Yttria Particles Research Articles

SEM Observation of Yttria Particle Morphology Changing by Calcination

SEM Observation of Yttria Particle Morphology Changing by Calcination

Improvement in the resistance of aluminum with yttria particles to sliding wear in air and in a corrosive medium

Effects of yttria addition on the performance of aluminum during sliding wear, friction, corrosion and corrosive wear were evaluated with the aim at enhancing Al alloys against corrosive wear using compounds of oxygen-active elements. The sliding wear and friction tests were performed in air and in a dilute HNO 3 solution on a pin-on-disc tribometer. Worn surfaces were examined using scanning electron microscopy (SEM). Mechanical properties, including micro-hardness and elastic modulus, of the Y 2O 3-containing aluminum were determined using nano/micro-indentation instruments. The performance of the Y 2O 3-containing aluminum was compared to those of Y-containing aluminum, pure aluminum and 6061 Al alloy. The experimental results showed that yttria considerably improved the resistance of aluminum to corrosion, corrosive wear and dry sliding wear, respectively.

Synthesis of nanocrystalline yttria by sol–gel method

A method for the synthesis of nanocrystalline yttria by sol–gel method has been developed and is described in the paper. X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigations confirm the grain sizes to be in the range of 20–40 nm. EDAX analysis from the yttria particles in TEM further confirmed the high chemical purity of the powders. Development of such an inexpensive and easy technique for synthesis of high purity nanocrystalline yttria can be of commercial significance.

Creep of magnesium strengthened with high volume fractions of yttria dispersoids

Creep experiments were performed on dispersion-strengthened-cast magnesium (DSC-Mg), consisting of unalloyed magnesium with 1 μm grain size containing 30 vol.% of 0.33 μm yttria particles. Strain rates were measured for temperatures between 573 and 723 K at compressive stresses between 7 and 125 MPa. DSC-Mg exhibits outstanding creep strength as compared with other magnesium materials, but is less creep resistant than comparable DSC-Al and other dispersion-strengthened aluminum materials. Two separate creep regimes were observed in DSC-Mg, at low stresses ( σ<30 MPa), both the apparent stress exponent ( n app≈2) and the apparent activation energy ( Q app≈48 kJ mol −1) are low, while at high stresses ( σ>34 MPa), these parameters are much higher ( n app=9–15 and Q app=230–325 kJ mol −1) and increase, respectively, with increasing temperature and stress. The low-stress regime can be explained by an existing model of grain-boundary sliding inhibited by dispersoids at grain-boundaries. The unexpectedly low activation energy (about half the activation energy of grain boundary diffusion in pure magnesium) is interpreted as interfacial diffusion at the Mg/Y 2O 3 interface. The high-stress regime can be described by dislocation creep with dispersion-strengthening from the interaction of the submicron particles with matrix dislocations. The origin of the threshold stress is discussed in the light of existing dislocation climb, detachment and pile-up models.

Electrokinetic Properties of Colloidal Zirconia Powders in Aqueous Suspension

Pure zirconia, yttria, and three yttria‐doped zirconia powders of submicrometer size have been dispersed in various aqueous solutions. The zeta potential (zeta) of the zirconia powders is determined primarily via streaming‐current (SC) detection and is confirmed using electrophoretic spectroscopy techniques. The results reveal that the isoelectric point (IEP) of these zirconia powders is in the pH range of 5.6‐7.2 and zeta is controlled primarily by the yttrium content of the zirconia powders and the type of electrolyte. In addition, the yttria content strongly affects the potential and SC in zirconia suspensions only at high solids contents (&gt;1 vol%). The electrokinetic data reveal that the surface of the yttria‐doped tetragonal zirconia powder (TZP) can be modified via the adsorption of ionic molecules or polymeric species in the suspension. The adsorption of an anionic polymer can stabilize zirconia particles in a solution that is almost neutral or weakly basic (in the IEP range of pure ZrO2). The interaction of the zirconia and yttria particles with the electrolytes in an aqueous suspension will be discussed to reveal the roles of hydrated oxide formation and zirconia surface interaction with polymeric dispersants.

Processing and Creep Resistance of Nickel/Yttria Composites

In this study, pure nickel and yttria (Y2O3) were selected as a model system to investigate the feasibility of processing metal matrix composites (MMCs) through a powder metallurgy approach for the in-situ formation of a continuous three-dimensional reinforcement network or the in-situ formation of discrete reinforcements with certain degrees of interconnected clusters. Composites with a volume fraction of Y2O3 ranging from 20 to 50 % were prepared through hot pressing. The density, microstructure and creep resistance of these composites were evaluated as a function of the yttria volume fraction. It was found that a continuous Y2O3 network was formed in composites with 40 and 50 vol % Y2O3, while yttria was discrete with some degrees of interconnected clusters in composites with 20 and 30 vol % Y2O3. The creep rate was reduced by two to three orders of magnitude with the addition of 20 to 30 vol % Y2O3, and it continued to decrease with increasing the volume fraction of yttria to 50%. The analysis indicated that the load transfer to isolated yttria particles could not account for the improved creep resistance of composites with 20 and 30 vol % Y2O3, while the load transfer to a continuous yttria network in composites with 40 and 50 vol % Y2O3 could not be approximated by the model of the load transfer to continuous fibres. The discrepancies are believed to be related to the presence of interconnected yttria clusters, the low relative density of the yttria phase in the composite, and the low load-carrying capability through a three-dimensional network in comparison with the load-carrying capability through continuous fibres. It is suggested that the density of the yttria phase and hence the creep resistance of the composite can be further improved over what have been obtained in this study by densifying the composite at high temperatures and pressures.

Mechanical and corrosion behaviour of powder metallurgy stainless steel based metal matrix composites

Stainless steel matrix composites were manufactured using powder metallurgy techniques. Matrixes of AISI 316L stainless steel were reinforced with yttria or alumina particles. Chromium diboride was added in some cases and boron nitride in others to obtain steels with densities close to theoretical, using reactive (liquid phase) sintering techniques. The composites showed very good densification and better hardness than the 316L stainless steel without additions. The 316L steel reinforced with 4 wt-% yttria chromium diboride showed the highest density and strength, with an acceptable corrosion resistance.

Mechanical and corrosion behaviour of powder metallurgy stainless steel based metal matrix composites

Stainless steel matrix composites were manufactured using powder metallurgy techniques. Matrixes of AISI 316L stainless steel were reinforced with yttria or alumina particles. Chromium diboride was added in some cases and boron nitride in others to obtain steels with densities close to theoretical, using reactive (liquid phase) sintering techniques. The composites showed very good densification and better hardness than the 316L stainless steel without additions. The 316L steel reinforced with 4 wt-% yttria chromium diboride showed the highest density and strength, with an acceptable corrosion resistance.

Colloidal Stability of Zirconia‐Doped Yttria–Silica Binary Aqueous Suspensions

Minor amounts of zirconia (up to 5 wt%) were fused with yttria and their effect on water solubility of yttria, and colloidal stability of yttria–silica aqueous binary suspensions, was examined. It is shown that incorporated zirconia reduces the water sensitivity of yttria while maintaining the hydrophilic nature of the surface. This fusion procedure makes yttria particles suitable for production of longlasting yttria–silica aqueous suspensions.

High-temperature deformation of a mechanically alloyed niobium-yttria alloy

Mechanical alloying (MA) and hot isostatic pressing have been used to process two Nb alloys containing yttria particles, Nb-2vol.%Y 2O 3 and Nb-10vol.%Y 2O 3. Similar to some thermomechanically processed nickel-based alloys, both alloys exhibit partially recrystallized microstructures, consisting of a ‘necklace’ of small recrystallized grains surrounding much larger but isolated, unrecrystallized, cold-worked grains. Hot compression tests from 1049 to 1347 °C (0.5–0.6 T MP ) of the 10% Y 2O 3 alloy show that MA material possesses a much higher yield and creep strength than its powder-blended, fully recrystallized counterpart. In fact, the density-compensated specific yield strength of the MA Nb-10Y 2O 3 exceeds that of currently available commercial Nb alloys.

High Temperature Deformation and Densification in Alumina-Yttria Composites

Pinning by second phase particles offers a potent means for limiting grain growth and enhancing superplasticity in alumina-based ceramics. In the present study, a colloidal technique was used to produce green bodies of alumina-yttria composites; at elevated temperatures, the yttria particles react with alumina to produce YAG particles. The densification and high temperature deformation characteristics of alumina-YAG composites were studied using conventional free sintering and sinter-forging, which involves the application of a compressive stress without any lateral constraints. It is shown that the YAG particles retard both densification and grain growth. The experiments indicate also that the presence of YAG particles does not significantly alter the stress exponent for creep deformation.

Sinterability of Y2O3-Al2O3 particulate stainless steel matrix composites

P/M 316L austenitic stainless steel has been reinforced with yttria and alumina particles. In order to improve the sintering behaviour of these composite materials, chromium diboride and boron nitride were added. The sinterability of the different materials has been characterised through dilatometry and sintering curves (sintered density vs. sintering temperature). A metallographic study by SEM coupled with microprobe has also been performed. Composites materials present a good densification. Chromium diboride and boron nitride react with the matrix in different manners, but they both greatly improve the sinterability of reinforced materials. The optimum sintering temperature for these composites materials is 1250 °C.

The role of dispersoids in maintaining the corrosion resistance of mechanically alloyed oxide dispersion strengthened alloys

Amongst the various commercial mechanically alloyed (MA) oxide dispersion strengthened (ODS) alloys available, ferritic alloys are most suitable for use at temperatures exceeding 1,100 C, since {gamma}{prime} particle dissolution results in the loss of strength of nickel based MA ODS alloys. In commercial MA ODS alloys, yttria particles are commonly added in the starting powers, and it is well known that they are not retained in the final product because they react with aluminum and oxygen present in the system during thermomechanical processing and form mixed (Y,Al) oxide particles. In this paper the role of (Y,Al) oxide particles in maintaining the overall aluminum concentration in the matrix necessary to repair the outer oxide scale is discussed. It is well known that during exposure in air, aluminum is lost from the bulk of MA ODS alloys to the growth of a surface alumina film. The extent of aluminum lost is dependent on the section thickness and can significantly affect the volume fraction of the aluminum rich dispersoids. Coarsening of the dispersoids at temperatures up to 1,200 C in the thick MA 956 bar occurs without significant aluminum loss, while their volume fraction increases continuously. However, coarsening of the dispersoids in themore » thinner ODM 751 tube at 1,200 and 1,300 C is accompanied by dissolution of the dispersoid particles in order to maintain the aluminum concentration in the matrix.« less

Experimental evidence of resputtering of the yttria layer in a YBa2Cu3O7−x/Y2O3/YBa2Cu3O7−x trilayer film

Using transmission electron microscopy and energy dispersive x-ray spectroscopy analysis, the microstructure and composition of a YBa2Cu3O7−x/Y2O3/YBa2Cu3O7−x trilayer film deposited on MgO by off-axis sputtering at 670 °C and 100 mTorr was investigated. The YBa2Cu3O7−x layer in contact with the substrate was found to be mainly single phase. However, the top YBa2Cu3O7−x layer was embedded with 5–10 nm crystalline Y2O3 (yttria) particles, which disturbed the local microstructure. The top YBa2Cu3O7−x layer also had barium copper oxide particles covering its surface. The partial decomposition of the top YBa2Cu3O7−x layer may have been due to resputtering of the yttria layer which locally altered the deposition conditions. In particular, the oxygen pressure in the vicinity of the growing film may have increased due to sputtering of the yttria layer by plasma gas atoms and the sputtered target atoms. As a result, deposition occurred under conditions away from the thermodynamic stability line.

High-temperature low-cycle fatigue behavior of superalloy MA 760

The high-temperature cyclic behavior of oxide dispersion-strengthened superalloy MA 760 was studied at different temperatures and specimen orientations. The cyclic hardening curves at different strain am-plitudes were determined at 650, 900, and 950 °C, and the stress-strain response of the material was sub-stantiated by detailed fractographical scanning electron microscopy (SEM) and microstructuralJoptical microscopy, STEM) observations. During cycling, MA 760 did not exhibit marked hardening or soften-ing, and also the final fracture occurred very rapidly without a significant decrease in the cyclic strength of the material. The “brittle” nature of deformation behavior of MA 760, especially at lower test tempera-tures, as well as oxidation-induced secondary cracking along grain boundaries, was clearly revealed by microscopical studies of the fracture surfaces. At 650 °C, evidence of dislocation cutting of γ was ob-served in transmission micrographs, but at higher temperatures, dislocation climb and by-passing of γ particles were found to become more prominent features. At all test temperatures, dislocation climb over yttria particles, as well as the departure side pinning effect of dislocations at nonshearable particles, was frequently observed.

The synthesis of metal hydrous oxide particles by multiphase electrodispersion

A multiphase electrodispersion precipitation reactor called the electric dispersion reactor (EDR) was developed to synthesize ultrafine particles for the production of precursor powders of advanced ceramic materials. A pulsed electric field is employed to create dispersions of micrometer- and submicrometer-scale conducting (aqueous-based) drops in non-conducting (organic) liquids. Species initially dissolved in the organic phase diffuse into the aqueous droplets containing the other reagents. Reaction ensues, leading immediately to precipitation and gelation in the aqueous droplets. The EDR has produced 0.1–5 μm particles of silica, alumina, zirconia, yttria, and composite hydrous oxide particles from metal alkoxide and metal salt precursors. Also considered is the application of the EDR to the control and modification of the morphological characteristics of these particles, with illustrative examples.

Deposition of colloidal sintering-aid particles on silicon nitride

The deposition of alumina (Al2O3) and yttria (Y2O3) particles on silicon nitride (Si3N4) by electrostatic and electrosteric interactions was studied using acoustophoretic measurements. The characteristics of Si3N4, Al2O3' and Y2O3 powders in an aqueous environment were studied by measuring the electrokinetic sonic amplitude (ESA), and appropriate conditions for deposition by adsorption were identified. The deposition of Al2O3' Y2O3, and Y(NO3)3 on Si3N4 particles was conducted at different pH values where these chemically dissimilar particles exhibit electrical charges of opposite sign. The deposition studies indicated that the particle size has a strong effect on the amount of coating on Si3N4 particles. The presence of polyacrylic acid as an electrosteric stabilizer for Si3N4 had no measurable effect on Al2O3 adsorption. The shift of the isoelectric point of Si3N4 with Al2O3 showed that the total quantity of Al2O3 adsorbed on Si3N4 reached a maximum at approximately 75% coverage of the total surface are...

Electron microscopy studies of phase transformations in nanostructured Yttriumoxide

The grain size distribution and grain morphology of nanostructured Y 2O 3 particles prepared by gas condensation have been determined by electron microscopy. The narrow size distribution of the monoclinic phase in the as-prepared state, which is centered at 4 nm, changes into a bimodal distribution after annealing at 700°C and partial transformation into the cubic structure. After complete m-c transformation at 875°C the size distribution is wide and all small grains disappeared. It is concluded that the high pressure monoclinic modification is stabilized by the Gibbs-Thomson pressure in Yttria particles smaller than 8 nm.

Zeta potential of anatase (TiO 2) in mixed solvents

The replacement of water with mixed solvents (aqueous methanol and ethanol, up to 30% wt.) does not change the surface charge density dependence on the pH of anatase suspensions in 10 −3−10 −1 mol dm −3 KCI solutions. The negative ξ-potentials are decreased and the isoelectric point is shifted towards higher pH values in systems containing 10 −1 mol dm −3 KCl, LiCl, CsCl, or KI in 30% ethanol/water or 30% methanol/water mixtures as compared to pure aqueous systems. The effect is observed only at a sufficiently high ionic strength and alcohol concentration and is similar to that previously found for silica. The charge of anatase can be reversed by BaCl 2 and K 2SO 4 in aqueous and mixed solvent systems. Finally, analogous phenomena were observed with uniform yttria particles dispersed in ethanol/water mixtures in the presence of 10 −1 mol dm −3 KCl.

Abnormal Grain Growth in Oxide Dispersion Strengthened Ni-Base Superalloy

To examine the predominant factor controlling abnormal grain growth of Ni-base oxide dispersion strengthened superalloys, microstructures were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Three kinds of particles were recognized; (1) fine, round shape of yttria particles of 10 to 80 nm in diameter, (2) large, round shape of Al and Ti oxide particles of 80 to 200 nm diameter, (3) flake shape of Cr rich phase with sized of 0.2 to 0.4 μm in width and 0.4 to 2.5 μm in length