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Interaction between point defects and edge dislocation in BCC iron

Description: We present results of atomistic simulations of the interaction between self interstitial atoms and vacancies with edge dislocations in BCC iron. The calculations are carried out using molecular dynamics with an energy minimization scheme based on the quasi-Newton approach and use the Finnis-Sinclair interatomic potential for BCC iron developed by Ackland et al. Large anisotropy in the strain field of self interstitials is observed and it causes strong interaction with edge dislocations even when the defect is located on the dislocation glide plane. For vacancies, the relaxation volume is smaller and much more isotropic, which results in a far weaker interaction with the dislocation. A temperature dependent capture radius for vacancies and self interstitials is extracted from the simulations. The difference between the capture radii of vacancies and self interstitials is used to define the sink strength of the dislocation. Large deviations are observed from the predictions of elasticity based on treating point defects as isotropic dilatational centers. Further, the capture radius of edge dislocations in BCC iron is observed to be small and is of the order of l-3 nm for self interstitials.
Date: October 12, 1998
Creator: Diaz de la Rubia, T. & Shastry, V.
Partner: UNT Libraries Government Documents Department

Comparison of AlxGa1-xN films grown on sapphire by MBE underN-rich and Ga-rich conditions

Description: Al{sub x}Ga{sub 1-x}N layers grown under N-rich and Ga-rich conditions were investigated using transmission electron microscopy (TEM) and photoluminescence (PL). Despite a high density of threading dislocations ({approx}10{sup 10} cm{sup -2}) these layers had high PL quantum efficiencies. Both, TEM and PL studies showed significant differences between layers grown under N- and Ga-rich conditions. The results indicate that edge dislocations do contribute to nonradiative recombination in Al{sub x}Ga{sub 1-x}N layers.
Date: October 15, 2002
Creator: Jasinski, J.; Liliental-Weber, Z.; He, L.; Reshchikov, M.A. & Morkoc, H.
Partner: UNT Libraries Government Documents Department

Microstructure of laterally overgrown GaN layers

Description: Transmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c-plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed new light on the optical properties of such samples.
Date: April 3, 2001
Creator: Liliental-Weber, Zuzanna & Cherns, David
Partner: UNT Libraries Government Documents Department

Orthodox etching of HVPE-grown GaN

Description: Orthodox etching of HVPE-grown GaN in molten eutectic of KOH + NaOH (E etch) and in hot sulfuric and phosphoric acids (HH etch) is discussed in detail. Three size grades of pits are formed by the preferential E etching at the outcrops of threading dislocations on the Ga-polar surface of GaN. Using transmission electron microscopy (TEM) as the calibration tool it is shown that the largest pits are formed on screw, intermediate on mixed and the smallest on edge dislocations. This sequence of size does not follow the sequence of the Burgers values (and thus the magnitude of the elastic energy) of corresponding dislocations. This discrepancy is explained taking into account the effect of decoration of dislocations, the degree of which is expected to be different depending on the lattice deformation around the dislocations, i.e. on the edge component of the Burgers vector. It is argued that the large scatter of optimal etching temperatures required for revealing all three types of dislocations in HVPE-grown samples from different sources also depends upon the energetic status of dislocations. The role of kinetics for reliability of etching in both etches is discussed and the way of optimization of the etching parameters is shown.
Date: August 10, 2006
Creator: Weyher, J.L.; Lazar, S.; Macht, L.; Liliental-Weber, Z.; Molnar,R.J.; Muller, S. et al.
Partner: UNT Libraries Government Documents Department

Theoretical investigation of extended defects in group-III nitrides

Description: The authors have investigated two types of extended defects commonly found in AlN, GaN and InN films using density-functional techniques. First, basal-plane stacking faults have been studied for all three compounds. Stacking-fault energies were found to be largest in AlN and smallest in GaN consistent with density-functional results for their wurtzite/zinc-blende energy differences. In addition, the 4H and 6H structures were found to have lower energies than zinc blende for all three compounds. Secondly, the authors have investigated the electronic structure and formation energy for an edge dislocation in AlN. The full-core dislocation structure was found to have a filled electronic level approximately 0.55 eV above the valence-band edge and an empty level 1.4 eV below the conduction-band edge. An open-core structure was found to have filled and empty electronic levels closer to the middle of the energy gap. Formation energies for these two geometries suggest that the full-core structure would be expected to form in p-type material whereas both are expected in n-type material.
Date: December 1, 1997
Creator: Wright, A. F.
Partner: UNT Libraries Government Documents Department

Charge Accumulation at a Threading Edge Dislocation in GaN

Description: We have performed Monte Carlo calculations to determine the charge accumulation on threading edge dislocations in GaN as a function of the dislocation density and background dopant density. Four possible core structures have been examined, each of which produces defect levels in the gap and may therefore act as electron or hole traps. Our results indicate that charge accumulation, and the resulting electrostatic interactions, can change the relative stabilities of the different core structures. Structures having Ga and N vacancies at the dislocation core are predicted to be stable under nitrogen-rich and gallium-rich growth conditions, respectively. Due to dopant depletion at high dislocation density and the multitude of charge states, the line charge exhibits complex crossover behavior as the dopant and dislocation densities vary.
Date: January 20, 1999
Creator: Leung, K.; Stechel, E.B. & Wright, A.F.
Partner: UNT Libraries Government Documents Department

A Hierarchical Upscaling Method for Predicting Strength of Materials under Thermal, Radiation and Mechanical loading - Irradiation Strengthening Mechanisms in Stainless Steels

Description: Stainless steels based on Fe-Cr-Ni alloys are the most popular structural materials used in reactors. High energy particle irradiation of in this kind of polycrystalline structural materials usually produces irradiation hardening and embrittlement. The development of predictive capability for the influence of irradiation on mechanical behavior is very important in materials design for next-generation reactors. Irradiation hardening is related to structural information crossing different length scale, such as composition, dislocation, crystal orientation distribution and so on. To predict the effective hardening, the influence factors along different length scales should be considered. A multiscale approach was implemented in this work to predict irradiation hardening of iron based structural materials. Three length scales are involved in this multiscale model: nanometer, micrometer and millimeter. In the microscale, molecular dynamics (MD) was utilized to predict on the edge dislocation mobility in body centered cubic (bcc) Fe and its Ni and Cr alloys. On the mesoscale, dislocation dynamics (DD) models were used to predict the critical resolved shear stress from the evolution of local dislocation and defects. In the macroscale, a viscoplastic self-consistent (VPSC) model was applied to predict the irradiation hardening in samples with changes in texture. The effects of defect density and texture were investigated. Simulated evolution of yield strength with irradiation agrees well with the experimental data of irradiation strengthening of stainless steel 304L, 316L and T91. This multiscale model we developed in this project can provide a guidance tool in performance evaluation of structural materials for next-generation nuclear reactors. Combining with other tools developed in the Nuclear Energy Advanced Modeling and Simulation (NEAMS) program, the models developed will have more impact in improving the reliability of current reactors and affordability of new reactors.
Date: July 1, 2011
Creator: Li, Dongsheng; Zbib, Hussein M.; Garmestani, Hamid; Sun, Xin & Khaleel, Mohammad A.
Partner: UNT Libraries Government Documents Department

Effect of point defects on mechanical properties of metals. Progress report, January 1, 1975--December 31, 1975

Description: Results of research activities are presented on electron irradiation softening in pure Fe, effects of impurities on electron irradiation softening, electron microscopic investigation of electron irradiation softening, surface film softening, and dislocation motion. (JRD)
Date: December 1, 1975
Creator: Meshii, M.
Partner: UNT Libraries Government Documents Department