Temperature Effects in Al 5083 with a Bimodal Grain Size
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Temperature Effects in Al 5083 with a Bimodal Grain Size Andrew C. Magee1 and Leila J. Ladani1 1 The University of Alabama, Mechanical Engineering, Tuscaloosa, AL 35487, U.S.A. ABSTRACT An Al 5083 alloy with a bimodal grain size has been previously synthesized using a lowtemperature milling process and consolidation via cold isostatic pressing (CIP). This material has been shown to exhibit greatly improved strength when compared to conventional aluminum alloys. Additionally, this material has shown sensitivity to test conditions. In this work, we studied the effects of temperature on the strain rate sensitivity of this material by examining its elastic and plastic properties though uniaxial tension tests conducted under a variety of conditions at temperatures up to 473 K. Serrated stress-strain curves were observed, indicating dynamic strain aging. Strain rate sensitivity was found to depend non-monotonically on the test temperature. INTRODUCTION The strength of metals can be improved through grain size refinement as described by the Hall-Petch relationship, which states that a material’s strength increases with the inverse square root of the grain size. This effect has been utilized to create nanocrystalline (NC) and ultrafine grained (UFG) materials that exhibit drastic increases in strength when compared to coarse grained (CG) analogs [1-3]. The cost of these improvements, however, is reduced ductility which limits the utility of these materials. One method of addressing this drawback has been to incorporate CGs into the NC or UFG microstructure, creating a bimodal grain size distribution. This has been shown to restore some of the ductility to the material, while maintaining much of the strength gained from grain size refinement [4-9]. Among other materials, this approach has been applied to Al-Mg alloys, of which the 5083 alloy is studied in this work. In order for this class of materials to enter mainstream usage, the effects of their unique microstructure on their mechanical properties must be better understood. The processing path has been shown to have marked effects on the final properties of the material [2]. Furthermore, the loading conditions themselves have also been shown to affect the material properties. Material orientation, CG volume ratio, and strain rate have all been shown to have effects on the material properties [6,10-13]. At room temperature, nanostructured Al-Mg alloys have been observed to exhibit negative strain rate sensitivity (SRS). That is, higher strain rates result in decreased strength. Additionally, the strain rate also has an effect on the ductility of the material, though the nature of the effect is not agreed upon. There are some reports of ductility increasing with decreasing strain rates [12-13] and other reports of the opposite occurring [10-11]. In this work, the effect of temperature on SRS is examined. EXPERIMENT UFG powder was created by cryomilling Al 5083 powder with the composition shown in Table I for 8 hours. The UFG powder was blended with unmilled CG powder to cr
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