TEM Investigation on Microstructural Characteristics in Nanostructured Al-Mg Alloy
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TEM Investigation on Microstructural Characteristics in Nanostructured Al-Mg Alloy Young S. Park1, Kyung H. Chung1, Nack J. Kim2, and Enrique J. Lavernia1 1 Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, CA 95616 2 Center for Advanced Aerospace Materials, Pohang University of Science & Technology, POSTECH, Pohang 790-784, South Korea ABSTRACT The microstructural evolution in a nanostructured Al-Mg alloy fabricated by cryogenic mechanical alloying (cryomilling), was investigated using transmission electron microscopy (TEM). Nanostructured Al-Mg powders were first synthesized by a mechanical alloying under liquid nitrogen media (cryomilling), and the powders were subsequently degassed, hot isostatically pressed, and extruded into a full dense, bulk form. The results showed that Si containing phases and (Fe,Ni)Al intermetallics existed in as-extruded Al-Mg alloy. In addition, the extrusion temperature has a strong influence on the formation of microstructural anisotropy. A lower extrusion temperature yields a microstructure that is more anisotropic relative to that present at the higher extrusion temperature. More specifically, at the lower temperature, the nano-sized Al grains have a tendency to rotate towards the direction, along the extrusion direction. INTRODUCTION In an effort to develop novel materials, as well as to improve the properties of existing ones, a number of synthesis techniques have emerged in recent decades. These include mechanical alloying (MA) [1], rapid solidification [2], plasma processing [3], vapor deposition [4], torsion straining under high pressure [5], and equal channel angular pressing [6]. Among them, mechanical alloying has attracted attention as a powerful tool for the fabrication of various materials, including equilibrium, non equilibrium (e.g., amorphous, quasicrystal, nanocrystalline, etc.), and composite materials [7]. Recently, the synthesis of nanostructured Al-Mg alloys by mechanical alloying in liquid nitrogen slurry was studied in order to improve their mechanical properties, and thermal stability [8,9]. In one particular study, the microstructural evolution during thermal annealing of a cryogenically ball milled Al-Mg alloy was studied [9]. However, inspection of the available literature reveals that there is limited fundamental understanding on the microstructural evolution during consolidation processes, such as extrusion, of cryomilled Al-Mg alloys. A comprehensive, fundamental understanding of microstructural evolution in bulk nanostructured Al-Mg alloys during consolidations will provide an opportunity for process optimization, and accordingly, result in enhanced properties. Moreover, from a fundamental standpoint, the non-equilibrium microstructures that are typically present in these materials are sensitive to process conditions. The objective of the present study is to provide fundamental insight into the microstructural evolution in bulk nanostructured Al-Mg alloys during extrusion and to investigate the in
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