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Open AccessArticle

Microstructure Evolution and the Resulted Influence on Localized Corrosion in Al-Zn-Mg-Cu Alloy during Non-Isothermal Ageing

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Beijing Institute of Aeronautical Materials, Beijing 100095, China
2
Beijing Engineering Research Center of Advanced Aluminum Alloys and Application, Beijing 100095, China
3
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
4
National Key Laboratory of Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China
*
Authors to whom correspondence should be addressed.
Materials 2018, 11(5), 720; https://doi.org/10.3390/ma11050720
Received: 16 March 2018 / Revised: 18 April 2018 / Accepted: 28 April 2018 / Published: 3 May 2018
A non-isothermal ageing process was proposed for an Al-Zn-Mg-Cu alloy aiming to accommodate the slow heating/cooling procedure during the ageing of large components. The evolution of microstructure and microchemistry was analyzed by using transmission electron microscopy, high-angle annular dark field imaging, and energy dispersive spectrometry. The age-hardening of the alloy was examined to evaluate the strengthening behavior during the non-isothermal process. The corrosion behavior was investigated via observing the specimens immersed in EXCO solution (solution for Exfoliation Corrosion Susceptibility test in 2xxx and 7xxx series aluminum alloys, referring ASTM G34-01). Secondary precipitation was observed during the cooling stage, leading to increased precipitate number density. The distribution of grain boundary precipitates transits from discontinuous to continuous at the cooling stage, due to the secondary precipitation’s linking-up effect. The solutes’ enrichment on grain boundary precipitates and the depletion in precipitate-free zones develops during the heating procedure, but remains invariable during the cooling procedure. The corrosion in NIA (Non-isothermal Ageing) treated specimens initiates from pitting and then transits to intergranular corrosion and exfoliation corrosion. The transition from pitting to intergranular corrosion is very slow for specimens heated to 190 °C, but accelerates slightly as the cooling procedure proceeds. The transition to exfoliation corrosion is observed to be quite slow in all specimens in non-isothermal aged to over-aged condition, suggesting a corrosion resistance comparable to that of RRA condition. View Full-Text
Keywords: Al-Zn-Mg-Cu alloy; non-isothermal ageing; precipitation; segregation; local corrosion Al-Zn-Mg-Cu alloy; non-isothermal ageing; precipitation; segregation; local corrosion
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Chen, J.-Z.; Li, G.-A.; Cai, X.; Jiang, J.-T.; Shao, W.-Z.; Yang, L.; Zhen, L. Microstructure Evolution and the Resulted Influence on Localized Corrosion in Al-Zn-Mg-Cu Alloy during Non-Isothermal Ageing. Materials 2018, 11, 720.

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