On construction of SPD stress-strain curve for bronze Cu85–Pb5–Sn5–Zn5

Abstract

Stress-strain (SS) curve is a necessary ingredient of any mathematical model describing metal forming processes. Severe plastic deformation (SPD) imposes additional requirements on SS-tests because some of deformation mechanisms, such as shear band sliding, micro-cracks, etc., prove to be suppressed due to specific character of stress-strain state: complex loading path, all-round high pressure, and high gradients of stress and strain tensor fields. In this work, a novel method of SPD SS-curve construction combining the classical uniaxial compression test with microhardness measurements is proposed. The barreling effect is negligibly small owing to specific experimental conditions. This renders spatially uniform the stress-strain state and allows avoiding the inverse finite element modeling. The cast leaded tin bronze alloy Cu85–Pb5 –Sn5–Zn5 has been studied during upsetting process. An experimental force-displacement diagram appears to be strictly linear up to the moment of destruction under uniaxial compression.The dendritic structure evolution in both axial and cross sections of a cylindrical sample was studied by optical microscope. Local distortions in elements of this structure are evidence of spatial fluctuations in plastic strain at macroscopic level. Microhardness in these sections was also measured. Shear bands appear at strainεp≈0.14 and they are responsible of the softening behavior. In this work, softening is contrary to the experiments on deep rolling treatment where the monotonic increase in microhardness was observed. Microhardness measurements allow expanding the total strain into dislocation sliding and shear band sliding summands. This expansion makes it possible to discard the shear band sliding item and obtain an initial segment of SS-curve (up to strain εp=0.32 for bronze Cu85–Pb5–Sn5–Zn5) available for SPD simulation.