Thermodynamic and structural properties of [001] twist boundaries in gold
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R. LeSar Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 12 April 1990; accepted 15 January 1991)
We have employed the Local Harmonic (LH) model and the Embedded Atom Method (EAM) to examine the structural and thermodynamic properties of a series of twelve [001] twist boundaries in gold for temperatures between 0 K and 700 K. For the majority of the grain boundary misorientations, metastable structures were observed with grain boundary energies that were typically less than 0.1% larger than the stable structures. Four of the twelve grain boundaries underwent first order structural phase transitions as seen by the crossing of the free energy versus temperature curves for the competing structures. Relatively small cusps or inflections in the grain boundary free energy versus misorientation curves were observed at 25 (36.87°) and 213 (22.62°) at low temperatures, at 213 (22.62°) and 217 (28.07°) at intermediate temperatures, and at 25 (36.87°) and 217 (28.07°) at elevated temperatures. A maximum in the grain boundary entropy versus misorientation was observed at 217 (28.07°) for all temperatures, and local minima were observed at 25 (36.87°) at low temperature and in 213 (22.62°) at high temperature. The excess volume associated with the grain boundary shows a roughly linear dependence on grain boundary free energy at each temperature examined. The room-temperature mean-square vibrational amplitude is approximately 25% larger than that for the bulk at the (002) plane adjacent to the boundary and decays to within 2% of the bulk value by the second (002) plane from the boundary. The room-temperature mean-square vibrational amplitude is dominated by the in-plane (parallel to the grain boundary) vibrations at the (002) plane nearest the grain boundary.
I. INTRODUCTION
The properties of grain boundaries play a major role in determining the properties of polycrystalline materials. Grain boundaries influence strength, toughness, phase transformations, diffusion, electronic properties, etc. Therefore, it is not surprising that the structure and properties of grain boundaries have received a great deal of attention from the research community. Many theoretical and simulation studies have been performed to elucidate the atomic structure of grain boundaries (for example, see the papers in Ref. 1). The majority of the simulation work has been performed at a temperature of absolute zero, owing to the efficiency of T = 0 simulation methods. Nonetheless, experimental studies of grain boundary structure and properties have usually been performed at room temperature or above.2"5 This discrepancy has been addressed in simulation studies which use molecular dynamics or Monte Carlo methods to obtain finite-temperature results.1 These same methods have been exploited to determine the thermodynamic properties of grain boundaries. Unfortunately, due to the substantial demand these methods place on J. Mater. Res., Vol. 6, No. 5, May 1991 http://journals.cambridge.org
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