Microstructural Aspects of Second Phases in As-cast and Homogenized 7055 Aluminum Alloy with Different Impurity Contents
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7XXX series aluminum alloys (Al-Zn-Mg-Cu) are widely used in aerospace industry because of their high specific strength.[1] To meet the demand of a superior combination of high strength, fracture toughness, and stress corrosion cracking resistance for aircraft materials, one of the important approaches has been to reduce the contents of impurity elements such as Fe and Si in alloy compositions. During casting of Al-Zn-Mg-Cu alloys, Fe and Si impurities give rise to the formation of coarse intermetallic particles such as Al7Cu2Fe, Al3Fe, a-AlFeSi, and Mg2Si with sizes larger than 1 lm at the grain boundaries.[2] Because of higher melting points, those coarse particles are insoluble during subsequent heat and thermomechanical treatments.[3,4] Moreover, thermomechanical treatments such as rolling or extrusion with a limited amount of deformation are difficult to fragment them to very small.[5] Therefore, they remain in commercial plates, sheets, and extrusions to deteriorate the toughness and stress corrosion cracking (SCC) resistance of materials. The coarse intermetallic particles, especially the Fe-rich and Si-rich phases are brittle, hard, and incoherent with the a-Al matrix. Thus, they are easy to crack or separate from the matrix to form HUAN SHE, Doctoral Student, DA SHU, Associate Professor, WEI CHU, Master Student, and JUN WANG and BAODE SUN, Professors, are with the State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] Manuscript submitted November 20, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
voids at a low stress, which then grow and link under the loaded stress, finally form the macrocrack to cause the fracture of the alloys.[6,7] The effects of Fe and Si impurities on the SCC resistance are relative to the pitting of Fe-rich and Sirich phases in Al-Zn-Mg-Cu alloys,[8,9] which are found to be the initial site for localized corrosion due to the difference of Volta potential causing the strong galvanic coupling forming with the surrounding matrix.[10] Besides, the coarse phases containing Cu and Fe are cathodes with respect to the matrix and promote dissolution of the matrix, while the phases rich in Mg are anodes with respect to the matrix and dissolve preferentially.[9] Anodic dissolution not only provides an initial crack[11] but also promotes the production of hydrogen on the cathode,[12] hence reducing the SCC resistance. The fracture toughness may be decreased due to the increase of volume fraction of coarse Fe-rich and Si-rich phases with increasing Fe and Si contents, while the strength and elongation are virtually independent of Fe and Si contents in Al-Zn-Mg-Cu alloys.[13–16] Even though the harmful impact of Fe and Si impurities on the microstructures and properties of Al-Zn-Mg-Cu alloys has been reported, less attention has been focused on the evolution of second phases in as-cast and homogenized alloys with variations of Fe and Si contents, especially for ultra-high strength aluminum alloys. The aim of
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