Severe Plastic Deformation Induced Sensitization of Cryo-milled Nanocrystalline Al-7.5 Mg
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INTRODUCTION
COMMERCIAL Aluminum 5000-series alloys are susceptible to stress corrosion cracking (SCC) as a result of the formation of the anodic grain boundary b-phase, Al3Mg2, due to thermal exposure, also known as sensitization, in the temperature range of around 323 K to 473 K (50 C to 200 C).[1–4] Structural failure occurs via anodic dissolution of this grain boundary phase upon exposure to seawater or other corrosive environments. The beta phase nucleates at the grain boundary with random orientation and grows along the boundary with collector plate mechanism. The Mg atoms are mostly collected by the grain boundary through volume diffusion from within the grain. At a relatively low temperature, £373 K (100 C), pipe diffusion in addition to volume diffusion might play a major role in transporting Mg to the grain boundary.[2,4] Efforts have been made to increase the SCC resistance of Al-Mg alloys by adding small amount of rare earth elements.[5] Recently, higher corrosion resistance in 3.5 pct saltwater has been reported on Al 5083 containing ultrafine grains (UFG), which was obtained by friction stir weld (FSW) process.[6] They showed that the polarization resistance increases as the grain size decreases. Such an increase in corrosion resistance of Al 5083 with ultrafine microstructure has been correlated with the faster passivation–repassivation kinetics. Also, higher pitting resistance has been reported on nanocrystalline Al-Mg binary alloys produced by high energy ball milling as compared to conventional Al 5083.[6] It has been implicated that the nanocrystalline grains in Al-Mg obtained by high energy ball milling in liquid nitrogen environment might decrease the susceptibility to[7] SCC. However, a series of as-extruded nanocrystalline Al-Mg
RAMASIS GOSWAMI, Scientist, and PETER S. PAO, SYED B. QADRI, and RONALD L. HOLTZ, Senior Scientists, are with the Materials Science and Technology Division, Naval Research Laboratory, Washington, DC 20375. Contact e-mail: [email protected] Manuscript submitted September 30, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys have been reported to be highly susceptible by Sikora et al.[8] as compared to the non sensitized Al-5083. They conducted ASTM-G67 mass loss test by immersing the specimens in concentrated HNO3 at 303 K (30 C) for 24 hours on nanocrystalline Al-7.5 Mg and Al-8.6Mg alloys, and recorded that the mass loss per unit area is in the range of 50 to 65 mg/cm2. These nanocrystalline Al-Mg alloys were synthesized by cryomilling and hot isostatic pressing. However they have not studied the fine scale microstructure of the as-extruded alloys. Detailed study of the nucleation and growth of the b phase is, therefore, vital to understand the mechanisms and extent of sensitization in nanocrystalline grains produced by high energy ball milling or severe plastic deformation (SPD). The objective of this work is thus to investigate the sensitization behavior of Al-7.5 Mg containing nanocrystalline grains synthesized by high energy ball milling. We sh
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