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NTRODUCTION

STRICTER vehicle emissions requirements and the demand for increased fuel efficiency consistently drive the industry to produce better-performing, faster vehicles while reducing carbon-based emissions. One of the ways manufacturers have met these demands is by reducing the weight of their vehicles utilizing low density materials. For example, vehicle components consisting of steel, with an approximate density of 7.75 g cm3, could be replaced by a lighter weight material improving fuel efficiency. Aluminum alloys, with an approximate

CLARA HOFMEISTER is with the Department of Materials Science and Engineering, University of Central Florida, Orlando, FL 32816 and also with the Oak Ridge Institute for Science and Education, Belcamp, MD 21017. Contact e-mail: [email protected] LE ZHOU and YONGHO SOHN are with the Department of Materials Science and Engineering, University of Central Florida. FRANK KELLOGG is with the SURVICE Engineering Company, Belcamp, MD 21017. ANIT GIRI and KYU CHO are with the U.S. Army Research Laboratory, Weapons & Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD 21005. Manuscript submitted August 15, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

density of 2.7 g cm3, are therefore of great interest for the development of lightweight vehicles. Increasing the strength of aluminum alloys via fundamental strengthening mechanisms such as grain size reduction, dislocation forest, and Orowan can further enhance their quasi static as well as high strain and high strain rate properties making them suitable for wide ranges of dual use Army ground vehicle and civil transport applications. Reducing the grains down to the nanoscale was shown to be a promising method to increase the overall strength of aluminum alloys and their composites.[1–3] One method to reduce the grains is to cryomill or attritor ball mill the powders in the presence of liquid nitrogen. In this process, powders undergo severe plastic deformation through a simultaneous fracturing and cold welding process that reduces the grain size of the aluminum powders to a minimum size of about 20 nm.[2] The liquid nitrogen provides environmental protection for the powders by minimizing oxidation, and produces nanometer-sized nitrogen-rich dispersoids.[2] These dispersoids may help pin grain boundaries and provide additional strengthening to the material.[2,4]

Prior to consolidation and secondary processing, aluminum powders, whether cryomilled or not, need to be degassed. Gases are adsorbed onto and absorbed into the material during gas atomization, subsequent processing, and storage. These gases are present in the surface oxide film of Al powders which has thickness ranging from 2 to 15 nm[5–7] and consists primarily of Al2O3 and Al2O3Æ3H2O as well as physisorbed water and oxygen gas.[7] For alloys containing magnesium, like AA5083, there is also an enrichment of magnesium in the form of MgO or Mg(OH)2 in the surface layer.[6] Volatiles, if not properly removed during the degassing step, will desorb fr