Effects of changes in strain path on work hardening in cubic metals

Abstract

Effects of abrupt changes in strain path on work hardening in stretching 1200 aluminum, OFHC copper, 70-30 brass, a low-carbon ferritic steel, and 310 and 304 austenitic steels have been investigated. Tests were made with first-stage extension in uniaxial and equibiaxial tension. Second-stage stretching was in uniaxial tension. It is shown that all of the above materials have some susceptibility to transient reductions in work-hardening rate,dσ/de, after changes in strain path. Some of the material and process variables which can influence the form and magnitude of the transients are identified. Within the group of materials tested, stacking fault energy (SFE) appears to be the most generally influential material variable. The magnitudes of prestrain required to give significant reductions indσ/de·1/σ after strain path changes are higher in the low SFE alloys. Changes in strain path are accompanied by a reduction in the effective extension at the first onset of strain localization, when the reduction indσ/de is sufficient to causedσ/de·1/σ to fall below unity. Within the ranges of prestrain explored, the maximum reductions indσ/de·1/σ were in the range of 0.5 to 1.0. Thus, none of the changes in strain path investigated caused a reduction in the limit of effective uniform elongation, until the prestrain was sufficient to reducedσ/de·1/σ in monotonic deformation to less than a value in the range of 1.5 to 2.0. For these reasons, the possible reductions in effective uniform elongation were much more severe in the high SFE materials than those in the low SFE alloys.