Publications

Over the past decade, numerous efforts have sought to understand the influence of twin boundaries on the behavior of polycrystalline materials. Early results suggested that twin boundaries within nanocrystalline face-centered cubic metals have a considerable effect on material behavior by altering the activated deformation mechanisms. In this work, we employ molecular dynamics simulations to elucidate the role of twin boundaries on the deformation of 〈100〉 columnar nanocrystalline copper at room temperature under uniaxial strain. We leverage non-local kinematic metrics, formulated from continuum mechanics theory, to compute atomically-resolved rotational and strain fields during plastic deformation. These results are then utilized to compute the distribution of various nanoscale mechanisms during straining, and quantitatively resolve their contribution to the total strain accommodation within the microstructure, highlighting the fundamental role of twin boundaries. Our results show that nanoscale twins influence nanocrystalline copper by altering the cooperation of fundamental deformation mechanisms and their contributed role in strain accommodation, and we present new methods for extracting useful information from atomistic simulations. The simulation results suggest a tension-compression asymmetry in the distribution of deformation mechanisms and strain accommodation by either dislocations or twin boundary mechanisms. In highly twinned microstructures, twin boundary migration can become a significant deformation mode, in comparison to lattice dislocation plasticity in non-twinned columnar microstructures, especially during compression.

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Molecular dynamics simulations are leveraged in this study to explore rate-dependent grain growth and deformation in nanocrystalline nickel due to surface indentation at room temperature. A 50. nm thin film with approximately 700 grains is indented with a 15. nm spherical indenter at rates of 0.2. m/s, 1.0. m/s, and 5.0. m/s. We simulate the indentation, hold, and removal of the indenter, as well as compute grain growth and distribution profiles during microstructure deformation. Novel algorithms are also developed in this work to accurately distinguish individual grains and provide quantitative data for the evolution of the microstructure. Results of the simulations show that lattice deformation mechanisms, such as dislocation slip and twinning, that accompany grain growth are also functions of indentation rate and equilibration time. This work shows that grain growth in this nanocrystalline nickel structure is indeed rate-dependent, and is most prominent for grains near the indentation surface during both the hold and removal of the indenter. © 2013 Elsevier B.V.

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It is well known that screw dislocation motion dominates the plastic deformation in body-centered-cubic metals at low temperatures. The nature of the nonplanar structure of screw dislocations gives rise to high lattice friction, which results in strong temperature and strain rate dependence of plastic flow. Thus the nature of the Peierls potential, which is responsible for the high lattice resistance, is an important physical property of the material. However, current empirical potentials give a complicated picture of the Peierls potential. Here, we investigate the nature of the Peierls potential using density functional theory in the bcc transition metals. The results show that the shape of the Peierls potential is sinusoidal for every material investigated. Furthermore, we show that the magnitude of the potential scales strongly with the energy per unit length of the screw dislocation in the material.

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