Dislocations In Iron Research Articles

Elastic interactions between screw dislocations in iron

The kinetics of elastically interacting screw dislocations has been studied in pure iron strained in situ at low temperature. Annihilating and expanding screw dipoles yield macroscopic activation areas which are substantially smaller than those deduced from conventional mechanical stress, but consistent with theoretical estimates. The kinetics of attractive intersecting screw dislocations indicates that their velocity is determined by the velocity of their most stressed parts. Repulsive screw dislocations with different Burgers vectors can move cooperatively at a surprisingly high velocity, probably on account of elastic torque interactions and twinning-anti-twinning effects. All these interactions are shown to play an important role in the description of macroscopic mechanical properties in terms of individual dislocation mechanisms.

Quantitative hydrogen trap states on high-angle grain boundaries and at dislocations in iron

Low-temperature thermal desorption spectrometry (L-TDS), by which specimens can be heated from -200 °C, was used to quantitatively evaluate the hydrogen desorption peak temperatures (Tp), the hydrogen desorption activation energies (Ea), and the hydrogen occupation ratios (θ) of high-angle grain boundaries and dislocations in iron. Cold-rolled iron was subjected to recrystallization and grain growth to produce three kinds of specimens with different areas of high-angle grain boundaries, in which low-angle grain boundaries were annihilated. L-TDS spectra of specimens containing dislocations and high-angle grain boundaries displayed two hydrogen desorption peaks, denoted as Peak 1 and Peak 2. Peak 1 corresponded to dislocations and displayed Tp, Ea, and θ values of 23 °C, 28.6 kJ·mol−1, and 1.1 to 2.7 atom·nm−1, respectively. In contrast, Peak 2 corresponded to high-angle grain boundaries and displayed Tp, Ea, and θ values of 85 °C, 43.7 kJ·mol−1, and 2.3 to 2.9 atom·nm−2, respectively.

Unveiling the Mechanisms of High-Temperature 1/2[111] Screw Dislocation Glide in Iron–Carbon Alloys

We have developed a self-consistent model for predicting the velocity of 1/2[111] screw dislocation in binary iron–carbon alloys gliding by a high-temperature Peierls mechanism. The methodology of modelling includes: (i) Kinetic Monte-Carlo (kMC) simulation of carbon segregation in the dislocation core and determination the total carbon occupancy of the core binding sites; (ii) Determination of kink-pair formation enthalpy of a screw dislocation in iron—carbon alloy; (iii) KMC simulation of carbon drag and determination of maximal dislocation velocity at which the atmosphere of carbon atoms can follow a moving screw dislocation; (iv) Self consistent calculation of the average velocity of screw dislocation in binary iron–carbon alloys gliding by a high-temperature kink-pair mechanism under a constant strain rate. We conduct a quantitative analysis of the conditions of stress and temperature at which screw dislocation glide in iron–carbon alloy is accomplished by a high-temperature kink-pair mechanism. We estimate the dislocation velocity at which the screw dislocation breaks away from the carbon cloud and thermally-activated smooth dislocation propagation is interrupted by sporadic bursts of dislocation activity.

Atomistic simulations of carbon effect on kink-pair energetics of bcc iron screw dislocations

The mechanical properties of body-centered cubic metals at low temperatures are strongly influenced by the solute atoms, which could be relatively complex as either hardening or softening effects would be introduced depending on the solute concentration and temperature. One major impact is that solute atoms can affect both kink formation and motion during screw dislocation gliding, which plays an important role in plastic behavior of bcc metals. In this study, atomistic simulations are conducted on the carbon-affected kinking of a screw dislocation in iron using the nudged elastic band method. When kink nucleates alongside a carbon atom, the kink formation energy decreases as the carbon transits to a stronger binding site, and vice versa. When a single kink meets the carbon during propagation, the sideward motion of the kink is impeded. With regard to the different temperatures and solute concentrations, the softening and hardening effects induced by carbon solutes in iron can be explained by the complex atomistic process.

Hydrogen diffusion in the elastic fields of dislocations in iron

The effect of dislocation stress fields on the sink efficiency thereof is studied for hydrogen interstitial atoms at temperatures of 293 and 600 K and at a dislocation density of 3 × 1014 m–2 in bcc iron crystal. Rectilinear full screw and edge dislocations in basic slip systems 〈111〉{110}, 〈111〉{112}, 〈100〉{100}, and 〈100〉{110} are considered. Diffusion of defects is simulated by means of the object kinetic Monte Carlo method. The energy of interaction between defects and dislocations is calculated using the anisotropic theory of elasticity. The elastic fields of dislocations result in a less than 25% change of the sink efficiency as compared to the noninteracting linear sink efficiency at a room temperature. The elastic fields of edge dislocations increase the dislocation sink efficiency, whereas the elastic fields of screw dislocations either decrease this parameter (in the case of dislocations with the Burgers vector being 1/2〈111〉) or do not affect it (in the case of dislocations with the Burgers vector being 〈100〉). At temperatures above 600 K, the dislocations affect the behavior of hydrogen in bcc iron mainly owing to a high binding energy between the hydrogen atom and dislocation cores.

Effect of carbon decoration on the absorption of 〈100〉 dislocation loops by dislocations in iron

This work closes a series of molecular dynamics studies addressing how solute/interstitial segregation at dislocation loops affects their interaction with moving dislocations in body-centred cubic Fe-based alloys. We consider the interaction of 〈100〉 interstitial dislocation loops decorated by different numbers of carbon atoms in a wide temperature range. The results reveal clearly that the decoration affects the reaction mechanism and increases the unpinning stress, in general. The most pronounced and reproducible increase of the unpinning stress is found in the intermediate temperature range from 300 up to 600K. The carbon-decoration effect is related to the modification of the loop–dislocation reaction and its importance at the technologically relevant neutron irradiation conditions is discussed.

Hydrogen-induced change in core structures of {110}[111] edge and {110}[111] screw dislocations in iron

Employing the empirical embedded-atom method potentials, the evolution of edge and screw dislocation core structure is calculated at different hydrogen concentrations. With hydrogen, the core energy and Peierls potential are reduced for all dislocations. A broaden-core and a quasi-split core structure are observed for edge and screw dislocation respectively. The screw dislocation and hydrogen interaction in body-centred cubic iron is found to be not mainly due to the change of elastic modulus, but the variation of dislocation core structure.

Publisher's Note: Dislocation core field. II. Screw dislocation in iron [Phys. Rev. B84, 224107 (2011)

Received 28 December 2011DOI:https://doi.org/10.1103/PhysRevB.85.029902©2012 American Physical Society

Determination of the activation energy by stochastic analyses of molecular dynamics simulations of dislocation processes

An investigation is reported of the probability and the probability density of thermal activation of stress-driven dislocation processes, as simulated using molecular dynamics (MD). Stochastic analyses of the survival probability are found to lead to simple relationships between the loading history and the distribution of the interaction time and strength. It is shown that the determination of the activation energy associated to a thermally activated event can be achieved by a reduction of the stochastic process to a process obeying the Poisson's distribution, preserving the activation probability at the survival time. The method is applied to the kink-pair mechanism for screw dislocations in iron. Predictions are compared with experimental results and with other methods reported in the literature, which allows the difference in the approximations and in the assumptions considered in these models to be underlined.

Relaxed energy and structure of edge dislocation in iron

With modified analytical embedded-atom method and molecular dynamics simulation, this paper simulates the strain energy and the equilibrium core structure of a〈100〉 edge dislocation in BCC metal iron on atomistic scale. In addition, the trapping effect of dislocation on vacancy is investigated as well. The results show that the equilibrium dislocation core is quite narrow and has a C2v symmetry structure. Calculated strain energy Es of the dislocation is a linear function of ln(R/2b) while R ≥ 5.16 Å (1 Å = 0.1 nm), in excellent agreement with the elasticity theory prediction. Determined core radius and energy are 5.16 Å and 0.62 eV/Å, respectively. The closer the vacancy to the dislocation line is, the lower the vacancy formation energy is, this fact implies that the dislocation has a trend to trap the vacancy, especially for a separation distance of the vacancy from dislocation line being less than two lattice constants.

Influence of the stress field due to pressurized nanometric He bubbles on the mobility of an edge dislocation in iron

Voids and He bubbles are strong obstacles to dislocation, which induce hardening and loss of ductility. In Fe, molecular dynamics simulation is used to investigate the basic mechanisms of the interaction between a moving edge dislocation and a void or He bubble, as a function of its He content, temperature, interatomic potentials and interaction geometry. Different interatomic potentials for Fe–Fe and Fe–He interactions are used. It appears that temperature eases the dislocation release, due to the increased mobility of the screw segments appearing on the dislocation line upon bowing from the void or He bubble. The mobility includes the cross-slipping of these segments, which leads to the formation of a jog. It appears that the He bubble induces an inhomogeneous stress field in its surroundings, which strongly influences the dislocation passage depending on the geometry of the interaction.

1018 経路積分分子動力学法による鉄中の転位近傍における水素挙動解析(OS10. 電子・原子・マルチシミュレーションに基づく材料特性評価,OS11. 材料の組織・強度に関するマルチスケールアナリシス 合同ポスターセッション,オーガナイズドセッション講演)

1018 経路積分分子動力学法による鉄中の転位近傍における水素挙動解析(OS10. 電子・原子・マルチシミュレーションに基づく材料特性評価,OS11. 材料の組織・強度に関するマルチスケールアナリシス 合同ポスターセッション,オーガナイズドセッション講演)

Effect of theα−γPhase Transition on the Stability of Dislocation Loops in bcc Iron

Body-centered-cubic iron develops an elastic instability, driven by spin fluctuations, near the alpha-gamma phase transition temperature T(c) = 912 degrees C that is associated with the dramatic reduction of the shear stiffness constant c' (c(11)-c(12))/2 near T(c). This reduction of c' has a profound effect on the temperature dependence of the anisotropic elastic self-energies of dislocations in iron. It also affects the relative stability of the a[100] and a/2[111] prismatic edge dislocation loops formed during irradiation. The difference between the anisotropic elastic free energies provides the fundamental explanation for the observed dominant occurrence of the a[100], as opposed to the a/2[111], Burgers vector configurations of prismatic dislocation loops in iron and iron-based alloys at high temperatures.

Effect of helium on irradiation-induced hardening of iron: A simulation point of view

Irradiation-induced hardening and loss of ductility of ferritic/martensitic materials envisaged for future fusion reactor is still difficult to understand. In particular, helium (He), produced by transmutation by the fusion neutrons of 14MeV, is known to impact mechanical properties, but its effect at the microstructural level is still unclear. Molecular dynamics simulations of the mobility of an edge dislocation in iron (Fe) are performed to study the effect of He, either as a gas in solid solution or in cavities. Obstacle to the dislocation, the cavity, in the form of a void or a He bubble, is compared to a a0 [100] dislocation loop, all being 2nm in size. Results show that He in solid solution up to 1.0at.% has a little effect on the dislocation mobility. Conversely, the cavities and the a0 [100] dislocation loop are strong obstacles to the passage of the edge dislocation. Interestingly He bubbles present a lower obstacle strength than voids for low He contents, while for high He content the bubble promotes loop punching, which induces a strong resistance to the passage of the dislocation.

2027 鉄中の銅析出物とらせん転位の相互作用の分子動力学シミュレーション(J01-1 材料や構造の破壊/損傷/マルチスケール解析(1),J01 材料や構造の破壊/損傷/マルチスケール解析)

Molecular dynamics simulations of interaction between a copper precipitate and a screw dislocation in iron were performed. The copper precipitates are considered as a harmful element in the neutron irradiation embrittlement of reactor pressure vessel steels. In the simulations, we applied a shear stress to atomic model, and gradually increase until the screw dislocation cuts through the copper precipitate. Then we investigated the critical resolved shear stress (CRSS) and the mechanism to break away from the copper precipitate.

Simulation of Screw Dislocation Motion in Iron by Molecular Dynamics Simulations

Molecular dynamics (MD) simulations are used to investigate the response of a/2<111> screw dislocation in iron submitted to pure shear strain. The dislocation glides and remains in a (110) plane; the motion occurs exclusively through the nucleation and propagation of double kinks. The critical stress is calculated as a function of the temperature. A new method is developed and used to determine the activation energy of the double kink mechanism from MD simulations. It is shown that the differences between experimental and simulation conditions lead to a significant difference in activation energy. These differences are explained, and the method developed provides the link between MD and mesoscopic simulations.

Dynamics of drag of self-interstitial clusters by an edge dislocation in iron

Self-interstitial atom (SIA) clusters are created in metals under fast neutron irradiation and are believed to interact with dislocations and increase the flow stress. In this paper, atomic-scale computer simulations are used to investigate the dynamic interaction between an edge dislocation and small SIA loops that do not intersect the dislocation glide plane and whose Burgers vector is parallel to it. Such loops can be dragged by a moving dislocation and this effect is simulated as a function of temperature and loop size, spacing, stand-off distance and Burgers vector orientation. The results can be understood in terms of the one-dimensional mobility of SIA loops.

放射性トレーサーを用いた低拡散係数の高精度測定

Experimental techniques for precise determination of low tracer diffusion coefficients in solid metals and alloys using radioisotopes are reviewed. Sputter-microsectioning method is most useful to measure submicron diffusion profiles at low temperatures. Measurements of tracer diffusivity along grain boundaries and dislocations in iron and stainless steels from the analysis of type C kinetics as well as type B kinetics are discussed.

Atomistic simulations of effect of hydrogen on kink-pair energetics of screw dislocations in bcc iron

Atomistic simulations are conducted on the hydrogen-affected kinking process of a screw dislocation in iron by the nudged elastic band method. We find that when a kink pair nucleates at hydrogen, the activation energy is decreased by the transition of hydrogen to a stronger binding site, while it is increased by the transition to a weaker binding site. When a kink pair meets hydrogen during expanding, the sideward motion of the kink pair is impeded by hydrogen. The simulation provides an insight into the complex atomistic process of hydrogen-induced softening and hardening.

Three-dimensional atomic-scale mapping of a cottrell atmosphere around a dislocation in iron

Three-dimensional atomic-scale mapping of a cottrell atmosphere around a dislocation in iron