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

Nanograin Size Effects on the Strength of Biphase Nanolayered Composites

In this work, we employ atomic-scale simulations to uncover the interface-driven deformation mechanisms in biphase nanolayered composites. Two internal boundaries persist in these materials, the interlayer crystalline boundaries and intralayer biphase interfaces, and both have nanoscale dimensions. These internal surfaces are known to control the activation and motion of dislocations, and despite the fact that most of these materials bear both types of interfaces. From our calculations, we find that the first defect event, signifying yield, is controlled by the intralayer spacing (grain size, d), and not the intralayer biphase spacing (layer thickness, h). The interplay of two internal sizes leads to a very broad transition region from grain boundary sliding dominated flow, where the material is weak and insensitive to changes in h, to grain boundary dislocation emission and glide dominated flow, where the material is strong and sensitive to changes in h. Such a rich set of states and size effects are not seen in idealized materials with one of these internal surfaces removed. These findings provide some insight into how changes in h and d resulting from different synthesis processes can affect the strength of nanolayered materials

Modeling and Simulation of Nanoindentation

Nanoindentation is a hardness test method applied to small volumes of material which can provide some unique effects and spark many related research activities. To fully understand the phenomena observed during nanoindentation tests, modeling and simulation methods have been developed to predict the mechanical response of materials during nanoindentation. However, challenges remain with those computational approaches, because of their length scale, predictive capability, and accuracy. This article reviews recent progress and challenges for modeling and simulation of nanoindentation, including an overview of molecular dynamics, the quasicontinuum method, discrete dislocation dynamics, and the crystal plasticity finite element method, and discusses how to integrate multiscale modeling approaches seamlessly with experimental studies to understand the length-scale effects and microstructure evolution during nanoindentation tests, creating a unique opportunity to establish new calibration procedures for the nanoindentation technique

Fracture Resistance of Cu/Nb Metallic Nanolayered Composite

In this work, molecular dynamics simulations to explore the crack propagation and fracture behavior of Cu/Nb metallic nanolayered composites (MNCs) were performed. The results of this study are consistent with the previous experimental results, which illustrated that cracks in Cu and Nb layers may exhibit different propagation paths and distances under the isostrain loading condition. The analysis reveals that the interface can increase the fracture resistance of the Nb layer in Cu/Nb MNCs by providing the dislocation sources to generate the plastic strain at the front of the crack. Increasing the layer thickness can enhance the fracture resistance of both Cu and Nb layers, as the critical stress for activating the dislocation motion decreases with the increment of the layer thickness. In addition, grain boundaries (GBs) in polycrystalline Cu/Nb samples would decrease the fracture resistance of Nb layer by promoting the crack propagate along the GBs, i.e., intergranular fracture, while the effect of interface and layer thickness on the fracture resistance of MNCs will not be altered by introducing the GBs in MNCs

Effect of Plastic Incompatibility on the Strain Hardening Behavior of Al-TiN Nanolayered Composites

The strain hardening behavior of Al-TiN nanolayered composites induced by plastic incompatibility was studied by 3-D discrete dislocation dynamics (DDD) simulations. Our simulations results indicate the strain hardening rate solely induced by the plastic incompatibility is independent of layer thickness and dislocation density at a constant layer thickness ratio, while the yield stress exhibits a strong size effect. Furthermore, the strain hardening rate increases with decreasing Al/TiN layer thickness ratio and our predicted results match well with prior experiment data

Deformation of Heterogeneous Nanocrystalline Lamella with a Preexisting Crack

Molecular dynamics simulations were performed on heterogeneous nanocrystalline Al lamellae composed of nanocrystalline (NC) and single-crystalline (SC) layers to study the effect of the heterogeneous microstructures on the propagation of preexisting cracks. Under tensile loading, the heterogeneous NC Al lamella exhibited higher crack growth resistance than the pure NC Al. In addition, a lower volume fraction of the NC layer provided better crack growth resistance in heterogeneous lamellae (HL) samples, which agrees well with previous experiment results. After analyzing the distribution of the atom-level virial stress and microstructure evolution during the deformation, we found that the average stress on grain boundary atoms was much lower in HL samples than that in pure NC sample. When the crack approaches the interface, the heterogeneous microstructure can reduce the stress concentration by emitting dislocations from the interface into the SC layer

Unusual Size Effects from Tilted Twin Boundaries in Nano-Twinned Metals

Tilted twin boundaries (TBs), whose plane normals are rotated at an angle from the parent grain axis, naturally occur in columnar-grained, nano-twinned (NT) metals. Here, using a combination of atomistic simulations and analytical modeling, we reveal that NT metals with the ideal, non-tilted TBs exhibit continuously increasing strength with decreasing twin thickness, and hence, no inverse twin thickness size effect on strength. In contrast, NT metals with tilted TBs exhibit an inverse size effect, and the critical twin thickness, below which strength decreases, increases as the TB-tilt angle increases. The analysis also identifies a critical value of TB tilt, for which strength becomes independent of twin thickness and is the weakest. The transition arises from a change in dislocation activity prevailing mostly on planes inclined to TBs to planes parallel to the TBs. These findings reveal a profound influence of TB tilt angle that could redirect the analysis and engineering of nano-twin structures

Thickness-Dependent Shear Localization in Cu/Nb Metallic Nanolayered Composites

We performed molecular dynamics simulations to study the effect of the layer thickness on the shear localization in Cu/Nb metallic nanolayered composites (MNCs). Our simulation results achieve good agreement with experimental results that the inverse size effect in the strength occurs in samples with layer thickness below 2.0 nm. The strain softening observed in those samples was triggered by the shear localization. The quantitative analysis revealed that the unsymmetrical dislocation transmission across the interface induces the shear localization and promotes the shear band formation in Cu/Nb MNCs. The plastic strain mainly comes from the interface sliding within the shear band

The Effect of the Chemical Composition on Mechanical Properties of CMAS Diopside Glass Ceramics

Molecular dynamics simulations were performed on CaO-MgO-Al2O3-SiO2 (CMAS) diopside glass ceramics (GCs) to study the effect of nanocrystal on glass and the effect of chemical composition on mechanical properties. Under tensile loading, the GCs demonstrated that the strength lay between its glass and ceramic counterparts and maintained considerable ductility. Moreover, high Mg/Ca ion ratios are conductive to both the strength and ductility of GCs. In addition, Al ions should be avoided as far as possible since they would promote fracture. After analyzing the shear strain and displacement vector map for ion structures, we found that the presence of nanocrystal in glass changes the original deformation pattern and led to the deformation concentration surrounding the nanocrystal. A high Mg/Ca ion ratio would make the deformation more homogeneous, while a high Ca/Mg ion ratio would aggregate the deformation in the glass region near the nanocrystal. The existence of Al ions near the interface between glass and crystal would promote the formation of voids