Nano-structure evolution of secondary Al3(Sc1-xZrx) particles during superplastic deformation and their effects on deformation mechanism in Al-Zn-Mg alloys

The nano-structure evolution of secondary Al-3(Sc1-xZrx) particles during high-strain-rate (0.01 s(-1)) superplastic deformation at 500 degrees C was investigated by high resolution transmission electron microscopy. The results show that by increasing the true stains from 0.69 to 2.40, the mean radii of spheroidal secondary Al-3(Sc1-xZrx) particles increase from 9.8 +/- 3.4 nm to 16.4 +/- 6.8 nm, and the lattice misfit value increases from 1.04% to 1.30%. Before deformation, Al-3(Sc1-xZrx) nano-particles are completely coherent with Al(a) matrix. As the deformation proceeds, the accumulated sever plastic deformation introduces misfit dislocations at the interface between particles and matrix. Superplastic flow deformation data indicate that with the increase of true strains, the strain rate sensitivity of Al-Zn-Mg alloy decreases from 0.29 to 0.19, and the deformation activation energy increases from 112 to 121 kJ/mol. However, for Al-Zn-Mg alloy with secondary Al3(Sc1-xZrx) nano-particles, the strain rate sensitivity increase from 0.33 to 0.45, and the deformation activation energy decreases from 107 to 84 kJ/mol. Based on the microstructural results and the established constitutive equations at different strains, it can be concluded that at the working hardening stage, two kinds of alloys are both controlled by dislocation viscous glide creep mechanism. At the dynamic softening stage, dislocation creep mechanism operates in Al-Zn-Mg alloy, whereas grain boundary sliding mechanism is dominant in Al-Zn-Mg-Sc-Zr alloy. Secondary Al-3(Sc1-xZrx) nano-particles play an important role in accelerating the cooperative grain boundary deformation and only affect dynamic softening deformation mechanism of Al-Zn-Mg alloys. (C) 2016 Elsevier B.V. All rights reserved.