scholarly journals Hydrogen behaviour at twist {110} grain boundaries in α -Fe

2017 ◽  
Vol 375 (2098) ◽  
pp. 20160402 ◽  
Author(s):  
Eunan J. McEniry ◽  
Tilmann Hickel ◽  
Jörg Neugebauer
Keyword(s):  
Grain Boundaries ◽  
Coincidence Site Lattice ◽  
Critical Role ◽  
Tight Binding ◽  
Structural Defects ◽  
Interface Plane ◽  
Extended Defects ◽  
Binding Model ◽  
Σ3 Boundary

The behaviour of hydrogen at structural defects such as grain boundaries plays a critical role in the phenomenon of hydrogen embrittlement. However, characterization of the energetics and diffusion of hydrogen in the vicinity of such extended defects using conventional ab initio techniques is challenging due to the relatively large system sizes required when dealing with realistic grain boundary geometries. In order to be able to access the required system sizes, as well as high-throughput testing of a large number of configurations, while remaining within a quantum-mechanical framework, an environmental tight-binding model for the iron–hydrogen system has been developed. The resulting model is applied to study the behaviour of hydrogen at a class of low-energy {110}-terminated twist grain boundaries in α -Fe. We find that, for particular Σ values within the coincidence site lattice description, the atomic geometry at the interface plane provides extremely favourable trap sites for H, which also possess high escape barriers for diffusion. By contrast, via simulated tensile testing, weakly trapped hydrogen at the interface plane of the bulk-like Σ3 boundary acts as a ‘glue’ for the boundary, increasing both the energetic barrier and the elongation to rupture. This article is part of the themed issue ‘The challenges of hydrogen and metals’.

Structural Disorder and Localized Gap States in Silicon Grain Boundaries from a Tight-Binding Model

1997 ◽  
Vol 491 ◽  
Author(s):  
F. Cleri ◽  
P. Keblinski ◽  
L. Colombo ◽  
S. R. Phillpot ◽  
D. Wolf
Keyword(s):  
Molecular Dynamics ◽  
Grain Boundaries ◽  
Tight Binding ◽  
Structural Disorder ◽  
High Energy ◽  
Tight Binding Model ◽  
Binding Model ◽  
Dynamics Simulations ◽  
Gap States ◽  
Forbidden Gap

ABSTRACTTight-binding molecular dynamics simulations of typical high-energy grain boundaries in silicon show that the atomic structure of the interface in thermodynamic equilibrium is similar to that of bulk amorphous silicon and contains coordination defects. The corresponding electronic structure is also amorphous-like, displaying extra states in the forbidden gap mainly localized around the coordination defects, where large changes in the bond-hybridization character are observed. It is proposed that such coordination defects in disordered high-energy grain boundaries are responsible for the experimentally observed gap states in polycrystalline Si.

scholarly journals Effects of Magnetic Field on the Residual Stress and Structural Defects of Ti-6Al-4V

Metals ◽  
2020 ◽  
Vol 10 (1) ◽  
pp. 141 ◽  
Author(s):  
Xu Zhang ◽  
Qian Zhao ◽  
Zhipeng Cai ◽  
Jiluan Pan
Keyword(s):  
Magnetic Field ◽  
Residual Stress ◽  
Grain Boundaries ◽  
Electron Backscatter Diffraction ◽  
Magnetic Measurement ◽  
Coincidence Site Lattice ◽  
Weak Ferromagnetism ◽  
Structural Defects ◽  
Test Results ◽  
The Status

In this work, the influences of a magnetic field of 2.4 T on the macro residual stress and the status of structural defects, including grain boundaries, dislocations and the Fe-rich clusters of Ti-6Al-4V were investigated by X-ray Diffraction (XRD), Electron Backscatter Diffraction (EBSD) and magnetic measurement. The XRD test results show that the applied magnetic field can cause the relaxation and homogenization of macro residual stress. The maps of Kernel Average Misorientation (KAM) values obtained by EBSD tests present a significant dislocation multiplication caused by a magnetic field, and the rise of dislocation density was estimated to be about 32% by XRD tests. The EBSD test results also show an increase in the fraction of Coincidence Site Lattice (CSL) grain boundaries and a decrease in the fraction of low-angle grain boundaries. The results of magnetic measurement show that Ti-6Al-4V has mixed magnetism consisting of paramagnetism and weak ferromagnetism, and that the ferromagnetic saturation magnetization decreased after exposing the alloy to the magnetic field, which suggests the dissolution of the Fe-rich clusters in the alloy. These magnetically-induced changes are related to magnetoplastic effects, a kind of phenomena on which there have been some research, and the possible mechanism of them is discussed in this paper.

A Tight-Binding Model for Optical Properties of Porous Silicon

1997 ◽  
Vol 491 ◽  
Author(s):  
M. Cruz ◽  
M. R. Beltran ◽  
C. Wang ◽  
J. Tagüeña-Martinez
Keyword(s):  
Optical Properties ◽  
Porous Silicon ◽  
Crystalline Silicon ◽  
Tight Binding ◽  
Basis Set ◽  
Extended Defects ◽  
Tight Binding Model ◽  
Binding Model ◽  
Semi Empirical ◽  
Good Agreement

ABSTRACTSemi-empirical tight-binding techniques have been extensively used during the last six decades to study local and extended defects as well as aperiodic systems. In this work we propose a tight-binding model capable of describing optical properties of disordered porous materials in a novel way. Besides discussing the details of this approach, we apply it to study porous silicon (p-Si). For this purpose, we use an sp3s* basis set and supercells, where empty columns are digged in the [001] direction in crystalline silicon (c-Si). The disorder of the pores is considered through a random perturbative potential, which relaxes the wave vector selection rule, resulting in a significant enlargement of the optically active k-zone. The dielectric function and the light absorption spectra are calculated. The results are compared with experimental data showing a good agreement.

scholarly journals INTERACTION OF HYDROGEN IMPURITY WITH NANOCRYSTALLINE PALLADIUM AND NICKEL

2018 ◽  
Vol 61 (8) ◽  
pp. 631-637
Author(s):  
G. M. Poletaev ◽  
I. V. Zorya ◽  
R. Y. Rakitin
Keyword(s):  
Binding Energy ◽  
Crystal Lattice ◽  
Grain Boundaries ◽  
Tight Binding ◽  
Triple Junctions ◽  
High Concentration ◽  
Hydrogen Atoms ◽  
Binding Model ◽  
Metal Atoms ◽  
Morse Potentials

The interaction of hydrogen atoms with nanocrystalline palladium and nickel in the work was studied by the molecular dynamics method. The nanocrystalline structure of palladium and nickel was created in the model by crystallization from the liquid state at the presence of several specially introduced crystalline embryos. After solidification, the calculation blocks, in addition to the crystalline phase, contained grain boundaries and triple junctions of grain boundaries. The interactions of metal atoms with each other were described by the multi-particle Cleri-Rosato potential constructed in the framework of the tight-binding model. Morse potentials were used to describe the interactions of hydrogen atoms with metal atoms and with each other. The parameters of Morse potentials were calculated from the experimental data of theabsorption energy, the activation energy of the above-barrier diffusion of hydrogen in a metal (at normal and high temperatures), the binding energy with a vacancy, dilatation. According to the results obtained in the present work, at a high concentration of hydrogen (the concentration of 10% from the metal atoms was considered), the hydrogen atoms combine into aggregates, which are formed predominantly near the surface of the metal. The aggregates contained, as a rule, several dozen hydrogen atoms and had low diffusion activity. The binding energy of hydrogen atoms with these aggregates was greater than with the metal crystal lattice or grain boundaries in it. In palladium, hydrogen aggregates were formed farther from the surface than in nickel. Apparently, this is due not so much to the relatively low energy of hydrogen absorption by palladium (–0.1 eV) in comparison with nickel (0.16 eV), but rather to the difference in lattice parameters of the metals under consideration: 3.89 Å for Pd and 3.524 Å for Ni. For the same reason, conspicuously, hydrogen aggregates in a pure crystal lattice were more often observed in Pd than in Ni. In Ni, aggregates, as a rule, were formed in defect areas containing an excess free volume: near the free surface, in grain boundaries and in triple junctions.

Characterization of Grain Boundaries in Recrystallized L12-Type Intermetallic Alloys

2005 ◽  
Vol 502 ◽  
pp. 151-156
Author(s):  
Yasuyuki Kaneno ◽  
Takayuki Takasugi
Keyword(s):  
Grain Boundaries ◽  
Electron Backscatter Diffraction ◽  
Stacking Fault ◽  
Grain Boundary Character Distribution ◽  
Grain Boundary Character ◽  
Disordered Alloys ◽  
Σ3 Boundary ◽  
Fcc Alloy ◽  
Backscatter Diffraction

Microstructural feature of the recrystallized Co-based (Co3Ti) and Ni-based (Ni3(Si,Ti) and Ni3Fe) ordered alloys with L12 structure was investigated by the electron backscatter diffraction (EBSD) method, with emphasis on grain boundary character distribution (GBCD). For comparison, the GBCDs of the recrystallized Co-Ni, Ni-Fe and 70/30 brass disordered alloys, and also copper, nickel and aluminum pure metals with A1 (fcc) structure, which have widely different stacking fault energies, were also determined. The frequency of Σ3 boundary for the Co-based alloys was higher than that for the Ni-based alloys, regardless of ordered L12 alloy or disordered fcc alloy, indicating that the frequency of Σ3 boundary was primarily dominated by stacking fault energy. Furthermore, the effect of ordering energy on structure and energy of the grain boundaries including Σ3 boundary in the ordered L12 alloys was discussed.

scholarly journals EFFECT OF DEFORMATION ON MIGRATION RATE OF GRAIN BOUNDARIES IN NICKEL

2019 ◽  
Vol 61 (12) ◽  
pp. 974-979
Author(s):  
G. M. Poletaev ◽  
I. V. Zorya ◽  
R. Y. Rakitin ◽  
D. V. Kokhanenko ◽  
M. D. Starostenkov
Keyword(s):  
Grain Boundaries ◽  
Migration Rate ◽  
Diffusion Processes ◽  
Tight Binding ◽  
Boundary Surface ◽  
Computer Experiment ◽  
Uniaxial Deformation ◽  
Binding Model ◽  
Directed Movement ◽  
And Migration

Effect of deformation along various directions against migrating  boundary on migration rate of edge boundaries with <100> and <111>  misorientation  axes  in  nickel  was  studied  by  means  of  molecular  dynamics  method.  Grain  boundaries  were  created  in  U-shaped  model.  Force of boundary surface tension, arising from the boundary intension  to  minimize  its  energy,  was  the  reason  of  directed  movement  of  the  boundary toward its area decrease. The force provoking migration and  migration rate of the boundary remained constant throughout the entire  movement  of  the  boundary,  gradually  decreasing  towards  the  end  of  computer  experiment,  which  made  it  possible  to  measure  migration  rate quite simply. Effect of uniaxial deformation along the X, Y, Z axes  on migration rate of the boundaries was considered. Uniaxial deformation in the model was set at beginning of the computer experiment by  changing  corresponding  interatomic  distances  along  one  of  the  axes.  Interactions of nickel atoms with each other were described with the aid  of Cleri Rosato many-particle potential constructed in the framework  of  tight  binding  model.  For  the  boundaries  considered,  dependences  of  migration  rate  on  misorientation  angle  at  temperature  of  1700 K  were obtained. It is shown that the high-angle <111> and <100> edge  boundaries migrate approximately at the same rate, while mobility of  low-angle  boundaries  differs  significantly:  low-angle  <111>  boundaries migrate about twice as fast as the <100> boundaries. It was found  that in almost all cases, both at elastic compression and tension deformation, migration rate of considered boundaries was slowed down. An  exception was the case of deformation along the <111> edge boundary  axis. When compressing along the edge axis, <111> boundary migrated faster, while on the contrary, it was slower at tension. The obtained  results testify to the fact that migration of edge boundaries is not due to  diffusion processes, such as climbing of dislocations, single migrations of  atoms,  but,  apparently,  by  collective  atomic  permutations:  shifts, slides and splittings of grain boundary dislocations.

Determination of the lattice translation across {110} APBs in GaAs

1988 ◽  
Vol 46 ◽  
pp. 600-601
Author(s):  
D.R. Rasmussen ◽  
N.-H. Cho ◽  
C.B. Carter
Keyword(s):  
Grain Boundaries ◽  
Lower Energy ◽  
Antiphase Boundary ◽  
The Other ◽  
Boundary Structure ◽  
Interface Plane ◽  
Inversion Symmetry ◽  
High Angle ◽  
Equilibrium Distance

Domains in GaAs can exist which are related to one another by the inversion symmetry, i.e., the sites of gallium and arsenic in one domain are interchanged in the other domain. The boundary between these two different domains is known as an antiphase boundary [1], In the terminology used to describe grain boundaries, the grains on either side of this boundary can be regarded as being Σ=1-related. For the {110} interface plane, in particular, there are equal numbers of GaGa and As-As anti-site bonds across the interface. The equilibrium distance between two atoms of the same kind crossing the boundary is expected to be different from the length of normal GaAs bonds in the bulk. Therefore, the relative position of each grain on either side of an APB may be translated such that the boundary can have a lower energy situation. This translation does not affect the perfect Σ=1 coincidence site relationship. Such a lattice translation is expected for all high-angle grain boundaries as a way of relaxation of the boundary structure.

High-resolution electron microscopy of nanostructured materials

1995 ◽  
Vol 53 ◽  
pp. 172-173
Author(s):  
M. José-Yacamán
Keyword(s):  
Electron Microscopy ◽  
High Resolution ◽  
Grain Boundaries ◽  
Nanostructured Materials ◽  
High Resolution Electron Microscopy ◽  
Hrem Image ◽  
Small Particles ◽  
Resolution Electron ◽  
Nanophase Materials

Electron microscopy is a fundamental tool in materials characterization. In the case of nanostructured materials we are looking for features with a size in the nanometer range. Therefore often the conventional TEM techniques are not enough for characterization of nanophases. High Resolution Electron Microscopy (HREM), is a key technique in order to characterize those materials with a resolution of ~ 1.7A. High resolution studies of metallic nanostructured materials has been also reported in the literature. It is concluded that boundaries in nanophase materials are similar in structure to the regular grain boundaries. That work therefore did not confirm the early hipothesis on the field that grain boundaries in nanostructured materials have a special behavior. We will show in this paper that by a combination of HREM image processing, and image calculations, it is possible to prove that small particles and coalesced grains have a significant surface roughness, as well as large internal strain.

Structural analysis of a Σ5 grain boundary in YBa2Cu3O7δ

1993 ◽  
Vol 51 ◽  
pp. 942-943
Author(s):  
J.-Y. Wang ◽  
Y. Zhu ◽  
A.H. King ◽  
M. Suenaga
Keyword(s):  
Grain Boundary ◽  
Lattice Mismatch ◽  
Weak Link ◽  
Coincidence Site Lattice ◽  
Large Angle ◽  
Dislocation Core ◽  
Superconducting Order Parameter ◽  
Outstanding Problem ◽  
Transmission Electron

One outstanding problem in YBa2Cu3O7−δ superconductors is the weak link behavior of grain boundaries, especially boundaries with a large-angle misorientation. Increasing evidence shows that lattice mismatch at the boundaries contributes to variations in oxygen and cation concentrations at the boundaries, while the strain field surrounding a dislocation core at the boundary suppresses the superconducting order parameter. Thus, understanding the structure of the grain boundary and the grain boundary dislocations (which describe the topology of the boundary) is essential in elucidating the superconducting characteristics of boundaries. Here, we discuss our study of the structure of a Σ5 grain boundary by transmission electron microscopy. The characterization of the structure of the boundary was based on the coincidence site lattice (CSL) model.Fig.l shows two-beam images of the grain boundary near the projection. An array of grain boundary dislocations, with spacings of about 30nm, is clearly visible in Fig. 1(a), but invisible in Fig. 1(b).

Characterization of Serrated Grain Boundaries in Ni-Based Alloy 690

2014 ◽  
Vol 52 (9) ◽  
pp. 695-704
Author(s):  
Yun Soo Lim ◽  
Dong Jim Kim ◽  
Seong Sik Hwang
Keyword(s):  
Grain Boundaries ◽  
Alloy 690
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