| 1. |
- Chang, Zhongwen, 1985-, et al.
(författare)
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Anomalous bias factors of dislocations in bcc iron
- 2015
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Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 461, s. 221-229
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Tidskriftsartikel (refereegranskat)abstract
- Dislocation bias factors in bcc Fe have been calculated based on atomistic interaction energy maps on three kinds of dislocations, namely the a0/2〈1 1 1〉{1 1 0} screw, a0/2〈1 1 1〉{1 1 0} and a0〈1 0 0〉{0 0 1} edge dislocations. The results show that the dislocation bias is higher for the a0/2〈1 1 1〉 edge dislocation than for the a0〈1 0 0〉 edge dislocation, even though the latter possesses a larger Burgers vector. This indicates the importance of the dislocation core contribution. For the a0/2〈1 1 1〉{1 1 0} screw dislocation, a negative dislocation bias has been obtained, which implies a more efficient absorption of vacancies than of SIAs. The effect of coexistence of both edge- and screw dislocations are assessed by a total bias. A possible complementary mechanism for explaining the long swelling incubation time in bcc metals is suggested and discussed.
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| 2. |
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| 3. |
- Chang, Zhongwen, 1985-, et al.
(författare)
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Dislocation bias factors in fcc copper derived from atomistic calculations
- 2013
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Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 441:1-3, s. 357-363
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Tidskriftsartikel (refereegranskat)abstract
- Atomistic calculations were employed in order to calculate the interaction energy of an edge dislocation with different point defects. The bias factor was calculated by applying a finite element method on the interaction energy landscapes obtained from the atomistic calculations. A comparison of the calculated bias factor with a model based on elasticity theory reveals around 30% discrepancy under conditions representative for electron irradiation at 600 degrees C. Possible reasons are discussed. The bias factor dependence on dislocation density and ambient temperature is presented and discussed.
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| 4. |
- Chang, Zhongwen, 1985-, et al.
(författare)
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Interaction Energy Calculations of Edge Dislocation with Point Defects in FCC Cu
- 2013
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Konferensbidrag (refereegranskat)abstract
- In order to improve the dislocation bias (DB) model of swelling under irradiation, a large scale of atomistic simulation of the interaction in face centered cubic (FCC) Cu model lattice between an edge dislocation (ED) and point defects such as a vacancy, a self-interstital atom (SIA) have been performed for various configurations. It is found dislocation core splits into partial cores after energy relaxation. Interactions with any SIA conficurations is one order of magnitute larger than with a vacancy. The reason that SIA creats a larger dilatation volumn than the vacancy is directly observed from calculation. Furthurmore, within the interaction range, an octahedron position rather than dumbbell in <100> direction is observed in the stable state after relaxation in interactions between a edge dislocation and a dumbbell SIA. Comparision of interaction energy in analytical and atomistic calculation shows that analytical one has a stronger interaction in vacancy-ED systems, suggesting that the bias factor (BF) from analytical calculation is larger than from atomistic calculation.
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| 5. |
- Chang, Zhongwen, 1985-, et al.
(författare)
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Multiscale calculation of dislocation bias in fcc Ni and bcc Fe model lattices
- 2014
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Ingår i: Nuclear Instruments and Methods in Physics Research Section B. - : Elsevier. - 0168-583X .- 1872-9584.
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Tidskriftsartikel (refereegranskat)abstract
- In order to gain more insights on void swelling, dislocation bias is studied in this work. Molecular staticsimulations with empirical potentials are applied to map the dislocation–point defects interactionenergies in both fcc Ni and bcc Fe model lattices. The interaction energies are then used to numericallysolve the diffusion equation and obtain the dislocation bias. The importance of the dislocation core regionis studied under a the temperature range 573–1173 K and the dislocation densities 1012—1015 m-2. Theresults show that larger dislocation bias is found in the fcc Ni than in the bcc Fe under differenttemperatures and dislocation densities. The anisotropic interaction energy model is used to obtain thedislocation bias and the result is compared to that obtained using the atomistic interaction model, thecontribution from the core structure is then shown in both the Ni lattice and the Fe lattice.
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| 6. |
- Chang, Zhongwen, 1985-
(författare)
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Multiscale modelling of radiation-enhanced diffusion phenomena in metals
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A multiscale modelling framework and an experiment campaign are used to study void swelling and Cu precipitation under irradiation. Several aspects regarding defect and solute diffusion under irradiation have been studied in this thesis.First, a self-diffusion model in bcc Fe has been constructed in order to describe the non-linear effects, especially the magnetic transition, around the Curie temperature. First principles calculations are applied to obtain the parameters in the model. The paramagnetic state is simulated by statistical sampling of randomly arranged spin states on each atom. The model fits well with the experimental observations.Then, a combination of atomistic calculations and the finite element method (FEM) is developed in order to solve the diffusion equations of point defects, which are under the influence of a dislocation strain field. The dislocation bias, a key parameter in void swelling models, is hence obtained numerically. The method has been applied in different structural lattices. In the bcc materials, anomalous bias factors have been found for both edge- and screw dislocations. For the edge dislocations, the traditional assumption that the dislocation bias value is proportional to the Burgers vector has been proven not appropriate. For the screw dislocation, a negative bias value is obtained. This implies that vacancies, instead of self-interstitials, are preferentially absorbed into the screw dislocations. Thus a possible complementary mechanism is here introduced for explaining the long swelling incubation time before the steady swelling in bcc materials compared to that in fcc materials.Edge dislocations in fcc materials split into partial dislocations due to their relatively low stacking fault energy. This feature complicates the analytical derivation of the dislocation bias. However, by transforming the analytical dislocation-point defect interaction energies to discrete interaction maps numerically applied in the FEM method, it is possible to perform a systematic study on typical fcc materials, i.e. Cu, Ni and Al. The impacts on the dislocation bias from elastic constants and stacking fault energy have been studied. It is found that the partial splitting distance is the dominating factor that determines the dislocation bias. A prediction method has been hence developed to obtain the dislocation bias of the austenitic alloys, for which it is difficult to use an atomistic description of the interaction maps. A prediction of about 8% dislocation bias of a typical austenitic 316 alloy has been made without performing specific atomistic calculations in the austenitic alloys.Finally, Cu precipitation under irradiation has been studied using both experiment and simulations. Cast iron and FeCu alloy samples were irradiated for a week with 2 MeV electrons. The resistivity of the samples was measured in situ. The microstructure of the samples was then examined by atom probe tomography. No Cu precipitation was found in the cast iron sample while small Cu clusters are observed in the FeCu model alloy. To simulate the clustering process, Kinetic Monte Carlo (KMC) and rate theory methods are used. Both the KMC and rate theory simulations show clearly the Cu clustering process in the FeCu alloy but not in cast iron within the irradiation dose.
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