Structural Disjoining Potential for Grain Boundary Premelting and Grain Coalescence from Molecular-Dynamics Simulations Saryu Fensin, 1 David Olmsted, 2 Dorel Buta, 1 Mark Asta, 3, 1 Alain Karma, 2, 4 and J. J. Hoyt 5 $^{1}$Department of Chemical Engineering and Materials Science, University of California, Davis, CA $^{2}$Department of Physics, Northeastern University, Boston, MA $^{3}$Department of Materials Science and Engineering, University of California, Berkeley, CA $^{4}$Center for Interdisciplinary Research on Complex Systems, Northeastern University, Boston, MA $^{5}$Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada Abstract We describe a molecular dynamics framework for the direct calculation of the short-ranged struc- tural forces underlying grain-boundary premelting and grain-coalescence in solidification. The method is applied in a comparative study of (i) a Σ9 〈 115 〉 120 o twist and (ii) a Σ9 〈 110 〉 { 411 } symmetric tilt boundary in a classical embedded-atom model of elemental Ni. Although both boundaries feature highly disordered structures near the melting point, the nature of the tempera- ture dependence of the width of the disordered regions in these boundaries is qualitatively different. The former boundary displays behavior consistent with a logarithmically diverging premelted layer thickness as the melting temperature is approached from below, while the latter displays behavior featuring a finite grain-boundary width at the melting point. It is demonstrated that both types of behavior can be quantitatively described within a sharp-interface thermodynamic formalism involving a width-dependent interfacial free energy, referred to as the disjoining potential. The disjoining potential for boundary (i) is calculated to display a monotonic exponential dependence on width, while that of boundary (ii) features a weak attractive minimum. The results of this work are discussed in relation to recent simulation and theoretical studies of the thermodynamic forces underlying grain-boundary premelting. arXiv:1001.0583v1 [cond-mat.mtrl-sci] 4 Jan 2010 1