High-entropy alloys, near-equiatomic solid solutions of five or more elements, represent a new strategy for the design of materials with properties superior to those of conventional alloys. However, ...their phase space remains constrained, with transition metal high-entropy alloys exhibiting only face- or body-centered cubic structures. Here, we report the high-pressure synthesis of a hexagonal close-packed phase of the prototypical high-entropy alloy CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed to suppression of the local magnetic moments, destabilizing the initial fcc structure. Similar to fcc-to-hcp transformations in Al and the noble gases, the transformation is sluggish, occurring over a range of >40 GPa. However, the behaviour of CrMnFeCoNi is unique in that the hcp phase is retained following decompression to ambient pressure, yielding metastable fcc-hcp mixtures. This demonstrates a means of tuning the structures and properties of high-entropy alloys in a manner not achievable by conventional processing techniques.
A major challenge to understanding the response of materials to extreme environments (e.g., nuclear fuels/waste forms and fusion materials) is to unravel the processes by which a material can ...incorporate atomic-scale disorder, and at the same time, remain crystalline. While it has long been known that all condensed matter, even liquids and glasses, possess short-range order, the relation between fully-ordered, disordered, and aperiodic structures over multiple length scales is not well understood. For example, when defects are introduced (via pressure or irradiation) into materials adopting the pyrochlore structure, these complex oxides either disorder over specific crystallographic sites, remaining crystalline, or become aperiodic. Here we present neutron total scattering results characterizing the irradiation response of two pyrochlores, one that is known to disorder (Er2Sn2O7) and the other to amorphize (Dy2Sn2O7) under ion irradiation. The results demonstrate that in both cases, the local pyrochlore structure is transformed into similar short range configurations that are best fit by the orthorhombic weberite structure, even though the two compositions have distinctly different structures, aperiodic vs. disordered-crystalline, at longer length scales. Thus, a material's resistance to amorphization may not depend primarily on local defect formation energies, but rather on the structure's compatibility with meso-scale modulations of the local order in a way that maintains long-range periodicity.
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Abstract
Atomic disordering in materials alters their physical and chemical properties and can subsequently affect their performance. In complex ceramic materials, it is a challenge to understand the ...nature of structural disordering, due to the difficulty of direct, atomic-scale experimental observations. Here we report the direct imaging of ion irradiation-induced antisite defects in M
n+1
AX
n
phases using double C
S
-corrected scanning transmission electron microscopy and provide compelling evidence of order-to-disorder phase transformations, overturning the conventional view that irradiation causes phase decomposition to binary fcc-structured M
n+1
X
n
. With the formation of uniformly distributed cation antisite defects and the rearrangement of X anions, disordered solid solution γ-(M
n+1
A)X
n
phases are formed at low ion fluences, followed by gradual transitions to solid solution fcc-structured (M
n+1
A)X
n
phases. This study provides a comprehensive understanding of the order-to-disorder transformations in M
n+1
AX
n
phases and proposes a method for the synthesis of new solid solution (M
n+1
A)X
n
phases by tailoring the disorder.
The structure, size, and morphology of ion tracks resulting from irradiation of five different pyrochlore compositions (A2Ti2O7, A = Yb, Er, Y, Gd, Sm) with 2.2 GeV 197Au ions were investigated by ...means of synchrotron X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Radiation-induced amorphization occurred in all five materials analyzed following an exponential rate as a function of ion fluence. XRD patterns showed a general trend of increasing susceptibility of amorphization with increasing ratio of A- to B-site cation ionic radii (rA/rB) with the exception of Y2Ti2O7 and Sm2Ti2O7. This indicates that the track size does not necessarily increase with rA/rB, in contrast with results from previous swift heavy ion studies on Gd2Zr2-xTixO7 pyrochlore materials. For Y2Ti2O7, this effect is attributed to the significantly lower electron density of this material relative to the lanthanide-bearing pyrochlores, thus lowering the electronic energy loss (dE/dx) of the high-energy ions in this composition. An energy loss normalization procedure was performed which reveals an initial increase of amorphous track size with rA/rB that saturates above a cation radius ratio larger than Gd2Ti2O7. This is in agreement with previous low-energy ion irradiation experiments and first principles calculations of the disordering energy of titanate pyrochlores indicating that the same trends in disordering energy apply to radiation damage induced in both the nuclear and electronic energy loss regimes. HRTEM images indicate that single ion tracks in Yb2Ti2O7 and Er2Ti2O7, which have small A-site cations and low rA/rB, exhibit a core-shell structure with a small amorphous core surrounded by a larger disordered shell. In contrast, single tracks in Gd2Ti2O7 and Sm2Ti2O7, have a larger amorphous core with minimal disordered shells.
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Grain size effects on irradiated CeO2, ThO2, and UO2 Cureton, William F.; Palomares, Raul I.; Walters, Jeffrey ...
Acta materialia,
November 2018, 2018-11-00, 2018-11, 2018-11-01, Letnik:
160, Številka:
C
Journal Article
Recenzirano
Odprti dostop
Microcrystalline and nanocrystalline UO2, ThO2, and CeO2 (∼2 μm and∼20 nm particle size, respectively) were irradiated with 946 MeV Au ions at room temperature and characterized by synchrotron X-ray ...diffraction, Raman spectroscopy, and transmission electron microscopy. All samples show a small increase in unit cell parameter as a function of ion fluence (0.17 ± 0.03% for CeO2 and 0.11 ± 0.03% for ThO2), except microcrystalline UO2, which displays a small contraction of the unit cell (−0.06 ± 0.02%). Raman spectroscopy measurements of microcrystalline UO2 indicate an increase in nonstoichiometry after irradiation. All bulk materials are subject to an increase in heterogeneous microstrain, most notably UO2, implying that the relatively small changes in unit cell parameter are accompanied by substantial local disorder induced by isolated defects. The magnitude of volumetric swelling for all materials is larger in the nanocrystalline form as compared with the microcrystalline form (0.38 ± 0.60% for CeO2, 0.14 ± 0.03% for ThO2, and 0.52 ± 0.13% for UO2). ThO2 shows the smallest difference in swelling between the microcrystalline and nanocrystalline samples (∼0.03%). All nanocrystalline materials exhibit irradiation-induced grain coarsening along with a decrease in heterogeneous microstrain with increasing ion fluence, except nanocrystalline CeO2, which shows no observable change in grain size and a slight increase in heterogeneous microstrain attributed to the accelerated formation of a secondary Ce11O20 phase evidenced in the X-ray diffraction data, present in both nanocrystalline and microcrystalline materials. Surprisingly, nanocrystalline UO2 exhibits a significant degree of swelling indicative of a decrease in oxygen content along with an increase in disorder induced by oxygen loss at grain boundaries during irradiation, based on the analysis of X-ray diffraction and Raman spectroscopy.
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Assessing the utility of hypersonic boost glide vehicles (BGVs) requires comparing their capabilities to alternative systems that could carry out the same missions, particularly given the technical ...difficulties and additional costs of developing BGVs compared to more established technologies. This paper discusses the primary motivations given for BGVs-most notably countering missile defenses-and summarizes current hypersonic development programs. It finds that evading the most capable current endo-atmospheric defenses requires that BGVs maintain speeds significantly higher than Mach 5 throughout their glide phase, which has implications for their mass and range. The paper then compares BGVs to maneuverable reentry vehicles (MaRVs) carried on ballistic missiles flown on depressed trajectories and shows that MaRVs can offer significant advantages over BGVs in a wide range of cases. Finally, the paper shows that BGV maneuvering during its glide phase can result in substantial costs in range and glide speed.
Atomic disordering in materials alters their physical and chemical properties and can subsequently affect their performance. In complex ceramic materials, it is a challenge to understand the nature ...of structural disordering, due to the difficulty of direct, atomic-scale experimental observations. Here we report the direct imaging of ion irradiation-induced antisite defects in M
AX
phases using double C
-corrected scanning transmission electron microscopy and provide compelling evidence of order-to-disorder phase transformations, overturning the conventional view that irradiation causes phase decomposition to binary fcc-structured M
X
. With the formation of uniformly distributed cation antisite defects and the rearrangement of X anions, disordered solid solution γ-(M
A)X
phases are formed at low ion fluences, followed by gradual transitions to solid solution fcc-structured (M
A)X
phases. This study provides a comprehensive understanding of the order-to-disorder transformations in M
AX
phases and proposes a method for the synthesis of new solid solution (M
A)X
phases by tailoring the disorder.
China has dominated the global production of rare-earth elements (REE) since the 1990s. This market dominance is often described as a Chinese victory over competitors—particularly the US—in a ...geopolitical contest, since REEs are critical components of numerous advanced technologies, including military systems, that great powers compete to control. However, this geopolitical explanation for recent trends in global REE market fails to account for domestic factors—technical, economic, political, and environmental—that significantly impact the exploitation of geologic resources. This paper assesses the causes of decline in US REE production, which facilitated China's domination of the global market. We show that the failure of the US REE production was primarily caused by domestic factors, including misalignment between the US government and corporate interests. At a time when the China-US competition for REE production is seen not only as an economic contest, but also as a matter of national and international security, we argue that strategic cooperation between China and the US could effectively diversify and improve the resilience of REE supply chains and mitigate economic and national security risks associated with potential future disruptions.
•Today, there is an imbalance of global rare-earth elements (REEs) supply chain.•China has a global monopoly on the production (70%) and processing (90%) of REEs.•China's dominance has become a focal point in great power competition with the US.•Decline of the US supply chain for REEs is analyzed in view of domestic factors.•Strategic cooperation between China and the US is recommended.