|Structural and corrosion characterization of biodegradable Mg-Zn alloy castings|
KUBASEK, J., VOJTĚCH, D., POSPÍŠILOVÁ, I.
vol. 50 (2012), no. 6, pp. 415 - 424
As-cast Mg, Mg-1Zn, Mg-4Zn and Mg-6Zn (wt.%) alloys were studied in this work. The alloys were prepared by gravity casting into a metal mold. The average cooling rate was 500 K min-1. Structural analysis revealed the presence of zinc concentration gradients and a strong zinc super-saturation in the α-Mg phase. In the Mg-4Zn and Mg-6Zn alloys, an inter-dendritic MgZn phase was present. The alloys’ Vickers hardness increased with increasing Zn-concentration from 25 HV10 (pure Mg) to 45 HV10 (Mg-6Zn). The corrosion rates were measured by exposure to simulated body fluid. The Mg-1Zn alloy showed the lowest corrosion rate of 0.08 mm/yr, which was significantly lower than the corrosion rates of pure Mg and AZ91HP alloys (0.5 and 0.6 mm/yr, respectively). Increasing the Zn concentration in Mg alloys increased the corrosion rate, and the as-cast Mg-6Zn alloy corroded quickly, at a rate of 3 mm/yr. The alloys were solution heat treated at 300 °C for 150 h. Structural analysis revealed a partial dissolution of the MgZn phase, precipitation of the MgZn phase in regions that were originally supersaturated with zinc, and homogenization of the α-Mg phase during the heat treatment process. The observed structural changes increased the corrosion resistance, particularly that of the Mg-6Zn alloy whose corrosion rate was reduced four-fold, to 0.8 mm/yr. The measured corrosion rates were related to the observed structural characteristics of the alloy materials.
magnesium alloys, biomaterials, corrosion, casting
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