Ensuring the in vitro degradation reproducibility of powder metallurgy processed Mg 0.6Ca system
Magnesium degradation is a complex phenomenon that is too difficult to be described by a single influential parameter. Magnesium degradation is often influenced by either overtaking or overlapping factors like the cell culture medium composition, physiological conditions, impurities, and material’s internal microstructure, etc. This poses a challenge in obtaining the reproducible degradation results. Hence, in the present work, microstructural features like porosity and grain size distributions in powder metallurgy (PM) Mg-0.6Ca system were discretely evaluated for their roles in altering the specimen in vitro degradation rates. Importance was also given to the specimen impurity and mechanical properties. Based on the results, the limitations in PM processing conditions towards obtaining robust degradation results or, in other words, the material parameter thresholds to be realized for obtaining reproducible degradation profiles in PM Mg-0.6Ca specimens were put forth. Additionally, using literature evidence, the mechanisms governing pore closure and grain growth during liquid phase sintering of Mg-0.6Ca specimens from the PM processing perspective were determined. PM Mg-0.6Ca specimens were fabricated via powder blending of pure magnesium and master alloy Mg-10Ca powders. Specimens of seven different porosities, from 3% to 21%, were produced by varying sintering temperatures. Specimens with heterogeneous grain size distributions were obtained by surface modification of pure magnesium powders by means of a mechanical sieving treatment. Degradation profiles were analyzed in vitro using a semi static immersion test for 16 days under physiological conditions of 37 °C, 20% O2, 5% CO2, 95% relative humidity. Dulbecco’s modified Eagle’s medium was used as cell culture medium with Glutamax and 10% fetal bovine serum as supplements. Mechanical properties were determined using micro tensile specimens. The results indicate that low mean degradation rates (MDR < 0.3 mm/year) with homogeneous and reproducible degradation are obtainable in PM Mg-0.6Ca specimens up to 10% porosity. The specimen porosity threshold to ensure degradation reproducibility is 12%, however, at the expense of an increased MDR (1.6 mm/year). μCT characterized 12% as the transition porosity, where the specimen surface open pore fraction drops from > 95% to ≤ 45% when falling below this value. Similarly, the pore interconnectivity sharply drops from > 95% to < 10% at this porosity, thereby enhancing the degradation reproducibility. From PM processing perspective, the sintering temperature of 570 °C is proven as beneficial to promote liquid fractions high enough to enhance specimen sinter density. The present work also showed that heterogeneous grain growth is prompted by the reduced oxide pinning effect at the grain boundaries during sintering of PM Mg-0.6Ca specimens. The heterogeneous grain growth additionally induced the formation of eutectic lamellar structure α-Mg + Mg2Ca at certain grain boundaries throughout the microstructure, which is otherwise not evident in specimens with a homogeneous grain size. Based on the literature and results of the present work, it is postulated that this eutectic structure is the major reason for a non-reproducible degradation in PM Mg-0.6Ca specimens possessing a heterogeneous grain structure. Though mechanical properties are not majorly affected, it is recommended that heterogeneous grain growth is to be avoided in PM Mg-0.6Ca specimens. The presented results also implicitly conveyed the flexibility of PM as a viable technique to design Mg-Ca materials with tailor made degradation and mechanical strengths.
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