Mechanism for enhanced precipitation strengthening due to the addition of copper to Al-Zn-Mg alloys with high Zn/Mg ratio

The mechanism for enhanced precipitation strengthening due to the addition of copper to Al-9.0Zn-1.5Mg (wt.%) alloys has been investigated by means of tensile test, transmission electron microscopy, anomalous small-angle X-ray scattering and three-dimensional atom probe. With the addition of copper increasing from 0% to 1.0%, and then 2.6%, the strength increment of peak-aged alloys due to precipitation strengthening increases from 274.8 MPa to 329.3 MPa, and then 343.0 MPa. Based on the copper-induced change of the parameters of clusters, GPI zones and η′ precipitates coexisting in the aged alloys, the enhanced precipitation strengthening has been discussed in detail by considering their coherent, modulus and short-order strengthening. In the alloy without copper, the coherent strengthening from η′ precipitates play a main role in precipitation strengthening. In copper-containing alloys, the contribution of clusters to strengthening become significant. The addition of copper decreases coherent strengthening and modulus strengthening from η′ precipitates, but increases modulus strengthening from GPI zones and clusters, and short-range order strengthening from clusters more apparently, and consequently enhances strength of the alloys after aging.