Beta (β) phase precipitation and subsequent intergranular corrosion and cracking present some of the main concerns for the usage of Al 5xxx series alloys. In this manuscript, β phase precipitation ...and changes in grain boundary interfacial structures are presented for Al 5083 samples sensitized at 100 °C for 6 months. It was observed that β phase precipitates are formed on/near different type of dispersoids and at high angle grain boundaries. Some precipitates formed directly on the surface of Al6Mn dispersoids, while some precipitates were detected near Al6(Mn,Fe) dispersoids and were connected to the β phase precipitate by dislocation bands. This may suggest that dislocation bands existing between the dispersoid and β phase precipitates can act as fast paths for atom diffusion and assist the growth of the β phase. The precipitation of β phase on/near preexisting dispersoids or at boundaries is ascribed to the decrease in nucleation energy needed for heterogeneous nucleation. After sensitization, layer structures containing a high density of dislocations and distortion regions were found at the grain boundaries. The thickness of the grain boundary layer increases at grain boundaries with higher misorientation angles.
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•Beta (β) precipitation on different dispersoids and grain boundaries is revealed.•Layer structures are observed at the grain boundaries after sensitization.•Layer structures contain a high density of dislocations and distortion regions.•Thickness of the layer structures increases with higher misorientation angles.
Abstract
An eastern boundary current (EBC) system driven by a large-scale meridional buoyancy gradient is simulated using an idealized eddy-resolving model. The EBC system consists of a pair of ...stacked meridional currents that flow poleward near the surface and equatorward at intermediate depths. Buoyancy advection in the EBC is primarily balanced by the shedding of eddies, with anticyclonic, warm-core eddies dominating near the surface and cyclonic, cold-core eddies found at intermediate depths. These boundary eddies play a significant role in both the eastern boundary circulation—by helping to trap the EBC near the coast—and the large-scale circulation through their effect on the downwelling limb of the overturning circulation. Momentum and thickness budgets analyzed using the thickness-weighted average framework highlight the role of eddy form drag in shaping and maintaining the EBC. The efficiency of the form drag increases dramatically at the offshore flank of the EBC. This zonal variation of the form drag is essential for maintaining a swift, narrow EBC. The essential physics of the EBC are illustrated using a simple, semianalytical model.
In this paper, the S32101 duplex stainless steel welded joints were produced by a K-TIG welding system. The weld geometry parameters under different welding speeds were analyzed by combining the ...morphological characteristics of the keyhole. The microstructure and impact toughness of the base metal and weld metal zone under different welding speeds were studied. The experiment results show that the welding speed has quite an effect on the geometry profile of the weld. In addition, the characteristic parameters of the keyhole can effectively predict the geometry profile of the weld. The test results prove that the microstructure, Σ3 coincidence site lattice grain boundary, and phase boundary of ferrite and austenite have an effect on the impact property of the weld metal zone. When the proportion of the austenite, Σ3 coincidence site lattice grain boundary and random phase boundary increased, the impact property of the weld metal zone also increased.
The evolution of cold air layers near the surface was investigated for a night with stable conditions near the surface. Spatial air temperature observations at 276 co-located vertical profiles were ...made using high-resolution fibre-optic based distributed temperature sensing (DTS) in a quasi three-dimensional geometry oriented along a shallow depression in the landscape and analysed for patterns in near-surface flow. Temperature stratification was observed to be interrupted by transient temperature structures on the scale of metres for which the flow direction and velocity could be quantified. The high spatial resolution and large spatial domain of the DTS revealed temperature structures in a level of detail that exceeded the capability of traditional point observations of air temperature at low wind speeds. Further, composition techniques were applied to describe wave-like motions in the opposite direction of the mean flow, at intervals of approximately 200 s (5 mHz). The DTS technique delivered tomography on a scale of tens of metres. The spatial observations at high spatial (fractions of a metre) and temporal (sec) resolution provided new opportunities for detection and quantification of surface-flow features and description of complicated scale interactions. High-resolution DTS is therefore a valuable addition to experimental research on stable and weak-wind boundary layers near the surface.
Achieving a combination of high mechanical strength and high electrical conductivity in low-weight Al alloys requires a full understanding of the relationships between nanoscaled features and ...physical properties. Grain boundary strengthening through grain size reduction offers some interesting possibilities but is limited by thermal stability issues. Zener pinning by stable nanoscaled particles or grain boundary segregation are well-known strategies for stabilizing grain boundaries. In this study, the Al–Ca system has been selected to investigate the way segregation affects the combination of mechanical strength and electrical resistivity. For this purpose, an Al–Ca composite material was severely deformed by high-pressure torsion to achieve a nanoscaled structure with a mean grain size of only 25 nm. X-ray diffraction, transmission electron microscopy and atom probe tomography data revealed that the fcc Ca phase was dissolved for large levels of plastic deformation leading mainly to Ca segregations along crystalline defects. The resulting microhardness of about 300 HV is much higher than predictions based on Hall and Petch Law and is attributed to limited grain boundary mediated plasticity due to Ca segregation. The electrical resistivity is also much higher than that expected for nanostructured Al. The main contribution comes from Ca segregations that lead to a fraction of electrons reflected or trapped by grain boundaries twice larger than in pure Al. The two-phase state was investigated by in-situ and ex-situ microscopy after annealing at 200 °C for 30 min, where precipitation of nanoscaled Al4Ca particles occurred and the mean grain size reached 35 nm. Annealing also significantly decreased electrical resistivity, but it remained much higher than that of nanostructured pure Al, due to Al/Al4Ca interfaces that reflect or trap more than 85% of electrons.
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•The Tb has been diffused into (Nd,Ce)FeB magnets through GBDP by magnetron sputtering, and obtained coercivity enhancement.•The effect of diffusion temperature, time and Tb coating ...thickness on the microstructure and magnetic properties of the diffused (Nd,Ce)FeB magnets have been discussed.•Ce enriched in REE-rich nodes at the triple junction, which has an inhibiting effect on the diffusion of Tb.
Grain boundary diffusion process (GBDP) of Tb on sintered Ce-substituted Nd-Fe-B magnets has been conducted by magnetron sputtering deposition of Tb film for coercivity enhancement. The microstructure and magnetic properties of the diffused (Nd,Ce)FeB magnets have been evaluated and the effect of diffusion temperature, time and Tb coating thickness on the Ce-substituted N35 magnets are discussed. The results show that a core-shell structure is formed around the main grains after GBDP, which promotes the coercivity enhancement of the magnets. A maximum coercivity enhancement from 10.9 kOe to 15.2 kOe is achieved in Ce-N35 magnets by deposition of 10 μm Tb films followed by diffusion processing at 900 °C for 300 min. The inhibition effect of Ce in GBDP of Tb is also evaluated using (Nd,Ce)FeB magnets with different Ce contents and discussed using diffusion model simulation implemented by Tb distributions through GD-OES. The rare earth element (REE)-rich nodes located at the triple junction of the grains may attribute to the Tb aggregation and the formation of TbFe2, resulting in the inhibition of further Tb diffusion.
To reduce or even eliminate the negative effects of grain boundaries (GBs) in nickel-based superalloys, the GB-segregation behaviors of 14 main alloying elements X of superalloys and their effects on ...the stability and fracture strength of Ni Σ5 001(210) GB were systematically investigated based on the concept of GB segregation engineering (GBSE), using first-principles calculations. We found that 10 alloying elements can segregate towards the GB. Their GB-segregation tendencies increase in the order of W < Mo < Al < Mn < Cu < Ti < Ru < Ta < Hf < Zr, due to the increase in atomic radius and electronegativity differences between X and host Ni atoms. The GB energies of X-segregated GBs depend linearly on the GB segregation energies of solutes X, the stronger segregation tendency of solute X could lower the GB energy and make the GB more stable. Except Cu, the segregations of the other 9 solutes increase the GB fracture strength. The segregation-induced GB stabilizing and strengthening mechanisms were further revealed on the electronic level. This work could be beneficial for manipulating the composition, structure and properties of Ni GBs and designing Ni-based superalloys with desired performances based on the GBSE.
•The grain boundary segregation of alloying elements-X is affected by atomic radius and electronegativity differenc.•The distribution map of grain boundary energy and work of separation of X-segregated grain boundary is obtained.•The mechanism of grain boundary X-segregation induced stabilization is the reduction of anti-bonding states.•The strengthening mechanism of the grain boundary segregation of X is the increase in bond strength of Ni-X.
The charge depletion near the grain boundary region in proton-conducting BaZrO3 electrolyte results in large potential barrier for the proton migration. Alternatively, a barium zirconate-cerate solid ...solution has been shown to improve the protonic conductivity by lowering the charge depletion along the grain boundary. Here, we report the modulation in grain boundary resistance of barium cerate – zirconate solid solution in the presence of trivalent rare-earth dopants differing in ionic radii. A hydrothermal assisted co-precipitation method was used for the preparation of two solid solutions, BaCe0.5−xZr0.5MxO3 and BaCe0.5Zr0.5−xMxO3 (M = La, Gd, Sm, Y; x = 0.10). The investigation of electrical properties revealed that the conductivity has a strong correlation with the ionic radii of the dopant and its interaction with the mobile protons. In comparison to larger sized dopants, the dopants with smaller ionic radii such as Y3+ has the lower trapping sites for proton, thereby, achieving the higher conductivity of 61.2 mS cm-1 at 600 °C under reducing atmosphere. The space charge quantification through Mott-Schottky approximation showed a negligible variation in barrier potential. However, the inclusion of Ce reduced the defect segregation towards the grain boundary due to the electrostatic interaction which in turn reduced the formation of the space charge layer thereby improving the conductivity.
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•The space charge potential with respect to addition of Ce has been investigated using Mott-Schottky model.•Ce forbids the segregation of charged defects towards the grain boundary and reduces the space charge layer.•10 mol% Y doped BaCe0.4Zr0.5O3 exhibited the higher protonic conductivity of 61.2 mS cm-1 with 0.6 eV activation energy.
Developing a facile method to prepare high‐quality perovskite films without using the antisolvent technique is critical for upscaling production of perovskite solar cells (PVSCs). However, the ...as‐prepared formamidinium (FA)‐based perovskite films often exhibit poor film quality with high density of defects if antisolvent is not used, limiting the photovoltaic performance and long‐term stability of derived PVSCs. Herein, this work adopts pre‐synthesized 3D methylammonium lead chloride (MAPbCl3) and 1D 2‐aminobenzothiazole lead iodide (ABTPbI3) microcrystals into self‐drying perovskite precursors, which serve as seed crystals to promote nucleation and growth of FAPbI3‐based perovskites without requiring antisolvent extraction. The combined binary microcrystals facilitate the formation of a dense and pinhole‐free perovskite film with a stable perovskite lattice and defect‐healed grain boundaries, enabling efficient charge carrier transfer and reduced non‐radiative recombination loss. As a result, the best‐performing inverted architecture device exhibits a champion power conversion efficiency of 23.27% for small‐area devices (0.09 cm2) and 21.52% for large‐area devices (1.0 cm2). These values are among the highest efficiencies reported for antisolvent‐free PVSCs. Additionally, the unencapsulated device shows enhanced moisture, thermal, and operational stabilities, and maintains 92% of its initial efficiency after being held at the maximum power point for 1000 h.
By employing pre‐synthesized 3D methylammonium lead chloride (MAPbCl3) and 1D 2‐aminobenzothiazole lead iodide (ABTPbI3) microcrystals into a self‐drying perovskite precursor, this work successfully modifies the crystallization process without antisolvents and reduces defects at grain boundaries. Furthermore, the best‐performing inverted device exhibits a champion power conversion efficiency of 23.27% for small‐area devices (0.09 cm2) and 21.52% for large‐area devices (1.0 cm2).
The properties of materials change, sometimes catastrophically, as alloying elements and impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that regular atomic ...patterns often arise as a result of this solute segregation at high-symmetry boundaries, but it is not known whether superstructures exist at general grain boundaries in polycrystals. In bismuth-doped polycrystalline nickel, we found that ordered, segregation-induced grain boundary superstructures occur at randomly selected general grain boundaries, and that these reconstructions are driven by the orientation of the terminating grain surfaces rather than by lattice matching between grains. This discovery shows that adsorbate-induced superstructures are not limited to special grain boundaries but may exist at a variety of general grain boundaries, and hence they can affect the performance of polycrystalline engineering alloys.
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