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The effect of annealing on the mechanical properties of copper and copper‑nickel alloys under high strain rate
The high strain rate compression experiments on cylindrical samples of pure copper and copper-nickel (Cu-19 at.% Ni) alloy using the split Hopkinson pressure bar are performed in the strain rate range of 103 to 2 × 104 s−1 . The behavior of the specimens is studied in both the as-received state and after annealing. Stress– strain curves of the alloys are determined as a function of strain rate as well as the plastic flow stress at 20% true strain. It is shown that the strain rate dependence of plastic flow stress in the range of 103 − 2 × 104 s−1 is weak, and no increase in yield strength is observed with increasing strain rate in this range. With the decrease in impurity concentration in annealed copper from 0.1 to 0.03 wt.%, no increase in yield stress with increasing strain rate is observed either. The yield strength of the alloys decreases with increasing annealing temperature, but for annealed copper with medium and coarse grain sizes, the yield strength values differ by no more than 10%. A decrease in grain size perpendicular to the deformation axis, as well as an increase in dislocation density within the grains, is found using electron and optical microscopy. Our findings imply that the weak strain rate sensitivity is likely related to the absence of twinning and recrystallization during deformation. A good agreement is shown between the experimental strain gauge data and the finite element simulation results using experimental stress–strain curves obtained by the standard Kolsky method.