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

Abstract

The static recrystallization/grain growth kinetics of the AZ31 (Mg-3Al-1Zn, wt%) alloy were investigated employing Vickers microhardness and electron backscatter diffraction (EBSD) measurements. The AZ31 alloy was subject to a hot-rolling for 70% of thickness reduction and then annealed at various temperatures (150°C, 250°C, and 350°C) from 5 min to 24 h. First, the static recrystallization kinetics were analysed by means of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The results showed that the recrystallization occurred under two regimes both involving their own Avrami exponent/ activation energy. In regime I, the Avrami exponent was found in the range of 1.5-0.35 depending on the annealing temperature and activation energy of 74.1±5.7 kJ×mol-1. In regime II, an identical value of Avrami exponent was found (0.1-0.2) and a very low activation energy of 14.8±0.7 kJ×mol-1 was found for all annealing conditions. Non-random nucleation sites such as shear bands were considered as the main factor responsible for the deviation from the JMAK model. Moreover, the grain growth kinetics was well fitted by the general equation where . Accordingly, Qg = 109± 0.2 kJ×mol-1 which is median between grain boundary diffusion and bulk diffusion values for Mg and its alloys. The derived activation energies were discussed in terms of influencing factors such as solute drag, formation of basal texture, and microstructural heterogeneities like shear bands and twinning.

Grain growth activation energy of ECAP-processed Mg-Ce alloy: Effect of deformation temperature

“…The activation energy value (111.4 ±5.7 kJ/mol) of the sample ECAP-processed at 300 °C is slightly higher than the activation energy for grain boundary diffusion in pure Mg (92 kJ/mol) [30]. A similar activation energy, Q = 109.2 kJ/mol, was found for a hot-rolled AZ31 alloy at the temperature range of 150-350 °C [19]. It is well known that the activation energy for grain boundary diffusion is higher in alloys containing stable second particles than in pure metals [7].…”

“…Theoretically, n = 2 for metals without defects and therefore a parabolic growth law is assumed [24]. Practically, the n value has been reported in a range from 2 to 15 in Mg-based alloys under various deformation and recrystallization conditions [19,[25][26][27][28][29]. For example, it was found previously that n = 8 for hot-rolled and annealed AZ31 alloy and it was attributed to the effect of microstructure heterogeneities such as presence of second phases, dislocation network and crystallographic texture on the static recrystallization and grain growth.…”

“…For example, it was found previously that n = 8 for hot-rolled and annealed AZ31 alloy and it was attributed to the effect of microstructure heterogeneities such as presence of second phases, dislocation network and crystallographic texture on the static recrystallization and grain growth. Based on the previous study [19], the n for the present investigation is chosen equal to 8.…”

“…Therefore, the recrystallization and grain growth kinetics must be inspected to control the mechanical properties of Mg-based alloys. The recrystallization and grain growth kinetics are typically evaluated by calculating the activation energy which controls the extent of the resistance to any grain growth [19]. So far, limited studies can be found in the literature dealing with the evolution of microstructure and the corresponding mechanical properties of ECAP-processed Mg alloys containing RE elements (Mg-RE alloys) after annealing treatments [16,20].…”

Primary recrystallization of a magnesium hybrid material fabricated by high-pressure torsion

“…The AZ31 and Mg-Gd alloys exhibit a retained deformation texture during recrystallization and grain growth phenomena [22,35,37,38]. The present results show that the recrystallization textures of the AZ31 and Mg-0.6Gd regions are different from the deformation texture [9] and also they can be modified with appropriate annealing treatments.…”