Heterogeneous nucleation of solidification of cadmium particles embedded in an aluminium matrix

A hypomonotectic alloy of Al-4.5wt%Cd has been manufactured by melt spinning and the resulting microstructure examined by transmission electron microscopy. As-melt spun hypomonotectic Al-4.5wt%Cd consists of a homogeneous distribution of faceted 5 to 120 nm diameter cadmium particles embedded in a matrix of aluminium, formed during the monotectic solidification reaction. The cadmium particles exhibit an orientation relationship with the aluminium matrix of {111}_Al//{0001}_Cd and 110_AlAl//11¯20> _Cd, with four cadmium particle variants depending upon which of the four {111}_Al planes is parallel to {0001}_Cd. The cadmium particles exibit a distorted cuboctahedral shape, bounded by six curved {100}_Al//{20¯23}_Cd facets, six curved {111}_Al/{40¯43}_Cd facets and two flat {111}_Al//{0001}_Cd facets. The as-melt spun cadmium particle shape is metastable and the cadmium particles equilibrate during heat treatment below the cadmium melting point, becoming elongated to increase the surface area and decrease the separation of the {111}_Al//{0001}_Cd facets.The equilibrium cadmium particle shape and, therefore, the anisotropy of solid aluminium-solid cadmium and solid aluminium -liquid cadmium surface energies have been monitored by in situ heating in the transmission electron microscope over the temperature range between room temperature and 420 °C. The anisotropy of solid aluminium-solid cadmium surface energy is constant between room temperature and the cadmium melting point, with the {100}_Al//{20¯23}_Cd surface energy on average 40% greater than the {111}_Al//{0001}_Cd surface energy, and 10% greater than the {111}_Al//{40¯43_Cd surface energy. When the cadmium particles melt at temperatures above 321 °C, the {100}_Al//{20¯23}_Cd facets disappear and the {111}_Al//{40¯43}_Cd and {111}_A1//{0001}_Cd surface energies become equal. The {111}_Al facets do not disappear when the cadmium particles melt, and the anisotropy of solid aluminium-liquid cadmium surface energy decreases gradually with increasing temperature above the cadmium melting point.The kinetics of cadmium solidification have been examined by heating and cooling experiments in a differential scanning calorimeter over a range of heating and cooling rates. Cadmium particle solidification is nucleated catalytically by the surrounding aluminium matrix on the {111}_Al faceted surfaces, with an undercooling of 56 K and a contact angle of 42 °. The nucleation kinetics of cadmium particle solidification are in good agreement with the hemispherical cap model of heterogeneous nucleation.     

X-ray fluorescence study of fine powders of wolframite ores with SiO2 and WO3 major phases

Single phase pure SiO₂ exhibits a strong and symmetric X-ray fluorescence (XRF) peak at 1.741 keV, which does not depend much on the size of the particles. Moreover, the separated particles of 30–110 m size strongly interact with 5–20 wt% WO₃ additions of similar particle sizes, and show dramatic changes of peak intensity (l _p), half-bandwidth 2_1/2, (i.e. the full width at the half peak-intensity), and integrated intensity (l) of the signal as a function of size of the particles. The WO₃ fluoresces at the relatively higher energy of 8.39 keV with adversely modified intensity l _p or l in such a peculiar way that the total intensity l _t in the two signals of the two WO₃ and SiO₂ phases is nearly constant. The results are tested and applied to XRF analyses of a wolframite ore having SiO₂ and WO₃ as the two major phases. In all these examples, the intensities of the SiO₂ and WO₃ signals vary, basically due to the expected macroscopic electromagnetic interactions between the two phases.     

Effects of Precursors with Fine BaZrO3 Inclusions on the Growth and Microstructure of Textured YBCO

We prepared precursors with fine BaZrO₃ inclusions for melt-texture growth of high-temperature superconductors with nominal composition of Y_1.5Ba₂Cu₃O _y + x BaZrO₃ (x = 0, 0.04, 0.08, and 0.1), by the process of liquid-phase powder melt-texture growth (LPMG). We then investigated the effects of the precursors on the texture growth and the microstructure of composite YBa₂Cu₃O_7-/Y₂BaCuO₅ (YBCO). The BaZrO₃ particles were about 100 nm in size in the textured YBa₂Cu₃O_7- (Y123), observed by the scanning electron microscopy (SEM). Owing to the pushing effects of growing fronts, fine particles were pushed out of and thus gathered in front of the growing fronts or between Y123 grains. The gathered particles in turn blocked the further growth of the Y123 grains. However, BaZrO₃ particles seeded Y123 grains, giving rise to multigrain growths. These negative effects of BaZrO₃ fine particles exclude themselves as effective flux pinning centers in the textured YBCO.     

Microstructural properties of Pt-doped YBa2Cu3O7−x high-T c superconductor prepared by melting method

We have studied the effect of platinum addition on the superconducting properties of YBa₂Cu₃O_7–x (123) compound and elucidated from the metallurgical point of view the mechanism of formation of the fine dispersion of Y₂BaCuO₅ (211) particles in YBa₂Cu₃O_7–x superconductor prepared by a melting method. In this study, the amounts of BaCuO₂ and CuO-rich phases unreacted during the peritectic reaction were markedly decreased by the 211 powder addition. The 211 particles of Pt-free sintered samples were 8–10 m in size, but in 1 wt% Pt-added samples 211 particles were finely dispersed in the 123 matrix and the size of 211 particle was about 1–2 m. The critical temperature (T _c,zero) of Pt-doped samples was 91.5 K and the transport critical current density (J _c) of Pt-doped samples was much more than 10⁴A cm^–2. The high J _c and fine dispersion of 211 particles of Pt-doped YBa₂Cu₃O_7–x superconductor are attributed to Ba₄CuPt₂O₉ compounds formed during the partial melting, which were considered as nucleation sites of 211 particles, rather than Pt itself.     

Preparation of rod-shaped BaTiO3 powder

Rod-shaped BaTiO₃ powder particles have been prepared from rod-shaped TiO₂ ·nH₂O and BaCO₃ in molten chloride. The morphology of BaTiO₃ particles was studied referring to the effects of the chemical species of the starting titanium compound, amount of chloride, particle size of the titanium compound and reaction conditions, and the preparation condition of rod-shaped BaTiO₃ has been determined: i.e., large TiO₂ ·nH₂O particles were heated at 700°C in molten salt with an equal amount of BaTiO₃. This condition was effective in suppressing the formation of BaTiO₃ by a solution-precipitation process as well as the deformation of either TiO₂ ·nH₂O or BaTiO₃, which are responsible for the formation of equiaxed BaTiO₃ particles. The obtained rod-shaped BaTiO₃ particles had a cubic symmetry. Electron diffraction analysis showed that the following topotactic relation is retained; 0 1 0_{potassium tetratitanate} 0 1 0_{hydrated titania} 1 0 0_anatase 1 0 0_{barium titanate}     

Condensation and morphology of magnesium particles in vapour plumes

A plume chamber study of condensation in magnesium vapour was performed in flowing argon at atmospheric pressure and source temperatures (T ₀) up to 1213 K. The wall temperature (T ) was at 300 K. Sampled particles closely resembled those from zinc and cadmium aerosols and included spheres and prisms, indicative of vapour-liquid and vapour-solid nucleation. The spheres solidify from single rafts, and in addition to the hexagonal prisms that grow from dendrites, flatter and elongated forms occur as twins. The presence or absence of either spheres or prisms was found to depend on the setting ofT ₀, in agreement with theory, which predicts thatT ₀ will determine the position of the nucleation threshold temperature (T _n) relative to the melting point (T _f). The occurrence of particles condensed as solid as well as liquid nuclei whenT _n was higher thanT _f showed that supersaturated vapour states can persist as the vapour cools belowT _n. The twinned particles observed with magnesium do not occur with zinc or cadmium at atmospheric pressure. It is suggested that the presence of hydrodynamic stresses causes twinning in magnesium whereas in zinc and cadmium it results in malformed hexagonal prisms.     

Fabrication and characteristics of AA6061/SiCp composites by pressureless infiltration technique

The tensile properties and microstructures of AA6061/SiC_p composites fabricated by the pressureless infiltration method under a nitrogen atmosphere were examined. Since the spontaneous infiltration of molten AA6061 into the powder bed containing SiC_p occurred at 800 °C for 1 hour under a nitrogen atmosphere, it was possible to fabricate composites reinforced with SiC_p. Reaction product (Al₄C₃) was formed at the interface between SiC_p and Al alloy matrix. In addition, the amount and size of the Al₄C₃ is increased significantly by increasing the infiltration temperature. The reaction product (AlN) was formed as a result of the in situ reaction in both the control alloy and the composite. A significant strengthening even in the control alloy occurred due to the formation of in situ AlN particle even without an addition of SiC_p. While a further strengthening of the composite was produced by the reinforced SiC_p, strain to failure of the composite fabricated at 800 °C showed the lowest value (1.3%) in the T6 condition due to the formation of the severe reaction product (Al₄C₃). The grain size of the control alloy significantly decreased to about 20 m compared to 50 m for the commercial alloy. In addition, the grain size in the composite reinforced with SiC_p further decreased to about 8.0 m. This grain refinement contributed to strengthening of the control alloy and composite.     

Anomalous magnetic features in α-Fe2O3-based ceramic composites containing B2O3 dispersoids

Ceramic composites based on BaO-Fe₂O₃-B₂O₃ starting compositions have been prepared by the solid-state reaction technique. Basically the samples comprise crystalline particles due to the-Fe₂O₃ phase. It is found that an easy control of the particle size and many of the physical properties of the system is possible by modifications of the composition and the sintering. At sintering temperatures below 1200° C the material is essentially non-magnetic. However, it exhibits surprisingly large magnetization (small particle size and low coercive fields) at sintering temperatures above 1200° C. The anomaly is explained by invoking a heat-induced Fe³⁺ Fe²⁺ reduction and considering a modified distribution of the cations in the-Fe₂O₃ precipitated particles. The results are consistent with microstructures and electron paramagnetic resonance measurements of the samples.     

On the thin-section size dependent creep strength of a single crystal nickel-base superalloy

The combined effects of thin-section size, D, and microcracks on the creep behaviour of the single crystal MAR-M002 were investigated at the creep conditions of 300 MPa and 900 °C. It was observed that the creep rupture life, t _R is controlled by the mean microcrack size to thin-section size, (d _c/D), (or the total number, (N _m), of the mean-sized microcrack particles across the diameter, assuming D/d _c=N_m); reducing N _m continuously improves t _R. The creep rupture strain (or ductility), _R, can be improved sharply by increasing the total number, N _T, of microcrack particles across the cross-section, N _T D ² N _A, where N _A is the number of microcrack particles (cavity density) per cross-section. The behaviour of the creep rupture ductility was interpreted in terms of the weakest link, or largest-flaw concept; the observation of the higher proportion of the less likely dangerous (smaller in size) microcracks with increasing N _T was the underlining reason for the improvement in ductility.     

The influence of grain size on the creep of uranium dioxide

The creep of uranium dioxide has been investigated as a function of grain size. At high stresses, when creep is controlled by dislocation movement, grain boundaries exert a strengthening effect and this strengthening is correlated with the Hall-Petch equation. The degree of strengthening diminishes with increases in temperature. At lower stresses, when creep is controlled by mass transport, grain boundaries exert a weakening effect owing to the reduction in diffusion path length as grain size is reduced. In this range behaviour is correlated with the Nabarro-Herring equation with stress replaced by an effective stress _E=–₀ where ₀ is a threshold stress for diffusional creep associated with the limitation of the ability of boundaries to emit and absorb vacancies. ₀ appears to decrease as grain size is increased.     

Effect of grain size upon flow and fracture in a precipitation-strengthened Ti-8 wt % Al-0.25 wt % Si alloy

The interaction of grain size and precipitation strengthening has been studied in a Ti-8 wt % Al-0.25 wt %Si alloy. Grain sizes varying from 9 to 90m were produced by warm-working and annealing the alloy in the single--phase field. A uniform distribution of the coherent ₂ particles in the matrix was produced by ageing the alloy in the two-phase ( + ₂) field. The yield strength Hall-Petch slopes of the alloys with and without the ₂ precipitates were found to be nearly equal, indicating that the precipitation and grain-boundary strengthening are linearly additive. While specimens containing no precipitates exhibited a high ductility for all grain sizes, the ductility of the specimens with the ₂ particles decreased drastically with increasing grain size. TEM examination of the specimens containing the precipitates revealed a highly planar, localized slip and SEM examination of the fracture surfaces of these specimens revealed a transition in fracture behaviour from highly dimpled to mixed cleavage and intergranular with increasing grain size.     

In situ production and defect characterization of laser PVD layers from YBaCuO HTSC targets inside a scanning electron microscope

In situ generation and defect characterization of YBa₂Cu₃O_7–x layers are carried out inside a scanning electron microscope (SEM) to investigate the process of pulsed laser PVD. A Nd-glass laser with a wavelength =1.06 µm and an energy densityE<200 MJ m^–2 was combined with a SEM and used for experiments. By the irradiation of a YBa₂Cu₃O_7–x target the laser PVD process and the deposition on Si substrates were originated. At high pulse energies and/or long laser pulses heating and evaporation processes of target material dominated. For laser irradiation without Q-switch (=10 ms) the defect particles incorporated into laser PVD layers were mainly big droplets originated from the liquid phase. Otherwise, for very short pulses (Q-switch mode) the layer defects were also particles of irregular size generated by material ablation on the target surface. In contrast to the droplets these irregular particles and also the layer itself showed the same stoichiometry as the target. Both the particle density and the layer thickness showed the same plane distribution function. Therefore the particle density in the plasma seemed to possess the same solid angle distribution as the plasma itself.     

Decomposition of some Si-Al-O-N powders during plasma spheroidization

An attempt to prepare Si-AI-O-N glasses with compositions lying along the mullite-Si₃N₄ junction, by subjecting powders to rapid melting in a nitrogen plasma followed by quenching (10⁶ C sec^–1), resulted in their decomposition. Powders (–53, + 37m) containing SiO₂/Si₃N₄ in the ratio 3/1 tended to decompose to give spheroidized-AI₂O₃/-AI₂O₃ particles, SiO and N₂. For SiO₂/Si₃N₄>3 the particles consisted of-AI₂O₃ stabilized by the presence of SiO₂ in solution, whereas if SiO₂/Si₃N₄<3, the particles consisted predominantly of the AI₂O₃-AIN spinel. Larger particle size powders (–75, + 53m) did not decompose to the same extent and there was evidence that a nitrogen-containing glass was formed with a devitrification temperature of 1100° C compared with 1000° C for AI₂O₃-SiO₂ glasses.     

Alumina-chromium carbide composite through an internal synthesis method

In situ formation of chromium carbide particles, through a solid state reaction between Cr₂O₃ and SiC, for strengthening AI₂O₃ has been studied. Three kinds of chromium compound, Cr₃Si, Cr₃C₂ and Cr₇C₃ and mullite were formed in the alumina matrix. The reaction behaviour during hot pressing depends on heating parameters such as temperature and atmosphere. In a vacuum environment, the Cr₃Si particles formed first and was the dominant dispersed phase below 1550°C, while the Cr₇C₃ phase was only dominant above 1600°C. The Cr₃C₂ phase emerged briefly then diminished at temperature 1500°C. In an argon environment, however, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450–1550 °C. The mullite phase formed concurrently through the diffusion of SiO₂ phase into the Al₂O₃ matrix, which is a by-product from the reaction between Cr₂O₃ and SiC. Incorporating chromium carbide or suicide particles into the Al₂O₃ matrix induces a strengthening effect. However, only when the content of dispersed phases is low and is mainly of Cr₃C₂ particles, is the strengthening effect significant. For instance, the composite, containing 5 vol% chromium carbide and hot-pressed at 1500°C in argon, gives a flexural strength and fracture toughness up to 600 MPa and 6.1 MPam^1/2, respectively.     

Effect of cationic substituents on particle morphology of goethite and the magnetic properties of maghemite derived from substituted goethite

Preparation of nanoparticles with the desired shape and size by the wet-chemical precipitation process is a challenging task. Thus, the effects of different substitutional impurities such as Al, Cr, Co or Ni on the particle morphology and phase stability of goethite have been investigated. Goethite is prepared by air oxidation of Fe(OH)₂ · xH₂O gel under near neutral conditions. Below certain concentration levels of dopants (Al³⁺ 10; Cr³⁺ 5; Co²⁺ 10 and Ni²⁺ 5 at.%) the samples remained monophasic as revealed by XRD, TEM, and IR studies. Above these levels, the substituents produce traces of secondary phases such as lepidocrocite, spinel ferrite and Mⁿ(OH)_n. The goethite structure is stable in spite of the iso- or aliovalent substitutions. The individual additives have divergent influence on the particle morphology; Al³⁺ and Cr³⁺ decreases the particle size to <50 nm and aspect ratio (AR) <2. Co-substitution produces slender particles with AR as high as 25. Whereas, Ni²⁺ does not have any influence on the particle morphology. The attributable factors in morphology control are the increased nucleation rate, restricted growth along needle axes, and the strain induced in the goethite lattice as a result of difference in ionic radii. Maghemite, -Fe₂O_3–, particles are obtained from goethite wherein the topotactic conversion renders the retention of the particle morphology of the precursor. Maghemite with substituted impurities showed substantial differences in magnetic properties. Saturation magnetization (_s) and coercivity (H _c) go down to very low values due to relaxation of spins on the surface atoms as revealed by Mössbauer spectroscopy. Decrease in coercivity is by way of the presence of diamagnetic ion (Al³⁺). Whereas, Co-substituted maghemite has enhanced H _c as a result of high magnetocrystalline anisotropy accompanied by the shape anisotropy.     

The effect of particles on the critical strain associated with the Portevin-LeChatelier effect in aluminium alloys

The effect of particles on the critical strain, _c, associated with the Portevin-LeChatelier (PL) effect of aluminium alloys is studied using Al-Mg-Ni and Al-Si alloys. Al-Mg-Ni and Al-Si alloy matrixes are composed of Al₃Ni and Si particles, respectively. Tensile tests were performed in the temperature range 223–273 K in which the critical strain decreases with increasing temperature, and strain rates between 10^–5 and 10^–2 s^–1 were chosen. According to the apparent activation energies, Q, Mg and Si solute atoms are responsible for the flow instability in Al-Mg-Ni and Al-Si alloys, respectively. The experimental results also show that the critical strain decreases with decreasing particle spacing, d _p. Since the particle spacing is small compared to the corresponding grain size, the decrease in critical strain should be ascribed to the effect of particles. Considering that the dislocation density is increased by the particles, a modified model showing the critical strain, _c, as a function of particle spacing, d _p, is proposed as T ^–1 exp (–Q/kT), in which , T and k are the strain rate, temperature and Boltzmann constant, respectively. Linear fit of the plots of In _c versus In d _p and In _c versus In d _p indicates that this equation is appropriate to rationalize the particle effect on the critical strain.     

A micromechanical method for predicting the precipitation hardening response of particle strengthened alloys hardened by ordered precipitates

Summary A micromechanical method was developed for predicting the precipitation hardening response of particle strengthened alloys hardened by ordered precipitates based on the microstructure, composition, and heat treatment, and utilizing a minimum number of experimental tests to evaluate the microstructural constants of the overall model. The overall approach was based on incorporating the dislocation particle interaction mechanics, particle growth and coarsening theory, thermodynamics, and particle strengthening mechanisms applicable to precipitation hardened alloys as part of the overall micromechanical method. The method/model evaluates, from a minimum number of experimental tensile tests, microstructural constants necessary in determining the precipitation srengthening response of a particle strengthened alloy. The materials that were used as vehicles to demonstrate and evaluate the model were precipitation hardenable aluminium-lithium-zirconium and nickel-aluminum alloys. Utilizing these demonstration alloys, the method used a total of four tensile tests to evaluate the necessary microstructural constants and thus predict the variation in strength as a function of aging time, aging temperature, and composition, for the underaged, the peak-aged, and the overaged conditions. Predictions of the precipitation strengthening response were made incorporating the Wagner particle distribution model to evaluate the size distributions of particles in the microstructures. The predicted variation of strength with aging practice and composition using the Wagner distribution model compared well with the corresponding experimental yield strength results.Notation b Burgers vector - average particle size diameter for a particle distribution - d _loop particle looping diameter for dislocation bypassing by Orowan looping - f _v volume fraction of precipitates - h() Wagner particle size distribution function - n _total total number of precipitate particles per unit area on a given microstructural plane - average particle size radius for a particle distribution - average planar particle size radius on a given microstructural plane - t aging time, in hours - average planar particle cross sectional area - G _t total shear modulus of the material - K _c particle growth rate constant - texture or Taylor grain orientation factor - N _v total number of precipitate particles per unit volume - Q _A activation energy for diffusion - R universal gas constant - T aging temperature - the interparticle separation or spacing - _y yield strength - _q as-quenched strength - _i intrinsic lattice strength - _c critical resolved shear strength - _loop critical resolved shear strength for dislocation particle bypassing via. Orowan looping - _particle total critical resolved shear strength for particle strengthening - _shear critical resolved shear strength for dislocation particle shearing, in underaged state     

γ′ particle coarsening and yield in Alloy 800

The age strengthening of Alloy 800 by ordered particles of-Ni₃(Al, Ti) phase has been studied by using both transmission electron microscopy and room-temperature tensile tests on aged specimens. The samples have been aged in the temperature range 525 to 650° C up to a maximum time of 10⁴ h. Two groups of samples with different Ti/Al ratios have been investigated in order to obtain more reliable information on the role played by these alloying elements on the age-hardening behaviour. The linear dimension increases with time,t, ast ^1/3 and an activation energy of 70.0 kcal mol^–1 was derived from the temperature dependence of the coarsening rate. The particle volume-fraction, as measured by electron microscopy, has been found to remain constant on ageing in the temperature range 525 to 600° C. The increase in the critical shear stress due to the particles is found to agree quantitatively with the equations of Brown and Ham which describe hardening by ordered particles. Antiphase boundary energies of 227 and 279 mJ m^–2 have been measured, respectively, for the two groups of samples investigated.     

Synthesis and photoluminescence properties of (Y,Gd)BO3 : Eu phosphor prepared by ultrasonic spray

The mixed solution of acetate of Y, Gd, Eu and boric acid diluted in water was used as the precursor for the ultrasonic spray for the synthesis of (Y,Gd)BO₃ : Eu phosphor. It was found that (Y,Gd)BO₃ : Eu phosphor made by ultrasonic spray pyrolysis had a spherical shape and a narrow size distribution having a mean size of 1.3 m, while it had irregular, coarse and non-uniform size distribution for the phosphor formed by solid-state reaction. The as-sprayed particles was amorphous but they converted into the same polycrystalline phase of solid state reaction after post heat treatment at 1100 °C for 2 hr. The emitting intensity under 147 nm VUV excitations for the spray-formed (Y,Gd)BO₃ : Eu phosphor, however, was inferior to the later one. It was found that the optimum concentrations of Gd and Eu were 30% and 5%, respectively in (Y_1–x Gd _x )_1–y BO₃ : Eu _y phosphors prepared both by spray and solid state reaction.     

The mechanical and electrical properties of ZrO2-Naβ″-Al2O3 composites

ZrO₂-Na-Al₂O₃ composites were prepared by a conventional method using two different powder routes and different milling liquids. The retained tetragonal-phase ZrO₂ was 85 to 90% for composites with 2.4 to 15 vol% ZrO₂. The fracture toughness (K _lc) and strength increased with increasing ZrO₂ content. At 20 vol % ZrO₂,K _lc and bend strength were 4.35 M Pa m^1/2 and 390 MPa, respectively. Stress-induced transformation toughening is the predominant toughening mechanism. Dispersion toughening also contributes to the increase ofK _lc. Surface strengthening was found to be an effective strengthening method for low ZrO₂ levels. The critical tetragonal ZrO₂ grain size was found to increase from 0.86 to 1.02gmm as the ZrO₂ content increased from 2.5 to 15 vol %. A detailed study of the ionic conductivity of the 15 vol % ZrO₂ dispersed sample was conducted by an a.c. technique between –124° C and 300° C. The bulk and total conductivities were calculated via complex-plane analysis. The total (grain and grain-boundary) ionic specific resistivity was 9 cm at 300° C. The activation enthalpies of the bulk and total conductivity processes were 0.30 and 0.32 eV, respectively.