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Title: Dislocation and Stacking Fault Core Fields in FCC Metals

Abstract

Atomistic models were used to determine the properties of dislocation core fields and stacking fault fields in Al and Cu using embedded atom method (EAM) potentials. Long-range, linear elastic displacement fields due to nonlinear behavior within dislocation cores, the core field, for relevant combinations of Shockley partial dislocations for edge, screw, and mixed (60° and 30°) geometries were obtained. Displacement fields of stacking faults were obtained separately and used to partition the core field of dissociated dislocations into core fields of partial dislocations and a stacking fault expansion field. Core field stresses were derived from which the total force, including the Volterra field plus core field, between dislocations for several dislocation configurations was determined. The Volterra field dominates when the distance between dislocations exceeds about 50b but forces due to core fields are important for smaller separation distances and were found to affect the equilibrium angle of edge dislocation dipoles and to contribute to the force between otherwise non-interacting edge and screw dislocations. Interactions among the components of a dissociated dislocation modify the equilibrium separation for Shockley partials suggesting that methods that determine stacking fault energies using measurements of separation distances should include core fields.

Authors:
;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
876872
Report Number(s):
PNNL-SA-42367
KC0201020; TRN: US200608%%327
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Philosophical Magazine A, 85(36):4477-4508; Journal Volume: 85; Journal Issue: 36
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ATOMS; DIPOLES; DISLOCATIONS; EDGE DISLOCATIONS; SCREW DISLOCATIONS; STACKING FAULTS; STRESSES

Citation Formats

Henager, Charles H, and Hoagland, Richard G. Dislocation and Stacking Fault Core Fields in FCC Metals. United States: N. p., 2005. Web. doi:10.1080/14786430500300181.
Henager, Charles H, & Hoagland, Richard G. Dislocation and Stacking Fault Core Fields in FCC Metals. United States. doi:10.1080/14786430500300181.
Henager, Charles H, and Hoagland, Richard G. Wed . "Dislocation and Stacking Fault Core Fields in FCC Metals". United States. doi:10.1080/14786430500300181.
@article{osti_876872,
title = {Dislocation and Stacking Fault Core Fields in FCC Metals},
author = {Henager, Charles H and Hoagland, Richard G},
abstractNote = {Atomistic models were used to determine the properties of dislocation core fields and stacking fault fields in Al and Cu using embedded atom method (EAM) potentials. Long-range, linear elastic displacement fields due to nonlinear behavior within dislocation cores, the core field, for relevant combinations of Shockley partial dislocations for edge, screw, and mixed (60° and 30°) geometries were obtained. Displacement fields of stacking faults were obtained separately and used to partition the core field of dissociated dislocations into core fields of partial dislocations and a stacking fault expansion field. Core field stresses were derived from which the total force, including the Volterra field plus core field, between dislocations for several dislocation configurations was determined. The Volterra field dominates when the distance between dislocations exceeds about 50b but forces due to core fields are important for smaller separation distances and were found to affect the equilibrium angle of edge dislocation dipoles and to contribute to the force between otherwise non-interacting edge and screw dislocations. Interactions among the components of a dissociated dislocation modify the equilibrium separation for Shockley partials suggesting that methods that determine stacking fault energies using measurements of separation distances should include core fields.},
doi = {10.1080/14786430500300181},
journal = {Philosophical Magazine A, 85(36):4477-4508},
number = 36,
volume = 85,
place = {United States},
year = {Wed Dec 21 00:00:00 EST 2005},
month = {Wed Dec 21 00:00:00 EST 2005}
}
  • Atomistic models were used to obtain dislocation core fields for edge, screw, and mixed dislocations in Al and Cu using EAM. Core fields are analyzed using a line force dipole representation, with dilatant and dipole terms. The core field contribution to the force between dislocations is shown to be significant for interactions within 50b.
  • Sheet specimens of nickel, copper, and 70/30 brass, representing a range of stacking-fault free energies of 128, 78, and 14 mJ/m/sup 2/, respectively, were variously annealed to produce a range of four different grain sizes and then cold-rolled to reductions in thickness of 5, 10, 15, 45, and 60%. Plots of the 0.2% offset yield stress as well as the ultimate tensile stress and residual microhardness versus the reciprocal square root of the grain size (D/sup -1///sup 2/) showed that a Hall-Petch-type relationship could be applied in all cases. A plot of the slopes of the Hall-Petch relationships for themore » annealed materials as a function of the stacking-fault free energy (..gamma../sub SF/ revealed a linear relationship of the form K = (Gb/2..pi..(1 - v)) X (..cap alpha.. - delta..gamma../sub SF/), where ..cap alpha.. and delta are dimensionally consistent constants. For prestrained materials, where the grain microstructure is altered somewhat systematically in the range of stacking-fault free energy as observed directly by transmission electron microscopy, the Hall-Petch relationship becomes delta = delta/sub o/ + ..beta..epsilon/sup 1///sup 2/ - KD/sup -1///sup 2/, where epsilon is the strain, delta/sub o/ is the ''friction stress'' for the annealed material, and ..beta.. is a constant which may be dependent upon the stacking-fault free energy. An equation of this form also applies in the description of the ultimate tensile stress and the hardness except that the additional term accounting for the strain-induced alterations in the microstructure is somewhat altered, and the exponent of this term depends upon the stacking-fault free energy.« less
  • This article investigates the microstructural variables influencing the stress required to produce deformation twins in polycrystalline fcc metals. Classical studies on fcc single crystals have concluded that the deformation-twinning stress has a parabolic dependence on the stacking-fault energy (SFE) of the metal. In this article, new data are presented, indicating that the SFE has only an indirect effect on the twinning stress. The results show that the dislocation density and the homogeneous slip length are the most relevant microstructural variables that directly influence the twinning stress in the polycrystal. A new criterion for the initiation of deformation twinning in polycrystallinemore » fcc metals at low homologous temperatures has been proposed as ({sigma}{sub tw} {minus} {sigma}{sub 0})/G = C(d/b){sup A}, where {sigma}{sub tw} is the deformation twinning stress, {sigma}{sub 0} is the initial yield strength, G is the shear modulus, d is the average homogeneous slip length, b is the magnitude of the Burger`s vector, and C and A are constants determined to have values of 0.0004 and {minus}0.89, respectively. The role of the SFE was observed to be critical in building the necessary dislocation density while maintaining relatively large homogeneous slip lengths.« less
  • Twenty-one {l_angle}110{r_angle} symmetric tilt grain boundaries (GB{close_quote}s) are investigated with atomistic simulations, using an embedded-atom method (EAM) potential for a low stacking-fault energy fcc metal. Lattice statics simulations with a large number of initial configurations are used to identify both the equilibrium and metastable structures at 0 K. The level of difficulty in finding the equilibrium structures is quantitatively assessed. The stability of the structures at an elevated temperature is investigated by Monte Carlo annealing. A form of GB dissociation is identified in a number of the boundaries. These structures are used to develop a dislocation model of GB dissociationmore » by stacking-fault emission. Also, an attempt is made to apply the structural unit model (SUM) to the simulated boundaries and problems that are encountered for GB structures in low stacking-fault energy metals are enumerated and discussed. {copyright} {ital 1996 The American Physical Society.}« less