Delamination of Sensitized Al-Mg Alloy During Fatigue Crack Growth in Room Temperature Air
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nesium (5xxx series) alloys are used in applications that include pressure vessels, aquatic hulls, and aquatic superstructure[1] due to their excellent balance of properties. The corrosion resistance of 5083 and similar alloys has been well documented by others.[2–8] Recent work has shown that 5xxx series alloys with greater than 3 to 4 wt pct Mg in solution (e.g., 5083) can exhibit chemical instability, as well as sensitization to corrosion and stress corrosion cracking[1,3,4,8,9] with exposure to temperatures between 323 K and 473 K (50 °C and 200 °C). This sensitization occurs via migration of Mg to grain boundaries and the eventual formation of a Mg-rich b-phase,[1,2] as documented in both binary Al-Mg and commercial 5xxx alloys.[3,9–14] The presence and growth of b-phase has been shown to severely degrade the corrosion resistance and stress corrosion behavior of 5083-H116 specimens via thermal exposures ranging from 353 K to 448 K (80 °C to 175 °C) for times up to 672 hours.[8,9] Reductions in Vickers hardness, yield strength, and tensile strength with increasing time and temperature were also noted,[8] resulting from a combination of reduction in dislocation density and sensitization. In the process of JUSTIN K. BROSI, formerly Graduate Student, with the Department of Materials Science & Engineering, Case Western Reserve University, White Metallurgy Building, Cleveland, OH 44106, is now with Bettis Atomic Power Laboratory, West Mifflin, PA. MOHSEN SEIFI, Graduate Student, and JOHN J. LEWANDOWSKI, Leonard Case, Jr., Professor of Metallurgy, are with the Department of Materials Science & Engineering, Case Western Reserve University. Contact email: [email protected] Manuscript submitted March 21, 2012. Article published online August 18, 2012 3952—VOLUME 43A, NOVEMBER 2012
sensitization and the diffusion of Mg to the grain boundaries to form b-phase, a Mg depletion zone has been proposed to develop,[11] contributing to the strength reductions due to removal of Mg from solid solution. Less work has examined the effects of extended thermal exposures at lower temperatures on the tensile and fatigue crack growth behavior. The present work examines the effects of thermal exposure on the mechanical behavior of commercially available 5083-H116 in tests conducted in room temperature laboratory air (relative humidity 65 to 70 pct). Similar work on 5456-H116 alloys has been presented previously.[10,15] Exposure temperatures of 343 K, 353 K, 373 K, and 448 K (70 °C, 80 °C, 100 °C, and 175 °C) were utilized for times ranging from 10 to 2000 hours. This paper highlights some of the key tension and fatigue crack growth observations and is part of larger effort underway on a wider range of 5xxx alloys. Material used for the present work was commercially available 6.35 mm thick 5083-H116 plate,[16] composition presented elsewhere.[15] Cylindrical tensile specimens 54.0 mm in length with a 30.0 mm gage length and 3.175 mm gauge diameter and rectangular bend bar specimens 115 mm 9 6.0 mm 9 12.0 mm were machined from the plate stock with the
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