Cluster/dislocation interactions in dilute aluminum-based solid solutions
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The influence of single solute atoms and solute clusters on an extended edge dislocation dipole in Al was studied by atomistic simulation. Single Cu and Ag solute/dislocation interaction energy calculations showed that Cu interacts strongly with an Al extended dislocation and prefers sites in the compressive region, in agreement with elasticity theory predictions. Single Ag atoms, however, are strongly repelled by an Al extended dislocation, in contrast with elasticity theory predictions. Monte Carlo simulations of Al: 1% Cu, Al: 2% Cu, Al: 1% Ag, Al: 0.5% Cu, 0.5% Ag, and Al: 0.75% Cu, 0.25% Ag were carried out in the presence of an extended dislocation dipole at 600 K allowing for solute segregation. Cu atoms in the binary alloys were observed to segregate to the compressive regions of the extended dislocation dipole, forming widespread "atmospheres" over the width of both extended dislocations which did not affect the partial dislocation spacing. Ag in the binary alloy formed small Ag zones which also had little influence on the spacing between the partials. The ternary systems, however, exhibited highly localized solute clusters that had a large impact on the extended dislocation dipole structure, increasing the separation between the partial dislocations. The resulting cluster structures are discussed along with their influence on the apparent stacking fault energy of the alloy systems.
I. INTRODUCTION The stacking fault energy (SFE) of a metal is widely used to classify mechanical behavior. Low SFE metals exhibit different slip mechanisms, deformation textures, and degrees of recovery and recrystallization compared to high SFE metals. In spite of these microstructural effects, the SFE has an electronic origin resulting from the atomic bond rearrangements across a faulted interface. Therefore, one might expect to be able to take advantage of the change in electronic character of a host metal upon alloying to tailor the SFE and, subsequently, the mechanical behavior of an alloy. Gallagher,1 however, made clear in an insightful review article more than 20 years ago the difficulties involved in experimentally measuring the composition dependence of the SFE. Although he noted some general trends (specifically in Ag-, Cu-, and Ni-based alloys), including that (a) the SFE tends to decrease with increasing solute content and, more specifically, with increasing valence electron to atom ratio, and (b) the SFE tends to change uniformly from the SFE of one host end point to that of the other in alloys exhibiting solid solubility across the entire phase diagram, Gallagher clearly differentiated between what he denoted the "effective" SFE measured experimentally and the "true" SFE. The difference arises because the SFE can only 578
http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 3, Mar 1995
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be experimentally measured indirectly. In some form, the dislocation structure—be it extended dislocation nodes, dislocation loops, dislocation density, or partial dislocation separation—is the di
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