Effects of Sc and Zr microalloying additions on the recrystallization texture and mechanism of Al-Zn-Mg alloys

Effects of Sc and Zr microalloying additions on the recrystallization texture and mechanism during annealing were investigated in detail by the methods of X-ray diffraction macrotexture measurements and electron backscattering diffraction local orientation observations in three kinds of Al-Zn-Mg cold rolled sheets (Al-Zn-Mg, Al-Zn-Mg-0.10 wt.%Sc-0.10 wt.%Zr and Al-Zn-Mg-0.25 wt.%Sc-0.10 wt.%Zr). The results show that the texture of the three cold rolled alloys all consists of cube and rolling orientations. During recrystallization annealing in the conventional Al-Zn-Mg alloy, rolling texture gradually weakens and finally disappears and cube texture is the dominant recrystallization texture. The nucleation mechanism of the Al-Zn-Mg alloy is cube nucleation. For Al-Zn-Mg-Sc-Zr alloys, with the increase of Sc and Zr contents and annealing temperatures, the intensity of rolling texture β-fiber increases. Rolling texture is dominant in the recrystallized Al-Zn-Mg-Sc-Zr alloy. The main microstructural features of the Al-Zn-Mg-Sc-Zr alloy during annealing are that lots of disperse, coherent and nano-scaled Al3(Sc, Zr) particles strongly pin dislocations and grain/sub-grain boundaries, inhibiting the occurrence of recovery and recrystallization. The microstructural observation results show that S and Brass orientations are the preferred nucleation orientations in the Al-Zn-Mg-Sc-Zr alloy. To calculate the orientation-dependent stored energies, a recrystallization nucleation model was established, based on a standard Read-Schockley equation. The calculated results show that S and Brass orientations have higher stored energies. The nucleation mechanism of the Al-Zn-Mg-Sc-Zr alloy is high stored energy nucleation.