Micron Grains Research Articles

Microstructural evolution of Y 2O 3 and MgAl 2O 4 ODS EUROFER steels during their elaboration by mechanical milling and hot isostatic pressing

Different ODS EUROFER steels reinforced with Y 2O 3 and MgAl 2O 4 were elaborated by mechanical milling and hot isostatic pressing. Good compromise between strength and ductility could be obtained but the impact properties remain low (especially for the Y 2O 3 ODS steel). The materials were structurally characterized at each step of the elaboration. During milling, the martensite laths of the steel are transformed into nano-metric ferritic grains and the Y 2O 3 oxides dissolve (but not the MgAl 2O 4 spinels). After the HIP, all the ODS steels remain ferritic with micrometric grains, surrounded by nano-metric grains for the Y 2O 3 ODS steels. The mechanisms in the Y 2O 3 ODS steels are complex: the Y 2O 3 oxides re-precipitate as nano-Y 2O 3 particles that impede a complete austenitization during the HIP. The quenchability of the ODS steels is modified by the milling process, the oxide nature and the oxide content. Eventually, the advantages and drawbacks of each oxide type are discussed.

Structural phase transition and magnetic properties of the Sr 2FeRe 1− xBx′O 6 ( B′=Nb, Ta; x=0, 0.1) double perovskites

The double perovskites, Sr 2FeReO 6 and Sr 2FeRe 0.9 M 0.1O 6 ( M=Nb, Ta) have been obtained by soft synthesis methods which yield homogeneous particles of micrometric grain size. The materials have been studied by X-ray and neutron powder diffraction, scanning electron microscopy and magnetic measurements. Rietveld refinements show that the compounds adopt a tetragonal I4/ mmm structure at high temperatures and monoclinic P2 1/ n below the transition temperature. The magnetic structures were determined by neutron powder diffraction at 100 and 300 K for the Sr 2FeReO 6, Sr 2FeRe 0.9Nb 0.1O 6 and Sr 2FeRe 0.9Ta 0.1O 6 phases, respectively. Evidence for a ferrimagnetic coupling between the Fe 3+ and Re 5+ sublattices has been observed. Magnetic measurements yield magnetic moments lower than the theoretical ones being in accord with the antisite disorder of 25% in the B– B′ positions.

Features of the local structure of fine-grained HTSC YBa2Cu2.985 57Fe0.015Oy

Polycrystalline high-Tc superconductors (HTSCs) with the composition YBa2Cu2.985 57Fe0.015Oy (y≈6.92, Tc=91–91.5 K) with micron and submicron grain sizes are investigated by the method of gamma resonance (Mössbauer) spectroscopy at temperatures of 293, 91, and 78 K. A correlation is found between the changes of the intensities of the components of the Mössbauer spectra and the mean grain sizes 〈D〉. It is shown that the observed changes in the spectra are caused by an interplane redistribution of oxygen wherein the oxygen content δ in the (CuOδ) planes increases and the lattice parameter c decreases. This effect is greater for smaller mean grain sizes 〈D〉. It is concluded that the observed changes in the anion subsystem of the fine-grained HTSCs as compared to bulk equilibrium samples of YBa2Cu3O6+δ are mainly due to a partial interchange of the Y3+ and Ba2+ cations in them. This interchange may be a result of the nonequilibrium synthesis conditions that must be used to produce HTSCs with micron and submicron grain sizes.

Evaluation of the performance of different nano-ceramic tool grades when machining nickel-base, inconel 718, alloy

High-speed machining of aerospace alloys can be enhanced by the use of advanced cutting tool materials such as nano-grain size ceramics that exhibit improved physical and mechanical properties than their micron grain counterparts. The performance of recently developed nano-grain size ceramic tool materials were evaluated when machining nickel base, Inconel 718, in terms of tool life, tool failure modes and wear mechanisms as well as component forces generated under different roughing conditions. The tools were rejected mainly due to wear on the tool nose. It is also evident that chemical compositions of the tool materials played significant role in their failure. The alumina base ceramics performed better than the silicon nitride base ceramics. Severe abrasion wear was observed on both rake and flank faces of the cutting tools while cutting forces increased with increasing cutting speed when machining with the silicon nitride base nano-ceramic tools. This is probably due to the lower superplastic flow temperature of the nitride base nano-ceramics. The alumina base ceramics are more susceptible to chipping at the cutting edge than the silicon nitride base ceramics despite their higher edge toughness.

Northwest Africa 428: Impact‐induced annealing of an L6 chondrite breccia

Abstract— Northwest Africa (NWA) 428 is an L chondrite that was successively thermally metamorphosed to petrologic type‐6, shocked to stage S4–S5, brecciated, and annealed to approximately petrologic type‐4. Its thermal and shock history resembles that of the previously studied LL6 chondrite, Miller Range (MIL) 99301, which formed on a different asteroid. The petrologic type‐6 classification of NWA 428 is based on its highly recrystallized texture, coarse metal (150 ± 150 μm), troilite (100 ± 170 μm), and plagioclase (20–60 μm) grains, and relatively homogeneous olivine (Fa24.4 ± 0.6), low‐Ca pyroxene (Fs20.5 ± 0.4), and plagioclase (Ab84.2 ± 0.4) compositions. The petrographic criteria that indicate shock stage S4–S5 include the presence of chromite veinlets, chromite‐plagioclase assemblages, numerous occurrences of metallic Cu, irregular troilite grains within metallic Fe‐Ni, polycrystalline troilite, duplex plessite, metal and troilite veins, large troilite nodules, and low‐Ca clinopyroxene with polysynthetic twins. If the rock had been shocked before thermal metamorphism, low‐Ca clinopyroxene produced by the shock event would have transformed into orthopyroxene. Post‐shock brecciation is indicated by the presence of recrystallized clasts and highly shocked clasts that form sharp boundaries with the host. Post‐shock annealing is indicated by the sharp optical extinction of the olivine grains; during annealing, the damaged olivine crystal lattices healed. If temperatures exceeded those approximating petrologic type‐4 (˜600–700°C) during annealing, the low‐Ca clinopyroxene would have transformed into orthopyroxene. The other shock indicators, likewise, survived the mild annealing. An impact event is the most plausible source of post‐metamorphic, post‐shock annealing because any 26Al that may have been present when the asteroid accreted would have decayed away by the time NWA 428 was annealed. The similar inferred histories of NWA 428 (L6) and MIL 99301 (LL6) indicate that impact heating affected more than 1 ordinary chondrite parent body.

Synthesis and characterisation of LiNi 1− x− yCo xAl yO 2 cathodes for lithium-ion batteries by the PVA precursor method

Polycrystalline powders of LiNi 1− x Co x O 2 and LiNi 1− x− y Co xAl y O 2 were synthesized at 720 °C from the nitrate-PVA precursor method. The water was removed by two different processes: (i) evaporation-drying at 110 °C, (ii) spray-drying at 150 °C. In both cases a gel is obtained wherein LiNO 3 crystallizes in evaporated–dried samples but not in spray–dried samples which are more amorphous. All precursors give single phase powders with micronic well-shaped grains. The best electrochemical performances are obtained from evaporated samples with a capacity remaining higher than 160 mAh/g for LiNi 0.82Co 0.18O 2 and 140 mAh/g for LiNi 0.82Co 0.13Al 0.05O 2 after 20 cycles. The lower performances of the spray-dried samples seem to be related to Li 2CO 3 formation with subsequent non-stoichiometry as a consequence of the improved homogeneous distribution of the constituent cations in the polymer.

Electrochemical oxidation of WC in acidic sulphate solution

The electrochemical oxidation of WC in aqueous solutions is critical for the functional stability of hardmetals in aggressive environments. The oxidation of WC and the nature of the oxides formed in 0.1 M sulphuric acid under electrochemical polarisation have been studied by electrochemical methods (cyclic voltammetry, chronoamperometry). The structure and composition of the electrochemically treated surfaces has been analysed by X-ray diffraction, surface Raman spectroscopy and energy dispersive analysis of characteristic X-rays. The morphology of the attacked surfaces was investigated by scanning electron microscopy. Anodic attack gives rise to hydrous WO 3 forming a continuous film which tends to develop pits. Alternated anodic and cathodic polarisation yields a very similar composition, but with a lower degree of hydration. The morphology is characterised by micrometric loose grains forming by disruption of the larger grains of the pristine sintered WC surface. The electrochemical conditions giving rise to the inception of WO 3 formation by WC oxidation could be accurately identified by monitoring the inception of the development of the cathodic loop corresponding to the electrochemical behaviour of the electrochromic material.

Superconductivity at [formula omitted] in compacted submicrometer platinum powders

The superconducting transition temperature and critical magnetic field observed in compacted Pt powder samples with submicrometer grain size are as high as 20 mK and 18 mT and thus more than one order of magnitude larger than for those with larger (micrometer) grain size. The submicrometer samples exhibit strong lattice strain and significantly smaller Debye temperatures compared to bulk platinum. Moreover, the ferromagnetic spin fluctuations of the conduction electrons are partially quenched in these compacts.

The influence of particle size of raw materials on the final WO 3 grain size

Obtaining of tungstic oxide from wolframite concentrates involves the production of synthetic scheelite and/or tungstic acid as intermediate products. The present work studies the influence of different factors affecting the particle size of these compounds and also investigates the correlation between its granulometry and the final grain size of WO 3. The research included as main goals (i) the study of the effect on the synthetic scheelite grain size of the following parameters—concentration and amount of calcium chloride; concentration of tungstate solution and type of mixing and stirring, (ii) the study of the correlation between granulometric sizes of synthetic scheelite and final tungstic oxide granulometric sizes, (iii) the study of the effect of temperature, type of agitation, concentration and amount of hydrochloric acid on the recovery and particle size of tungstic oxide. Conditions to produce WO 3 powders with tailor-made micrometric grain size from 10 to 0.5 μm are presented.

Nanostructured polymer/nanophase ceramic composites enhance osteoblast and chondrocyte adhesion.

Osteoblast (bone-forming cell) and chondrocyte (cartilage-synthesizing cell) adhesion on novel nanostructured polylactic/glycolic acid (PLGA) and titania composites were investigated in the present in vitro study. Nanostructured polymers were created by chemically treating micron-structured PLGA with select concentrations of NaOH for various periods of time. Dimensions of ceramics were controlled by utilizing either micron or nanometer grain size titania. Compared with surfaces with conventional or micron surface roughness dimensions, results provided the first evidence of increased osteoblast and chondrocyte adhesion on 100 wt% PLGA films with nanometer polymer surface roughness dimensions. Results also confirmed other literature reports of enhanced osteoblast adhesion on 100 wt% nanometer compared with conventional grain size titania compacts; however, the present study provided the first evidence that decreasing titania grain size into the nanometer range did not influence chondrocyte adhesion. Finally, osteoblast and chondrocyte adhesion increased on 70/30 wt% PLGA/titania composites formulated to possess nanosurface rather than conventional surface feature dimensions. The present study, thus, provided evidence that these nanostructured PLGA/titania composites may possess the ability to simulate surface and/or chemical properties of bone and cartilage, respectively, to allow for exciting alternatives in the design of prostheses with greater efficacy.

Discrete rocket effect and its implication for micron grain acceleration

The rocket equation breaks down for micron grains because the number of particles per grain is small compared with that within macroscopic objects. Therefore, the discrete rocket effect replaces the classical continuous rocket effect for micron grains. The discrete rocket effect is due to the fact that ejected mass from a micron grain can be comparable to the total mass for these intermediate size objects. Electron beam and laser ablation are possible means to accelerate micron grains to 106 m/s and higher velocity.

A Strategic Two-Stage Low-Temperature Thermal Processing Leading to Fully Dense and Fine-Grained Doped-ZnO Varistors

Fully dense doped-ZnO varistors with sub- micrometer grain size (see Figure for a TEM image) can be fabricated by a strategic two-stage, one-cycle, thermal processing method at temperatures as low as 825 °C. The nonlinear coefficients α of such fully dense varistors are as high as 270, which is more than one order of magnitude higher than those currently given for commercial ZnO varistors.

Evaluation of the Machinability of Nickel-Base, Inconel 718, Alloy with Nano-Ceramic Cutting Tools

The machinability of difficult-to-cut aerospace alloys can be enhanced by the rapid development of cutting tool materials that can withstand machining at high-speed conditions. The performance of nano-grain size ceramic tool materials were evaluated when machining nickel base, Inconel 718, in terms of tool life, tool failure modes and wear mechanisms as well as component forces generated under different roughing conditions. Comparison tests were carried out with commercially available ceramic tool materials of micron-grain composition. The test results show that the micron grain size commercially available tool materials generally gave the longest tool life. The dominant failure mode is nose wear, while some of the nano-ceramic tools were rejected mainly due to chipping at the cutting edge. This suggests that physical properties and mechanical stability of the cutting edge of the ceramic tools influence their overall performance. It is also evident that chemical compositions of the tool materials played a significant role in their failure. The alumina base ceramics are more susceptible to premature fracture than the silicon nitride base ceramics with higher fracture toughness. Presented as a Society of Tribologists and Lubrication Engineers Paper at the ASME/STLE Tribology Conference in Cancun, Mexico October 27–30, 2002

Microstructural Changes of the As-Quenched Zn-22 mass%Al-2 mass%Cu Alloy during Cold Rolling

A study of the microstructural changes of a Zn-22 mass%Al-2 mass%Cu alloy was conducted during rolling at room temperature. Cold rolling was pursued on the as-quenched and room temperature aged samples. A reduction in thickness of about 97.6% and a total elongation of 3510% were obtained in the former sample while a sudden crack was present in the later sample. The X-ray diffraction and TEM results showed that nanometric equiaxial grains of the α and η phases in a matrix of the β phase were present during the rolling process of the as-quenched sample. Aged sample showed the presence of micrometric equiaxial grains of the a and η phases. The formation of nanometric grains of the α and η phases seems to be occurred by a strain-induced phase transformation. These nanometric grains seem to be the responsible for the superplasticity observed at room temperature in this kind of alloys.

Results from bending tests on submicron and micron WC–Co grades at elevated temperatures

The bend behaviour of WC–Co cemented carbides with micron and submicron grain size and Co content ranging from 6 to 15 wt% was investigated in the temperature range from 20 to 1000 °C from the load–deflection curves in vacuum. Transverse rupture stress σ f (TRS), yield stress σ s and strain at fracture δ were determined. The ductile–brittle transition temperature T d as well as the strain-hardening parameters at 800 and 1000 °C were also obtained. The temperature T d was found to be 200–300 °C for micron grades and 300–700 °C for submicron grades, but essential plastic strain at fracture was observed only at temperatures higher than 600–700 °C. The influence of grain size, cobalt content and temperature on σ s, σ f, δ and strain hardening is discussed. It is shown that the WC–Co alloys are characterized by very high strain hardening. The predominant deformation mechanism for submicron grades at 1000 °C and deformation rate 7×10 −5 s −1 is grain boundary sliding, that is typical of superplastic deformation.

Processing and characterisation of nanocrystalline iron aluminide coatings prepared by thermal spraying of milled powders

This paper presents the first results of an ongoing research which was set up to investigate the possibility of processing an ultra fine grain Fe-40Al (Zr, B) alloy using a thermal spraying route. The coating produced here by high velocity oxy-fuel (HVOF) thermal spraying of milled powder had a highly metastable microstructure, inherited from the initial milling, that was characterised essentially by the nanometer grain size of its FeAl phase. The hardness of this nanocrystalline coating was 35% higher than that of a conventional coating obtained from atomised powder and close to that of a fully dense bulk material having a micrometer grain size.

GEOCHEMISTRY AND MINERALOGY OF GOLD-RICH HYDROTHERMAL PRECIPITATES FROM THE EASTERN MANUS BASIN, PAPUA NEW GUINEA

Three of the five hydrothermal vent fields currently known in the Manus back-arc basin of Papua New Guinea are enriched in gold. Samples of precipitates from the PACMANUS and Susu Knolls hydrothermal fields in eastern Manus Basin contain up to 56.8 ppm gold, and average 11.3 ± 13 (1� ) ppm (n = 103). Chimney samples dominated by sphalerite, barite and chalcopyrite have significantly higher contents of gold than sulfide-poor samples. Gold in the sulfide chimneys occurs as micrometric grains of silver-poor (1.4 to 4.2 wt.% Ag) native gold in three distinct associations. In copper-rich chimney samples, gold occurs as inclusions in chalcopyrite associated with tennantite, bornite and covellite. In zinc-rich chimney samples, gold is present mai nly as inclusions in tennantite associated with chalcopyrite and sphalerite. A high activity of sulfur is indicated for the gold-ri ch eastern Manus Basin precipitates by the sulfide assemblages, the low iron content of sphalerite (average = 2.3 ± 3 mole % FeS), and the silver-poor nature of gold grains. Evaluation of possible complexes of gold indicates that AuHS ° is the dominant complex, which is consistent with the high activity of sulfur in the hydrothermal system. According to available experimental data , AuHS° is the dominant complex up to 325°C. This complex thus is the only one required for gold enrichment in both the Zn–Au and the Cu–Au associations. Gold transport as the AuHS° complex is likely to be equally important in other seafloor hydrothermal systems with acidic end-member hydrothermal fluids.

Amorphous phase formation in spray deposited AlYNiCo and AlYNiCoZr alloys

Recycled high-strength aluminum alloys have limited use as structural materials due to poor mechanical properties. Spray forming remelting followed by hot extrusion is a promising route for reprocessing 7xxx alloys. The 7050 alloy machining chips were spray formed, hot extruded, rotary swaged and heat-treated in order to improve mechanical properties. Microstructures, tensile properties and fatigue strength results for a 2.7 mm-diameter recycled wire are presented. Secondary phases and precipitates were investigated by XRD, SEM, EBSD, TEM and DSC. As-swaged and heat-treated (solution and aging) conditions were evaluated. Mechanical properties of both conditions outperformed AA7050 aerospace specification. Substantial grain refinement resulted from the extensive plastic deformation imposed by rotary swaging. Refined micrometric and sub-micrometric Al grains, as well as coarse and fine intermetallic precipitates were observed. Subsequent solution treatment resulted in a homogeneous, recrystallized and equiaxed microstructure with grain size of 9 μm. Nanoscale GP(I) zones and η′ phase precipitates formed after aging at 120 °C, imparting higher tensile (586 MPa) and fatigue (198 MPa) strengths.

Structural determination of sintered Si3N4/SiC nanocomposite using the XPS differential charge effect

We report a study using X-ray Photoemission Spectroscopy (XPS) of a nanocomposite which is a mixture of silicon nitride (Si3N4) micrometric grains (mean diameter of 0.2 μm) and silicon carbide (SiC) nanoparticles (from 0.01 μm to 0.06 μm). A differential charge effect was detected in this compound for the C1s and Si2p core levels. We concluded that the existence of such an effect was because the first raw acquired spectra display selective partial reductions of the charge for 6–7 h. Contributions to the XPS core levels from silicon carbide (Si–C4 and C–Si4) are not affected by charge shifts, in contrast to other contributions. This differential charge effect was not observed for SiCN nanopowders which display a rigid charge shift of all the contributions in the same core levels. This is a good fingerprint for the structural organization in the compound. We concluded the existence of SiC nanoparticles dispersed in a vitreous YSiAlON[C] phase located at the grain boundaries in the material which made up a conducting cement/skeleton surrounding large silicon nitride grains (0.2 μm).

Microstructural refinement of Ti-44Al-11Nb by severe plastic deformation

Recent studies have demonstrated that Seere Plastic Deformation (SPD) can be successfully used to produce submicrometer sized grains. Equal Channel Angular Extrusion (ECAE) has proven to be an effective technique for producing severe plastic deformation. With the objective of producing sub micron grains and determining the beneficial effects of such microstructures on mechanical properties, the authors have an ongoing study of severe plastic deformation effects in several TiAl and Ti{sub 3}Al based intermetallics. The microstructural modifications in a Ti-44Al-11Nb (all compositions are in atomic percent) alloy processed by equal channel angular extrusion are evaluated.