Showing papers on "Directional solidification published in 1994"

Progress in melt texturing of YBa2Cu3Ox superconductor

Abstract: Melt processing is the most prominent method utilized in the fabrication of bulk YBa2Cu3Ox(123) because of the superior transport and magnetic properties of this material. Due to the Y-diffusion-controlled growth mechanism of 123, the maximum allowable growth rate is sensitive to the size and distribution of the Y2BaCuO5 precipitates. In addition, a slow cooling rate and a slow growth rate have to be employed to maintain the stability of the 123 planar growth front. Among the many melt-texturing methods and modifications, seeded directional solidification has the potential to produce long single-domained samples of large cross-sectional area with the strongly superconducting a-b planes aligned along the sample axis. Steady progress has also been achieved in flux-pinning enhancement through defect engineering. This includes the introduction and modification of defects such as twins, irradiation damage and second-phase inclusions. In particular, high-temperature mechanical deformation has been shown to be effective in increasing the dislocation density and results in improved Jc as well as reduced Jc anisotropy.

Enhanced critical currents by CeO2 additions in directionally solidified YBa2Cu3O7

Abstract: The microstructure and critical currents of superconducting YBa2Cu3O7‐Y2BaCuO5 composites with small CeO2 additions prepared using a directional solidification procedure have been investigated. The small CeO2 additions cause the decomposition of the Y2BaCuO5 phase leading to the formation of Y2O3 and BaCeO3. Experimental evidence is given for the subsequent nucleation of the Y2BaCuO5 phase on the Y2O3 particles. This new decomposition‐nucleation mechanism leads to textured YBa2Cu3O7 having enhanced critical currents. An addition of 0.3 wt % CeO2 can increase Jabc by a factor of 2 in samples having similar concentration of Y2BaCuO5 precipitates. Critical currents above 105 A/cm2 at 77 K and zero field are obtained by this method.

Convective instability in ammonium chloride solution directionally solidified from below

Abstract: The stabilities of salt-finger and plume convection, two major flows characterizing the fluid dynamics of NH4Cl solutions cooling from below, are investigated by theoretical and experimental approaches. A linear stability analysis is implemented to study theoretically the onset of salt-finger convection. Special emphasis is placed on the competition between different instability modes. It is found that in most of the cases considered, the neutral curve consists of two separated monotonic branches with a Hopf bifurcation branch in between; the right-hand monotonic branch corresponding to the boundary-layer-mode convection is more unstable than the left-hand monotonic branch corresponding to the mushy-layer mode. We also conducted a series of experiments covering wide ranges of bulk fluid concentration C∞ and bottom temperature TB to study the stability characteristics of plume convection. From the measurement of both temperature and concentration of the interstitial fluid in the mushy layer, we verify that during the progress of solidification the melt in the mush is in a thermodynamic equilibrium state except at the melt/mush interface where most of the solidification occurs. The critical Rayleigh number of the onset of plume convection is found to be Rccm = 1.1 × 107Π* (see (22)), where Π* is the permeability of the mush. This relation is believed to be valid up to supereutectic NH4Cl solutions.

Effect of solidification conditions on MC carbides in a nickel-base superalloy IN 738 LC

Abstract: Transition metal carbides (MC) formed in the solidification process of cast nickel-base superalloys were found to affect greatly the mechanical properties and boundary performance of alloys. Thus, investigations of their growth behavior have received considerable concern from metallurgists. Earlier studies on such carbides were mostly confined to normal solidification conditions, i.e. equiaxed and directionally solidified conditions. Recently, some researchers also investigated the carbides with the rapid solidification method. However, there is lack of systematic study of growth characters of MC carbides under various solidification conditions. The intent of this paper is to establish a complete description for carbide growth. Accordingly, the variations of morphology, size, amount and distribution of MC carbides in the nickel-base superalloy Inconel 738 LC will be studied under a broad range of cooling rates from 10[sup [minus]2] to 10[sup 5][degree]C/s by use of directional solidification, investment casting, chill casting and splat cooling methods.

Crystal orientation and growth anisotropy of YBa2Cu3O6+x fabricated by directional solidification method

Abstract: The unidirectional solidification by a zone-melting method was performed to investigate the growth anisotropy of YBa2Cu3O6+x (123) superconductive oxides. Large faceted interfaces were obtained at growth rates lower than 2 mm/h. The three-dimensional interface morphology and the onset of growth were observed. The crystal faces toward the growth direction were revealed to be close to the {112} face, which was selected as the result of the preferential growth. The growth rate of the {100} faces was evaluated to be about 1.5 times faster than that of the {001} faces from the steady growth condition.

Melt texturing of preferentially aligned YBa2Cu3Ox superconductor by a seeded directional solidification method

Abstract: Directional solidification has been shown to be capable of producing long lengths of melt‐textured YBa2Cu3Ox. In order to maximize the current carrying capability of these superconductors, grain alignment needs to be directed along the sample axis. So far, this preferential grain alignment can only be achieved in samples of small cross‐sectional areas. We have developed a seeded directional solidification method which allows the fabrication of long lengths of single‐domained YBa2Cu3Ox with large cross‐sectional areas and predetermined grain alignment. The current carrying capability of these superconductors is superior to that of the misaligned ones, and the Ic is estimated to be a few thousand amperes for samples of 4×8 mm2 cross‐sectional area.

Evolution of microstructure and local thermal conditions during directional solidification of A356-SiC particle composites

Abstract: Solidification microstructures of aluminium silicon alloy (A-356) containing 0, 10, 15 and 20 vol % silicon carbide particles formed during directional solidification from a chill have been studied and compared with the structures obtained during solidification of the base alloy under similar mould and chill conditions. Columnar dendritic structure was observed during solidification of the base alloy at all distances from the chill. In the case of composites, the presence of silicon carbide particles disturbs the orderly aligned arrangement of dendrites observed in the base alloy, under similar solidification conditions, except near the chill surface where a particle-free zone is observed due to probable pushing of particles by the macroscopic solidification front with cell spacings finer than the particle size. During the entire range of solidification conditions studied in this work, the silicon carbide particles are pushed by growing dendrites of α-aluminium into the last freezing eutectic liquid. The observations on pushing of silicon carbide particles have been examined in relation to existing models on particle pushing by planar solidification fronts. Even in the regions away from the chill, where silicon carbide particles are present, there are large regions covering several dendrite arm spacings where there are no particles representing another form of macrosegregation of particles. It is observed that the secondary dendrite arm spacings (DAS)of α-aluminium are related to cooling rate by an equation DAS =b (T) n for the base alloy as well as for the composite. The coefficientb is generally higher for composites than for base alloy, and it is found to be a function of particle content. The value ofn for the composite is close to the value of the base alloy and is not significantly influenced by the presence of particles. Cooling rate, temperature gradients and the rate of advancement of the solidification front have been experimentally measured for the base alloy as well as for the composites during unidirectional solidification. The study indicates that the presence of particles themselves alters the cooling rates, temperature gradients and growth rate of the macroscopic solidification front under identical thermal surroundings during solidification. The possible influences of these alterations in growth condition on the solidification microstructure due to the presence of particles are discussed together with the other possible direct influences of particles on dendritic growth of aluminium-silicon alloys.

Thermal stability of advanced Ni-base superalloys

Abstract: Exposures consisting of 1 to 900 h at 1000 and 1100 °C after an ageing treatment of 16 h at 870 °C were used to study the thermal stability of selected γ′-strengthened Ni-based superalloys representing conventional, directional solidification, and single-crystal castings. Various techniques of microscopy, spectroscopy and diffraction were used to characterize the microstructure. Primary MC carbides in the alloys studied were found to be stable toward decomposition into lower carbides. In the aged condition, the strengthening γ′ phase assumed a cuboidal morphology; however, all alloys also contained varying proportions of coarse lamellar γ′ and hyperfine cooling γ′. On an atomic scale, the nature of the cuboidal γ′-matrix interface was found to vary from coherent to partially coherent. However, the overall lattice mismatch varied from one alloy to another depending upon its composition and the distribution of various elements in carbide phases and lamellar γ′ phase. Directional growth of the cuboidal γ′ phase upon exposure to higher temperatures was found to be accelerated by a large initial lattice mismatch leading to a considerable loss of coherency, as indicated by the observation of dislocation networks around the γ′ particles. Although the composition of the γ′ phase remained essentially unchanged, there was a marked change in matrix composition. Sigma phase was found to precipitate in all alloys, but its thermal stability was a function of alloy composition. The initial decrease in hardness followed by a hardening effect during exposure could be explained in terms of the partial dissolution of the γ′ phase and precipitation of sigma phase.

High Temperature Mechanical Properties of Al2O3/ZrO2(Y2O3) Fibers

Abstract: In-situ composite fibers produced by directional solidification of two phase oxide eutectics are one means of producing fibers with good strength and higher creep resistance than single crystal fibers. In this work, directionally solidified alumina-yttria stabilized zirconia eutectic fibers have been grown by the laser heated float zone (LHFZ) method at NASA Lewis. The average tensile strength of the alumina-zirconia (60.8 m/o Al2O3; 39.2 m/o ZrO2 (9.5 m/o Y2O3)) eutectic fibers was 1.2 GPa at room temperature. The high temperature tensile strength and creep resistance of the eutectic fiber were determined and compared to single crystal Al2O3.

Microstructures and Properties of Ds in-Situ Composites of Nb-Ti-Si Alloys

Abstract: In-situ composites based on binary Nb-Si alloys and consisting of a Nb solid solution with Nb3Si or Nb5Si3 have shown a promising combination of low temperature and high temperature mechanical properties. The environmental resistance and room temperature fracture toughness of these composites can be further enhanced by additions such as Ti, Hf, Cr, and Al. In the present study, ternary Nb-Ti-Si alloys were prepared by directional solidification to generate aligned two and three phase composites containing a Nb solid solution with Nb3Si and/or Nb5Si3. The present paper will describe microstructures, phase equilibria and fracture toughness of these composites. The improvement in the room temperature fracture toughness over binary Nb-Nb5Si3 composites is discussed.

Processing, Microstructures, and Properties of Cr-Cr3Si, Nb-Nb3Si, and V-V3Si Eutectics

Abstract: The present paper examines the potential of refractory metal-A15 silicide composites as structural materials for high temperature applications. Three eutectic systems are considered, Cr-Cr3Si, Nb-Nb3Si, and V-V3Si, since they all have melting points above 1700°C and densities lower than Ni-based superalloys. Eutectic compositions were selected because directional solidification of eutectics can be used to generate aligned composite microstructures. The present paper describes processing, microstructures and properties of directional ly solidified Cr-Cr3Si, Nb-Nb3Si, and V-V3Si. Based on microstructural and chemical analyses, some phase diagram modifications are suggested. The orientation relationships between the refractory metal and A15-silicide phases were characterized using transmission electron microscopy. In the Cr-Cr3Si eutectic a strong orientation relationship was observed; there was excellent lattice matching of the (110) in Cr and (210) in Cr3Si. Room temperature and elevated temperatu...

Metastable eutectic formation in Ni- Al alloys

Abstract: Directional solidification experiments have been carried out at low rates in alloys near the N13AI composition in the Ni-AI binary alloy system. Results confirm an earlier study at high rates showing that the eutectic in this system occurs between the β and γ′ phases. It is shown that under certain composition and growth conditions a metastable eutectic between the β and γ phases forms in these alloys. Analysis of the decomposition of this metastable eutectic into the γ′ phase and additional composition measurements are used to provide an improved version of the phase diagram near the N13AI composition.

Influence of Calcium-treatment on Sulfide- and Oxide-inclusions in Continuous-cast Slab of Clean Steel—Dendrite Structure and Inclusions

Abstract: A model proposed in another publication1) for steady state uni-directional solidification is evolved to a relationship between solidification variables and dendrite structure/morphology in non-steady state directional solidification. Dendrite structure and inclusion behaviors in columnar zone of continuous cast slabs are discussed with the model. Behaviors of inclusions in calcium-treated clean steels are discussed based on equilibrium considerations as well as widely accepted results and knowledges hitherto. An idea is introduced to interpret competitive precipitations of oxides and sulfides for calcium in residual melts between dendrite arms where the inclusions precipitate. Quantitative predictions are enabled with the present model, and several equations are given to volume fraction, size and morphology of the inclusions in calcium-treated clean steels in relation with Ca-S-O balance and solidification variables.

The effect of enhanced gravity levels on microstructural development in Pb-50 wt pct Sn alloys during controlled directional solidification

Abstract: The Pb-50 wt pct Sn alloys were directionally solidified at 21.1 µm s−1 through a temperature gradient of ∼3.5 K mm−1. With the aid of a centrifuge, the solidifying dendritic interfaces were subjected to constant gravity levels, opposite to the growth direction, up to 15.3 times that of Earth’s. Microstructural examination revealed no significant change in the secondary dendrite arm spacing, the interdendritic eutectic spacing, or the primary dendrite trunk diameter as a function of increasing gravity level; the primary dendrite arm spacing, however, decreased significantly. The primary spacing decrease is argued to result from suppressing convection in the bulk liquid or by modification of the rejected solute layer as a result of enhanced buoyancy.

Easy glass formation in Mg64Ni21Nd15 by Bridgman solidification

Abstract: Easy glass-forming Mg 64 Ni 21 Nd 15 alloy was subjected to Bridgman steady state directional solidification at growth velocities between 0.5 and 2.4 mm s −1 through a temperature gradient of about 15 K mm −1 . X-ray diffraction (XRD) indicated crystalline solidification structure at 0.5 and 1.0 mm s −1 , amorphous with some residual crystallinity at 1.7 mm s −1 and fully amorphous at 2.4 mm s −1 . The strongest XRD peaks at 0.5 and 1.0 mm s −1 correspond to Mg 3 Nd, the other phases present being, as yet, unidentified. Values of T g and T x of 467 K and 507 K respectively, determined by differential scanning calorimetry for the amorphous condition, match well our previous determinations from chill-cast or melt-spun ribbon of similar alloy composition. The significance of these results for an improved understanding of glass formation in this and related easy glass-forming systems is discussed.

Solidification morphology and semi-solid deformation in superalloy Rene 108

Abstract: A high temperature nickel-base superalloy (Rene 108) was directionally solidified by imposing various growth rates and thermal gradients using a modified Bridgeman apparatus. The scaling of the solidification structure was recorded as a function of the imposed growth variables. A special Gleeble testing procedure, developed previously where the solidified samples were quickly raised to a predetermined temperature in the semi-solid zone and fractured, was used for the measurement of fracture conditions in the semi-solid region. The effect of the solidification process variables, namely, the temperature gradient and velocity, on the fracture stress in the transverse direction was to increase the fracture stress at a given temperature. The upper hot-tearing temperature was noted to be a function of the solidification variables. The amount of strain accommodation and the hot tearing resistance was found to be influenced by the solidification microstructure. Fracture maps, which include the temperature, transverse fracture stress and temperature gradient during solidification (T-σT-G), for the directionally solidified microstructures are presented. Castability maps are created from the microstructure and the fracture data and display the porosity and semi-solid strength as a function of the casting variables.

Microscopic Pattern of Ice Crystal Growth in the Presence of Thermal Hysteresis Proteins

Abstract: This study examines the effect of thermal hysteresis proteins (THPs) from the winter flounder (Psuedopleuronectes americanus) on the ice-water interface morphology during freezing of aqueous solutions. Experiments were performed using a directional solidification stage, and the development of the two-phase interface was observed through a microscope and recorded by a video system. Unusual ice crystal morphologies were observed, including faceted ice crystal growth along the (1100) crystal plane; spicular or needlelike growth in the (1010) direction; and growth parallel to the c-axis, (0001), consisting of incorporated liquid inclusions bounded by hexagonal prism faces. The observed crystallographic structures can be explained as an effect of the interaction between the THPs and the primary prism faces of ice crystals. This results in an increase in the Gibbs free energy of these planes, followed by ice growth into the supercooled liquid adjacent to these faces.

Nonlinear morphological instabilities in directional solidification—an integral approximation

Abstract: In this article, we derive a strongly nonlinear evolution equation by using the integral method to study the instabilities in a directionally solidified binary mixture. This equation can not only describe the interfacial behaviors of all the long‐wave limits, but can also provide the possibility of strongly nonlinear instability analyses. Weakly nonlinear analyses proceeded from the critical conditions are undertaken to investigate the two‐dimensional bifurcation types and a transition curve separating subcritical and supercritical ranges is obtained.

Directional solidification apparatus and method

Abstract: Method and apparatus for directional solidification (e.g columnar grain or single crystal casting) of a melt involves refractory particulates, such as SiC particles, provided about at least one article molding cavity region of a casting mold wherein the particulates are heated to an elevated temperature above a melt casting temperature. The article molding cavity region of the casting mold is heated from an initial preheat temperature to an elevated casting temperature by heat from the particulates prior to communication of the article molding cavity to a chill member. For single crystal casting, a radiation transparent crystal selector member is disposed in a mold grain nucleation and growth cavity located between the article molding cavity and the chill member to select a single crystal for propagation in the article molding cavity region. A melt permeable, thermal insulation member may be disposed between the mold grain nucleation and growth cavity region and the chill member.

The effect of silicon on the microstructure and segregation of directionally solidified IN738 superalloy

Abstract: The effect of silicon on the microstructure and solidification segregation of directionally solidified IN738 nickel-based superalloy was studied. Directional solidification at various solidification rates and partial directional solidification plus rapid quenching were applied. Metallographic analysis and an electron microprobe were mainly used to observe and measure the microstructure and elemental segregation of the alloy, respectively. It was found that silicon affected the morphology of the liquid-solid interface of the alloy during solidification and gave the alloy a tendency to form well-developed dendrites. The addition of silicon enlarged the solidus-liquidus temperature interval, and the solidification rate also greatly influenced the interval. The interval increased with increasing solidification rate. Silicon promoted the precipitation of the gamma/gamma' eutectic, and also affected its precipitation temperature. Silicon segregated mainly in interdendritic regions, and promoted the segregation of other elements. All of the effects of silicon on the alloy related to the solidification rate.

Effects of Pt doping on microstructure of YBa2Cu3Ox superconductor prepared by directional solidification

Abstract: YBa2Cu3Ox superconductor was prepared by directional solidification of Y2BaCuO5–BaCuO2–CuO–Pt powder mixture. Pt doping produced a microstructure which contained fine Y2BaCuO5 particles in textured YBa2Cu3Ox grains. The same effect was not observed when a Pt wire was inserted in the sample instead of powders. Energy-dispersive X-ray spectroscopy (EDS) revealed that a compound containing Ba–Cu–Pt was formed around the wire as well as around the powders. The compound was identified as Ba4Pt1+x Cu2-x O9-z .