Characterizing the Microstructure and Strengthening Mechanisms in Cryomilled Al 5083
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Characterizing the Microstructure and Strengthening Mechanisms in Cryomilled Al 5083 G. Lucadamo, N.Y.C. Yang, C. SanMarchi, and E. J. Lavernia1 Materials and Engineering Sciences Center, Sandia National Laboratories, Livermore, CA 94551-0969 1 Department of Materials Science and Engineering, UC Davis, Davis, CA ABSTRACT Cryomilling is a method of producing nanostructured morphologies from a range of starting metals and alloys. This process substantially increases the strength of lightweight alloys. In this work, we characterize the microstructure of Al alloy 5083 following cryomilling, hot isostatic pressing (HIP), and extrusion. The yield strength of the cryomilled 5083 Al is approximately twice that of conventional wrought 5083 Al and the room temperature microhardness essentially is unchanged following annealing at temperatures that approach 0.8 Tm. Using complementary transmission electron microscopy (TEM) techniques such as energy filtered (EFTEM) and weak beam imaging we investigate the mechanisms responsible for the mechanical properties. A survey of the microstructure identifies several sources of strengthening. These include: submicron grain sizes in the as-extruded material, precipitates, and metal-oxide phases. Also, the Mg in the alloy is expected to contribute some solid solution strengthening. TEM images show that lattice dislocations frequently are pinned at the precipitate interfaces. Continued precipitation and grain boundary pinning by oxide particles at elevated temperatures may account for the persistence of hardness following annealing. INTRODUCTION Processing metals by inducing severe plastic deformation (SPD) yields nanostructured alloys with substantial improvements in mechanical strength [1-3]. The SPD is introduced into the material by different methods including: high-reduction rolling, equal channel angular pressing (ECAP), and ball milling. Rolling and ball milling performed at cryogenic temperatures limit dynamic recovery and recrystallization and produce a nanocrystalline microstructure [3,4]. In particular, the microstructure and mechanical properties of Al-Mg alloys produced from powder compacts have been the subject of several recent studies [4-11]. The addition of Mg to Al increases strength without compromising ductility [12]. Al-Mg alloys also possess good weldability and corrosion resistance. Small amounts of Mn and Cr (< 0.7 wt.%) in addition to Mg, such as in the 5xxx series of Al, increase the amount of precipitation in the microstructure [12]. A new process using cryomilling followed by hot isostatic pressing (HIP) and extrusion has produced Al-Mg alloys with improved mechanical strength and good ductility [10]. Other important and intriguing characteristics of alloys processed by SPD at cryogenic temperatures are good creep resistance and the retention of nearly room temperature mechanical strength following heat treatment [4,6]. In the present work, an Al 5083 alloy was processed using cryomilling and HIP consolidation. While the cryomilled powder is compositiona
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