バイオテクノロジーとバイオマテリアル

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Biomaterials 2019: Study on mechanical properties and corrosion resistance of Mg-Zn-Nd alloys as potential biodegradable implants- O. Hakimi- SCE

O. Hakimi

Mg alloys are considered as potential structural materials for biodegradable implants mainly due to their excellent biocompatibility, degradation behavior in in vivo conditions and adequate mechanical properties. However, their accelerated corrosion rate in physiological environments may lead to premature loss of mechanical integrity and cytotoxic effects. Here we characterized the corrosion behavior and mechanical properties of a novel magnesium-zinc alloy, Mg-5%Zn-0.13%Y-0.35%Zr with up to 3% Nd additions following a homogenizing treatment and extrusion process, with regards to serving as a biodegradable implant.

The microstructural characteristics were examined by optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction analysis. The corrosion performance examination was carried out under in in vitro conditions, including immersion testing, electrochemical analysis, and stress corrosion cracking (SCC) assessments in terms of slow strain rate testing (SSRT), all in PBS solution. The mechanical evaluations included hardness and tensile examinations.

The obtained results clearly demonstrated an optimal combination of strength and ductility for the new alloy at 2% Nd concentration. This was attributed to an optimal concentration of the secondary phase, W-phase (Mg3(Nd,Y)2Zn3), generated at grain boundaries.

The addition of different concentrations of Nd to the base alloy, resulted in minor effect on the corrosion resistance, nevertheless, the calculated corrosion resistance of all tested alloys was within the range which can be considered as suitable for biodegradable applications. Therefore, it is believed that the new alloy at 2% Nd concentration, Mg-5Zn-2Nd-0.13Y-0.35Zr, can be considered as a potential candidate for biodegradable implants.

Discussions: In this study, we demonstrate the effect of Nd additions on the mechanical properties and corrosion performance of a novel base alloy, Mg-5%Zn-0.13%Y-0.35%Zr, following a homogenizing treatment and extrusion process. We found that the base alloy with 2%wt Nd yields the optimal combination of strength (YS = 340 MPa, UTS = 360 MPa) and ductility (16% elongation). This can be explained by the different microstructure introduced into the various alloys.

The presence of a single α-Mg phase in the Mg-5%Zn alloy, as demonstrated by SEM and XRD analysis, suggests that Zn mainly exits as a soluble element in the α-Mg solid solution phase. When 1%Nd is added, the W-phase forms at the grain boundaries, which increases the strength of the alloy. Additions of 2–3%wt Nd further increase the strength, which may be related to differences in size and morphology of the W and T-phase. Based on XRD results, the lattice parameter of the W-phase was gradually increased relative to the value previously reported for the Mg-Zn-Y material system. This may be due to a substitution of Y with Nd in the W-phase, which introduces the larger Nd atomic radius. These results are in accordance with a previous microstructural investigation of the Mg-Zn-Nd material system.

The relationship between the strength behavior of Mg-Zn-Nd alloys, the volume fraction and arrangement of secondary phases was made apparent by the fractography analysis. We believe that the fracture of these Mg alloys demonstrates a competition between two distinct mechanisms: brittle cleavage and ductile dimples rupture. The fracture of the base alloy exhibits mainly brittle cleavage fracture with minor dimples rupture, which indicates its limited potential for plastic deformation. The precipitated particles have the common effect of increasing the yield and tensile strength as well as increasing material embrittlement due to nucleation along grain boundaries. However, small and round particles may enhance ductility of the material by initiating dimples that grow by plastic deformation. This potential is revealed by the addition of 1%wt Nd to the base alloy. Notably, the addition of 2%wt Nd to the base alloy significantly reduces the cleavage planes while enhancing dimple rupture, as observed by SEM fractography Thus, high values of strength (YS and UTS) are obtained due to a strengthening effect that concurrently improves elongation. This effect may be due to a “particle induced plasticity”. An increase of Nd content to 3%wt stimulates denser and smaller dimples covering the fracture surface, as observed. The growth and expansion of the dimples are limited due to their large density. Thus, the measured values of UTS and YS were retained while the ductility was slightly decreased compared to the 2%wt Nd alloy.

The additions of up to 3%wt Nd does not have any significant effect on corrosion performance in terms of immersion test, EIS analysis, potentiodynamic polarization measurements and SSRT examination. This indicates that Nd is less effective as a corrosion protective through precipitation in W and T phases compared to its effect through a dissolved element in α-Mg solid solution. However, the addition of Nd exerts a substantial effect on strength, increasing the YS by ~24% and the UTS by 12%. This beneficial effect increases the specific strength (strength/density) of the base alloy and hence reduces the amount of material required to produce a structural implant. Reducing the Mg implant’s weight will consequently reduce the amount of hydrogen formation, which is considered as a significant drawback for Mg to serve as an implant material.