Large-strain Bauschinger effects in fcc metals and alloys

Abstract

We have performed Bauschinger experiments on a variety of fcc metals and alloys, after large amounts of prestrain, using torsion and a short thin-walled tube geometry. The materials we studied were 99.99 pct Al, OFE copper, 70:30 brass, Al-1 pct Mg, Al-2 pct Mg, Al-0.17 pct Fe-0.07 pct Si, Al-0.8 pct Mn, and two Al-Cu alloys (Al-2.6 pct Cu and Al-4 pct Cu) given different heat treatments. For the material systems other than the Al-Cu alloys, the stress reversal was after a prestrain in shear of ≈3.0. Two stress reversals were performed on the Al-Cu alloys. The first was at γ = 0.3 and the second at γ = 1.2. Thus, for the Al-Cu, the prestrain and the final increment of deformation were in the same direction. The Bauschinger yield stress in these experiments was characterized by a very large offset shear strain of 0.05. This definition of reverse yield minimizes the effects of heterogeneous deformation and long-range internal elastic stresses that arise mainly from second-phase particles. We attributed the effects we observed to “isotropic hardening” associated with the dislocation substructures that developed in the different materials. We found that the behavior of these materials could be divided into two categories: those which deform by planar slip and those that form a “cell” structure and are characterized as having wavy slip. When the deformation was wavy in nature, we attributed the observed Bauschinger effects to be a result of the untangling of the “cells” formed during the prestrain. Different morphologies of cells had different behaviors when the stress was reversed. The behavior of the planar slip alloys depended on whether or not the barriers to dislocation activity were rigid or shearable. The θ′ precipitates in the Al-Cu alloys and the twin boundaries in the 70:30 brass constituted rigid barriers to dislocation motion, and a very large Bauschinger effect was observed. The solid solution Al-Cu material and that containing Guinier-Preston (GP) zones and θ″ had almost no Bauschinger effect when the yield stress in reverse deformation was considered. After yield, these materials hardened very rapidly and the flow stress in the reverse direction exceeded that for the equivalent amount of monotonie deformation.