Evaluation of Al 3 Mg 2 Precipitates and Mn-Rich Phase in Aluminum-Magnesium Alloy Based on Scanning Transmission Electr
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TRODUCTION
IT is known that the desirable mechanical properties of aluminum 5083 alloys result from the presence of various intermetallic precipitates.[1,2] However, the precipitate Al3Mg2 (b-phase) is one important reason behind the reduced corrosion resistance which limits the functionality of this material.[3,4] With a corrosion potential of around 1.29 V (SCE), the b-phase is typically more active than 5083 Al matrix, which has a potential of 0.73 V (SCE). The b-phase is thus preferentially attacked by corrosive environments.[4–7] Considerable research has been conducted to understand the mechanism behind the b-phase-related corrosion phenomena.[4–13] The precipitation behavior of intergranular and intragranular b-phase in AA5083-H131 has been previously studied, but most of these studies were based on elevated temperatures that are much higher than actual application temperatures.[9,11] Another study reported the effect of sensitization on the chemical and mechanical properties of Al-Mg alloy AA5083, but did not provide direct evidence of Al3Mg2 intermetallic compound formation.[14] Previous work suggests that Mg-rich b-phase precipitation along grain boundaries is the only reason for intergranular corrosion and material failure.[15,16] However, it has been reported that b-phase does not have to be continuous for crack propagation.[6] YAKUN ZHU, Master Candidate, SOUMYA KAR, Researcher, and MICHAEL L. FREE, Professor, are with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT. Contact e-mail: [email protected] DAVID A. CULLEN and LAWRENCE F. ALLARD, Scientists, are with the Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN. Manuscript submitted January 14, 2012. Article published online August 3, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
It is well accepted that the decomposition of Al-Mg supersaturated solid-solution follows a four-stage process: solid solution to Guinier–Preston zones to b¢¢ to b¢ to b.[17,18] In Al-Mg alloy aged between 373 K and 523 K (100 C and 250 C), the b¢-phase forms first around 373 K (100 C) and b-phase appears around 523 K (200 C) in the matrix of the nearly complete depletion of Mg.[19,20] However, it has been observed that b-phase precipitates below 473 K (200 C).[8,11] In addition, there is a controversy regarding whether defects, such as dislocations, provide nucleation sites for b¢-phase precipitation.[21,22] A wealth of experimental research has been performed, but the detailed interpretation still remains disputed. To shed further light on the matter, we performed a detailed study using scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS). The variety of precipitates and their subsequent effects on b phase nucleation and growth kinetics highlight the need to characterize this materials system as a function of heat treatment time and temperature. For this, prolonged heat treatment was performed on 5083-H131 alloy samples at 343 K and 448 K (70 C and 175 C) in
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