Atomistic models of triple junctions and the origin of topological changes in microstructural evolution
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Atomistic models of triple junctions and the origin of topological changes in microstructural evolution 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 and 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 Triple junctions are crucial elements in microstructural evolution: for example, their mobility can be rate-limiting if lower than that of grain boundaries. However, very little is known about their atomic-level structure and properties. We studied the atomic structure of multiple-twin triple junctions in silicon, formed by the convergence of two {111} and one {221} symmetric-tilt grain boundaries. Molecular dynamics simulations with the Stillinger-Weber potential and constant-traction border conditions were performed on several triple junction configurations, obtained by different combinations of the three grain boundaries. All the configurations have a positive excess line energy, a measurable volume contraction and display regions of opposite, tensile and compressive, residual stress. Moreover, we tried to elucidate the role of triple junctions as being the seeds of the only microscopic events that can lead to topological changes in the microstructure. Such events, usually dubbed T1 and T2 in mesoscopic models, correspond to grain switching (in the Ashby-Verrall sense) and grain-disappearance events, respectively. We present preliminary results for the atomic-scale modelling of both classes of topological events and discuss the connection between atomistic and mesoscopic modelling of microstructural evolution.
INTRODUCTION Triple junctions in a polycrystalline microstructure correspond to the one-dimensional 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. Indeed, this amounts to establishing whether a triple junction is to be truly regarded as a line defect or simply as a geometric locus [1,2]. Moreover, such defect-like triple junctions are eventually going to affect the kinetics of microstructure evolution The atomic-level study of triple junctions would elucidate if they could carry an excess volume and line energy , e.g. with a positive sign as implied by the description of triple junctions as disclination structures [3]; such a line energy would necessarily condition the arrangement of the grain-boundary network, and could also affect materials properties [4]. A further consequence of a non vanishing line energy, if positive, could be the destabilization of the grain boundary network at the
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