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.