The absence of steady-state flow during large strain plastic deformation of some Fcc metals at low and intermediate temperatures

Abstract

A study of the plastic deformation of several fcc metals and alloys at large strains was conducted. The purpose of this study was to take a critical look at the assumption of steady-state flow at low and intermediate temperatures. For this purpose, large strain data were obtainedvia torsion testing of thin-walled tubes. The stress-strain results from these tests followed two distinct trends: at low temperatures, strain hardening continued at shear strains of 8; at higher temperatures strain softening occurred. Continued strain hardening was observed in pure nickel, nickel-cobalt solid-solutions, pure aluminum, and two aluminum alloys. A laminar arrangement of closely spaced dislocation walls arises at large strains and low temperatures, which differs from the well-recovered equiaxed subgrain structure observed at high temperature. Thus, it appears that dynamic recovery processes are not sufficient to establish a steady-state dislocation structure at low temperatures. Strain softening in nickel and nickel-cobalt at higher temperatures was attributed to dynamic recrystallization. In none of the large strain tests conducted was a steady-state flow stress, independent of strain, observed. Torsion test data were found to match data from steady-state tensile creep and compression tests. In the case of the large strain torsion tests of nickel, recrystallization occurred. This suggests that recrystallization and boundary migration can be important processes in creep.