Showing papers on "Directional solidification published in 1995"

A metallographic study of porosity and fracture behavior in relation to the tensile properties in 319.2 end chill castings

Abstract: A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching. The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al5FeSi, α- Al15(Fe,Mn)3Si2, and Al2Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties to DAS allows for the use of the latter as an indicator of the expected properties of the alloy.

Solidification processing of high temperature intermetallic eutectic-based alloys

Abstract: This paper describes the role of solidification processing of high-temperature eutectic alloys in the development of high-temperature structural materials. Particular emphasis is placed on directional solidification of eutectic-based alloys using cold crucible Czochralski crystal growth. Alloys with melting temperatures up to 2250 °C were directionally solidified by this technique for study of the microstructure, phase equilibria and mechanical properties. The attributes and limitations of the Czochralski technique are compared with those of other methods for solidification of high-temperature alloys. The microstructures and fracture toughness data of directionally solidified in situ composites generated from NbSi and CrNb alloy systems are presented. Property enhancements that can be achieved by directional solidification of high-temperature materials are discussed.

Modified melt texturing process for YBCO based on the polythermic section YO1.5“Ba0.4Cu0.6O” in the YBaCuO phase diagram at 0.21 bar oxygen pressure

Abstract: Y 2 O 3 admixed to 123 significantly influences the mechanism of melt texturing of YBCO. Based on phase diagram considerations, it is shown that the process window for stable growth conditions can be considerably increased. Furthermore, a lower process temperature ( T a =960°C to 985°C) becomes possible. Samples have been grown by both directional solidification and zone melting techniques. The superconducting properties are determined by local and integral levitation force measurements.

Experimental study of convection in a mushy layer during directional solidification

Abstract: Results of experiments using a number of techniques to study the nature of convection in a mushy layer generated by directional solidification of aqueous ammonium chloride solutions are reported. The techniques include flow visualization using a dye tracing method to study convection within the mushy layer before and after the onset of plume convection, and X-ray tomography to measure the solid fraction of a growing mush. The principal results are as follows. (i) Prior to the onset of chimneys, there is no convective motion in the mush, in spite of the vigorous finger convection at the mush-liquid interface. (ii) When the plume convection is fully developed, the flow of fluid in the mush consists of a nearly uniform downward motion toward the bottom of the tank, horizontal motion along the bottom toward the chimneys, then upward plume motion through the chimneys in the liquid region above the mush. (iii) The solid fraction of a growing mush as determined by X-ray tomography shows a significant decrease toward the bottom of the tank after the chimneys are fully developed. As a result, the concomitant increase in the local permeability can be as much as 50%. Some of the results reported herein confirm theoretical predictions of Worster (1992) and Amberg & Homsey (1993). Others reveal phenomena not observed heretofore.

On the creep of directionally solidified MoSi2-Mo5Si3 eutectics

Abstract: The high temperature deformation behaviour of MoSi2-Mo5Si3 eutectics has been investigated as a function of lamellar spacing over the temperature range 1100–1400°C and strain rates (∈) of 1 × 10−4 to 1 × 10−6 s−1. Specimens with lamellar morphologies were produced by directional solidification using the Czochralski method at pull rates of 25–210 mm/h giving lamellar spacings (λ) of 2.6 to 1.09 μm. The measured flow stress was found to increase as the lamellar spacing decreased for a given strain rate. A constitutive model for creep that incorporates reinforcement spacing for creeping fibers in a creeping matrix was found to describe the creep behaviour of the eutectic, i.e. ∈αλ withm = 1. Creep deformation of the eutectic was controlled by ½〈110〉 (001) partial dislocations in the Mo5Si3 phase. The creep behaviour of a [314] oriented Mo5Si3 single crystal was also investigated.

In-situ refractory intermetallic-based composites

Abstract: With the ultimate objective of exploiting refractory intermetallics for high-temperature structural materials, several binary and ternary two-phase intermetallics/refractory-metal solid solutions were explored. The ductile solid solution is used to toughen the composite microstructure via in-situ phase separation. While the viability of ductile phase separation in solid state was briefly considered for systems such as Nb 3 Al/Nb, much of the work focused on processing eutectic systems such as Cr 2 Nb/Nb and (Nb,Mo) 5 Si 3 /Nb,Mo). This paper describes results obtained via containerless directional solidification of these high-melting eutectic alloys using an optical float-zone furnace. The observations are explained on the basis of solidification theory and parameters unique to the optical float-zone furnace. It is demonstrated that, by this technique, casting-defect- and macrosegregation-free material, with well-aligned microstructure, can be readily produced. Moreover, the potential to approach sub-micron laminate spacing at high growth rate in alloys with very high melting eutectics has also been established. Room-temperature bend test evaluation of directionally solidified material is discussed in light of prevailing theories of ductile phase toughening. The results of a preliminary exploration of the NbMoCrSiAl multicomponent system are presented, showing the prevalence of eutectic phase separation and the potential for improving oxidation resistance.

Modeling of inverse segregation and porosity formation in directionally solidified aluminum alloys

Abstract: A model has been developed which simulates inverse segregation and microporosity formation in directionally solidified alloys. Based upon a finite difference scheme, the model takes into account volume changes associated with density variations during solidification. The continuity equations for the mass, the solute, and the energy together with the Darcy equation describing the flow in the mushy zone are solved in a mixed Lagrangian-Eulerian representation. All nodal points within the liquid phase move with the fluid velocity, whereas nodes are fixed in space as soon as they are reached by dendrite tips. When the dendrite tips arrive at the end of the ingot, the remaining interdendritic liquid partially compensates for the solidification shrinkage occurring deeper within the volume. Since the size of the ingot remains fixed from that point on (absence of a purely liquid region), air (macroporosity) is introduced at the mesh points to satisfy the mass balance, starting from the top of the mushy zone. The formation of microporosity is also accounted for in the model through a calculation of local hydrogen segregation. Using this model, it is shown that inverse segregation decreases with increasing hydrogen content (or volume fraction of microporosity). The results of the simulation are compared with experimental results obtained on an Al-Cu alloy solidified under well-controlled directional conditions.

A controllable temperature gradient furnace for the fabrication of large grain YBCO ceramics

Abstract: Large grain high temperature superconducting (HTS) materials have the potential to trap relatively large magnetic fields at liquid nitrogen temperatures (77 K), which makes them particularly attractive for a number of permanent magnet type engineering applications [1, 2]. As a result, a variety of melt processing techniques have been developed in recent years to fabricate large grain YBa2Cu307_ ~, (YBCO) ceramic which superconducts at 92 K. In general, melt processing techniques are based on the principles of semi-solid growth (solidification of a material consisting of liquid and solid [3, 4] and directional solidification (propagation of a macroscopic solidification front along specific directions [5]). The advantages of using semi-solid processes in the fabrication of electronic ceramics include the potential to (i) implement net shape processing, (ii) control the growth pattern and crystallographic orientation of individual grains, (iii) fabricate new kinds of composites and (iv) engineer the properties of the as-grown materials. YBCO is particularly well suited to growth by a melt process since it undergoes a peritectic transition at 1015°C (i.e. the solid decomposes to form a second solid phase and a liquid) [1]. Melt processedYBCO samples containing grains up to several centimetres in diameter have trapped fields in excess of 2 T at 77 K [6], which underlines the potential of this technique for the fabrication of practical HTS materials. The solidification rate in melt growth processes can be influenced significantly by the presence of a temperature gradient, which may be adjusted to facilitate heterogeneous grain nucleation along the direction of the thermal gradient. A variety of specialist techniques may be employed to generate the required gradient, including zone heating [3], infrared heating [7] and laser heating [3]. The controllability of the growth morphology of the specimen may be enhanced further by the use of either a "cold finger" or a seed crystal technique to generate a single heterogeneous nucleation site in a partially molten ceramic. This is common practice in the growth of single crystals by the Czochralski method, in particular [8]. It is important that temperature gradients used in the solidification process are both large and available over a wide temperature range if a single furnace is

On the microstructure and crystallography of directionally solidified MoSi2-Mo5Si3 eutectics

Abstract: Directionally solidified MoSi2-Mo5Si3 eutectic rods were produced by the Czochralski method. A script lamellar microstructure was produced and an inverse square root dependence upon the pull rate (39–210 mm/h) was observed for the lamellar spacing. The eutectic rods were textured with [001] of Mo5Si3 and 〈110〉 of MoSi2 parallel to the rod axis and an orientation relationship consisting of [110]MoSi2[110]Mo5Si3 and (1¯10)MoSi2 ||(002)Mo5Si3 was present in all of the samples. The eutectic grows with a lamellar plate morphology inclined at 15° relative to the rod axis and four growth variants are possible. A script lamellar microstructure, as viewed in the transverse plane, was produced by periodic branching of the Mo5Si3 lamellae and this branching was not associated with any change in crystallography. Addition of 0.35 at.% erbium produced a more fibrous microstructure and an erbium-rich compound, Er2Mo3Si4 with space group 21/c, formed congruently with the Mo5Si3-MoSi2 during solidification. A ledge-terrace growth mechanism was proposed to explain the observed microstructures.

Solidification condition of bulk glassy Zr60Al10Ni10Cu15Pd5 alloy by unidirectional arc melting

Abstract: The relation between the formation of a glassy phase and the solidification parameters of moving velocity of liquid/ solid interface (V), temperature gradient (G) and cooling rate (R) was examined for a Zr 60 Al 10 Ni 10 cu 15 Pd 5 alloy, with the aim of clarifying a solidification condition for formation of a bulk glassy alloy by a unidirectional arc-melting method. The glassy phase was obtained in the condition of V> 4 mm/s, G>4 K/mm and R>40K/s. The decrease in G causes the formation of equiaxed dendrites, oriented dendrites and cell structure. The supercooling for the present alloy was measured to be as large as 385 K at a low cooling rate of 40K/s. The large supercooling ability is presumably due to the formation of a highly dense random packed structure where the nucleation of a crystalline phase and the atomic rearrangement for growth reaction are difficult. The glass formation of the present multicomponent alloy in the unidirectional arc melting method seems to be dominated by the ease of the supercooling ability rather than the achievement of high cooling rate.

Orientation of Y2BaCuO5 precipitates during unidirectional solidification of Y1Ba2Cu3O7- delta under a magnetic field

Abstract: The effect of a vertical magnetic field on Y2BaCuO5 (211) precipitates during horizontal unidirectional solidification of YBaCuO has been investigated. EPR shows first that some of the residual 211 particles are not randomly oriented in the 123 matrix when the sample is processed under a magnetic field. Secondly, the analysis of the angular dependency of the EPR spectra shows that these particles tend to be oriented in the 123 crystal with their b211 crystallographic axis parallel to the applied magnetic field and also to the C123 axis. X-ray pole figures on the same sample lead to an identical conclusion. The consequences of this observation regarding Y1Ba2Cu3O7- delta (123) crystallization are discussed.

Steady-state cellular solidification of Al-Cu reinforced with alumina fibers

Abstract: The steady-state directional solidification of aluminum-4.5 wt pct copper and aluminum-1.0 wt pct copper alloys reinforced with parallel,continuous, closely spaced alumina fibers is investigated under growth conditions that produce a plane front or cells in corresponding unreinforced alloys. Specimens were designed to have a central reinforced region surrounded by unreinforced metal of the composite matrix composition. Each was produced by pressure infiltration, subsequently remelted, directionally solidified, and quenched to reveal the liquid/solid metal interface. Both unreinforced and composite sections were characterized to determine solidification front morphology and degree of microsegregation. In the unreinforced portion of the samples, the transition from plane-front to cellular solidification was observed to correspond to a coefficient of diffusion of copper in liquid aluminum of 5 − 10−9 m2 − s−1, in agreement with published values. Cell lengths, analyzed using a finite-difference model of microsegregation, are in agreement with the Bower-Brody-Flemings (BBF) model for cell tip undercooling. In the composite portion of the samples, the alloys solidify free of lateral microsegregation for all solidification conditions investigated, in agreement with theory. The shape of the liquid/solid metal interface near the fibers indicates a much lower fiber/liquid metal interfacial energy than fiber/solid metal interfacial energy. In the composite, plane front solidification is therefore not observed even when plane front solidification obtains in the unreinforced alloy. It is shown that geometrical constraint imposed on deep cells by the fibers causes significant increases in cell tip undercoolings, in agreement with current analyses of deep cell solidification.

Thermal and diffusion-induced stresses in crystalline solids

Abstract: The stressed crystal that has been obtained during the crystallization process is considered with particular attention given to the comparative numerical analysis of composition and temperature gradients as the causes of the stressed state. The numerical calculations are referred to the directional solidification method. It is shown that the contribution of diffusion induced stresses to the total stressed state of a crystal in the growth process may be comparable to the contribution from thermostresses. In this article, as a consequence of the main thermodynamical laws the relations obtained by Larche and Cahn [Acta Metall. 21, 1051 (1973)] are generalized to the case of a coupling dependence of elastic moduli on temperature and composition. The relations provide the important connection between the elastic modulus of compound materials measured using acoustic methods (at constant entropy and chemical potentials) and those that are used in practice (at constant temperature and composition).

Formation and suppression of channels during upward solidification of a binary mixture

Abstract: Characteristics of the mushy zone were experimentally investigated in the upward solidification of a binary mixture. The formation of channels and techniques for their suppression were ex-plored. The experiments were carried out in a rectangular test section using aqueous ammonium chloride as the phase-change material at hypo- and hypereutectic concentrations ranging from 15 to 33 wt pct of salt. The cold-plate temperature was varied in the range of -60 ‡C to-14 ‡C. Transient temperature profiles as well as the positions of the liquidus and solidus interfaces were obtained. The mushy-zone characteristics, the nature and distribution of chan-nels, and the associated fluid flows were studied as a function of initial solution concentration and cold-plate temperature. The application of low-amplitude vibration to the test cell was found to reduce the mushy-zone thickness and the number of channels for all concentrations; chan-neling was almost completely suppressed at the lower concentrations. Channel suppression was influenced by vibration amplitude rather than frequency, with the larger-amplitude vibration being the more effective. The vibration results suggest that the formation and sustenance of channels is influenced more by developments within the mushy zone than by bulk liquid behavior.

Directional solidification into static stability

Abstract: Consider the directional solidification of a binary alloy rejecting a heavy solute as it solidifies upward. If the solidification front is planar, the fluid melt ahead of the front is stably stratified and convection is not expected. In this paper we analyse the linear stability of planar solidification asymptotically in the limit of large solutal Rayleigh number, R. Three distinct linear modes are found which correspond to internal waves, buoyancy edge waves, or morphological modes. Of these three modes, only the morphological modes are subject to an instability. We find that for large Rayleigh number this instability first occurs at long wavelengths with wavenumbers that scale on R -1/14 . The scalings derived from the linear analysis are used to construct a nonlinear theory for the morphological instability in the large Rayleigh number limit. Similarity solutions are found which describe steadily convecting, non-planar growth reminiscent of an observed phenomenon known as steepling.

Mechanical properties and high-temperature deformation behaviour of particle-strengthened NiAl alloys

Abstract: Alloying of NiAl with Ta or Nb leads to an alloy with high high-temperature strength. Additional alloying with Cr leads to additional precipitation hardening. Highest strength is obtained by directional solidification, while alloys with improved ductility have been prepared by powder metallurgy. High-temperature deformation results in dynamic and meta-dynamic recrystallization of the Laves phase. Prototype components for engine application have been produced.

Effect of phosphorus on solidification process and segregation of directionally solidified IN738 superalloy

Abstract: The effect of phosphorus on the solidification and the solute segregation of a directionally solidified IN738 Ni-based superalloy was investigated experimentally employing a method of partially directional solidification and subsequent quick quenching. It was found that the phosphorus addition widened the solidus-liquidus temperature interval of the alloy. Both the P content and the solidification rate affected the morphology of the solid-liquid interface and the precipitation of phosphides in the alloy. Phosphorus segregated largely in the intercellular/interdendritic regions and promoted the segregation of other alloying elements in the solidified structure.

A boundary conforming grid generation system for interface tracking

Abstract: The development of an algebraic grid generation system to track a solid-liquid interface during directional solidification of a binary alloy is discussed. A single mapping, constructed with tensor product B-splines, is proposed for calculations of both shallow and deep solidification cells. The initial spline coefficients for the coordinate mapping are modified to minimize a discrete functional that regulates the smoothness and orthogonality of the mesh. The use of transfinite blending function interpolation to obtain an initial grid is examined.

Preferred-pattern formation during the initial transient in cellular solidification

Abstract: Critical information on the pending problem of preferred-pattern formation and wavelength selection in cellular directional solidification is currently expected from the study of initial transients. Therefore, experiments were carried out in lead—thallium alloys to analyse the dynamical process by which a periodic array of cells is formed by using two classical but different experimental procedures, one violent and one soft. The statistical analysis of pattern characteristics (average wavelength, level of disorder) shows that both procedures converge towards a common asymptotic state, which means that a preferred pattern does exist. Although different, the initial transients are qualitatively similar. It is found that in both cases, the solid—liquid interface is first dendritic and then restabilizes into a cellular array. Oscillations, in periodicity and in the sense of disorder, are observed, that are associated to the repetition of phases of cell divisions, probably by tip-splitting, separated by rearrangement periods.