Texture evolution and tension–compression yield symmetry of AZ31 magnesium alloy during cyclic extrusion and compression: Experiments and polycrystal plasticity simulations

Abstract

Cyclic extrusion and compression (CEC) was selected as a representative variable-strain-path process for mitigating tension–compression yield asymmetry in extruded AZ31 magnesium alloy. A machine-learning-assisted VPSC-dDRX(CA) framework was developed to characterize the associated microstructural and texture evolution. Velocity-gradient histories for representative conditions were obtained using finite-element modeling (FEM), while a gradient-boosting tree model reconstructed histories for intermediate conditions not explicitly simulated by FEM. These inputs enabled predictions of deformation-mechanism activity, dynamic recrystallization (DRX), grain-size evolution, and texture development. A modified Hall–Petch relation then calculated tensile yield strength (TYS), compressive yield strength (CYS), and TYS/CYS. Under representative validation conditions, the maximum deviation between the predicted and experimental grain sizes was within 0.5 μm. Low extrusion ratios and limited cycles produced incomplete DRX, forming a deformation-grain-dominated structure (DGS) with mixed texture (MT). Intermediate ratios and appropriate cycles balanced DRX nucleation and grain-boundary migration, producing a fine recrystallized structure (FDRXS) with an average grain size of 5.7–5.8 μm. Basal 〈a〉 slip retained part of the fiber texture component during extrusion, whereas enhanced pyramidal 〈c+a〉 slip and {10-12} tensile twinning rotated the c-axes away from the extrusion direction during compression, promoting deviated annular texture (DAT). Higher ratios or additional cycles accelerated grain-boundary migration and the growth of recrystallized grains, resulting in an enlarged dynamically recrystallized structure (EDRXS) and reconcentration of annular components toward a compression-type texture (CT). The framework thus resolved the sequential DGS–MT, FDRXS–DAT, and EDRXS–CT transitions across the processing window and quantitatively connected them to the resulting yield response. Consequently, DGS–MT retained a tensile-yield advantage, with TYS of 142 ± 2 MPa; FDRXS–DAT combined grain-refinement strengthening and crystallographic coordination, yielding TYS/CYS of 1.00 ± 0.05; and EDRXS–CT exhibited predicted CYS of 153 ± 7 MPa. This multiscale strategy provides a mechanistic basis for coordinating microstructure, texture, and mechanical response under complex deformation paths.