Microanalytical study of the heterogeneous phases in commercial Al-Zn-Mg-Cu alloys
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I.
INTRODUCTION
THE second
phase particles in precipitation hardening aluminum alloys are commonly classified into three types: constituents, dispersoids, and fine strengthening precipitates. The constituent phases are relatively large (2 to 5/~m) and consist of insoluble and/or partially soluble intermetallic compounds. These compounds primarily form as a result of interaction of the alloying elements with the impurity elements such as Si and Fe. Dispersoids, which are also intermetallic compounds, are relatively smaller in size (0.5 to 2/zm) and typically contain elements such as Mg, Cu, Zn, and Cr which are intentionally added for either strengthening or retardation of the recrystallization kinetics. The strengthening precipitates form during the aging treatments in an extremely fine and homogeneous dispersion. All these particles are complex both in chemistry and crystal structure and critically control strength, toughness, and stress corrosion cracking resistance of the A1 alloys. ~-2a Therefore, proper characterization of the structure and chemistry of these phases is essential to our understanding of the microstructure-property relationship of the commercial age hardenable alloys. The crystal structure and chemical composition of pure binary, ternary, and quaternary aluminum compounds have been reviewed by Mondalfo. ~5 However, in commercial quality multicomponent alloy systems both the structure and chemistry of the phases could be modified by the presence of other elements and also impurities. Therefore, there is a significant impetus to study the crystal structure and compositions of the phases in commercial grade alloys. Modem probe-forming analytical electron microscopes are capable of defining the crystal structure and chemistry of submicron size particles using convergent beam diffraction and X-ray microanalysis, respectively. The large constituent phases can be readily characterized in thin foils. The analysis of submicron dispersoid phases, however, can be carried RAGHAVAN AYER and J.Y. KOO are with Exxon Research and Engineering Company, Route 22 East, Annandale, NJ 08801. J.W. STEEDS is with the Department of Physics, University of Bristol, Bristol, United Kingdom. B. K. PARK is with Westinghouse Electric Corporation, P.O. Box 3399, Pittsburgh, PA 15230. Manuscript submitted February 8, 1985. METALLURGICAL TRANSACTIONS A
out only on extraction residues to avoid interference from the surrounding matrix. Until recently, extraction of the second phase particles in aluminum alloys using conventional electrolytic procedure was not satisfactory since the electrolyte invariably dissolved both the aluminum matrix and many of the second phase particles. Recently, Cocks et al. and others I8"~9have developed a chemical extraction method which has been shown to extract successfully the second phase particles in the 6000 series A1 alloys. When this procedure was used to extract the particles in 7075 and 7475 A1 alloys in the present study, it was found that the constituent and dispersoid phases were succes
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