Analytical electron microscopy studies of the CMSX–4 single crystal superalloy subjected to high temperature annealing

The aim of the present work is to investigate the development of the γ/γ' microstructure and the changes in chemical composition of γ and γ' phases during high temperature annealing of CMSX-4 superalloy at a temperature of 1100°C in the time range from 500 to 2500 h. The studies were performed by means of scanning electron microscopy and the modern scanning-transmission electron microscopy with Super-X EDS system of four SDD detectors. Results of scanning electron microscopy and scanning-transmission electron microscopy analyses have shown that the microstructure of CMSX-4 superalloy is unstable during ageing at high temperature and the coalescence of cuboidal γ' precipitates was observed. Energy dispersive X-ray microanalysis revealed the distribution of particular alloying elements in the γ and γ' phases and the differences in their concentration in the function of the annealing time

Analytical electron microscopy investigation of topologically close-packed phases in CMSX-4 single crystal superalloy

In this work the topologically close-packed phases precipitated during annealing of CMSX-4 single-crystal superalloy at temperature 1100°C were investigated. Microstructural analyses were carried out by means of scanning- and transmission electron microscopy as well as scanning-transmission electron microscopy in high angle annular dark field mode. Chemical composition in nanoareas was determined using energy dispersive X-ray spectroscopy. Scanning electron microscopy investigation has shown that the topologically close-packed precipitates were formed already after 50 h of annealing at temperature 1000°C. With prolongation of the annealing time up to 2500 h the change of the morphology of topologically close-packed particles from blocky to needle-like occurred. Selected area electron diffraction analysis indicated that the topologically close-packed precipitates are the orthorhombic P phase. Quantitative energy dispersive X-ray spectroscopy analysis revealed that the topologically close-packed precipitates are enriched mostly in Re and W

Ni−W/ZrO2Ni-W/ZrO_2 nanocomposites obtained by ultrasonic DC electrodeposition

Composite coatings consisting of a nanocrystalline Ni–W alloy matrix reinforced with ZrO2 particles (average size of 50 nm) were synthesized by electrochemical deposition assisted by an external ultrasonic field. The Ni–W/ZrO2 coatings were deposited from aqueous sulphate–citrate electrolytes containing zirconia nanopowder in suspension on steel substrates in a system with a rotating disk electrode (RDE). The influence of relevant processing parameters (i.e., concentration of zirconia powder in plating bath, current density, hydrodynamic conditions, ultrasonic field frequency) on the composite characteristics was discussed. Based on micromechanical (microhardness, Young’s modulus) and microstructural (morphology, phase composition, crystallite size) properties of the coatings, the conditions for electrodeposition of crack-free, homogeneous Ni–W/ZrO2 nanocomposites with enhanced functional properties have been developed

Elektroosadzanie i właściwości nanokrystalicznych stopów na osnowie niklu z trudnotopliwym metalem z kąpieli cytrynianowych

Głównym celem pracy było ustalenie optymalnych warunków procesu elektroosadzania metalicznych powłok Ni-Mo o podwyższonych właściwościach mikromechanicznych. Charakteryzowane stopy zostały osadzone na podłożu ze stali ferrytycznej, w warunkach galwanostatycznych, w modelowym układzie z wirujaca elektroda dyskowa (WED), z wodnych kompleksowych roztworów cytrynianowych zawierających sole niklu i molibdenu. Określono wpływ pH elektrolitu (regulowanego przez dodatek kwasu siarkowego lub amoniaku) na zawartość molibdenu w stopie, jakość osadów, jak również wydajnosc pradowa procesu elektroosadzania. Stwierdzono, że wzrost pH jest związany ze stopniowym zwiększaniem zawartości molibdenu w powłokach. Maksymalna zawartość molibdenu uzyskano w stopach wydzielonych z kąpieli galwanicznej o pH 7, gdzie jednocześnie zaobserwowano najwyższe stężenie cytrynianowych, elektroaktywnych kompleksów molibdenu typu [MoO4(Cit)H]4?-(Cit=C6H5O3-7 ). Dla wybranej kąpieli galwanicznej o optymalnym pH badano wpływ gęstości prądu katodowego (kluczowego parametru operacyjnego, kontrolującego między innymi skład chemiczny oraz mikrostrukturę, w tym skład fazowy i rozmiar krystalitów) na właściwości mechaniczne i tribologiczne wytworzonych powłok. Wykazano, że w całym analizowanym zakresie gęstości prądu, otrzymano powłoki Ni-Mo bez siatki mikropęknięć, o dobrej adhezji do stalowego podłoża, charakteryzujące się podwyższoną twardością w zakresie 6.5 do 7.8 GPa. Ponadto, powłoki osadzane przy wyższych gęstościach prądu (powyżej 3.5 A/dm2) odznaczają się zwartą i jednorodną mikrostrukturą, a tym samym najwyższą odpornością na zużycie przez tarcie.The main aim of the present work was to determine the optimal conditions for electrodeposition of metallic Ni-Mo coatings of enhanced micromechanical properties. These alloys were electrodeposited on the ferritic steel substrate, under galvanostatic regime in a system with a rotating disk electrode (RDE), from an aqueous citrate complex solution containing nickel and molybdenum salts. The effect of the electrolyte solution pH (adjusted by sulphuric acid or ammonia) on the molybdenum content and on deposit quality as well as on the current efficiency of the electrodeposition process, has been studied. It was established that increase of bath pH is correlated with gradual increase of molybdenum content in deposits up to pH 7, where the maximum concentration of Mo(VI) electroactive citrate complex ions [MoO4(Cit)H]4- (Cit= C6H5O7-3 ) in plating bath was observed. In the selected bath of the optimum pH value, the effect of cathodic current density, as a crucial operating parameter which strongly controls the chemical composition and microstructure parameters (e.g. phase compositions, crystallite size), on the mechanical and tribological properties of the resulting coatings has been determined. It has been shown that - under all investigated current density range - crack-free, well adherent Ni-Mo coatings, characterized by microhardness of 6.5-7.8 GPa, were obtained. Alloys deposited at higher tested current densities (above 3.5 A/dm2) were characterized by compact and uniform microstructure, and thus had the highest wear and friction resistance

Strong dispersion effect of cobalt spinel active phase spread over ceria for catalytic N2O decomposition: The role of the interface periphery

A series of Co3O4/CeO2 catalysts with increasing cobalt spinel loading in the range of 1–20 wt.% was prepared by incipient wetness impregnation of CeO2. The obtained catalysts were thoroughly examined by XRD, XPS, XRF, RS, TEM/EDX/EELS, TPR and BET techniques. The catalytic tests in deN2O reaction revealed that the 10 wt.% of cobalt spinel in supported system is able to reproduce the activity of bare Co3O4 catalyst. However, it was found that the catalyst with the lowest content of Co3O4 equal to 1 wt.% exhibits the highest apparent reaction rate per mass of the spinel active phase. The observed activity was explained basing on the transmission electron microscopy analysis in terms of the dispersion of spinel phase over ceria support. A simple model that accounts for the observed strong dispersion effect is proposed. It consists in a two-step mechanism, where N2O is dissociated on the spinel nanograins and the resultant oxygen species are preferentially recombined at the Co3O4/CeO2 interface periphery.The authors would like to acknowledge the Polish National Centre for Research and Development funding awarded by the decision number PBS2/A5/38/2013. On the Polish part the research was partially carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no.POIG.02.01.00-12-023/08)

High-resolution cryo-EM structures of plant cytochrome b6fb_{6}f at work

Plants use solar energy to power cellular metabolism. The oxidation of plastoquinol and reduction of plastocyanin by cytochrome $b_{6}f$ (Cyt $b_{6}f$) is known as one of the key steps of photosynthesis, but the catalytic mechanism in the plastoquinone oxidation site ($Q_{p}$) remains elusive. Here, we describe two high-resolution cryo-EM structures of the spinach Cyt $b_{6}f$ homodimer with endogenous plastoquinones and in complex with plastocyanin. Three plastoquinones are visible and line up one after another head to tail near $Q_{p}$ in both monomers, indicating the existence of a channel in each monomer. Therefore, quinones appear to flow through Cyt $b_{6}f$ in one direction, transiently exposing the redox-active ring of quinone during catalysis. Our work proposes an unprecedented one-way traffic model that explains efficient quinol oxidation during photosynthesis and respiration. Structures of cytochrome $b_{6}f$ with and without plastocyanin imply a one-way traffic of quinones for efficient photosynthesis

Probing the electronic transport on the reconstructed Au/Ge(001) surface

By using scanning tunnelling potentiometry we characterized the lateral variation of the electrochemical potential $\mu _{ec}$ on the gold-induced Ge(001)-c(8 × 2)-Au surface reconstruction while a lateral current flows through the sample. On the reconstruction and across domain boundaries we find that $\mu _{ec}$ shows a constant gradient as a function of the position between the contacts. In addition, nanoscale Au clusters on the surface do not show an electronic coupling to the gold-induced surface reconstruction. In combination with high resolution scanning electron microscopy and transmission electron microscopy, we conclude that an additional transport channel buried about 2 nm underneath the surface represents a major transport channel for electrons