Realistic time-scale fully atomistic simulations of surface nucleation of dislocations in pristine nanopillars

We use our recently proposed accelerated dynamics algorithm (Tiwary and van de Walle, 2011) to calculate temperature and stress dependence of activation free energy for surface nucleation of dislocations in pristine Gold nanopillars under realistic loads. While maintaining fully atomistic resolution, we achieve the fraction of a second time-scale regime. We find that the activation free energy depends significantly and non-linearly on the driving force (stress or strain) and temperature, leading to very high activation entropies. We also perform compression tests on Gold nanopillars for strain-rates varying between 7 orders of magnitudes, reaching as low as 10^3/s. Our calculations bring out the perils of high strain-rate Molecular Dynamics calculations: we find that while the failure mechanism for compression of Gold nanopillars remains the same across the entire strain-rate range, the elastic limit (defined as stress for nucleation of the first dislocation) depends significantly on the strain-rate. We also propose a new methodology that overcomes some of the limits in our original accelerated dynamics scheme (and accelerated dynamics methods in general). We lay out our methods in sufficient details so as to be used for understanding and predicting deformation mechanism under realistic driving forces for various problems

Multiscale modelling of martensitic phase transformation: Example of Si I to Si II

Martensitic phase transformations (PTs), amorphization, twinning, and dislocation motion are the main deformation mechanisms in many crystalline materials. However, the interaction between these material behaviors are not well understood. In this work, we try to understand the interaction between dislocation motion and PTs through multiscale modelling using density functional theory (DFT, in collaboration with Drs. Zarkevich and Johnson from Ames Laboratory), molecular dynamics (MD), continuum mechanics and concurrent atomistic-continuum (CAC) method. The following topics are discussed. 1. A continuum/atomistic approach for predicting lattice instability during martensitic PTs is developed for the general loading with an arbitrary stress tensor and large strains. It was applied to the transformation between semiconducting Si I and metallic Si II phases. The instability criterion represents the critical value of the modified transformation work, which is linear in normal to cubic faces components of the true stress tensor and is independent of shear stresses. 2. Starting with thermodynamic predictions and combining with MD simulations, special triaxial compression-tension states were found at which the stresses for the instability of the crystal lattice of silicon are the same for direct and reverse Si I to Si II PTs. This leads to unique homogeneous and hysteresis-free first-order PTs. Zero hysteresis and homogeneous transformations are the optimal property for various PT applications, which drastically reduce damage and energy dissipation. 3. DFT calculations were carried out. It turned out that the instability stress plane also exists in the stress space, which demonstrates the reliability of the MD results. 4. A nonlinear elastic model which reproduce lattice instability of Si under normal stress and shear stress correctly was developed. 5. To scale up the simulation size, parallel algorithm was developed for a CAC method which has been demonstrated to be able to reproduce phase transformation in Si. Using spatial decomposition method, the parallel efficiency reached 90% with the maximum number of processors that can be accessed, i.e., 768 from Condo at ISU. The parallel efficiency as well as the applicability of CAC in predicting dislocation-mediated plasticity in submicron-sized specimen containing billions of atoms are demonstrated. 6. Under shear, the 60 degree dislocation pile-up against different grain boundaries in Si induces amorphous shear band. The shear stress needed for amorphization decreases almost linearly with the number of dislocations in a pile-up. 7. The shuffle screw dislocation transmit into the neighboring grain for all GBs in silicon. The critical shear stress averaged over the whole sample for the transmission is dependent on the local structure of the GBs

Understanding the Atomistic Deformation Mechanisms of Cu-Nb Multilayered Nanocomposites using Molecular Dynamics.

Metal-metal multilayered nanocomposites exhibit a remarkable improvement in strength, thermal stability, and irradiation resistance. This superior response of multilayered nanocomposites is attributed to various factors, such as the high density of interfaces, layer thickness, and nature of interfaces. In the current work, molecular dynamic simulations will be utilized to study the uniaxial (tension and compression) mechanical response of Cu-Nb multilayered nanocomposites (MNCs). In specific, the aim is to understand the deformation mechanisms of accumulative roll bonded vs. physical vapor deposited interfaces with nanolayered Cu and Nb. In addition, the effect of loading mode (tension vs. compression) and crystallography on the deformation of MNCs is discussed in detail with emphasis on atomic-scale interactions between dislocations, dislocation-interfaces, and dislocations-twins. The outcome of this work would serve as a supplement to plasticity literature which provides deep insights into interfacial deformation mechanisms and defect formation in metal-metal multilayered nanocomposite under various loading scenarios

Atomistic simulations of deformation in Metallic Nanolayered Composites

The mechanical behavior of Metallic Nanolayered Composites (MNCs) is governed by their underlying microstructure. In this dissertation, the roles of the interlayer spacing (grain size, d) and the intralayer biphase spacing (layer thickness, h) on mechanical response of Cu/Nb MNCs are examined by Molecular Dynamics (MD) simulations. The study of the strength of MNCs show that small changes in both d and h play a profound role in the relative plastic contributions from grain boundary sliding and dislocation glide. The interplay of d and h leads to a very broad transition region from grain boundary sliding dominated flow, where the strength of the material is weak and insensitive to changes in h, to grain boundary dislocation emission and glide dominated flow, where the strength of the material is strong and sensitive to changes in h. The study of the fracture behavior of MNCs shows that cracks in Cu and Nb layers may exhibit different propagation paths and distances under the same external loading. Interfaces can improve the fracture resistance of the Nb layer in Cu/Nb MNCs by providing mobile dislocation sources to generate the plastic strain at the crack tip necessary for crack blunting. Increasing the layer thickness can further enhance the fracture resistance of both Cu and Nb layers, since the critical stress for activating dislocation motion decreases with increasing the layer thickness. A novel atomistic-informed interface-dislocation dynamics (I-DD) model has been developed to study Metal-Ceramic Nanolayered Composites (MCNCs) based on the key deformation process and microstructure features revealed by MD simulations. The I-DD predicted results match well with the prior experimental results where both yield stress and strain hardening rate increase as the layer thickness decreases. This I-DD model shows great potential in predicting and optimizing the mechanical properties of MNCs --Abstract, page iv