High-strain-rate superplasticity in bulk cryomilled ultra-fine-grained 5083 Al

Abstract

The ductility and creep of bulk ultra-fine-grained (UFG) 5083 Al (grain size ∼440 nm) processed by gas atomization, cryomilling, and consolidation were studied in the temperature range 523 to 648 K. Also, the creep microstructure developed in the alloy was examined by means of transmission electron microscopy (TEM). The ductility as a function of strain rate exhibits a maximum that shifts to higher strain rates with increasing temperature. An analysis of the experimental data indicates that the true stress exponent is about 2, and the true activation energy is close to that anticipated for boundary diffusion in 5083 Al. These creep characteristics along with the ductility behavior of 5083 Al are a reflection of its creep behavior as a superplastic alloy and not as a solid-solution alloy. In addition, the observation of elongations of more than 300 pct at strain rates higher than 0.1 s−1 is indicative of the occurrence of high-strain-rate (HSR) superplasticity. Microstructural evidence for the occurrence of HSR superplasticity includes the retention of equiaxed grains after deformation, the observation of features associated with the occurrence of grain boundary sliding, and the formation of cavity stringers. Grain size stability during the superplastic deformation of the alloy is attributed to the presence of dispersion particles that are introduced during gas spraying and cryomilling. These particles also serve as obstacles for dislocation motion, which may account for the threshold stress estimated from the creep data of the alloy.