Through-thickness gradient microstructure and its effect on mechanical properties and corrosion behavior of a water-quenched AZ31 Mg alloy extruded at different temperature

Balancing mechanical properties and corrosion resistance in wrought Mg alloys remains a challenge, since microstructural heterogeneities that enhance strength and ductility often increase electrochemical heterogeneity. Gradient microstructures induced by thermomechanical processing provide a promising strategy for performance tailoring, yet their influence on corrosion behavior remains poorly understood. Here, the gradient-structured AZ31 Mg alloy was fabricated via low-ratio extrusion at 150 °C, 250 °C, and 350 °C. The gradient microstructure evolution and its effects on mechanical and corrosion properties were systematically investigated. All alloys developed a through-thickness gradient structure with a fine-grained surface layer, a transition region, and a coarse-grained or unrecrystallized core, yielding a U-shaped hardness profile across the thickness. Increasing extrusion temperature promoted recrystallization and weakened gradient structure. Mechanical and corrosion properties exhibited a clear trade-off with increasing extrusion temperature. The E150 sample (strongest gradient) exhibited the highest yield strength (225.5 MPa) but lowest ductility (7.0%) and poorest corrosion resistance, whereas the E350 sample (weakest gradient) showed reduced grain boundary strengthening, giving the highest elongation (15.9%) but lowest yield strength (155.6 MPa) and best corrosion resistance. The E250 sample (moderate gradient) achieved a favorable balance among yield strength (188.2 MPa), ultimate tensile strength (256.8 MPa), ductility (9.8%) and intermediate corrosion resistance. These comprehensive performances were governed by the synergistic effects of grain size, recrystallization fraction, dislocation density and texture gradients, rather than any single factor. This study reveals that tuning extrusion temperature effectively tailors through-thickness gradient microstructures, providing a pathway for optimizing the mechanical-corrosion balance in extruded Mg alloys.