Ogata Lab., Dept. Mech. Sci. and Bioeng., Grad. School Eng. Sci., Osaka Univ.

Dislocation modelling

Masato Wakeda, Tomohito Tsuru, Masanori Kohyama, Taisuke Ozaki, Hideaki Sawada, Mitsuhiro Itakura, and Shigenobu Ogata, “Chemical misfit origin of solute strengthening in iron alloys”,Acta Materialia, 131 (2017) 445-456.

In this first-principles study, we investigate the effect of many kinds of substitutional solute species on screw dislocation motion in bcc-Fe, dominating the strength of dilute Fe alloys. Most of the solute species show a significant interaction with the dislocation core, while only several solute species among them, such as Si, P, and Cu, significantly lower the Peierls potential of the screw dislocation motion. A firstprinciples interaction energy with the “Easy-core” structure excellently correlates with the change in the g-surface caused by solute atoms (i.e., chemical misfit). Based on the interaction energy, we predicted the effect of each species on macroscopic critical resolved shear stress (CRSS) of the dilute Fe alloy. The CRSS at low and high temperature for various alloys basically agree with experiment CRSS. These results provide a novel understanding of the interaction between a screw dislocation and solute species from the first-principles.

Jun-Ping Du, Yun-Jiang Wang, Yu-Chieh Lo, Liang Wan, and Shigenobu Ogata, “Mechanism transition and strong temperature dependence of dislocation nucleation from grain boundaries: An accelerated molecular dynamics study”,Physical Review B, 94 (2016) 104110-1-8.

Accelerated molecular dynamics reveals a mechanism transition and strong temperature dependence of dislocation nucleation from grain boundaries (GBs) in Cu. At stress levels up to ~ 90% of the ideal dislocation nucleation stress, atomic shuffling at the E structural unit in a GB acts as a precursor to dislocation nucleation, and eventually a single dislocation is nucleated. At very high stress levels near the ideal dislocation nucleation stress, a multiple dislocation is collectively nucleated. In these processes, the activation free energy and activation volume depend strongly on temperature. The strain-rate dependence of the critical nucleation stress is studied and the result shows that the mechanism transition from the shuffling-assisted dislocation nucleation mechanism to the collective dislocation nucleation mechanism occurs during the strain rate increasing from 10^-4 s-1 to 10^10 s-1. The findings of the mechanism transition and strong temperature dependence of GB dislocation nucleation have been missed in the previous conventional molecular dynamics studies.

Kazuki Matsubara, Hajime Kimizuka, Shigenobu Ogata, “Formation of {1121} twins from I1-type stacking faults in Mg: A molecular dynamics study”,Computational Materials Science, 122 (2016) 314-321.

The nature of the reaction and transient behavior of I1-type stacking faults (SFs) under shear stress was investigated to understand the role of I1 SFs in non-basal plastic deformation of Mg. This was because it has been suggested that I1 SFs formed via solute (such as Y) additions to Mg may act as the nucleation source of non-basal dislocations, which serve as important deformation modes to improve the workability. Crystal models of pure Mg containing an I1 SF were deformed by simple shear along the basal plane using molecular dynamics simulations with many-body interatomic potentials. The characteristic dissociation reaction was observed at the boundaries of the I1 SF, where < c+a > dislocations on the pyramidal plane, Shockley partial dislocations on the basal plane, and stair-rod dislocations were nucleated. Interestingly, instead of activation of the non-basal dislocations, {1121} twins were nucleated in theearly stage of the reaction and grew steadily as the applied stress was increased. It was suggested that the I1 SF was likely to act as a ‘‘reactive” defect to assist and accommodate the c-axis deformation. Further, the deformation-induced twin boundaries were found to act as moderate obstacles to dislocation movement on the basal planes in the Mg matrix, without significantly sacrificing its plasticity.

Ming-Shuai Ding, Jun-Ping Du, Liang Wan, Shigenobu Ogata, Lin Tian, Evan Ma, Wei-Zhong Han, Ju Li, and Zhi-Wei Shan, “Radiation-Induced Helium Nanobubbles Enhance Ductility in Submicron-Sized Single-Crystalline Copper”, Nano Letters, (2016) availiable online.

The workability and ductility of metals usually degrade with exposure to irradiation, hence the phrase “radiation damage”. Here, we found that helium (He) radiation can actually enhance the room-temperature deformability of submicron-sized copper. In particular, Cu single crystals with diameter of 100–300 nm and containing numerous pressurized sub-10 nm He bubbles become stronger, more stable in plastic flow and ductile in tension, compared to fully dense samples of the same dimensions that tend to display plastic instability (strain bursts). The sub-10 nm He bubbles are seen to be dislocation sources as well as shearable obstacles, which promote dislocation storage and reduce dislocation mean free path, thus contributing to more homogeneous and stable plasticity. Failure happens abruptly only after significant bubble coalescence. The current findings can be explained in light of Weibull statistics of failure and the beneficial effects of bubbles on plasticity. These results shed light on plasticity and damage developments in metals and could open new avenues for making mechanically robust nano- and microstructures by ion beam processing and He bubble engineering.

Wei-Zhong Han, Ling Huang, Shigenobu Ogata, Hajime Kimizuka, Zhao-Chun Yang, Christopher Weinberger, Qing-Jie Li, Bo-Yu Liu, Xi-Xiang Zhang, Ju Li, Evan Ma, Zhi-Wei Shan, “From “smaller is stronger” to “size-independent strength plateau”: towards measuring the ideal strength of iron”, Advanced Materials, published online 17 APR 2015.

The trend from “smaller is stronger” to “size-independent strength plateau” is observed in the compression of spherical iron nanoparticles. When the diameter of iron nanospheres is less than a critical value, the maximum contact pressure saturates at 10.7 GPa, corresponding to a local shear stress of ≈9.4 GPa, which is comparable to the theoretical shear strength of iron.

Shigeto Takebayashi, Kohsaku Ushioda, Naoki Yoshinaga, and Shigenobu Ogata, “Impact toughness of 0.3 mass % carbon tempered martensitic steels evaluated by instrumented Charpy test”, Mater. Sci. Forum, 783-786 (2014), 1033-1038.

The effect of tempering temperature on the impact toughness of 0.3 mass% carbon martensitic steels with prior austenite grain (PAG) size of about 6 μm and 30 μm were investigated. Instrumented Charpy impact test (ICIT) method was used to evaluate the impact toughness. The tempering temperature of 723K gives the largest difference in the Charpy impact energy at room temperature (RT) between the specimens with two different PAG sizes. Investigation of the test temperature dependence of Charpy impact energy in the 723K tempered steels shows a steep transition at around 200 K for the 6 μm PAG specimen, while it shows a continuous slow transition in a wide range of temperature for the 60 μm PAG specimen. ICIT waveform analysis shows that the fracture propagation energy in stead of the fracture initiation energy mainly controls the temperature dependence of the impact energy. The carbide size distribution in these two specimens was investigated by SEM and TEM. The 60 μm PAG specimen shows the distribution of coarser carbides than does the 6 μm PAG specimen, which seems to be the main reason for the observed difference between them in the Charpy impact energy and the other properties of impact fracture.

Masato Wakeda, Hajime Kimizuka and Shigenobu Ogata, "Atomistic Study of Interaction between Screw Dislocation and Si Atom in Fe-Si Alloy", Journal of the Japan Institute of Metals, 77-10 (2013) 409-414.

Atomistic details of the interaction between screw dislocation and substitutional solute Si in Fe-Si alloy was investigated by atomistic modeling method. We developed an embedded atom method (EAM) potential for Fe-Si interaction based on the density functional theory calculations and then evaluated the interaction energy between Si and screw dislocation using the developed potential. The interaction energy is found to become larger as the Si atom approaches the screw dislocation. This indicates that attractive driving force acting between the Si atom and screw dislocation can lead to the athermal resistance. Using Nudged Elastic Band (NEB) method, we computed the energy barrier for screw dislocation glide associated with double kink formation, and found that the energy barrier is reduced by nearby existing Si atom. These results suggest that solute Si causes both solid-solution hardening and softening in Fe-Si alloys.

Shin Yamamoto, Yun-Jiang Wang, Akio Ishii, and Shigenobu Ogata, "Atomistic Design of High Strength Crystalline-Amorphous Nanocomposites", Materials Transactions, 54-9 (2013) 1592-1596.

There is a long-standing demand for materials which could simultaneously demonstrate multiple promising properties like high strength, good ductility and toughness. In this study, a three-dimensional bulk nanocomposite material which is composed of nanoscale crystalline metal and metallic glass is revealed to present high strength and potentially good ductility by molecular dynamics. A critical high strength is achieved by varying the ratio between crystalline and amorphous phase. The critical strength is revealed to be higher than that expected from the rule of mixture. The mechanism underlying the occurrence of critical strength in the nanocomposite is elucidated by the interaction between dislocation and matrix of amorphous phase. Our concept could guide the engineers to design more advanced bulk nanostructured materials.

Akio Ishii, Ju Li, and Shigenobu Ogata, "”Conjugate Channeling” Effect in Dislocation Core Diffusion: Carbon Transport in Dislocated BCC Iron", PloS one, 8-4 (2013) e60586-1-7.

Dislocation pipe diffusion seems to be a well-established phenomenon. Here we demonstrate an unexpected effect, that the migration of interstitials such as carbon in iron may be accelerated not in the dislocation line direction ξ, but in a conjugate diffusion direction. This accelerated random walk arises from a simple crystallographic channeling effect. c is a function of the Burgers vector b, but not ξ, thus a dislocation loop possesses the same everywhere. Using molecular dynamics and accelerated dynamics simulations, we further show that such dislocation-core-coupled carbon diffusion in iron has temperature-dependent activation enthalpy like a fragile glass. The 71° mixed dislocation is the only case in which we see straightforward pipe diffusion that does not depend on dislocation mobility.

Yun-Jiang Wang, Guo-Jie Jason Gao, and Shigenobu Ogata, "Size-dependent transition of deformation mechanism, and nonlinear elasticity in Ni3Al nanowires", Applied Physics Letters, 102 (2013) 041902-1-5.

A size-dependent transition of deformation mechanism is revealed in Ni3Al nanowire under atomistic uniaxial tension. Deformation twinning is replaced by phase transformation when the diameter of Ni3Al nanowire reduces to a critical value near 4?nm. Enhanced size-dependent nonlinear elasticity is observed in the nanowires, in comparison to their bulk counterpart which is benchmarked by combined density functional and atomistic study. This study provide fundamental understanding on the size-dependent deformation mechanisms of nanostructured alloys.

Yun-jiang Wang, Akio Ishii, and Shigenobu Ogata, "Grain Size Dependence of Creep in Nanocrystalline Copper by Molecular Dynamics", Materials Transactions, 53-1 (2012) 156-160.

The grain size dependence of creep is critical to understand the plastic deformation mechanism of nanoscale metals. Here we used molecular dynamics to study the stress-induced grain size exponent transition in creep of nanocrystalline copper. The grain size exponent was found to initially increase with increasing stress, then decrease after some critical stress. The derived grain size exponents are in agreement with experimental results for diffusional and grain boundary sliding creep at low stress. While, the founded decreasing grain size exponent with increasing stress for dislocation nucleation creep in nanocrystal is in contrast with conventional materials. We propose a constitutive equation for dislocation nucleation governed creep in nanocrystal to explain its grain size dependence transition with stress.

Yun-Jiang Wang, Akio Ishii, and Shigenobu Ogata, "Transition of creep mechanism in nanocrystalline metals", Physical Review B, 84-22 (2011) 224102-1-7.

Understanding creep mechanisms with atomistic details is of great importance to achieve the mechanical and thermodynamical stabilities of nanocrystalline (NC) metals over a wide temperature range. Here we report a molecular dynamics analysis of creep in NC copper dominated by competing deformation mechanisms. We found the dominating creep mechanism transits from grain boundary (GB) diffusion to GB sliding, and then dislocation nucleation with increasing stress. The derived stress exponent, small activation volume of 0.1?10b3, and grain size exponent all agree quantitatively with experimental values. We proposed a stress-temperature deformation map in NC metals accommodated by the competition among different stress-driven, thermally activated processes. The model is general to answer the question why deformation mechanism transits with stress in NC metals.

Hideki Mori, Hajime Kimizuka, and Shigenobu Ogata, "Microscopic Phase-Field Modeling of Edge and Screw Dislocation Core Structures and Peierls Stresses of BCC Iron", Journal of the Japan Institute of Metals, 75-2 (2011) 104-109. [in Japanese].

We investigate edge and screw dislocation core structures and Peierls stresses of BCC iron using microscopic phase-field (MPF) modeling. Parameters needed for the MPF modeling, such as the generalized-stacking-fault (GSF) energy, are determined based on first principles density functional theory (DFT) calculations. Screw dislocation core has six-fold symmetric structure and 0.05 nm width. The edge dislocation core is three times wider than the screw dislocation core. The Peierls stresses of edge and screw dislocations are estimated as 0.07 GPa and 2.8 GPa, respectively.

Hideki Mori, Hajime Kimizuka, and Shigenobu Ogata, "Dislocation Properties and Peierls Stress of BCC Iron Based on Generalized-Stacking-Fault Energy Surface by Using First Principles Calculations", Journal of the Japan Institute of Metals, 73-8 (2009) 595-600. [in Japanese]

We calculate the generalized-stacking-fault (GSF) energy surface of BCC iron by using first principles density functional theory (DFT). We employ the semidiscrete variational Peierls-Nabarro (SVPN) model to investigate the edge dislocation properties of BCC iron. The dislocation core width and Peierls stress are estimated as 0.20 nm and 80 MPa, respectively. We also estimate the GSF energy surface and dislocation properties using two different embedded atom method (EAM) potentials and compare with the DFT results.

Futoshi Shimizu, Shigenobu Ogata, Hajime Kimizuka, Takuma Kano, Ju Li, and Hideo Kaburaki, "First-Principles Calculation on Screw Dislocation Core Properties in BCC Molybdenum", Journal of Earth Simulator, 7 (2007) 17-21.

Predicting atomistic properties of a dislocation is a first step toward an understanding of plastic behavior of materials, in particular BCC metals. The core structure and Peierls stress of a screw dislocation in BCC metals have been studied over the years using the first-principles and empirical methods, however, their conclusions vary due to the inefficiency of the methods. We have executed first-principles calculations based on the density functional method, employing the most accurate 1×1×20 k-point samplings, to determine thecore structure and Peierls stress of the a0/2[111] screw dislocation of molybdenum. We have concluded that the core has a 6-fold structure, and determined the Peierls stress of 1.8 GPa for the simple shear strain along the (-110)<111> direction.