A molecular dynamics investigation on grain disappearance at a triple junction in polycrystalline silicon
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A molecular dynamics investigation on grain disappearance at a triple junction in polycrystalline silicon Alessandra Satta, Luciano Colombo Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica, Università di Cagliari Cittadella Universitaria, I-09042, Monserrato - Cagliari, Italy Fabrizio Cleri Ente Nuove Tecnologie, Energia e Ambiente, Divisione Materiali, Centro Ricerche Casaccia, CP 2400, I-00100 Roma, Italy, and Istituto Nazionale per la Fisica della Materia, Roma, Italy. ABSTRACT Topological changes in microstructure are strictly related to the microscopic evolution of triple junctions (TJ). The three-sided grain disappearance, usually called T2 process, is here investigated via 3D-atomistic modeling. In particular the stability of a three-sided grain insertion in a triple junction in silicon is studied within the framework of Molecular Dynamics simulations. The Stillinger-Weber interatomic potential is adopted and constant-traction border conditions are considered to ensure a proper embedding of the atomistic region in a virtually infinite bulk continuum. Dealing with the T2-event, the critical radius below which the threesided inner grain become unstable is evaluated to be three to four times the lattice constant of silicon. Moreover, we show that the instability sets in through the amorphization of the central shrinking grain. INTRODUCTION Triple junctions in a polycrystalline microstructure correspond to the regions of space where three grain boundaries meet. Despite being such a fundamental component of any polycrystal, the role of triple junctions is still rather unclear. In particular, there is an ongoing discussion whether triple junctions may carry any special properties, such as an excess (according to Gibbs) line energy and volume, a peculiar mobility and so on. Recently [1], an atomic-level model of both structure and elastic properties of triple junctions has been proposed. By means of molecular dynamics, such a model predicts a triple junction to have a positive excess line energy, in agreement with the description of triple junctions as disclination structures [2] and a negative excess volume, i.e. a volume contraction of the junction region. Such a line energy would necessarily condition the arrangement of the grainboundary network, and could also affect materials properties [3]. The mobility of triple junctions is extremely difficult both to be defined and measured; a recent experiment [4] seems to demonstrate that triple junctions can have a finite mobility resulting in a drag of grain boundary motion, thus potentially affecting grain size evolution and other microstructure features [5]. Taking advantage of the model previously obtained through atomistic simulation of triple junctions [1], we pursued the investigation on the class of events characterized by the topological invariance of the microstructure. Among the several different topological elementary events affecting the microstructure evolution, we restrict our present discussion on the so-called T2 event [6,7], nam
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