Abstract
Biodegradable and absorbable orthopedic implants support bone regeneration, in case of fracture or disorder, without a necessary removal operation. Magnesium has unique mechanical advantages compared to polymers, and better biological response than conventional non degradable metals such as titanium and steel. Magnesium wrought alloys are processed by extrusion or rolling into pins and plates. For new potential applications from textiles, scaffolding such as wire meshes, bone cages and stents, fine wires with a wide range of customizability and diameters down to 50 μm are needed. Efficient process routes for cold drawn magnesium (Mg) wires with the alloying elements silver (Ag) and gadolinium (Gd) are investigated. Binary Mg alloys with 0.5, 0.9 and 1.4at.% (2, 4 and 6wt.%) Ag and 0.3 and 0.7at.% (2 and 5wt.%) Gd and ternary 0.5at.% Ag and 0.1at.% Gd (2wt.% Ag and 1wt.% Gd) were used for an alloy benchmark. The experimentally measured drawing stresses in the range of 105 to 120 MPa are in agreement with the wire drawing model of Geleji. Geleji’s model allows the quantification of experimentally limited process parameters such as the strain per drawing step, friction and the opening angle of the drawing dies. Under suitable process conditions, all alloy compositions can be drawn to more than 0.6 strain, before heat treatment is necessary. The electrical resistivity correlates with multiple microstructural features and can be examined nondestructively by 4 wire sensing. The resistivity increase is alloydependent and 6.61 (0.34) nΩm/at.% Ag and 90.85 nΩm/at.% Gd in solid solution. Furthermore, it is linearly dependent on the strain hardening state of around 9.22 (0.64) nΩm per unit strain for all binary alloy systems. Dislocation density has the greatest influence, accounting for over 97% of the increase, ahead of crystal orientation change, which was investigated by synchrotron diffraction, grain boundary density, and vacancy concentration. A dislocation resistivity of 9.7 (1.0)·10−25 Ωm3 can be reported experimentally for magnesium (Mg), while calculations by the Friedel model suggested about 20% of the found values. Precisely timed heat treatment up to recrystallization makes high ductility and hardenability of Mg wires possible. The degree of recrystallization is monitored by the decreasing resistivity and short range ordering mechanisms can be registered. Recrystallized microstructures with finest grains around 15 μm are found for Mg0.5Ag after 20 to 30 s and in all Gd containing alloys after 3 min. The proposed 4 wire resistance setup allows the determination of the in situ degradation rate of wires in PBS and DMEM within the first two days. Under physiological conditions, recrystallized Mg0.7Gd and Mg0.5Ag0.1Gd are the most stable and promising for application.
