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Abstract
Recent improvements in the strength of aluminum alloys have been obtained through the development of complex microstructures containing reduced grain sizes. However, these ultra-fine grained (UFG) materials display additional mechanical properties that make them susceptible to heterogeneous deformation and instabilities that lead to failure. Understanding the thermo-mechanical response of these materials is especially important for high strain rate loading conditions, where the localized increase in plastic work and nearly adiabatic conditions lead to local temperature rises in the material. The goal of this work is to better understand the mechanisms associated with the thermal softening (defined as the decrease in flow stress with increasing temperature) of UFG aluminum under high strain rate loading conditions. A modified Kolsky bar system was used to perform high temperature (298 — 573 K) compression experiments at strain rates on the order of 103 s–1 on two UFG aluminums: Al-5083 (an Al-Mg-Mn alloy) and Al-1100 (a commercially pure aluminum). The thermal softening behaviors of both UFG Al-5083 and Al-1100 were found to be controlled by thermally activated dislocation motion over the entire range of temperatures tested.
Microstructure analysis was used to investigate the microstructural stability of these materials during the preheating period required for high temperatures experiments. It was found that significant grain growth and recovery can occur even over short (< 60 seconds) preheating times. An experimental procedure was developed to investigate the influence of this microstructural evolution on the mechanical response of the material. Care must be taken to account for these effects when evaluating the thermo-mechanical behavior of these materials.
Finally, a physics-based thermo-mechanical-viscoplastic model with two internal state variables, the dislocation density and the grain size, was constructed. The evolution equations for these variables capture the history response of the deformation using known physical mechanisms. The scalar form of the model was then used to identify the important physical mechanisms behind the thermo-mechanical behavior of UFG Al-1100 loaded at high strain rates.
