Transforming detrimental inclusions into in-situ reinforcing phases: Microstructural evolution and strengthening mechanisms in thermo-mechanically processed recycled AZ31 alloys

The inevitable introduction of exogenous oxide inclusions typically severely degrades the mechanical properties of recycled magnesium alloys. In this study, we propose a novel thermomechanical strategy to transform these detrimental impurities into in-situ reinforcing phases. By incorporating 50 wt.% surface-contaminated AZ31B machining chips, we systematically investigated the microstructural evolution, crystallographic orientation, and underlying strengthening mechanisms during multi-pass hot rolling and subsequent annealing. The results indicate that prolonged chip immersion (7 days) introduces a high density of rigid MgO and C-rich inclusions. These act as potent heterogeneous nucleation sites during solidification but trigger severe macroscopic strain localization during hot rolling. This strain partitioning induces the formation of dense shear bands governed by continuous dynamic recrystallization (CDRX), resulting in a drastic ductility degradation, with the elongation plummeting to 5.3%. Crucially, the subsequent post-deformation annealing (400°C, 10 min) reverses this embrittlement through a selective microstructural transition. The exogenous inclusions exert an anisotropic Zener pinning effect, which severely restricts the boundary mobility of highly defective non-basal grains while granting a kinetic advantage to the basal matrix via oriented growth, ultimately culminating in an exceptionally sharp basal texture (Max MUD = 14.98). Thermodynamically preserved, un-recrystallized non-basal grains serve as discrete "soft domains" within the hard basal matrix. Upon tensile loading, the rheological mismatch between these domains activates a robust Hetero-Deformation Induced (HDI) strengthening mechanism. Consequently, the optimized recycled alloy (7-day immersion + annealing) achieves a superior strength-ductility synergy, exhibiting a yield strength of 192.8 MPa, an ultimate tensile strength of 273.3 MPa, and a recovered elongation of 15.0%, providing a viable mechanistic framework for high-performance magnesium alloy recycling.