Abstract
Magnesium alloys are contemporary candidates for many structural applications of which medical applications, such as bioresorbable implants, are of significant interest to the community and a challenge to materials scientists. The generally poor resistance of magnesium alloys to environmentally assisted fracture, resulting, in particular, in faster-than-desired bio-corrosion degradation in body fluids, strongly impedes their broad uptake in clinical practice. Since temporary structures implanted to support osteosynthesis or healing tissues may experience variable loading, the resistance to bio-corrosion fatigue is a critical issue that has yet to be understood in order to maintain the structural integrity and to prevent the premature failure of implants. In the present communication, we address several aspects of the corrosion fatigue behaviour of magnesium alloys, using the popular commercial ZK60 Mg-Zn-Zr alloy as a representative example. Specifically, the effects of the testing frequency, surface roughness and metallic coatings are discussed in conjunction with the fatigue fractography after the testing of miniature specimens in air and simulated body fluid. It is demonstrated that accelerated environmentally assisted degradation under cyclic loading occurs due to a complicated interplay between corrosion damage, stress corrosion cracking and cyclic loads. The occurrence of corrosion fatigue in Mg alloys is exaggerated by the significant sensitivity to the testing frequency. The fatigue life or strength reduced remarkably with a decrease in the test frequency.
Highlights
Osteosynthesis is one of the most common techniques in traumatology and orthopaedic surgery
The elastic moduli of Mg-based alloys (40–45 GPa) are better compared to the stiffness of cortical bones (3–20 GPa) than those of conventional metallic materials used as permanent implants (c.f., ~200 GPa for stainless steel, ~230 GPa for cobalt-based alloys, and ~115 GPa for titanium alloys [10]), which helps to eliminate the stress-shielding effect hindering the healing process
As it is commonly seen in the transverse sections of the ex-extruded ZK60 alloys (Figure 1), the microstructure is not uniform due to incomplete dynamic recrystallisation occuorcrcinurgridnugrdinugrinhgotheoxtterxutsruiosnio.nS.oSmome ceocaorasreseuunnrereccrryyssttaalllliisseedd ggrraaiinnssaarreessuurrrorouundneddedbyby fine eqfiunieaxeeqduiarexcerdyrsetcarlylissteadllisgerdaignrsa,inrse,prreepsreensetnintignga atytyppicicaalleexxttrruussiioonnmmicicrorostsrturcutcutruer. eF.roFmrom the EBthSeDEBgSraDingrmainapmanpdantdhethceocrorrersepspoonnddiinngg diissttrriibbuutitoionnofogfrgairnasinizessizsehsowshnoiwn FniginurFei1gau,cr,e 1a,c, rersepsepcetcitviveelyly,tthhe meeaannggrraaininsiszieze(e(degdeggergairnasinesxcelxucsilvues)ivisea)bisouatb3o.u9 tμ3m.9, aμnmd,thaendstathndeasrtdandard dedveivaitaitoionnisis22.2.2μμmm
Summary
Osteosynthesis is one of the most common techniques in traumatology and orthopaedic surgery. Since the main purpose of implants is to provide the necessary mechanical support only during the bone regeneration process, biodegradable metals shift the existing paradigm in orthopaedy and traumatology in the direction of temporary structures, including bone implants and cardiovascular stents [4,5,6,7]. Do Mg alloys exhibit excellent biocompatibility and natural biodegradability, they exhibit low density (1.7–2.0 g/cm3) and strength in excess of 250 MPa. Notably, the elastic moduli of Mg-based alloys (40–45 GPa) are better compared to the stiffness of cortical bones (3–20 GPa) than those of conventional metallic materials used as permanent implants (c.f., ~200 GPa for stainless steel, ~230 GPa for cobalt-based alloys, and ~115 GPa for titanium alloys [10]), which helps to eliminate the stress-shielding effect hindering the healing process
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