Large-strain Bauschinger effects in fcc metals and alloys
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I.
INTRODUCTION
THIS work was undertaken to quantify the Bauschinger effect in a variety of fcc metals after large plastic deformations. We reversed the direction of stressing after "yon Mises" equivalent strains about 1.0 and continued the reverse deformation for a large increment of plastic strain, approximately equivalent to that in the forward direction. Because of the large strains involved in these experiments, effects resulting from heterogeneities in plastic flow and long-range elastic stresses were minimized. Thus, we could interpret the results of these experiments purely in terms of the "isotropic hardening" of the dislocation substructures associated with each of the metals involved in this study. This is different than most of the previous Bauschinger studies which have reversed the direction of stressing after only small amounts of prestrain and which used small-offset definitions of reverse yield. In these cases, the results are complicated by two contributions to the Bauschinger effect: directional (kinematic) long-range elastic stresses and polarized hardening (isotropic) from the dislocation substructure. The materials studied included OFE copper, 70:30 brass, 0.9999 pure A1, AI-1 pct Mg, A1-2 pct Mg, A1-0.8 pct Mn, A1-4 pct Cu, A1-2.6 pct Cu, and AI0.17 pct Fe-0.07 pct Si. The materials were selected to give different microstructures, single phase and two phases M.G. S T O U T and A.D. ROLLETT, Staff Members, are with the Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted March 7, 1990. METALLURGICAL TRANSACTIONS A
with incoherent and coherent precipitates of different sizes and spacings, and different hardening mechanisms, dislocation forest hardening, solid solution strengthening, and hardening via second phases. Much of the work described in the literature has been on the A1-4 pct Cu system. Abel and Ham, t~] Wilson, t21 Sleeswyk et al., t3~ Stoltz and Pelloux, t41 and Moan and Embury tS~ have studied this system. The different authors have used various methods to quantify the degree of the Bauschinger effect. A . Small Prestrains
Moan and Embury tSl defined a Bauschinger stress as half the difference between the forward and reverse flow stresses. The reverse yield stress was defmed by a 0.1 pct deviation from proportional behavior. They related this stress to the total work hardening and conducted an experimental investigation of A1-3.6 pct Cu single and polycrystals heat-treated to produce a 0' substructure as well as a single-phase solid solution. Moan and Embury found that the Bauschinger stress (referred to as back stress) for the solid solution was about 25 pct of the total work hardening after 3 pct prestrain. The stress was much larger for the 0' material, accounting for 60 pct of the work hardening. Stoltz and Pelloux t4] discussed their results both in terms of the stress at "reverse yielding" and also in terms of a "Bauschinger strain" or the strain at which the stress, in the reverse direction, reaches 0.75 of the forward flow stress. They found that for
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