Mechanical behavior of microcrystalline aluminum-lithium alloy under superplasticity conditions

Aluminum-lithium alloy 1420, which, after equal-channel angular pressing, has a grain size of about 3 µm, is shown to possess superplasticity in a temperature range of T=320–395°C upon tension at a constant relative strain rate of 10^?2–10^?3 s^?1. The axial deformation at fracture can exceed 1800%. The data processing at such large deformations should be carried out using true strains ?_t and stresses σ_t. In the flow curve, a short stage of hardening is followed by a long softening stage. They can be described by the relation $dot varepsilon _t sim sigma _t^n exp ( - U/kT)$ with a constant exponent n≈2 and activation energies U≈1 eV for the softening stage and U≈1.4 eV for the hardening stage. The deformation is supposed to be controlled by grain-boundary sliding at the stage of softening and by self-diffusion in the bulk of grains at the hardening stage.