High-resolution Transmission Electron Microscopy Observations Research Articles

Microstructure and mechanical properties of the ZrB2–SiC eutectic phase obtained via induction plasma spheroidization

The microstructure, phase composition and interfaces of ZrB2-30 vol%SiC eutectic were investigated using TEM fabricated via induction plasma spheroidization(IPS). The obtained ZrB2–SiC hypereutectic consists of the alternating H–ZrB2 and 6H–SiC lamellas, and its composition is ZrB2-65.04 mol.%SiC. 3 types of interfaces including SiC lamella-ZrB2 granule, ZrB2 lamella-SiC lamella and ZrB2 lamella-ZrB2 granule present a preferential orientation relationship. High-resolution TEM observations show that the ZrB2 phase in the eutectic prefers to growth along the (101) plane. In addition, the mechanical properties of each phase in the IPS-treated ZrB2–SiC powder were measured by nanoindentation. Hardness and elastic modulus of ZrB2 granules were measured 17.86 GPa and 168.97 GPa, respectively, while these values were obtained 9.99 GPa and 120.66 GPa for ZrB2–SiC eutectic phase.

Second phase structure analysis and wear behavior of the ultra-high manganese steel

The high manganese steel is widely used in wear-resisting parts of the cone crusher, such as the grinding mill wall and crushing wall. In this work, the microstructure and wear resistance of an ultra-high manganese steel (Mn18Cr2Mo) were analyzed by electron diffraction and high-resolution transmission electron microscope (HRTEM) observation and impact abrasive wear test. The results show that there are spherical or block Mo2C and strip-like (Fe, Cr)3C phases in Mn18Cr2Mo manganese steel. The orientation of the Mo2C phase and the austenite matrix are [001]austenite// Mo2C, (020)austenite//(020)Mo2C, austenite//(202)Mo2C and austenite//(242)Mo2C. The that of the (Fe, Cr)3C phase and the austenite matrix are [011]austenite// (Fe, Cr)3C and austenite//(210)(Fe, Cr)3C. Besides, austenite is approximately parallel to (Fe, Cr)3C. The interface relation of (002)austenite is approximately parallel to (Fe, Cr)3C. Owing to high density dislocations, deformation bands, microcrystal and non-crystalline, the wear resistance of the Mn18Cr2Mo manganese steel is 1.83 times that of the Mn13 manganese steel under the impact abrasive wear test.

Temperature-Adaptive Ultralubricity of a WS2/a-C Nanocomposite Coating: Performance from Room Temperature up to 500 °C.

This study reports on the ultralubricity of a high-temperature resilient nanocomposite WS2/a-C tribocoating. The coefficient of friction of this coating remains at around 0.02 independently of a thermal treatment up to ∼500 °C, as confirmed by high-temperature tribotests. Moreover, the coating annealed at 450 °C keeps exhibiting a similar ultralubricity when cooled back down to room temperature and tested there, implying a tribological self-adaptation over a broad temperature range. High-resolution TEM observations of the tribofilms on the wear track unveil that WS2 nanoplatelets form dynamically via atomic rearrangement and extend via unfaulting geometrical defects (bound by partial climb dislocations). The (002) basal planes of the WS2 nanoplatelets, reoriented parallel to the tribo-sliding direction, contribute to a sustainable ultralubricity. The declining triboperformance beyond 500 °C is associated with sulfur loss rather than the transformation of WS2 into inferior WO3 via oxidation as suggested earlier. This self-adaptive WS2/a-C tribocoating holds promise for a constant ultralubrication with excellent thermal performance.

Effect of Lamellar Spacing on Creep Strength of <i>α</i>-Mg/C14-Mg<sub>2</sub>Ca Eutectic Alloy

In tensile tests, α-Mg/C14-Mg2Ca eutectic alloy with a lamellar structure is plastically deformed above 473 K but ruptures before yielding at temperatures below 423 K. This study investigates the effect of the α/C14 interface on the creep strength of α-Mg/C14-Mg2Ca eutectic alloy at 473 K under 40 MPa stress. The creep curves of the alloy exhibited three stages: a normal transient creep stage, minimum creep-rate stage, and accelerating stage. The minimum creep rate was proportional to the lamellar spacing, indicating that the α/C14 lamellar interface plays a creep-strengthening role. In high-resolution transmission electron microscope observations of the specimens after the creep test, a dislocations appeared within the α-Mg lamellae and were randomly distributed on the α/C14 interface. It was deduced that the α/C14 interface presents a barrier to dislocation glide and does not annihilate and/or rearrange its dislocation caused by the creep test.

Severe plastic deformation-produced gradient nanostructured copper with a strengthening-softening transition

Low-excess energy twin boundary can effectively stabilize the conventional grain boundary. It has been reported that deformation-activated nanotwins in nanograined metals produced by severe plastic deformation techniques can significantly enhance mechanical-thermal stability. However, fabrication, structural evolution, and the effect of grain size and twin thickness on the mechanical stability of nanograined-nanotwinned metals, where both the grain size and twin thickness reach the nanometer scale (especially grain size is lower than 40 nm), remain unclear. In this study, a gradient nanostructured layer containing a nanograined-nanotwinned sub-layer region and an extremely refined twin-free nanograined top surface layer with grain size as small as ~10 nm is achieved on copper by using an ultrahigh-strain rate single point diamond turning technique. High-resolution transmission electron microscope observations, atomistic molecular dynamic simulations, and nanoindetation tests were performed to reveal the size-dependent mechanisms of grain refinement and hardness along the gradient direction. The propensity of deformation multifold twinning is increased firstly in large-size nanograins and then decreased once grain size is below ~48 nm, finally replaced by detwinning to form extremely fine twin-free nanograins at the topmost surface layer. In other words, both the zero-macrostrain-induced deformation multifold twinning and symmetry-breaking-based detwinning processes can continuously refine nanograins along the gradient direction. Critical grain sizes for deformation multifold twinning and detwinning are discussed. Interestingly, a Hall-Petch strengthening-softening transition is discovered at a critical grain size of ~30 nm in the gradient nanostructured layer. The softening mechanisms are elucidated to be attributed to the twin thickness effect on deformation mode in nanograined-nanotwinned structures and the pure grain boundary-mediated plasticity in extremely fine twin-free nanograins. A series of critical twin thicknesses for softening in nanograins with different grain sizes are discussed; that is, the smaller the grain size is, the smaller the critical twin thickness will be. This study offers the potential for understanding and developing stable nanostructured metals.

Electronic states and metallic character of carbide Co/MoS2 catalytic interface

We report computer assisted density functional theory computations of electronic states in carbide Co9S8/MoS2 interface model. The interface model was previously proposed using crystallographic information from experimental high-resolution TEM observations; and directly observed by in-situ heating to confirm carbon deposit occurs at the sulfur edge of Co9S8/MoS2 which creates a thin carbide layer. In here, the total energy for carbon adsorption results eight times more favorable to occur at sulfur edge in comparison to molybdenum or cobalt replacement by permutation of carbon atoms as contained in C, CH, CH2 species by exothermic energies. Amorphous carbon excites 2 pz orbitals as observed on density of states near Fermi level (F E ), and 13% decreased in terms of charge carriers available, causing to decrease its chemical catalytic reactivity and bending of MoS2 slabs as carbon starts to accumulate at the sulfur edge mainly, which is attributed to charge distribution around the adsorption site caused by the foreign atoms, confirmed by electron density plots, that acts as ‘electron traps’. This helps us concluding that carbon replacement can induce change of selectivity in direct desulfurization pathway.

Microstructure evaluation of shear bands of microcutting chips in AA6061 alloy under the mechanochemical effect

The mechanochemical effect on the microcutting of AA6061 alloy is studied through characterization on the chip. A pronounced reduction of machining forces and chip thickness was observed with mechanochemical effect during microcutting. Furthermore, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) observations were performed on the chips and shear bands. The result reveals much coarser grains (24.6 μm in size) in the surfactant-affected chip than that in the surfactant-free chip (13.5 μm). Different grain orientations are induced by microcutting. {100}<001> and {110}<112> grain orientations are majority for surfactant-free chip, and {110}<001> and {100}<110> dominate most for all grain orientations for surfactant-affected chip. Additionally, since less localized shear strain and lower temperature are generated inside the shear band with mechanochemical effect, almost no recrystallization phenomena can be observed in this region. TEM analysis shows that fewer subgrains and dislocations could be observed inside the shear band of the surfactant-affected chip in comparison with the surfactant-free chip. Based on the high-resolution transmission electron microscopic (HRTEM) observations, dislocations were observed at the atomic scale. The results show that the main dislocation motion mode in shear bands of surfactant-free and surfactant-affected chips are dislocation climb and dislocation glide, respectively.

Analysis of interface properties and associated void of nanoscale Al precipitates in Al-Si alloys: First-principles calculations and experiment

In this paper, the properties of the interface between nanoscale Al precipitate and Si matrix were studied by the first-principles calculations combined with high-resolution transmission electron microscopy (HRTEM) observation. The purpose of this work is to reveal its role on the mechanical properties of the eutectic Si particle in Al-Si alloys. Meanwhile, the formation mechanisms of the void associated with every Al precipitate were also discussed in depth. The most thermodynamically stable interface structure was obtained by the calculation of the work of adhesion (Wad) and interfacial energy (γint) of the 12 models of Al(11¯2)/Si(220) interface. By analyzing the electronic structure of the interface, such as charge density difference, and density of states (DOS), it is found that strong AlSi covalent bonds are formed at the interface, making the bonding strength higher than the inside of the Al precipitate. Therefore, the microcracks tend to form and propagate firstly inside the nanoscale Al precipitates instead of Al/Si interface. In order to explain the interesting phenomenon that there is always a void associated with every Al particle, the diffusion barrier of Al atom in the Si matrix and the binding energy of solute atom and a vacancy were calculated. The results show that the vacancies can be introduced by the change of the volume and the migration of the solute-vacancy pair during the diffusion and precipitation progress. Then these vacancies will be discharged along the close-packed direction (Al<111>) and gather at the Al{111}/Si{001} interface, forming the visible void. This agrees very well with the experimental observation.

Low-power memristors based on layered 2D SnSe/graphene materials

The emerging two-terminal memristor with a conductance-adjustable function under external stimulation is considered a strong candidate for use in artificial memory and electronic synapses. However, the stability, uniformity, and power consumption of memristors are still challenging in neuromorphic computing. Here an Au/SnSe/graphene/SiO2/Si memristor was fabricated, incorporating two-dimensional graphene with high thermal conductivity. The device not only exhibits excellent electrical characteristics (e.g., high stability, good uniformity and a high ROFF/RON ratio), but also can implement biological synaptic functions such as paired-pulse facilitation, short-term plasticity and long-term plasticity. Its set and reset power values can be as low as 16.7 and 2.3 nW, respectively. Meanwhile, the resistance switching mechanism for the device, which might be associated with the formation and rupture of a filamentary conducting path consisting of Sn vacancies, was confirmed by high-resolution transmission electron microscopy observations. The proposed device is an excellent candidate for use in high-density storage and low-power neuromorphic computing applications.

Deformation Twinning in Octahedron-Based Face-Centered Cubic Metallic Structures: Localized Shear-Force Dipoles Drive Atomic Displacements

Twinning is found to impart favorable mechanical, physical and chemical properties to nanostructured materials. One important twinning mode, deformation twinning, prevails in coarse-grained hexagonal close-packed (HCP) crystalline materials and body-centered cubic (BCC) and face-centered cubic (FCC) nanomaterials under high-stress conditions. In FCC structures, the {111} deformation twinning is traditionally believed to nucleate and grow through layer-by-layer emission of 1/6 Shockley partial dislocations on consecutive {111} planes. Here, we report that by conducting high-resolution transmission electron microscopy (HRTEM) observation, deformation twinning is, for the first time, found to occur in nanocrystalline (Fe, Nb)23Zr6 particles with a Mn23Th6-type FCC structure that is composed of a Zr-octahedron-based FCC network connected by alloying elements Fe and Nb like the large FCC structure such as metal-organic-framework (MOF). Based on direct atomic-scale observations, we discover a new mechanism for the {111} deformation twinning in FCC structures. To form a [112]/(111) twin, for example, short ( (‾1‾11) planes within two adjacent (111) plane layers in the repeated three-layer sequence of (111) planes are shear deformed continuously by a shear-force dipole along the [112] direction like a domino effect, whereas the other (111) plane in the repeated sequence remains intact. Through this route, a small energy for twinning is expected because only 2/3 (111) planes need to be transformed to form a twin. In addition, a loading criterion for deformation twinning of a FCC NP under uniaxial compression is proposed based on our results. Our work here not only provides a fundamental understanding on deformation twinning in FCC structures, but also opens up studies of deformation behaviors in a class of Mn23Th6-type FCC materials.

Bastnäsite nanoparticles in carbonatite-syenite-hosted REE deposit： implication for La and Ce migration and bastnäsite growth

High-resolution transmission electron microscopy observations of bastnäsite from the Maoniuping rare earth element (REE) deposit at Panxi, SW China, revealed the presence of nanoparticles in the surface of bastnäsite crystal. The nanoparticles are identified as the bastnäsite nanocrystals, with 5–30 nm in length, by energy-dispersive spectrometry and fast fourier transform patterns. This represents the first observation of bastnäsite nanoparticle in nature, confirming a new form of migration and precipitation of La and Ce in the hydrothermal fluids. Meanwhile, we herein reveal that random attachment of small bastnäsite nanoparticles during aggregation-based growth initially produces large bastnäsite nanoparticles, driven by the tectonic stress. Subsequent evolution of the random aggregation and orientation of large bastnäsite nanoparticles result in bastnäsite crystal formation, driven by the tectonic stress and inherent surface stress. This underlines the bastnäsite crystallization by random aggregation and the significant role of tectonic stress in forming the bastnäsite crystal in REE deposit.

Atomic-scale dissecting the formation mechanism of gradient nanostructured layer on Mg alloy processed by a novel high-speed machining technique

Severe plastic deformation (SPD)-induced gradient nanostructured (GNS) metallic materials exhibit superior mechanical performance, especially the high strength and good ductility. In this study, a novel high-speed machining SPD technique, namely single point diamond turning (SPDT), was developed to produce effectively the GNS layer on the hexagonal close-packed (HCP) structural Mg alloy. The high-resolution transmission electron microscopy observations and atomistic molecular dynamics simulations were mainly performed to atomic-scale dissect the grain refinement process and corresponding plastic deformation mechanisms of the GNS layer. It was found that the grain refinement process for the formation of the GNS Mg alloy layer consists of elongated coarse grains, lamellar fine grains with deformation-induced-tension twins and contraction twins, ultrafine grains, and nanograins with the grain size of ∼70 nm along the direction from the inner matrix to surface. Specifically, experiment results and atomistic simulations reveal that these deformation twins are formed by gliding twinning partial dislocations that are dissociated from the lattice dislocations piled up at grain boundaries. The corresponding deformation mechanisms were evidenced to transit from the deformation twinning to dislocation slip when the grain size was below 2.45 μm. Moreover, the Hall-Petch relationship plot and the surface equivalent stress along the gradient direction estimated by finite element analysis for the SPDT process were incorporated to quantitatively elucidate the transition of deformation mechanisms during the grain refinement process. Our findings have implications for the development of the facile SPD technique to construct high strength-ductility heterogeneous GNS metals, especially for the HCP metals.

Exploring (1overline{1}2)-related ordered structure in oxidation-synthesized \u03b1-Fe2O3 nanowhiskers

Hematite (α-Fe2O3) nanowhiskers (NWs) synthesized via oxidation of iron-based substrates are a promising photoanode material for photoelectrochemical water splitting. Such synthesized α-Fe2O3 NWs have been found to contain ordered axial structures. Herein, we reveal that the known (1overline{1}2)-related ordered structure actually exists in bicrystalline α-Fe2O3 NWs instead of single-crystalline α-Fe2O3 NWs and that it is associated with another known (3overline{3}0)-related ordered structure. Through a spherical aberration (CS)-corrected high-resolution transmission electron microscopy (HR-TEM) investigation, the microstructural characteristic of the (1overline{1}2)-related ordered structure is verified to be periodic atomic column displacements serving as tensile strain accommodation. The HR-TEM observation are also supported by a monochromated O K-edge EELS analysis, which indicates that α-Fe2O3 NWs hosting the (1overline{1}2)-related ordered structure are indeed associated with lattice expansion. In sum, our microstructural study elucidates the root cause of the long-asserted relationship between the (1overline{1}2)-related ordered structure and oxygen vacancy ordering.

Dependence of the Generation Behavior of Charged Nanoparticles and Ag Film Growth on Sputtering Power during DC Magnetron Sputtering

Effects of sputtering power on the deposition rate and microstructure, crystallinity, and electrical properties of Ag films during direct current (DC) magnetron sputtering are investigated. Thin films (~ 100 nm) are deposited at sputtering powers of 10, 20, 50, 100, 200 and 300 W and analyzed by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and a four-point probe. The film deposited at a sputtering power of 10 W has the lowest growth rate, but the highest crystalline quality, with the lowest full width at half maximum (FWHM) and the lowest resistivity. The film deposited at a sputtering power of 200 W has the highest growth rate, and the second best crystalline quality in view of FWHM and resistivity. The film deposited at a sputtering power of 50 W has the moderate growth rate, and the worst crystalline quality in view of FWHM and resistivity. High-resolution TEM observations reveal that films deposited at sputtering powers of 10 and 200 W have far fewer defects, such as grain boundaries, dislocations and stacking faults than those deposited at a sputtering power of 50 W. Such deposition behavior could be explained by sputtering power, which affected the generation of the charged nanoparticles. And the high quality of films could be obtained at a high deposition rate, in which charge plays an important role.Graphic

Grain boundary-mediated plasticity accommodating the cracking process in nanograined gold: In situ observations and simulations

In this study, the underlying atomic-scale plastic deformation mechanisms responsible for the crack propagation process in nanograined gold thin film with an average grain size of ~10 nm (ranging from ~2 nm to ~22 nm) is investigated by the in situ high-resolution transmission electron microscope observations (i.e., a homemade device with atomic force microscope inside transmission electron microscope) and atomistic molecular dynamic simulations. The real-time results based on the experimental observations and simulations uncover consistently that the crack propagation in nanograined gold thin film is accommodated by the grain boundary-mediated plasticity, which may result in the grain coalescence between neighboring nanograins. Furthermore, we find that the grain boundary-mediated plasticity is grain size-dependent, i.e., GB dislocation activities-induced grain rotation in relative larger grains and GB migration in relative smaller grains in comparison with a critical grain size of ~ 10 nm.

Studies of magnetic behavior of chemically synthesized interacting superparamagnetic copper ferrite nanoparticles

The magnetic property of nanocrystalline copper ferrite (CuFe2O4, CF) with narrow size distribution synthesized by co-precipitation technique is investigated. The occupancy of cations among the tetrahedral and octahedral sites is determined from the Rietveld analysis of the X-ray diffraction (XRD) pattern. Microstructural, optical, and magnetic properties of the ferrite nanoparticles (NPs) are analyzed in detail using high-resolution transmission electron microscopy (HRTEM), UV–Vis absorption spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, and superconducting quantum interference device (SQUID) magnetometer. The blue shift in optical band gap compared to bulk one is observed due to the smaller particle size. The particle size distribution inferred by mathematical analysis from the variation of difference between field cooled (FC) and zero field cooled (ZFC) magnetization with temperature supports the HRTEM observation. M–T curve initially proposes the presence of superparamagnetic (SPM) NPs with non-interacting core and interacting surface spin. Therefore magnetic behavior of the sample is explained by core–shell model. This fact is substantiated from the good fitting of M–H curves at different temperatures using a combined equation generated from ‘Law of approach’ to saturation magnetization and Langevin function. With the lowering of temperature magnetic blocking and random spin freezing process occurs. The ratio of $$M_{{\text{R}}} /M_{{\text{s}}}$$ (remanent/saturation magnetization) less than 0.5 for all temperatures implies the presence of uniaxial anisotropy of the single-domain NPs. The unsaturated nature of the M–H curves may be due to the combined contribution from SPM core and spin canting in the mixed spinel CuFe2O4.

Structural Characterization of Al65Cu20Fe15 Melt-Spun Alloy by X-ray, Neutron Diffraction, High-Resolution Electron Microscopy and Mössbauer Spectroscopy.

The aim of the work was to characterize the structure of Al65Cu20Fe15 alloy obtained with the use of conventional casting and rapid solidification-melt-spinning technology. Based on the literature data, the possibility of an icosahedral quasicrystalline phase forming in the Al-Cu-Fe was verified. Structure analysis was performed based on the results of X-ray diffraction, neutron diffraction, 57Fe Mössbauer and transmission electron microscopy. Studies using differential scanning calorimetry were carried out to describe the crystallization mechanism. Additionally, electrochemical tests were performed in order to characterize the influence of the structure and cooling rate on the corrosion resistance. On the basis of the structural studies, the formation of a metastable icosahedral phase and partial amorphous state of ribbon structure were demonstrated. The possibility of the formation of icosahedral quasicrystalline phase I-AlCuFe together with the crystalline phases was indicated by X-ray diffraction (XRD), neutron diffraction (ND) patterns, Mössbauer spectroscopy, high-resolution transmission electron microscopy (HRTEM) observations and differential scanning calorimetry (DSC) curves. The beneficial effect of the application of rapid solidification on the corrosive properties was also confirmed.

Monolayer goldene intercalated in graphene layers

Non-independent metalenes cannot maintain structural stability, but they are expected to possess extraordinary properties that provide excellent prospects for applications in physics, chemistry, and materials science. In this study, a simple physical method is used to prepare a metalene sandwiched between graphene layers. High-resolution transmission electron microscopy observation reveals Au (111) single-layer goldene intercalated with graphene monolayers. Density functional theory calculations indicate that the Au atoms in goldene are stabilized through weak interactions between graphene and Au atoms. The discovery of this structure consisting of single layers of goldene sandwiched between graphene layers suggests ideas for the preparation of such layered materials and opens up the possibilities for their wider application in physics and materials science.

Morphological Modulation of Conducting Polymer Nanocomposites with Nickel Cobaltite/Reduced Graphene Oxide and Their Subtle Effects on the Capacitive Behaviors.

This work reports on the urchin-like architecture-based nickel cobaltite (NiCo2O4)/reduced graphene oxide (rGO)/conducting polymer [polyaniline (PANI) or polypyrrole (PPy)] nanocomposites prepared through a hydrothermal synthesis procedure, followed by in situ polymerization techniques. Subsequently, these materials are subjected to electrochemical investigation to search for promising electrode materials for energy storage applications. Interestingly, the morphology of NiCo2O4 varies upon the addition of rGO as well as nitrogen-doped rGO (N-rGO). When it is composite with rGO, it forms an urchin-like architecture, and with N-rGO, it forms nanoparticle structures having a diameter of 90 ± 10 nm. Further, these nanostructures are intricately coated by conducting polymers (such as PANI and PPy) as evidenced from the field emission scanning electron microscopy and high-resolution transmission electron microscopy observations, and the overall shape does not alter after the modification. All composites are investigated thoroughly by Fourier transform infrared, Raman, X-ray powder diffraction, and X-ray photoelectron spectroscopies to confer the formation and presence of polymers. The NiCo2O4/rGO/PPy nanocomposites exhibit an excellent specific capacitance of 1547 ± 5 F/g at a current density of 0.5 A/g, a better energy density of 34.37 ± 0.11 W h/kg at 0.5 A/g, a notable power density of 99.98 ± 0.31 W/kg at 0.5 A/g, and retains its 94 ± 1% specific capacitance after 5000 charge-discharge cycles. The uniform coating over the urchin-like architecture by conducting polymers constructs a typical morphology, and possibly, it is the key factor behind gaining such superior electrochemical behavior owing to (a) the enhancement of surface area and (b) the combination of double-layer capacitive and pseudocapacitive properties. Furthermore, a symmetric flexible supercapacitor device made of NiCo2O4/rGO/PPy nanocomposites provides superior electrochemical behavior.

Structure and Stability of the (001) Surface of Co3O4

Atomic-scale structures and energetics of the Co3O4 (001) surface are studied combining high-resolution transmission electron microscopy observations and density functional theory calculations. The...