Constitutive modeling for the dynamic recrystallization evolution of AZ80 magnesium alloy based on stress–strain data

In order to improve the understanding of the dynamic recrystallization (DRX) behaviors of as-cast AZ80 magnesium alloy, a series of isothermal upsetting experiments with height reduction 60% were performed at the temperatures of 523 K, 573 K, 623 K and 673 K, and the strain rates of 0.01 s−1, 0.1 s−1, 1 s−1 and 10 s−1 on a Gleeble 1500 thermo-mechanical simulator. Dependence of the flow stress on temperature and strain rate is described by means of the conventional hyperbolic sine equation. By regression analysis, the activation energy of DRX in the whole range of deformation temperature was determined to be Q = 215.82 kJ mol−1. Based on dσ/dɛ versus σ curves and their processing results, the flow stress curves for AZ80 magnesium alloy were evaluated that they have some characteristic points including the critical strain for DRX initiation (ɛc), the strain for peak stress (ɛp), and the strain for maximum softening rate (ɛ*), which means that the evolution of DRX can be expressed by the process variables. In order to characterize the evolution of DRX volume fraction, the modified Avrami type equation including ɛc and ɛ* as a function of the dimensionless parameter controlling the stored energy, Z/A, was evaluated and the effect of deformation conditions was described in detail. Finally, the theoretical prediction on the relationships between the DRX volume fractions and the deformation conditions were validated by the microstructure graphs.

► ɛc and ɛ* were described as the functions of Z/A, not constants. ► A large range of strain, strain rate and temperature were considered. ► The method of using flow curves has higher operability than optical observation. ► Deformation conditions corresponding to complete/incomplete DRX were predicted. ► Experiment data validate theoretical prediction.