Effect of Zn Content on the Microstructure and Properties of Super-High Strength Al-Zn-Mg-Cu Alloys
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TRODUCTION
A l-Zn-Mg-Cu alloys have been widely used as important structural materials in the aeronautical industries because of their attractive comprehensive properties, such as high specific strength, high yield strength ratio, formability, and high resistance to fatigue.[1,2] AlZn-Mg-Cu alloys are precipitation-hardening Al alloys. The usual precipitation sequence from the Supersaturated Solid Solution (SSS) in Al-Zn-Mg-Cu alloys can be summarized as follows[3]: SSS ! GP zones ! Metastable g0 ! Stable g ðMgZn2 Þ GP zones are the first forming precipitates and fully coherent with the matrix. Two types of GP zones (GP I and GP II zones) have been identified.[4–7] The GP I zones nucleate on Mg-Zn clusters in the temperature range less than ~373 K (100 C), which have a spherical shape of up to 6 nm in diameter, and dissolve in a temperature range from 373 K to 413 K (100 C to
ZIYONG CHEN and ZUOREN NIE, Professors, are with the College of Materials Science and Engineering, Beijing University of Technology, Pingleyuan 100, Chaoyang District, Beijing 100124, People’s Republic of China. Contact e-mail: [email protected] YUANKE MO, formerly Postgraduate Student with the College of Materials Science and Engineering, Beijing University of Technology, and is now Ph.D. Student with the Institute for Advanced Materials and Technology, University of Science and Technology Beijing, No. 30 Xueyuan Road, Haidian District, Beijing 100083, People’s Republic of China. Manuscript submitted February 27, 2011. Article published online April 17, 2013 3910—VOLUME 44A, AUGUST 2013
140 C).[3,7,8] The GP II zones seem to nucleate on Vacancy Rich Clusters (VRCs) that are formed during quenching after solution treatment at temperatures above 723 K (450 C). These clusters are more enriched with Zn and start to dissolve at higher temperatures or longer aging time than those of GP I.[4,5,7] The ThreeDimensional Atom Probe (3DAP) analysis identifies GP II as spherical precipitates being similar to GP I.[5,6] Both metastable GP I and GP II are the heterogeneous nucleation cores for thermodynamically more stable precipitates g¢, and transform to g¢ during aging at longer time or higher temperature.[3,5,7,9] The metastable phase g¢ has a plate-like shape (size in thickness and in diameter being dependent on the heat-treatment time and temperature), and it is the precursor of the equilibrium g phase. The morphology of the g is lath or cigar-like (high aspect ratios and elongated precipitate).[3,8–11] GP zones (GP I and GP II zones) and g¢ are the phases that contribute to the precipitation hardening of the alloys in the under-aging and peak-aging conditions, while g forms during over-aging conditions.[4,5,7,8,11] The properties of Al-Zn-Mg-Cu alloys were affected by the contents of the major alloy elements.[12] In order to reach the high levels of strength, the contents of major alloy elements in the super-high strength Al-ZnMg-Cu alloys are increased gradually. The content of Zn in 7055 alloy exceeds 8 wt pct, and those in B96w as well as 7A60 alloy
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