THERMODYNAMIC AND STRUCTURAL-PROPERTIES OF [001] TWIST BOUNDARIES IN GOLD

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 SIGMA-5 (36.87-degrees) and SIGMA-13 (22.62-degrees) at low temperatures, at SIGMA-13 (22.62-degrees) and SIGMA-17 (28.07-degrees) at intermediate temperatures, and at SIGMA-5 (36.87-degrees) and SIGMA-17 (28.07-degrees) at elevated temperatures. A maximum in the grain boundary entropy versus misorientation was observed at SIGMA-17 (28.07-degrees) for all temperatures, and local minima were observed at SIGMA-5 (36.87-degrees) at low temperature and in SIGMA-13 (22.62-degrees) 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.