Low-angle Twist Research Articles

Structures and energies of twist grain boundaries in Mg[formula omitted]SiO4 forsterite

In this work, we investigate twist grain boundary (GB) energies and structures in Mg2SiO4 forsterite using atomistic simulations. We first present a new bond orientational order parameter allowing to highlight disordered regions in this low-symmetry crystal for which classical visualization tools are ineffective. Then we examine three GB planes, (010), (120) and (001), corresponding to the most favorable free surfaces of this crystal. We show that twist GB follow the same energy ordering as corresponding free surfaces. In addition, except for some misorientation angles and for the (120) GB plane, GB energies and structures are quite insensitive to microscopic translational degrees of freedom. The dislocation composition of low-angle twist GB can be related to γ-surfaces in the corresponding planes, and are in good agreement with first-principle calculations. It is also shown that the dislocation core structures in low angle twist GB can strongly differ from the ones of intracrystalline dislocations.

A survey of the crystallography-dependency of twist grain boundary mobility in Al based on atomistic simulations

The correlations between the mobility and crystallographic parameters of twist grain boundaries (GBs) are investigated based on a mobility dataset created by molecular dynamics simulations using a modified synthetic driving force method. Statistically, low-angle twist GBs are not less or more mobile than high-angle ones, but twist GBs with axes near 〈111〉 or boundary plane close to {111} have low mobilities. These tendencies are significantly different from those revealed for symmetrical tilt GBs. The mobility overall does not exhibit any visible correlation with the coincidence index or energy of a GB.

Shock deformation and spallation of Cu bicrystals with (1 1 1) twist grain boundaries

The shock deformation and spallation of Cu bicrystals with (111) twist grain boundary (GB) of different misorientation angles are investigated by using molecular dynamics simulations. The selected GBs include low angle (LAGB), near twin (NTGB) and intermediate angle (IAGB) twist GBs, and the shock direction is along the GB normal. It is demonstrated that the bicrystals are of lower Hugoniot elastic limit than single crystal, as the twist GBs provide dislocation sources for deformation. At the shock compression stage, GB plasticity at the LAGBs and NTGBs is initiated via reaction and dissociation of GB dislocations, and can be triggered by elastic shock wave more readily than that at the IAGBs, on which dislocation emission is homogeneous and irrelevant to initial GB dislocations. At the tension stage, the incipient spall for different twist GBs occurs at an identical shock strength, but is of different details in dislocation and void evolutions. With varying shock strength and misorientation angle, spall strength of the Cu bicrystals depends on whether GB plasticity is produced at the compression stage, and the values are consistent with that of Cu single crystal under similar shock loading conditions.

Dissociated vacancies and screw dislocations in MgO and UO2: atomistic modeling and linear elasticity analysis

Understanding the effect of dislocations on the mass transport in ionic ceramics is important for understanding the behavior of these materials in a variety of contexts. In particular, the dissociated nature of vacancies at screw dislocations, or more generally, at a wide range of low-angle twist grain-boundaries, has ramifications for the mechanism of defect migration and thus mass transport at these microstructural features. In this paper, a systematic study of the dissociated vacancies at screw dislocations in MgO is carried out. The important role of stress migration in the atomistic modeling study is identified. Another aspect of the current work is a rigorous treatment of the linear elasticity model. As a result, good agreement between the atomistic modeling results and the linear elasticity model is obtained. Furthermore, we demonstrate that the proposed vacancy dissociation mechanism can also be extended to more complicated ionic ceramics such as UO2, highlighting the generality of the mechanism.

Structure and Mobility of Dissociated Vacancies at Twist Grain Boundaries and Screw Dislocations in Ionic Rocksalt Compounds

Interfaces, grain boundaries, and dislocations are known to have significant impact on the transport properties of materials. Even so, it is still not clear how the structure of interfaces influences the mobility and concentration of carriers that are responsible for transport. Using low angle twist grain boundaries in MgO as a model system, we examine the structural and kinetic properties of vacancies. These boundaries are characterized by a network of screw dislocations. Vacancies of both types, Mg and O, are strongly attracted to the dislocation network, residing preferentially at the misfit dislocation intersections (MDIs). However, the vacancies can lower their energy by splitting into two parts, which then repel each other along the dislocation line between two MDIs, further lowering their energy. This dissociated structure has important consequences for transport, as the free energy of the dissociated vacancies decreases with decreasing twist angle, leading to an increase in the net migration barri...

Role of point defects in self-diffusion along low-angle twist boundaries in fcc metals: A molecular dynamics study

The interaction of point defects with low-angle (100), (110) and (111) twist boundaries in fcc metals Ni, Cu, Al and role of the point defects in self-diffusion along considered boundaries were studied by the molecular dynamics method. The binding energies of point defects with low-angle twist boundaries were calculated. It was found that the point defects are mainly fixed in the nodes of grain-boundary screw dislocations network. It was shown that the introduced vacancies play an important role in diffusion along twist grain boundaries. The contribution of introduced interstitial atoms to grain-boundary diffusion under thermodynamic equilibrium condition is less by several orders of magnitude in comparison with the contribution of vacancies.

Strengthening effects of various grain boundaries with nano-spacing as barriers of dislocation motion from molecular dynamics simulations

Strengthening in metals is traditionally achieved through the controlled creation of various grain boundaries (GBs), such as low-angle GBs, high-angle GBs, and twin boundaries (TBs). In the present study, a series of large-scale molecular dynamics simulations with spherical nanoindentation and carefully designed model were conducted to investigate and compare the strengthening effects of various GBs with nano-spacing as barriers of dislocation motion. Simulation results showed that high-angle twist GBs and TBs are similar barriers and low-angle twist GBs are less effective in obstructing dislocation motion. Corresponding atomistic mechanisms were also given. At a certain indentation depth, dislocation transmission and dislocation nucleation from the other side of boundaries were observed for low-angle twist GBs, whereas dislocations were completely blocked by high-angle twist GBs and TBs at the same indentation depth. The current findings should provide insights for comprehensive understanding of the strengthening effects of various GBs at nanoscale.

Atomic simulations of twist grain boundary structures and deformation behaviors in aluminum

The structures and behaviors of grain boundaries (GBs) have profound effects on the mechanical properties of polycrystalline materials. In this paper, twist GBs in aluminum were investigated with molecular dynamic simulations to reveal their atomic structures, energy and interactions with dislocations. One hundred twenty-six twist GBs were studied, and the energy of all these twist GBs were calculated. The result indicates that <001> and <111> twist GBs have lower energy than <101> twist GBs because of their higher interplanar spacing. In addition, 12 types of <001> twist GBs in aluminum were chosen to explore the deformation behaviors. Low angle twist GBs with high density of network structures can resist greater tension because mutually hindering behaviors between partial dislocations increase the twist GB strength.

Dislocation Nucleation on Grain Boundaries: Low Angle Twist and Asymmetric Tilt Boundaries

We investigate the mechanisms of incipient plasticity at low angle twist and asymmetric tilt boundaries in fcc metals. To observe plasticity of grain boundaries independently of the bulk plasticity, we simulate nanoindentation of bicrystals. On the low angle twist boundaries, the intrinsic grain boundary (GB) dislocation network deforms under load until a dislocation segment compatible with glide on a lattice slip plane is created. The half loops are then emitted into the bulk of the crystal. Asymmetric twist boundaries considered here did not produce bulk dislocations under load. Instead, the boundary with a low excess volume nucleated a mobile GB dislocation and additional GB defects. The GB sliding proceeded by motion of the mobile GB dislocation. The boundary with a high excess volume sheared elastically, while bulk-nucleated dislocations produced plastic relaxation.

Atomistic view of thin Ni/Ni3Al (0 0 1) under uniaxial tension of twist grain boundaries

Atomic motion and the structure response of grain boundaries (GBs) are essential to the plastic deformation of small-volume polycrystal systems, especially for thin materials that exhibit some dramatic characteristics. Here, the microstructure and properties of thin Ni/Ni3Al (0 0 1) with the uniaxial tension of twist GBs are investigated using molecular dynamics (MD) simulations with an embedded atom (EAM) potential. We find that low angle twist GB dislocations propagate mainly from the corners to the center of the interface, and for high angle twist GBs they originate from the edge of the interface but do not extend to the center. In the process of plastic deformation, both low angle and high angle twist GB fractures occur in the center of the interface, which is completely different from the situation of thick materials reported by other references. More interestingly, the fracture patterns between low angle and high angle twist GBs exhibit extremely different performance due to the different characteristics of the initial dislocation structures in the interfaces.

Misorientation dependence of Al2O3 grain boundary thermal resistance

The room-temperature thermal resistance of model low-angle (0001) twist bicrystal Al2O3 grain boundaries was measured as a function of misorientation angle using the 3ω thermal conductivity testing technique. The work yields interfacial thermal resistances of 0.44 × 10−8, 1.2 × 10−8, and 1.54 × 10−8 Km2 W−1 for the ∼1.3°, ∼8.0°, and ∼13° twist grain boundaries, respectively. The interfacial thermal resistance correlates with the grain boundary energy calculated from the Read-Shockley model. The results indicate significant anisotropy in the Kapitza resistance whose magnitude is dominated by local structural defects at the grain boundary rather than spectral mismatch between grains.

Structure and energy of (1 1 1) low-angle twist boundaries in Al, Cu and Ni

We study the structure and energy of (111) low-angle twist boundaries in face-centered cubic Al, Cu and Ni, using a generalized Peierls–Nabarro model incorporating the full disregistry vector in the slip plane and the associated stacking fault energy. It is found that dislocation network structures on these twist boundaries can be determined by a single dimensionless parameter, with two extreme cases of a hexagonal network of perfect dislocations and triangular network of partial dislocations enclosing stacking faults. We construct a simple model of these networks based upon straight partial dislocation segments. Based on this structural model, we derive an analytical expression for the twist boundary energy as a function of the twist angle θ, intrinsic stacking fault energy and parameters describing the isolated dislocation core. In addition to the θ and θlogθ terms [3] and the θ2 term [17] in the boundary energy, our new energy expression also includes additional terms in θ that represent the partial dissociation of the dislocation nodes and the effect of the stacking fault energy. The analytical predictions of boundary structure and energy are shown to be in excellent agreement with the Peierls–Nabarro model simulations.

Effects of lattice mismatch on interfacial structures of liquid and solidified Al in contact with hetero-phase substrates: MD simulations

In this study, the effects of the misfit on in-plane structures of liquid Al and interfacial structure of solidified Al in contact with the heterophase substrates have been investigated, using molecular dynamics (MD) simulations. The MD simulations were conducted for Al/fcc (111) substrates with varied misfits. The order parameter and atomic arrangement indicated that the in-plane ordering of the liquid at the interface decreases significantly with an increase of the misfit, i.e., solid-like for small misfit and liquid-like for large misfit. Further, our MD simulation results revealed that a perfect orientation relationship forms at the interface between the substrate and the solidified Al for a misfit of less than -3% and the boundary is coherent. With an increase in the misfit, Shockley partial and extended dislocations form at the interface, and the boundary becomes a semi-coherent or low-angle twist boundary.

Structure of screw dislocations in a (0 0 0 1)/[0 0 0 1] low-angle twist grain boundary of alumina (α-Al2O3)

An alumina (α-Al2O3) bicrystal with a (0001)/[0001] low-angle twist grain boundary was fabricated by bonding two (0001) substrates at 1500°C in air. Dislocation structures in the resultant grain boundary were extensively observed in plan view and cross-sectional direction by transmission electron microscopy (TEM). The grain boundary consisted of a hexagonal dislocation network of 1/3〈12¯10〉 basal screw dislocations and wavy belt structures. The atomic core structure of the screw dislocations was analyzed in end-on view by high-resolution TEM. It was evident that the 1/3〈12¯10〉 basal screw dislocation does not dissociate into partial dislocations, which is in contrast to the 1/3〈12¯10〉 basal edge dislocation that dissociates into 1/3〈11¯00〉 and 1/3〈01¯10〉 partial dislocations. Also, TEM observation revealed that the wavy belt structures correspond to planar voids. From the morphology of the planar voids, it is considered that they accommodate a slight tilt component of the boundary.

The angular dislocation parallel to a free surface. Application to a (1 1 1)Si low-angle twist boundary

The elastic displacement field of a sharply angular dislocation with its two legs parallel to a planar free surface is given in a practical analytical form. Its expression gives access, in transmission electron microscopy (TEM), to computed images of numerous interacting dislocations all located at a distance h close to the free surface. As an application, this field is used repeatedly to study, in dark-field TEM and a g(3g) diffraction mode with g{2 2 0}, the contrast of dissociated triple nodes of a low-angle twist boundary in silicon extended over a (1 1 1) plane. It is shown that free surface elastic effects can influence the contrasts of some 30° partials if h is at the nanometer scale.

Tunable helium bubble superlattice ordered by screw dislocation network

Helium bubble nucleation at low-angle twist boundaries in gold has been investigated. It is found that the helium bubbles preferentially nucleate at screw dislocation nodal points and result in helium bubble superlattice formation, which is completely isomorphic with the screw dislocation network along the twist-grain boundary. Molecular statics calculations reveal that defect formation/solution energies along the screw dislocations, especially at the nodal points, are lower than their bulk counterparts. It is believed that this driving force is responsible for the helium bubble superlattice formation. Our study suggests that grain boundary engineering via adjustable twist angles in parallel boundaries to form tunable 3D bubble superlattices could afford a very promising approach for design of radiation tolerant materials.

Helium bubble growth at BCC twist grain boundaries

We study the growth of helium bubbles in α-Fe at low angle twist grain boundaries and in bulk using molecular dynamics and Metropolis Monte Carlo simulations. We describe the pressures and volumes of the helium bubbles and analyze the maximum pressure a bubble can sustain before emitting interstitial loops. We give a quantitative analysis of how these emitted loops behave differently in the bulk and at the grain boundary.

Атомный механизм диффузии по малоугловым границам кручения в ГЦК металлах

Атомный механизм диффузии по малоугловым границам кручения в ГЦК металлах

Evolution of the interface structure of bonded Si wafers after high temperature annealing

The evolution of the interfaces of bonded Si wafers and the corresponding low-angle twist boundary have been analysed in dependence on thermal annealing. Two orientation relations were investigated: i) Si(001)/SiO2/Si(001) and ii) Si(110)/SiO2/Si(001). The interfaces were analysed by TEM and STEM/EDX and EELS. It is found that the decomposition rate of the intermediate oxide layer and the formation of a Si-Si bonded interface depend very much on the lattice mismatch and on the twist angle. A dissolution of the oxide and the formation of Si-Si boundaries occur much faster in the case of Si(110)/Si(001) bonding than in Si(001)/Si(001). The process of interface fusion and the dissolution of the oxide layer are discussed.

Molecular dynamical investigation on plastic behavior of Cu(100) twist-grain boundary under shear load

By using molecular dynamics with EAM potential, yield strength of copper column with (100) twist grain boundary and twist grain boundary effect are investigated under shear load. The results reveal that dislocations nucleate at the misfit dislocation network of low-angle twist grain boundary and dislocations stacking influence the yield strength of copper column. For high-angle twist grain boundary, twist grain boundary sliding decreases the yield stress of copper column. Meanwhile it is found that, with increasing twist angle, copper column yielding strength increases firstly. When twist angle reaches the critical value, the yielding strength decreases with increasing twist angle. It is concluded that different mechanisms mediate the yielding of copper column. Dislocation nucleation controls the yield stress of copper column for low-angle twist grain boundary and yielding of copper column is dominated by grain boundary sliding for high-angle twist grain boundary.