Molecular dynamics study of grain boundary structure and properties at high temperatures

This thesis reports research involving the development and application of atomistic simulation methods to study the effects of high homologous temperatures on the structural, thermodynamic, kinetic and mechanical properties of grain boundaries in metals. Our interest in these properties is due to the role they play in governing the evolution of microstructure and deformation of metals during solidification processing. The interest in developing more predictive models for the formation of solidification defects highlights a need to better understand the thermodynamic driving forces underlying grain-boundary premelting and the mobility and shear strength of these interfaces at high temperatures. In this work we study two different elemental systems, namely Ni and Cu, and consider a variety of grain boundary structures characterized by different misorientation angles, twist/tilt character and zero-temperature energies.;A method to calculate the disjoining potential from molecular dynamics (MD) is developed and applied to grain boundaries in Ni. The disjoining potential characterizes the variation in grain-boundary free energy as a function of the width of a premelted interfacial layer. The MD method for the calculation of this property is applied to grain boundaries that display continuous premelting transitions, as well as a boundary characterized by a disordered atomic structure displaying a finite interfacial width at the melting temperature. The disjoining potential represents an important input property to larger scale models of solidification and grain coalescence.;We further develop analysis methods to characterize the change in the atomic structure of an asymmetric tilt grain boundary in elemental Cu as a function of temperature. This boundary is characterized by a potential-energy surface with multiple minima as a function of the relative translation of the grains parallel to the interface plane. The more complex structure of this boundary, relative to the idealized tilt and symmetric tilt boundaries commonly studied by atomistic simulations, better represents grain boundaries in real polycrystalline materials. The structure of this boundary is found to display both disordering and roughening with increasing temperature. The mobility was calculated at high homologous temperatures using a method based on the analysis of the random walk of the interface in equilibrium MD simulations. The magnitude of the calculated mobilities matched well with previous experiments and simulations. However, an analysis of the temperature dependence of the mobility using an Arrhenius form with a constant prefactor and activation energy (Q) leads to a value of Q significantly larger than reported previously in atomistic simulations. Finally, the temperature dependent response of this grain boundary to shear was investigated. The dynamical behavior of the boundary in response to an applied shear strain was observed to undergo qualitative changes as the temperature was increased above 700 K. A decrease in shear resistance of the grain boundaries as a function of temperature was characterized by a decrease in the critical shear stress, an upper bound for which was estimated to be roughly a factor of five lower at the melting temperature than at zero temperature.