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1.
Materials (Basel) ; 10(1)2017 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-28772453

RESUMO

Single-crystal Ni-base superalloys, consisting of a two-phase γ/ γ ' microstructure, retain high strengths at elevated temperatures and are key materials for high temperature applications, like, e.g., turbine blades of aircraft engines. The lattice misfit between the γ and γ ' phases results in internal stresses, which significantly influence the deformation and creep behavior of the material. Large-scale atomistic simulations that are often used to enhance our understanding of the deformation mechanisms in such materials must accurately account for such misfit stresses. In this work, we compare the internal stresses in both idealized and experimentally-informed, i.e., more realistic, γ/ γ ' microstructures. The idealized samples are generated by assuming, as is frequently done, a periodic arrangement of cube-shaped γ ' particles with planar γ/ γ ' interfaces. The experimentally-informed samples are generated from two different sources to produce three different samples-the scanning electron microscopy micrograph-informed quasi-2D atomistic sample and atom probe tomography-informed stoichiometric and non-stoichiometric atomistic samples. Additionally, we compare the stress state of an idealized embedded cube microstructure with finite element simulations incorporating 3D periodic boundary conditions. Subsequently, we study the influence of the resulting stress state on the evolution of dislocation loops in the different samples. The results show that the stresses in the atomistic and finite element simulations are almost identical. Furthermore, quasi-2D boundary conditions lead to a significantly different stress state and, consequently, different evolution of the dislocation loop, when compared to samples with fully 3D boundary conditions.

2.
MethodsX ; 3: 279-88, 2016.
Artigo em Inglês | MEDLINE | ID: mdl-27114926

RESUMO

The accurate and fast identification of crystallographic defects plays a key role for the analysis of atomistic simulation output data. For face-centered cubic (fcc) metals, most existing structure analysis tools allow for the direct distinction of common defects, such as stacking faults or certain low-index surfaces. For body-centered cubic (bcc) metals, on the other hand, a robust way to identify such defects is currently not easily available. We therefore introduce a new method for analyzing atomistic configurations of bcc metals, the BCC Defect Analysis (BDA). It uses existing structure analysis algorithms and combines their results to uniquely distinguish between typical defects in bcc metals. In essence, the BDA method offers the following features:•Identification of typical defect structures in bcc metals.•Reduction of erroneously identified defects by iterative comparison to the defects in the atom's neighborhood.•Availability as ready-to-use Python script for the widespread visualization tool OVITO [http://ovito.org].

3.
Nano Lett ; 16(1): 105-13, 2016 Jan 13.
Artigo em Inglês | MEDLINE | ID: mdl-26569137

RESUMO

Silica (SiO2) glass, an essential material in human civilization, possesses excellent formability near its glass-transition temperature (Tg > 1100 °C). However, bulk SiO2 glass is very brittle at room temperature. Here we show a surprising brittle-to-ductile transition of SiO2 glass nanofibers at room temperature as its diameter reduces below 18 nm, accompanied by ultrahigh fracture strength. Large tensile plastic elongation up to 18% can be achieved at low strain rate. The unexpected ductility is due to a free surface affected zone in the nanofibers, with enhanced ionic mobility compared to the bulk that improves ductility by producing more bond-switching events per irreversible bond loss under tensile stress. Our discovery is fundamentally important for understanding the damage tolerance of small-scale amorphous structures.


Assuntos
Vidro/química , Nanofibras/química , Dióxido de Silício/química , Resistência à Tração , Humanos , Transição de Fase , Temperatura
4.
Data Brief ; 3: 209-15, 2015 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-26217746

RESUMO

Experimental and theoretical studies on nanowires have reported a size-dependence of the Young׳s modulus in the axial direction, which has been attributed to the increasing influence of surface stresses with decreasing wire diameter. Internal interfaces and their associated interface stresses could lead to similar changes in the elastic properties. In Kobler et al. [1], however, we reported results from atomistic calculations which showed for Ag that twin boundaries have a negligible effect on the Young׳s modulus. Here, we present data of density-functional theory calculations of elastic constants and Young׳s modulus for defect-free bulk Ag as well as for bulk Ag containing dense arrays of twin boundaries. It is shown that rigorous convergence tests are required in order to be able to deduce changes in the elastic properties due to bulk defects in a reliable way.

5.
PLoS One ; 9(4): e93309, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-24691379

RESUMO

We report molecular dynamics simulations of shear in a biomimetic hydroxyapatite-collagen composite. Our model exhibits elastic properties fully dominated by the inorganic component. However, beyond the elastic regime the biomolecules along with the hierarchical nature of the composite account for the formation of structure-inherent slip zones. These accommodate shear without compromising the overall structure and lead to the sliding of intrinsically defined rods at roughly constant restoring force. Upon releasing load, rod displacement is reversible and backcreep is observed as gradual ionic rearrangement in the slip zone, subjected to an activation barrier.


Assuntos
Apatitas/química , Biomimética , Proteínas/química , Resistência ao Cisalhamento , Materiais Biomiméticos/química , Colágeno/química , Durapatita/química , Simulação de Dinâmica Molecular
6.
Nat Commun ; 5: 3033, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-24398783

RESUMO

Mechanical response of metal nanowires has recently attracted a lot of interest due to their ultra-high strengths and unique deformation behaviours. Atomistic simulations have predicted that face-centered cubic metal nanowires deform in different modes depending on the orientation between wire axis and loading direction. Here we report, by combination of in situ transmission electron microscopy and molecular dynamic simulation, the conditions under which particular deformation mechanisms take place during the uniaxial loading of [110]-oriented Au nanowires. Furthermore, by performing cyclic uniaxial loading, we show reversible plastic deformation by twinning and consecutive detwinning in tension and compression, respectively. Molecular dynamics simulations rationalize the observed behaviours in terms of the orientation-dependent resolved shear stress on the leading and trailing partial dislocations, their potential nucleation sites and energy barriers. This reversible twinning-detwinning process accommodates large strains that can be beneficially utilized in applications requiring high ductility in addition to ultra-high strength.

7.
ACS Nano ; 8(2): 1629-38, 2014 Feb 25.
Artigo em Inglês | MEDLINE | ID: mdl-24417379

RESUMO

Recent progress in achieving high degrees of monodispersity in chemical synthesis of complex nanostructures creates the unique situation in which individual nanostructures become representative for the whole ensemble. Under these conditions, atomistic simulations can play a completely new role in interpreting structural data obtained from averaging techniques. We apply this approach to fivefold twinned Ag nanowires for which the existence of an ambient-stable tetragonal phase in the nanowire core has been recently proposed. Quantitative comparison of experimental X-ray diffraction data with atomistic calculations unequivocally shows that the diffractograms can be fully explained by the complex strain state and defect structure of fivefold twinned Ag nanowires with fcc crystal structure. In addition, our approach enables rapid and accurate determination of wire diameters by a modified Scherrer analysis which uses a database generated by atomistic simulations.

8.
Phys Rev Lett ; 97(17): 170201, 2006 Oct 27.
Artigo em Inglês | MEDLINE | ID: mdl-17155444

RESUMO

We introduce a simple local atomic structure optimization algorithm which is significantly faster than standard implementations of the conjugate gradient method and often competitive with more sophisticated quasi-Newton schemes typically used in ab initio calculations. It is based on conventional molecular dynamics with additional velocity modifications and adaptive time steps. The surprising efficiency and especially the robustness and versatility of the method is illustrated using a variety of test cases from nanoscience, solid state physics, materials research, and biochemistry.


Assuntos
Algoritmos , Modelos Químicos , Fenretinida/química , Conformação Molecular , Termodinâmica
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