Nanoscale characterization of biodegradable, lean magnesium alloys: a detailed study of their microstructure–property correlations
Open access
Autor(in)
Datum
2019Typ
- Doctoral Thesis
ETH Bibliographie
yes
Altmetrics
Abstract
Micro- and nanoscale intermetallic phases play a critical role for the properties of magnesium (Mg) alloys because they may initiate and control the local corrosion processes via microgalvanic coupling and govern the mechanical properties via precipitation hardening or their active role in grain refinement. This thesis aims at establishing a thorough mechanistic understanding of the role of the micro- and nanostructure, specifically that of intermetallic phases, for the properties of Mg-based lean alloys with an alloying-element content below 1 at.% of two alloying systems: the Mg–Zn–Ca (ZX) system, with a strong focus on the correlations of the alloys’ microstructure and their corrosion susceptibility, and the Mg–Al–Ca–Mn (AXM) system for its correlations of microstructure and mechanical performance. In both cases analytical transmission electron microscopy (TEM), which allows simultaneous access to morphological, chemical and structural information, was utilized to characterize the chemical distribution of the alloying elements and the type of intermetallic phases, which provides the basis for their correlation with the alloys’ properties. Biodegradable ZX-lean alloys have the potential to change as temporary implants the way how cardiovascular and muscoskeletal conditions are treated. Their general suitability for biomedical applications has been demonstrated, but so far the material characterization mainly focused on the macroscopic length scale with little insights into the microstructure-induced corrosion mechanisms at play. However, their in-depth understanding is crucial for a tight control over the degradation behavior in physiological environment and thus critical to a successful clinical outcome. A quasi-in situ TEM method was developed, which allows for the local investigation of the same area before and after corrosion attack at its very early stages. It was combined with ex situ cross-sectional TEM analysis of the corrosion products that formed as a well-developed surface layer after prolonged immersion in simulated body fluid. Both methods applied in concert allowed accessing the alloy dissolution on the nanoscale from seconds to hours. Nanometric intermetallic particles (IMPs) composed of Mg2Ca were found to dissolve preferentially and rapidly after a few seconds of immersion. In contrast, those composed of a ternary IM1-phase were observed to act as local cathodes, which facilitated the hydrogen-reduction reaction on their surface and accelerated Mg matrix dissolution in their vicinity. High-resolution TEM analysis of the IM1-phase IMPs revealed that electrochemically active Ca and Mg dissolve preferentially, whereas Zn is cathodically protected and gradually enriches, leading to dealloying of the IMPs and their gradual ennoblement. This mechanism of cathodically polarized dealloying was documented for the first time to occur in Mg alloys and is believed to apply for other intermetallic phases and other active alloying systems. The role of nanometric IMPs in the macroscopic degradation behavior of ZX-lean alloys (at a scale relevant to implants, i.e. surface areas of the order of mm2 to cm2) was investigated in vitro by means of electrochemical methods and complemented by TEM analysis. To this end, two model alloys were used that differed solely in their IMP type, hosting exclusively either Mg2Ca- or IM1-type IMPs. A significant shift of the corrosion potential Ecorr towards higher values and an increase of the cathodic reaction rates upon polarization marked the impact of cathodically active IM1-type IMPs on the macroscopic electrochemical response. Despite the nanometric size of the IMPs, their phase type was found to govern the rate of degradation on a macroscopic scale, determined from hydrogen evolution, and charge and current transients. The degradation rate was higher when the IMPs were composed of the IM1 phase compared to the Mg2Ca phase. The in vitro observations were confirmed by in vivo investigations upon implantation of the two ZX-model alloys with tailored IMP types into femoral shafts of rats. Micro-computed tomography and histological analysis showed that both alloys are well tolerated by the surrounding tissue with early implant–bone contact and no signs of excessive hydrogen-gas release or inflammation, and are thus rated suitable for their use as temporary bone-implant materials. Irrespective of the IMP type, both ZX alloys showed a dynamically increasing cathodic activity with progressing dissolution. High-resolution analytical TEM analysis revealed nanometric Zn clusters, which likely formed via noble-element redeposition at the interface of the Mg metal and the corrosion-product-layer. These facilitate as additional nano-cathodes the gradual increase of the alloys’ cathodic activity. In light of the mechanical properties of ZX-lean alloys, the IMPs were found to play a crucial role as effective obstacles to grain-boundary motion in ensuring a fine-grained microstructure. In fact, grain-boundary strengthening was identified to be the far dominating hardening mechanism in ZX-lean alloys, with a Hall–Petch constant determined at 255 MPa μm½. In contrast, the contribution of IMPs to the alloys’ strength by precipitation hardening is negligibly small, 3 MPa at maximum, owing to their insufficient number density. Motivated by a lack of hardenability in ZX-lean alloys, an alternative alloy was developed based on the AXM system, also lean in composition, for an intended application in the transportation sector, which imposes a high demand on easy processability and low costs. The alloy-design concept was assisted by thermodynamic calculations and directed towards a microstructure that contains an Al– Mn-pinning phase and an Al–Ca-hardening phase for grain-growth control and age-hardenability, respectively. The optimized alloy Mg–Al0.6–Ca0.28–Mn0.25 (in wt.%) (AXM100) shows a remarkable age-hardening response of 100 MPa in the T6 peak-aged condition, corresponding to an increase of ~62%, thus generating a tensile yield strength of 253 MPa. A multiscale microstructural analysis combining light microscopy, TEM, and atom probe tomography, related the superior hardening to the precipitation of nanometric and monolayered Guinier–Preston zones composed of Al and Ca. Grain growth during high-temperature treatments was successfully retarded by thermally stable Al–Mn dispersoids, the composition of which was determined to be β-Mn. However, the grain size still needs to be reduced overall to make AXM100 a competitive Mg alloy for commercial deployment. In sum, the identification of the microstructural contributors to the macroscopic alloy properties in ZX- and AXM-lean alloys presented in this thesis has generated a detailed understanding of the corresponding microstructure–property correlations in these alloys. This sets the premises for tailoring and further optimizing their degradation behavior and mechanical performance. Mehr anzeigen
Persistenter Link
https://doi.org/10.3929/ethz-b-000408846Publikationsstatus
publishedExterne Links
Printexemplar via ETH-Bibliothek suchen
Verlag
ETH ZurichThema
Biomaterial; Magnesium alloy; Biocorrosion; Transmission electron microscopy; Material characterization; Microstructure; Biodegradation; Medical materials; ImplantOrganisationseinheit
03661 - Löffler, Jörg F. / Löffler, Jörg F.
Förderung
157058 - Biodegradable Mg alloys with optimized properties via microstructural design (SNF)
Zugehörige Publikationen und Daten
Is source of: http://hdl.handle.net/20.500.11850/372726
Is source of: http://hdl.handle.net/20.500.11850/381785
Is source of: http://hdl.handle.net/20.500.11850/281743
ETH Bibliographie
yes
Altmetrics