Characterization and analysis of low-temperature superplasticity in 8090 Al-Li alloys

The 8090 Al-Li alloys, after a special thermomechanical process (TMP), exhibited low-temperature superplasticity (LTSP) from 350 °C to 450 °C and behaved differently from the conventional high-temperature superplasticity (HTSP). The LTSP sheets after ~700 pct 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, resulting in a post-SP T6 strength of ~500 MPa, significantly higher than that of the 8090 alloys tested at normal superplastic temperature of 525 °C or above. Examination from the movement of surface marker lines in LTSP samples confirmed the role of grain boundary sliding (GBS), coupled with grain rotation and migration. During the initial stage (<150 pct), GBS along certain higher-angled boundaries was proceeded along a plane ±45 deg with respect to the sample surface. With increasing straining, sliding between individual grains or grain groups was observed on other planes, forming a zigzag morphology. Transmission electron microscopy (TEM) observations revealed appreciable dislocation activities, suggesting the involvement of dislocation creep. The tensile behavior and deformation mechanisms of the HTSP and LTSP sheets were investigated and analyzed over the strain rates range 10^-5 to 10^-2 s^-1. 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. Based upon these results, the primary deformation and accommodation mechanisms for the HTSP and LTSP sheets are GBS and dislocation creep, respectively.