Thermal stresses and microstructural transformations in thin copper samples irradiated with a high-current electron beam

Irradiation of metals and alloys by pulsed electron beams modifies their structure and properties. When irradiating a copper foil with a high-current electron beam of energy ∼20 keV, power density ∼10⁷ W/cm², and pulse duration 50 ns, we observed microtwins in the near-surface region of the target, testifying to the fact that the loading was high-speed in character and the peak pressure was over an order of magnitude higher than the dynamic yield stress. However, calculations have shown that the amplitude of the compression-rarefaction pulse was not over 2 MPa, which is below the dynamic yield stress by two orders of magnitude. To explain these results, we have calculated thermal stresses in terms of the theory of thermal deformation of thin plates. The calculations have shown that these stresses reach 3 GPa in magnitude at the target surface. The stresses relax with equalization of the temperature (the equalization time is ∼10⁻⁵ s for a copper sample). The stress magnitude is consistent with experiments on determination of the threshold for twinning in copper foils under shock-wave loading.