Abstract
The amount of intermetallic phases largely influences corrosion resistance of degradable magnesium alloys. In order to investigate their effects on the degradability, alloying elements Nd (low solubility in Mg) and Gd (high solubility in Mg) were selected, which were reported to have good cytocompatibilities. Pure Mg and binary Mg-Nd (0.5, 2, 5 wt. %) alloys with different amount of intermetallics were prepared by hot extrusion. In binary Mg-Gd alloys, intermetallics with different percentages were achieved by heat treatments. The degradation behaviour was explored by immersion in DMEM + 10% FBS under cell culture conditions. In Mg-Nd alloys, the formed equilibrium intermetallic Mg41Nd5 phase distributes dispersive in matrix and its amount increases with increasing content of Nd. Micro-galvanic corrosion occurred between the intermetallic phase Mg41Nd5 and matrix. During degradation, the Mg matrix was preferentially dissolved. Contrary to this, the Mg41Nd5 phase was stable and embedded by degradation products, indicating the intermetallic Mg41Nd5 was not degradable in simulated body fluid. The degradation rate of Mg-Nd alloys in¬crea¬sed with increasing amount of intermetallics at the initial stage of corrosion. As extruded Mg-5Nd with maximum amount of intermetallics suffered from the most severe corrosion. However, after longer immersion durations (7 and 14 days), the degradation rate decreased, which was comparable to that of pure Mg. In this alloy with highest percentage of intermetallics, the micro-galvanic corrosion and formation of protective corrosion products came to the balance.In Mg-Gd alloys, the formed precipitate during T6 treatment is primarily a nano-scale metastable β' (Mg7Gd) phase. Its amount in the selected alloys ranges from 2.9% to 14.3%. The dispersed β' precipitates are gradually replaced by precipitate chains and grow into an interlaced network with a triangular arrangement with increasing aging time. When intermetallic amount was below 10.3%, corrosion accelerated with increasing amount of βʹ-Mg7Gd phase. The most severe degradation occurred in the alloy with 10.3% of intermetallic. When intermetallic amount of βʹ-Mg7Gd was above 10.3% and increased to a maximum of 14.3%, its degradation behavior was reversed and slowed down. In the alloy with the greatest amount of intermetallics, the distribution of a dense interlaced network with a triangular arrangement was observed for β' precipitates. It worked as a new proposed barrier to prevent corrosion from progressing deeper into the matrix.
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