Structural and corrosion characterization of biodegradable Mg-RE (RE=Gd, Y, Nd) alloys

Binary Mg-Gd (up to 5% Gd in mass fraction), Mg-Nd (up to 9% Nd in mass fraction) and ternary Mg-Gd-Y (up to 5% Gd, 1% Y) alloys with precisely determined contents of cathodic impurities (Fe, Ni, Cu, Co) were studied. The alloys were studied in the as-cast state (cooling rate of 500 K/min) and after solution heat treatment (T4). Structures were investigated by optical and scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction and glow discharge spectrometry. Structural investigation was completed by Vickers hardness measurements. Corrosion behavior in the simulated physiological solution (9 g/L NaCl) was assessed by immersion tests and potentiodynamic measurements. It was found that the structures of the as-cast alloys were dominated by fine α-Mg dendrites and eutectic Mg-RE phases. The dendrites exhibited RE-concentration gradients which were most pronounced in the Mg-Gd alloys. For this reason, the T4 heat treatment of the Mg-Gd alloy led to the formation of a new cuboidal Mg5Gd phase. The corrosion resistance was significantly improved by Gd. The effect of Nd was weak and the addition of Y to Mg-Gd alloys had harmful effect on the corrosion resistance. The T4 heat treatment strongly accelerated the corrosion of Mg-Gd alloys. Its effect on the corrosion of Mg-Nd alloys was not significant. The observed corrosion behavior of the alloys was discussed in relation to their structural states and contents of cathodic impurities.