Nanometer-sized Grains Research Articles

Effect of fast multiple rotation rolling on friction surfaced Al[sbnd]Si[sbnd]Cu alloy

In this study, the impact of four linear speeds (75, 95, 115, and 135 mm/min) during fast-multiple rotation rolling (FMRR) on the A390 coating applied via friction surfacing (FS) on the AA1050 substrate was investigated. The microstructure, mechanical properties, and wear resistance were examined using optical microscopy, scanning electron microscopy, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), nanoindentation tests, microhardness tests, and pin-on-disc tests. Results showed that as the linear speed increased to 135 mm/min, the FMRR-treated layer became 48.6 % thinner. At a linear speed of 135 mm/min, the grain size reduced to 1.24 ± 0.15 μm, and the sizes of Al2Cu and Si particles decreased to 0.51 ± 0.02 μm and 3.45 ± 0.11 μm, respectively. The nano-hardness and Young's modulus increased to 9.11 GPa and 218.9 GPa, respectively. Moreover, the wear rate decreased by 43.53 % compared to the consumable rod and by 65.96 % compared to the AA1050 substrate at a linear speed of 135 mm/min. EBSD results indicated the presence of recrystallization and shear texture components in the FMRR-treated layer. TEM analysis confirmed the presence of nanometer-sized Si particles (<10 nm) and grains (100–500 nm) in the FMRR-processed layer.

Influence of grain size on electromechanical properties of (Ba,Ca)(Zr,Ti)O3: A multiscale analysis using spark plasma sintering and aerosol deposition

The piezoelectric and ferroelectric properties of aerosol deposited (AD) thick ceramic films are significantly reduced in comparison to bulk materials, which is assumed to be largely related to the nanometer-sized grains (∼0.01 μm) observed in the resulting microstructures. This work, therefore, focuses on understanding the role of grain size in electromechanical properties within the range of AD films based on Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT). In a comprehensive compilation of the piezoelectric constant (d33) of samples from various studies the grain size dependency of the d33 was shown for the lead-free ferroelectrics BCZT, BaTiO3, and (K,Na)NbO3. In this, it can be seen that no BCZT sample with a grain size below 0.4 μm was analyzed regarding their d33 before, thereby leaving a gap to the grain size of AD films. In this study, the grain size of ceramic bulk samples was varied using spark plasma and conventional sintering (0.1 μm – 11 μm) and their properties compared to those of AD films. To enable the fabrication of bulk samples in the same range as AD films, a hydrothermal powder with an average particle size of 0.075 μm was synthesized. A continuous increase of the permittivity, the polarization, and the piezoelectric coefficient was observed through grain size increase. These results suggest that the grain size of AD films needs to be enhanced to improve their properties. Importantly, the insights of this multiscale analysis on the grain size can help to tailor also the properties of other BCZT films and bulk samples.

An experimental simulation of oxygen isotope exchange reaction between amorphous silicate dust and carbon monoxide gas in the early Solar System

The reaction mechanism and kinetics of oxygen isotope exchange between tens of nanometer-sized amorphous silicate grains with forsterite composition (amorphous forsterite) and low-pressure carbon monoxide (CO) gas (PCO) of 0.05–1 Pa at 643–883 K were examined to investigate oxygen isotopic evolution in the protosolar disk that led to the mass-independent oxygen isotopic variation of planetary materials. Both CO gas supply- and diffusion-controlled isotope exchange reactions were observed. At 753–883 K and PCO of 0.05–1 Pa, the supply of CO gas controls the isotope exchange reaction, and its rate is 2–3 orders of magnitude smaller than that of the H2O supply-controlled isotope exchange reaction. The diffusion-controlled isotope exchange occurred at 643–703 K and PCO of 0.3 Pa, and the reaction rate of D (m2/s) = (3.1 ± 2.3) × 10−23 exp[−41.7 ± 9.6 (kJ mol−1) R−1 (1/T − 1/1200)] was obtained.We found that the oxygen isotope exchange rates of amorphous forsterite with CO and H2O gases are larger than those of gaseous isotope exchange between CO and H2O gases at a wide range of temperatures, wherein amorphous forsterite crystallization does not precede the isotope exchange reaction of amorphous forsterite with these gases. The most sluggish isotope exchange rate between H2O and CO in the gas phase suggests that amorphous forsterite would play a role in accelerating gaseous isotopic equilibrium through the isotope exchange of amorphous forsterite with both CO and H2O. We found that the oxygen isotopic equilibrium between 0.1 μm-sized amorphous forsterite, CO, and H2O would be accomplished through the isotope exchange of amorphous forsterite at temperatures as low as ∼600–700 K in the dynamically accreting protosolar disk, which is significantly lower than expected for the case of gaseous isotope exchange (>∼800 K).

Enhancing AISI 321 stainless steel's high-temperature oxidation resistance at 600 °C through surface nanocrystallization and pre-oxidation synergy

This study explores the high-temperature oxidation behavior of AISI 321 austenitic stainless steel (ASS) at 600 °C in an Ar-21vol.%O2 atmosphere, considering nanometer-sized surface grain size and pre-oxidation conditions. After initial solution annealing at 1050 °C, AISI 321 ASS samples were water-quenched for homogenized microstructure (SAed samples). Moreover, the surface nanocrystallization of AISI 321 ASS was performed through high-energy shot peening (SNCed samples), reducing the surface grain size to ~75 ± 5 nm. Pre-oxidation occurred at 400 °C for 150 h in Ar-21%O2 for both SAed and SNCed samples. High-temperature oxidation kinetics was analyzed at 600 °C for 150 h using thermal gravimetric analysis (TGA) on pre-oxidized and non-preoxidized SAed and SNCed samples. FESEM/GDOES and GI-XRD revealed that SNCed samples exhibited improved oxidation resistance, showing no Cr depletion in the subsurface oxide layer up to 100 h, and only minor signs after 150 h. Pre-oxidized SNCed samples demonstrated superior oxidation performance, showing no Cr depletion and subsurface Kirkendall pore formation even after 150 h. These findings are coupled with diffusion relationships, and propose potential mechanisms for enhancing the oxidation behavior of pre-oxidized SNCed AISI 321 stainless steel.

Supergene gold transformations of the placer gold deposits from the Gamba district in northern Cameroon: Implications for secondary and nano-particulate gold formations

The northern part of Cameroon is a Sudanese or Sudano-Sahelian type characterized mainly by a dry climate, a significant landscape position, and distance from the primary gold (Au) sources, giving the occasion to explore the study of complex supergene Au transformations. This study on the biogeochemical transformation of Au in Cameroon will expand understanding of this process and provide advantageous results for Au recovery from materials that are ultimately regarded as waste. In this study, placer Au particles were collected from four individual sites, Djaba, Bigbal, Bougma, and Mayo Salah, and analyzed using a scanning electron microscope (SEM), electron probe microanalysis (EPMA) and LA-ICP-MS Time of Flight (ToF) to observe nanometer-sized Au grain morphologies, the Au mobility and spatial association between Au and related major and minor elements by element distributions in Au particles, respectively. The results from the examination of Au samples across all sampling sites show that Au particles are formed by the binary Au/Ag alloys and that the formation of Au nano- and micro-particles is associated with Au/Ag dissolution and Au re-precipitation. The abundant nano- and micro-particles of Au morphotypes within the cavities of primary Au grains across all the sampling locations were associated with the polymorphic layers formed by clays, iron-oxides, and silicate minerals. The nano- and micro-particles of Au overgrow as colloids and crystalline phases of triangular, spherical, hexagonal, and octahedral platelet aggregates of bacteriomorphic Au and were indicative of biogeochemical transformation. The high Au concentrations in the rim suggest the dissolution of Ag and Cu involving Au mobility identified in placer Au grains across the individual sites. Hence, the minor elements, such as Ag and Cu, occur in solid solution in the Au cores, while trace elements, such as Si, Al, Fe, Cr, Ti, Ga, Ni, Pb, Mn, V, and As, form impurities or micro inclusions in the rim of native gold which was interpreted as evidence of Au mobility. Furthermore, the Au grains are commonly associated with pyrite, chalcopyrite, bornite, and galena. This can only be attributed to the hypogene orogenic precipitation environment, implying that Au grains were transported from primary sources. Overall, the Au particle transformation and mobility of placer Au highlighted in this study was a good indicator for understanding biogeochemical mechanisms affecting gold particles in the northern Cameroon supergene environments.

Energy Efficiency of the Carbonate Hydroxyapatite Nanoparticle Synthesis Using Microwave Heating Treatment and Its Effect on Material Characteristics

This work aimed to study the energy efficiency of the synthesis process of carbonated hydroxyapatite (CHA) nanoparticles using microwave heating treatment and its effect on material characteristics. Microwaves can provide heat quickly, so it is expected to increase the efficiency of CHA synthesis through the heat provided. The CHA nanoparticles were synthesized using precipitation and heated using a microwave oven. The unheated and hydrothermal-heated precipitation methods were also conducted for comparison purposes. The microwave-heated precipitations were done at 270 W for 0.05, 0.10, and 0.15 h, while the hydrothermal-heated precipitations were done at 100 °C for 1, 2, and 3 h. The CHA materials were characterized using an infrared spectrophotometer, X-ray diffractometer, and electron microscope. The X-ray diffractogram and infrared spectra confirmed that the synthesized materials had a hydroxyapatite crystal phase with a CO32− functional group in their spectra. Microscopic images revealed that the materials were nanometer-sized grain aggregates. The heat treatment and duration increased the material characteristics, i.e., crystallinity, crystallite, and grain size. The CHA with microwave heat treatment had the highest crystallinity and crystallite size. The electrical energy calculation revealed microwave heating had better energy efficiency than hydrothermal heating.

Deuterium retention and transport in ion-irradiated tungsten exposed to deuterium atoms: Role of grain boundaries

The influence of grain boundaries on deuterium (D) retention and transport was investigated in nanocrystalline tungsten (W) by exposing the samples to sub eV D atoms. Thin tungsten films with nanometer-sized grains were produced by pulsed laser deposition on tungsten substrates. Their grain size was increased up to one micrometer by thermal annealing in vacuum up to 1223 K. Irradiation damage was created by 20 MeV W ions at 290 K. The transmission electron microscopy analysis showed one order of magnitude larger dislocation density in nanometer-grained samples compared with the larger-grained samples. The samples were after W irradiation exposed to 0.3 eV D atoms at 600 K. D retention and D depth profiles were measured by nuclear reaction analysis. In the as-deposited nanometer-grained samples, D populated the damaged region more than three times faster than in the samples with larger grains, indicating that grain-boundaries increase D transport through the material. The concentration of defects was assessed by the final D concentration in the samples. The sample with the smallest grain size showed slightly larger D concentration in the irradiated area, but the difference in the D concentration was not substantial between different-grained samples. A large D concentration in the non-irradiated nanometer-grained sample was measured which is an indication for a high defect density in the initial material. From our observations, it can be postulated that the nanocrystalline microstructure did not substantially influence the generation of irradiation-induced defects by defect annihilation at grain boundaries.

Experimental Phase Function and Degree of Linear Polarization of Light Scattered by Hydrogenated Amorphous Carbon Circumstellar Dust Analogs

Astronomical observations of the polarized intensity of scattered visible light have revealed the presence of dust envelopes around different types of evolved stars. These observations have helped determine the diameter and width of dust shells around stars with unprecedented accuracy. Simple geometric particle models are used in order to retrieve dust properties from these observations. In this work, we have synthesized and characterized a particulate sample of hydrogenated amorphous carbon (HAC), which is considered to be a realistic carbonaceous interstellar dust analog based on infrared absorption spectroscopy, and we have measured its phase function and degree of linear polarization curves at 514 nm using the CODULAB apparatus at IAA-CSIC. The experimental light-scattering data has been examined in order to explore possible improvements in the interpretation of astronomical observations of circumstellar dust from the point of view of the retrieval of dust properties, including size and porosity. Our results suggest that circumstellar dust observations of linearly polarized scattered light, which are commonly attributed to a population of spherical grains with a radius of ∼0.1 μm, are consistent with larger porous aggregates composed of nanometer-sized grains. In addition, an internal 50wt% mixture of HAC and ultrafine forsterite powder has been generated to study the effect of the mixing of these two components on the light-scattering behavior of dust in cometary environments and protoplanetary disks. In this case, the HAC component, which is not very absorbent, has a very small effect, and the mixture scatters light similarly to the forsterite sample.

Non-magmatic sulfur source of hydrothermal colloform pyrite: Insights from the Jinshandian iron skarn deposit, Daye district, eastern China

Colloform pyrite occurs widely in many ore deposits in the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in eastern China, but its genesis is still debated. In this study, we present detailed textures and geochemical compositions of newly identified colloform pyrite (Py1) and late coarse-grained pyrite (Py2) from the Jinshandian iron skarn deposit in the Daye district to decipher the genesis of the pyrite grains. Colloform pyrite is composed of nanometer-sized (<1 μm) grains, showing typical colloform textures, and occurs as interstitial fillings between magnetite grains. In contrast, Py2 commonly has a large grain size (up to 200 μm) and is usually distributed along the margins of colloform pyrite. These two types of pyrite have relatively high Co/Ni (1.24–13.36) ratios. δ34SCDT values of Py1 (17.22‰–20.63‰, mean 19.42‰) and Py2 (18.96‰–19.95‰, mean 19.43‰) are close to those of gypsum (anhydrite) from evaporites in the Daye district, indicating the incorporation of evaporite-derived materials into the formation of colloform pyrite. On the basis of the textures, trace-element compositions, and sulfur isotope characteristics of the colloform pyrite, it is inferred that this pyrite from the Jinshandian iron deposit is of hydrothermal origin. The sulfur isotope composition of colloform pyrite from Jinshandian is heavier relative to other magmatic-hydrothermal colloform pyrites, which indicates colloform pyrite could formed in diverse hydrothermal fluids (magmatic to no-magmatic fluid). Our study further confirmed that hydrothermal colloform pyrite has widely occurred in Fe and Cu-Au deposits associated with the Cretaceous magmatism in the MLYRMB.

The effect of nanocrystalline microstructure on deuterium transport in displacement damaged tungsten

The influence of grain boundaries (GBs) on the deuterium (D) transport and the creation of defects in nanocrystalline tungsten (W) films deposited on a W substrate was studied. Samples with three different grain sizes were produced for this purpose: a sample with a film having nanometer-size grains, a sample with hundred nanometer-grained film and a sample with micrometer-grained film. Samples were irradiated by 20 MeV W ions at 300 K to create displacement damage and exposed to 300 eV D ions at 450 K to populate the created and any pre-existing defects. The D transport and retention was assessed by measuring D depth profiles after certain exposure times by nuclear reaction analysis (NRA) using a 3He ion beam. From the final D concentration in the damaged area we could determine the concentration of defects that trap hydrogen, showing that the sample with the smallest grain size had the highest D concentration and it decreases with the increase of the grain size. Therefore, in nanocrystalline tungsten irradiated at 300 K, GBs do not improve radiation resistance, which would lead to fewer defects. For the first time, we show experimentally, that D transport is faster inside the nanometer-grained sample as compared to the micrometer-grained sample, meaning that D atoms have enhanced bulk diffusion along GBs. Accidentally, the film thickness was so thin that the W irradiation reached the interface between the W film and substrate, where NRA showed enhanced retention of oxygen. At that depth, two times higher D concentration was observed compared to D concentration in the damaged area in the middle of the film indicating on defect stabilization due to the presence of oxygen.

Nanotwin stability in alloyed copper under ambient and cryo-temperature dependent deformation states

Nanometer size grains, solid solution strengthening, low temperatures, and a high density of nanotwins can provide significant strengthening to a material. Nevertheless, under loading the microstructure may evolve altering the effect of grain size, twin density, etc. on this strengthening. In some cases, these effects can be coupled as in detwinning, which is a mechanism for cryogenic grain growth. In this work, a series of copper nanotwinned columnar grains are solid solution alloyed with aluminum and subject to ambient and cryogenic (−196 °C) environments during mechanical indentation, with the influence of alloying on the detwinning evolution studied. Under indentation, at a lower aluminum solute content (2 at.%), the columnar grains bent corresponding to a loss of twins. At a higher aluminum solute content (8 at.%), the grains shear and retain the nanotwins. Atomistic-scale simulations reveal an increase in the incoherent twin boundary velocity with decreasing temperature, but this boundary velocity is reduced with increasing alloy solute content. The coupling of solid solution strengthening and nanotwin stabilization is then used to explain the microstructural stability as a function of indentation at different temperature conditions.

Smooth Thin Film of a CoNiCu Medium-Entropy Alloy Consisting of Single Nanometer-Sized Grains Formed by Electrodeposition in a Water-in-Oil Emulsion

High-entropy and medium-entropy alloys are new classes of alloys that exhibit excellent strength and toughness. These alloys are expected to be used as multifunctional catalysts given that methods for producing nanoparticles of such alloys have been established. By contrast, the production methods to form thin films of such alloys are still very limited. Here, we show that electrodeposition in a chemically stabilized water-in-oil emulsion functions as a general method to form thin films of such alloys. The key to forming thin films of these alloys by electrodeposition is to conduct reduction under diffusion-limited conditions and to confine the distribution of concentration in a droplet. This situation is not achieved in conventional bulk aqueous or nonaqueous single-phase solution but is achieved in a water-in-oil emulsion. Here, we found aerosol OT as a suitable surfactant for the present water-in-oil emulsion to stabilize the emulsion during electrodeposition. It is also clarified that the increase in viscosity of the aqueous solution within water droplets by the addition of glycerol decreases the roughness greatly. The strategy demonstrated here is expected to spur further applied research into thin films of high- and medium-entropy alloys.

Particle entrainment and rotating convection in Enceladus’ ocean

Observations from Cassini have identified nanometer-sized silica grains in Saturn’s E-ring although their origin is unclear. Tidal deformation within Enceladus’ silicate core has been predicted to generate hot hydrothermal fluids that rise from the core-ocean boundary and traverse the subsurface ocean. This raises the possibility that the particles observed by Cassini could have been produced by hydrothermal alteration and ejected via the south polar plumes. Here, we use an analytical model to quantify potential for particle entrainment in Enceladus’ ocean. We use scaling relations to characterize ocean convection and define a parameter space that enables particle entrainment. We find that both the core-ocean heat fluxes and the transport timescale necessary to drive oceanic convection and entrain particles of the observed sizes are consistent with observations and predictions from existing thermal models. We conclude that hydrothermal alteration at Enceladus’ seafloor could indeed be the source of silica particles in Saturn’s E-ring.

Simulation of surface regolith gardening and impact associated melt layer production under ns-pulsed laser ablation

The effect of surface regolith gardening and melt layer production produced by space weathering (SW) (owing to micrometeorite bombardment) of surficial regolith layers of airless planetary surfaces was investigated in an experimental setup by using laser-induced ablation of powdered analog material (synthetic Fo100) under vacuum with a ns-pulsed infrared laser. The investigated analog pellets were prepared from the fine fraction (< 1 μm) up to a grain size of 280 μm, which resembles the uppermost regolith surface of many airless planetary bodies. The Fo-powder was pressed into shape to form a pellet. We focused here on nanometer-sized structural modifications that are induced in the relocated grains, sputtered off ejecta material and melt sprinkles that formed away from the craters caused by laser irradiation of the pressed pellet surface. The ejecta particles were redistributed over the entire pellet surface and beyond. The forming sputter film, melt sprinkles and ballistically ejected grains were caught on carbon film grids positioned nearby the craters. The grids were investigated with a transmission electron microscope (TEM) to discern between the distinct deposition types that were formed by ejecta condensate and partially molten ejected nanometer-size analog grains. Apart from a heavily modified pellet surface, we found that deposited droplets are mostly amorphous with minor nanocrystalline subdomains. Eight out of ten droplets show distinct incipient crystallization stages. This indicates at a relatively high amount of amorphous regolith material at the incipient stage of SW for airless bodies, if the regolith is altered via micrometeorite bombardment.

Conversion of MgO nanocrystal surfaces into ceramic interfaces: Exsolution of BaO as photoluminescent interface probes.

Ion exsolution can be instrumental to engineer intergranular regions inside ceramic microstructures. BaO admixtures that were trapped inside nanometer-sized MgO grains during gas phase synthesis undergo annealing-induced exsolution to generate photoluminescent surface and interface structures. During their segregation from the bulk into the grain interfaces, the BaO admixtures impact grain coarsening and powder densification, effects that were compared for the first time using an integrated characterization approach. For the characterization of the different stages the materials adopt between powder synthesis and compact annealing, spectroscopy measurements (UV-Vis diffuse reflectance, cathodo- and photoluminescence [PL]) were complemented by an in-depth structure characterization (density measurements, X-ray diffraction [XRD], and electron microscopy). Depending on the Ba2+ concentration, isolated impurity ions either become part of low-coordinated surface structures of the MgO grains where they give rise to a characteristic bright PL emission profile around λ=500nm, or they aggregate to form nanocrystalline BaO segregates at the inner pore surfaces to produce an emission feature centered at λ=460nm. Both types of PL emission sites exhibit O2 gas adsorption-dependent PL emission properties that are reversible with respect to its pressure. The here-reported distribution of BaO segregates between the intergranular region and the free pore surfaces inside the MgO-based compacts underlines that solid-based exsolution strategies are well suited to stabilize nanometer-sized segregates of metal oxides that otherwise would coalesce and grow in size beyond the nanoscale.

Thermally stable and corrosion resistant nanolaminated Al-Mn alloy with low angle boundary structures

Metals with nanometer-sized grains are strong but often suffer from poor thermal stability. Herein, nanolaminated (NL) structure with an average thickness of 110 nm and a low angle grain boundaries (LAGBs) fraction of 80% was obtained in an Al-1Mn (in wt%) alloy produced by high strain rate deformation at cryogenic temperature. The NL structures are thermally stable up to 523 K, which can effectively suppress GB precipitation during aging, thus improving the pitting corrosion resistance in 0.6 M NaCl solution. The high thermal stability of NL structures can be mainly attributed to the formation of a large fraction (80%) of LAGBs.

Radial distribution of the carbonaceous nano-grains in the protoplanetary disk around HD 169142

Context. HD 169142 is part of the class of (pre-)transitional protoplanetary disks showing multiple carbon nanodust spectroscopic signatures (aromatic, aliphatic) dominating the infrared spectrum. Precise constraints on the spatial distribution and properties of carbonaceous dust particles are essential to understanding the physics, radiative transfer processes, and chemistry of the disk. The HD 169142 disk is seen almost face-on and thus it offers a unique opportunity to study the dust radial evolution in disks. Aims. We investigate the spatial distribution of the carriers of several dust aromatic emission features of the disk across a broad spatial range (10–200 AU) as well as their properties. Methods. We analysed imaging and spectroscopic observations in the 8–12 µm range from the VLT Imager and Spectrometer for mid-Infrared (VISIR) at the Very Large Telescope (VLT), as well as adaptive optics spectroscopic observations in the 3–4 µm range from the Nasmyth Adaptive Optics System – Near-Infrared Imager and Spectrograph (NACO) at VLT. The data probe the spatial variation of the flux in the 3.3 µm, 8.6 µm, and 11.3 µm aromatic bands. To constrain the radial distribution of carbonaceous nano-grains, the observations were compared to model predictions using The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS), which is integrated into the POLARIS radiative transfer code by calculating the thermal and stochastic heating of micro-and nanometer-sized dust grains for a given disk structure. Results. Our data show predominant nano-particle emission at all radii (accessible with our resolution of about 0.1″ or ~12 AU at 3 µm and ~0.3″, 35 AU at 10 µm) in the HD 169142 disk. This unambiguously shows that carbonaceous nano-grains dominate radiatively the infrared spectrum in most of the disk, a finding that has been suggested in previous studies. In order to account for both VISIR and NACO emission maps, we show the need for aromatic particles distributed within the disk from the outermost regions to a radius of 20 AU, corresponding to the outer limit of the inner cavity derived from previous observations. In the inner cavity, these aromatic particles might be present but their abundance would then be significantly decreased.

Nanostructured Metallic Glass in a Highly Upgraded Energy State Contributing to Efficient Catalytic Performance.

Metallic glasses (MGs), with high density of low coordination sites and high Gibbs free energy state, are novel promising and competitive candidates in the family of electrochemical catalysts. However, it remains a grand challenge to modify the properties of MGs by control of the disordered atomic structure. Recently, nanostructured metallic glasses (NGs), consisting of amorphous nanometer-sized grains connected by amorphous interfaces, have been reported to exhibit tunable properties compared to the MGs with identical chemical composition. Here, it is demonstrated that electrodeposited Ni-P NG is characterized by an extremely high energy state due to its heterogeneous structure, which significantly promotes the catalytic performance. Moreover, the Ni-P NG with a heterogeneous structure is a perfect precursor for the fabrication of unique honey-like nanoporous structure, which displays superior catalytic performance in the urea oxidation reaction (UOR). Specifically, modified Ni-P NG requires a potential of mere 1.36V at 10mA cm-2 , with a Tafel slope of 13mV dec-1 , which is the best UOR performance in Ni-based alloys. The present work demonstrates that the nanostructurization of MGs provides a universal and effective pathway to upgrade the energy state of MGs for the design of high-performance catalysts in energy conversion.

Thermodynamic stabilization of nanocrystallinity

Abstract Nanocrystalline materials are polycrystals made up of nanometer-sized grains separated by a network of interfaces – grain or phase boundaries – that generally make a positive contribution to the total energy of the system. Consequently, there exists a thermodynamic driving force for reducing the overall interface area, which renders such systems intrinsically unstable against coarsening. The latter process entails interface migration and the concomitant deterioration of any property enhancements effected by the ultrafine grain size. We describe a strategy for significantly reducing or even eliminating the driving force for grain growth in nanocrystalline materials via the deliberate segregation of solute atoms into the core region of boundaries. Applied to Pd – Zr solid solutions containing up to 20 at.% Zr, the strategy yields nanocrystalline specimens manifesting an unusually high thermal stability with respect to grain growth, extending to the vicinity of the melting point. Parallels are drawn between the migration of grain boundaries in segregation-stabilized systems and antiphase domain boundaries in ordered alloys.

Cold Spray Additive Manufacturing: Microstructure Evolution and Bonding Features

ConspectusMost metal additive manufacturing (AM) methods involve the melting or sintering of feedstock powder or wire using an energy source (laser, electron beam, or electric arc). Solid-state AM, sometimes also known as non-beam-based AM, is a process in which the deposited material does not melt and is built up layer-by-layer, typically through severe plastic deformation. Initially considered to be a coating technique, by virtue of its high deposition rate, cold spray additive manufacturing (CSAM) is attractive as a solid-state AM technology. In the CSAM process, metal powder particles are impacted onto a substrate at a supersonic velocity and relatively low temperature. The CSAM process reduces or eliminates many problems associated with melting or beam-based AM methods, making the CSAM technically attractive for a wide range of applications, such as in the aerospace, automobile, marine, biomedical, machinery, and energy sectors.In this Account, the author briefly reviews the setup of a cold spray system and discusses the strengths and drawbacks of CSAM compared with thermal sprays and beam-based AM processes, as well as applications in relevant industries. The author summarizes the bonding mechanisms proposed for the cold spray process. The focus of this Account is to review the microstructure evolution of several typical model metals (copper, nickel, aluminum, and titanium) during the cold spray process. The author shows a large variety of microstructure characteristics (recrystallized grains, annealing twins, shear bands, submicron-sized grains, deformation twins, and nanometer-sized grains) in cold-sprayed copper, dynamic recrystallization of cold-sprayed nickel, and the formation of refined grains even below 10 nm in size in cold-sprayed aluminum. The magnitude of the stacking fault energy of as-sprayed materials considerably influences the microstructure after cold spray and postprocessing. Due to the relatively low thermal conductivity and ductility of titanium, cold spraying of titanium shows both a high deposition efficiency and relatively high porosity of as-sprayed parts, as well as a heterogeneous microstructure. Moreover, the Account introduces the postprocessing heat treatment and nanoindentation characterization of cold-sprayed materials. A fundamental understanding of microstructure evolution during and after the cold spray process is essential for optimal mechanical properties. Lastly, the author provides a perspective on applying old lessons and taking advantage of new techniques and materials, including powder characterization, hybrid additive manufacturing, machine learning for searching process windows, nanomechanical testing, and emerging alloys (high-entropy alloys, nanocrystalline alloys, and quasicrystals), to advance the research and applications of CSAM.