The structure of a rapidly solidified Al-Fe-Ti-C alloy

The microstructures of melt-spun Al-2.03 Fe-0.46 Ti-0.35 C (at %) superheated to 1523 K (ribbon I), and 1673 K (ribbon II), respectively, before quenching, have been characterized using analytical electron microscopy and X-ray diffraction. A duplex microstructure has been observed for ribbon I, consisting of a microcellular region, across a sharp transition, followed by a coarser cellular or dendritic structure. The intercellular phases consisted mostly of Al₆Fe (few of Al₃Fe) and the dispersed TiC particles distributed in the -Al matrix with an exact orientation relationship. However, the microstructure of ribbon II comprised uniform, fine-scale dispersions of Al₆Fe phase in -Al grains, and larger size, elongated amorphous phase particles located along the grain boundaries, and approximately 0.46 at % Ti and 0.35 at % C dissolved in the -Al matrix. During the annealing of ribbon II, the amorphous phase transformed to _T-AlFeSi phase, the Al₆Fe dispersoids grew upwards and Al₃Fe, TiC particles precipitated in the -Al matrix. TiC phase formed both in ribbon I and in annealed ribbon II all had an atomic composition of TiC_0.79 (the nominal atomic percent ratio for the alloy was 0.74) and a lattice parameter of 0.424 nm. Moreover, there is a cube-cube orientation relationship between TiC and -Al matrix with a disregistry =0.049. In addition, the solidification characteristics of rapid solidification processing (RSP) Al-Fe-Ti-C alloy and mechanism of TiC formation have been discussed.     

Influence of stabilizers on Na-β′′-Al2O3 phase formation in Li2O(MgO)-Na2O-Al2O3 ternary systems

The influences of stabilizers on - and -Al2O3 phase formations in Li2O(MgO)-Na2O-Al2O3 systems were investigated. When stabilized with 4MgCO3Mg(OH)25H2O, most of the -Al2O3 phase formed below 1200°C and further - to -Al2O3 transformation with an increase of temperature was not observed. On the other hand, when stabilized with Li2CO3,-Al2O3 formation occurred by two steps. First, -Al2O3 was partly formed below 1200°C, and, second, noticeable transformation from -Al2O3 to -Al2O3 occurred at higher temperature ranges. It was shown that transient eutectic liquid in the Li2O-Na2O-Al2O3 system promoted the - to -Al2O3 transformation at higher temperatures. Uniform distribution of both Mg2+ and Li+ stabilizing ions enhanced -Al2O3 formation at low temperatures. In the Li-stabilized systems, however, homogeneous distribution of Li+ ions hindered both the formation of transient eutectic liquid and the second - to -Al2O3 phase transformation at high temperatures.     

Enhancing effect of Cl2 atmosphere on transition aluminas transformation

In the present study we examine the changes in phase composition and microstructure undergone by solids after thermal treatments performed on transition aluminas in chlorine and air atmospheres. The thermal transformation to the stable -Al2O3 was faster for the chlorine-heated samples than for the air-heated ones, and for chlorinated samples single crystals of -Al2O3 were observed. We propose a mechanism by which AlCl3(g) formation allowed vapour mass transport that contributed to the growth of crystals of -Al2O3. Hydroxyls contained in transition aluminas could enhance gaseous transport and could also react with chlorine molecules. The subsequent -Al2O3 crystallization would be assisted by water vapour.     

Fabrication of β- and β′′-Al2O3 tubes by pressureless powder packing forming and salt infiltration

A new forming method, pressureless powder packing (PLPP), was studied to fabricate the - and -Al2O3 tubes. Alkali sources were infiltrated into the pores of -Al2O3 tube preforms that had been prepared by the PLPP forming method. The composition for the synthesis of -Al2O3 phase was Na2O · 0. 138Li2 · 4.4Al2O3. The -Al2O3 fraction of calcined and sintered bodies was increased with the increase of calcination temperature, and phase formation was largely affected by the type of starting -Al2O3. Large particle size and narrow size distribution of fused -Al2O3 resulted in uniform green microstructure that enhanced the homogeneity of alkali salts after infiltration, which was very important for the -Al2O3 formation. Sintered microstructure was uniform in all specimens but further development was required for density improvement.     

Cellular microstructure and heterogeneous coarsening of δ′ in rapidly solidified Al-Li-Ti alloys

Four melt-spun Al-Li-Ti alloys with 2 wt% lithium and 0.10 to 0.35 wt% titanium have been obtained and heat-treated at 473 K for up to 1000 h. Rapid solidification gives rise to a matrix with titanium in solid solution which drastically alters the coarsening rate. While TEM studies of samples aged for short times show a homogeneous distribution of metastable phase, as ageing time is increased, and depending on the ribbon section, three different microstructures can be distinguished: (i) on the wheel side, the distribution is homogeneous; (ii) intermediate regions show particles delineating cells with narrow walls; (iii) on the gas side, particles delineate circular cells. A higher titanium content in the cell centres than on cell walls has been determined. The coarsening rate of in microstructure (i) above is slower than in binary Al-Li alloys. Cellular microstructures (ii) and (iii) show the preferential coarsening of particles on the walls, which is faster the higher the titanium concentration. Taking into account the fact that the partition coefficient of titanium in aluminium in the peritectic region is > 1, an explanation of phase evolution is given which leads to the conclusion that the effect of titanium in solid solution is to retain vacancies, restricting lithium diffusion.     

Reinforcement of molten carbonate fuel cell matrixes by adding rod-shaped γ-LiAlO2 particles

Improvement in strength of -LiAlO₂ matrixes for a molten carbonate fuel cell (MCFC) via addition of rod-shaped -LiAlO₂ particles has been investigated. The rod-shaped -LiAlO₂ particles with the aspect ratio of 9–15 were obtained by heat-treating rod-shaped -LiAlO₂ particles at 800 °C for 3 h, synthesized from the reaction mixture of LiOH--Al₂O₃-NaOH. The -form was very stable for more than 700 h in a molten carbonate environment at 650 °C, while the -form showed both a phase change from - to -LiAlO₂ and a morphology change from rod- to bipyramid-shapes. The average pore size and the porosity of the matrix (550 m thickness) fabricated by tape-casting of the slurry with weight ratio 1.0 of rod-shaped -LiAlO₂ particles to commercial -LiAlO₂ powders (2.0 m dia.) were about 0.12 m and 54%, respectively, after being heat-treated at 650 °C for 2 h. The strength (197 g_f/mm²) of the rod- shaped particle reinforced matrixes was enhanced more than twice as much as that of the non-reinforced -LiAlO₂ matrixes (91 g_f/mm²).     

Spray atomization of two Al–Fe binary alloys: solidification and microstructure characterization

The effect of solidification history on the resultant microstructures in atomized Al–3Fe and Al–7Fe powders is studied, with particular emphasis on the relationships between droplet size, undercooling and phase stability. The atomized Al–Fe powders exhibit four microstructural features, i.e. Al3Fe phase, Al + Al6Fe eutectic, -Al dendrite and a predendritic structure. The presence of these is noted to depend on a kinetic phase competitive growth mechanism, which was determined by the initial undercooling experienced by the powders. The results of scanning electron microscope analysis demonstrate that the content of Fe in the -Al phase increases with decreasing powder particle size, i.e. for Al–3 wt% Fe powders, the content of Fe in the -Al phase is 2.21 and 2.56 wt% corresponding to powder particle sizes of 90 and 33 m, respectively; for Al–7 wt% Fe powders, the content of Fe in the -Al phase is 5.51 and 5.98 wt% corresponding to powder sizes of 90 and 33 m, respectively. In the present study, homogeneous nucleation undercooling, corresponding to the -Al phases, is also estimated using an existing correlation.     

Microstructural evolution in rapidly solidified Al-Cu-Si ternary alloys

Several Al-Cu-Si alloys were melt spun to produce stable, fine scale microstructures suitable for superplastic deformation and consolidation. Scanning electron microscopy of the ribbon cross-sections reveal two distinct alternating microstructural morphologies, suggesting transitions in solidification behavior. One structure consists of intimately interlocked -Al and (Al₂Cu) phases with dispersed spheroids of (Si). The other structure consists of equiaxed or cellular-dendritic -Al with interdendritic and (Si). The latter was found in the middle portion of the ribbon cross-section when cast at a low speed, and throughout the ribbon cross-section when cast at high speed. The dendritic structure appears to result from independent nucleation events in the undercooled liquid ahead of the solid-liquid interface. The solidification mechanism for the interlocked structure appears to involve multiple nucleation of the phase followed by its cooperative growth with the -Al phase. This cooperative growth is unlike that which forms a lamellar structure, as it results in a branched, randomly oriented network. We postulate that the (Si) phase is the first phase to form from the undercooled liquid, and it is uniformly dispersed throughout the undercooled melt. The (Si) spheroids provide nucleation sites for the phase because of its observed association with the phase. The -Al grain size varies from 1 m near the wheel side surface of the ribbon to 8 m with sub-grains near the free surface. The size of the and (Si) phases is on the order of a m and less. The microstructural size scale appears to be small enough for this material to exhibit superplastic behavior when deformed.     

Microstructural development in cast and aged Ni–Al–Cr-based alloys derived from the B2 type β-NiAl structure

The formation of intermetallic compounds consisting of nickel-rich B2-type NiAl (-phase) ductilized by two-phase A1 ()/L12 () regions provides the possibility of combining ductility and high-temperature performance. Similar microstructures can also form the basis of high-temperature shape memory alloys, due to martensitic transformation of the -phase to an L10-type product. One route by which –/ microstructures can be produced involves the use of chromium as a -stabilizer. However, microstructural development in such a case is complicated by the formation of -Cr precipitates.This paper examines microstructural development and stability in cast Ni-25 at % Al-14 at % Cr, Ni-29 at % Al-22 at % Cr and Ni-27 at % Al-8 at % Cr alloys, together with a more complex material, namely, Ni-20 at % Al-13 at % Co-9 at % Cr-4 at % Ti-1 at % Mo-1 at % V. Both the as-cast condition and samples aged at 850 and 1100 °C for 140 h are examined using transmission electron microscopy. The paper discusses the formation of L10 martensite, intradendritic ', interdendritic /' and -Cr precipitation.     

On a simple wave approximation of a set of linear dispersive wave equations

Summary The validity of an approximation ₀ of one of the solutions of a set of two linear coupled dispersive wave equations has been discussed. ₀ is the solution of a linear Korteweg-de Vries equation and satisfies the same initial condition as . It is shown that for square integrable solutions having a spectral range not exceeding [–, ] the approximation is useful if ^{5 2}t«1 in the sense that –₀(t)« (t)(L ₂ -norm). is a measure for the dispersion. The approximation fails in that sense ast . Some remarks to a similar nonlinear problem are made.     

Temperature dependence of the experimental penetration depth of superconducting thin films

Experimental magnetic field penetration depths (t, d, H) of the stable and superheated Meissner state were calculated as a function of temperature for various applied magnetic fields and various film thicknesses for two cases: (1) (t)/d and (2) 2(t)/d ( is the Ginzburg-Landau penetration depth,d is the film thickness, is the GL parameter). The results of the first case should be a useful tool for obtaining (0) of amorphous superconducting thin films.¹ This work was supported in part by NSF Grant No. INT 8006927.     

Approximate stress intensity factor for an embedded elliptical crack near two parallel free surfaces

An approximate stress intensity factor is derived for an embedded elliptical crack in a plate which is subjected to uniaxial tension in the direction perpendicular to the crack surface. The major axis of an eccentrically located elliptical crack is assumed to be parallel with the two plate surfaces. The approximate stress intensity factors on the minor axis of the elliptical crack are then determined as Ba where a is a correction factor due to the curvature of the ellipse and 6 is a correction factor due to the eccentricity of the crack in the wall.

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Zusammenfassung Es wird ein angenaeherter Faktor fuer die Spannungskonzentration an einem elliptischen Riss, der in einen Platte unter einachsiger Zugspannung eingeschlossen ist, abgeleitet. Die Zugspannungsrichtung ist senkrecht zur Rissoberflaeche. Es wird angenommen, dass die Hauptachse des exzentrisch gelagerten Risses parallel zu den beiden Plattenoberflaechen ist. Fuer den angenaeherten Faktor der Spannungskonzentration an der kleineren Hauptachse des elliptischen Risses ergibt sich dann Ba wobei ein Korrekturfaktor fuer die Ellipsenkruemmung ist und einen Korrekturfaktor fuer die Exzentrizitaet des Risses in der Platte bedeutet.

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Résumé Un facteur approché d'intensité de contrainte est obtenu pour une fracture elliptique encastrée dans une plaque soumise à une tension uniaxiale dans la direction perpendiculaire à la surface de la fracture. Le grand axe d'une fracture elliptique excentrique est supposé être parallèle aux deux surfaces de la plaque. Les facteurs approches d'intensité de contrainte, le long du petit axe de la fracture elliptique sont déterminés par Ba où a est un facteur de correction dû à la courbure de l'ellipse et est un facteur de correction dû à l'excentricité de la fracture dans le mur.

     

Microstructure-property studies in friction-stir welded, Thixomolded magnesium alloy AM60

Thixomolded magnesium alloy AM60 plates joined by friction-stir welding were observed to be as strong or stronger than the unwelded base material. The thixotropic microstructures of the as-molded plates consisted of either 3% or 18% primary solid fraction of -Mg globules (45 m average size) in a eutectic mixture consisting of -Mg grains (10 m) surrounded by Mg₁₇Al₁₂ intermetallic grains in the -Mg grain boundaries (having an average size of 1–2 m). This complex, composite microstructure became a homogeneous (Mg + 6% Al)), recrystallized, equiaxed grain (10–15 m) microstructure in the weld zone.     

The mechanism of continuous dissolution of the lamellar structure in aged Al 22 at% Zn alloy

The mechanism of continuous dissolution was studied using transmission electron microscopy, X-ray diffraction and hardness measurements. The metastable phase was identified in the first stage of dissolution nucleating at the - boundaries. Interface dislocations at the - boundaries forming a perpendicular net in the 110 directions were identified as of screw character and of Burgers vector b=1/2a 110. During dissolution a change of shape of precipitates occurs leading to their fragmentation and to a change of the direction of boundaries into 110 directions.     

Evaluation of the fatigue fracture resistance of unfilled and filled polytetrafluoroethylene materials

Polytetrafluoroethylenes (PTFEs) and their composites are a special class of fluorocarbons with very high chemical resistance and wide service temperature. This makes them good candidate materials for load-bearing components exposed to harsh environments, including some space applications. In the present work, fatigue crack propagation (FCP) behavior of four materials from the fluorocarbon family, including PTFE without filler (virgin PTFE), PTFE with 15% glass fiber, PTFE with 15% graphite particles, and PTFE with 25% glass fiber, were studied. Tension/tension FCP experiments were carried out using single-edge notch (SEN) specimens under load control. The maximum stress was kept constant at 8 MPa for each material at a frequency of 3 Hz. The minimum to maximum stress ratio was 0.27. FCP data such as the number of cycles, crack length, and hysteresis loops were recorded in order to establish the crack speed, the energy release rate, J*, and the change in work Wi. Parameters that characterize the resistance of PTFEs to FCP have been successfully determined by the modified crack layer (MCL) model. These parameters are , the specific energy of damage, which reflects the FCP resistance of the PTFE materials, and the dissipative characteristic of the materials, . It has been found that the MCL model describes the behavior of the PTFEs over the entire range of the energy release rate and discriminates the subtle effects introduced by changing the filler type and dosage as well as the processing conditions. The values of the specific energy of damage have been found to decrease by increasing the dosage of the fiberglass fillers. Graphite particulate filler also reduced the value of more than fiberglass filler for the same dosage. Microscopic analysis of the fracture surface in the stable crack propagation region of each material revealed that there exists a strong correlation between the value of and the amount of damage energy manifested by different mechanisms and species during the fatigue process.     

Chemical Transformations of γ-Al(OH)3during Closed-System Heat Treatment

The chemical transformations of -Al(OH)₃during closed-system heat treatment in a self-generated gaseous atmosphere (water vapor) were studied. At t 200°C, -Al(OH)₃was found to convert into -AlOOH, which, in turn, converts into the equilibrium phase -Al₂O₃at t 400°C. The processes underlying the effect of water vapor on the kinetics and mechanisms of the chemical transformations of -Al(OH)₃and -AlOOH in a closed system are discussed.     

Effect of the Neutron Fluence on the Rate of Transfer of 115m In Atoms, Yielded by Transmutation of Nuclei, in Irradiated Metallic Cadmium

A study was made of migration of ^115m In atoms formed by nuclear transmutation [¹¹⁴Cd(n')¹¹⁵Cd ^- decay]. It was shown that the rate of ^115m In transfer tends to increase with increasing total radiation load (the fluence of the bombarding particles) and, thereby, specific activity of the preparations.     

Investigation of the backscatter coefficient of Β radiations from an aluminum backing in a 2π geometry

Conclusions It follows from the investigation of backscatter of radiations (at maximum energies of 10–290 fJ, i.e., of 0.155–1.711 MeV) from an aluminum backing with a 2 geometry that for weightless samples the intensity of the saturation backscatter is virtually independent of the -radiation energy, whereas for weightable samples it increases with a rising energy of particles.The coefficients thus obtained for -radiators with a finite active-layer thickness, which include the effects of self-scatter and self-absorption of output radiation over an angle of 2, can be used for approximate computation of the activity of radiators with a known active-layer thickness from the measured external output of -particles (and vice versa).     

Oxalate materials prepared by evaporation as precursors to superconducting ceramics

Some unexpected chemistry will be discussed which occurs during the production of highT _c superconductors by an oxalate route. Evaporation (rather than filtration) of a 123 YBaCu nitrate solution containing excess oxalic acid was done, in order to make sure that the solid had the same composition as the solution. It was assumed that the removal of water and nitric acid by vacuum would quantitatively yield the salts as oxalates. Surprisingly, the evaporation changed what was initially a homogeneous, but off-composition, material into a rather inhomogeneous material. During the evaporation, the yttrium and barium were apparently redissolved from their initial form as submicrometre particles containing all three metals. They were redeposited as 10–100 m rectangular and octahedral crystals. However, this material yields very good superconductors upon simple grinding and heat treatment as measured by conductivity and a.c. induction.     

Effect of Intense Self-Irradiation on the Phase Composition of Metallic Plutonium

The crystal structure of film samples of "high-level" (based on ²³⁸Pu) and low-level (based on ²³⁹Pu) metallic plutonium during their prolonged (up to 343 days) storage (self-irradiation) at room temperature was studied by X-ray diffraction. In the samples of high-level plutonium, the -Pu and -Pu lattices coexist. In the period of 40-60 days, the other known crystal modifications of plutonium (-Pu, -Pu, -Pu, and -Pu) are also present. Low-level plutonium had only the -Pu lattice. A possible origin of this phenomenon is discussed.