Development of functionally graded AZ31B–Y₂O₃ composites via novel friction stir processing: comparison with conventional FSP

Abstract

Friction stir processing (FSP) with yttrium oxide (Y₂O₃) as the reinforcement was employed to fabricate a functionally graded material (FGM) of AZ31B magnesium alloy. To fabricate FGM, three different routes were used: (1) single-pass FSP with reinforcement, (2) double-pass FSP with reinforcement introduced during the first pass only, and (3) double-pass FSP with reinforcement added during both passes. All the FSP samples had much higher microhardness and corrosion resistance than the untreated alloy. The third route produced a uniform dispersion of Y₂O₃ particles that refined grains, disrupted β-Mg₁₇Al₁₂ networks, and significantly increased surface nobility. Scanning Kelvin probe force microscopy (SKPFM) confirmed a higher Volta potential across the FGM surface, indicating a reduced micro-galvanic driving force compared to the base alloy. As a result, the FGM exhibited the highest microhardness (142.4 HV) and substantial reductions in corrosion rate under both electrochemical (1.44 mm/year) and cyclic atmospheric (1.799 mm/year) conditions. In contrast, double-pass FSP specimen was significantly affected by particle clustering, increased heat input, and β-phase coarsening. Localized corrosion resistance and increased surface nobility of the FGM demonstrate that the controlled embedding of particles and microstructural grading are the two major factors in enhancing durability and performance in AZ31B magnesium alloys simultaneously.