Sohail Shah1
NTNU1
Al-Zn-Mg-(Cu) (Al 7XXX) is an ultra-high strength class of aluminum alloy which is used for various applications like aerospace and automobile industry[1]. These alloys are mainly strengthened through precipitation of nanoscale metastable Guinier-Preston (GP) zones and eta’ precipitates which are key in controlling the strength as well as other properties. The composition, size and structure of these nanoparticles are equally important , which can further influence the properties like corrosion resistance[2]. These clusters and precipitate formation require heat treatment or high temperature bakes to form high density of precipitates which is not only time consuming but high in cost as well. A novel technique of controlled cyclic deformation at room temperature is sufficient to mediate dynamic precipitation of solute clusters which also enhance the material properties[3]. Here, we investigate the 7XXX series of aluminum alloys, one with copper and another with little or no copper using the Atom Probe Tomography technique (APT) after subjecting them to cyclic ageing. The APT is a powerful analytical characterization tool at the atomic scale for various characterization including local chemical composition and 3D morphology of individual grain boundaries and precipitates[4]. The objective of this work is to carry out cyclic loading fatigue tests and look at the composition and structure of clusters and compare them to the ones formed by conventional Natural ageing (NA) phenomenon after Solution Heat Treatment (SHT).<br/>Preliminary results show a strong correlation between hardening and the clusters formed while the clusters formed immediately after cyclic ageing seem quite unstable rendering a poor mechanical response. A 10 day NA allows the clusters to stabilize in structure and size as confirmed by APT results which enhances the mechanical properties. A severe reduction in ductility (fracture strain) is observed for cyclic aged samples which can be understood by studying the microstructure using correlative TEM and APT.<br/><br/><b>References</b><br/>[1] T. F. Chung <i>et al.</i>, “Transmission electron microscopy investigation of separated nucleation and in-situ nucleation in AA7050 aluminium alloy,” <i>Acta Mater.</i>, 2018.<br/>[2] Y. L. Wang, Y. Y. Song, H. C. Jiang, Z. M. Li, D. Zhang, and L. J. Rong, “Variation of nanoparticle fraction and compositions in two-stage double peaks aging precipitation of Al−Zn−Mg alloy,” <i>Nanoscale Res. Lett.</i>, 2018.<br/>[3] W. Sun <i>et al.</i>, “Precipitation strengthening of aluminum alloys by room-temperature cyclic plasticity,” <i>Science (80-. ).</i>, 2019.<br/>[4] D. Blavette, A. Bostel, J. M. Sarrau, B. Deconihout, and A. Menand, “An atom probe for three-dimensional tomography,” <i>Nature</i>, 1993.