Thermal stability in bulk cryomilled ultrafine-grained 5083 Al alloy
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I. INTRODUCTION
NEAR nanostructured or ultrafine-grained (UFG) materials are defined as materials having grain sizes whose linear dimensions are in the range of 100 to 500 nm. The physical properties of these materials are potentially superior to those of their coarse-grained counterparts.[1–4] This potential superiority results from the reduced size or dimensionality of the near nanometer-sized crystallites as well as from the numerous interfaces between adjacent crystallites.[3] In this article, experimental data obtained on the thermal stability of the bulk UFG 5083 Al alloy and its strength at high temperatures are reported and analyzed. The alloy was processed by gas atomization followed by cryomilling, consolidation, hot isostatic pressing (hipping), and extrusion. The investigation was motivated by several considerations as described herein. First, the properties associated with UFG alloys are closely related to the extremely fine grain size and the large volume fraction of grain boundaries. As a result, it is important to study the thermal stability of UFG alloys such as UFG 5083 Al for the purpose of identifying various factors that can contribute to maintaining the grain size at the ultrafine scale during high-temperature applications. Second, UFG alloys exhibit grain sizes that fall between the macro (1 m) and the nano (100 nm) ranges. Accordingly, studying UFG alloys
such as UFG 5083 Al can provide guiding information regarding the thermal stability of material systems in the transition region between these two ranges. Third, previous research focused on studying thermal stability and microstructures of cryomilled (milled under liquid nitrogen temperature) UFG alloys in different material systems in the powder form.[5–8] However, for structural applications, such studies need to be conducted on materials in the bulk form. At present, there are no data regarding thermal stability of UFG 5083 Al alloy in the bulk form. Finally, in order to study UFG 5083 Al alloy in the bulk form, the alloy was prepared by gas atomization followed by cryomilling, consolidation, and extrusion. The preparation of the bulk alloy using this procedure leads to introducing several substructural features including (a) oxide particles resulting from the breakup of oxide layers that are formed around metal particles during gas atomization, and (b) carbides, nitrides, and other impurities during cryomilling. Accordingly, an investigation of thermal stability of UFG 5083 Al alloy prepared by gas atomization followed by cryomilling, consolidation (hipping), and extrusion can provide information that may lead to the identification of the role played by the aforementioned substructural features regarding grain growth mechanisms and high-temperature strength. II. EXPERIMENTAL PROCEDURE
INDRANIL ROY and MANISH CHAUHAN, Graduate Research Assistants, and FARGHALLI A. MOHAMED, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697. Contact e-mail: [email protected] E
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