The dynamic microstructure evolution of Mg-3Sn-2Al-1Zn-5Li magnesium alloy during hot deformation is studied by hot compression tests over the temperature range of 200–350 °C under the strain rate of 0.001–1 s−1, whereafter constitutive equations and processing maps are developed and constructed. In most of cases, the material shows typical dynamic recrystallization (DRX) features, with a signal peak value followed by a gradual decrease. The value of Q (deformation activation energy) is 138.89414 kJ/mol, and the instability domains occur at high strain rate but the stability domains are opposite, and the optimum hot working parameter for the studied alloy is determined to be 350 °C/0.001 s−1 according to the processing maps. Within 200–350 °C, the operating mechanism of dynamic recrystallization (DRX) of Mg-3Sn-2Al-1Zn-5Li alloy during hot deformation is mainly affected by strain rate. Dynamic recrystallization (DRX) structures are observed from the samples at 300 °C/0.001 s−1 and 350 °C/0.001 s−1, which consist of continuous DRX (CDRX) and discontinuous DRX (DDRX). However, dynamic recovery (DRV) still dominates the softening mechanism. At the grain boundaries, mass dislocation pile-ups and dislocation tangle provide sites for potential nucleation. Meanwhile, flow localization bands are observed from the samples at 200 °C/1 s−1 and 250 °C/0.1 s−1 as the main instability mechanism.

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In this work, the Mg-3Sn-2Al-1Zn (TAZ321, wt. %) alloy with excellent high temperature resistance was compressed using a Gleeble-3500 thermo-mechanical simulator at a wide temperature and the strain rate range. The kinetics analyses showed that the dominant deformation mechanism was likely caused by the cross slipping of dislocations. A constitutive equation which expressed the relationship between the flow stress, deformation temperature, and strain rate was established, and the average activation energy Q was calculated to be 172.1 kJ/mol. In order to delineate the stability and instability working domains, as well as obtain the optimum hot working parameters of the alloy, the hot processing maps in accordance with Prassad’s criterion are constructed at the true strain of 0.2, 0.4, 0.6, and 0.8, respectively. Based on the hot processing map and microstructure observation, the optimum hot working parameter was determined to be 350 °C/1 s−1. The continuous fine dynamic recrystallization (CDRX) grains occurred in the optimum deformation zone. The predicted instability domains was identified as T = 200–300 °C, ε ˙ = 10−2–1 s−1, which corresponded to the microstructure of deformation twinning and micro cracks at the intersection of grain boundaries.

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In this study, the grain boundary character distribution (GBCD) of the γ-TiAl-based alloy was modified through the thermomechanical processing route. A compression test was conducted to identify the mechanical property improvements based on the grain boundary engineering (GBE) treatment. After multidirectional isothermal forging (MDIF) and annealing, the average grain size was refined to 20 µm. The length ratio of the low Σ (1 ≤ Σ ≤ 29) coincidence site lattice (CSL) boundaries increased to 64.01%, with the alloy exhibiting a higher compressive strength (2913 MPa) and maximum compression ratio (45.3%), owing to the strengthening mechanism of grain boundaries and dislocations. Consequently, the combination of MDIF and annealing could be used for grain refinement and tuning GBCD of γ-TiAl-based alloys.