Microstructural and corrosion rate evolution of Mg-Nd-Zr alloy during multiaxial forging processing

Abstract

Bioresorbable magnesium alloys represent a promising class of materials for temporary osteosynthesis implants; however, their clinical application is limited by excessively rapid degradation rates. This study investigates the effect of severe plastic deformation via multiaxial forging («abc»-pressing) on the microstructure, microhardness, and corrosion behaviour of an Mg-Nd-Zr system alloy (Nd–3.0 wt%, Zr–0.4 wt%, Zn–0.2 wt%) for biomedical applications. Deformation processing was performed at 350 °C with up to 9 cycles. Microstructural characterisation was conducted using optical microscopy and scanning electron microscopy with energy-dispersive spectroscopy. Corrosion behaviour was evaluated by potentiodynamic polarisation in 0.9% NaCl solution simulating physiological conditions. Multiaxial forging induced grain refinement from 56 ± 21 μm to 48 ± 18 μm after 5 deformation cycles, accompanied by transformation of the continuous intergranular Mg₁₂Nd β-phase network into uniformly distributed spheroidised particles. Decomposition of the supersaturated α−Mg solid solution resulted in Nd depletion of the matrix (from 2.2 to <0.6 wt%) and concurrent β-phase enrichment (up to 30.6 wt% Nd). Microhardness measurements revealed an increase in mean values across all specimen faces following deformation processing, with a significant reduction in data scatter, indicating improved structural homogeneity attributable to increased dislocation density and uniform distribution of secondary phases. Electrochemical testing demonstrated a 30% reduction in corrosion current density (from 347 to 242 μA/cm²) and corrosion rate (from 7.94 to 5.54 mm/year), with electrochemical data suggesting a tendency toward more uniform degradation, consistent with post-corrosion observations from the same alloy processed by an alternative SPD method. The improved corrosion resistance is attributed to reduced galvanic potential differences between the α−Mg matrix and β-phase precipitates, secondary-phase fragmentation, and modification of the grain boundary network. These findings demonstrate that multiaxial forging is an effective thermomechanical processing route for optimising the property profile of Mg-Nd-Zr alloys for bioresorbable implant applications.