| 1. |
- Bakaev, A., et al.
(författare)
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Effect of isotropic stress on dislocation bias factor in bcc iron : an atomistic study
- 2018
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Ingår i: Philosophical Magazine. - : Taylor and Francis Ltd.. - 1478-6435 .- 1478-6443. ; 98:1, s. 54-74
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Tidskriftsartikel (refereegranskat)abstract
- The effect of externally applied stress on the dislocation bias factor (BF) in bcc iron has been studied using a combination of atomistic static calculations and finite element integration. Three kinds of dislocations were considered, namely, a0/2〈1 1 1〉{1 1 0} screw, a0/2〈1 1 1〉{1 1 0} edge and a0〈1 0 0〉{0 0 1} edge dislocations. The computations reveal that the isotropic crystal expansion leads to an increasing or constant dislocation bias, depending on the Burgers vector and type of dislocation. On the other hand, compressive stress reduces the dislocation bias for all the dislocations studied. Variation of the dislocation BF depending on dislocation type and Burgers vector is discussed by analysing the modification of the interaction energy landscape and the capture efficiency values for the vacancy and self-interstitial atom.
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| 2. |
- 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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| 3. |
- Chang, Zhongwen, et al.
(författare)
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Assessment of the dislocation bias in fcc metals and extrapolation to austenitic steels
- 2015
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 465
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Tidskriftsartikel (refereegranskat)abstract
- A systematic study of dislocation bias has been performed using a method that combines atomistic and elastic dislocation-point defect interaction models with a numerical solution of the diffusion equation with a drift term. Copper, nickel and aluminium model lattices are used in this study, covering a wide range of shear moduli and stacking fault energies. It is found that the dominant parameter for the dislocation bias in fcc metals is the width of the stacking fault ribbon. The variation in elastic constants does not strongly impact the dislocation bias value. As a result of this analysis and its extrapolation, the dislocation bias of the widely applied austenitic stainless steels of 316 type is predicted to be about 0.1 at temperature close to the swelling peak (815 K) and typical dislocation density of 1014 m-2. This is in line with the bias calculated using the elastic interaction model, which implies that the prediction method can be used readily in other fcc systems even without EAM potentials. By comparing the bias values obtained using atomistic- and elastic interaction energies, about 20% discrepancy is found, therefore a more realistic bias value for the 316 type alloy is 0.08 in these conditions.
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| 4. |
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| 5. |
- 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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| 6. |
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| 7. |
- 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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| 8. |
- Chang, Zhongwen
(författare)
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Modelling of Dislocation Bias in FCC Materials
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Irradiation induced void swelling is problematic for the application of austenitic steels under high dose irradiation. In this thesis, the swelling is characterized by dislocation bias. The dislocation bias is obtained using the finite element method, accounting for fcc copper and nickel under electron irradiation. The methodology is implemented with the interaction energies between an edge dislocation and point defects. Analytically derived interaction energies, which are based on elasticity theory, are compared with interaction energies obtained from atomistic model using semi-empirical atomic potentials as physics basis. The comparison shows that the description of analytical interaction energies is inaccurate in the dislocation core regions. The bias factor dependence on dislocation density and temperature is presented and discussed. At high temperatures or low dislocation densities, the two approaches tend to converge. However, the dislocation bias based on the interaction energies from the two approaches, reveals larger discrepancy for nickel than for copper. The impact on dislocation bias from the different stacking fault energies of copper and nickel is elaborated. Nickel, which has a larger stacking fault energy, is predicted to have larger swelling rate than copper under the same irradiation conditions.
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| 9. |
- 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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| 10. |
- 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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