Effects of Combined Additions of Mn and Zr on Dispersoid Formation and Recrystallization Behavior in Al-Zn-Mg Alloys

The effects of Mn or Zr additions on the dispersoid formation and recrystallization behavior in Al-Zn-Mg alloys were investigated. The combined additions of Mn and Zr are expected to increase the uniformity of dispersoids and increase the sizes of Al3Zr and Al6Mn dispersoids but decrease their volume fraction, which reduces the pinning force of dispersoids to grain boundaries. Al6Mn dispersoids are incoherent with the Al matrix and result in lower pinning force compared to that of Al3Zr dispersoids. Consequently, the inhibiting effect of combined additions of Mn and Zr on recrystallization is not as strong as that due to Zr addition alone. After hot rolling, the long axis of most ellipsoidal Al6Mn dispersoids tends to be parallel to the rolling direction (RD), resulting in a larger pinning force in the normal direction (ND) than in the RD; therefore, the aspect ratios of recrystallized grains are larger in Mn-containing alloys than in Mn-free alloys. Further, the addition of Mn in Al-Zn-Mg alloy can lead to coarse alpha-AlFeMnSi constituent particles and, therefore, a higher area fraction of second phase in Mn-containing alloys; as a result, particle-stimulated nucleation (PSN) is the dominant recrystallization mechanism in Mn-containing alloys, while both PSN and strain-induced grain boundary migration (SIBM) exist in Zr-containing alloys.