Kinetics of recrystallization for twin-roll casting AZ31 magnesium alloy during homogenization

Abstract

The kinetics of recrystallization for twin-roll casting AZ31 magnesium alloy with different thicknesses during homogenization was analyzed. It is shown that fine grains are first formed at the boundaries of deformed bands in the twin-roll casting slab. The recrystallized grains with no strain are gradually substituted for the deformed microstructure of twin-roll casting AZ31 magnesium alloy. The incubation temperature and time for the recrystallization of a twin-roll casting AZ31 magnesium alloy strip with a thickness of 3 mm are lower and shorter than those of the 6-mm thick strip, respectively. The 3-mm thick twin-roll casting magnesium alloy has finer grains than the 6-mm thick strip. The activation energies of recrystallization for twin-roll casting AZ31 magnesium alloy slabs with the thickness of 3 and 6 mm are 88 and 69 kJ/mol, respectively. The kinetics curves of recrystallization for twin-roll casting AZ31 magnesium alloy were obtained.

Evaluation of static recrystallization and grain growth kinetics of hot-rolled AZ31 alloy

“…The activation energy of regime I is equal to 74.1±5.7 kJmol -1 which is lower than the boundary self-diffusion energy (92 kJmol -1 ) and the lattice self-diffusion energy (135 kJmol -1 ) [24]. Low activation energy of 85.9 kJmol -1 was also reported in heavily cold-drawn and annealed AZ31 alloy [19] and in the range of 65 kJmol -1 to 88 kJmol -1 depending on the thickness of the twin-roll casting AZ31 alloy [26]. The reduced activation energy found in the various AZ31 alloys is mostly attributed to the acceleration of static recrystallization due to the occurrence of heterogeneities such as shear bands.…”

“…Many experimental studies on various metals reported the deviation of Avrami exponent from the theoretical values [9,12,18,19,21,[26][27][28][51][52][53][54]. For example, the Avrami exponent was found in the range of 0.27-3.4 for AZ31 alloy under different deformation and annealing conditions [9,18,19,21,[26][27][28]51]. Moreover, a lower n range (0.6-0.9) was also reported in processed and annealed Mg-RE alloys [12,51].…”

“…Globally, the n values obtained from the present investigation are lower than the ideal values (Table 1), except for the sample annealed at the lowest temperature of 150°C. Many experimental studies on various metals reported the deviation of Avrami exponent from the theoretical values [9,12,18,19,21,[26][27][28][51][52][53][54]. For example, the Avrami exponent was found in the range of 0.27-3.4 for AZ31 alloy under different deformation and annealing conditions [9,18,19,21,[26][27][28]51].…”

“…For Mg-based alloys, the calculated activation energies for recrystallization/grain growth are generally compared with grain boundary diffusion activation energy (92 kJmol -1 ) or with the lattice self-diffusion energy (135 kJmol -1 ) [24]. However, a deviation from these values due to the presence of inhomogeneous nucleation sites such as twinning and shear bands formed during deformation processing is often reported [9,12,18,19,21,[25][26][27][28]. Besides, the static recrystallization/grain growth kinetics of Mg-based alloys are strongly related to their metallurgical and microstructural states and parameters such as alloying elements, grain size, grains orientation, nature and geometry of grain boundaries, and the distribution of particles [29][30][31][32][33].…”

“…In addition to the importance of annealing conditions, i.e., temperature and duration, deformation processing conditions including strain rate, deformation temperature, and thickness reduction in the case of rolling process do greatly affect the static recrystallization/ grain growth kinetics. For example, it was expected that the static recrystallization rate might increase with increasing thickness reduction because of the increase of the stored energy at higher thickness reduction [26]. However, this is true only when the DRX did not occur during deformation processing [21].…”