An analytical electron microscopy study of constituent particles in commercial 7075-T6 and 2024-T3 alloys
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INTRODUCTION
IN commercial aluminum alloys, pitting corrosion has been observed to occur at intermetallic constituent particles.* For example, pitting occurs at Al7Cu2Fe particles in *The term ‘‘constituent particles’’ is used to designate the insoluble, undissolved, or precipitated coarse particles that are formed and distributed heterogeneously in aluminum alloys from impurity elements, excess alloying elements, or improper heat treatment.
7075 alloy.[1] Iron-rich constituent particles, such as Al3Fe and a-AlFeSi, were found to be more noble than the matrix and enhanced the development of pitting.[2,3] MgZn2 particles were found to be initiation sites for pitting in powder metallurgy Al-Mg-Zn alloys.[4] The role of constituent particles in the pitting corrosion of aluminum alloys has been confirmed by recent studies of 7075-T6 and 2024-T3 (bare) alloys in an aerated 0.5M NaCl solution.[5,6] Two types of particles were identified. Type A particles were anodic with respect to the matrix and tended to dissolve themselves, while type C particles were cathodic to the matrix and tended to promote dissolution of the adjacent matrix.[5,6] Analyses by scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) have shown that, in the 7075-T6 alloy, type A particles were those that contain Al, Cu, Mg, and Zn and type C particles were those with Al, Fe, Cu, Mn, and Zn. In the 2024-T3 alloy, on the other hand, types A and C particles contained Al, Cu, and Mg and Al, Fe, Cu, and Mn, respectively.[5,6,7] Because the particles are important in promoting pitting corrosion, characterization of their crystal structure, chemical composition, and electrochemical behavior is needed to better understand the particle-pitting relationship in commercial aluminum al-
MING GAO, Engineering Advisor, is with Mobil Exploration & Producing Center (MEPTEC), Material, Corrosion and Inspection Group, Farmers Branch, TX 75381-9047. C.R. FENG, Research Physicist, is with the Materials Science Division, United States Naval Research Laboratory, Washington, DC 20375-5320. ROBERT P. WEI, Professor, is with the Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015. Manuscript submitted July 1, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
loys. Parallel studies were carried out, therefore, to (1) identify the constituent particles and determine their crystal structure and chemical composition and (2) determine the nature and extent of their electrochemical interactions with the matrix following exposure to electrolytes. The first of these studies is described herein and the results of the second study are reported in a companion article.[8] The crystal structure and composition of pure binary, ternary, and quaternary aluminum compounds have been extensively studied.[9,10] Constituent particles in commercial aluminum alloys, however, are complex. Their crystal structure and composition can be modified by alloying elements and impurities in the alloys and are commonly determined by X-ra
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