Rate-dependent properties of aluminum-manganese and aluminum-magnesium alloys during dynamic strain aging, and of OFHC-copper and 304-stainless steel.

摘要

The purpose of this work was to obtain a consistent set of experimental data to evaluate relaxation, creep, and cyclic properties of metallic materials experiencing dynamic strain aging at room temperature. The experimental results were used to corroborate predictions made by Viscoplasticity Theory Based on Overstress (VBO) as modified to account for zero or negative strain rate sensitivity experienced during dynamic strain aging. The modified VBO model predicts that relaxation and creep are present and "normal" during dynamic strain aging. However, few data confirm these predictions.; Dynamic strain aging is typically manifested by serrated stress-strain curves (a.k.a. Portevin-LeChatelier effect). Other effects are increased strength with increasing temperature and zero or negative strain rate sensitivity. Zero (negative) strain rate sensitivity is characterized by an unchanged (decreasing) flow stress with a change in strain rate. VBO, a "unified" state variable theory, is not capable of reproducing serrations on the stress-strain curve.; Aluminum 3003 and 5005 specimens were tested in an MTS servohydraulic testing machine. Strains were measured by a clip-on extensometer attached to the gage section. Tests included monotonic loading and unloading, strain rate change tests, stress relaxation tests, short-time "cold" creep tests, and cyclic tests between +/-0.8% strain. A statistical evaluation of test results confirmed that the aluminum alloys experienced dynamic strain aging and, as predicted, relaxation and creep are present and "normal." Cyclic hardening, although small, was also evident.; Additional room temperature testing using annealed oxygen-free high conductivity copper showed deformation-induced strain rate sensitivity, which is in agreement with predictions made by the modified VBO. Finally, evaluation of the stress-strain curve of 304 stainless steel showed that the unloading was not linear as the slope varied from nearly twice the modulus of elasticity (E) to less than the E. Analysis of the unloading behavior with VBO indicates that the point where the slope equals E corresponds to where the stress equals the equilibrium stress.