The Besnus transition in 4C pyrrhotite revisited Gehring, Andreas U; Firlus, Alexander; Koulialias, Dimitrios ...
Geophysical journal international,
03/2022, Letnik:
228, Številka:
3
Journal Article
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SUMMARY
Ferrimagnetic, monoclinic 4C pyrrhotite (Fe7S8) is the only iron sulphide with high relevance for palaeomagnetism and rock magnetism that can be identified in rock materials by its ...characteristic low-temperature anomaly. Despite its relevance in natural magnetism and the many magnetic studies over the last decades, the physics and the crystallography behind this anomaly, also denoted Besnus transition, is a matter of debate. In this study, we analyse the static and dynamic magnetization associated with the Besnus transition in conjunction with low-temperature structural data of 4C pyrrhotite reported in the literature. The correlation between the Fe–Fe bonds causing spin-orbit coupling and the dynamic magnetic properties show that the magnetic characteristics of the Besnus transition stem from the interaction of two magnetocrystalline anisotropy systems triggered by thermally induced structural changes on an atomic level in monoclinic 4C pyrrhotite. This refutes the widespread view that the Besnus transition is caused by a crystallographic change from monoclinic to triclinic.
Bulk metallic glasses (BMGs) are high-strength, highly elastic materials due to their disordered atomic structure. Because BMGs require sufficiently high cooling rates to bypass crystallization, ...laser-based additive manufacturing (AM) methods have recently been employed for the fabrication of BMGs. In this study, we present an optimized Laser Powder-Bed Fusion (LPBF) process on a Zr-based BMG (Zr59.3Cu28.8Al10.4Nb1.5, in at.%), with a focus on characterization, mechanical properties, and dimensional accuracy. A volumetric density of 99.82% was achieved. Although the sample was qualified as amorphous via laboratory X-ray diffraction experiments, a more meticulous study using synchrotron radiation revealed nanocrystals in the heat-affected zones (HAZs) of the melt pool. Fast differential scanning calorimetry (FDSC) and numerical simulations were then employed to illustrate the mechanism of crystallization. The LPBF-processed alloy revealed excellent mechanical properties, such as high hardness, wear resistance, compressive strength, and flexural strength. Apart from vein-like patterns, the fracture surfaces of the compression test samples showed liquid-like features, which indicate a significant local temperature increase during fracture. The dimensional accuracy was assessed with a benchmark exhibiting complex geometrical features and reached at least 40 μm. The results indicate that LPBF processing is a promising route for the manufacturing of BMGs for various applications.
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•Advanced characterization is required to detect nanocrystals formed during the Laser powder-bed fusion process•Results of finite element simulation and fast differential scanning calorimetry unveil the mechanism of crystallization•Despite the presence of nanocrystals in the Zr-based BMG, excellent mechanical properties are achieved•Using optimized parameters, a benchmark with complex features is fabricated with excellent geometrical accuracy
In this study, two biodegradable Mg-Zn-Ca alloys with alloy content of less than 1 wt % were strengthened via high pressure torsion (HPT). A subsequent heat treatment at temperatures of around 0.45
...led to an additional, sometimes even larger increase in both hardness and tensile strength. A hardness of more than 110 HV and tensile strength of more than 300 MPa were achieved in Mg-0.2Zn-0.5Ca by this procedure. Microstructural analyses were conducted by scanning and transmission electron microscopy (SEM and TEM, respectively) and atom probe tomography (APT) to reveal the origin of this strength increase. They indicated a grain size in the sub-micron range, Ca-rich precipitates, and segregation of the alloying elements at the grain boundaries after HPT-processing. While the grain size and segregation remained mostly unchanged during the heat treatment, the size and density of the precipitates increased slightly. However, estimates with an Orowan-type equation showed that precipitation hardening cannot account for the strength increase observed. Instead, the high concentration of vacancies after HPT-processing is thought to lead to the formation of vacancy agglomerates and dislocation loops in the basal plane, where they represent particularly strong obstacles to dislocation movement, thus, accounting for the considerable strength increase observed. This idea is substantiated by theoretical considerations and quenching experiments, which also show an increase in hardness when the same heat treatment is applied.
Over the last decade, demand has increased for developing new, alternative materials in pediatric trauma care to overcome the disadvantages associated with conventional implant materials. Magnesium ...(Mg)-based alloys seem to adequately fulfill the vision of a homogeneously resorbable, biocompatible, load-bearing and functionally supportive implant. The aim of the present study is to introduce the high-strength, lean alloy Mg‒0.45Zn‒0.45Ca, in wt% (ZX00), and for the first time investigate the clinical applicability of screw osteosynthesis using this alloy that contains no rare-earth elements. The alloy was applied in a growing sheep model with osteotomized bone (simulating a fracture) and compared to a non-osteotomy control group regarding degradation behavior and fracture healing. The alloy exhibits an ultimate tensile strength of 285.7 ± 3.1 MPa, an elongation at fracture of 18.2 ± 2.1%, and a reduced in vitro degradation rate compared to alloys containing higher amounts of Zn. In vivo, no significant difference between the osteotomized bone and the control group was found regarding the change in screw volume over implantation time. Therefore, it can be concluded that the fracture healing process, including its effects on the surrounding area, has no significant influence on degradation behavior. There was also no negative influence from hydrogen-gas formation on fracture healing. Despite the proximal and distal screws showing chronologically different gas release, the osteotomy showed complete consolidation.
Conventional implants involve several disadvantages in pediatric trauma care. Magnesium-based alloys seem to overcome these issues as discussed in the recent literature. This study evaluates the clinical applicability of high-strength lean Mg‒0.45Zn‒0.45Ca (ZX00) screws in a growing-sheep model. Two groups, one including a simulated fracture and one group without fracture, underwent implantation of the alloy and were compared to each other. No significant difference regarding screw volume was observed between the groups. There was no negative influence of hydrogen-gas formation on fracture healing and a complete fracture consolidation was found after 12 weeks for all animals investigated.
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Implant removal is unnecessary for biodegradable magnesium (Mg)-based implants and, therefore, the related risk for implant-induced fractures is limited. Aging, on the other hand, is associated with ...low bone-turnover and decreased bone mass and density, and thus increased fracture risk. Osteoporosis is accompanied by Mg deficiency, therefore, we hypothesized that Mg-based implants may support bone formation by Mg2+ ion release in an ovariectomy-induced osteoporotic rat model. Hence, we investigated osseointegration and implant degradation of a low-alloyed, degrading Mg–Zn–Ca implant (ZX00) in ovariectomy-induced osteoporotic (Osteo), old healthy (OH), and juvenile healthy (JH) groups of female Sprague Dawley rats via in vivo micro-computed tomography (µCT). For the Osteo rats, we demonstrate diminished trabecular bone already after 8 weeks upon ovariectomy and significantly enhanced implant volume loss, with correspondingly pronounced gas formation, compared to the OH and JH groups. Sclerotic rim development was observed in about half of the osteoporotic rats, suggesting a prevention from foreign-body and osteonecrosis development. Synchrotron radiation-based µCT confirmed lower bone volume fractions in the Osteo group compared to the OH and JH groups. Qualitative histological analysis additionally visualized the enhanced implant degradation in the Osteo group. To date, ZX00 provides an interesting implant material for young and older healthy patients, but it may not be of advantage in pharmacologically untreated osteoporotic conditions.
Magnesium-based implants are promising candidates for treatment of osteoporotic fractures because of their biodegradable, biomechanical, anti-bacterial and bone regenerative properties. Here we investigate magnesium‒zinc‒calcium implant materials in a rat model with ovariectomy-induced osteoporosis (Osteo group) and compare the related osseointegration and implant degradation with the results obtained for old healthy (OH) and juvenile healthy (JH) rats. The work applied an appropriate disease model for osteoporosis and focused in particular on long-term implant degradation for different bone conditions. Enhanced implant degradation and sclerotic rim formation was observed in osteoporotic rats, which illustrates that the setting of different bone models generates significantly modified clinical outcome. It further illustrated that these differences must be taken into account in future biodegradable implant development.
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The nanostructural adaptation of bone is crucial for its biocompatibility with orthopedic implants. The bone nanostructure also determines its mechanical properties and performance. However, the ...bone's temporal and spatial nanoadaptation around degrading implants remains largely unknown. Here, we present insights into this important bone adaptation by applying scanning electron microscopy, elemental analysis, and small-angle X-ray scattering tensor tomography (SASTT). We extend the novel SASTT reconstruction method and provide a 3D scattering reciprocal space map per voxel of the sample's volume. From this reconstruction, parameters such as the thickness of the bone mineral particles are quantified, which provide additional information on nanostructural adaptation of bone during healing. We selected a rat femoral bone and a degrading ZX10 magnesium implant as model system, and investigated it over the course of 18 months, using a sham as control. We observe that the bone's nanostructural adaptation starts with an initially fast interfacial bone growth close to the implant, which spreads by a re-orientation of the nanostructure in the bone volume around the implant, and is consolidated in the later degradation stages. These observations reveal the complex bulk bone-implant interactions and enable future research on the related biomechanical bone responses.
Traumatic bone injuries are among the most frequent causes of surgical treatment, and often require the placement of an implant. The ideal implant supports and induces bone formation, while being mechanically and chemically adapted to the bone structure, ensuring a gradual load transfer. While magnesium implants fulfill these requirements, the nanostructural changes during bone healing and implant degradation remain not completely elucidated. Here, we unveil these processes in rat femoral bones with ZX10 magnesium implants and show different stages of bone healing in such a model system.
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This study presents an extensive series of novel bulk metallic glasses (BMGs) which broadly span all three corners of the Mg–Cu–Yb ternary system. Over 30 alloys were synthesised within a wide ...composition range from (at.%): Mg: 13–55, Cu: 17.5–45.5, and Yb: 9–70. In terms of composition, this ternary system is considered to be one of the broadest for bulk glass formation known - this probably due to the three elements' thermodynamic compatibility and unique combinations of atomic radii, which generate specific, favoured structural topologies. The investigation reports the design method, critical casting size, thermophysical characteristics and mechanical properties of these new BMGs.
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•Quantitative predictions with a physical model of AMZ4 BMG crystallization induced by the LPBF process over a wide range of crystallized fractions.•FEM simulations with a proper TTT ...diagrams measured by Flash DSC upon heating on the exact same powder used for LPBF experiments.•Highlight of two thermal effects operating at different scales: a global one, originating from heat accumulation in the whole sample, and a local one, due to the effect of each individual laser track pass.•Comparison experiments vs FEM simulations -> good agreement on the crystallized fraction.•Validation of the model on an optimized sample with 0% crystallization.
One of the main challenges in the fabrication of bulk metallic glasses (BMGs) via laser-based additive manufacturing (AM) is undesirable crystal phase formation, which usually deteriorates the mechanical properties of the BMG fabricated parts. Understanding the crystallization process therefore helps to manufacture parts with desirable properties. In this study, partially crystallized Zr-based BMG (AMZ4) samples were fabricated via laser powder-bed fusion (LPBF). Samples with a different time delay between each adjacent laser tracks were produced to vary the thermal history during the manufacturing process. Two characteristic thermal effects were decoupled, a global one and a local one. The global thermal effect originates from heat accumulation in the whole sample, increasing the overall sample temperature, reducing local cooling rates from the melt and changing thermal cycles in the heat-affected zones (HAZs). The local thermal effect refers to the contribution of each individual laser-track pass, happening even in the absence of the global effect. As the time delay is increased, the sample has more time to dissipate heat, which implies a reduced influence of the global thermal effects, and therefore lower crystalline fractions. The experiments were designed such as to allow for detailed validations of the thermal fields predicted by a Finite Element (FEM) model of the LPBF process. These were indeed used as an input to predict the crystallized fraction in each AMZ4 sample, using previously measured TTT diagrams. For the first time, quantitative predictions with a numerical model could be made over a wide range of crystallized fractions, and were in good agreement with those measured by DSC. To validate the model, a 0 % crystallized fraction was also simulated, corresponding to optimized printing conditions despite the high oxygen content (>1000 ppm) of the AMZ4 chosen for the experiments. It therefore represents a reliable tool for finding optimal processing parameters of BMGs known to be challenging to print.
Accurate compression tests with a piezoelectric load cell and an acquisition rate of up to 10
kHz were performed on a Zr-based bulk metallic glass in the temperature range 210–320
K at a strain rate ...of 10
−3 s
−1. Information about the stress drop magnitude and the associated size of shear displacements as a function of temperature and strain provides detailed insights into the shear band characteristics, which can be described by a stick–slip process. The average shear slip displacement is on average about 1–2
μm, irrespective of temperature, whereas the associated slip time (or stress drop time) increases from ∼1
ms at 320
K to ∼0.4
s at 213
K, yielding values on the deformation kinetics and the shear viscosity. Scanning electron microscopy investigations on shear surfaces and in situ acoustic emission measurements provide further understanding into the complex multistep shear slip process.
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