Cathodic reduction process of Al–Cu–Y alloy in fluoride-oxide eutectic system via molten salt electrolysis

Al-Cu-Y alloys were prepared by molten salt electrolysis in fluoride-oxide system composed of electrolyte(Na_3 AlF_6-AlF_3-LiF-MgF_2) and oxide(Al_2 O_3-CuO-Y_2 O_3). Cathodic reduction process of Al_2 O_3,CuO and Y_2 O_3 were analyzed by cyclic voltammetry and chronoamperometry. Components and phase composition of alloy samples prepared by potentiostatic electrolysis were characterized by scanning electron microscopy and energy dispersive spectroscopy. The results show that the Al-Cu-Y alloy can be prepared in the AIF_3-NaF-5 wt%LiF-5 wt%MgF2(NaF/AlF_3 = 2.2, molecular ratio) eutectic system with mixed oxide(Al_2 O_3-CuO-Y_2 O_3) through 2 h at the conditions of a temperature of 1208 K, cell voltage3.0 V, cathode current density 0.7 A/cm~2. Al(Ⅲ) and Cu(Ⅱ) ions can be reduced to zero valence Al(0) and Cu(0) directly on carbonaceous electrode surface by instantaneous nucleation, respectively, the reduction process is controlled by diffusion. The reduction potential of Y(Ⅲ) ions is close to the active ions of fluoride melts, but strengthened phase AI3 Y can be formed through electrochemical reduction and alloyed process with active Al(Ⅲ) and Cu(Ⅱ) ions, meanwhile, the Al_2 Cu and Al_3 Y phases are distributed at the grain boundary of Al matrix.     

Structural Studies of Dispersoids in Fe–15 wt% Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–5 wt% Ti Model ODS Alloys During Milling and Subsequent Annealing

Oxide dispersion strengthened (ODS) steels have very high thermal stability and creep resistance due to reinforcement of hard and stable nano-sized ceramic dispersoids in metallic matrix which act as barriers to dislocation motion. This study established the role of Ti in the structural evolution of yttria during mechanical milling and subsequent annealing in a Fe–15 wt% Y₂O₃–5 wt% Ti model ODS alloy, using electron microscopy and XRD techniques. The alloy was synthesized in a high energy planetary ball mill in Ar atmosphere by varying the milling durations in the range of 0 (un-milled) to 60 h. The XRD result revealed amorphisation of Y₂O₃/Ti during milling and evolution of YTiO₃ complex oxide upon annealing at 1273 K for 1 h. The electron microscopy studies revealed the refinement of alloy powders from ~50  μm to few nanometers during milling. Electron diffraction analysis and high resolution transmission electron microscopy of 60 h milled as well as and annealed powder showed formation of different types of Y–Ti–O complex oxides such as Y₂Ti₂O₇, Y₂TiO₅ and YTiO₃.     

ODS ferritic steels produced by an alternative route (STARS): microstructural characterisation after atomisation,HIPping and heat treatments

The conventional PM ODS Ferritic Steel (FS) processing route includes gas atomisation of steel powder and its mechanical alloying (MA) with Y₂O₃ powder particles to dissolve yttrium and form, during consolidation, a dispersion of oxide nanoparticles (Y–Ti–O) in a nanostructured matrix. This work presents an alternative route to produce ODS steels avoiding MA: STARS (Surface Treatment of gas Atomized powder followed by Reactive Synthesis). STARS FS powders with composition Fe–14Cr–2W–0.3Ti–0.23Y, already containing the nanoparticles precursors, were gas-atomized. Oxygen, Y and Ti contents were tailored to the required values to form Y–Ti–O nanoparticles during processing. Powders were HIPped at 900, 1220 and 1300°C. Specimens HIPped at 900 and 1220°C were heat treated (HT) at temperatures ranging from 1200 to 1320°C. The microstructural evolution with HIP and HT temperatures, including characterisation of nanoparticles and feasibility of achieving complete dissolution of prior particle boundaries (PPBs) were assessed.     

Evaluation of Microstructure and Mechanical Properties of Nano-Y2O3-Dispersed Ferritic Alloy Synthesized by Mechanical Alloying and Consolidated by High-Pressure Sintering

In this study, an attempt has been made to synthesize 1.0 wt pct nano-Y₂O₃-dispersed ferritic alloys with nominal compositions: 83.0 Fe-13.5 Cr-2.0 Al-0.5 Ti (alloy A), 79.0 Fe-17.5 Cr-2.0 Al-0.5 Ti (alloy B), 75.0 Fe-21.5 Cr-2.0 Al-0.5 Ti (alloy C), and 71.0 Fe-25.5 Cr-2.0 Al-0.5 Ti (alloy D) steels (all in wt pct) by solid-state mechanical alloying route and consolidation the milled powder by high-pressure sintering at 873 K, 1073 K, and 1273 K (600°C, 800°C, and 1000°C) using 8 GPa uniaxial pressure for 3 minutes. Subsequently, an extensive effort has been undertaken to characterize the microstructural and phase evolution by X-ray diffraction, scanning and transmission electron microscopy, and energy dispersive spectroscopy. Mechanical properties including hardness, compressive strength, Young’s modulus, and fracture toughness were determined using micro/nano-indentation unit and universal testing machine. The present ferritic alloys record extraordinary levels of compressive strength (from 1150 to 2550 MPa), Young’s modulus (from 200 to 240 GPa), indentation fracture toughness (from 3.6 to 15.4 MPa√m), and hardness (from13.5 to 18.5 GPa) and measure up to 1.5 through 2 times greater strength but with a lower density (~7.4 Mg/m³) than other oxide dispersion-strengthened ferritic steels (<1200 MPa) or tungsten-based alloys (<2200 MPa). Besides superior mechanical strength, the novelty of these alloys lies in the unique microstructure comprising uniform distribution of either nanometric (~10 nm) oxide (Y₂Ti₂O₇/Y₂TiO₅ or un-reacted Y₂O₃) or intermetallic (Fe₁₁TiY and Al_9.22Cr_2.78Y) particles' ferritic matrix useful for grain boundary pinning and creep resistance.     

Effect of the method of introduction of Y2O3 into NiAl-based powder alloys on their structure: I. Agitation in a ball mill

The effect of the sintering temperature (1100–1400°C) of NiAl alloy samples with oxide Y₂O₃ produced by hydrostatic pressing on their structure and phase composition and the distribution of oxide particles in a NiAl-based intermetallic matrix alloyed with ～0.5 at % Fe is considered. It is found that dispersed oxide particles in the compact material prepared from a mixture of oxide Y₂O₃ powder and a NiAl alloy (produced by calcium hydride reduction of a mixture of nickel and aluminum oxides) powder in a standard ball mill are nonuniformly distributed in the volume. The morphology of oxides changes during sintering: sintered samples contain rounded particles, which differ strongly from the clearly faceted angular particles of oxide Y₂O₃ added to a mixture (they represent conglomerates of single crystals). In the sintered samples, large aggregates of oxides are revealed along grain boundaries. Mass transfer is possible at the NiAl/Y₂O₃ interface in the system: it leads to partial substitution of aluminum and/or iron atoms for yttrium atoms in the Y₂O₃ lattice and to the formation of submicroscopic particles of (Fe,Al)₅Y₃O₁₂-type oxides.     

Synthesis of high quality Ce:YAG nanopowders by graphene oxide nanosheet-assisted co-precipitation method

(Ce_(0.04)Y_(2.96))Al_5O_(12) phosphor nanoparticles were prepared by a modified co-precipitation method with graphene oxide(GO) nanosheets used as dispersing agent. The GO concentration is controlled at 0.0.005,0.01, 0.02, and 0.03 g/L. The addition of lamellar GO nanosheets in the precipitant solution possibly enhances both the dispersity of precursor particles and the crystallinity of phosphor nanoparticles. Pure Ce-doped YAG phase is obtained by calcining the precipitate at 1000 ℃ for 3 h. The(Ce_(0.04)Y_(2.96))Al_5 O_(12)phosphor nanoparticles have an average size of 64 nm and there is no significant change on particle size with increase of the GO concentration in precipitant solution. The luminescence property of(Ce_(0.04)Y_(2.96))Al_5O_(12) phosphor nanoparticles varies with different concentrations of GO. The photoluminescence emission intensity of the optimum sample with 0.02 g/L GO is about 1.6 times higher than the sample without using GO.     

Fabrication of Al-3 Wt pct Mg matrix composites reinforced with Al2O3 and SiC particulates by the pressureless infiltration technique

In Al-3 wt pct Mg/Al₂O_3p (or SiC _p ) composites fabricated by the pressureless infiltration method, the infiltration behavior of molten metal, the mechanical properties, and the interfacial reactions were investigated. The spontaneous infiltration of the molten Al-3 wt pct Mg alloy into the powder bed occurred at a relatively low temperature (700 °C for 1 hour under a nitrogen atmosphere). Spontaneous infiltration of the molten metal is related to the formation of Mg₃N₂ by the reaction of Mg and nitrogen. The tensile strength and 0.2 pct offset yield strength and elongation tend to decrease with increasing infiltration temperature and time, because of an increased interfacial reaction. In Al-3Mg/Al₂O₃ composites, MgAl₂O₄ was observed at interfaces between Al₂O₃ and the matrix, as well as at oxide films of the Al powder surface. In addition, MgO was observed at interfaces between Al₂O₃ and the matrix. On the other hand, Al₄C₃ was formed at interfaces between SiC and the matrix in Al-3Mg/SiC composites. In addition, MgAl₂O₄ was observed as a reaction product at the interfaces between oxide films of SiC and the matrix, as well as at oxide films of the Al powder surface. Since the Si released as a result of the interfacial reaction is combined with Mg, age hardening can occur by the precipitation of Mg₂Si via T6 treatment.     

Highly transparent ytterbium doped yttrium lanthanum oxide ceramics

To prepare ytterbium doped lanthania yttria nanopowder a method of laser evaporation of mixed oxides was used. After calcinations of the powder at 1200 °C a pure single-phase solid solution Yb3+:(LaxY1–x)2O3 was formed in the nanoparticles. Influence of lanthanum oxide as an isovalent additive on the yttria structure was investigated. The lanthanium ions were proved to be a good aid to sinter yttria ceramics doped with Yb3+ at moderate temperatures about 1650 °С. The ceramics with relative density higher than 99.99% and grain size about 40 μm were fabricated. Full transmittance of 1.8 mm thick Yb0.11La0.23Y1.66O3 ceramics reached 82.5% at 800 nm. This material could be a good gain medium for ytterbium high power pulse lasers.     

Isothermal Grain Growth Studies on Nanostructured 9Cr-1Mo and 9Cr-1W Ferritic Steels Containing Nano-sized Oxide Dispersoids

Analysis of isothermal grain growth kinetics of nanocrystalline Fe-9Cr-1Mo and Fe-9Cr-1W-based ferritic oxide dispersion strengthened alloys is reported. Fe-9Cr-1Mo-0.25Ti-0.5Y₂O₃ alloy exhibited ~900 and ~250 pct enhancement in grain-coarsening resistance at 1073 K (800 °C) in comparison with Fe-9Cr-1Mo-0.5Y₂O₃ alloy and Fe-9Cr-1W-0.5Y₂O₃ alloy, respectively. Comparison of grain growth time exponents also revealed that addition of Ti and Y₂O₃ to nanocrystalline Fe-9Cr alloy has significantly enhanced the grain growth resistance. This is attributed to the possible presence of Y-Ti-O-based nanoclusters (<5 nm).     

High-Temperature Oxidation Resistance of a Nanoceria Spray-Coated 316L Stainless Steel Under Short-Term Air Exposure

Nanoceria coatings using a spray method were implemented on a 316L stainless steel (SS). Coated and uncoated coupons were exposed to dry air at 1073 K to 1273 K (800 °C to 1000 °C) for short time periods (up to 24 hours) and in situ measurements of oxidation were carried out using a highly sensitive thermogravimetric balance. From the experimental outcome, activation energies were determined in both, coated and uncoated 316 SS coupons. The estimated exhibited activation energies for oxidation in the coated and uncoated conditions were 174 and 356 kJ/mol, respectively. In addition, the developed scales were significantly different. In the coated steel, the dominant oxide was an oxide spinel (Fe, Mn)₃O₄ and the presence of Fe₂O₃ was sharply reduced, particularly at 1273 K (1000 °C). In contrast, no spinel was found in the uncoated 316L SS, and Fe₂O₃ was always present in the scale at all the investigated oxidation temperatures. The coated steels developed a highly adherent fine-grained scale structure. Apparently, the nanoceria particles enhanced nucleation of the newly formed scale while restricting coarsening. Coarse grain structures were found in the uncoated steels with scale growth occurring at grain ledges. Moreover, the oxidation rates for the coated 316L SS were at least an order of magnitude lower than those exhibited by the steel in the uncoated condition. The reduction in oxidation rates is attributed to a shift in the oxidation mechanism from outward cation diffusion to inward oxygen diffusion.     

Reoxidation on the Surface of Molten Low‐Carbon Aluminum‐Killed Steel

The unwanted formation of oxide on the surface of molten steel is a process that plagues virtually all grades of steel and often results in unacceptable defect distributions in the finished product. The kinetic rates of the reaction remain largely unknown due to the difficulties in experimental measurements at high temperature involving reactive molten melts. This paper presents recent experimental work and analysis of oxide evolution on the surface of Al‐killed steel melts at oxygen partial pressures of Po₂= 1~5×10^?5atm, by using a Confocal Scanning Laser Microscope (CSLM) equipped with a gold image furnace. The effects of gas flow rate (170~300 cm³/min) and temperature (1580~1630°C) were investigated. The oxide phase formed on the melt surface was in all cases Al₂O₃ but not the thermodynamically stable FeAl₂O₄. It was found, under the range of experimental conditions in this study, that the rate controlling mechanism for oxide nucleation and growth was gas phase mass transfer of oxygen to the melt surface. The morphology of the oxide changed gradually from distinctly dendritic at low gas flow rates to aggregates as the flow rate was increased.     

Preparation and luminescence properties of orange-red Ba3Y4O9:Sm3+ phosphors

A novel orange-red emitting Ba₃Y₄O₉:Sm³⁺ phosphors were prepared by a high temperature solid-state reaction in air. X-ray diffraction (XRD), photoluminescence spectra, fluorescence decay and temperature-dependent emission spectra were utilized to characterize the structure and luminescence properties. The results show that the excitation spectrum includes a series of linear peaks at 350, 367, 382, 410, 424, 445, 470 and 495 nm, respectively. Under 410 nm excitation, the emission peaks were located at 574 nm (⁴G_5/2—⁶H_5/2), 608 nm (⁴G_5/2—⁶H_7/2), 659 nm (⁴G_5/2—⁶H_9/2) and 722 nm (⁴G_5/2—⁶H_11/2), respectively. The concentration quenching occurs when x equals 0.08 for Ba₃Y_4–xO₉:xSm³⁺ phosphor and its mechanism is ascribed to the dipole–dipole interaction. The chromaticity coordinates of Ba₃Y_3.92O₉:0.08Sm³⁺ phosphor are in the orange-red region. The temperature-dependent study shows that this phosphor has excellent luminescence thermal-stability. And the luminescence intensity of Ba₃Y_3.92O₉:0.08Sm³⁺ phosphor at 473 K only declines by about 25.75% of its initial intensity. The experimental data indicate that Ba₃Y₄O₉:Sm³⁺ phosphor may be promising as an orange-red emitting phosphor for white light emitting diodes.     

Dynamic deformation behavior of an oxide-dispersed tungsten heavy alloy fabricated by mechanical alloying

The objective of this study is to investigate the dynamic deformation and fracture behavior of an oxide-dispersed (OD) tungsten heavy alloy fabricated by mechanical alloying (MA). The tungsten alloy was processed by adding 0.1 wt pct Y₂O₃ powders during MA, in order to form fine oxides at triple junctions of tungsten particles or at tungsten/matrix interfaces. Dynamic torsion tests were conducted for this alloy, and the test data were compared with those of a conventional liquid-phase sintered (LPS) specimen. A refinement in tungsten particle size could be obtained after MA and multistep heat treatment without an increase in the interfacial area fraction between tungsten particles. The dynamic test results indicated that interfacial debonding between tungsten particles occurred over broad deformed areas in this alloy, suggesting the possibility of adiabatic shear-band formation. Also, oxide dispersion was effective in promoting interfacial debonding, since the fine oxides acted as initiation sites for interfacial debonding. These findings suggest that the idea of forming fine oxides would be useful for improving self-sharpening and penetration performance in tungsten heavy alloys.     

Oxide Transformation in Cr-Mn-Prealloyed Sintered Steels: Thermodynamic and Kinetic Aspects

The main obstacle for utilization of Cr and Mn as alloying elements in powder metallurgy is their high oxygen affinity leading to oxidation risk during powder manufacturing, handling, and especially during further consolidation. Despite the high purity of the commercially available Cr- and Mn-prealloyed iron powder grades, the risk of stable oxide formation during the sintering process remains. Thermodynamic and kinetic simulation of the oxide formation/transformation on the former powder surface during heating and sintering stages using thermodynamic modeling tools (Thermo-Calc and HSC Chemistry) was performed. Simulation is based on the results from the analysis of amount, morphology, and composition of the oxide phases inside the inter-particle necks in the specimens from interrupted sintering trials utilizing advanced analysis tools (HRSEM + EDX and XPS). The effect of the processing parameters, such as sintering atmosphere composition, temperature profile as well as graphite addition on the possible scenarios of oxide reduction/formation/transformation for Fe-Cr-Mn-C powder systems, was evaluated. Results indicate that oxide transformation occurs in accordance with the thermodynamic stability of oxides as follows: Fe₂O₃ → FeO → Fe₂MnO₄ → Cr₂FeO₄ → Cr₂O₃ → MnCr₂O₄ → MnO/MnSiO _x  → SiO₂. Spinel MnCr₂O₄ was identified as the most stable oxide phase at applied sintering conditions up to 1393 K (1120 °C). Controlled conditions during the heating stage minimize the formation of stable oxide products and produce oxide-free sintered parts.     

Electrochemical preparation and characterization of brain-like nanostructures of Y2O3

Nanostructured Y2O3 was successfully prepared via a two-step and template-free method.Firstly,yttrium hydroxide precursor was galvanostatically grown on the steel substrate from chloride bath by direct and pulse current deposition modes.Direct current deposition was carried out at the constant current density of 0.1 A/dm2 for 600 s.The pulse current was also performed at a typical on-time and off-time(ton=1 s and toff=1 s)with an average current density of 0.05 A/dm2(Ia=0.05 A/dm2)for 600 s.The obtained hydroxide films were then scraped from the substrates and thermally converted into final oxide product via heat-treatment.Thermal behaviors and phase transformations during the heat treatment of the hydroxide powder samples were investigated by differential scanning calorimetry(DSC)and thermogravimetric analysis(TGA).The final oxide products were characterized by means of X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FTIR)and scanning electron microscopy(SEM).The results showed that the well-crystallized Y2O3 with brain-and sphere-like morphology were achievable via pulse and direct deposition modes,respectively.It was concluded that pulse current cathodic electrodeposition offered a facile route for preparation of nanostructured Y2O3.     

Energy transfer and 2 μm emission in Tm~(3+)/Ho~(3+) co-doped(Y_(0.87)La_(0.1)Zr_(0.03))_2O_3 nanopowders

(Y_(0.87)La_(0.1)Zr_(0.03))_2O_3 nanopowders doped with various concentrations of Tm~(3+) and Ho~(3+) were prepared by the citrate method. The standard cubic Y_2O_3 phase can be matched in the Tm~(3+)/Ho~(3+) co-doped(Y_(0.87)La_(0.1)Zr_(0.03))_2 O_3 nanopowders. The nanopowders exhibit average particle sizes of 40,60, 80 and 100 nm after calcinated at 900,1000,1100 and 1200℃,respectively. The energy transfer from Tm~(3+) to Ho~(3+) and the optimum fluorescence emission around 2 μm were investigated. Results indicate that the emission bands at around 1.86 and 1.95 μm correspond to ~3 F_4→~3 H_6 transition of Tm~(3+) and ~5 I_7→~5 I_8 transition of Ho~(3+), respectively.Better spectral properties were achieved in Tm~(3+)/Ho~(3+) co-doped(Y_(0.87)La_(0.1)Zr_(0.03))_2O_3 nanopowders with the average size of 100 nm obtained at the conditions of the treatment of precursors calcinated at 1200 ℃ for 2 h doped with 1.5 mol% Tm~(3+) and 1 mol% Ho~(3+).     

Synthesis and characterization of yttrium and antimony codoped SnO2 conductive nanoparticles

Y was used as a dopant in preparing conductive powder to improve its performance. Y and Sb co-doped SnO₂ conductive nanoparticles were prepared by the complexation-coprecipitation method with Sn, Sb₂O₃ and Y₂O₃ as the raw materials. Crystal phase, thermal behavior and structure of the prepared conductive nanoparticles were characterized by X-ray diffraction (XRD), thermal analysis (TG-DSC), Fourier transform infrared (FTIR) and transmission electron microscopy (TEM) techniques, respectively. The Y and Sb co-doped SnO₂ conductive nanoparticles with a structure of tetragonal rutile had intense absorption in 4000-2500 cm^?1, and the diameter ranged from 10 to 30 nm. The resistivity of Y and Sb co-doped SnO₂ conductive nanoparticles was as low as 0.09 Ω·cm which was 4.6 times lower than that of Sb doped SnO₂ conductive nanoparticles.     

Fundamental Understanding of the Dissolution of Oxide Film on Ti Powder and the Unique Scavenging Feature by LaB<Subscript>6</Subscript>

The surface oxide film on Ti powder consists of TiO₂, Ti₂O₃, and TiO and starts to dissolve into the titanium matrix underneath from ~ 943 K (670 °C). LaB₆ can scavenge O prior to the active dissolution of the surface oxide film. The scavenging process begins by forming an interfacial LaBO₃ layer due to reaction between surface oxide layer and LaB₆, followed by diffusion of oxygen through the LaBO₃ layer. The unique oxygen-scavenging feature of LaB₆ produces better tensile elongation.     

Peculiarities of helium porosity formation in the surface layer of the structural materials used for the first wall of fusion reactor

Transmission electron microscopy is used to study the formation of helium porosity in the nearsurface layer of ferritic–martensitic steels and vanadium irradiated by 40-keV He⁺ ions at a temperature of 923 K up to fluence of 5 × 10²⁰ He⁺/m² and, then, by 7.5-MeV Ni²⁺ ions at 923 K up to dose of 100 dpa. Large gas bubbles are found to form in the zone with the maximum concentration of radiation vacancies during He⁺ ion irradiation. Moreover, small bubbles form in some grains at the depths that are larger than the He+ ion range in the irradiated material. Sequential irradiation by He⁺ and Ni²⁺ ions leads to the nucleation of helium bubbles at still larger depths due to helium atom transport via recoil and/or ion mixing. The precipitation hardening of the steels by Y₂O₃ oxide nanoparticles is found to suppress helium swelling substantially.