Ion Beam Transmission Electron Microscopy Research Articles

How important is carbon sequestration in phytoliths within the soil?

Abstract Background and aims An overlooked fraction of the terrestrial carbon (C) pool is that associated with biogenic silica deposited in plants (phytoliths), so-called PhytOC. This fraction is small compared with the main C pools, but is of interest because it could be a long-term C sink as phytoliths may protect organic C from mineralization. However, the topic is hotly contested and unclear due to both methodological and theoretical limitations. Scope We aim to review this topic, with specific emphasis on: (i) the range of C concentrations associated with phytoliths; (ii) soil phytolith preservation and subsequent organic C mineralization; and (iii) global estimates of C sequestration within PhytOC. Conclusions Recent work has suggested that [PhytOC] could be much greater than currently acknowledged, but also highly variable and dependent on cell silicification types. A short case study using cryo‐Scanning Electron Microscopy (cryo-SEM), X‐ray microanalysis (EDX), plus Focused Ion Beam (FIB) and Scanning Transmission Electron Microscopy (STEM) on the culms of a sedge (Schoenus caespititius) confirmed this thinking. Understanding of both phytolith and PhytOC fates in soil is poor. We suggest that phytolith residence time should be seen as a gradient. Such a continuum is explained by different phytolith sizes, types and chemistry, which will also have contrasting PhytOC. Our estimation of C sequestration as PhytOC each year (11–190 Tg C yr−1) represents between < 1% and 13% of the C that could be sequestered globally in soils (estimated at 1400 Tg C yr−1). We conclude that (1) more research is needed to improve our understanding of the formation and fate of PhytOC in terrestrial ecosystems and (2) it would be unwise to put our faith in PhytOC sequestration or other related methodologies to “solve” the climate crisis.

Maximization of the blue-white contrast of ancient porcelain decorations from Jingdezhen imperial kiln by Co-spinel formation

By analyzing the chromaticity values of the ancient blue-and-white porcelain combined with the imitated experiment, the color information is quantified and ‘color dispersion’ mechanism is clarified. Energy Dispersive X-Ray Fluorescence (EDXRF), field emission scanning electron microscopy (FE-SEM), the advanced focused ion beam transmission electron microscopy (FIB-TEM) technique, etc, were used to study the characteristics and the ‘color dispersion’ mechanism of the blue-and-white porcelain in Jingdezhen imperial kiln. The ‘color dispersion’ of the blue-and-white porcelain is mainly caused by Co-element structure stability, which is verified by the archaeological experiment results. Blue-and-white decoration lines diffusion length (named as ‘color dispersion’) are long (i.e. fuzzy) which has few or no spinel crystals in the body-glaze interaction layer of the blue-and-white porcelain. However, the blue and white porcelain with the clear lines has a large number of spinel crystals. The experimental results enrich the literature on the history of ceramic manufacturing techniques in Jingdezhen.

Spontaneous Reconstruction of Copper Active Sites during the Alkaline CORR: Degradation and Recovery of the Performance.

Understanding the reconstruction of electrocatalysts under operational conditions is essential for studying their catalytic mechanisms and industrial applications. Herein, using spatiotemporally resolved Raman spectroscopy with CO as a probe molecule, we resolved the spontaneous reconstruction of Cu active sites during cathodic CO reduction reactions (CORRs). Quasi-in situ focused ion beam transmission electron microscopy (FIB-TEM) revealed that under prolonged electrolysis, the Cu surface can reconstruct to form nanometer-sized Cu particles with (111)/(100) facets and abundant grain boundaries, which strongly favor the formation of an inactive *CObridge binding site and deteriorate the CORR performance. A short period of anodic oxidation can efficiently remove these reconstructed nanoparticles by quick dissolution of Cu, thus providing an effective strategy to regenerate the Cu catalysts and recover their CORR performance. This study provides real-time in situ observations of Cu reconstruction and changes in the binding of key reaction intermediates, highlighting the decisive role of the local active site, rather than the macroscopic morphology, on adsorption of key reaction intermediates and thus CORR performance.

Insight into continuous growth mechanisms of ZrO2-Al2O3 composite PEO coatings with superior cavitation erosion resistance

The ZrO2-Al2O3 composite coatings were fabricated on TiB2/2024Al composite by the plasma electrolytic oxidation (PEO) treatment with either monoclinic (m-ZrO2) or tetragonal zirconia (t-ZrO2) nanoparticles (NPs) addition. Such PEO coatings showed a double layer morphology due to the limited physical motion ability of ZrO2 colloidal particles into coatings. t-ZrO2 remained as the main phase in ZrO2-Al2O3 PEO composite coatings whether t-ZrO2 or m-ZrO2 NPs were added. Based on the microstructure analyses by focus ion beam plus scanning electron microscopy and transmission electron microscopy, ZrO2 showed netlike morphology in the outer layer with NPs partially melted at the mesh boundary and lines-like morphology fully melted at the inner mesh in the coating. A prominent morphological transition of ZrO2 was clearly observed at the interface between outer layer and inner layer. Therefore, the continuous growth mechanisms of ZrO2-Al2O3 composite PEO coatings were proposed in terms of morphological evolution process. Furthermore, electrochemical measurements and cavitation erosion tests were conducted to study the comprehensive surface properties (corrosion and impact resistance) of ZrO2-Al2O3 composite coatings. As a result, the ZrO2-Al2O3 composite PEO coatings with m-ZrO2 addition exhibited the greatly reduced corrosion rate, and superior cavitation erosion resistance. Accordingly, the underlying mechanisms for these improved properties were intensively discussed.

Effect of cooling rate on the composition and chemical heterogeneity of quench-induced grain boundary η-phase precipitates in 7xxx aluminium alloys

The mechanical properties and stress corrosion cracking (SCC) resistance of 7xxx series aluminium alloys are significantly affected by the composition and distribution of precipitates formed during heat treatment. In particular, their quench sensitivity is related to the formation of η-phase precipitates that nucleate heterogeneously on grain boundaries at lower cooling rates after solution treatment, which has been a key factor restricting the gauge of hot rolled plates in the aerospace industry. To better understand the effects of slower cooling rates on the composition of quench-induced grain boundary precipitates (Q-GBPs) found in thick plate 7xxx alloys, plasma focused ion beam and high-resolution scanning transmission electron microscopy were used to obtain accurate composition data. The η-phase Q-GBPs have a complex- branched morphology, which develops higher aspect ratios and secondary arms as the cooling rate is reduced. Only a small change in average composition of Q-GBPs was found with cooling rate; but a large scatter was observed. This is caused by significant Zn/Cu/Al composition gradients developing along their principal growth directions in both AA7050 and AA7085 alloys. This concentration gradient did not reduce significantly after a T76 treatment. Simulations of Q-GBP growth with different cooling rates using a CALPHAD-informed phase-field model, with the η-phase represented by a two-sublattice model, gave results consistent with experimental observations. Chemical gradients were predicted to develop in the Q-GBPs due to the changing local equilibrium at the growth front during the cooling. The influence of this non-homogeneous microchemistry on the SCC behaviour of 7xxx alloys is briefly discussed.

Microstructural study on the effect of thermo-physical properties for plasma sprayed Ni and Ni[sbnd]20Cr splats formed on Cu substrates

Ni and Ni20Cr powder particles were air plasma sprayed onto polished copper substrates to gain insight into the flattening behaviour of both Ni and Ni20Cr particles (splats) formed on a relatively low melting point material (i.e. copper) substrate. The extent of any interfacial bonding, along with any splat-substrate intermixing derived from the different thermo-physical properties of the sprayed particles were characterised. The plan-view features of the splats were captured using scanning electron microscopy (SEM), while splat-substrate interfacial properties were examined using focused ion beam microscopy (FIB) and transmission electron microscopy (TEM). The chemical characteristics of the splat and substrate were analyzed using energy-dispersive X-ray spectroscopy (EDS). The addition of ∼20 wt% chromium to Ni (i.e the Ni20Cr particles) was observed to significantly influence the spreading behaviour of the particles on impact with the substrate, both the extent of any splat splashing and the splat flattening ratio, as well as the microstructure of the splat-substrate interface, including the degree of substrate melting and the likelihood of material from the substrate jetting into the body of the splat. The evidence of intermixing between the splat and the copper substrate for both feedstocks is an intriguing feature of this study. The presence of a thermodynamically formed copper oxide layer during substrate preheating also influenced both the splat formation behaviour and substrate melting as observed by both microstructural examination and investigation through numerical models.

The origin of microstructural alterations in M50 bearing steel undergoing rolling contact fatigue

M50 bearing steel designed for aerospace applications exhibits unique microstructural alterations at the subsurface during rolling contact fatigue (RCF). In this work, three types of microstructural alterations, light etching region (LER), white etching band (WEB) and white etching area (WEA) are systematically studied. A hardness profile at the subsurface is obtained by microindentation with material softening being detected. It is found that the material softening is associated with the formation of WEBs. The microstructural nature of the three types of microstructural alterations is revealed by detailed characterisation using focused ion beam milling and transmission electron microscopy. It is found that LERs, WEBs and WEAs are a manifestation of the decay of the parent martensite and all consist of dislocation cells or fine grains, indicative of plastic deformation. Through subsurface stress field analysis, LER, WEBs and WEAs are found to be caused by different stress components. The similarities and differences of the microstructural alterations in M50 and in widely studied 100Cr6 are discussed where the stability of carbides is believed to play a key role. The difference in material response in the microstructural alterations in M50 is further analysed and modelled based on Kocks and Mecking theory, with different types of activated dislocation slip systems found to be the root cause for such difference. Based on the material response analysis for WEBs, a material softening model for M50 during RCF is established by considering residual plastic strain accumulation, yielding prediction results in agreement with experimental measurement.

Failure mechanism and life correlation of Inconel 718 in high and very high cycle fatigue regimes

High and very high cycle fatigue (HCF and VHCF) behaviours and failure mechanism of Inconel 718 were investigated via ultrasonic fatigue tests and microscopic characterization techniques. It is shown that the S-N curve continuously decreases in the HCF regime, while forming a plateau for VHCF. Cracks initiate from a single site on the surface or subsurface. Site-specific focused ion beam sampling and transmission electron microscopy characterization reveal that the crack initiation is governed by slip-induced localized plastic deformation. Microcracks initiate near non-metallic inclusions or slip band-grain boundary site. Plus, a life correlation model considering the size of micro-defect was proposed to better correlate the fatigue life.

Halloysite-gold core-shell nanosystem synergistically enhances thermal conductivity and mechanical properties to optimize the wear-resistance of a pheonlic-PBO/PTFE textile composite liner

Polymer-textile liner composites have potential applications in aerospace applications for reducing the abrasion damage of moving parts during operation owing to their self-lubrication, light weight, and high loading capacity. Herein, Au nanoparticles (AuNPs) are successfully loaded into the lumen of halloysite nanotubes (HNTs) to construct an HNTs-Au peasecod core-shell nanosystem to optimize the wear resistance of phenolic resin-based poly(p-phenylene benzobisoxazole) (PBO)/polytetrafluoroethylene (PTFE) textile composites. Transmission electron microscope (TEM) characterization reveals that the AuNPs are well-dispersed inside the HNTs, with an average diameter of 6–9 nm. The anti-wear performance of the HNTs and Au-reinforced PBO/PTFE composites is evaluated using a pin-on-disk friction tester at 100 MPa. Evidently, the addition of HNTs-Au induces a 27.9% decrease in the wear rate of the composites. Possible anti-wear mechanisms are proposed based on the analyzed results of the worn surface morphology and the cross-section of the tribofilm obtained by focused ion beam transmission electron microscopy.

Direct Atomic-Scale Insight into the Precipitation Formation at the Lanthanum Hydroxide Nanoparticle/Solution Interface

Understanding precipitation formation at lanthanum hydroxide (La(OH)3) nanoparticle-solution interfaces plays a crucial role in catalysis, adsorption, and electrochemical energy storage applications. Liquid-phase transmission electron microscopy enables powerful visualization with high resolution. However, direct atomic-scale imaging of the interfacial metal (hydro)oxide nanostructure in solutions has been a major challenge due to their beam-driven dissolution. Combining focused ion beam and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we present an atomic-scale study of precipitation formation at La(OH)3 nanoparticle interfaces after reaction with phosphate. The structure transformation is observed to occur at high- and low-crystalline La(OH)3 nanoparticle surfaces. Low-crystalline La(OH)3 mostly transformed and high-crystalline ones partly converted to LaPO4 precipitations on the outer surface. The long-term structure evolution shows the low transformation of high-crystalline La(OH)3 nanoparticles to LaPO4 precipitation. Because precipitation at solid-solution interfaces is common in nature and industry, these results could provide valuable references for their atomic-scale observation.

Promoting bonding strength between internal Al-Si based gradient coating and aluminum alloy cylinder bore by forming homo-epitaxial growth interface

The reliability of interface bonding strength between internal thermal sprayed coating and cylinder bore substrate causes special concern in the field of internal thermal spraying. In this study, a new approach is proposed to create a large area of metallurgical bonding at the coating-substrate interface by the design of Al-Si based gradient coating on A383 alloy cylinder bore. The internal coating comprises hypereutectic Al-Si-Cu-Fe top layer, near-eutectic Al-Si bond layer, and transition layers, which were fabricated by an advanced internal rotating plasma spraying technology. The tensile adhesive and interfacial indentation tests were conducted to investigate the adhesion properties of the coating. The crystallographic relationship of the bond layer-substrate interface was further investigated using the focused ion beam and high-resolution transmission electron microscopy. Results show that the Al-Si based gradient coating presents both desirable surface hardness (385.1 ± 46.6 HV0.025) and bonding strength (46.4–58.1 MPa). The tensile failure mode is the fracture at the subsurface of the top layer, without fracture at the coating-substrate interface. The superior bonding strength results from the formation of chemical interdiffusion (estimated thickness of 700–1500 nm) and homo-epitaxial growth at the interface. This study provides a framework for forming a reliable metallurgical bonding interface of internal thermal-sprayed coating.

Nanostructural domains in martian apatites that record primary subsolidus exsolution of halogens: Insights into nakhlite petrogenesis

Abstract The microstructures of selected F-, Cl-, and OH-bearing martian apatite grains, two in Northwest Africa (NWA) 998 (cumulus apatites, embedded in pyroxene) and a set of four in Nakhla (intercumulus apatites), were studied by focused ion beam–transmission electron microscopy (FIB-TEM) techniques. Our results show that the nanostructure of martian apatite is characterized by a domain structure at the 5–10 nm scale defined by undulous lattice fringes and slight differences in contrast, indicative of localized elastic strain within the lattices and misorientations in the crystal. The domain structure records a primary post-magmatic signature formed during initial subsolidus cooling (T <800 °C), in which halogens clustered by phase separation (exsolution), but overall preserved continuity in the crystalline structure. Northwest Africa 998 apatites, with average Cl/F ratios of 1.26 and 2.11, show higher undulosity of the lattice fringes and more differences in contrast than Nakhla apatites (average Cl/F = 4.23), suggesting that when Cl/F is close to 1, there is more strain in the structure. Vacancies likely played a key role stabilizing these ternary apatites that otherwise would be immiscible. Apatites in Nakhla show larger variations in halogen and rare-earth element (REE) contents within and between grains that are only a few micrometers apart, consistent with growth under disequilibrium conditions and crystallization in open systems. Nakhla apatite preserves chemical zonation, where F, REEs, Si, and Fe are higher in the core and Cl increases toward the outer layers of the crystal. There is no evidence of subsolidus ionic diffusion or post-magmatic fluid interactions that affected bulk apatite compositions in NWA 998 or Nakhla. The observed zonation is consistent with crystallization from a late-stage melt that became Cl-enriched, and assimilation of volatile-rich crustal sediments is the most plausible mechanism for the observed zonation. This work has broader implications for interpreting the chemistry of apatite in other planetary systems.

Hydrogen-prompted heterogeneous development of dislocation structure in Ni

First documented in 1875, the deterioration of mechanical properties of hydrogen-containing metals is a longstanding yet unsolved problem in materials science. In this work, the evolution of dislocation structures in differently orientated grains (i.e., near [100], [110], and [111]) of the uncharged and hydrogen-charged (400 and 1200 ppm) polycrystalline Ni were systematically investigated by combining electron backscatter diffraction, focused ion beam and scanning transmission electron microscopy. By using site-specific characterization methods, for the first time, we discover that hydrogen-enhanced localized plasticity (HELP) is orientation-dependent, with the following sequence: [100] > [111] > [110]. Massive incompatibility between differently orientated grains, induced by the orientation dependence of HELP, contributes to the premature intergranular fracture of Ni, especially for the 400 ppm H-charged Ni. Our results suggest that optimizing orientation distribution is a potential approach for enhancing metals' resistance to hydrogen damage. The relative contribution of HELP and hydrogen-enhanced decohesion (HEDE) mechanisms in hydrogen embrittlement of Ni is also analyzed quantitatively for 400 and 1200 ppm H-charged samples. In the 400 ppm H-charged Ni, a strong synergistic interaction exists between HELP and HEDE mechanisms, and the HELP mechanism plays a critical role in premature fracture. By contrast, in the 1200 ppm H-charged Ni, the HELP effect on final failure is much less significant and HEDE is the dominant embrittlement mechanism.

Mapping mineralogical heterogeneities at the nm-scale by scanning electron microscopy in modern Sardinian stromatolites: Deciphering the origin of their laminations

Stromatolites are found throughout the geological record and have received strong attention because they provide precious information about paleoenvironments and microbial paleobiodiversity. However, while this information is interpreted based on our knowledge about modern analogs, the latter remains incomplete. Here, we investigated the environmental and biological information recorded by modern stromatolites in Mari Ermi, a coastal pond in Western Sardinia that experiences severe seasonal evaporation and large salinity variations. For this purpose, we combined a variety of analytical tools, allowing to characterize the mineralogical composition of these stromatolites from the bulk, centimeter-scale to the nanometer-scale and assess the spatial distribution of the mineral phases. In particular, we used quantified x-ray elemental maps provided by energy dispersive x-ray spectrometry analyses coupled with scanning electron microscopy (SEM-EDXS). These maps were processed using an innovative data treatment allowing (i) mineral phase recognition, (ii) assessment of the mineral formula of each phase and (iii) mapping of their spatial distribution with a spatial resolution down to a hundred nanometers. Overall, this proved as an efficient approach to unravel mineralogical heterogeneities and detect informative mineral phases overlooked by bulk analyses. Mari Ermi stromatolites were mostly composed of magnesian calcite with an average Mg/Ca of ~0.1–0.2 as indicated by bulk analyses. However, microscopy observations revealed variable Mg-contents of calcite with a specific distribution, the most Mg-enriched calcites (Mg/Ca up to ~0.5) being systematically distributed around microbial remnants. Moreover, Mari Ermi stromatolites comprised an alternation of Mg-richer and Mg-poorer laminae, which paralleled the varying enrichment in microbial remnants of alternating laminae. The combination of SEM-EDXS, focused ion beam milling and transmission electron microscopy allowed to detect several minor phases, hardly or not detected at all by bulk analyses, such as aragonite, a variety of clay minerals as well as a gypsum-like phase. An additional calcium sulfate phase, best interpreted as S-apatite (cesanite), was detected by SEM-EDXS. Overall, this mineral assemblage and its definite spatial distribution record only discrete stages of the evaporation of Mari Ermi lagoon. Moreover, microorganisms seem to have a major control on Mg substitution in calcite. As a result, the dynamics of microbial populations, influenced by the salinity variations induced by evaporation in the lagoons, generates the formation of laminae in Mari Ermi stromatolites.

Transmission electron microscopy study of a high burnup U-10Zr metallic fuel

To support the development of U-10 wt.% Zr (U-10Zr) metallic fuel for Generation IV sodium-cooled fast reactors, we analyzed a Na-bonded solid U-10Zr fuel cross-section that was irradiated to a burnup of approximately 13.2 at.% at the Fast Flux Test Facility (FFTF). Advanced characterization techniques, including site-specific sample preparation by focused ion beam (FIB) and scanning transmission electron microscopy (STEM), were used to reveal the Zr redistribution and characterize the fuel matrix and secondary phases (such as solid fission products) present at the end of life. Results showed that the fuel pin cross-section is divided into three major concentric zones: a Zr-rich central region, a Zr-lean intermediate region, and a Zr intermediate peripherical region. The phase characterization revealed that the irradiation environment enhanced the development and stabilization of phases not predicted by the standard equilibrium U-Zr phase diagrams. Comparing the current results with the ones from previous studies, it is reaffirmed that the radial temperature profile and the time spent in the reactor, rather than the fuel burnup, are the two factors that most influence the formation of redistribution zones and their extension along the fuel cross-section. Various solid fission products, such as lanthanides, ZrRu, BaTe, CsI, and Ba and Sr oxides, precipitated inside the fission gas pores. This study provides unprecedented nanoscale understandings in the irradiated U-10Zr fuel system that may benefit fuel performance modelling and advanced fuel development.

Lubrication mechanism of a strong tribofilm by imidazolium ionic liquid

Friction modifiers (FMs) are surface-active additives added to base fluids to reduce friction between rubbing surfaces. Their effectiveness depends on their interactions with rubbing surfaces and may be mitigated by the choice of the base fluid. In this work, the performance of an imidazolium ionic liquid (ImIL) additive in polyethylene-glycol (PEG) and 1,4-butanediol for lubricating steel/steel and diamond-like-carbon/diamond-like carbon (DLC—DLC) contacts were investigated. ImIL-containing PEG reduces friction more effectively in steel—steel than DLC—DLC contacts. In contrast, adding ImIL in 1,4-butanediol results in an increase in friction in steel—steel contacts. Results from the Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and focused ion beam-transmission electron microscopy (FIB-TEM) reveal that a surface film is formed on steel during rubbing in ImIL-containing PEG. This film consists of two layers. The top layer is composed of amorphous carbon and are easily removed during rubbing. The bottom layer, which contains iron oxide and nitride compound, adheres strongly on the steel surface. This film maintains its effectiveness in a steel—steel contact even after ImIL additives are depleted. Such film is not observed in 1,4-butanediol where the adsorption of ImIL is hindered, as suggested by the quartz crystal microbalance (QCM) measurements. No benefit is observed when the base fluid on its own is sufficiently lubricious, as in the case of DLC surfaces.This work provides fundamental insights on how compatibilities among base fluid, FM, and rubbing surface affect the performance of IL as surface active additives. It reveals the structure of an ionic liquid (IL) surface film, which is effective and durable. The knowledge is useful for guiding future IL additive development.

In Situ FIB-TEM-TOF-SIMS Combination Technique: Application in the Analysis of Ultra-Light and Trace Elements in Phyllosilicates

At present, a single technical method has difficulty in obtaining microscopic data of ultra-light elements, trace elements, and crystal structures in samples simultaneously. This work combined an in situ focused ion beam—transmission electron microscopy—time of flight secondary ion mass spectrometry (FTT) technique and analyzed the composition and crystal structure of four phyllosilicate samples. These materials were comprised of antigorite, clinochlore, and cookeite phases. An FIB sample preparation technique was found to provide a sample thickness suitable for TEM observations and a degree of surface roughness appropriate for TOF-SIMS analysis. In addition, the relative amounts and distributions of various elements could be obtained, as well as crystal structure data, such that the composition and crystal structure of each specimen were determined. The in situ FTT method demonstrated herein successfully combines the advantages of all three analytical techniques and offers unique advantages with regard to analyzing ultra-light and trace elements as well as the structural data of phyllosilicates.

Microstructural study of Ni and Ni-20Cr particles plasma sprayed on stainless steel substrate at 300 ˚C

In thermal spray coatings, the bonding and adherence of the spray particles (called splats) to the substrate surface significantly influence the overall quality of the finished coating. In this study, both elemental Ni and Ni-20Cr powders were plasma sprayed onto polished stainless-steel substrates held at 300 ˚C. Splat formation for both feedstock materials was investigated and compared. Detailed studies of the microstructure of single splats as well as the splat-substrate interface were performed using scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). The additions of Cr to nickel promoted improved adherence of the splat to the substrate and modified splat formation by improving wettability during spreading. More intriguingly, energy dispersive spectroscopy (EDS) mapping and line scans, performed on TEM cross-sections, revealed splat-substrate inter-diffusion and chemical inter-mixing. Of particular note, a ‘jet’ of iron/chromium-based material, arising presumably from the substrate, was noted to be intimately intermixed into the interior of the nickel splat.

Understanding the pitting mechanism of super ferritic stainless steel in bromide solutions: The role of Ti/Nb–Mo precipitates with a core–shell structure

The core–shell structure of Ti/Nb precipitates in UNS S44660 was revealed by focused ion beam–transmission electron microscopy (FIB–TEM). In the bromide solution, the Nb in the shell is preferentially and selectively eluted, leaving a micro-crevice. The combined effect of the strong anodic adsorption of Br− and the large strain field in the shell promotes the free dissolution of residual Mo. Micro-crevices surround the Ti-rich nucleus and expand into pits. Consequently, pitting preferentially initiates at the Ti/Nb precipitates, which was verified by scratching electrode tests and immersion tests in bromide solutions.

Microstructure and properties of the C/C-superalloy brazed joint by Ni-based filler

Stable and reliable C/C-superalloy joints that can withstand high-temperature service will become extremely important. Ni-based filler was employed for successful brazing of C/C composites and In738LC superalloy in this study. The brazed joints contained isothermal solidification zone (ISZ), diffusion affected zone (DAZ), and penetration zone close to the C/C side. The ISZ contained lots of carbide-borides besides Ni(s,s) and Cr(s,s). The specific composition and atomic arrangement of these phases were characterized by the focused ion beam (FIB) method and transmission electron microscopy (TEM) analysis. Results showed the obvious lattice distortion of the Ti–Mo–B compound indicated the presence of the severe internal residual stress. The microhardness distribution of the brazed joint was analyzed in combination with the microstructure. Brittle Cr–Ti–Mo carbide-borides formed in the reaction layer contributed to the high microhardness, and the shape of borides influenced the DAZ microhardness significantly When the brazing temperature was 1100 °C, the highest joint shear strength was obtained due to a matched proportion of the width and the microhardness of ISZ.