the course-grained realm (d > 100nm). Knowledge concerning methods for stabilizing the microstructure of nc materials exposed to elevated thermal environments is highly desirable. In alloy systems, the solubility of the substitutional solute component can be notably different in the lattice and grain boundary regions (that difference being the enthalpy of segregation, ∆Hseg). As a result, interfacial segregation occurs leading to enrichment of the grain boundaries with solute atoms and consequentially a reduction of the specific grain boundary energy, γ [5]. It has been proposed by Weissmuller and coworkers [6-8] that it is theoretically plausible for nc metallic alloys to be m
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