Precipitation Behavior of AA2618
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g ternary system is the basis for the development of the 2XXX series and related commercial aluminum alloys.[1] Additions of other elements, e.g., Si, Ag, Fe, and Ni, to the ternary system produce a variety of strengthening phases in these alloys. The concentration ratio of Cu to Mg, as well as their total contents, also affects the evolution of phases in the alloy system. Such constitutional complexity normally translates into a complex precipitation sequence during age hardening, which has yet to be fully understood for many of Al-CuMg–based commercial alloys. Aluminum alloy 2618, which is widely used in the automotive and aerospace industries, falls into such a group of alloys. The precipitation during the artificial aging of Al-CuMg alloys with a Cu/Mg mass ratio of 2.2 proceeds in the sequence[2,3] supersaturated solid solution (SSSS) ! zones ! S00 ! S0 ! S The zones, which are coherent with the matrix and enriched with copper and magnesium, were named the Guinier–Preston–Bagaryatsky (GPB) zones by H. LU, formerly Graduate Student, Department of Mechanical and Industrial Engineering, Northeastern University, is Senior Engineer, Altran Solutions, 451 D Street, Boston, MA 02210. P. KADOLKAR, formerly with the Materials Science and Technology Division, Oak Ridge National Laboratory, is with Contech Metal Forge, 901 Alfred Thun Road, Clarksville, TN 37040. K . NAKAZAWA, formerly with the Department of Mechanical and Industrial Engineering, Northeastern University, is retired. T. ANDO, Professor, is with the Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA 02115. C.A. BLUE, Deputy Division Director of Technology, is with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted March 26, 2007. Article published online August 29, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
Silcock.[4] The ternary S phase (Al2CuMg) is the equilibrium intermetallic phase, which has an orthorhombic structure with lattice parameters: a = 0.400 nm, b = 0.923 nm, c = 0.714 nm.[5] The two intermediate phases, S¢¢and S¢, are considered to have a slightly distorted S structure with different degrees of coherency with the a-Al matrix.[2,3] The S¢ phase precipitates with orientation relationships [100]S¢//[100]a, [010]S¢//[021]a, and [001]S¢//[012]a.[2] Silcock, however, found no evidence for a phase resembling S¢¢ in Al-Cu-Mg alloys.[4] Instead, an intermediate phase was identified between the GPB zones and the S¢ phase in the precipitation sequence. This phase, designated GPB2 zones,[4] may develop in situ from the GPB zones. Later, Charai et al.[6] identified the S¢¢ phase in an Al-0.9 mass pct Cu-1.4 mass pct Mg alloy by high-resolution electron microscopy (HREM) with localized Fourier analysis, while GPB2 zones were identified in different Al-CuMg alloys.[7,8] Shih et al.[9] and Wang et al.[10] designated GPB2/S¢¢ for the intermediate phase between GPB and S¢. Trace additions of Si, Fe, and Ni modify the aging process in Al-Cu-Mg alloys in v
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