LOW TEMPERATURE SUPERPLASTICITY IN 8090 Al-Li ALLOYS

The normal forming temperature for aircraft-used superplastic aluminum lithium base alloys usually needs to be within the range of 510-530 °C. Rapid recrystailization, grain growth, and solute-loss from surface layers cannot be prevented from at this high temperature. Thus, the development for low temperature superplastic Al-Li base alloys becomes meaningful. Through a series of thermomechanical treatments, the 8090 AI-Li base alloy becomes superplastic at temperatures from 350 to 450 °C. The TMT-processed sheets exhibit uniaxial tensile elongations over 600% while tested at 350 °C and 8x10^-4 s^-1, without adding back pressure. The m-value within the test conditions usually ranged from 0.3 to 0.4, and the flow stress was much higher than normal cases. The grain growth, cavitation and solute depletion effects were seen to be improved greatly. Finally, the microstructure of the superplastically deformed specimens was interesting and is worth detailed characterization in order to explain the superplastic mechanism.