Mechanism for Gamma-Precipitation in Aluminum-Silver Alloys and Self-Assembly of Polyelectrolytes: Modeling of Complex Layered Materials
Finkenstadt, Daniel Kris
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https://hdl.handle.net/2142/80511
Description
Title
Mechanism for Gamma-Precipitation in Aluminum-Silver Alloys and Self-Assembly of Polyelectrolytes: Modeling of Complex Layered Materials
Author(s)
Finkenstadt, Daniel Kris
Issue Date
2005
Doctoral Committee Chair(s)
Johnson, Duane D.
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
In fcc Al, stacking fault energy (SFE) is high at ∼150 mJ/m 2, inhibiting stacking fault (SF) formation and dislocation motion. Yet hcp precipitates form rapidly in Al-rich face centered cubic (fcc) Al-Ag, even as the energy difference DeltaEhcp-fcc between hcp and fcc solid solution increases with Ag content. Using ab initio methods, based on electron density functional theory (DFT), I have calculated SFE versus distance of a Ag (111) plane from intrinsic (isf), extrinsic (esf) and twin (tsf) defects. I have found that Ag solute adjacent to (111) shear planes greatly reduces the ideal shear strength in fcc Al-Ag solid solution. Now with an inhomogeneous distribution of solute, SF formation is favorable, especially when a Tungsten-Carbide-like structure of alternating AlAg hcp decorates defects. I have found that solute position relative to defect plane, i.e. lattice symmetry, combined with favorable solute mixing, i.e. alloy chemistry, significantly lower SFE. My results indicate that commonly quoted arguments relating gammaesf ∼ gammaisf ∼ 2gamma tsf ∼ DeltaEhcp-fcc hold only in cases that maintain symmetry of the underlying Bravais lattice, such as elemental metals and homogeneous solid-solution. I then provide a generalization of the relation that is applicable to more realistic systems. I show that this defect/solute-mediated, low energy pathway provides a local mechanism in inhomogeneous solid solutions for rapid hcp precipitation as observed in Al-rich fcc Al-Ag. I have used approximate unit cells with DFT calculation to estimate hcp precipitate/fcc interface energy in Al-Ag and found that classical nucleation theory in combination with dendritic growth of secondary, nucleating edge structures gives √t increase in hcp precipitate width-to-thickness aspect ratio with time t. My aspect-ratio model is the first theory of which I am aware that accurately predicts an increasing aspect ratio for Al-Ag hcp precipitates, as observed.
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