From Crystalline to Glassy: Crack Propagation Modes in Decagonal Quasicrystals

2003 ◽  
Vol 805 ◽  
Author(s):  
Christoph Rudhart ◽  
Peter Gumbsch ◽  
Hans-Rainer Trebin
Keyword(s):  
Crack Extension ◽  
Amorphous Solids ◽  
Linear Scaling ◽  
Dislocation Emission ◽  
Medium Temperature ◽  
Decagonal Quasicrystals ◽  
Shear Transformation ◽  
Shear Transformation Zones ◽  
Dynamics Simulations ◽  
Increasing Temperature

ABSTRACTThe propagation of mode-I cracks in a two-dimensional decagonal model quasicrystal is studied by molecular dynamics simulations. The samples are endowed with an atomically sharp seed crack and a temperature gradient. Subsequently the crack is loaded by linear scaling of the displacement field. The response of the crack running into regions of increasing temperature is monitored.For low temperatures below 30% of the melting temperature Tm the model-quasicrystal fails by brittle fracture. We observe that the crack follows the path of dislocations nucleated at its tip. The crack propagates along well defined planes and circumvents tightly bound clusters. In the medium temperature regime from 30% to 70% Tm the crack is blunting spontaneously by dislocation emission. In the range of 70%-80% Tm the quasicrystal fails by nucleation, growth and coalescence of micro-voids. This gradual, dislocation-free crack extension is caused by plastic deformation which is mediated by localized rearrangements comparable to so-called shear transformation zones in amorphous solids.

scholarly journals Shear-Transformation Zone Activation during Loading and Unloading in Nanoindentation of Metallic Glasses

Materials ◽  
2019 ◽  
Vol 12 (9) ◽  
pp. 1477 ◽  
Author(s):  
Karina E. Avila ◽  
Stefan Küchemann ◽  
Iyad Alabd Alhafez ◽  
Herbert M. Urbassek
Keyword(s):  
Dynamics Simulation ◽  
Minor Effect ◽  
Rate Analysis ◽  
Loading And Unloading ◽  
Shear Transformation ◽  
Shear Transformation Zones ◽  
Experimental Findings ◽  
A Minor ◽  
Increasing Temperature ◽  
Activity Cluster

Using molecular dynamics simulation, we study nanoindentation in large samples of Cu–Zr glass at various temperatures between zero and the glass transition temperature. We find that besides the elastic modulus, the yielding point also strongly (by around 50%) decreases with increasing temperature; this behavior is in qualitative agreement with predictions of the cooperative shear model. Shear-transformation zones (STZs) show up in increasing sizes at low temperatures, leading to shear-band activity. Cluster analysis of the STZs exhibits a power-law behavior in the statistics of STZ sizes. We find strong plastic activity also during the unloading phase; it shows up both in the deactivation of previous plastic zones and the appearance of new zones, leading to the observation of pop-outs. The statistics of STZs occurring during unloading show that they operate in a similar nature as the STZs found during loading. For both cases, loading and unloading, we find the statistics of STZs to be related to directed percolation. Material hardness shows a weak strain-rate dependence, confirming previously reported experimental findings; the number of pop-ins is reduced at slower indentation rate. Analysis of the dependence of our simulation results on the quench rate applied during preparation of the glass shows only a minor effect on the properties of STZs.

scholarly journals Super Ductility of Nanoglass Aluminium Nitride

Nanomaterials ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 1535 ◽  
Author(s):  
Zhao ◽  
Peng ◽  
Huang ◽  
Yang ◽  
Hu ◽  
...  
Keyword(s):  
Strain Rate ◽  
Ultimate Strength ◽  
Scaling Law ◽  
Aluminium Nitride ◽  
Grain Sizes ◽  
Shear Transformation ◽  
Effects Of Temperature ◽  
Shear Transformation Zones ◽  
Function Relationship ◽  
Dynamics Simulations

Ceramics have been widely used in many fields because of their distinctive properties, however, brittle fracture usually limits their application. To solve this problem, nanoglass ceramics were developed. In this article, we numerically investigated the mechanical properties of nanoglass aluminium nitride (ng-AlN) with different glassy grain sizes under tension using molecular dynamics simulations. It was found that ng-AlN exhibits super ductility and tends to deform uniformly without the formation of voids as the glassy grain size decreases to about 1 nm, which was attributed to a large number of uniformly distributed shear transformation zones (STZs). We further investigated the effects of temperature and strain rate on ng-AlNd = 1 nm, which showed that temperature insignificantly influences the elastic modulus, while the dependence of the ultimate strength on temperature follows the T2/3 scaling law. Meanwhile, the ultimate strength of ng-AlNd = 1 nm is positively correlated with the strain rate, following a power function relationship.

scholarly journals Collisions between amorphous carbon nanoparticles: phase transformations

2020 ◽  
Vol 641 ◽  
pp. A159
Author(s):  
Maureen L. Nietiadi ◽  
Felipe Valencia ◽  
Rafael I. Gonzalez ◽  
Eduardo M. Bringa ◽  
Herbert M. Urbassek
Keyword(s):  
Amorphous Carbon ◽  
Contact Radius ◽  
Central Collisions ◽  
Dust Clouds ◽  
Collision Zone ◽  
Atomic Surface ◽  
Shear Transformation ◽  
Shear Transformation Zones ◽  
Dynamics Simulations ◽  
20 Nm

Context. Collisions of nanoparticles (NPs) occur in dust clouds and protoplanetary disks. Aims. Sticking collisions lead to the growth of NPs, in contrast to bouncing or even fragmentation events and we aim to explore these processes in amorphous carbon NPs. Methods. Using molecular-dynamics simulations, we studied central collisions between amorphous carbon NPs that had radii in the range of 6.5–20 nm and velocities of 100–3000 m s−1, and with varying sp3 content (20–55%). Results. We find that the collisions are always sticking. The contact radius formed surpasses the estimate provided by the traditional Johnson-Kendall-Roberts model, pointing at the dominant influence of attractive forces between the NPs. Plasticity occurs via shear-transformation zones. In addition, we find bond rearrangements in the collision zone. Low-sp3 material (sp3 ≤ 40%) is compressed to sp3 > 50%. On the other hand, for the highest sp3 fraction, 55%, graphitization starts in the collision zone leading to low-density and even porous material. Conclusions. Collisions of amorphous carbon NPs lead to an increased porosity, atomic surface roughness, and changed hybridization that affect the mechanical and optical properties of the collided NPs.

scholarly journals Influence of grain size and composition, topology and excess free volume on the deformation behavior of Cu–Zr nanoglasses

2015 ◽  
Vol 6 ◽  
pp. 537-545 ◽  
Author(s):  
Daniel Şopu ◽  
Karsten Albe
Keyword(s):  
Mechanical Properties ◽  
Grain Size ◽  
Free Volume ◽  
Deformation Behavior ◽  
Alloy Composition ◽  
Topological Order ◽  
Shear Transformation ◽  
Shear Transformation Zones ◽  
Dynamics Simulations ◽  
Topological Disorder

The influence of grain size and composition on the mechanical properties of Cu–Zr nanoglasses (NGs) is investigated by molecular dynamics simulations using two model glasses of different alloy composition, namely Cu64Zr36 (Cu-rich) and Cu36Zr64 (Zr-rich). When the grain size is increased, or the fraction of interfaces in these NGs is decreased, we find a transition from a homogeneous to an inhomogeneous plastic deformation, because the softer interfaces are promoting the formation shear transformation zones. In case of the Cu-rich system, shear localization at the interfaces is most pronounced, since both the topological order and free volume content of the interfaces are very different from the bulk phase. After thermal treatment the redistribution of free volume leads to a more homogenous deformation behavior. The deformation behavior of the softer Zr-rich nanoglass, in contrast, is only weakly affected by the presence of glass–glass interfaces, since the interfaces don’t show topological disorder. Our results provide clear evidence that the mechanical properties of metallic NGs can be systematically tuned by controlling the size and the chemical composition of the glassy nanograins.

Polymers contamination by heavy metal compounds

2002 ◽  
Vol 56 (11) ◽  
pp. 483-488
Author(s):  
Sasa Jovanic ◽  
Dragoslav Stoiljkovic ◽  
Ivanka Popovic
Keyword(s):  
Heavy Metal ◽  
Aqueous Medium ◽  
Metal Compound ◽  
Medium Temperature ◽  
Metal Compounds ◽  
Metal Type ◽  
Copper Manganese ◽  
Increasing Temperature

The contamination of important synthetic (surface unmodified) polymers by various heavy metal compounds (such as copper, manganese and lead) in aqueous medium was investigated in this study. The influence of the pH of the aqueous medium, temperature and metal type on contamination was investigated during a 10 day period. It was found that increasing pH contributed to higher polymer contamination (at higher pH 100 times for copper and up to 400 times for lead), as well as contact with easily penetrable substances. Increasing temperature decreased contamination by the metal compound for PELD and PET which was not the case for PEHD and PR.

Uncertainties of Thermal Conductivities From Equilibrium Molecular Dynamics Simulations

2016 ◽  
Author(s):  
Zuyuan Wang ◽  
Xiulin Ruan
Keyword(s):  
Molecular Dynamics ◽  
Correlation Time ◽  
Domain Size ◽  
Material System ◽  
Dynamics Simulations ◽  
Increasing Temperature ◽  
Solid Argon ◽  
Simulation Parameters ◽  
Simulation Time ◽  
Thermal Conductivities

The Green-Kubo method in the framework of equilibrium molecular dynamics (EMD) simulations is an effective method that has been widely used to calculate thermal conductivities of materials. The previous studies focused on the thermal conductivity values or the average values from repetitive simulations. Little research has been done to investigate the uncertainties of the thermal conductivities from EMD simulations. In this paper, we use solid argon as the material system to study the factors influencing the uncertainties of the predicted thermal conductivities. We find that the uncertainties decrease with the total simulation time as (ttotal)−α and increase with correlation time as (tcorre)β, where 0.48 < α, β < 0.52. We also find that the uncertainties decrease with increasing temperature, but the simulation domain size has a negligible effect. We propose some guidelines for selecting appropriate simulation parameters (e.g., the correlation time and total simulation time) to achieve a desired level of uncertainty. This work is potentially useful for future studies on calculating the thermal conductivities of materials using EMD simulations.

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