Showing papers on "Directional solidification published in 2002"

Directional solidification of aluminium -copper alloys

Abstract: Directional solidification experiments have been carried out on different Al–Cu alloys as a function of solidification parameters, temperature gradient G, growth rate V, and composition C0. The specimens were solidified under steady state conditions with a constant temperature gradient (7.4 K mm−1) at a wide range of growth rates (9–490 μm s−1) and with a constant growth rate of 9.5 μm s−1 at a wide range of temperature gradients (1.0–7.4 K mm−1). Microstructural parameters, the primary dendrite arm spacing λ1, secondary dendrite arm spacing λ2, dendrite tip radius R, mushy zone depth d were measured and expressed as functions of solidification parameters, G, V and C0 by using a linear regression analysis. The results were in good agreement with previous experimental work and current theoretical models suggested for dendritic growth.

Solidification thermal parameters affecting the columnar-to-equiaxed transition

Abstract: Experiments were conducted to analyze the columnar-to-equiaxed transition (CET) during the upward unsteady-state directional solidification of Al-Cu and Sn-Pb alloys, under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal parameters: transient heat-transfer coefficients, tip growth rates, thermal gradients, and cooling rates. The observed results do not give support to CET criteria based individually either on tip growth rate or temperature gradients ahead of the liquidus isotherm. Rather, the analysis has indicated that a more convenient criterion should encompass both thermal parameters through the tip cooling rate. The columnar growth is expected to prevail throughout the casting for a tip cooling rate higher than a critical value, which depends only on the alloy system and was observed to be about 0.2 K/s for Al-Cu alloys and 0.01 K/s for Sn-Pb alloys in the present investigation.

Microstructure of Y2O3 doped Al2O3–ZrO2 eutectics grown by the laser floating zone method

Abstract: Al 2 O 3 –ZrO 2 eutectics containing 9 mol% Y 2 O 3 (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. The eutectic microstructures were investigated as a function of the growth variables. Using a solidification-axis thermal gradient of 600 °C/mm a homogeneous, colony-free, interpenetrating lamellar microstructure was obtained for growth rates less than 50 mm/h. Higher growth rates produce cellular structures. Colonies grew with the [0001] alumina and the [110] zirconia axis parallel to the growth direction. The uniform lamellar microstructure obtained at low growth rates is stable during thermal treatment at 1500 °C.

Microstructure control of TiAl alloys containing β stabilizers by directional solidification

Abstract: Phase transformation at elevated temperatures in Ti-Al-X(X=Mo, Re, W) systems has been investigated by analyzing the dendrite morphology of arc-melted buttons. It was found that the addition of β stabilizers, such as Mo, Re and W, shifted the β phase field at liquid/solid temperatures to the high Al composition side. So, the β phase formed as a primary crystal even in higher Al compositions in Ti-Al-X ternary systems compared with Ti-Al binary. The addition of W was found to be the most effective β (bcc) stabilizer among Mo, Re, W, and the Ti-47Al-2W composition was selected for β solidification. In the directional solidification of Ti-47Al-2W alloy at the growth rate of 90 mm/h using the Bridgman type DS apparatus, it was found that the lamellar orientation in the columnar grains was parallel or inclined with the angle of 45° to the growth direction. This means that the β phase forms at the solid/liquid interface in the Ti-47Al-2W alloy. The tensile elongation of DS alloys was clearly improved compared with that of the polycrystalline alloy with a fully lamellar microstructure, while maintaining the same yield stress level.

Temperature dependence of flexural strength and microstructure of Al2O3/Y3Al5O12/ZrO2 ternary melt growth composites

Abstract: New Al2O3/Y3Al5O12(YAG)/ZrO2 ternary Melt Growth Composites (MGCs) with a novel microstructure have been fabricated by unidirectional solidification. These MGCs displayed superior high-temperature strength characteristics. The flexural strength increases progressively in the range 650–800 MPa with a rise in temperature from room temperature up to 1873 K. These excellent high-temperature characteristics are closely linked to such factors as: a microstructure consisting of three-dimensionally continuous and complexly entangled single-crystal Al2O3 with a hexagonal structure, single-crystal YAG with a garnet structure and fine ZrO2 with a cubic structure; characteristic dimensions of the microstructure of Al2O3/YAG/ZrO2 ternary eutectic ceramics of around 2–3 μm for YAG, around 2–3 μm for Al2O3 and around 0.4–0.8 μm for ZrO2; and the fact that no amorphous phase is formed at interfaces between any of the phases.

Morphological aspects of competitive grain growth during directional solidification of a nickel-base superalloy, CMSX4

Abstract: Quenched directional solidification of specially oriented bi-crystals of the Ni-base superalloy CMSX4, was carried out in an attempt to understand the role of the dendritic morphology in the process of competitive grain growth. For the range of misorientations considered (primary 〈001〉 misoriented by up to 7° from the uniaxial thermal gradient), there was no evidence of overgrowth of the primary misoriented dendrite by the secondary arms on the leading aligned primary. In fact, it was observed that for this range of misorientations, the tip of the retarded primary suppresses the growth of secondaries on its leading neighbour. This subsequently simply restricts the growth of the mis-aligned crystal to its original boundary, rather than reducing its size and is suggested as a possible reason for the range of stable axial orientations encountered during directional solidification of CMSX4.

Large area orientation of block copolymer microdomains in thin films

Abstract: A method and apparatus for orientation of block copolymer microdomains via rapid solidification. Rapid solidification from a solvent may include directional solidification and/or epitaxy to form patterns of microdomains in a film of block copolymer. Microdomains may include various structures formed by components of a block copolymer, such as vertical lamellae, in-plane cylinders, and vertical cylinders, and may depend on film thickness. Orientation of structures in microdomains may be controlled to be approximately uniform, and spatial arrangement of microdomains may be controlled.

Fabrication of Porous Iron by Unidirectional Solidification in Nitrogen Atmosphere

Abstract: Porous iron whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a pressurized mixture gas of nitrogen and argon. Nitrogen dissolved in the molten iron is rejected at the solid-liquid interface during the solidification due to the solubility difference of nitrogen between the liquid and solid. The gas pores are evolved from the nitrogen insoluble in the solid iron, which grow unidirectionally. The porosity is controlled by the partial pressures of nitrogen and argon during melting and solidification. The porosity decreases with increase of the partial pressure of argon at a given nitrogen pressure according to the Boyle’s law. At a constant total pressure of the mixture gas, the porosity increases with increasing partial pressure of nitrogen and no pores are formed during solidification below a critical partial pressure of nitrogen. The nitrogen concentration in the solid iron increases with increasing partial pressu re of nitrogen. The solid-solution hardening has been observed in as-cast porous iron, while more significant hardening has also been found in the porous iron quenched from a high temperature 1273 K, which is due to the martensitic transformation.

Niobium Silicide High Temperature In Situ Composites

Abstract: The requirements for greater aircraft-engine performance, greater thrust-to-weight ratios, and greater fuel efficiency have resulted in significant increases in turbine gas-path temperatures. Present-day aircraft engines have combustion-gas temperatures well in excess of the melting temperatures of the airfoil alloys, and therefore rely on sophisticated cooling methods to keep the alloys solid. Both materials and airfoil blade designs have evolved to sustain these increasing demands (Bewlay et al., 1999a; Subramanian et al. 1996). Advances in high-temperature materials have had a major impact on the efficiency of gas-turbine engines, so that currently superalloys provide a maximum surface temperature capability of*1150 8C. The evolution in HPTB (high pressure turbine blade) cooling technology is illustrated schematically in Figure 1. In the 1960s, equiaxed-grain Ni-based superalloys were cooled with radial cooling passages and with film cooling holes at the leading and trailing edges to reduce the interaction with the combustion gases. This advance over uncooled hardware improved blade durability, and allowed an increase in turbine inlet temperatures to greater than 1100 8C (2000 8F). Once cooling of hardware became routine, and the ability was developed to cast airfoils with more complex cooling schemes, these gains in cooling effectiveness were coupled with improved investment casting technology, and with the introduction of directional solidification for the production of HPTBs with either columnar or single-crystal microstructures. In the past decade, the potential of new alloys strengthened with intermetallic compounds with low densities, high elastic moduli, and high melting ranges (Dimiduk et al., 1993; Subramanian et al., 1997) has been explored. Intermetallic-based compound materials, such as Nb or Mo silicides, have been combined with metallic second phases in order to generate composites with a combination of attractive hightemperature properties and acceptable low-temperature properties. Nb-silicide based in-situ composites with Nb3Si and/or Nb5Si3 silicides have been shown to have great potential because of their attractive balance of highand low-temperature mechanical properties (Mendiratta et al., 1993; Bewlay et al., 1996, 1997). These materials have the potential to surpass the performance of Ni-based superalloys. This chapter will describe directional solidification and single-crystal technologies, with particular consideration to their role in present and future aircraftengine applications. Areas that will be covered include Ni-based superalloys strengthened by Ni3Al, DS eutectics of Ni-based superalloys, and DS Nb-silicide-based in-situ composites. This chapter will compare the role that Ni-based superalloys and Nbsilicide composites play in improving the performance of gas-turbine engines. Microstructures, phase compositions, and mechanical behavior will be reviewed.

Effects of Rotation on Convection in a Porous Layer During Alloy Solidification

Abstract: The topic of this chapter is that of the effects of an external constraint of rotation on convection, driven mainly by compositional buoyancy, in a porous layer adjacent to the solid–liquid interface during directional solidification of a binary alloy. In the solidification literature such a porous layer is referred to as a mushy layer. The analyses carried out by several studies in the past on the subject of this chapter and the subsequent results are reviewed first, the latest results unpublished elsewhere are presented briefly, and then in the second-half of the chapter the investigation of effects of rotation on convection in a horizontal porous layer of melt and dendrite solids is carried out subject to a simple model. The results based on this simple model indicate that the Coriolis force can have stabilizing effects on both the stationary and oscillatory mode of convection, while the oscillatory mode of convection experiences an additional destabilization due to the Coriolis force effect.

Dynamics of low anisotropy morphologies in directional solidification.

Abstract: We report experimental results on quasi-two-dimensional diffusion limited growth in directionally solidified succinonitrile with small amounts of poly(ethylene oxide), acetone, or camphor as a solute. Seaweed growth, or dense branching morphology, is selected by growing grains close to the ${111}$ plane, where the in-plane surface tension is nearly isotropic. The observed growth morphologies are very sensitive to small anisotropies in surface tension caused by misorientations from the ${111}$ plane. Different seaweed morphologies are found, including the degenerate, the stabilized, and the strongly tilted seaweeds. The degenerate seaweeds show a limited fractal scaling range and, with increased undercooling, suggests a transition from ``fractal'' to ``compact'' seaweed. Strongly tilted seaweeds demonstrate a significant twofold anisotropy. In addition, seaweed-dendrite transitions are observed in low anisotropy growth.

Hydrogen evolution during directional solidification and its effect on porosity formation in aluminum alloys

Abstract: Most of the models for predicting porosity formation in aluminum alloy castings use a simple mass balance, such as the lever rule, to track hydrogen enrichment in the interdendritic liquid. However, the hydrogen concentration predicted by the lever rule is typically too low to satisfy the threshold concentration for pore nucleation based on classical nucleation and growth theory. As a result, important features of microporosity such as the size and spacing of pores cannot be treated properly. In this article, the hydrogen concentration during the directional solidification of an Al-4.5 pct Cu alloy is calculated, assuming hydrogen rejection during solidification and diffusion in the mushy zone. The calculation shows that the use of the lever rule greatly underestimates the hydrogen concentration at the eutectic front. This is due to the fact that the eutectic front also rejects hydrogen and that this is not considered in the use of the lever rule. Results of numerical simulations that consider hydrogen rejection and diffusion are compared with results obtained using the lever rule. The comparison indicates that actual hydrogen concentrations may be orders of magnitude higher than that predicted by the lever rule. It is suggested that the lever rule should not be used in predicting porosity nucleation. The model outlined in this article is used to propose and explain the formation of a wavelike distribution of pores during directional solidification.

Primary dendrite distribution and disorder during directional solidification of Pb-Sb alloys

Abstract: Pb-2.2 wt pct Sb and Pb-5.8 wt pct Sb alloys have been directionally solidified from a single-crystal seed with its [100] orientation parallel to the growth direction, to examine the primary dendrite distribution and disorder of the dendrite arrays. The dendrite distribution and ordering have been investigated using analysis techniques such as the Gauss-amplitude fit to the frequency distribution of nearest and higher-order spacings, minimum spanning tree (MST), Voronoi polygon, and Fourier transform (FT) of the dendrite centers. Since the arrangement of dendrites is driven by the requirement to accommodate side-branch growth along the 〈100〉 directions, the FT images of the fully developed dendrite centers contain spots which indicate this preferred alignment. A directional solidification distance of about three mushy-zone lengths is sufficient to ensure a steady-state dendritic array, in terms of reaching a constant mean primary spacing. However, local dendrite ordering continues throughout the directional solidification process. The interdendritic convection not only decreases the mean primary spacing, it also makes the dendrite array more disordered and reduces the ratio of the upper and lower spacing limits, as defined by the largest 5 pct and the smallest 5 pct of the population.

The Effect of Convection on Disorder in Primary Cellular and Dendritic Arrays

Abstract: Directional solidification studies have been carried out to characterize the spatial disorder in the arrays of cells and dendrites. Different factors that cause array disorder are investigated experimentally and analyzed numerically. In addition to the disorder resulting from the fundamental selection of a range of primary spacings under given experimental conditions, a significant variation in primary spacings is shown to occur in bulk samples due to convection effects, especially at low growth velocities. The effect of convection on array disorder is examined through directional solidification studies in two different alloy systems, Pb-Sn and Al-Cu. A detailed analysis of the spacing distribution is carried out, which shows that the disorder in the spacing distribution is greater in the Al-Cu system than in Pb-Sn system. Numerical models are developed which show that fluid motion can occur in both these systems due to the negative axial density gradient or due the radial temperature gradient which is always present in Bridgman growth. The modes of convection have been found to be significantly different in these systems, due to the solute being heavier than the solvent in the Al-Cu system and lighter than it in the Pb-Sn system. The results of the model have been shown to explain experimental observations of higher disorder and greater solute segregation in a weakly convective Al-Cu system than those in a highly convective Pb-Sn system.

Influence of directional solidification variables on the cellular and primary dendrite arm spacings of PWA1484

Abstract: A series of directional solidification experiments have been performed to elucidate the effects of thermal gradient G and growth velocity V on the solidification behavior and microstructural development of the multicomponent Ni-base superalloy PWA 1484. A range of aligned as-cast microstructures were exhibited by the alloy: (i) aligned dendrites with well developed secondary and tertiary arms; (ii) flanged cellular dendrites aligned with the growth direction and without secondary arms; and (iii) cells with no evidence of flanges or secondary arms. The role of the imposed process parameters on the primary arm spacings that developed in the Bridgman-grown samples were examined in terms of current theoretical models. The presence of secondary arms increases the spacings between dendrites and leads to a greater sensitivity of λ1 on G−1/2V−1/4. The exponent of V was analyzed and found to depend upon the imposed gradient G. High withdrawal velocities and low thermal gradients were found to cause radial non-uniformity of the primary dendrite arm spacing. Such behavior was associated with off-axis heat flows.

Effect of directionally solidified microstructures on the room-temperature fracture-toughness properties of Ni-33(at. pct)Al-33Cr-1Mo and Ni-33(at. pct)Al-31Cr-3Mo eutectic alloys grown at different solidification rates

Abstract: Directionally solidified (DS) Ni-33(at. pct)Al-33Cr-1Mo and Ni-33(at. pct)Al-31Cr-3Mo eutectic alloys were grown at different rates varying from 7.6 to 508 mm h−1. The microstructures consisted of eutectic colonies with parallel lamellar NiAl/(Cr,Mo) plates for solidification rates at and below 12.7 mm h−1. Cellular eutectic microstructures were observed at higher solidification rates, where the plates exhibited a radial pattern. Room-temperature fracture-toughness tests were conducted using a modified ASTM E-399 technique. The average fracture-toughness values for specimens with planar eutectic and cellular microstructures were about 12 to 15 and 17 MPa\(\sqrt m \), respectively, for both alloys. However, the Ni-33(at. pct)Al-33Cr-1Mo specimens grown at and above 254 mm h−1 exhibited fracture toughness values of about 8 MPa\(\sqrt m \) due to the presence of short (Cr,Mo) plates. The fracture toughness values for the Ni-33(at. pct)Al-31Cr-3Mo alloy were also correlated with quantitative microstructural data in an attempt to identify the relevant elements of the microstructure determining resistance to fracture. A phenomenological fracture model is presented in an attempt to rationalize the present observations.

A composition window in the TiAl–Mo–Si system suitable for lamellar structure control through seeding and directional solidification

Abstract: The thermal stability of the lamellar microstructure of cast Ti–46Al–1.5Mo–(0–3)Si (at.%) alloys was investigated to find suitable seed compositions for growing ingots of these alloys with the aligned lamellar microstructure through seeding and directional solidification. In order for the seeding to be made successfully, the original orientation of the lamellar microstructure must be restored upon heating to and cooling from the melting temperature. In this study, the lamellar stability was determined by examining whether or not the lamellar structure in the as-cast alloys is preserved after quickly heating to just below melting temperature, holding, and then cooling to room temperature. This requirement was found to be fulfilled for the Si contents larger than 1.0 at.%. Ti–46Al–1.5Mo–1Si and Ti–46Al–1.5Mo–1.5Si (at.%) alloys are typical examples of alloys which can be used as seed materials. Directional solidification experiments of Ti–46Al–1.5Mo–1Si alloy were performed using Ti–46Al–1.5Mo–1.5Si alloy as a seed material and ingots with the aligned lamellar microstructure were obtained.

Formation of Pores during Unidirectional Solidification of Water Containing Carbon Dioxide

Abstract: Water-carbon dioxide solutions of various concentrations of carbon dioxide are unidirectionally solidified upwards in a glass cell at various growth rates, and the formation of the pores of carbon dioxide is in-situ observed. Columnar pores are formed at low growth rates. The length of the columnar pores becomes shorter as the growth rate increases or as the carbon dioxide concentration increases. The distribution of the pores is not uniform but periodic along the solidification direction, except under the conditions of high growth rate and high concentration. The change in the length of the columnar pores and the periodic formation of the pores are explained in terms of the rate of supply of carbon dioxide to the growing pores from the surrounding liquid.