Papers by Author: C. Hénon

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.

Abstract: RRA treatment was applied to a high-Zn, 7XXX alloy under development for aerospace applications. Microstructure of the alloy is studied at different stages of the 3-step ageing process, by Transmission Electron Microscopy, in order to understand the corresponding evolution of mechanical and corrosion properties. The Compression Yield Strength at the end of the high temperature step was found higher than at the end of the 1st step, contrary to the conventional RRA treatment. After re-ageing, the final CYS turned out significantly higher than at the T6 temper of the alloy, while the material remained sensitive to exfoliation corrosion.

Abstract: It is generally considered that exfoliation corrosion is due to the build-up of corrosion products that create a wedging stress that lifts up the surface grains. However, the exfoliation mechanism is still under discussion: possible operating mechanisms include intergranular corrosion of in plane grain boundaries accelerated by the wedging effect, or crack propagation by a “purely” stress corrosion mechanism. The sensitivity to exfoliation corrosion of AA7449 in relation to the intergranular and stress corrosion cracking sensitivity has been addressed in a program of controlled quenches followed by thermal treatments. Our observations demonstrate that the quench rate has a strong effect on intergranular corrosion and exfoliation corrosion sensitivity and in a lesser extent on stress corrosion cracking. In the first moments of the EXCO test, the initiation of corrosion follows the same trends as those revealed by the ASTM G110 test. We observe intergranular initiation for the slow quench rate (~5°C/s) and pitting initiation for samples quenched between 50 to 500°C/s. On the contrary, the final EXCO corrosion quotations do not seem to correlate with the intergranular resistance but rather with SCC resistance.