Materials Science Forum Vols. 519-521 (2006) pp. 455-460 online at http://www.scientific.net © (2006) Trans Tech Publications, Switzerland

Understanding the compromise between strength and exfoliation corrosion in high strength 7000 alloys T. Marlaud1,2,a, B.Baroux1,b, A. Deschamps1,c, J.L Chemin1,d and C. Hénon2,e 1

LTPCM, CNRS UMR 5614, Institut National Polytechnique de Grenoble, BP 75, 38402 St Martin d’Hères Cedex, France 2

Alcan Centre de Recherches de Voreppe, 725 rue Aristide Bergès – BP27 – 38341 Voreppe Cedex, France a

c

[email protected], [email protected], [email protected], [email protected], [email protected]

Keywords: 7000 alloys, exfoliation corrosion, precipitation hardening, SAXS, TEM, DSC

Abstract. The microstructural evolution has been investigated in three alloys of the 7000 series possessing increasing zinc contents by combining small-angle X-ray scattering, differential scanning calorimetry and transmission electron microscopy, in order to gain understanding on the evolution of the compromise between yield strength and corrosion resistance. We show that the three materials show qualitatively identical precipitation sequences; however the precipitated volume fraction is shown to increase in parallel to the Zn content. Moreover, the precipitate size evolution is faster in the high Zn alloy. The precipitate composition is inferred to vary in the three materials, and this difference is shown to explain the differences in calorimetric behavior, precipitation kinetics and corrosion resistance. Introduction Alloy development in the high strength 7000 series alloys aims at increasing particularly the yield strength. However, it is known that for a given alloy the yield strength has a negative correlation with corrosion resistance [1], and particularly exfoliating corrosion, as measured for instance by the EXCO test. A usual way to reduce the corrosion susceptibility is to over age the material [2], to the cost of a 15% decrease of strength. The aim of the present work is to investigate in detail the influence of microstructure on the compromise between exfoliating corrosion and strength in three different alloys of increasing strength, with increasing Zn content (7150, 7449 and PA766, an alloy under development at Alcan). This will be achieved notably by a detailed characterization of the precipitation kinetics during heat treatment. Materials and Methods The composition of the three studied materials is given in Table 1. They all have similar Mg contents, whereas the Zn content steadily increases from 7150 to PA766 and the Cu follows an opposite tendency. PA766 7449 7150 >8.4 Zn 8,325 6,22 Composition