TEM Observations Research Articles

Cyclic hardening/softening and deformation mechanisms of a twip steel under reversed loading

Push-pull tests at fixed plastic strain amplitude or fixed stress amplitude were run on a TWIP steel, and followed by TEM observations, to analyze its cyclic behavior in relation with the deformation mechanisms. The kinematic and isotropic components of the flow stress were measured throughout the whole cyclic hardening/softening stages, and their evolution with the cumulated plastic strain was compared to those measured in tension. The rise of the internal stress was found responsible for the initial cyclic hardening, but this stress reached at most 50% of the flow stress, as compared to nearly 70% in tension. Special constitutive equations were identified to capture these evolutions, as well as the transition from hardening to softening. Both components of the flow stress slightly decrease during the softening stage, whose origin is discussed, based on TEM observations at peak stress amplitude, or after softening. The present measurements and TEM observations, combined with those from a previous study of twinning/detwinning kinetics in push-pull on the same steel [29], suggest that under cyclic loading, mechanical twinning cannot be responsible for significant kinematic hardening of intragranular nature (or “dynamic Hall-Petch effect”), but rather contributes to a back stress of intergranular origin.

Low-temperature synthesis of ultra-high-temperature HfC and HfCN nanoparticles

Hafnium carbide (HfC) and carbonitride (HfCxNy) nanoparticles were synthesized via reduction of HfO2 oxide with calcium hydride in the presence of carbon or carbon/nitrogen, refractory phases were synthesized at 1473 K, respectively. The average particle size of the obtained materials was in the range of 50–90 nm, allowing them to be ranged in a class of nanodispersed powders. The synthesized HfC had a lattice parameter of 0.4640 nm, which corresponds to the HfC1.0 stoichiometry. The diffraction spectra of the HfCxNy can be described by two isomorphic phases of approximate stoichiometry HfC0.91N0.09 (0.4629 nm) and HfC0.58N0.42 (0.4591 nm). Both HfC and HfCxNy nanoparticles had a predominantly cubic morphology, TEM observations revealed the presence of the continuous amorphous film with a thickness of 4–5 nm on their surface. The excess of reducing agent and external atmospheric pressure (argon) above the reaction volume were identified as the technological parameters, which strongly affect the quality of the synthesized HfCN powder. Also, it was shown that the formation of refractory HfC and HfCN particles under the presented experimental conditions is controlled by the dissolution-precipitation mechanism.

Biological characterization of the melanin biosynthesis gene Bcscd1 in the plant pathogenic fungus Botrytis cinerea

The gray mold fungus Botrytis cinerea produces dark-colored conidia and sclerotia due to deposition of melanin on the cell wall of these structures. However, the role of melanin biosynthesis on development and function of conidia and sclerotia have not been well elucidated in this fungus. This study disrupted the melanin biosynthesis gene Bcscd1 (for scytalone dehydratase) in the wild type B05.10, and the resulting mutants were compared with B05.10 and complementary mutants (COM) for growth and development, virulence and response to biotic/abiotic stresses. Three disruption mutants were obtained, and they did not differ from B05.10 and COM in mycelial growth rate on potato dextrose agar, however, they formed brownish conidia and scleotia deficient in melanogenesis, whereas B05.10 and COM formed grayish conidia and black sclerotia with normal melanogenesis. The disruption mutants were as aggressive as B05.10 and COM in infection of tobacco leaves. TEM observation showed that the disruption mutant ΔScd1-85 formed numerous tiny grooves in the conidial cell wall, thereby causing uneven thickness in the cell wall. In contrast, B05.10 and COM rarely formed tiny grooves in their conidial cell wall with even thickness. Moreover, the sclerotial cortex cell wall of ΔScd1-85 lost rigidity and the cells became collapsed, whereas the sclerotial cortex cell wall of B05.10 and COM appeared rigid, and the cells appeared plump in shape. The disruption mutants were more sensitive than B05.10 and COM in response to chemical stresses (H2O2, NaCl, SDS, sorbitol) for conidial germination and sclerotial survival. The sclerotia of the disruption mutants were more susceptible than the sclerotia of B05.10 and COM to infection by the mycoparasite Trichoderma koningiopsis. These results confirmed previous studies about the effect of melanin production on pathogenicity of B. cinerea, and expanded our knowledge about the role of Bcscd1 in cell wall integrity and in response to biotic and abiotic stresses.

<i>In Situ</i> TEM Observation and MD Simulation of Frank Partial Dislocation Climbing in Al–Cu Alloy

<i>In Situ</i> TEM Observation and MD Simulation of Frank Partial Dislocation Climbing in Al–Cu Alloy

Hierarchical structure and deformation behavior of a novel multicomponent β titanium alloy with ultrahigh strength

Based on the general [Mo] equivalent criterion and d-electron orbital theory, a new ultrahigh-strength β titanium alloy with eight major elements (Ti-4.5Al-6.5Mo-2Cr-2.6Nb-2Zr-2Sn-1V, TB17) for industrial applications was developed. An ingot of five tons was successfully melted by thrice vacuum consumable arc melting. The microstructure and elements partitioning of different conditions were investigated systematically. The results suggest that the hierarchical structures of micro-scale first α phase (α f ), nano-scale secondary α phase (α s ), and ultrafine FCC substructures can be tailored by solution plus aging (STA) heat treatment. The lateral and epitaxial growth of α f phase promotes the HCP-α to FCC substructure transformation with the help of elements partitioning during the aging process. Moreover, the element V, generally regarded as β stabilizer, is found to mainly concentrate in the Al-rich α f phase in this study probably due to its relatively lower content and the strong bonding energy of Al-V. The hierarchical structure has a strong interaction with dislocations, which contributes to achieve a superhigh strength of 1376 MPa. In addition, the plastic strain is partitioned in the multi-scale precipitates (such as the α and FCC substructures) and β matrix, resulting in a considerable plasticity. TEM observation demonstrates that high density entangled dislocations at interfaces and mechanical twins exist in the STA sample after tensile test. It can be deduced that both dislocation slipping and twinning mechanisms are present in this alloy. Therefore, TB17 alloy can serve as an excellent candidate for structural materials on aircrafts that require high strength and lightweight. • A novel β titanium alloy with ultrahigh strength of 1376 MPa was developed. • The multi-scale hierarchical structures were tailored by heat treatment. • The transformation from HCP-α to FCC-substructure was observed. • The V element mainly concentrated in the micro-scale α lath.

In-situ TEM investigation of dislocation loop reaction and irradiation hardening in H2+-He+ dual-beam irradiated Mo

• Irradiation can directly cause the formation of 1/2<111> loops and <100> loops. • Irradiation can directly induce 1/2<111>loop to transform into <100> loop. • Two 1/2<111> loops react to form <100> loop without strict size similarity condition. • Yield strength increment caused by irradiation-induced loops is analyzed. • Loop evolution and its relationship with irradiation dose are established. Through in-situ TEM observation during 30 keV H 2 + -He + dual-beam irradiation at 723 K, the reaction and transformation of dislocation loops in pure Mo were investigated, especially for <100> loops. Irradiation could directly cause the formation of 1/2<111> loops and <100> loops, but 1/2<111> loops were dominant. In-situ observation confirmed the formation mechanism of <100> loops, including direct irradiation induced mechanism, 1/2<111> loop direct conversion mechanism, and reaction mechanism of two 1/2<111> loops. Meanwhile, the reaction of two 1/2<111> loops to produce <100> loop should not require the strict size similarity condition. The reaction between 1/2<111> loops could also produce 1/2<111> loop, which was essentially a process in which one loop absorbed another one. The yield strength increment caused by irradiation-induced loops was analyzed, and its saturation value reached 0.48 GPa at 0.06 dpa. Compared with single He + irradiation, the number density and average diameter of loops increased significantly and more serious damage was caused under the synergistic effect of hydrogen and helium. The mechanism based on in-situ experimental observation was discussed in depth. Graphical Abstract .

Strengthening CrFeCoNiMn0.75Cu0.25 high entropy alloy via laser shock peening

For most metallic materials, surface hardening via modifying the near-surface microstructure is an effective method for improving mechanical properties. Among these processes, laser shock peening (LSP), which is versatile and nondestructive to the fabricated product shape, has received much attention. The present work has shown that the mechanical properties of CrFeCoNiMn0.75Cu0.25 high entropy alloy (HEA) have a significant improvement after LSP treatment. Yield strength more than two times of as-cast alloys is achieved in HEA samples treated by LSP for 4 cycles, due to the introduction of a gradient microstructure comprising subgrains, dense dislocations and nano-twins near the treated surface. The combination of dislocation hardening and mechanical twinning improves the strain hardening ability for the LSP-treated HEA, leading to excellent plasticity during tensile loading. Grain refinement also takes place via a faulting process of dislocation dissociation on nearly every {111} plane. Then, TEM observations and finite simulation of the LSP processed HEA samples after different levels of accumulated strain demonstrate that further tensile deformation is mainly accommodated by dislocations in the core region of the sample unaffected by the LSP, while the top LSP-affected layer accommodates little further plasticity as it is sufficiently hardened by the LSP. The present results indicate the possibility to develop gradient-structured HEAs with excellent mechanical properties using laser shock peening.

Salinity regulates the formation and hatching of Cryptocaryon irritans tomonts, affecting infectivity to Larimichthys crocea

Cryptocaryon irritans are the main pathogens of “white spot disease” on the body surface of marine teleosts, but the seawater environment affects pathogen development and virulence. This study investigated C. irritans tomont formation, cell division, hatching, and infectivity to Larimichthys crocea under eight salinity gradients (S0, S5, S10, S20, S30, S40, S45, and S50) to understand how C. irritans grows, develops and harms host fish in a variable natural environment. The results showed that the protomont hatching rate in S5, S10, S20 and S30 significantly increased over time, but the highest and lowest values appeared in S20 and S30, respectively; and the hatching rate in S20 is significantly higher than S30.The rate of tomont formation increased over treatment time, and the highest and lowest rates were observed in S20 and S0 groups, respectively. Seawater at extreme salinity (S0, S40, S45, and S50) significantly reduced the encystment duration to <5 min. In addition, the tomont division rate at S5, S10, S20, S30, S40 and S45 significantly increased over time, and the highest and the lowest values were recorded in S20 and S45, respectively; and the division rate at S20 is significantly higher than S45. The hatching rate in S5, S10, S20, S30, and S40 also significantly increased over time, but the highest and lowest values appeared in S20 and S5, respectively; and the hatching rate in S20 is significantly higher than S5. The tomonts treated with seawater of 5–45‰ salinity divided evidently, but most tomonts under 0 and 50‰ salinity dissolved. TEM observation showed the pellicular alveoli of tomonts in S40 and S45 were wider than S20, the chromatin was loose, and the cyst wall was separately compressed into thin-layered layers. Finally, the aspect ratios of the theronts hatched at S5, S10, S20, S30, and S40 significantly increased than those at S20, and the theronts became long and thin. The infectivity of the theronts hatched from tomonts under S5 and S40 salinity treatments were significantly lower than the control (S20) because of the changes of theront number and morphology. In conclusion, salinity changes regulate the formation and development of C. irritans tomonts. Low (S5) or high (S40) salinity inhibits the infectivity of incubated theronts.

The Largest Quasicrystalline Tiling with Dodecagonal Symmetry from a Single Pentablock Quarterpolymer of the AB1CB2D Type.

A quasicrystalline tiling pattern with tile size of ca. 60 nm has been discovered in the bulk state of a four-component pentablock polymer molecule of the AS1IS2P type, where A, S, I, and P denote poly(4-vinylbenzyldimethylamine), polystyrene, polyisoprene, and poly(2-vinylpyridine), respectively. The polymer samples used were prepared by anionic polymerizations and have narrow molecular weight distribution. The sample films were obtained by an extremely slow solvent-cast process from dilute solutions of tetrahydrofuran for 14 days. It has been found by TEM observation that the quarterpolymer, AS1IS2P-4 (Mn = 149 kg/mol, ϕA/ϕS1/ϕI/ϕS2/ϕP = 0.12/0.27/0.20/0.29/0.13), reveals two final stable structures, i.e., a 3.3.4.3.4 periodic tiling pattern as a minor component and a quasicrystalline (QC) tiling with dodecagonal symmetry as a major component, where the former includes a triangle/square number ratio of 2 and the latter has one of approximately 2.28, which is close enough to the ideal ratio, 4/ 2.31, for the triangle/square random tiling of the dodecagonal QC tiling (DDQC). Two structures were also clearly proved by SAXS diffraction patterns. Here, it should be noted this QC structure having a tile side length of ca. 60 nm was created with a single block polymer molecule.

In-situ TEM observations on interaction of basal dislocations and β′ phases in a Mg–Gd binary alloy

Here we report in-situ TEM observations on interaction between dislocations and β′ phases in aged Mg–Gd alloys. The slipping <a> dislocations squash into the interface and bow out to bypass the β′ phase, leading to a zigzag route of plastic flow. Orowan mechanism is determined as the main strengthen mechanism.

Smooth rims in Queen Alexandra Range (QUE) 99177: Fluid–chondrule interactions and clues on the geochemical conditions of the primordial fluid that altered CR carbonaceous chondrites

Queen Alexandra Range (QUE) 99177 is one of the least altered CR carbonaceous chondrite known, with mineralogical and isotopic characteristics that indicate a high level of pristinity. In this study, we have examined the so-called smooth rims that surround many type I chondrules in QUE 99177, using SEM, EPMA, and FIB-TEM techniques. We have characterized their constituent phases to unravel the precursor material(s) of smooth rims, assess their formation mechanisms, and constrain the conditions of the altering fluid. Smooth rims are the most common type of rims around type I chondrules and exclusively occur around chondrules with Silica-rich Igneous Rims (SIRs). Smooth rims consist of an Fe-rich, hydrous silicate material that is Si- and Fe-rich, with minor Mg, Al, Ca, and Mn, and gives low analytical totals measured by EPMA. TEM observations reveal that the Fe-rich silicate phase is an amorphous gel that contains unaltered crystalline phases and igneous glass. Crystalline phases consist of igneous, unaltered, zoned pyroxenes with compositions consistent with pyroxenes in SIRs, as well as albite and chromite. The amorphous gel preserves previous crystal outlines with morphologies consistent with silica (cristobalite) grains in SIRs and has a composition identical to pseudomorphic silica replacements in SIRs. Based on these observations, we conclude that smooth rims derive from low-temperature aqueous alteration of silica in SIRs by an Fe-rich fluid. We suggest that the Fe was derived by leaching of amorphous silicates in the matrix, which reacted rapidly with melted water ice, although alteration of Fe,Ni metal blebs in SIRs could potentially be an additional source of Fe. Silica underwent dissolution and replacement whereas feldspar and glass remained unaltered because (1) the fluid was slightly alkaline, (2) cristobalite has a reaction rate much higher than quartz and feldspar, and (3) the alteration was very limited and fast, indicating that it was due solely to melting of accreted water ice and there was no introduction of additional fluid from external sources.

Peptoid‐Directed Formation of Five‐Fold Twinned Au Nanostars through Particle Attachment and Facet Stabilization

AbstractWhile bio‐inspired synthesis offers great potential for controlling nucleation and growth of inorganic particles, precisely tuning biomolecule–particle interactions is a long‐standing challenge. Herein, we used variations in peptoid sequence to manipulate peptoid–Au interactions, leading to the synthesis of concave five‐fold twinned, five‐pointed Au nanostars via a process of repeated particle attachment and facet stabilization. Ex situ and liquid‐phase TEM observations show that a balance between particle attachment biased to occur near the star points, preferential growth along the [100] direction, and stabilization of (111) facets is critical to forming star‐shaped particles. Molecular simulations predict that interaction strengths between peptoids and distinct Au facets differ significantly and thus can alter attachment kinetics and surface energies to form the stars. This work provides new insights into how sequence‐defined ligands affect particle growth to regulate crystal morphology.

Impact of nano-oxides and injected gas on swelling and hardening of 18Cr10NiTi stainless steel during ion irradiation

The radiation performance of 18Cr10NiTi-ODS steel with the addition of Y2O3-ZrO2 nano-oxides was compared with that of non-ODS 18Cr10NiTi austenitic steel in terms of radiation-induced hardening and cavity swelling. TEM observations were performed to evaluate the effect of grain refinement and nano-sized oxide precipitates on microstructural changes and radiation tolerance in specimens exposed to 1.8 MeV Cr3+ or 1.4 MeV Ar+ irradiations over a wide range of temperatures. Formation of homogenously distributed voids was observed in both alloys upon Cr3+ irradiation but the ODS steel exhibited a considerably broader temperature range of void formation and lower overall swelling. Cavity formation in steels irradiated with Ar+ showed a strong dependence on Ar/dpa ratio. Argon addition was found to enhance the nucleation of cavities, which act as point defects sinks that reduce void swelling. The largest swelling resistance was observed in the ODS steel due to the combined effect of mechanical alloying and Ar gas co-injection. The hardening behavior was investigated in both non-irradiated and Ar+ irradiated samples at room temperature and elevated temperatures by means of nano-indentation. Along with the strengthening effect of radiation-induced dislocations, gas-filled nano-cavities strongly contribute to the hardening of irradiated alloys with a tendency to increase the barrier strength of nano-cavities with decreasing cavity size. The ODS variant was found to be somewhat less susceptible to radiation hardening compared to non-ODS 18Cr10NiTi steel.

Peptoid-Directed Formation of Five-Fold Twinned Au Nanostars through Particle Attachment and Facet Stabilization.

While bio-inspired synthesis offers great potential for controlling nucleation and growth of inorganic particles, precisely tuning biomolecule–particle interactions is a long-standing challenge. Herein, we used variations in peptoid sequence to manipulate peptoid–Au interaction, leading to synthesis of concave five-fold twinned, five-pointed Au nanostars via a process of repeated particle attachment and facet stabilization. Ex situ and liquid-phase TEM observations show that a balance between particle attachment biased to occur near the star points, preferential growth along the [100] direction, and stabilization of (111) facets is critical to forming star-shaped particles. Molecular simulations predict that interaction strengths between peptoids and distinct Au facets differ significantly and thus can alter attachment kinetics and surface energies to form the stars. This work provides new insights into how sequence-defined ligands affect particle growth to regulate crystal morphology.

TEM Observations of the Microstructural Changes in the Interfacial Zone of Explosively Welded Titanium/Steel Before and After Ex Situ and In Situ Heat Treatment.

This paper presents the comparison of the microstructure of the interface zone formed between titanium (Ti Gr. 1) and steel (P265GH+N) in various processing stages—directly after explosive welding versus the annealing state. Transmission electron microscopy technique served as an excellent tool for studies of the sharp interface in-between the waves. Directly after the welding process in this area, a thin layer of the metastable β-Ti (Fe) solid solution was observed. In the next step, two variants of annealing have been employed: ex situ and in situ in TEM, which revealed the complete information on the interface zone transformation. The results have shown that during the annealing at 600°C for 1.5 h, the diffusion of carbon towards titanium caused the formation of titanium carbides with a layered arrangement. Compared to our previous studies, the carbides found here have a hexagonal structure. Furthermore, changes in the dislocation structure were observed, indicating the occurrence of recovery processes. Possible reasons for differences observed in the microstructure of the interface formed due to ex situ and in situ annealing are also discussed. The microstructure observations are accompanied by the microhardness measurements, which showed that the annealing caused a significant reduction in the microhardness values.

In-situ TEM investigation of dislocation loop evolution in Al-forming austenitic stainless steels during Fe+ irradiation: Effects of irradiation dose and pre-existing dislocations

Alumina-forming austenitic (AFA) stainless steel with excellent properties has been regarded as a promising candidate of fuel claddings in Supercritical carbon dioxide (S-CO2) gas cooled nuclear reactor. In current work, in-situ irradiation with 400 keV Fe+ in transmission electron microscope were performed on AFA stainless steel to investigate the influences of irradiation dose and pre-existing dislocation lines on loop evolution at 773 K. The irradiation damage caused by irradiation-induced loops was also analyzed. In-situ TEM observation displayed the evolution and characteristics of dislocation loops, including initiation, migration, aggregation, growth, annihilation, and combination. Both Frank loops with b = 1/3<111> and perfect loops with b = 1/2<110> were formed, and at 0.54 dpa, 1/2<110> loops accounted for the majority and was up to 64.6%. Pre-existing dislocation lines obviously affected not only the evolution and distribution of dislocation loops, but also the degree of irradiation hardening. At the same irradiation dose, not only the size and number density of dislocation loops in the region with dislocation lines were smaller than those in the region without dislocation lines, but also the increment of yield strength and irradiation damage was lower before the formation of dislocation network. Increasing pre-existing dislocation density should be beneficial to improve the irradiation resistance of materials, but it needed to be comprehensively considered with the mechanical properties of the material. Al component played an important role in irradiation behavior of AFA steels and the corresponding mechanism was discussed subsequently.

TEM Study of Bainitic Low-Carbon HSLA Steel: The Orientation Relationships of Cementite

Abstract Two different bainitic structures can be present in steel depending on the heat treatment to which the samples are subjected. The two different types of bainitic structures exhibit a different orientation relationship between the cementite and the ferrite matrix. The Pitsch orientation relationship is observed in upper bainite while the Bagaryatski orientation relationship is observed in lower bainite [1]. Different heat treatment samples of low-carbon High Strength Low Alloy (HSLA) steel were studied using TEM observations in order to ascertain the orientation relationship between ferrite and carbide in the different bainitic structures and to determine whether this relationship may indicate the type of bainitic structure.

Accelerated Ionic and Charge Transfer through Atomic Interfacial Electric Fields for Superior Sodium Storage.

Atomic interfacial electric fields hold great potential for boosting ionic and charge transfer and accelerating electrochemical reaction kinetics. Here, built-in electric fields within the heterostructure are created by electrostatic assembly of unilamellar titano-niobate/graphene (reduced graphene oxide) nanosheets as building blocks. Scanning Kelvin probe microscopy confirms the existence of built-in electric fields by detecting the unbalanced surface potential of disparate nanosheets in the heterostructure, which facilitates ion and electron transfer, thus enabling an excellent reversible sodium storage capacity of 245 mAh g-1 at 0.05 A g-1. Theoretical analysis also confirms that the electric field can enhance the electric conductivity and facilitate electron transfer at the atomic interface. Moreover, in situ TEM observations confirm the homogeneous intercalation of sodium ions and very small volume expansion of the hybrid materials. As a result, a highly stable lifetime of 3000 cycles is achieved with capacity retention of 98.8%. This work attests the importance of accelerating ionic and charge transfer through atomic interfacial electric field for superior sodium storage.

Synthesis and Characterization of Supramolecular Nanotubes of Tetraphenylethylene-Porphyrin Conjugates

A series of tetraphenylethylene-porphyrin conjugates ((TPE)nH2Ps, n = 1, 2, and 4) are synthesized by Lindsey condensation and Adler-Longo methods. (TPE)nH2Ps are structurally characterized by 1H-NMR and MALDITOF mass spectrometry, and the photophysical and electrochemical properties are analyzed by UV-Vis absorption and fluorescence spectroscopies, and cyclic voltammetry. As the number of TPE increase, the porphyrin band gaps were slightly decreased, together with a small bathochromic shift of absorption and red shift in emission bands. The theoretical DFT calculations also showed good agreement with the experimental results. From TEM observations, (TPE)2H2P and (TPE)4H2P formed nanotubes with a diameter of 10 to 20 nm and a length of about 100 nm in a chlorobenzene/n-hexane (1:1 v/v) mixture and found that the wall thickness of nanotubes increased with the number of TPE moieties.

Photocatalytic hydrogen generation using polydiacetylene crystal nanostructures

Hydrogen generation has been demonstrated successfully using polydiacetylene (PDA) crystal nanostructures as a photocatalyst. Especially, PDA nanofibers (NFs) exhibited reproducible hydrogen generation. TEM observation clarified that surfactant SDS would promote adsorption of co-catalyst Pt NPs on the surface of PDA NFs, and Pt-deposited PDA NFs reproducibly generated hydrogen gas using methanol as a hole scavenger under light irradiation with Xenon lamp.