Characterization of Al-Mg Alloy Aged at Low Temperatures
- PDF / 3,140,996 Bytes
- 11 Pages / 593.972 x 792 pts Page_size
- 16 Downloads / 245 Views
RODUCTION
ALUMINUM 5xxx alloys are widely used in naval ships and offshore structures due to their excellent balance of strength, ductility, weldability, and corrosion resistance.[1,2] Unfortunately, because the major alloy element, Mg, is supersaturated in the alloy matrix, Al 5xxx alloy (e.g., 5083 and 5456) will become sensitized to intergranular corrosion (IGC), and stress corrosion cracking (SCC) when exposed to temperatures between 323 K and 473 K (50 C and 200 C) for extended periods of time.[3–5] The IGC/SCC behavior occurs as a consequence of the precipitation of grain boundary (GB) b phase (fcc), Al3Mg2, which is anodic relative to the Al matrix, and will be selectively dissolved in seawater.[3–6] Considerable research has been conducted to identify the mechanism of b phase-related corrosion in Al 5xxx alloys.[7–10] Based on the previous research on Al-Mg alloys, the equilibrium b phase forms by the following reaction sequence: solid solution to Guinier–Preston (GP) zones to b¢¢ to b¢, and finally to b phase.[11–15] GP zones form at relatively low temperature and have the same structure as the Al matrix (but enriched in Mg atoms).[16,17] However, there is controversy regarding the critical Mg concentration required for GP zone GAOSONG YI, ERIK SUNDBERG, and MICHAEL L. FREE are with the Department of Metallurgical Engineering, University of Utah, 135 S 1460 E, Rm412, Salt Lake City, UT, 84112. Contact e-mail: [email protected] DAVID A. CULLEN is with the Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831. KENNETH C. LITTRELL is with the Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831. WILLIAM GOLUMBFSKIE is with the Naval Sea Systems Command Carderock Division, West Bethesda, MD, 20817. Manuscript submitted June 14, 2016. Article published online February 6, 2017 2040—VOLUME 48A, APRIL 2017
formation.[18,19] Nozato and Ishihara[18] found that GP zones were able to form in Al alloy with 7.6 to 12.5 at.pct Mg when the aging temperature is lower than 313 K (40 C). In contrast, Autono et al.[19] stated that GP zones can only form in Al alloys with Mg concentrations higher than 13 pct (from room temperature to 573 K (300 C)). Based on TEM analyses and diffraction studies, b¢¢ phase (Al3Mg) has been shown to have an L12 crystal structure,[20–22] and it is coherent with the Al matrix.[16] b¢ phase is a metastable phase with a stoichiometry of Al3Mg2 (hcp), and it is semi-coherent with the Al matrix.[17] Studies by Nebti et al.[23] and Goswami et al.[24] suggest that b phase appears around 448 K (175 C). Additional information indicates that given sufficient aging time (as long as 12 months), b¢ and b phases are also found to form at 343 K (70 C) on pre-existing particles as well as at grain boundaries.[25] In addition, b¢¢ phase has been observed to form directly (without the appearance of GP zones) from highly concentrated (Mg‡ 20 at.pct)Al-Mg alloys.[16] The sensitization behavior of Al 5456 H116 has been investigated
Data Loading...
