Dislocation Core Structure Research Articles

Effect of hydrogen on the basal dissociation of 〈a〉-type screw dislocation core in titanium

The existence of basal dissociated screw dislocation core structure of titanium with different H concentrations is investigated by first-principles calculation. The calculation of stacking fault energy of the basal plane with H in different layers suggests that a high H concentration would be beneficial for basal dissociation. The direct calculation based on the dislocation core instead proved that a small amount of H cannot stabilize the basal dissociated core structure. When H concentration increases, the basal dissociated core configuration and some mixed cores with basal faults could be more stable than the pyramidal dissociated structure.

Electronic descriptors for dislocation deformation behavior and intrinsic ductility in bcc high-entropy alloys

Controlling the balance between strength and damage tolerance in high-entropy alloys (HEAs) is central to their application as structural materials. Materials discovery efforts for HEAs are therefore impeded by an incomplete understanding of the chemical factors governing this balance. Through first-principles calculations, this study explores factors governing intrinsic ductility of a crucial subset of HEAs—those with a body-centered cubic (bcc) crystal structure. Analyses of three sets of bcc HEAs comprising nine different compositions reveal that alloy chemistry profoundly influences screw dislocation core structure, dislocation vibrational properties, and intrinsic ductility parameters derived from unstable stacking fault and surface energies. Key features in the electronic structure are identified that correlate with these properties: the fraction of occupied bonding states and bimodality of the d-orbital density of states. The findings enhance the fundamental understanding of the origins of intrinsic ductility and establish an electronic structure–based framework for computationally accelerated materials discovery and design.

Core structure of dislocations in ordered ferromagnetic FeCo

Core structure of dislocations in ordered ferromagnetic FeCo

Mobility of dislocations in carbon nanotube bundles

Horizontally aligned carbon nanotubes (CNTs) have a unique combination of mechanical and physical properties and find numerous applications. The shear deformation of a horizontally aligned CNT bundle is studied under plane-strain conditions using the method of molecular dynamics. The CNTs in the bundle are not perfectly packed, and it is important to see how defects alter the mechanisms of their deformation under applied load. The defects in the form of edge dislocations are introduced into the bundle. It is found that the dislocation core structure depends on the CNT diameter, as the deformability of the CNT cross section increases with the diameter. Different deformation mechanisms (dislocation sliding or cracking) are revealed in the bundles with CNTs of different diameters. The results presented describe how the dislocation core structure affects the dislocation mobility in CNT bundles and the mechanisms of their deformation.

Distorted dislocation cores and asymmetric glide resistances in titanium

The determination of Critical Resolved Shear Stress (CRSS) in titanium for basal, prismatic, and pyramidal slip-planes without empirical constants is presented by combining Density Functional Theory (DFT) and anisotropic elasticity. A new mechanism leading to tension-compression (T-C) asymmetry in the CRSS levels has been revealed for the first time. The conditions for this asymmetry are established, involving a complex interplay between the dislocation core-structure and core-advance behavior. The three conditions for T-C asymmetry that need to be simultaneously satisfied can be summarized as: (1) a medium stacking fault width, d, (3<d/bF<10, where bF is the magnitude of the Burgers vector of the full dislocation), (2) an asymmetric core-structure of the extended dislocation (ξ1≠ξ2, where ξ1 and ξ2 are the core-widths of the first and second partials, respectively), and (3) intermittent motion of the partials (Δd/bF≠0, where Δd is the magnitude of fluctuation in stacking fault width during intermittent motion). Pyramidal-slip in titanium satisfies all three conditions, resulting in significant T-C asymmetry. The CRSS theory considers a Wigner-Seitz (W-S) cell based domain area assigned to each lattice site for the calculations of core-energies accurately capturing the slip-plane lattice. The W-S based approach is essential due to the lower symmetry of the HCP crystal circumventing potential errors associated with the one-dimensional atomic-row approximation. Dislocation core structures are obtained for all the slip-systems in titanium showing significant distortion of the disregistry profile governing the core-advance behavior. The CRSS values predicted from the theory show agreement with the experimental CRSS levels reported in the literature.

Synergistic influence of dislocations and helium cluster on hydrogen atom in tungsten

Dislocations are common line defects in the preparation and irradiation processes of tungsten (W), and they can serve as trapping sites for hydrogen (H) and helium (He) atoms. Existing studies have indicated that dislocations preferentially capture He atoms to form dislocation-He cluster structures. The focus of this study was on the effects of this new structure on the trapped and diffusion behavior of H atoms using molecular dynamics (MD) method. The study considered the number of He atoms from 1 to 150, and they were classified as small He cluster and large He cluster. By calculating the binding energy, the study revealed that the capture strength is positively correlated with the He clusters’ size. However, it is negatively correlated with the distance from the dislocation core and the captured H atoms’ number. This study output the stable configurations to analyze the growth patterns of He cluster in dislocations, and it was observed that the interstitial atoms caused by trap mutation preferentially aggregate into dislocation loops due to the dislocation core structure. Moreover, the study investigated the diffusion behavior of H atoms around the dislocation-He cluster structure at 600 K within 1 ns. The hydrogen atoms tend to migrate towards the He cluster and distribute around it uniformly. The distribution range narrows with the helium cluster's size, and the distribution pattern depends on the dislocations’ type. This demonstrated the formation of dislocation-He cluster structure, promoting the increased H retention.

Mechanical quenching phenomenon in diamond

The structure of dislocation cores, the fundamental knowledge on crystal plasticity, remains largely unexplored in covalent crystals. Here, we conducted atomically resolved characterizations of dislocation core structures in a plastically deformed diamond anvil cell tip that was unloaded from an exceptionally high pressure of 360 GPa. Our observations unveiled a series of nonequilibrium dislocation cores that deviate from the commonly accepted "five-seven-membered ring" dislocation core model found in FCC-structured covalent crystals. The nonequilibrium dislocation cores were generated through a process known as "mechanical quenching," analogous to the quenching process where a high-energy state is rapidly frozen. The density functional theory-based molecular dynamic simulations reveal that the phenomenon of mechanical quenching in diamond arises from the challenging relaxation of the nonequilibrium configuration, necessitating a large critical strain of 25% that is difficult to maintain. Further electronic-scale analysis suggested that such large critical strain is spent on the excitation of valance electrons for bond breaking and rebonding during relaxation. These findings establish a foundation for the plasticity theory of covalent materials and provide insights into the design of electrical and luminescent properties in diamond, which are intimately linked to the dislocation core structure.

Combined effect of interstitial-substitutional elements on dislocation dynamics in nitrogen-added austenitic stainless steels

Combined addition of interstitial-substitutional elements has been acknowledged to contribute to the increase in the strengths of steels. For further improvements in mechanical properties, their atomic-scale interaction mechanisms with dislocations are required to be examined. In this study, both high-resolution transmission electron microscopy and atom-probe tomography were used to correlate interstitial-substitutional elements with dislocation characteristics in austenitic stainless steels. Three types of dislocation core structures are identified and associated with their strain fields as well as N and Cr atoms in the N-added steels. It is revealed that N atoms interact elastically with the dislocations, followed by the segregation of Cr atoms via the chemical interaction between N and Cr atoms. This insight significantly improves the understanding of the multiple alloying mechanism in metallic materials such as interstitial alloys and high-entropy alloys.

An atomistic study on the HELP mechanism of hydrogen embrittlement in pure metal Fe

The Hydrogen Enhanced Localized Plasticity (HELP) mechanism is one of the most important theories explaining Hydrogen Embrittlement in metallic materials. While much research has focused on hydrogen's impact on dislocation core structure and dislocation mobility, its effect on local dislocation density and plasticity remains less explored. This study examines both aspects using two distinct atomistic simulations: one for a single edge dislocation under shear and another for a bulk model under cyclic loading, both across varying hydrogen concentrations. We find that hydrogen stabilizes the edge dislocation and exhibits a dual impact on dislocation mobility. Specifically, mobility increases below a shear load of 900 MPa but progressively decreases above this threshold. Furthermore, dislocation accumulation is notably suppressed at around 1 % hydrogen concentration. These findings offer key insights for future research on Hydrogen Embrittlement, particularly in fatigue scenarios.

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.

Finite-temperature screw dislocation core structures and dynamics in α-titanium

A multiscale approach based on molecular dynamics (MD) and kinetic Monte Carlo (kMC) methods is developed to simulate the dynamics of an 〈a〉 screw dislocation in α-Ti. The free energy barriers for the core dissociation transitions and Peierls barriers for dislocation glide as a function of temperature are extracted from the MD simulations (based on machine learning interatomic potentials and optimization); these form the input to kMC simulations. Dislocation random walk trajectories from kMC agree well with those predicted by MD. On some planes, dislocations move via a locking-unlocking mechanism. Surprisingly, some dislocations glide in directions that are not parallel with the core dissociation direction. The MD/kMC multiscale method proposed is applicable to dislocation motion in simple and complex materials (not only screw dislocations in Ti) as a function of temperature and stress state.

Deformation behaviors in light of dislocation core characteristics with respect to the compositional-dependent misfit potentials of aluminum alloys

Dislocation core dominates dislocation mobility and mechanical properties of crystalline solids. To date, a complete landscape for describing dislocation with narrow core in metals like aluminum (Al) remains elusive, and thus deformation mechanisms of Al alloys are still unclear. This work investigates the dislocation core structure and deformation behaviors of Al alloyed with solutes X (X = Mg, Si, Cu, Zn, and Fe) within the framework of semi-discrete variational theory combined with first-principles calculations. Depending on the dislocation core characteristics, the deformation modes of Al alloys are governed by the compositional-dependent misfit potentials. Except for Fe, all other solutes in Al decrease the intrinsic SF energy, with considerable effects being Mg and Si. The deformation tends to occur via cross-slip of dislocations in Al alloyed with Mg, whereas Si additions can benefit to deform via emission of partial dislocations. Our investigation offers theoretical guidance for choosing solutes favorable to mechanical performances of Al alloys.

New understanding of one-dimensional thermal glide of a nano-sized prismatic dislocation loop in bcc iron: an atomic scale study

Vacancy-type prismatic dislocation loops (PDLs) can be formed directly through cross-slip mechanisms of screw dislocations or via the accumulation and transformation of high-density vacancy-type defects. The concentration of vacancy-type defects in body-centered cubic (BCC) iron can be increased through plastic deformation and in the presence of hydrogen. Nano-sized PDLs can act as high-speed vacancy transport in BCC iron, causing embrittlement. Based on molecular dynamics simulations, this study comprehensively analyzes the one-dimensional diffusion of vacancy-type PDLs composed of 37–169 vacancies (V37-V169) at temperatures ranging from 100 to 1000 K. The activation energy of diffusion increased with decreasing PDL size for small loops (V37–V61) but remained almost constant for larger loops (>V91). This behavior was found to be associated with the tensile stress produced by the opposing edge dislocation segment in the loop where the distance is closer for a smaller PDL. That is, the tensile stress along the slip direction of edge dislocation causes the unstable stacking fault energy to increase, and the change of dislocation core structure further contributes to an increase in the activation energy for the diffusion of PDL. In contrast, an external compressive stress reduces the activation energy and pre-exponential factor of diffusion in small PDLs, as exemplified in V37 PDL.

An atomistic-to-microscale characterization of the kink-controlled dislocation dynamics in bcc metals through finite-temperature coarse-grained atomistic simulations

Adopting bcc tungsten (W) as a model material, we characterize the temperature and stress dependence of kink dynamics on a dislocation line with length L ranging from 60 nm to 1 μm using finite-temperature coarse-grained (FT-CG) atomistic simulations. The main novelty of this work is to accommodate major salient aspects, namely the motion of μm-long dislocation lines, the atomic-scale kink dynamics, and the full spectrum of phonon dynamics, all in one single FT-CG model. At a fraction of the cost of molecular dynamics (MD) calculations, the FT-CG simulation predicts: (a) a dislocation-induced degeneration of the phonon density of states (PDoS) of W; (b) the kink-induced dislocation core structure transition from a “soft” (non-planar, compact) to a “hard” configuration (planar, split); and (c) the crossover from the line tension (LT) regime to the elastic interaction (EI) regime in the temperature dependence of the flow stress. Several findings arise from the simulations: (1) the kink activation stress, σf, not only depends on the temperature, T, but also exhibits a sensitivity to the dislocation line length, L. For μm-long dislocations, it approaches experimental results but σf for kink activation on nm-long dislocations does not; (2) upon an increase of T, the σf reduction for the sample containing μm-long dislocations is significantly larger than that for the one with nm-long dislocations; (3) based on data extracted from FT-CG simulations of “temperature jump tests”, the l- dependence of the kink activation enthalpy, ΔH, is characterized. It can be as high as ∼3 eV for a dislocation with a length of tens of nm but reduces to an experimentally comparable level of ∼1.5 eV when L is 0.3 μm or longer. This suggests an easier kink activation on a longer dislocation. Such a dislocation line length dependence of ΔH can be further amplified at an even lower applied stress; (4) the entropic kink activation barrier, ΔHT, is linearly proportional to T. The slope of the ΔHT – T relation, however, will be largely underestimated in nanoscale MD simulations, but can be comparable with that from experiments when L is ∼ 0.3 μm or longer. These findings highlight the limitations of nanoscale MD models in simulating kink-controlled dislocation dynamics. The knowledge gained here can support the development of mobility laws that incorporate the stress-, temperature-, and line length-dependence all into one formulation for understanding plasticity in bcc metals and other high-Peierls-stress alloys.

Asymmetric Domain Nucleation from Dislocation Core in Barium Titanate: Molecular Dynamics Simulation Using Machine‐Learning Potential through Active Learning

Barium titanate (BaTiO3) is a ferroelectric material without toxic elements, whose ferroelectric properties such as permittivity, coercive field, and spontaneous polarization are affected by the nucleation of domains of reversed polarization and the motion of domain walls. Dislocations can act as obstacles to domain‐wall migration or as active sites for domain nucleation. Thus, studies are conducted on the utilization of dislocations to improve the ferroelectric properties of BaTiO3. However, the atomistic mechanism of domain nucleation around the dislocation core is still unclear. In this article, a machine learning (ML) potential is developed to study the influence of dislocations on domain nucleation. The potential is trained using an active‐learning approach to ensure accuracy in the bulk properties of the ferroelectric and paraelectric phases, as well as in the dislocation core structures in BaTiO3. Molecular dynamics simulations using the ML potential show that the influence of dislocations on polarization reversal depends on the directional relationship between the external electric field and the dislocation. Furthermore, strong local polarizations exist surrounding the dislocation core, owing to vacancies in the core. These polarizations can act as both domain nucleation sites and obstacles for domain migration when ordered along the dislocation line.

Direct observation of intrinsic core structure of a partial dislocation in ZnS

Direct observation of intrinsic core structure of a partial dislocation in ZnS

Development of an interatomic potential for the W–Ta system

A physics-inspired, data-driven interatomic potential framework for multi-element system is presented. This potential is based on the generalization of the embedded atom method potential and systematically incorporates two-, three- and many-body effects. Different atomic environments are described by atom-centered Gaussian basis sets which provides sufficient flexibility to capture diverse atomic environments and allows for easy optimization of the free parameters. An interatomic potential model for the tungsten–tantalum (W–Ta) system is developed using this framework by training on data from ab initio density functional theory (DFT) calculations. The potential is thoroughly tested at various compositions by comparing bulk and defect properties from experimental and DFT data. It is shown that the potential predicts the elastic constants, defect properties, such as vacancy and interstitial formation energies, core structures of dislocations, and melting points for both pure and tantalum and tungsten with an accuracy comparable to other single element machine learning based potentials. The potential model is used to investigate the formation energies of ordered alloys and vacancy formation energies for chemically random W–Ta alloys.

Thickness-dependent creep rate caused by climbing control of a superdislocation in a Ni-based single crystal superalloy

Creep tests are carried out on Ni-based single crystal superalloy sheet samples with different thicknesses at 1100 °C/130 MPa. The results indicate that the secondary creep rate at constant stress exhibits a strong thickness dependence and increases with the decrease in sample thickness. Under load in [001] direction, two types of a<100> family superdislocations with Burgers vectors a[100] and a[010] enter γ′ rafts in thin samples, whereas only the type of a[100] superdislocation enter γ′ rafts in thicker samples. The density of a<100> superdislocations is higher in thin sheets than in thick ones. It is proved that the movement mode of a<100> superdislocations in γ′ rafts is pure climb according to non-compact dislocation core structure. In addition, the climb velocity of the a<100> superdislocations in thin samples is determined to be greater than that in thick ones caused by the fact that the a<100> superdislocations in thin samples experience a higher driving force. On these bases, it is revealed that more types of Burgers vectors, higher density and greater climb velocity of a<100> superdislocations in γ′ rafts contributes to higher secondary creep rate in thinner samples.

Phase-field modeling of dislocation–interstitial interactions

The mechanical behavior of body-centered cubic (BCC) materials can be dramatically affected by the presence of interstitial solute atoms. We present a new phase-field dislocation dynamics formulation to include the diffusion of interstitials. Short-range interactions are accounted for by a concentration-dependent lattice energy, and long-range interactions are accounted for by modifications to the elastic energy. The interstitial diffusion law introduces gradients that require methods for minimizing Gibbs oscillations, which is done via a modified Green’s function. The formulation is general to any solute-solvent system and is applied here to Nb-O as a model system, whose interstitial parameters are obtained from ab initio calculations. The effect of O on the core structures of Nb edge and screw dislocations is calculated. The diffusion of O to form interstitial atmospheres around dislocation cores is simulated, as well as the critical stresses required for dislocations to break away or cross slip from these atmospheres. Future applications of the method to simulate complex interstitial embrittlement mechanisms are discussed.

Hydrogen-Induced Attractive Force Between Two Partials of Edge Dislocation in Nickel

AbstractHydrogen embrittlement is an important technological problem underpinning failure of many structural elements. It has been extensively investigated in the literatures; however, several open issues remain that prevent a full understanding of this phenomenon. One important issue is the uncertain knowledge of how hydrogen atoms affect the dislocation core structure. Here, by exploring the hydrogen role on the dissociated edge dislocation, we reveal that there exists an additional attractive force between two partials due to the hydrogen atmosphere, which would reduce the equilibrium separation distance. This hydrogen-induced attractive force is quantitatively estimated by means of continuum mechanics. Furthermore, molecular statics simulations also capture the hydrogen-reduced separation distance under varying hydrogen background fractions, qualitatively verifying the theoretical prediction of attractive force. These findings at the atomistic scale will inform the hydrogen embrittlement modeling and experiments, especially on the hydrogen effect on the dislocation glide, climb, dynamics strain ageing, and so on.