Mechanical and thermal properties and corrosion behaviour of heat-treated Mg–Y–Nd–Ag alloys

Abstract

The magnesium alloys are promising candidates for biodegradable medical implants which reduce the necessity of second surgery to remove the implants. Yttrium in solid solution is an attractive alloying element because it improves mechanical properties and exhibits suitable corrosion properties. Silver was shown to have an antibacterial effect. The effect of Y and Nd solutes on the corrosion, mechanical and thermal properties of Mg–4Y–2Nd–1Ag (in mass%) and Mg–2Y–1Nd–1Ag (in mass%) alloys prepared by gravity casting has been investigated. The alloys were isothermally annealed at two temperatures: 500 °C/24 h and 525 °C/24 h. Microhardness (HV 0.1) together with differential scanning calorimetry measurements were compared to microstructure development that was observed by transmission and scanning electron microscopy. Corrosion behaviour was studied by using electrochemical measurements and hydrogen evolution test. In the Mg–4Y–2Nd–1Ag alloy annealed at 500 °C/24 h, one exothermic process was observed; in the Mg–4Y–2Nd–1Ag alloy annealed at 525 °C/24 h, two exothermic processes were observed. The activation energies of these processes were calculated by Kissinger method as ~ 140 kJ mol⁻¹ for the alloy annealed at 500 °C/24 h and ~ 115 kJ mol⁻¹ and ~ 120 kJ mol⁻¹ for the alloy annealed at 525 °C/24 h. No thermal processes were observed in the Mg–2Y–1Nd–1Ag alloys. The results showed that the alloys with the lower amount of Y and Nd solutes exhibit considerably higher corrosion resistance. Unlike in the Mg–2Y–1Nd–1Ag alloy, the corrosion rate of the Mg–4Y–2Nd–1Ag in physiological environment increases with the time of immersion. It was found that isothermal heat treatments lead to an improvement of corrosion properties in both studied alloys but affect microhardness only moderately.