Quantitative Measurement of Hardening Precipitate State in Al-Zn-Mg Alloys Using Atom Probe Tomography at Different Ageing Conditions

Aluminium alloys have broad applications in the aerospace, automobile and high speed railway industry due to its outstanding strength to weight ratio and other mechanical properties[1-2]. The general precipitation sequence may be written as:<br/>SSSS à GP(I)/GP(II) à eta’ + eta(1–12) à eta(MgZn2). Here, we investigate different Natural Ageing (NA) times (3 months and 6 months and 1 year ) along with early stage thermal heat treatment to characterize the GP zones formed in terms of size distribution as a function of composition along with Zn/Mg ratios for 2 different 7xxx series alloys, one with copper as an alloying element and the other without copper.<br/>Preliminary results from Atom Probe Tomography reveals that there is an evolution of Zn/Mg ratio from 3-6 months.<br/>We further subject these alloys after 1 year to high temperature ageing leading to decrease in hardness which can be attributed to dissolution of the temperature sensitive GP zones and evolution of eta' which restores hardness, all characterized with APT and TEM.