Degassing Behavior of Nanostructured Al and Its Composites

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INTRODUCTION

ULTRAFINE-GRAINED (UFG) and nanostructured Al alloys (e.g., grain size 100 to ~1000 nm) produced via cryomilling have attracted considerable interest in the past decade due to their reported high room-temperature strength, high specific strength, and enhanced high-strain-rate properties.[1,2] The observed improvements in mechanical response over those of conventional materials are in general attributed to various mechanisms, including: microstructural refinement (i.e., Hall–Petch strengthening), extended solid solubility, the presence of nonequilibrium structures (e.g., twins), and Orowan strengthening due to the presence of nanoscale secondary dispersoids. Cryomilled powders typically require a degassing procedure prior to consolidation into bulk form, in order to minimize the presence of hydrogen and hydrated compounds (e.g., Al2O3Æ3H2O) that are adsorbed on the powder. Moreover, the degassing conditions (e.g., temperature, time, and vacuum level) must be selected so as to retain the original microstructure to the extent possible. It is well established that the presence of entrapped gases in powder metallurgy (PM) Al alloys can lead to cracking of the compact during thermomechanical ZHIHUI ZHANG and YING LI, Postdoctoral Researchers, RUSTIN VOGT and TROY D. TOPPING, Graduate Student Researchers, YIZHANG ZHOU, Associate Researcher, JULIE M. SCHOENUNG, Professor, and ENRIQUE J. LAVERNIA, Distinguished Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail: [email protected] STEVEN DALLEK, Research Scientist, formerly with the Naval Surface Warfare Center, Carderock Division, West Bethesda, MD 20817, is with G/J Associates, Annapolis, MD 21401. Manuscript submitted July 20, 2009. Article published online November 6, 2009 532—VOLUME 41A, FEBRUARY 2010

processing or can result in the formation of blisters that severely degrade ductility. Reduction of the gas content in Al alloy powders prior to compaction is, therefore, critically important.[3–6] Thermodynamic calculations reveal that it is practically impossible to produce oxide-free powder because in the Al-O system, the partial pressure of O2 in equilibrium with Al2O3 is far below the range attainable under protective gases or in vacuum systems (Al oxide requires an oxygen partial pressure of 10144 atm at 100 C and 1064 atm at 500 C[7]). Consequently, oxidation always takes place on the fresh surface of Al powders. Furthermore, oxide formation is associated with a fast kinetic process[8] and the oxide layer exhibits a strong tendency for hydration reactions when exposed to humid environments.[9] Accordingly, Al alloy powder is in general covered with an oxide layer bonded with chemisorbed water (Al2O3Æ 3H2O) and physisorbed water (H2O).[3,9] For gas-atomized powders, high-temperature degassing has been investigated in Al alloys such as Al-Zn-Mg-Cu-Co (Al 7091),[3,10,11] Al-Si-Cu-Mg-Fe,[12,13] Al-Fe-Mo-Zr,[12] and Al-Fe-Mg alloys.[14] Although there is wide vari

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