The influence of stacking fault energy on the mechanical behavior of Cu and Cu-Al alloys: Deformation twinning, work har
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THE role of stacking fault energy (SFE) in the deformation of fcc metals has commonly been studied in Cu and Cu-Al solid solution alloys[1–19] since the SFE of Cu (⬃78 ergs/cm2) decreases by more than an order of magnitude when alloyed with Al (6 wt pct Al, SFE ⬃6 ergs/cm2)[20] without exceeding the solid solubility limit of Al in Cu.[21] A higher SFE results in a smaller separation between the partial dislocations and vice versa.[22] During the deformation of fcc metals, cross-slip is believed to occur by the pinching of partial dislocations in their original slip plane and their subsequent extension on the cross-slip plane.[23,24] Thus, the requirement of pinching the partial dislocations for cross-slip controls the dependence of dislocation substructures on SFE. A smaller interpartial spacing (high SFE) facilitates partial dislocation pinching, and, consequently, a greater tendency for cross-slip resulting in a wavy dislocation substructure. At larger strains and at room temperature, the dislocations in a high SFE material tend to rearrange themselves into cell-like structures with the majority of the dislocations residing in cell walls and the interior of the cells relatively dislocation-free.[2] On the other hand, a large separation between the partials (low SFE) inhibits cross-slip and causes dislocations to organize themselves into planar arrays or planar slip bands (i.e., a more uniform dislocation distribution). At larger strains in low SFE materials, the AASHISH ROHATGI, Postgraduate Researcher, and KENNETH S. VECCHIO, Professor, are with the Department of Mechanical and Aerospace Engineering, University of California–San Diego, La Jolla, CA 92093-0411. GEORGE T. GRAY III, Team Leader, is with Dynamic Materials Properties: Testing and Modeling, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted December 21, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
dislocation arrangement can become random along with the formation of stacking faults. Grace and Inman[5] compiled data from various studies on the dislocation configurations observed in various materials subjected to quasi-static or shock deformation (Table I). They concluded that the transition between the cell-type dislocation configuration (high SFE materials) and the planar-type dislocation configuration (low SFE materials) in quasi-statically or shock-deformed materials (shock pressure below 10 GPa) occurs at similar values of SFE, between ⬃16 and 32 ergs/cm2 and ⬃25 and 36 ergs/cm2, respectively. In addition to controlling the dislocation substructure, SFE also affects the propensity of a material to form deformation twins, with low SFE favoring deformation twinning.[4,6] Twinning and slip are competitive deformation processes with, generally, slip dominating. However, at high strain rates (such as those encountered during shock loading) and/or at low temperatures, twinning can contribute to the plastic deformation in addition to the slip mechanism.[4,25] For example, Cu (high SFE) does not twin at moderate levels of stra
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