Corrosion is normally an undesirable phenomenon in engineering applications. In the field of biomedical applications, however, implants that 'biocorrode' are of considerable interest. Deploying them ...not only abrogates the need for implant-removal surgery, but also circumvents the long-term negative effects of permanent implants. In this context magnesium is an attractive biodegradable material, but its corrosion is accompanied by hydrogen evolution, which is problematic in many biomedical applications. Whereas the degradation and thus the hydrogen evolution of crystalline Mg alloys can be altered only within a very limited range, Mg-based glasses offer extended solubility for alloying elements plus a homogeneous single-phase structure, both of which may alter corrosion behaviour significantly. Here we report on a distinct reduction in hydrogen evolution in Zn-rich MgZnCa glasses. Above a particular Zn-alloying threshold ( 28 at.%), a Zn- and oxygen-rich passivating layer forms on the alloy surface, which we explain by a model based on the calculated Pourbaix diagram of Zn in simulated body fluid. We document animal studies that confirm the great reduction in hydrogen evolution and reveal the same good tissue compatibility as seen for crystalline Mg implants. Thus, the glassy Mg60+xZn35−xCa5 (0≤x≤7) alloys show great potential for deployment in a new generation of biodegradable implants.
Via fast differential scanning calorimetry using an Au-based glass as an example, we show that metallic glasses should be classified into two types of amorphous/monolithic glass. The first type, ...termed self-doped glass (SDG), forms quenched-in nuclei or nucleation precursors upon cooling, whereas in the so-called chemically homogeneous glass (CHG) no quenched-in structures are found. For the Au-based glass investigated, the critical cooling and heating rates for the SDG are 500 K s
and 20,000 K s
, respectively; for the CHG they are 4000 K s
and 6000 K s
. The similarity in the critical rates for CHG, so far not reported in literature, and CHG's tendency towards stochastic nucleation underline the novelty of this glass state. Identifying different types of metallic glass, as is possible by advanced chip calorimetry, and comparing them with molecular and polymeric systems may help to elaborate a more generalized glass theory and improve metallic glass processing.
This study presents a unique Mg-based alloy composition in the Mg–Zn–Yb system which exhibits bulk metallic glass, metastable icosahedral quasicrystals (iQCs), and crystalline approximant phases in ...the as-cast condition. Microscopy revealed a smooth gradual transition from glass to QC. We also report the complete melting of a metastable eutectic phase mixture (including a QC phase), generated via suppression of the metastable-to-stable phase transition at high heating rates using fast differential scanning calorimetry (FDSC). The melting temperature and enthalpy of fusion of this phase mixture could be measured directly, which unambiguously proves its metastability in any temperature range. The kinetic pathway from liquid state to stable solid state (an approximant phase) minimizes the free-energy barrier for nucleation through an intermediate state (metastable QC phase) because of its low solid–liquid interfacial energy. At high undercooling of the liquid, where diffusion is limited, another approximant phase with near-liquid composition forms just above the glass-transition temperature. These experimental results shed light on the competition between metastable and stable crystals, and on glass formation via system frustration associated with the presence of several free-energy minima.
We demonstrate experimentally that a part-per-million addition of Sn solutes in Al-Mg-Si alloys can inhibit natural aging and enhance artificial aging. The mechanism controlling the aging is argued ...to be vacancy diffusion, with solutes trapping vacancies at low temperature and releasing them at elevated temperature, which is supported by a thermodynamic model and first-principles computations of Sn-vacancy binding. This "diffusion on demand" solves the long-standing problem of detrimental natural aging in Al-Mg-Si alloys, which is of great scientific and industrial importance. Moreover, the mechanism of controlled buffering and release of excess vacancies is generally applicable to modulate diffusion in other metallic systems.
We report the ultrasensitive detection of adenine using deep-UV surface-enhanced resonance Raman scattering on aluminum nanostructures. Well-defined Al nanoparticle arrays fabricated over large areas ...using extreme-UV interference lithography exhibited sharp and tunable plasmon resonances in the UV and deep-UV wavelength ranges. Theoretical modeling based on the finite-difference time-domain method was used to understand the near-field and far-field optical properties of the nanoparticle arrays. Raman measurements were performed on adenine molecules coated uniformly on the Al nanoparticle arrays at a laser excitation wavelength of 257.2 nm. With this technique, less than 10 amol of label-free adenine molecules could be detected reproducibly in real time. Zeptomole (∼30 000 molecules) detection sensitivity was readily achieved proving that deep-UV surface-enhanced resonance Raman scattering is an extremely sensitive tool for the detection of biomolecules.
A key question in materials science is how fast properties evolve, which relates to the kinetics of phase transformations. In metals, kinetics is primarily connected to diffusion, which for ...substitutional elements is enabled via mobile atomic-lattice vacancies. In fact, non-equilibrium vacancies are often required for structural changes. Rapid quenching of various important alloys, such as Al- or Mg-alloys, results for example in natural aging, i.e. slight movements of solute atoms in the material, which significantly alter the material properties. In this study we demonstrate a size effect of natural aging in an AlMgSi alloy via atom probe tomography with near-atomic image resolution. We show that non-equilibrium vacancy diffusional processes are generally stopped when the sample size reaches the nanometer scale. This precludes clustering and natural aging in samples below a certain size and has implications towards the study of non-equilibrium diffusion and microstructural changes via microscopy techniques.
Biodegradable magnesium alloys generally contain intermetallic phases on the micro‐ or nanoscale, which can initiate and control local corrosion processes via microgalvanic coupling. However, the ...experimental difficulties in characterizing active degradation on the nanoscale have so far limited the understanding of how these materials degrade in complex physiological environments. Here a quasi‐in situ experiment based on transmission electron microscopy (TEM) is designed, which enables the initial corrosion attack at nanometric particles to be accessed within the first seconds of immersion. Combined with high‐resolution ex situ cross‐sectional TEM analysis of a well‐developed corrosion‐product layer, mechanistic insights into Mg‐alloys' degradation on the nanoscale are provided over a large range of immersion times. Applying this methodology to lean Mg–Zn‒Ca alloys and following in detail the dissolution of their nanometric Zn‐ and Ca‐rich particles the in statu nascendi observation of intermetallic‐particle dealloying is documented for magnesium alloys, where electrochemically active Ca and Mg preferentially dissolve and electropositive Zn enriches, inducing the particles' gradual ennoblement. Based on electrochemical theory, here, the concept of cathodic‐polarization‐induced dealloying, which controls the dynamic microstructural changes, is presented. The general prerequisites for this new dealloying mechanism to occur in multicomponent alloys and its distinction to other dealloying modes are also discussed.
Dealloying of intermetallic nanoprecipitates governs the electrochemical reactivity of multicomponent Mg alloys. TEM‐based analyses allow the direct observation of intermetallic‐particle (IMP) dealloying at the nanoscale. Electrochemically active Ca preferentially dissolves, while electrochemically more noble Zn is cathodically protected. Zn enrichment leads to a gradual ennoblement of the IMP concomitant with its enhancing cathodic reactivity.
Fracture treatment in children needs new implant materials to overcome disadvantages associated with removal surgery. Magnesium-based implants constitute a biocompatible and bioresorbable ...alternative. In adults and especially in children, implant safety needs to be evaluated. In children the bone turnover rate is higher and implant material might influence growth capacity, and the long-term effect of accumulated particles or ions is more critical due to the host's prolonged post-surgery lifespan. In this study we aimed to investigate the degradation behavior of ZX00 (Mg-0.45Zn-0.45Ca; in wt.%) in a small and a large animal model to find out whether there is a difference between the two models (i) in degradation rate and (ii) in bone formation and in-growth. Our results 6, 12 and 24 weeks after ZX00 implantation showed no negative effects on bone formation and in-growth, and no adverse effects such as fibrotic or sclerotic encapsulation. The degradation rate did not significantly differ between the two growing-animal models, and both showed slow and homogeneous degradation performance. Our conclusion is that small animal models may be sufficient to investigate degradation rates and provide preliminary evidence on bone formation and in-growth of implant materials in a growing-animal model.
The safety of implant material is of the utmost importance, especially in children, who have enhanced bone turnover, more growth capacity and longer postoperative lifespans. Magnesium (Mg)-based implants have long been of great interest in pediatric orthopedic and trauma surgery, due to their good biocompatibility, biodegradability and biomechanics. In the study documented in this manuscript we investigated Mg-Zn-Ca implant material without rare-earth elements, and compared its outcome in a small and a large growing-animal model. In both models we observed bone formation and in-growth which featured no adverse effects such as fibrotic or sclerotic encapsulation, and slow homogeneous degradation performance of the Mg-based implant material.
A design strategy deployed in developing new biodegradable Mg-Y-Zn alloys is summarized and the key factors influencing their suitability for medical applications are described. The Mg-Y-Zn alloys ...reveal microstructural features and mechanical characteristics expected to be appropriate for vascular intervention applications. The focus of this article lies in the evaluation of the degradation performance and biological response of the alloys with respect to their potential as implant materials (stents). The degradation characteristics analyzed by immersion testing and electrochemical impedance spectroscopy in simulated physiological media reveal slow and homogeneous degradation. In vitro cell tests using human umbilical vein endothelial cells indicate good cytocompatibility on the basis of the alloys' eluates (extracts). Animal studies carried out with pigs on Mg-2Y-1Zn (in wt.%) reveal an auspicious in vivo performance. Evaluation of preparations derived from implants in various types of tissues indicates homogeneous degradation and only limited gas formation during in vivo testing. The characteristics of the tissue reactions indicate good biocompatibility. The new Mg-Y-Zn alloys show an interesting combination of preferred microstructural, mechanical, electrochemical and biological properties, which make them very promising for degradable implant applications.
We predict that complete magnetization reversal in simple metallic ferromagnetic nanoparticles is directly linked to the pair creation of topological point defects in the form of ...hedgehog-antihedgehog pairs. These dynamical point defects move at exceptionally high speeds in excess of 1500 m/s, faster than any other known magnetic object. Their rapid motion generates unprecedented solenoidal emergent fields on the order of megavolts per meter, in analogy to the magnetic field of a moving electric charge, providing a striking example that a moving hedgehog constitutes an emergent magnetic monopole.
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