Transmission Electron Microscopy System Research Articles

The formation mechanism of the nano-equiaxed (α+β) duplex phases structure during aging process after high pressure torsion deformation

The aim of this study was to investigate the effects of the nano equiaxed (α+β) duplex phases (NEDP) structure on the mechanical properties of metastable β type titanium alloy. To achieve this, the Ti-3.5Al-5Mo-4 V (Ti-B20) alloy was subjected to high pressure torsion (HPT) deformation and aging treatment. The results demonstrated that the NEDP (α+β) structure exhibited high thermal stability even after prolonged aging. Through systematic transmission electron microscopy (TEM) characterization, it was confirmed that the formation of the NEDP structure was attributed to the generation of crystal defects such as dislocation cells and dislocation pile-ups during HPT deformation. In addition, it was found that the orientation relationship between the α and β phases was [21̅1̅0]α//[111]β after prolonged aging, and the interlocked of coherent interface between the α and β phases allowed the NEDP (α+β) structure to maintain its thermal stability during long-term aging. Therefore, this paper provides valuable guidance for the fabrication of nano-equiaxed dual-phase structures.

Magnificent tensile strength and ductility synergy in a NiCoCrAlTi high-entropy alloy at elevated temperature

The study reported a novel L12-strengthening NiCoCrAlTi high-entropy alloy (HEA) with an outstanding synergy of tensile strength and ductility at both ambient and high temperatures. The HEA was prepared by arc melting and cold-rolling, followed by isothermal aging at designed precipitation temperatures to achieve coexistence of recrystallized and non-recrystallized grains. Transmission electron microscopy (TEM) characterization revealed a high density of rod-like and spheroidal L12 precipitates distributing in the micro/nanograins and non-recrystallized regions in the annealed specimens. The yield stress, ultimate tensile stress, and ductility of the HEA were ∼1300 MPa, 1610 MPa, and 14 % at room temperature, respectively. Such excellent mechanical properties of ∼1060 MPa, 1271 MPa, and 25 % were maintained to 600 °C, which was superior to most reported HEAs and Co- and Ni-based superalloys to date. Systematic TEM analysis unveiled at low strain, the deformation mechanism was mainly controlled by dislocations and stacking faults (SFs). With the increase in strain, the deformation mode gradually transferred to dislocations, deformation twins (DTs), and SFs. Moreover, the complex interaction between SFs and L12 precipitates, including the shearing and blocking effects, was frequently observed, contributing to the high tensile strength. Thus, the extremely high tensile strength and sustained ductility at 600 °C mainly originate from the cooperation among interaction between L12 precipitation and dislocations and extensive SFs, DTs, immobile Lomer-Cottrell (L-C) locks formed from interactions between SFs and SFs/DTs, hierarchical SFs/DTs networks, as well as hetero-deformation-induced strengthening. Such a unique deformation mechanism breaks the strength-ductility trade-off of the HEA at high temperatures.

Unlocking the mysterious polytypic features within vaterite CaCO3

Calcium carbonate (CaCO3), the most abundant biogenic mineral on earth, plays a crucial role in various fields such as hydrosphere, biosphere, and climate regulation. Of the four polymorphs, calcite, aragonite, vaterite, and amorphous CaCO3, vaterite is the most enigmatic one due to an ongoing debate regarding its structure that has persisted for nearly a century. In this work, based on systematic transmission electron microscopy characterizations, crystallographic analysis and machine learning aided molecular dynamics simulations with ab initio accuracy, we reveal that vaterite can be regarded as a polytypic structure. The basic phase has a monoclinic lattice possessing pseudohexagonal symmetry. Direct imaging and atomic-scale simulations provide evidence that a single grain of vaterite can contain three orientation variants. Additionally, we find that vaterite undergoes a second-order phase transition with a critical point of ~190 K. These atomic scale insights provide a comprehensive understanding of the structure of vaterite and offer advanced perspectives on the biomineralization process of calcium carbonate.

Creep-induced redistribution of alloying elements in CZ1 zirconium alloys

Introducing minor alloying elements is an effective strategy to improve the corrosion and mechanical properties of zirconium alloys for nuclear applications. During in-reactor service, external environment and stress can affect the distribution of alloying elements, substantially changing the degradation process of zirconium alloys. To date, there is a lack of in-depth understanding of the interaction between creep and microchemistry changes. Here, we conducted systematic transmission electron microscopy (TEM) and atom probe tomography (APT) investigations to address creep-induced redistribution of alloying elements in CZ1 (Zr-Sn-Nb-Fe-Cr-Cu) zirconium alloy with different initial microstructures. Nb, Fe, Sn, and Cu are found to co-segregate at grain boundaries. The higher the intermediate annealing temperature, the larger the Gibbsian interfacial excesses of solute elements are. We further demonstrate that creep can reduce the excess value of Fe at grain boundaries due to the coarsening of Zr-Fe-Cr second phase particles via grain boundary and dislocation pipe diffusion. At the same time, the excess value of Sn is increased by diffusing from the matrix to grain boundaries. Moreover, Cu as a minor element in the concentration range of 0.05–0.3 wt.% is found to segregate at dislocations to form the Cottrell atmosphere and develop Cu-rich nanoclusters for suppressing dislocation motion. The new understanding of the segregation and clustering of minor alloying elements provides guidance for developing zirconium alloys with enhanced creep resistance.

Effect of grain size on critical twinning stress and work hardening behavior in the equiatomic CrMnFeCoNi high-entropy alloy

While the impact of grain boundary strengthening on dislocation slip is particularly effective in the equiatomic CrMnFeCoNi high-entropy alloy (HEA), its effect on deformation twinning remains unclear. To better understand how a grain size reduction affects the onset of deformation twinning and the work hardening behavior of the CrMnFeCoNi HEA, chemically homogeneous, nearly untextured, and single-phase face-centered cubic alloys with different grain sizes were investigated. Tensile tests were performed at 293 and 77 K and interrupted at different strains followed by systematic transmission electron microscopy observations. In all cases, deformation twinning occurs above a critical stress that is independent of temperature. This uniaxial twinning stress decreases from ∼785 to ∼615 MPa when the grain size increases from 6 to 242 µm, respectively, following the Hall-Petch equation. The resistance of the grain boundaries against slip and twinning is found to be nearly identical (Hall-Petch slope: ∼500 MPa·µm1/2) but the twinning stress extrapolated to infinite grain size (592 ± 30 MPa) is larger than the uniaxial friction stress against dislocation glide at 293 and 77 K (130 and 320 MPa, respectively). Deformation twinning at 77 K is found to sustain a high work hardening rate when it is triggered in a plastic regime dominated by planar glide of dislocations. In contrast, it does not significantly contribute to the work hardening rate at 293 K when dislocation cells have already formed and the dislocation mean free path is smaller than the mean twin spacing.

Arsenic Precipitation in Heavily Arsenic‐Doped Czochralski Silicon

Heavily arsenic‐doped Czochralski (HAs‐CZ) silicon is an important substrate material for manufacturing power electronic devices. The arsenic impurities may be in a supersaturated status in a certain temperature range during the HAs‐CZ silicon crystal growth or the device fabrication. Then, whether and how the arsenic impurities can precipitate in HAs‐CZ silicon is an intriguing and practically significant issue that has never been addressed. Herein, it is first found that arsenic precipitation can occur in HAs‐CZ silicon when subjected to appropriately prolonged anneals at 550–950 °C. The resulting second‐phase precipitates are confirmed to be of orthorhombic SiAs phase, with the lattice parameters of a = 7.12 Å, b = 9.11 Å, and c = 9.00 Å, through systematic transmission electron microscopy characterizations. Moreover, it is discovered that the presence of vacancies/interstitial silicon atoms in HAs‐CZ silicon promotes/inhibits arsenic precipitation.

New dislocation dissociation accompanied by anti-phase shuffling in the α″ martensite phase of a Ti alloy

Martensite phase plays a crucial role in the shape memory effect and superelasticity. In this study, a new dislocation dissociation was found in the α″ martensite phase of a Ti-based biomedical shape memory alloy by systematic transmission electron microscopy analysis and first-principles calculations. The slip direction of dislocation was along 〈101〉, which was recently found in a single-crystalline martensite fabricated using stress-induced martensitic transformation. The 〈101〉 dislocation dissociated into two partial dislocations, and the Burgers vector for both partial dislocations was equal to 1/2<101>. A planar fault with an anti-phase shuffling of basal plane was observed between the dissociated partial dislocations. The dissociation reaction of the 〈101〉 dislocation was summarized as 〈101〉 → 1/2<101> + anti-phase shuffling boundary + 1/2<101>. Most of the observed dislocations were in the form of screw dislocations, and the dissociated partial dislocations were twisted around the dislocation line.

Atomic layer deposition of void-free ZnFe2O4 thin films and their magnetic properties

Atomic layer deposition (ALD) is an excellent method for depositing single-component thin films, but it is not a suitable technique for directly depositing multi-component films because different precursor sources usually require incompatible growing conditions. Therefore, an effective method to grow multi-component films by ALD is to prepare multi-layers films and then post-anneal them to form multi-component films. However, as has been reported so far, Kirkendall voids always inevitably form during post-annealing of ZnO/Fe3O4 thin films to form pure-phase zinc ferrite (ZnFe2O4) films, making it hard to obtain a compact zinc ferrite films without voids. Therefore, it is still challenging to prepare high-quality spinel films without voids by ALD. In this work, we used ALD to grow pure-phase and Kirkendall void-free ZnFe2O4 spinel thin films. The systematic transmission electron microscopy and selected area electron diffraction investigations prove that the as-grown ZnO and Fe3O4 thin films are composed of a number of nano-crystalline particles embedded in an amorphous matrix. This kind of unique microstructure is favorable for the thermal diffusion of elements during the solid-state reaction to form high-quality spinel films at a low temperature. The magnetic characterizations indicate that the obtained ZnFe2O4 thin films have a net magnetization, exhibiting ferrimagnetic behavior at room temperature and a very low blocking temperature TB=17K±2K.

STEM observation of a multiphase nucleus of spheroidal graphite

As part of a study on nucleation and growth of graphite in spheroidal graphite irons, systematic transmission electron microscopy observations are carried out. For this, focused ion beam is used to prepare thin foils that are precisely located and present an even thickness thus allowing high quality characterization.In this work, a nucleus with an overall elongated shape that was found in a perfectly round graphite spheroid is investigated by scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy. The acquisition of STEM images at different tilt of this nucleus is achieved which allows illustrating the precipitation process of the many phases constituting it. For the first time, a remaining of inoculant could be identified within the nucleus. Furthermore, the elongated shape of the nucleus is a definitive proof that the shape of nucleus does not determine that of the emerging graphite precipitate.

Thin-film growth and structural characterization of a novel layered iridate Ba7Ir3O13+δ

Iridates have attracted immense interest since their strong spin–orbit coupling (SOC) can lead to rich exotic phenomena such as a Jeff = ½ Mott insulating state. Here we report a novel iridate discovered in our efforts which aimed to synthesize Ba2IrO4 thin films. Through systematic transmission and scanning transmission electron microscopy studies, we have shown this new compound possesses a layered orthorhombic structure with the composition of Ba7Ir3O13+δ (BIO). This material is an insulator with an optical band gap of ∼1.3 eV. Furthermore, we found that the thin films of this material can be grown on differently orientated perovskite substrates or MgO substrates. Although all these films maintain an identical crystallographic orientation, i.e. its c-axis perpendicular to the substrate surface, they form various domain structures dependent on the substrate. When (001)-oriented LaAlO3 and (111) oriented SrTiO3 perovskites are used as substrates, the domains show 12 fold and 6 fold symmetry respectively, and the domain orientations are highly coherent and the domain-walls are atomically sharp. However, the films on the (110) oriented MgO substrates feature much less coherent domain walls and thread dislocations occur at the domain boundaries. These findings not only reveal a new playground for the study of the novel SOC physics of iridates, but also provide a route to tailor the domain wall structure via epitaxial lattice mismatch in films.

Precipitation of secondary phase in Mg-Zn-Gd alloy after room-temperature deformation and annealing

Mg-Zn-RE alloys reinforced with quasicrystals have been investigated extensively because they show excellent balance in mechanical properties. Here, we perform deformation and annealing for Mg-1.50Zn-0.25Gd (at.%) alloy at various temperatures, aimed to gain nanoscale precipitates. We find that after the room-temperature compression, the non-basal dislocation, stacking fault and twining are identified in the as-deformed samples, offering clear evidence that these deformation mechanisms can accommodate room-temperature deformation. We also perform systematic transmission electron microscopy observations of the precipitates in both the as-deformed and as-annealed samples. The results identify the formation of a large amount of secondary-phase precipitates, I-phase and MgZn2, when annealed at 200°C, and precipitation of a small amount of W-phase when annealed at 400°C.

Simultaneously Induced Self‐Assembly of Poly(3‐hexylthiophene) (P3HT) Nanowires and Thin‐Film Fabrication via Solution‐Floating Method on a Water Substrate

AbstractA single‐step fabrication of thin film of conjugated polymers (CPs) cannot be employed because inducement of self‐assembly of CPs and a thin‐film fabrication process are both required. This study introduces a simple and facile one‐pot strategy for fabricating uniform, large‐area organic semiconducting thin film without complex process. During the solvent evaporation of floated poly(3‐hexylthiophene) (P3HT) solution on water substrate, well‐dissolved P3HT chains are self‐assembled into 1D P3HT nanowires (NWs) with formation of thin film simultaneously. Through systematic transmission electron microscopy (TEM) experiments, this study also reveals for the first time the unique self‐assembly mechanism behind the formation of P3HT NWs from the incorporation of P3HT nanoparticles (NPs). In addition, highly uniform organic–inorganic hybrid thin films consisting of well‐defined P3HT/quantum dots (QDs) hybrid NWs can be created easily via a solution‐floating method by dropping P3HT and QDs dissolved solution. The floated P3HT NWs thin film on the surface of water can be easily transferred onto a Si wafer, bestowing advantages for the fabrication of a high‐performance organic field‐effect transistor device with a carrier mobility of 0.0856 cm2 V−1 s−1.

Evidence of Staphylococcus Aureus Deformation, Proliferation, and Migration in Canaliculi of Live Cortical Bone in Murine Models of Osteomyelitis

Although Staphylococcus aureus osteomyelitis is considered to be incurable, the major bacterial reservoir in live cortical bone has remained unknown. In addition to biofilm bacteria on necrotic tissue and implants, studies have implicated intracellular infection of osteoblasts and osteocytes as a mechanism of chronic osteomyelitis. Thus, we performed the first systematic transmission electron microscopy (TEM) studies to formally define major reservoirs of S. aureus in chronically infected mouse (Balb/c J) long bone tissue. Although rare, evidence of colonized osteoblasts was found. In contrast, we readily observed S. aureus within canaliculi of live cortical bone, which existed as chains of individual cocci and submicron rod-shaped bacteria leading to biofilm formation in osteocyte lacunae. As these observations do not conform to the expectations of S. aureus as non-motile cocci 1.0 to 1.5 μm in diameter, we also performed immunoelectron microscopy (IEM) following in vivo BrdU labeling to assess the role of bacterial proliferation in canalicular invasion. The results suggest that the deformed bacteria: (1) enter canaliculi via asymmetric binary fission; and (2) migrate toward osteocyte lacunae via proliferation at the leading edge. Additional in vitro studies confirmed S. aureus migration through a 0.5-μm porous membrane. Collectively, these findings define a novel mechanism of bone infection, and provide possible new insight as to why S. aureus implant-related infections of bone tissue are so challenging to treat. © 2016 American Society for Bone and Mineral Research.

Selective-area growth of GaN nanocolumns on Si(111) substrates for application to nanocolumn emitters with systematic analysis of dislocation filtering effect of nanocolumns

The growth of highly uniform arrays of GaN nanocolumns with diameters from 122 to 430 nm on Si (111) substrates was demonstrated. The employment of GaN film templates with flat surfaces (root mean square surface roughness of 0.84 nm), which were obtained using an AlN/GaN superlattice (SL) buffer on Si, contributed to the high-quality selective-area growth of nanocolumns using a thin Ti mask of 5 nm thickness by rf-plasma-assisted molecular beam epitaxy. Although the GaN template included a large number of dislocations (dislocation density ∼1011 cm−2), the dislocation filtering effect of nanocolumns was enhanced with decreasing nanocolumn diameters (D). Systematic transmission electron microscopy (TEM) observation enabled us to explain the dependence of the dislocation propagation behavior in nanocolumns on the nanocolumn diameter for the first time. Plan-view TEM analysis was performed for nanocolumns with D = 120–324 nm by slicing the nanocolumns horizontally at a height of ∼300 nm above their bottoms and dislocation propagation through the nanocolumns was analyzed by the cross-sectional TEM observation of nanocolumns with D ∼ 200 nm. It was clarified that dislocations were effectively filtered in the bottom 300 nm region of the nanocolumns, the dislocation density of the nanocolumns decreased with decreasing D, and for narrow nanocolumns with D < 200 nm, dislocation-free crystals were obtained in the upper part of the nanocolumns. The dramatic improvement in the emission properties of GaN nanocolumns observed with decreasing diameter is discussed in relation to the decreased dislocation density. The laser action of InGaN/GaN-based nanocolumn arrays with a nanocolumn diameter of 170 nm and a period of 200 nm on Si under optical excitation was obtained with an emission wavelength of 407 nm. We also fabricated red-emitting InGaN-based nanocolumn light-emitting diodes on Si that operated at a wavelength of 652 nm, demonstrating vertical conduction through the AlN/GaN SL buffer to the Si substrate.

Nanoscale icosahedral quasicrystal phase precipitation mechanism during annealing for Mg–Zn–Gd-based alloys

We have performed a systematic transmission electron microscopy study of precipitated particles in Mg–Zn–Gd-based alloys after annealing. We find that the Mg–Zn phases and ellipsoidal-shaped icosahedral quasicrystal phases (I-phase) are precipitated and distributed by partition in the grains, which can be attributed to the composition segregation of the alloying elements in the as-cast alloys. The content of alloying element is found to be crucial to the precipitation of nanoscale I-phase. The nanoscale I-phase ranges in size from a few to dozens of nanometers, revealing that the precipitation process satisfies the classical nucleation and growth theory.

Fe-site substitution effects on superconductivity and microstructure of phase-separated K0.8Fe1.75Se2 superconductor

Series of Fe-site–substituted K0.8Fe1.75−xMxSe2 (M = Cu, Ni, Mn, Zn, Co, Cr) single crystals of nominal composition have been prepared and microstructural features have been studied. Systematic Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) investigations reveal that the substitution effect can be classified into two types: Ni, Co and Cr atoms can be homogenously doped into the superconducting stripes of the phase-separated K0.8Fe1.75Se2 system, while the Cu, Mn and Zn atoms can hardly be introduced into the superconducting stripes, which instead give rise to the formation of new non-superconducting K-M-Se phases embedded in the sample. The complex physical measurement data reported in the literature can be nicely explained by the microstructure picture.

Precise Control of Quantum Dot Location within the P3HT-b-P2VP/QD Nanowires Formed by Crystallization-Driven 1D Growth of Hybrid Dimeric Seeds

Herein, we report a simple fabrication of hybrid nanowires (NWs) composed of a p-type conjugated polymer (CP) and n-type inorganic quantum dots (QDs) by exploiting the crystallization-driven solution assembly of poly(3-hexylthiophene)-b-poly(2-vinylpyridine) (P3HT-b-P2VP) rod-coil amphiphiles. The visualization of the crystallization-driven growth evolution of hybrid NWs through systematic transmission electron microscopy experiments showed that discrete dimeric CdSe QDs bridged by P3HT-b-P2VP polymers were generated during the initial state of crystallization. These, in turn, assemble into elongated fibrils, forming the coaxial P3HT-b-P2VP/QDs hybrid NWs. In particular, the location of the QD arrays within the single strand of P3HT-b-P2VP can be controlled precisely by manipulating the regioregularity (RR) values of P3HT block and the relative lengths of P2VP block. The degree of coaxiality of the QD arrays was shown to depend on the coplanarity of the thiophene rings of P3HT block, which can be controlled by the RR value of P3HT block. In addition, the location of QDs could be regulated at the specific-local site of P3HT-b-P2VP NW according to the surface characteristics of QDs. As an example, the comparison of two different QDs coated with hydrophobic alkyl-terminated and hydroxyl-terminated molecules, respectively, is used to elucidate the effect of the surface properties of QDs on their nanolocation in the NW.

Formation of Intermetallics in Sn-0.9Cu and Sn-0.7Cu-0.08Ni Solders

A study of the crystallography and composition of the constituent intermetallics in a binary eutectic Sn-0.9Cu and a ternary Sn-0.7Cu-0.08Ni solder alloy was carried out by systematic transmission electron microscopy. The results are analyzed and discussed in relation to the phase diagrams available in the literature. The intermetallic phases formed at different stages during solidification are present in different sizes and morphologies and also appear to exhibit different crystallographic properties and composition. The fine eutectic intermetallic in the Ni-containing alloy could not be matched with any of the known intermetallics in the ternary Sn-Cu-Ni system.

Direct Observation of an Ordered Phase in (1120) Plane InGaN Alloy

A polar-dependent phase with spontaneous atomic ordering is found in a nonpolar (1120) plane In0.08Ga0.92N film grown by metalorganic vapor-phase epitaxy. An atomic arrangement is a periodic sequence of group-III sublattices, such as In0.04Ga0.96N/In0.12Ga0.88N, that is only observed in a nitrogen polar region through systematic transmission electron microscopy investigation. Cathodoluminescence (CL) in the nitrogen polar region, i.e., spontaneously ordered atomic structure of InGaN, reveals anomalous emission behavior, specifically an S-shape-like (increase–decrease) temperature dependence of CL peak energy. It is suggested that the spontaneous ordered atomic structure of InGaN plays the role of a localized center owing to band gap shrinkage, which has been reported in other III–V alloy systems.

Probing the Characteristic Deformation Behaviors of Transformation-Induced Plasticity Steels

The characteristic micromechanical behaviors of contrasting transformation-induced plasticity (TRIP) steels were investigated under tensile loading by in-situ neutron diffraction and transmission electron microscopy in detail. As demonstrated by the lattice strain development from the neutron diffraction, in the TRIP steel with ∼10 pct RA, microyielding of soft ferrite was responsible for the first stress partition, but a second stress sharing was caused by effective martensitic transformation. In the TRIP steel with less than 5 pct RA, where the contribution from the martensitic transformation was minor, stress partition took place virtually between the ferrite and bainite phase. Probing with systematic transmission electron microscopy (TEM) observations, we pin down the inherent correlation between the microstructural evolutions and the stress partition mechanism. Based on the experimental observations, the factors influencing the work-hardening behavior of TRIP steels are discussed.