Bainitic microstructures in high-strength steels are obtained either by continuous cooling or isothermal holding. Both scenarios necessitate faster cooling to keep the parent austenite phase untransformed till the bainite-start temperature. The present study reports the development of bainitic microstructure in a low-carbon steel with minimal alloying additions, under continuous cooling at very slow rates, similar to furnace cooling. For understanding the related transformation pathways, samples from the forged-steel ingot are austenitized and cooled at different rates, viz. water quenching, air cooling, and furnace cooling. Microstructural characterization reveals development of acicular microstructures in all samples including the forged one, with gross absence of carbides. X-ray diffraction confirms the ferritic nature of acicular plates and also indicated retained austenite present in some samples, the content of which could be correlated to the extent of bainitic transformation. Thermodynamic calculations together with microstructural observations (e.g., ferrite plate size) and hardness data established the development of fully martensitic microstructure on water quenching, while that of a mixed microstructure comprising predominantly of bainite in the forged, air cooled, and furnace-cooled condition. The aforementioned findings could have wider implications in developing fully bainitic microstructures in large components, where uniform rapid cooling is not practically feasible. A low-carbon (0.23 wt%) carbide-free bainitic steel is successfully developed with a total alloying addition of 4.9 wt%, which is relatively lower than alloying additions reported in similar category of steels. The present findings indicate that the currently designed low-alloy steel composition is amenable to generation of nearly carbide-free bainitic microstructure in large-sized components under normal cooling conditions. Microstructures of steel after a,b) water quenching, c,d) air cooling, and e,f) furnace cooling, where bainite is obtained in both air and furnace cooled conditions.image (c) 2024 WILEY-VCH GmbH

Silver indium selenide (AgInSe2) has been proposed as an efficient absorber material for sustainable solar cells. Phase pure preparation of this exotic material in the nano regime is highly desirable. This report describes the design and synthesis of a new heterobimetallic complex [(Ph3P)(2)Ag(mu-SeCH2Ph)(2)In(SeCH2Ph)(2)] along with its structural characterization. This complex acts as an air enduring, normal temperature and pressure (NTP) stable versatile single source precursor for synthesizing bulk, nanocrystalline and thin film of AgInSe2. The crystal structure, phase purity, morphology, elemental composition and band gap of the synthesized materials were determined from powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and diffuse reflectance spectroscopy (DRS-UV), respectively. The band gap of the nanocrystalline AgInSe2 lies in the range suitable for solar cell applications. A prototype photoelectrochemical cell fabricated with the pristine AgInSe2 nanoparticles exhibits high photoresponsivity, which makes them a suitable candidate for clean energy applications.

Surface functionalization of semiconducting metal oxides has emerged as a highly effective approach for enhancing their sensing capabilities. In the present work, the surface of randomly oriented zinc oxide (ZnO) nanowires is modified with an optimized thickness (7 nm) of magnesium oxide (MgO), which exhibits an exceptionally sensitive and selective behavior toward NO2 gas, yielding a response of approximately 310 for 10 ppm concentration at room temperature. The synergistic interplay between ZnO and MgO leads to a remarkable 20-fold improvement in sensor response compared to a pristine ZnO film and allows the detection of concentrations as low as 50 ppb. The ZnO-MgO composite was characterized using X-ray diffraction (XRD), XPS, and SEM-EDS to gain structural, compositional, and morphological insights. The interaction of the NO2 molecule with the sensor film was investigated using density functional theory (DFT) simulations, revealing that oxygen vacant sites on the MgO surface are most favorable for NO2 adsorption, with an adsorption energy of -3.97 eV and a charge transfer of 1.74e toward NO2. The XPS, photoluminescence (PL), and EPR measurements experimentally verified the presence of oxygen vacancies in the sensing material. The introduction of localized levels within the band gap by oxygen vacancies significantly promotes the interaction of gas molecules with these sites, which enhances the charge transfer toward NO2 gas molecules. This augmentation has a profound influence on the space charge region at the ZnO-MgO interface, which is pivotal for modulating the charge transport in the ZnO layer, resulting in the substantial improvement of NO2 response at room temperature.

Microstructural quantification of austenite and ferrite phases during isothermal bainite transformation (IBT) is paramount to tailor properties in low-carbon carbide-free bainitic steel. Such quantification carried out at the end of IBT heat treatment through microstructural analyses is fraught with limitations, such as non-consideration of transformation of the untransformed austenite on cooling post IBT. The current work has, therefore, devised a simple model to quantify volume fractions and the average carbon contents of bainitic ferrite and different austenite regions, namely film and blocky present between ferrite platelets and between bainitic sheaves, respectively, formed during IBT treatments at 375 to 450 degrees C from dilatometric strain itself. Such quantification, through other means would be very demanding in terms of experimentation and characterization. Microstructural quantification from the devised model is validated with experimental findings and highlights the difference in carbon contents of film and blocky austenite during IBT treatments.

Two-dimensional (2-D) Bi2Se3 nanostructures have emerged as fine candidates due to their appealing performance toward energy conversion and storage applications. In view of this, a simple and economically viable method for their preparation is highly desirable. Herein, we present the synthesis, characterization, and structural elucidation of a new air-stable Bi-nicotinamide selenolate complex: [Bi{SeC5H3(3-CONH2)}(3)] (1). This complex serves as an efficient single-source molecular precursor (SSP) for the facile preparation of phase-pure Bi2Se3 nanostructures. The as-prepared Bi2Se3 nanostructures were characterized using microstructural analyses such as powder X-ray diffraction (PXRD), energy dispersive X-ray spectroscopy (EDS), and electron microscopy techniques to assess their phase purity, elemental composition, crystal structure, and morphology. This study also attempts to understand the effect of reaction conditions on the crystallinity, size, and morphology of nanostructures. The optical band gap of the nanostructures was tuned within the range of 1.9-2.0 eV, which is blue-shifted with respect to the corresponding bulk band gap and is suitable for energy conversion applications. Liquid junction photoelectrochemical cells fabricated from the as-prepared Bi2Se3 nanostructure exhibit good photoresponsivity and decent photostability, which project them as amenable candidates for alternative photon absorber materials.

Sigma (sigma) phase is considered as one of the most deleterious phases in stainless steels (SS) due to its detrimental effects on mechanical and corrosion properties. In this work, sigma phase formation and growth in annealed SS 321 (without delta-ferrite) and its effects on impact toughness and intergranular corrosion (IGC) were investigated, following thermal aging at 750 degrees C up to 5500 h. Microstructural characterization using X-ray diffraction and electron microscopy techniques revealed (a) precipitation of intergranular Cr-rich M23C6 and fine Ti-rich intra & intergranular carbides after short aging time of 27 h, (b) sigma phase nucleation at the Cr-rich M23C6 - austenite interfaces after 100 h of aging and (c) sigma phase at the austenite grain boundaries (GBs) beyond 700 h of aging. The experimental results, combined with thermodynamic calculations (ThermoCalc) showed that the meta stable Cr-rich carbide is the source of sigma phase nucleation. sigma phase started to grow at the expense of Cr-rich M23C6, and further, its size increased preferentially along the GB and into one of the austenite grains with increase of aging duration. Hardness remained in the range of 190-200 HV for all the specimens aged beyond 100 h and is shown to be due to stable Ti-rich intragranular carbides of 7-20 nm size. Impact toughness values decreased with the growth of sigma phase, and up to 50% reduction in toughness was found by about 1% sigma phase, with a ductile-brittle cleavage fracture. The normalized values of sigma phase fraction, determined from its size and the toughness values, with aging duration followed Kolmogorov-Johnson-Mehl-Avrami (KJMA) model and both yielded the same empirical rate constant (k) of 2.7 x 10(-7)/s and the n value of similar to 1. The lower k value and the n value of 1 suggest slower kinetics and one-dimensional growth of sigma phase, which is in well agreement with microstructural observations. Susceptibility to IGC was seen in specimens with sub-microscopic sigma phase (aged up to 100 h). However, aging beyond 700 h decreased resistance to uniform corrosion due to sigma phase growth.

Ni–Mo–Cr–Ti alloys are one of the proposed structural materials for molten salt breeder reactors. In the current work, the evolution of precipitates in the alloy at various stages of thermo-mechanical processing followed by thermal ageing at 750 °C is studied. The microstructure after thermal ageing for various durations shows the presence of ‘MC’-type carbides with different morphologies and compositions. Formation of carbide networks with bulky titanium-rich ‘MC’ carbides of irregular morphology and spherical ‘MC’ around it is observed in the early stages, while fine uniform dispersion of ‘MC’-type needle-like carbides that possess a cube-on-cube orientation relationship with the FCC matrix is observed in the latter stages. High-resolution transmission electron micrograph reveals the presence of misfit dislocations on the semi-coherent interface between the fine needle-like MC carbides and the matrix which is attributed to the lattice parameter mismatch between them.

The competing ordering and clustering tendencies in Ni–Mo alloy system are manifested by having nearly equal stabilities of superlattice structures whose enthalpies are dictated by interaction energies between near neighbours in different coordination shells. These effective cluster interaction parameters (ECIs) dictate the complex ordering/clustering processes. In the present investigation, the evolution of D1 a structure of Ni 4 Mo from the < 1½0 > short-range ordered structure in the fcc matrix of same composition was studied using electron microscopy. Cluster expansion formalism in conjunction with density functional theory-based calculations was employed to determine the ECIs for the alloy. It was concluded that pair, triplet and quadruplet ECIs stabilize unlike atoms in the first coordination. Finally, it was shown that these ECIs could be used with reasonable accuracy in the Monte Carlo simulations to evaluate Gibbs free energies of formation as a function of temperature, short- and long-range-order parameters to understand ordering/clustering tendencies and hence transformation pathways in the alloy.

Anisotropic silver nanostructures having complex morphologies are of immense importance due to its application in plasmon-enhanced photocatalysis, transparent conducting electrodes, and surface-enhanced Raman spectroscopy (SERS). We report the generation of silver nanowires, nanobelts, and nanostructures having dendritic fibrous morphologies by galvanic replacement reaction on copper thin-film coated dendritic fibrous nanosilica (DFNS). DC magnetron sputtering was used to deposit copper thin- film on DFNS. Electron microscopy revealed that galvanically grown silver nanostructures followed the wrinkled fibrous morphology of DFNS. In addition to dendritic fibrous nanosilver (DFNAg), silver nanobelts, nanowires with high aspect ratio also grew over large areas when the silver precursor concentration and the copper film thickness were varied. The SERS performance of DFNAg substrates was evaluated using a self-assembled monolayer of p-aminothiophenol (p-ATP). The SERS and plasmon-enhanced photocatalytic performance of DFNAg were compared with a standard SERS substrate, Ag film on nanosphere (AgFON). The ensemble-averaged Raman enhancement factor for DFNAg was found to be of the order of 105, on par with AgFON. Variation in Raman peak intensities revealed that plasmon-assisted photodimerization of p-ATP to p,p′-dimercaptoazobisbenzene was higher on DFNAg than AgFON which could be attributed to a high number of hot spots on DFNAg caused by its anisotropic structure.

Presented here is a technique of producing an ultra-high strength steel starting with an annealed low strength AISI 304L stainless steel. Annealed AISI 304L stainless steel was severely rolled at room temperature to a thickness reduction of 93%. Microstructure after rolling of SS304L showed subgrain formation with high dislocation density. The resulting strength of this steel after rolling was 1.7 GPa. Subsequent aging of this at 400 degrees C for 120 h further increased the strength to 2.2 GPa. The ductility of the as-rolled and subsequently aged conditions was similar and in the range of 5 to 7%. Electron back scatter diffraction analysis and vibrating-sample magnetometer showed that in the as-rolled condition the fraction of strain induced martensite was approximate to 72% with the remaining being untransformed austenite in the deformed state. The median spacing between boundaries (low as well as high angle) varied between 700 nm for the as-rolled condition to 900 nm after rolling and subsequent aging at 400 degrees C for 120 h. Atom probe tomography along with small angle neutron scattering showed that the process of aging resulted in a spinodal phase separation of Cr rich regions with 7 nm wavelength and precipitation of the G-phase having 1.75 nm radius. It was estimated that G-phase precipitation, rather than spinodally decomposed Cr-rich regions, is the dominant hardening phase. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In the present study, using electron channelling contrast imaging (ECCI) technique dislocation densities were estimated in Ti-modified Hastelloy-N deformed to a nominal strain of 0.1. These dislocations were imaged using the backscattered electrons signal under controlled two beam diffraction conditions in SEM. To obtain such a condition, the crystallographic orientation of the grain is evaluated from the electron backscattered diffraction (EBSD) patterns. The grain is then oriented to a desired diffracting condition w.r.t electron beam using the tilt and rotation angles of the SEM stage. For this purpose, a software has been developed which helps in manoeuvring the crystal to different diffracting conditions by estimating the permissible tilt and rotation angles of the SEM stage due to its geometry. The dislocation densities obtained from this technique are corroborated with that of TEM.

Binary and ternary Ni-Mo based alloys under the service conditions of high temperature show precipitation and chemical ordering which influence their microstructure, mechanical and corrosion properties. In the present work, the phase transformation behaviour of Ni-Mo and Ni-Mo-Cr alloys is studied to identify the role of Cr in altering the stability of brittle ordered intermetallic phases. TEM characterization demonstrates that the addition of Cr destabilizes not only the long-range ordered D1(a) phase but also the short-range order represented by < 1 1/2 0 > ordering wave vectors. EXAFS studies on Ni-Mo and Ni-Mo-Cr alloys establish preferential bonding of Cr with Ni as the first nearest neighbour in the fcc lattice replacing Mo. The cluster-expansion based calculations on Ni-Mo, Ni-Cr and Cr-Mo systems show that the nearest neighbour pair and multisite interaction energies are changed in magnitude as well as in sign for Ni-Mo and Ni-Cr systems signifying that the D1(a) type ordering tendency is reduced with Cr addition and the disordered fcc phase is stabilised. A combination of experimental and theoretical studies unequivocally establishes the stability of the disordered fcc structure, which is crucial for the long-term use of the selected Ni-Mo-Cr alloy at elevated temperatures. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In this paper, the insupressible eutectoid transformations in Ti–Cu and Zr–Cu alloys are studied. Under non-equilibrium cooling conditions, the parent β (bcc) phase decompose into several fine colonies of lamellar eutectoid of α (hcp) and M2Cu (tetragonal) phases (M = Ti/Zr) nucleated within the grain. The resultant lamellar eutectoid microstructures in both these alloy systems show a finer lamellar spacing and orientation relationship between the product phases. This article discusses the favourable conditions during non-equilibrium conditions that helps in maintaining the integrity of transformation front between the parent and product phases during the transformation resulting in lamellar morphology.

The present study investigates the microstructural changes and corrosion behavior of 13Cr martensitic stainless steel (MSS), austenitized at 1040 °C followed by cooling to room temperature through oil-quenching, air-cooling and furnace-cooling. Oil-quenching and air-cooling produced a lath martensitic microstructure, while, the oil-quenched condition showed δ-ferrite stringers and quench cracks. The slow furnace-cooling produced a widmanstatten type ferrite with Cr-rich carbides. Air-cooled condition developed a higher hardness than that for the oil-quenched and furnace-cooled conditions due to martensitic structure and fine carbide precipitation along the prior austenitic grain boundaries (PAGB) during cooling from austenitization temperature. Double loop-electrochemical potentiokinetic reactivation (DL-EPR) tests and immersion tests followed by scanning electron microscope examination indicated that furnace-cooled and air-cooled conditions are susceptible to intergranular corrosion (IGC). Slow cooling is highly detrimental to IGC. Air-cooled condition showed preferential attack along the PAGB boundaries in both DL-EPR and immersion tests whereas furnace-cooled condition showed combined heavy uniform corrosion and IGC. No preferential attack is observed in oil-quenched condition, and it exhibited uniform corrosion. The present results show that the austenitization followed by cooling in air is sufficient to cause sensitization in 13Cr MSS. However, controlled (fast) air-cooling (avoiding sensitization during cooling) would be better a choice for austenitization heat treatment of 13Cr MSS.

The present study investigated the microstructural evolution in alpha + beta(Zr) regime of Zr-2.5 wt% Nb (a dual phase alloy with alpha (hcp) and beta (bcc)) as a function of heat treatment temperature and holding time. The microstructures were studied in terms of composition and phase fraction evolution and the morphological changes of the constituent phases. The composition and quantification of phases revealed that, they significantly deviated from the corresponding equilibrium values for all the temperatures and holding times employed in the present study. In addition, an anomalous behavior was noticed in the time evolution of the phase fraction. The observed anomaly was rationalized by suggesting a possible sequence in attaining the equilibrium as: the crystal structure transformation of alpha to beta phase followed by Nb enrichment in the beta phase. Widmanstatten and (i) phase transformations were exhibited by beta phase at certain heat treatment conditions. The critical temperature for the widmanstatten transformation was found to be 750 degrees C, below which the high temperature beta phase retained to room temperature. Despite wide variations in the microstructures, flow curves were characteristically similar for all the heat treatment conditions. However, yield strength was found to be more sensitive to the beta phase morphology and the transformations associated with it. The presence of fine transformed products inside the beta phase was observed to promote higher extent of deformation twinning. (C) 2020 Elsevier B.V. All rights reserved.

A few layer/multilayer two-dimensional (2D) Tellurium (Te) nanoflakes were mechanically exfoliated and their structural and vibrational properties were probed. Layer thickness reduction was confirmed using optical microscopy, atomic force microscopy and kelvin probe force microscopy. Raman spectroscopy measurements revealed anomalous shift in the frequencies of their intrachain vibrational modes, where, after an initial increasing trend (blue shift) with thickness reduction from bulk, they changed to decreasing trend (red shift) on further layer thinning. Such unusual vibrational behaviour of 2D Te, exhibiting reversal from blue shift to red shift, is attributed mainly to the deformations occurring in the Te chains on layer thinning, followed by modifications in the local structures and bondlengths of the corresponding Te atoms. The deformation under relaxed conditions are also confirmed from first principle calculations. Red shift can also be correlated to lesser hole concentration for thinner flakes, as revealed by work function changes measured on the nanoflakes surfaces. Interestingly, in spite of such anomalous behavior, the difference in frequencies of both modes showed initial increase with thickness and saturation beyond a certain thickness, as generally observed in other 2D materials. Temperature dependent Raman spectroscopic measurements further confirmed anomalous behaviour for thinner flakes.

Recent research suggests that molybdenum carbide (beta-Mo2C) has the potential to be a cheap and active substitute for Pt-based electrocatalyst for hydrogen evolution reaction. In this article molybdenum carbide (Mo2C) electrocatalysts immobilized on carbon support were synthesized and evaluated for hydrogen evolution reaction (HER). The quantity of Mo in the samples was varied to understand the effect of Mo content in Mo2C/C electrocatalyst on the structure, morphology, electrochemical properties and HER. The Mo weight percentages determined by ICP-OES technique in four Mo2C/C samples prepared were found as similar to 9.3, 15.8, 20.4 and 28.0. SAXS studies revealed that the pore size of the carbon increased with an increase in Mo content, most probably to accommodate the Mo2C motifs. X-ray photoelectron spectra showed that the amount of low valent Mo increased as we increased the Mo content up to 20 wt % but decreased in the 28 wt % sample. All the samples were active for electrochemical HER with the sample having similar to 20 wt % Mo showing the highest activity and exhibited a Tafel slope of 69 mVdec(-1). Among all samples the 20 wt% Mo sample exhibited the highest electrochemical surface area (ECSA) of -2.92 mFcm(-2) and minimum charge transfer resistance for the HER. Thus, it is concluded that 20 wt% Mo in Mo2C/C electrocatalyst evolves with ideal pore size, highest ECSA, smooth charge transfer and thus exhibits the best electrochemical properties for HER. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Using an indigenously built DC magnetron sputtering system short period (similar to 42 angstrom) Co/Ti multilayer and Co/Ti multilayer with similar to 4 angstrom Cu buffer layer at both interfaces with 20 bilayers in each case have been prepared for use as reflecting optics in the water window soft X-ray regime of 23-44 angstrom. The samples have been subsequently characterized by specular and diffused grazing incidence X-ray reflectivity (GIXR) measurements as well as cross-sectional transmission electron microscopy (TEM) to carry out detail investigations on the interfaces. Analysis of GIXR data and TEM measurements clearly reveal that interface roughness is the major contributor to the interface imperfections in these short period multilayers and interface roughness in the multilayer are significantly reduced after the introduction of Cu barrier layers.

Silver nanoparticles (Ag NPs) have found wide-spread applications as antibacterial agents, catalysts and in chemical detection via surface-enhanced Raman spectroscopy (SERS). To avoid agglomeration and unwanted release to the environment, Ag NPs are usually loaded onto various substrates. However, simple procedures for large-scale synthesis of this important material are still lacking. Here, we demonstrate a facile, green and scalable synthesis of Ag NPs loaded to ceramic substrates. Environment-friendly biopolymer (gum arabic) and ceramic (silica) nanoparticles were spray-dried to obtain surface-functionalized mesoporous silica substrate. Thereafter, the silver precursor was reduced in situ to obtain Ag NPs attached to the silica substrate. Gum arabic played a crucial role in reducing and attaching Ag NPs to the substrate. The resulting composite material was characterized using X-ray diffraction, high-resolution transmission electron microscopy, field emission scanning electron microscopy and Fourier transform infrared spectroscopy. The antimicrobial efficacy of the synthesized composite was evaluated against a gram-negative bacterium, Escherichia coli and a gram-positive bacterium, Staphylococcus aureus. Complete bactericidal effect was observed for composite harboring silver concentration as low as 19 µg ml−1 and 76 µg ml−1 for Escherichia coli and Staphylococcus aureus, respectively. Furthermore, the composite was used as a catalyst in the reduction reaction of 4-nitrophenol and rhodamine B by sodium borohydride. The rate constant for 4-nitrophenol reduction was found to be 1750 s−1 g−1. The composite demonstrated its applicability as a substrate for surface-enhanced Raman spectroscopy (SERS) wherein very dilute concentration (10−9 M) of crystal violet and rhodamine 6G could be easily detected.

Evidences of both sluggish eutectoid and active eutectoid (not suppressible under rapid cooling) transformations have been found for the first time in a single hyper-eutectoid Ti-Cu alloy. Both of these types of eutectoid reactions have been investigated in detail, at different length scales, by coupling scanning electron microscopy (SEM),transmission electron microscopy (TEM) and atom probe tomography (APT). The unique three phase crystallographic relationship between the parent β (bcc) and the two product phases, α (hcp) and Ti2Cu, has been established. The extent of partitioning of the solute (Cu) between the two product phases has been determined by APT and is rationalised in terms of thermodynamic considerations. Based on the observed lattice site correspondence and the extent of solute partitioning, a possible mechanism of active eutectoid transformation is proposed.