SUPERPLASTICITY IN AEROSPACE MATERIALS
Keywords:
Superplasticity, Al-Li alloys, Superalloy, Thermomechanical Treatment, Constitutive Relationship, Superplastic FormingAbstract
Superplastic flow property and microstructural evolution of aerospace materials, with the examples of AA8090 Al-Li alloy and IN718 superalloy, are presented. Thermo-mechanical treatment of quasi-single phase material like this is used for producing fine grains required for superplasticity. In as-processed stage the microstructures are noted to have banded structure, which changes to equiaxed grains with concurrent grain growth and cavitation. Under this state, the flow stress varies with strain but the pseudo-steady state is attained after suitable prestraining at elevated temperatures. The maximum values of strain rate sensitivity index for deformation of AA8090 and IN718 superalloy are determined to be 0.82 and 0.70, respectively, with the corresponding values of ductility to be 475% and 579%. The activation energy for deformation suggest the role of grain boundary diffusion as an accommodation process for grain boundary sliding but, in the case of IN718 superalloy, grain boundary sliding is accommodated by lattice diffusion above a critical temperature.
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