Paulo S. Branicio

Structural characterization of deformed crystals by analysis of common atomic neighborhood

Simulations of crystal deformation and structural transformation may generate complex datasets involving networks with million to billion chemical bonds which makes local structure analysis a challenge. An ideal analysis method must recognize perfect crystal structures, such as face-centered cubic, body-centered cubic and hexagonal close packed, and differentiate structural defects such as dislocations, stacking faults, grain boundaries, cracks and surfaces. Currently a few methods are used for this purpose, e.g., the Common Neighbor Analysis (CNA) and the Centrosymmetry Parameter (CSP). This paper proposes an alternative method based on the calculation of a single parameter that depends on the common atomic neighborhood. We validate the method characterizing local structures in complex molecular-dynamics datasets, clarifying its advantages over the CNA and the CSP methods.

Brittle dynamic fracture of crystalline cubic silicon carbide "3C-SiC… via molecular dynamics simulation

Brittle fracture dynamics for three low-index crack surfaces, i.e., (110), (111), and (100), in crystalline cubic silicon carbide (3C-SiC) is studied using molecular dynamics simulation. The results exhibit significant orientation dependence: (110) fracture propagates in a cleavage manner; (111) fracture involves slip in the {111¯} planes; and crack branching is observed in (001) fracture. Calculated critical energy release rates, which characterize fracture toughness, are compared with available experimental and ab initio calculation data.

A transition from localized shear banding to homogeneous superplastic flow in nanoglass

A promising remedy to the failure of metallic glasses (MGs) by shear banding is the use of a dense network of glass-glass interfaces, i.e., a nanoglass (NG). Here we investigate the effect of grain size (d) on the failure of NG by performing molecular dynamics simulations of tensile-loading on Cu50Zr50 NG with d = 5 to 15 nm. Our results reveal a drastic change in deformation mode from a single shear band (d ∼ 15 to 10 nm), to cooperative shear failure (d ∼ 10 to 5 nm), to homogeneous superplastic flow (d ≤ 5 nm). Our results suggest that grain size can be an effective design parameter to tune the mechanical properties of MGs.

On the notch sensitivity of CuZr metallic glasses

Atomistic simulations are performed to study the effects of size and shape of a superficial or internal notch on the strength and failure mechanism of CuZr metallic glass (MG) under tensile loading. Our results show that plastic deformation originating at the notch root reduces the stress concentration there and leads to a notch-insensitive normalized tensile strength. The notch, however, dictates the failure location as the plastic zone at the notch root serves as a nucleation site for shear band (SB) formation. It is shown that when the plastic zone size reaches a critical value, a SB starts to propagate from the notch root across the entire sample, causing the material failure. These results provide useful guidelines for the design, testing, and engineering of MG for structural applications.

Large-scale molecular dynamics simulations of wear in diamond-like carbon at the nanoscale

We perform large-scale molecular dynamics simulations on diamond-like carbon to study wear mechanism and law at the nanoscale. Our simulations show that material loss during sliding varies linearly with normal load and sliding distance, consistent with Archards law. Our simulations also show that the number of chemical bonds across the contact interface during sliding correlates well with friction force, but not with material loss, indicating that friction and wear follow different mechanisms. Our analysis reveals the following wear mechanism: the shear traction causes mass accumulation at the trailing end of contact, which is then lost by a cluster detachment process.

Deformation mechanisms and damage in α-alumina under hypervelocity impact loading

Deformation mechanisms in α-alumina under hypervelocity impact are investigated using molecular dynamics simulations containing 540×106 atoms. A cylindrical projectile impacting normal to the (0001) surface at 18km∕s generates large temperature and pressure gradients around the impact face, and consequently local amorphization of the substrate in a surrounding hemispherical region is produced. Away from the impact face, a wide range of deformations emerge and disappear as a function of time under the influence of local stress fields, e.g., basal and pyramidal slips and basal and rhombohedral twins, all of which show good agreement with the experimental and theoretical results. New deformation modes are observed, such as twins along {01¯11}, which propagate at a roughly constant speed of 8km∕s and nucleate a large amount of defects where subsequent fractures initiate. The relation between deformation patterns and local stress levels is investigated. During unloading, we observe that microcracks nucleate ex...

Local stress calculation in simulations of multicomponent systems

The virial and Hardy methods provide accurate local stresses for single component materials such as monatomic metals. In contrast to the elemental material case, both methods provide poor estimates of the local stress for multicomponent materials. Using binary materials such as CaO, SiC and AlN and homogeneous strain, we demonstrate that there are several sources for the slow convergence of the virial and Hardy local stresses to the bulk values. Different approaches such as enforced stoichiometry, atomic localization functions and the atomic voronoi volume are used to improve the convergence and increase the spatial resolution of the local stress. The virial method with enforced stoichiometry and atomic voronoi volumes is the most accurate, giving exact stress values by the first atomic shell. In the general case, not assuming stoichiometry, the virial method with localization functions converge to 93% of the bulk value by the third atomic shell. This work may be particularly useful for the real-time description of stresses in simulations of shock waves and deformation dynamics.

Composition and grain size effects on the structural and mechanical properties of CuZr nanoglasses

Nanoglasses (NGs), metallic glasses (MGs) with a nanoscale grain structure, have the potential to considerably increase the ductility of traditional MGs while retaining their outstanding mechanical properties. We investigated the effects of composition on the structural and mechanical properties of CuZr NG films with grain sizes between 3 to 15 nm using molecular dynamics simulations. Results indicate a transition from localized shear banding to homogeneous superplastic flow with decreasing grain size, although the critical average grain size depends on composition: 5 nm for Cu36Zr64 and 3 nm for Cu64Zr36. The flow stress of the superplastic NG at different compositions follows the trend of the yield stress of the parent MG, i.e., Cu36Zr64 yield/flow stress: 2.54 GPa/1.29 GPa and Cu64Zr36 yield/flow stress: 3.57 GPa /1.58 GPa. Structural analysis indicates that the differences in mechanical behavior as a function of composition are rooted at the distinct statistics of prominent atomic Voronoi polyhedra. T...

Accelerating dislocations to transonic and supersonic speeds in anisotropic metals

The dynamics of stress-accelerated dislocations in copper is investigated using molecular dynamics simulations. The structure and motion of dissociated edge dislocations are analyzed using the common neighborhood parameter and local stresses. Dislocations are accelerated by high shear stresses and reach stable velocities in the two transonic regimes. Supersonic dislocations are generated but are transient, as they require high stresses, which trigger nucleation of multiple dislocation dipoles. A velocity plateau in the first transonic regime indicates a radiation-free state in agreement with theoretical predictions.

Structural, mechanical, and vibrational properties of Ga1−xInxAs alloys: A molecular dynamics study

Structural, mechanical, and vibrational properties of Ga1−xInxAs (0⩽x⩽1) random solid solutions are investigated with classical and ab initio molecular-dynamics simulations. We find that the Ga–As and In–As bond lengths change only slightly as a function of x, despite the large lattice mismatch (∼7%) between GaAs and InAs crystals. The nearest cation–cation distance has a broad distribution, whereas the nearest neighbor anion–anion distance distribution has two distinct peaks. The elastic constants exhibit a significant nonlinear dependence on x. The phonon density-of-states exhibits two high-frequency optical modes. These results are in excellent agreement with experiments.