Effect of Sc on Aging Kinetics in a Direct Chill Cast Al-Zn-Mg-Cu Alloy
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NEWER materials with high specific properties and low cost are always in high demand in the aerospace industry. Al-Zn-Mg-Cu alloys of the 7000 series have the highest strength among all commercial Al alloys.[1] Their strength is mainly controlled by the precipitation reactions accompanying heat treatment. The precipitation during aging of the supersaturated solid solution in these alloys is recognized to occur in the following sequence (with an increase in the aging time and/or temperature):[2] supersaturated solid solution ! Guinier Preston ðGPÞzones ! metastable g0 or T0
! equilibrium gðMgZn2 Þ or T Mg32 ½Al,Zn49 The interfacial energy between the coherent GP zones and the matrix is low, so that zones of a very small size can form. The metastable g¢ and T ¢ phases are semicoherent with the matrix and have higher interfacial energies and, O.N. SENKOV, Senior Scientist, and S.V. SENKOVA, Junior Scientist, are with UES, Inc., Dayton, OH 45432-1894. Contact e-mail: [email protected] M.R. SHAGIEV, Materials Scientist, formerly with UES, Inc., is with the Institute for Metals Superplasticity Problems, Russian Academy of Sciences, Ufa 450001, Russia. Manuscript submitted April 4, 2007. Article published online March 7, 2008 1034—VOLUME 39A, MAY 2008
therefore, require larger critical nucleus sizes to form and grow. The g¢[3] and T¢[4] phases have hexagonal crystal structures. The formation of the incoherent equilibrium g and T phases, which have a hexagonal and a cubic crystal structure,[5] respectively, generally coincides with overaging, i.e., with a decrease in both strength and hardness. The g¢ and g phases are formed during aging of the alloys having high Zn:Mg ratios, while the T¢ and T phases are formed in the alloys with moderate to low Zn:Mg ratios (generally less than 3:1). Two types of GP zones, GP I and GP II, have recently been identified.[6–8] The GP I zone nucleates on Mg-Zn clusters in the temperature range below ~100 C, has a spherical shape of up to 6 nm in diameter, and dissolves in a range from 100 C to 140 C, depending on the holding time.[7,9,10] The Mg:Zn ratio in GP I is about 1:2,[11] and the atoms are internally ordered on {100}Al planes.[7] The GP II zones seem to nucleate on vacancyrich clusters (VRCs) that are formed during quenching after solution treatment at temperatures above 450 C.[12] These clusters are more enriched with Zn (Mg:Zn ratio is ~1:5 to 1:4[11]) and start to dissolve at higher temperatures than GP I.[7–9,11] The morphology of GP II has not been well defined. High-resolution transmission electron microscopy (HRTEM) images indicate that these are disk- or platelike zones a few atoms thick located on {111}Al planes.[7] On the other hand, three-dimensional atom probe (3DAP) analysis identifies GP II as spherical precipitates, similar to GP I.[6,8] At longer aging times, both GP I and GP II can transform to g¢.[7–11,13] The g¢ precipitates may also METALLURGICAL AND MATERIALS TRANSACTIONS A
nucleate directly from the supersaturated solid solution. The addition o
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