Controlled rippling of graphene via irradiation and applied strain modify its mechanical properties: a nanoindentation simulation study

Ripples, present in free standing graphene, have an important influence in the mechanical behavior of this two-dimensional material. In this work we show through nanoindentation simulations, how out-of-plane displacements can be modified by strain resulting in softening of the membrane under compression and stiffening under tension. Irradiation also induces changes in the mechanical properties of graphene. Interestingly, compressed samples, irradiated at low doses are stiffened by the irradiation while samples under tensile strain do not show significant changes in their mechanical properties. These simulations indicate that vacancies, produced by the energetic ions, cannot be the ones directly responsible for this behavior. However, changes in roughness induced by the momentum transferred from the energetic ions to the membrane, can explain these differences. These results provide an alternative explanation to recent experimental observations of stiffening of graphene under low dose irradiation, as well as paths to tailor the mechanical properties of this material via applied strain and irradiation.This work is supported by the Generalitat Valenciana through grant reference PROMETEO2012/011 and the Spanish government through grant FIS2010-21883. CJR and EMB thanks support from SeCTyP-UNCuyo grant M003, and ANPCyT grant PICT-2014-0696. CJR thanks CONICET and the 310 Group at FCEN-UNCuyo

Boosting materials science simulations by high performance computing

Technology development is often limited by knowledge of materials engineering and manufacturing processes. This scenario spans across scales and disciplines, from aerospace engineering to MicroElectroMechanical Systems (MEMS) and NanoElectroMechanical Systems (NEMS). The mechanical response of materials is dictated by atomic/nanometric scale processes that can be explored by molecular dynamics (MD) simulations. In this work we employ atomistic simulations to prove indentation as a prototypical deformation process showing the advantage of High Performance Computing (HPC) implementations for speeding up research. Selecting the right HPC hardware for executing simulations is a process that usually involves testing different hardware architectures and software configurations. Currently, there are several alternatives, using HPC cluster facilities shared between several researchers, as provided by Universities or Government Institutions, owning a small cluster, acquiring a local workstation with a high-end microprocessor, and using accelerators such as Graphics Processing Units (GPU), Field Programmable Gate Arrays (FPGA), or Intel Many Integrated Cores (MIC). Given this broad set of alternatives, we run several benchmarks using various University HPC clusters, a former TOP500 cluster in a foreign computing center, two high-end workstations and several accelerators. A number of different metrics are proposed to compare the performance and aid in the selection of the best hardware architecture according to the needs and budget of researchers. Amongst several results, we find that the Titan X Pascal GPU has a ∼3 x speedup against 64 AMD Opteron CPU cores.Publicado en: Mecánica Computacional vol. XXXV, no. 10.Facultad de Ingenierí

Boosting materials science simulations by high performance computing

Technology development is often limited by knowledge of materials engineering and manufacturing processes. This scenario spans across scales and disciplines, from aerospace engineering to MicroElectroMechanical Systems (MEMS) and NanoElectroMechanical Systems (NEMS). The mechanical response of materials is dictated by atomic/nanometric scale processes that can be explored by molecular dynamics (MD) simulations. In this work we employ atomistic simulations to prove indentation as a prototypical deformation process showing the advantage of High Performance Computing (HPC) implementations for speeding up research. Selecting the right HPC hardware for executing simulations is a process that usually involves testing different hardware architectures and software configurations. Currently, there are several alternatives, using HPC cluster facilities shared between several researchers, as provided by Universities or Government Institutions, owning a small cluster, acquiring a local workstation with a high-end microprocessor, and using accelerators such as Graphics Processing Units (GPU), Field Programmable Gate Arrays (FPGA), or Intel Many Integrated Cores (MIC). Given this broad set of alternatives, we run several benchmarks using various University HPC clusters, a former TOP500 cluster in a foreign computing center, two high-end workstations and several accelerators. A number of different metrics are proposed to compare the performance and aid in the selection of the best hardware architecture according to the needs and budget of researchers. Amongst several results, we find that the Titan X Pascal GPU has a ∼3 x speedup against 64 AMD Opteron CPU cores.Publicado en: Mecánica Computacional vol. XXXV, no. 10.Facultad de Ingenierí

Micro-mechanical response of ultrafine grain and nanocrystalline tantalum

In order to investigate the effect of grain boundaries on the mechanical response in the micrometer and submicrometer levels, complementary experiments and molecular dynamics simulations were conducted on a model bcc metal, tantalum. Microscale pillar experiments (diameters of 1 and 2 μm) with a grain size of ~100–200 nm revealed a mechanical response characterized by a yield stress of ~1500 MPa. The hardening of the structure is reflected in the increase in the flow stress to 1700 MPa at a strain of ~0.35. Molecular dynamics simulations were conducted for nanocrystalline tantalum with grain sizes in the range of 20–50 nm and pillar diameters in the same range. The yield stress was approximately 6000 MPa for all specimens and the maximum of the stress–strain curves occurred at a strain of 0.07. Beyond that strain, the material softened because of its inability to store dislocations. The experimental results did not show a significant size dependence of yield stress on pillar diameter (equal to 1 and 2 um), which is attributed to the high ratio between pillar diameter and grain size (~10–20). This behavior is quite different from that in monocrystalline specimens where dislocation ‘starvation’ leads to a significant size dependence of strength. The ultrafine grains exhibit clear ‘pancaking’ upon being plastically deformed, with an increase in dislocation density. The plastic deformation is much more localized for the single crystals than for the nanocrystalline specimens, an observation made in both modeling and experiments. In the molecular dynamics simulations, the ratio of pillar diameter (20–50 nm) to grain size was in the range 0.2–2, and a much greater dependence of yield stress to pillar diameter was observed. A critical result from this work is the demonstration that the important parameter in establishing the overall deformation is the ratio between the grain size and pillar diameter; it governs the deformation mode, as well as surface sources and sinks, which are only important when the grain size is of the same order as the pillar diameter

Propiedades mecánicas de metales nanoporosos .

Este trabajo de tesis doctoral trata sobre el estudio de las propiedades mecánicas de espumas metálicas nanoporosas. Empleando simulaciones con la técnica de dinámica molecular se estudia la compresión a alta velocidad de un monocristal de tantalio bcc con poros distribuidos al azar. En comparación con estudios de un poro aislado, la interacción entre los poros produce una disminución en las tensiones necesarias para el inicio del régimen plástico. La plasticidad se manifiesta mediante la emisión de dislocaciones desde la superficie de los poros, en forma de lazos de corte, y su interacción deriva en endurecimiento. La evolución de la plasticidad conlleva a una disminución de la porosidad hasta que los poros desaparecen completamente. Las densidades de dislocaciones resultantes se corresponden con resultados experimentales. Con el fin de evaluar y cuantificar la evolución de la plasticidad se realizó un análisis de las dislocaciones observadas. Los nanoporos actúan como fuentes para la emisión de dislocaciones. Los lazos de corte se nuclean en la superficie de los poros y se expanden por el avance de la componente de borde. A partir de las simulaciones con dinámica molecular se predicen las configuraciones de las dislocaciones y sus densidades y éstas últimas resultan comparadas frente a modelos basados en dislocaciones geométricamente necesarias, con resultados satisfactorios. Se realizaron cálculos de tensiones críticas resueltas para todos los sistemas de deslizamiento, empleándose para identificar el vector de Burgers operante en los lazos de dislocación. Los cambios en la temperatura de la muestra durante la deformación plástica fueron empleados para estimar la densidad de dislocaciones móviles. Los resultados obtenidos son comparados con una variedad de modelos constitutivos basados en dislocaciones. Además, se estudió una nanoespuma de oro fcc, sobre la cual se realizó una caracterización empleando microscopía electrónica de transmisión y microscopía electrónica de barrido. En vista de los resultados de la caracterización experimental y empleando dinámica molecular, se estudiaron las propiedades mecánicas de una nanoespuma de oro fcc sujeta a compresión a alta velocidad. Para esta parte del estudio se empleó una nanoespuma con porosidad del orden del 75 %, identificándose distintas etapas en la deformación plástica. Con base en las simulaciones atomísticas, se ha observado un régimen de densificación en todas las muestras nanoporosas estudiadas. Con estos resultados, se propone un nuevo modelo de cambio de porosidad respecto a la deformación para su uso en simulaciones a escala del continuo

Defect production in Ar irradiated graphene membranes under different initial applied strains

Irradiation with low energy Ar ions of graphene membranes gives rise to changes in the mechanical properties of this material. These changes have been associated to the production of defects, mostly isolated vacancies. However, the initial state of the graphene membrane can also affect its mechanical response. Using molecular dynamics simulations we have studied defect production in graphene membranes irradiated with 140 eV Ar ions up to a dose of 0.075 × 1014 ions/cm2 and different initial strains, from −0.25% (compressive strain) to 0.25% (tensile strain). For all strains, the number of defects increases linearly with dose with a defect production of about 80% (80 defects every 100 ions). Defects are mostly single vacancies and di-vacancies, although some higher order clusters are also observed. Two different types of di-vacancies have been identified, the most common one being two vacancies at first nearest neighbours distance. Differences in the total number of defects with the applied strain are observed which is related to the production of a higher number of di-vacancies under compressive strain compared to tensile strain. We attribute this effect to the larger out-of-plane deformations of compressed samples that could favor the production of defects in closer proximity to others.This work is supported by the Generalitat Valenciana through grant reference PROMETEO2012/011 and the Spanish government through grant FIS2010-21883. CJR and EMB thank support from SeCTyP-UNCuyo and ANPCyT grant PICT-2014-0696

Uniaxial-deformation behavior of ice I-h as described by the TIP4P/Ice and mW water models

CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOUsing molecular dynamics simulations, we assess the uniaxial deformation response of ice I-h as described by two popular water models, namely, the all-atom TIP4P/Ice potential and the coarse-grained mW model. In particular, we investigate the response to both tensile and compressive uniaxial deformations along the [0001] and [0 (1) over bar 10] crystallographic directions for a series of different temperatures. We classify the respective failure mechanisms and assess their sensitivity to strain rate and cell size. While the TIP4P/Ice model fails by either brittle cleavage under tension at low temperatures or large-scale amorphization/melting, the mW potential behaves in a much more ductile manner, displaying numerous cases in which stress relief involves the nucleation and subsequent activity of lattice dislocations. Indeed, the fact that mW behaves in such a malleable manner even at strain rates that are substantially higher than those applied in typical experiments indicates that the mW description of ice I-h is excessively ductile. One possible contribution to this enhanced malleability is the absence of explicit protons in the mW model, disregarding the fundamental asymmetry of the hydrogen bond that plays an important role in the nucleation and motion of lattice dislocations in ice I-h.1491619CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOsem informaçãosem informação2013/08293-72016/23891-

Atomistic simulation of the mechanical properties of nanoporous gold

7–109 s−1 using molecular dynamics simulations. We consider the low-porosity regime (porosity of ∼5%), which is characterized by several stages of plastic deformation. At the onset of plasticity, pores act as if isolated by emitting “shear” dislocation loops. At higher deformations, the mechanical response is determined by the interactions between dislocations in the dense dislocation forest, leading to strain hardening. Increasing the strain rate results in an increasing flow stress ranging from 0.4 to 0.7 GPa within the range of applied strain rates. The von Mises stress σVM in the hardening regime features two possible power-law dependencies as a function of dislocation density ρd: in the initial stages of plastic deformation we obtained σVM∝ρd2, but changes to Taylor hardening σVM∝ρd1/2 at higher dislocation densities. The velocity of dislocations is estimated to be ∼60% of the speed of sound in the early stages of plastic deformation, but later decreases dramatically due to dislocation–dislocation and dislocation–pore interactions. The unloading of the complex dislocation and stacking fault network leads to the production of vacancies. As a result, we propose that the vacancy clusters observed experimentally in recovered samples and attributed to “dislocation-free” plasticity are instead due to the aggregation of those vacancies left behind during recovery.Fil: Rodriguez Nieva. J. F.. Massachusetts Institute of Technology; Estados Unidos. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Ruestes, Carlos Javier. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; ArgentinaFil: Tang, Yizhe. Shanghai University. Shanghai Institute of Applied Mathematics and Mechanics; ChinaFil: Bringa, Eduardo Marcial. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentin