Degradation behavior of as cast and powder metallurgy processed Mg-Ca alloysTuesday (09.05.2017) 18:24 - 18:27 Part of:
Magnesium alloys have gained importance as potential orthopedic implant materials mainly owing to their biodegradability and mechanical properties matching to that of bone. Various Mg alloys have been investigated over the recent years for biomedical applications. Mg-Ca alloys, in particular, are of interest as Ca is an essential element in human body and favors the formation of hydroxyapatite layer. However, the increased Mg2Ca secondary phase content with increasing Ca addition creates galvanic circuits between matrix and secondary phase, resulting in pitting corrosion in these alloys. Previous experiments on as cast Mg-Ca alloys showed promising results as potential orthopedic implant materials at ≤ 1wt.% Ca additions in Mg. Hence, different Mg-xCa alloys (x= 0.3, 0.6, 0.9 wt.%) were processed by permanent mould chill casting and powder metallurgy (PM) routes. Their ‘mean degradation depth, MDD (µm)’ values were measured by three day immersion test procedure. Physiological cell-culture medium (DMEM+ Glutamax) with protein content (10% FBS) and 1% Penicillin Streptomycin was employed as cell-culture medium. Grain size and porosity values of these alloys were measured by optical microscopy. The amount of secondary phase and its distribution was evaluated by SEM and EDX analysis in conjunction with AnalySIS Pro image analysis software. The immersion test results reveal that the amount of secondary phase is crucial in determining the degradation behavior of PM processed Mg-xCa alloys. However, in cast alloys, the counter acting effect of reduction in grain size with increase in distribution of secondary phase resulted in similar MDD values of 0.64-0.74 µm in these alloys. Of all the six alloys investigated, the PM processed Mg-0.3Ca alloy shows the highest degradation resistance with a low MDD value of 0.39 µm. Further on, a 6 week long term immersion test performed on this alloy revealed the ‘mean degradation rate, MDR (µm/day)’ as 0.51 µm/day.