Browsing by Author "Monk, J."
Now showing 1 - 3 of 3
Results Per Page
Sort Options
- Linear grain growth kinetics and rotation in nanocrystalline NiFarkas, Diana; Mohanty, S.; Monk, J. (American Physical Society, 2007-04-20)We report three-dimensional atomistic molecular dynamics studies of grain growth kinetics in nanocrystalline Ni. The results show the grain size increasing linearly with time, contrary to the square root of the time kinetics observed in coarse-grained structures. The average grain boundary energy per unit area decreases simultaneously with the decrease in total grain boundary area associated with grain growth. The average mobility of the boundaries increases as the grain size increases. The results can be explained by a model that considers a size effect in the boundary mobility.
- Metastability of multitwinned Ag nanorods: Molecular dynamics studyMonk, J.; Hoyt, J. J.; Farkas, Diana (American Physical Society, 2008-07-16)Nanoscale rods have been shown to exhibit a multiple twinned structure. The rods grow along a [110]-type crystallographic direction and have a pentagonal cross section with five (111) twins connecting the wire center to the corners of the pentagon. Here, we use molecular dynamics simulations with an embedded atom method interatomic potential for Ag to compute the ground-state energies of the multitwinned rods and compare with the bulk equilibrium crystal shape, as estimated from a Wulff construction. The excess energy of the nontwinned equilibrium nanorods and the multitwinned nanorods was obtained as a function of the wire length (L) as well as the cross sectional area (A(cs)). Various contributions to the total energy, such as twin boundary energy and surface energies, are discussed and included in an analytical model that compares favorably with the simulation results. Our results show that for infinitely long nanowires with A(cs)< 1500 nm(2), the nontwinned structure is always energetically favorable. However, if the energy of the dipyramidal atomic structure at the nanorod ends is included in the model then the twinned nanorods are stable with respect to the nontwinned rods below a critical aspect ratio (L/root A(cs)).
- Strain-induced grain growth and rotation in nickel nanowiresMonk, J.; Farkas, Diana (American Physical Society, 2007-01-10)Virtual tensile tests of nanocrystalline nickel wires of initial 5 nm grain size were simulated at strain rates varying from 3x10(7) s(-1) to 1x10(9) s(-1) at 300 K, reaching deformation levels up to 36%. The virtual tensile tests allowed the study of the strain rate sensitivity of these nanowires, yielding an activation volume of similar to 2b(3), where b is the Burger's vector, consistent with grain boundary mechanisms of plasticity. Most importantly, after 3% deformation the grain size increased significantly during the deformation, with larger grains growing at the expense of the smaller ones as the deformation levels increase. The volume of each grain was monitored as a function of time and stress level. The results clearly indicate that the observed grain growth is stress driven, with grain size versus stress behavior being only weakly dependent on the strain rate and simulation time. Grain growth is also accompanied by grain rotation. The observations are interpreted in terms of the coupling of the relative motion of the grains parallel to the boundary and the motion of the boundary in the direction perpendicular to itself. Our results are consistent with a model where the deformation is accommodated by grain boundary sliding which in turn is coupled with grain boundary migration and rotation, producing grain growth.