The microstructure of rapidly solidified Ti-32wt% Fe melt-spun ribbon

A Ti-32wt% Fe alloy was rapidly solidified by the melt-spinning technique. Three microstructures are found in the melt-spun ribbon of the alloy: the first contains small FeTi particles in a matrix, the second is a cellular microstructure composed of and FeTi and the third is a eutectic of these phases. The correlation between various solidification and cooling rates across the ribbon width to the three microstructures is explained.     

Non-equilibrium microstructure of hyper-eutectic Al-Si alloy solidified under superhigh pressure

The non-equilibrium microstructures of hyper-eutectic Al-26.6wt%Si solidified under superhigh pressure (5.5 GPa) have been investigated. The results show that there exists a great deal of primary phase in hyper-eutectic Al-Si alloy. The non-equilibrium microstructure for hyper-eutectic Al-Si alloy is composed of primary phase, phase and ( + ) eutectic phase. The solid solubility of Si in phase and the solid solubility of Al in phase increase significantly. The effects of high pressure on the solidification structures of Al-Si alloy are discussed.     

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.     

Microstructure of laser melted/rapidly solidified γ/Cr3Si metal silicide “in situ” composites

In order to improve the ductility of Cr₃Si metal silicide alloys, rapidly solidified /Cr₃Si metal silicide in situ composites were fabricated by laser melting/rapid solidification technology using Cr-Si-Ni alloy powders. Microstructure of the /Cr₃Si in situ composites was characterized by OM, SEM, XRD and EDS. The effect of Ni content in the alloy powder on microstructure and hardness of the /Cr₃Si composites was investigated. The /Cr₃Si metal silicide in situ composites have high hardness and rapidly solidified fine microstructure consisting of primary Cr₃Si dendrites and the interdendritic /Cr₃Si eutectics. The volume fraction of the Cr₃Si primary dendrites in the laser melted/rapidly solidified /Cr₃Si metal silicide in situ composites decreases with the increasing nickel content. Because of the presence of the ductile nickel-base solid solution and the rapidly solidified fine microstructure, the /Cr₃Si metal silicide in situ composites are expected to have adequate combination of strength and toughness. The results demonstrate that laser melting/rapid solidification for /Cr₃Si metal silicide in situ composites is a promising toughening method for improving the ductility of Cr₃Si metal silicide alloys.     

Effect of swaging on the 1000‡C compressive slow plastic flow characteristics of the directionally solidified eutectic alloy γ/γ'′-α

Swaging between 750 and 1050 C has been investigated as a means to introduce work into the directionally solidified eutectic alloy /gg- (Ni-32.3 wt % Mo-6.3 wt % Al) and increase the elevated temperature creep strength. The 1000 C slow plastic compressive flow stress-strain rate properties in air of as-grown, annealed, and worked nominally 10 and 25% materials have been determined. Swaging did not improve the slow plastic behaviour. In fact large reductions tended to degrade the strength and produced a change in the deformation mechanism from uniform flow to one involving intense slip band formation. Comparison of 1000 C tensile and compressive strength-strain rate data reveals that deformation is independent of the stress state.     

Asymptotic expansions for constant-composition dew-bubble curves near the critical locus

Explicit functional representations are developed for constant-composition dew and bubble curves near critical according to the modified Leung-Griffiths theory. The pressure and temperature incrementsP=P–P _c andT= T–T _c, where c denotes critical, are linearly transformed to new variablesP andT. In the transformed space, the coexistence curves are no longer double-valued and can be expressed as a nonanalytic expansion, where the coefficients are functions of the critical properties and their derivatives. A similar asymptotic expansion is developed forT in terms of the density increment=– _c. In the approximation that the critical exponents=0 and=1/3, the critical point in temperature-density space is shown to be a point of maximum concave upward curvature, rather than an inflection point as previously conjectured.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.Formerly National Bureau of Standards     

Ceramic eutectics in the systems ZrO2-Ln2O3 (Ln=Lanthanide): unidirectional solidification,microstructural and crystallographic characterization

Unidirectionally solidified refractory eutectics (melting point between 2100 and 2300° C) in the systems ZrO₂-Ln₂O₃ (Ln=Nd, Sm, Dy) were prepared by skull-melting. Depending on the growth conditions, different eutectic microstructures were obtained and studied. In particular, well-aligned lamellar eutectics with a great regularity (interlamellar spacing about 10 m) were observed with sufficiently slow solidification rates (e.g. 0.4 cm h^–1). Preferred crystallographic directions for the eutectic growth and epitaxial relationships between phases were determined by X-ray diffraction. Microhardness measurements showed significant reinforcement for eutectics with regard to the pure lanthanide oxide hardness.     

A finite volume technique to simulate the flow of a viscoelastic fluid

Hydrodynamically developing flow of Oldroyd B fluid in the planar die entrance region has been investigated numerically using SIMPLER algorithm in a non-uniform staggered grid system. It has been shown that for constant values of the Reynolds number, the entrance length increases as the Weissenberg number increases. For small Reynolds number flows the center line velocity distribution exhibit overshoot near the inlet, which seems to be related to the occurrence of numerical breakdown at small values of the limiting Weissenberg number than those for large Reynolds number flows. The distributions of the first normal stress difference display clearly the development of the flow characteristics from extensional flow to shear flow.List of symbols D rate of strain tensor - L slit halfheight - P pressure, indeterminate part of the Cauchy stress tensor - R the Reynolds number - t time - U average velocity in the slit - u velocity vector - u,v velocity components - W the Weissenberg number based on the difference between stress relaxation time and retardation time - W ₁ the Weissenberg number based on stress relaxation time - x,y rectangular Cartesian coordinates - ratio of retardation time to stress relaxation time - zero-shear-rate viscosity, ₁ + ₂ - ₁ non-Newtonian contribution to - ₂ Newtonian contribution to - ₁ stress relaxation time - ₂ retardation time - density - (, , ) xx, yy and xy components of ₁, respectively - determinate part of the Cauchy stress tensor - ₁ non-Newtonian contribution to - ₂ Newtonian contribution to      

Response dynamics of the lamellar spacing for Al-Si eutectic during directional solidification

The response of the lamellar spacing to abrupt change of the solidifying rate for Al-Si eutectic during directional solidification has been investigated both experimentally and theoretically. When the solidifying rate was changed abruptly, the response of the lamellar spacing was gradual and retarded, which was attributed to the cluster branching and cluster terminating mechanisms of the lamellar phases. The retarded distance, as a function of the abrupt change factor, , for both >1 and <1, has been evaluated, and the uniqueness of the spacing selection has been verified. A theoretical approach of the response dynamics has been presented by considering the solute diffusion in liquid and the growth anisotropy effect of the eutectic lamella. A dynamic factor has been introduced to characterize the growth anisotropy. Excellent agreement between the theoretical approach and the measured results has been shown. Finally, this theory has been successfully applied to describe non-steady-state directional solidification of Al-Si eutectic with constant accelerating solidification rate.     

Effect of microstructure and γ′ precipitate from undercooled DD3 superalloy on mechanical properties

The mechanical properties of DD3 superalloy solidified at various undercoolings were measured to investigate the effects of precipitate and microstructure on alloy properties. The influence of melt undercooling on the precipitation is also studied. It is found that not only the size of particle, but its distribution in the as-solidified structure is also drastically controlled by the melt undercooling. The analysis indicates that alloy solidified at a low undercooling is brittle, thus leading to a lower toughness and tensile strength. With increasing undercooling, the toughness and strength of the alloy increased accordingly, which may be attributed to the strengthening effect of precipitate and the reduced micro-segregation.     

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.     

Eutectic solidification characteristics of Bridgman grown AI-3Fe-0.1V alloy

Eutectic solidification characteristics of Al-2.85 wt%Fe-0.12 wt% Valloy have been investigated by steady-state growth over the range of solidification front velocity from 51 to 1030m/s and temperature gradient 8 to 15 K/mm. Increasing growth velocity displaced the Al-Al3Fe eutectic by Al-AlxFe eutectic rather than by the Al-Al6Fe eutectic obtained for the binary Al-3 wt%Fe alloy. A fully Al-AlxFe eutectic structure has been obtained for the first time in the vanadium-containing alloy over the growth velocity range from 71 to1030m/s except at 100 and 510m/s where some Al dendrites were present in the eutectic matrix. TheAl-AlxFe eutectic was observed to undergo a morphological transition from lamellar to rod-like with increasing growth velocity concurrently with formation of a cellular eutectic structure. It was found that the relationship = Av–1/2, between eutectic spacing and growth velocity v, was applicable with A = 22.4 ± 1.8 and 13.8 ± 2.1 m3/2s–1/2 for lamellar and rod-like Al-AlxFe eutectics, respectively.     

Resonant Tunneling of Holes in GaMnAs-Related Double-Barrier Structures

Using the multiband quantum transmitting boundary method (MQTBM), hole resonant tunneling through AlGaAs/GaMnAs junctions is investigated theoretically. Because of band-edge splitting in the DMS layer, the current for holes with different spins are tuned in resonance at different biases. The bound levels of the light hole in the quantum well region turned out to be dominant in the tunneling channel for both heavy and light holes. The resonant tunneling structure can be used as a spin filter for holes for adjusting the Fermi energy and the thickness of the junctions.     

Dislocation structures in Zr3Al-based alloys

Transmission electron microscopy has been used to investigate dislocation structures in deformed binary and ternary Zr₃Al-based alloys. In the binary alloy deformed at temperatures between 293 and 673 K the dislocations in the Zr₃Al phase consisted of a/31 1 2-type partial dislocations bounding superlattice intrinsic stacking fault on {1 1 1} planes. The {111} a/31 1 ¯2 stacking fault energy was approximately 2mJ m^–2 at 673 K. In binary specimens deformed between 873 and 1073 K cube slip predominated. Dislocations consisted mainly of a/2 1 1 0 pairs separated by antiphase boundary. For this temperature range the {1 0 0} a/201 1 antiphase boundary energy was between 30 and 45 mJ m^–2. Alloying with niobium or titanium was found to increase the {111} a/31 1¯2 stacking fault energy and thus increase the propensity for antiphase boundary-type dissociation.     

Sapphire matrix composites reinforced with single crystal YAG phases

An investigation of fabrication technology on eutectic composites consisting of Al₂O₃ phases and YAG (Y₃Al₅O₁₂) phases was carried out by applying the unidirectional solidification process. Unidirectionally solidified eutectic composites consisting of 110 sapphire phases and 420 single crystal YAG phases could be fabricated successfully by lowering a Mo crucible at a speed of 5 mm h^–1 under a pressure of 10^–5 mmHg of argon. These eutectic composites have excellent high-temperature properties up to 1973 K. For example, the flexural strength is 360–500 MPa independent of testing temperature from room temperature to 1973 K. Oxidation resistance at 1973 K in an air atmosphere is superior to SiC and Si₃N₄ and the microstructure of these eutectic composites is stable even after heat treatment at 1773 K for 50 h in an air atmosphere.     

Paramagnetic resonance in a “dirty” spin-glass

For a spin-glass with nonmagnetic defects (n _m ^1/3l 1, where n _m is the magnetic impurity concentration and l is the mean free path) an absorption function () is derived. Three ranges of temperature and external magnetic field are considered. In the vicinity of the transition the value of () d is estimated as a function of temperature and field.     

The setting of gypsum plaster

The setting process of gypsum plasters has been followed using strength testing, acoustic techniques and scanning electron microscopy. Gypsum plasters were found to set in three stages: formation of the calcium sulphate dihydrate crystal matrix followed by relief of internal stress, and finally evaporation of excess water. The -calcium sulphate hemihydrate set faster than the -form, giving a higher initial and final set strength. The set -plaster consisted of relatively short stubby dihydrate needles with a greater degree of intercrystalline bonding, resulting in greater strength. The microstructure of set -plaster appeared to be the result of a slower precipitation rate of dihydrate during the hydration reaction of hemihydrate.     

Eutectic reaction and microstructural characteristics of Al (Li)-Mg2Si alloys

Thermal analysis, directional solidification and metallographic techniques were applied to investigate the pseudobinary eutectic reaction process and the microstructural characteristics of Al(Li)-Mg₂Si alloys. It was demonstrated that the eutectic reaction curve for L Al(Li) + Mg₂Si in the Al(Li)-Mg-Si system moves to the Al-rich corner with the increase of Li additions. The pseudobinary eutectic point with the highest melting temperature and null temperature range of solidification ( T) and the ternary eutectic point for L Al(Li) + Mg₂Si + Si all move towards the Si-rich direction. Li additions widen the range of yielding binary eutectic of Al(Li)-Mg₂Si and depress the appearing of ternary eutectic efficiently. Al(Li)-Mg₂Si eutectic alloys have the best aligned structure when the directional solidification occurs with a T = 0. The Mg₂Si phase has a diversity of morphologies such as rod-like, crossed and rooftop-like. These various morphologies has the same preferred growth direction, i.e. [100].     

The athermal α → ω transformation in Zr-2 at% Nb alloy

The athermal transformation in Zr-2 at.% Nb alloy has been investigated by transmission electron microscopy. Analysis of the selected-area diffraction pattern has shown that the orientation relationships between the omega and the parent-phase in quenched Zr-2 at.% Nb alloy are the same as have been previously observed for the reaction in pure zirconium. Thus it was deduced that the direct transition has taken place in the alloy during cooling. The-originated -particles were visualized using the dark-field technique. The formation of the athermal omega in the-region of-stabilized Zr-Nb alloy is discussed in terms of the relative positions of the free energy equilibrium curvesT ₀ ,T ₀ ,T ₀ and the correspondingM _s ,M _s andT _s start curves. It is concluded that the omega phase can occur over a much wider range of alloy compositions than is usually recognized on the basis of transformation data.     

Structural transformation during sintering and annealing of beta alumina

Changes in the phase content, microstructure and lattice parameter are observed in stabilized/ alumina specimens following extended sintering and annealing treatments. The resulting state is dependent on composition of the starting powder and on temperature and duration of heat treatment; the kinetics of transformation between and alumina are generally slow and certain/ ceramics remain in a metastable state even after a prolonged high temperature anneal. Following post-sinter heat treatment, splitting of X-ray diffraction peaks reveals a segregation of the phase into two components of differing lattice parameter. With sintering schedules of a long duration the splitting may even be present in the as-fired condition as recently reported by Harbach [1]. The splitting is attributed to a structural change resulting from the expulsion of Na₂O from supersaturated grains.