Atomistic simulations of the role of dopant atoms in grain growth and deformation in nanocrystalline materials

Many enhancements in properties of nanocrystalline materials due to refined grain size are lost if grain growth occurs during application. This dissertation offers a theoretical investigation of the effect of dopant segregation on interfacial properties -- namely grain boundary (GB) energy, structure and mechanical strength -- using atomistic calculations. Three primary objectives are considered. First, molecular statics and molecular dynamics (MD) are used to study the energy of a two-dimensional coincident site lattice Sigma7 symmetric tilt grain boundary in a copper bicrystal using the Lennard-Jones (LJ) potential. Dopant(s) of various characteristics and concentration are added to the near vicinity of the GB to determine the structural and energetic affect. It is shown that the dopants are capable of altering the energy and structure of the GB even if several atomic spacings separate the two. Moreover, with increased dopant content, the GB energy is reduced to zero, which has been theoretically proposed as necessary for achieving microstructural stability. Second, MD simulations of bulk nanocrystalline Cu with dopants segregated in the GB regions are performed to investigate the impediment to grain growth caused by the dopants during annealing at constant temperature of 800K. In this parametric study, the concentration and atomic radii mismatch between the dopants and the host atoms is systematically varied. It is found that the excess enthalpy (DeltaH) of the system decreases linearly with increasing dopant content, and samples with zero DeltaH undergo no grain growth. Finally, MD simulations are used to study GB sliding in pure and doped copper bicrystals with both the LJ and Embedded-Atom Method potentials. Two tilt [100] GB's are considered: the coincident site lattice Sigma5 (310) interface and a random high angle interface. Shear stress between 0.69 and 1.61 GPa is applied to the bicrystals at ambient temperature (300K) and high temperature (800K). The results indicate that interstitial dopants and substitutional dopants with larger atomic radius are effective in retarding GB sliding. The quantifiable results obtained by these simulations can be used to guide experimental efforts to synthesize appropriately doped nanocrystalline metals with stable microstructures and improved mechanical properties.