Asteroid shapes and hydration levels can serve as tracers of their history and origin. For instance, the asteroids (162173) Ryugu and (101955) Bennu have an oblate spheroidal shape with a pronounced ...equator, but contain different surface hydration levels. Here we show, through numerical simulations of large asteroid disruptions, that oblate spheroids, some of which have a pronounced equator defining a spinning top shape, can form directly through gravitational reaccumulation. We further show that rubble piles formed in a single disruption can have similar porosities but variable degrees of hydration. The direct formation of top shapes from single disruption alone can explain the relatively old crater-retention ages of the equatorial features of Ryugu and Bennu. Two separate parent-body disruptions are not necessarily required to explain their different hydration levels.
In the present work, mechanical properties and low temperature superplasticity behavior of a nano/ultrafine grained AA6063 alloy fabricated by accumulative roll bonding (ARB) was investigated. To ...that end, superplasticity was evaluated at various deformation temperatures and for different strain rates in an AA6063 alloy ARBed up to seven cycles. Results showed that the nano/ultrafine grained AA6063 alloy exhibited an excellent low temperature superplasticity (low peak stress of 40 MPa and maximum elongation up to 270%) at 300 °C & 350 °C and under the nominal strain rates of 5 × 10−3s−1, 5 × 10−2s−1, 5 × 10−1s−1. The best condition of superplasticity, together with a stable microstructure, was obtained at 300 °C. Results also showed that at the deformation temperature of 250 °C, and under all strain rates, the elongation does not exceed 95%, hence for the absence of proper superplasticity at this temperature. It was found that although high amount of total elongation can be obtained at 350 °C and under low strain rates of 5 × 10−2s−1, 5 × 10−3s−1, the microstructural instability of these cases made them unsuitable for industrial applications. During superplastic deformation, low angle grain boundaries (LAGBs) gradually transformed into high angle grain boundaries (HAGBs) to sustain grain boundary sliding and to accommodate dynamic recovery. Constitutive equations were built, and strain rate sensitivity, as well as apparent activation energy variation were calculated. Using Zener-Hollomon parameter, the dominant hot deformation mechanism(s) in each deformation temperature was explained in conjunction with work hardening, dynamic recovery, dynamic recrystallization (DRX) and grain boundary sliding. As well, results indicated that the grain boundary sliding was the predominant deformation mechanism at 350 °C. Finally, a truly superplastic regime was achieved at the temperature of 300 °C and for the strain rate of 5 × 10−2 s−1.
In the present paper the role of shear reversal on microstructure, texture and mechanical properties of pure copper during a single pass of the simple shear extrusion (SSE) process was investigated. ...For SSE processing an appropriate die with a linear die profile was designed and constructed, which imposes forward shear in the first half and reverse shear in the second half channels. Electron back-scattering diffraction (EBSD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to evaluate the microstructure of the deformed samples. The geometrical nature of this process imposes a distribution of strain results in the inhomogeneous microstructure and the hardness throughout the plane perpendicular to the extrusion direction. Strain reversal during the process results in a slight reduction in dislocation density, the hardness and mean disorientation angle of the samples, and an increase in the grain size. After a complete pass of SSE, dislocation density decreased by ~14% if compared to the middle of the process. This suggests that the dislocation annihilation occurred by the reversal of the shear strain. The simple shear textures were formed gradually and the strongest simple shear textures were observed on the middle of the SSE channel. The degree of the simple shear textures decreases with the distance from the middle plane where the shear is reversed, but the simple shear textures are still the major components after exit of the channel. Hardness variation was modeled by contributions from dislocation strengthening and grain boundary strengthening, where dislocation density is approximated by the misorientation angle of LAGBs which are regarded as dislocation cell boundaries. As a result, the hardness can be predicted successfully by the microstructural features, i.e. the low-angle boundaries, the mean misorientation angle and the fraction of high-angle grain boundaries.
In the present paper the variation of microstructural parameters and tensile properties of ultrafine-grained copper processed by simple shear extrusion (SSE) via namely route C in 1, 2, 4, 6, 8 and ...12 passes is described. TEM analysis showed that the microstructure evolves from lamellar boundaries and elongated cells towards a more equiaxed homogeneous microstructure. After 12 passes, the grain fragmentation occurred in all the directions without any significant elongation in the grains. The minimum cell size is achieved after eight passes. Evaluation of dislocation density using scanning transmission electron microscopy observations shows a gradual increase of dislocation from one to eight passes following a reduction afterward. Yield stress and ultimate tensile stress reach a maximum after eight passes. The uniform elongation attains its minimum after eight passes. Reduction in dislocation density, grain growth, formation of Moiré fringes and twinning after twelve passes of SSE are some of the evidences for the softening. The critical grain size for the formation of nano twins (the onset of grain growth) is predicted. Prosperous prediction of yield stress using a strength–structure relationship helps in the understanding of the effect of dislocation density and microstructural observations.
Summary
Root amputation, immunosuppressive therapy, mandibular tooth extraction, pre-existing inflammation, and longer duration of treatment with bone-modifying agents were significantly associated ...with an increased risk of medication-related osteonecrosis of the jaw. Hopeless teeth should be extracted without drug holiday before the development of inflammation in cancer patients receiving high-dose bone-modifying agents.
Introduction
No studies have comprehensively analyzed the influence of pre-existing inflammation, surgical procedure–related factors such as primary wound closure, demographic factors, and drug holiday on the incidence of medication-related osteonecrosis of the jaw (MRONJ). The purpose of this study was to retrospectively investigate the relationships between these various factors and the development of MRONJ after tooth extraction in cancer patients receiving high-dose bone-modifying agents (BMAs) such as bisphosphonates or denosumab.
Methods
Risk factors for MRONJ after tooth extraction were evaluated with univariate and multivariate analyses. The following parameters were investigated in all patients: demographics, type and duration of BMA use, whether BMA use was discontinued before tooth extraction (drug holiday), the duration of such discontinuation, the presence of pre-existing inflammation, and whether additional surgical procedures (e.g., incision, removal of bone edges, root amputation) were performed.
Results
We found that root amputation (OR = 22.62), immunosuppressive therapy (OR = 16.61), extraction of mandibular teeth (OR = 12.14), extraction of teeth with pre-existing inflammation, and longer duration (≥ 8 months) of high-dose BMA (OR = 7.85) were all significantly associated with MRONJ.
Conclusions
Tooth extraction should not necessarily be postponed in cancer patients receiving high-dose BMA. The effectiveness of a short-term drug holiday was not confirmed, as drug holidays had no significant impact on MRONJ incidence. Tooth extraction may be acceptable during high-dose BMA therapy until 8 months after initiation.
The microstructure and deformation behavior of the FeCoNiMnV high entropy alloy (HEA) was investigated using the compressive test at temperatures ranging from room temperature to 800 °C. The X-ray ...powder diffraction (XRD), field emission scanning electron microscope (FESEM), and electron backscatter diffraction (EBSD) were used to analyze the structure and microstructure of the compressed samples. According to the XRD results, no specific phase transformation occurred during the hot deformation except for the second phase's partial dissolution. Up to 400 °C, the samples exhibited a relatively high strength with a small amount of ductility, demonstrating good thermal and structural stability. At 600 °C and 800 °C, the flow behavior of the alloy changed remarkably. Peak stress, followed by a gradual decline in flow stress, was observed at elevated temperatures, suggesting the occurrence of the dynamic recrystallization (DRX). The formation of grain boundaries with a high misorientation angle, the evidence of the grain growth, and the large fraction of the coherent twin boundaries confirmed the occurrence of DRX during the compression test. The flow curves at high temperatures were analyzed based on the Avrami kinetics of recrystallization. The Avrami exponent was determined as 1.668 and 1.135 for the samples compressed at 600 °C and 800 °C, respectively.
•FeCoNiMnV alloy shows superior strength up to 400 °C.•Rapid hardening to peak stress and subsequent softening is observed at 600 °C and 800 °C.•Both cDRX and dDRX occurs during the high temperature deformation.•The DRX kinetics is analyzed based on the JMAK model.•The Avrami exponent was obtained as 1.668 and 1.135 for the samples compressed at 600 °C and 800 °C, respectively.
The mapping of available water–ice is a crucial step in the lunar exploration missions. Ground penetrating radars have the potential to map the subsurface structure and the existence of water–ice in ...terms of the electromagnetic properties, specifically, the permittivity. Slight differences in permittivity can be significantly important when applied in a dry environment, such as on the Moon and Mars. The capability of detecting a small fraction of putative water–ice depends on the permittivity changes in terms of its dependent parameters, such as the frequency, the temperature, the porosity, and the chemical composition. Our work aims at mitigating false detection or overlooking of water–ce by considering their conditions that previous researches did not cover. We measured the permittivity of different lunar regolith relevant analogue samples with a fixed 40 % porosity in the ultra-high-frequency–super-high-frequency band. We used the coaxial probe method to measure anorthosite, basalt, dunite and ilmenite at
20
∘
C
,
-
20
∘
C
and
-
60
∘
C
, and we find that, at
-
60
∘
C
, the permittivity decreases about 6–18 % compared with the values at
20
∘
C
. Within this temperature range, the permittivity is quite similar to the permittivity of water–ice. We find that the conventional calculation would overestimate the permittivity in the low temperature areas, such as the permanently shadowed regions. We also find that each component in the lunar regolith has different temperature-dependent permittivity, which might be important for radar data analysis to detect lunar polar water–ice. Our results also suggest that it should be possible to estimate the water–ice content from radar measurements at different temperatures given an appropriate method.
Graphical Abstract
Rubble-Pile Asteroid Itokawa as Observed by Hayabusa Fujiwara, A; Kawaguchi, J; Yeomans, D.K ...
Science (American Association for the Advancement of Science),
06/2006, Letnik:
312, Številka:
5778
Journal Article
Recenzirano
During the interval from September through early December 2005, the Hayabusa spacecraft was in close proximity to near-Earth asteroid 25143 Itokawa, and a variety of data were taken on its shape, ...mass, and surface topography as well as its mineralogic and elemental abundances. The asteroid's orthogonal axes are 535, 294, and 209 meters, the mass is 3.51 x 10¹⁰ kilograms, and the estimated bulk density is 1.9 ± 0.13 grams per cubic centimeter. The correspondence between the smooth areas on the surface (Muses Sea and Sagamihara) and the gravitationally low regions suggests mass movement and an effective resurfacing process by impact jolting. Itokawa is considered to be a rubble-pile body because of its low bulk density, high porosity, boulder-rich appearance, and shape. The existence of very large boulders and pillars suggests an early collisional breakup of a preexisting parent asteroid followed by a re-agglomeration into a rubble-pile object.
In the present paper commercially pure copper was processed by simple shear extrusion (SSE) technique up to 12 passes using the so-called route C. For SSE processing an appropriate die with a linear ...die profile was designed and constructed. Effect of SSE passes on isotropy and uniformity of microstructures are focused. Electron back-scattering diffraction (EBSD) was used to evaluate the microstructure of the deformed samples in three orthogonal planes. To investigate the microstructural uniformity EBSD maps were taken from center to periphery of the extrusion direction plane (ED-plane) samples. Significant evolution in grain refinement was achieved down to sub-micron grain size in all planes. Hardness measurements show a considerable increase in hardness of the material after the processing, which confirms the microstructural evolutions. EBSD scans revealed a homogeneous ultrafine grained microstructure after 12 passes. Micro-shear bands were found as potential sites for accelerating the formation of new grains by fragmentation of the initial grains. The total frequency of coincidence site lattice (CSL) boundaries including Σ3 boundaries increased by the increasing of SSE passes. The higher fraction of low to high angle grain boundaries of SSE compared to equal channel angular pressing is an evidence for the cyclic behavior of SSE technique.
The effect of particle distribution non-uniformity on the microstructure and flow stress in nanostructured metal matrix composites produced by severe plastic deformation was investigated. The Al/SiC ...composite fabricated by accumulative roll bonding was considered as a case. Transmission electron microscope (TEM) and scanning transmission electron microscope (STEM) were used for microstructural characterizations. Based on the particle distribution, three different zones were considered: particle-free zone, single particle zone, and cluster zone. In comparison to the particle free zone, finer grain sizes with higher boundary misorientation angle were observed in the single particle zone. The lowest grain size (342/260 nm by TEM/STEM) was obtained in the cluster zone approximately between two near particles. The selected area diffraction showed the highest misorientation angle at these regions. A combined microstructure strengthening analysis was considered for the prediction of the flow stress. The flow stress is predicted based on the grain sizes at various microstructural zones, and the results were compared with the experimental yield stress of the composite.
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