DEFORMATION MECHANISMS OF LOW-TEMPERATURE SUPERPLASTICITY IN 8090 Al-Li ALLOYS

The 8090 Al-Li alloys, after special thermomechanical processes, exhibited low-temperature superplasticity (LTSP) from 350 to 450 °C, behaving differently from the conventional high-temperature superplasticity (HTSP). The LTSP sheets after ~700% elongation at 350 °C and 8×10^-4 s^-1 still possessed fine (sub)grains 3.7 μm in size and narrow surface Li-depletion zones 11 μm in width. It results in a post-SP T6 strength of ~500 MPa, which is higher than that of the conventional superplastic 8090 alloys tested at 525 °C or above. The tensile behavior and deformation mechanisms of the LTSP and HTSP sheets were investigated over the strain-rate range 10^-5-10^-2s^-1 and strain range 0.5-1.0. At ε=0.5, the strain rate sensitivity (m-value) for the LTSP and HTSP materials was found to be ~0.33 and 0.50, respectively. The activation energy was extracted to be 92 kJ/mole for the LTSP sheets and to be 141 kJ/mole for the HTSP sheets. As straining to ε=1.0, the m-value of the LTSP materials increased to 0.37 and the activation energy decreased slightly to 82 kJ/mole. By SEM examinations, the movement of surface grains in LTSP samples confirmed the role of grain boundary sliding (GBS). TEM observations revealed that the deformation mechanism consists of a large amount of dislocation motion resulting in the subgrain formation and rotation; and TEM observations from the transverse section might explain the anisotropic deformation behavior during the initial superplastic strain. The primary and rate-controlling deformation mechanisms for the HTSP and LTSP sheets are considered to be GBS and dislocation creep, respectively.