Compared to cubic metals, whose primary slip mode includes twelve equivalent systems, the lower crystalline symmetry of hexagonal close-packed metals results in a reduced number of equivalent primary slips and anisotropy in plasticity, leading to brittleness at the ambient temperature. At higher temperatures, the ductility of hexagonal close-packed metals improves owing to the activation of secondary ⟨c+a⟩ pyramidal slip systems. Thus, understanding the fundamental properties of corresponding dislocations is essential for the improvement of ductility at the ambient temperature. Here, we present the results of large-scale ab initio calculations for ⟨c+a⟩ pyramidal screw dislocations in magnesium and show that their slip behavior is a stark counterexample to the conventional wisdom that a slip plane is determined by the stacking fault plane of dislocations. A stacking fault between dissociated partial dislocations can assume a nonplanar shape with a negligible energy cost and can migrate normal to its plane by a local shuffling of atoms. Partial dislocations dissociated on a {21[over ¯]1[over ¯]2} plane "slither" in the {011[over ¯]1} plane, dragging the stacking fault with them in response to an applied shear stress. This finding resolves the apparent discrepancy that both {21[over ¯]1[over ¯]2} and {011[over ¯]1} slip traces are observed in experiments while ab initio calculations indicate that dislocations preferably dissociate in the {21[over ¯]1[over ¯]2} planes.

The cross-slip process of a screw    dislocation from the basal to the prismatic plane in magnesium was studied using the density functional theory and the molecular dynamics calculations. An atomistic method for calculating the total Peierls energy map has been devised to track the transition path of a dissociated and/or constricted screw    dislocation in the cross-slip process. The barrier of a screw    dislocation from the basal to the prismatic plane is estimated by the density functional theory for the first time to be 61.4±2.0 ?> meV per Burgers vector length. The activation enthalpy for the cross slip is calculated using a line tension model based on the density functional theory to be 1.4–1.7 eV, which is in reasonable agreement with experiments. On the basis of the results, the effect of temperature on the cross-slip process of the dissociated screw    dislocation on the basal plane is studied in detail using the molecular dynamics method with the embedded-atom-method (EAM) interatomic potential, in which the critical resolved shear stress for the cross slip is evaluated. It is confirmed that the bowed-out dislocation line on the prismatic plane consists of slightly dissociated rectilinear segments with connecting jogs at low temperatures and, as the temperature rises, the curved dislocation line becomes smooth with many segments. The motion of an    dislocation on the prismatic plane is jerky in the low temperature region, while it is retarded by the formation of the largely dissociated plateau segment above the room temperature. A large reduction of the critical shear stress for the cross slip is obtained when the    screw dislocation interacts with a hard-sphere particle placed on the basal plane in the low temperature region.

Development of He bubbles at grain boundaries (GBs) may lead materials to serious embrittlement, but the nucleation mechanism of such bubbles is not well understood. In the present paper, we analyzed the stability of various He-vacancy clusters, which are precursors of He bubbles, at several kinds of GBs of alpha-Fe using a set of empirical potentials. The results clearly indicated the preferred nucleation of He bubbles at GBs. We also found that the dissociation energy of vacancies from He-vacancy clusters at GBs is generally less than that for the intragranular counterparts, and that the He-to-vacancy ratio in equilibrium for various GB cases becomes larger in comparison with the intragranular case. The results obtained here are invaluable in establishing a kinetic model of He bubble nucleation at GBs. (C) 2013 Elsevier B. V. All rights reserved.

We investigate the effect of hydrogen on the mobility of a screw dislocation in body-centered cubic (bcc) iron using first-principles calculations, and show that an increase of screw dislocation velocity is expected for a limited temperature range. The interaction energy between a screw dislocation and hydrogen atoms is calculated for various hydrogen positions and dislocation configurations with careful estimations of the finite-size effects, and the strongest binding energy of a hydrogen atom to the stable screw dislocation configuration is estimated to be 256 ± 32 meV. These results are incorporated into a line tension model of a curved dislocation line to elucidate the effect of hydrogen on the dislocation migration process. Both the softening and hardening effect of hydrogen, caused by the reduction of kink nucleation enthalpy and kink trapping, respectively, are evaluated. A clear transition between softening and hardening behavior at the lower critical temperature is predicted, which is in qualitative agreement with experimental observation.

There is a pressing need to improve the ductility of magnesium alloys so that they can be applied as lightweight structural materials. In this study, a mechanism for enhancing the ductility of magnesium alloys has been pursued using the atomistic method. The generalized stacking fault (GSF) energies for basal and prismatic planes in magnesium were calculated by using density functional theory, and the effect of the GSF energy on the dislocation core structures was examined using a semidiscrete variational Peierls-Nabarro model. Yttrium was found to have an anomalous influence on the solution softening owing to a reduction in the GSF energy gradient.

Numerical modeling of thermal desorption spectra of hydrogen, which is used for identifying the hydrogen state in metals, is reviewed. The previously proposed models are described in the historical perspective by categorizing them according to the rate-determining processes of hydrogen detrapping and diffusion in the thermal desorption spectra. The range of validity of each model is also described.

In this paper, kinetic Monte Carlo method was applied to investigate the long time evolution of cascade damage prepared by molecular dynamics simulations in alpha-Fe up to recoil energy of more than 200 keV. We conducted thorough investigation on how the surviving defects vary with cascade damage energy and annealing temperature. The results can be used for input parameters of rate equations to simulate microstructural evolution under irradiation. The study also suggested that neighboring sub-cascades evolves almost independently during annealing, and that the temperature dependence of the annealing results can be explained by the temperature dependence of vacancy-migration and vacancy-dissociation probabilities. (C) 2012 Elsevier BM. All rights reserved.

Numerical modeling of thermal desorption spectra of hydrogen, which is used for identifying the hydrogen state in metals, is reviewed. The previously proposed models are described in the historical perspective by categorizing them according to the rate-determining processes of hydrogen detrapping and diffusion in the thermal desorption spectra. The range of validity of each model is also described.

Atomistic mechanisms of hydrogen-induced cracking along a bcc Fe Sigma 3(111) symmetrical tilt grain boundary (GB) have been studied by first-principles calculations. The mobile and immobile effects of hydrogen on the GB decohesion are analyzed by calculating the dependence of hydrogen segregation energy on the coverage relevant to the repulsive interaction among segregated hydrogen atoms at the GB and on its fracture surfaces, together with generalizing McLean's formula. It was found that the segregation of combined mobile and immobile hydrogen atoms from the bulk and/or GB on the fracture surfaces causes much stronger reduction (70-80%) in the GB cohesive energy. It can occur even at a very low bulk hydrogen content of about 10(-9) atomic fraction during slow cracking. This is in contrast to only 10-20% decohesion induced by immobile hydrogen at much higher hydrogen content during fast cracking. The mobile effect of hydrogen, giving rise to a profound reduction in the GB cohesive energy, is a key factor controlling the mechanism of hydrogen-induced GB cracking.

Following the Green-Kubo formalism in linear response theory, the lattice thermal conductivity of solid argon is determined by using classical molecular dynamics simulation to calculate the heat current correlation function. Comparing the absolute conductivities obtained using the Lennard-Jones potential with experiments, we find the predicted results to uniformly underestimate the measurements in magnitude, whereas the calculated temperature dependence corresponds well with the data. The temporal behavior of the heat current autocorrelation function shows that while a single exponential decay description is appropriate at elevated temperatures, below the half of the Debye temperature, the heat current relaxation clearly consists of two stages, an initial rapid decay associated with local dynamics followed by a slower component associated with the dynamics of lattice vibrations (phonons).

The decohesion model in which hydrogen segregating at grain boundaries reduces cohesive energy is considered to explain hydrogen embrittlement. Although there are several experimental and theoretical supports of this model, its total process is still unclear. In order to understand hydrogen embrittlement in terms of the decohesion model, therefore, it is necessary to evaluate stress and hydrogen concentration at grain boundaries under experimental conditions and to verify the grain boundary decohesion process. Under this consideration, we calculated the stress and the hydrogen concentration at grain boundaries in the three-dimensional polycrystalline model which was generated by the random Voronoi tessellation. The crystallographic anisotropy was given to each grain as a characteristic. As the boundary conditions of the calculations, data extracted from the results calculated in the notched round-bar specimen model under the tensile test condition in which fracture of the steel specimen is observed was given to the polycrystalline model. As a result, it was found that the evaluated stress does not reach the fracture stress estimated under the condition of the evaluated hydrogen concentration by first-principles calculations. Therefore, it was considered that the initiation of grain boundary fracture needs some factors except the stress concentration due to the crystallographic anisotropy.

We proposed a kinetic theory model of helium segregation from the bulk to the grain boundary in α-Fe. This model is constructed on the basis of the ab initio results of large segregation energy and large deduction in strength at the grain boundary due to helium. The simulation results of the model showed the typical segregation features as function of bulk concentration of helium and temperature in general agreement with experiment. In addition, the results are compared with McLean’s model for monoatomic-layered grain boundary segregation in equilibrium, and the validity of this equilibrium model is discussed.

Molecular dynamics is a useful tool for simulating cascade damage in metals and alloys, but the time scale accessible to molecular dynamics is only about 10 -10 s.Kinetic Monte Carlo can be used to simulate annealing of cascade damage to permit analysis of the longer time evolution of cascade damage.We conducted a series of such annealing simulations in α-Fe.The number of surviving displacements before annealing is ~0.3 of the Norgett-Robinson-Torrens (NRT) value in the case of primary knock-on atoms with energy more than ~10 keV, and it decreased by ~30% during the annealing at 300 K because of recombination of vacancies and self-interstitial atoms.The recombination ratio increased as the annealing temperature increased.These results can be meaningfully applied in models such as mean field reaction rate theory used to simulate long-term radiation damage accumulation.We also demonstrated that 1D motion of small SIA clusters can substantially influence the long-term accumulation of cascade damage.

Equilibrium molecular dynamics is performed to obtain the thermal conductivity of crystalline argon using the Green‐Kubo formalism, which permits the study of dynamical details of the transport process. A large system run to longer times is used to derive the heat flux autocorrelation functions from the low temperature solid to the liquid state. The power spectrum of an autocorrelation function reveals the change in the nature of the underlying atomic motions across the temperature range.

The mechanism of hydrogen embrittlement of Al grain boundaries is not well understood for many years. From first-principles calculations, we found that the inner hydrogen in fcc Al can segregate (be trapped) at an Al grain boundary and then decrease its fracture surface energy. On the other hand, we also found that gaseous (outer) hydrogen can not decrease the fracture surface energy of Al by surface adsorption (trappping) from H2 gas. These results are in good agreement with the experimental facts that the grain boundary embrittlement of Al can be caused by the inner hydrogen but cannot be caused by the outer hydrogen. Our preliminary calculations for Al-(Hg, Ga) systems show that Hg and Ga atoms can also decrease the fracture surface energy by the adsorption on the Al surface. It indicates that both of hydrogen embrittlement and liquid metal (Hg, Ga) embrittlement of Al grain boundaries are caused by the same mechanism: the reduction of fracture surface energy. The comparison with the calculations for other metals (Fe, Cu) are also discussed.

The new boundary condition for a single screw dislocation core in BCC iron has been devised to calculate the diffusion of hydrogen using the first principles calculation. It is found that the core structure of a screw dislocation changes drastically by extension due to the presence of high density hydrogen atoms in the core region. Also, we searched the diffusion path of a hydrogen atom from the surrounding trapping lattice site to the core of a screw dislocation, and found that there is the large energy barrier of 0.25eV. We conclude that the diffusion of hydrogen atoms along the core of a screw dislocation is very unlikely to occur in BCC iron.

The microscopic mechanism of grain boundary (GB) embrittlement in metals by hydrogen segregation (trapping) has been not well understood for many years. From first-principles calculations, we show here that the calculated cohesive energy of bcc Fe Σ3(111) and fcc Al(Cu) Σ5(012) symmetrical tilt GBs can be significantly reduced if many hydrogen atoms segregate at the GBs. This indicates that the reduction of the cohesive energy of the GB may cause the hydrogen-induced GB embrittlement in Fe, Al, and Cu. Considering the “mobile” effect of hydrogen during fracture, especially for the Fe system, more hydrogen atoms coming from solid solution state can segregate on the gradually formed two fracture surfaces and reduce further the cohesive energy. We suggest a new idea about the upper and lower critical stresses observed in the constant-load test of hydrogen-induced delayed fracture in high-strength steels; the upper critical stress is determined by the amount (density) of “immobile” hydrogen atoms segregated at the GB before fracture, and the lower critical stress is determined by the total amount (density) of immobile and mobile hydrogen atoms, the latter of which segregate on the two fracture surfaces during fracture.