An experimental study of the recrystallization mechanism during hot deformation of aluminium

Discontinuous dynamic recrystallization (involving nucleation and grain growth) is rarely observed in metals with high stacking fault energies, such as aluminium. In this metal, two other types of recrystallization have been observed: continuous dynamic recrystallization (CDRX, i.e. the transformation of subgrains into grains); and geometric dynamic recrystallization (due to the evolution of the initial grains). The main purpose of this work was to bring clearly into evidence and to better characterize CDRX. Uniaxial compression tests were carried out at 0.7 Tm and 10−2 s−1 on three types of polycrystalline aluminium: a pure aluminium (1199), a commercial purity aluminium (1200) and an Al-2.5wt.%Mg alloy (5052), and also on single crystals of pure aluminium. In addition, 1200 aluminium specimens were strained in torsion. The deformed microstructures were investigated at various strains using X-ray diffraction, optical microscopy, scanning electron microscopy and electron back-scattered diffraction. Observations of the single crystalline samples confirm that subgrain boundaries can effectively transform into grain boundaries, especially when the initial orientation is unstable. In the case of polycrystalline specimens, after separating the effects of the initial and new grain boundaries, it turns out that CDRX operates faster in the 1200 aluminium compared to the two other grades. Moreover, it appears that the strain path does not alter noticeably the CDRX kinetics.