Improving mechanical properties of lean Mg–Zn–Ca alloy for absorbable implants via Double Equal Channel Angular Pressing (D-ECAP)

Abstract

Mg alloys are suitable materials for biodegradable osteosynthesis implants which stabilize bone fractures as long as the fracture is healing but are absorbed by the human body after they have fulfilled this purpose. However, high degradation rates and/or low strength values can be seen as drawbacks of such alloys. The present study deals with a lean Mg–Zn–Ca alloy which exhibits a desired, low degradation rate and explores the possibilities to improve its mechanical properties by Severe Plastic Deformation (SPD) via Double Equal Channel Angular Pressing (D-ECAP). The effects of different D-ECAP process parameters on the microstructure, mechanical properties and degradation behavior are investigated in detail. The results show that grain refinement during D-ECAP occurs via dynamic recrystallization. Lowering the processing temperature leads to a reduction of the size of the smaller grains of the initial bimodal grain size distribution while a higher degree of deformation (higher number of D-ECAP passes) reduces almost only the area fraction of the larger grains. While the increase in hardness is mainly due to the small size of the recrystallized grains, the tensile strength is additionally determined by the remaining larger grains. Thus, the mechanical properties of the lean Mg–Zn–Ca alloy can be tailored by D-ECAP according to the requirements of the final application. For example, the tensile strength can be increased to more than 370 MPa while maintaining the elongation at fracture at 7%. The degradation rate is not affected by D-ECAP as the process does not change the number or size of intermetallic particles. The optimized mechanical properties, in combination with the low degradation rate, make the D-ECAP-processed alloy very suitable for biomedical applications as absorbable material for osteosynthesis implants.