Temperature dependence and size effects on strain hardening mechanisms in copper polycrystals

The thermomechanical behavior of polycrystalline sheets of high purity copper with various numbers of grains (d) across the thickness (t) is experimentally analyzed. As for other face centered cubic materials, the t/d ratio strongly affects the work hardening of copper, especially for t/d values lower than a critical one around five. These results are in agreement with previous ones concerning nickel and can be correlated with surface effects which progressively take place in the material when few grains are present across the thickness. The t/d ratio mainly affects the second work hardening stage and an increase in temperature tends to reduce this effect due to the early beginning of cross slip which leads to a generalization of the surface properties on the overall volume of the material. This result is supported by the analysis of the dislocation structures during the second and third hardening stages for polycrystalline and multicrystalline specimens. As a consequence, the forming of thin metallic products should be ideally performed at moderate temperatures in order to avoid size effects which can deteriorate the reliability of the final product.