The Limits of Low Temperature Superplasticity in AA 5083 Produced By Accumulative Roll Bonding (ARB)

Accumulative roll bonding (ARB) is a severe plastic deformation technique used to produce microstructures conducive for low temperature superplasticity. This processing technique not only produces sub-micron grains but also non-equilibrium grain boundaries with increased grain boundary diffusivity. Low temperature superplasticity (225 < T < 250 degrees C) was achieved with activation energies between 40 and 80 kJ/mol; significantly lower than what is commonly reported for grain boundary sliding limited by grain boundary diffusion (84 kJ/mol). This reduction in activation energy is a direct result of non-equilibrium grain boundary development which allows for enhanced diffusion rates at substantially lower temperatures. Activation energies increased above 100 kJ/mol after 15 min of thermal exposure between 225 degrees C and 250 degrees C, providing further evidence that low temperature superplasticity relies heavily on the metastable grain boundary structure produced by ARB. The cavitation void area fraction for optimal low temperature superplastic conditions was well below 1 pct for thinning ratios (t(o)/t(f)) around 2.0 (epsilon = 0.75, e = 1.12), which is far superior compared to the approximate to 5 pct achieved in conventionally processed material (T = 500 degrees C) strained to similar levels. This work not only provides a framework for the temperature and strain rate limits of superplasticity of submicron grained material, but also investigates critical parameters pertinent to the forming industry, including damage accumulation, strain localization and thermal stability of microstructure.

Automated summary

Accumulative roll bonding (ARB) is a plastic deformation technique that can produce microstructures suitable for low temperature superplasticity. This technique creates sub-micron grains and non-equilibrium grain boundaries with increased grain boundary diffusivity. The activation energy for low temperature superplasticity is lower than commonly reported values, and this reduction is attributed to the non-equilibrium grain boundary structure.