Showing papers on "Directional solidification published in 1997"

Nucleation ahead of the advancing interface in directional solidification

Abstract: During directional solidification of an alloy, it is possible to nucleate the growing phase or a new phase at or ahead of the interface. This is critical in the phase selection, in the columnar to equiaxed transition under casting, welding or rapid solidification conditions and the formation of bands in peritectic systems. Following Hunt, an appropriate theoretical model is developed to determine the conditions under which nucleation can occur in the liquid close to a moving solid-liquid interface for both, low and high interface velocities. At high growth rates, non-equilibrium effects are shown to play an important role in predicting such transitions.

Modeling of micro- and macrosegregation and freckle formation in single-crystal nickel-base superalloy directional solidification

Abstract: The formation of macrosegregation and freckles by multicomponent thermosolutal convection during the directional solidification of single-crystal Ni-base superalloys is numerically simulated. The model links a previously developed thermodynamic phase equilibrium subroutine with an existing code for simultaneously solving the macroscopic mass, momentum, energy, and species conservation equations for solidification of a multicomponent alloy. Simulation results are presented for a variety of casting speeds and imposed thermal gradients and for two alloy compositions. It is found that for a given alloy composition, the onset of convection and freckle formation occurs at a critical primary dendrite arm spacing, which agrees well with previous experimental findings. The predicted number and shape of the freckle chains in the unstable cases also agree qualitatively with experimental observations. Finally, it is demonstrated how the onset and nature of convection and macrosegregation vary with alloy composition. It is concluded that the present model can provide a valuable tool in predicting freckle defects in directional solidification of Ni-base superalloys.

Composition and growth rate effects in directionally solidified TiAl alloys

Abstract: Effects of alloy compositions and growth rates on the microstructure and the deformation behavior of fully lamellar TiAl alloys were systematically investigated by directional solidification (DS) techniques. Some β stabilizing elements, such as Mo, Nb and Cr, have been added in order to increase the β phase field in the binary system. It was found that the lamellar orientation was aligned nearly 0–45° to the growth direction in Ti–46Al–2Nb and Ti–46Al–2Cr(at.%) DS ingots grown at the growth rate of 90 mm h −1 . Therefore, it could be concluded that there is a good possibility to control the lamellar orientation by adding some β stabilizing elements, such as Mo, Nb and Cr, in the TiAl system. In the Ti–47.5Al–2.5Mo DS alloy, the lamellar boundary was nearly perpendicular to the growth direction at growth rates of 180 and 360 mm h −1 , however, it was nearly parallel to the growth direction at the growth rate of 90 mm h −1 . These results indicate that the lamellar boundary orientation of DS ingot has been affected by the growth rate as well as the alloy composition.

Motion and Remelting of Dendrite Fragments during Directional Solidification of a Nickel-Base Superalloy

Abstract: The formation of spurious grains during the directional solidification of a Ni-base superalloy is studied by modeling the movement and remelting of dendrite fragments originating in channels inside the mush. Such channels exist because of thermosolutal convective instabilities during solidification and persist as freckle chains in the solidified material. The fragment model is linked to a phase equilibrium subroutine for multicomponent Ni-base superalloys, as well as to a previously developed solidification and convection code. A parametric study is performed to investigate the effects of initial fragment location and size on the fragment paths and survivability in the melt for one of the channels predicted in a typical directional solidification simulation. It is found that only a small window of initial conditions exists which leads to spurious grain formation. This window corresponds to medium-sized fragments originating near the mouth of the channel. Other fragments either remelt completely or sink into the channel. The need for an accurate fragment generation model is discussed.

Prediction of a process window for the investment casting of dendritic single crystals

Abstract: Large dendritic single crystal blades operating in jet engines or land-based turbines are commonly produced in industrial plants. However, the production cost of some of these parts remains high due to the several types of defect which may appear during the different stages of the investment casting process. The prevention of stray crystal formation during solidification is the main problem which has to be faced by engineers. Analytical developments are first presented in order to define conditions under which single crystal growth is favored. This process window is described in a space where the thermal gradient and the withdrawal speed of a Bridgman furnace apparatus are the controlling parameters. The limits of this process window are then refined in order to determine the influence of other parameters such as the crystallographic orientation of the single dendritic grain, the size of the platform in which the single grain has to extend, the growth kinetics of the dendrite tips, or the existence of a lateral thermal gradient.

Precipitate size refinement by CeO2 and Y2BaCuO5 additions in directionally solidified YBa2Cu3O7

Abstract: Directional solidification of YBa2Cu3O7 has been carried out through a Bridgman technique, and the influence of Y2BaCuO5 and CeO2 additives on the size of Y2BaCuO5 precipitates has been investigated. It is demonstrated in this work that the most efficient procedure to reduce the size of the Y2BaCuO5 precipitates is to increase the concentration of nucleation centers present in the peritectic decomposition of YBa2Cu3O7−x. A small concentration (0.3−1 wt. %) of CeO2 has a strong influence on the solidification process and on the size of Y2BaCuO5 precipitates. It is shown that when CeO2 is added, further refinement of the size of precipitates results from the formation of nanometric Y2O3 particles which further enhance the multinucleation effect. We have also observed that coarsening effects are avoided with CeO2 additives.

Eutectic solidification processing via bulk melt undercooling

Abstract: The control of eutectic microstructures is generally achieved via controlling the growth velocity, thermal gradients and the alloy composition. While current efforts in producing in-situ nanocomposites are centered on conventional rapid solidification processing (RSP), we describe here a more non-traditional processing of eutectics by RSP via bulk melt undercooling of near-eutectic compositions. It will be shown that a wide variety of microstructures can be produced in alloys with the same composition. These include the formation of metastable phases, growth of coupled eutectic compositions and the formation of mixed metastable/stable eutectics. These effects are discussed using examples of rapidly solidified Nb-Si alloys processed in a containerless environment.

Directional solidification of (Re = Y, Nd): microstructure and superconducting properties

Abstract: Single-domain ceramics of (Re = Y, Nd) with high and high critical currents have been prepared in air by means of directional solidification and the leading microstructural factors controlling the pinning of vortices have been analysed. The interface pinning mechanism at secondary inclusions has been found to be more effective in than in in agreement with HRTEM pictures showing sharper interfaces in the first case. The observation of high and a vanishingly small fishtail effect in Nd123/422 composites suggest that the concentration of Nd - Ba antisite defects is minimized during the directional solidification process in air. Finally, it is demonstrated by TEM analysis that Nd123/422 ceramics show distinctive microstructural features compared with Y123/211 which could enhance the high-field critical currents and the irreversibility line. Frequency-dependent ac susceptibility studies provide evidence, however, that the thermal activation of vortices is enhanced in Nd123 as compared with Y123. It is suggested that incomplete oxygenation reduces the effective pinning energies.

Solidification and Structural Characteristics of α(Al)–Mg 2 Si Eutectic

Abstract: Solidification and structural characteristics of α(Al)-Mg 2 Si eutectic in the Al-Mg-Si system were investigated by thermoanalysis and directional solidification techniques. The two-phase eutectic line of L ⇔ α(Al) + Mg 2 Si, the temperature-change section of a pseudobinary α(Al)-Mg 2 Si system and the eutectic coupled zone within this section have been measured accurately. It was demonstrated that the coupled zone in the pseudobinary α(Al)-Mg 2 Si system could be described as C= 1.35 x 10 3 V + C 0 , where C is the molar percentage of Mg 2 Si, V is the solidification velocity in m/s, C 0 is the molar percentage of Mg 2 Si when the eutectic structure is obtained through a near-equilibrium solidification, and 3.65 mol% < C 0 < 5.0 mol%. In this system, Mg 2 Si is the nucleating and leading phase during eutectic growth, and the primary crystals exhibits a morphology of regular hexahedron with (100) faces. In the eutectic alloy, the morphology of phase Mg 2 Si transforms from lamellae to rods when the solidification velocity is higher than 1.1 x 10 -5 m/s. The relationship between the lamellar or interrod spacing, λ of eutectic Mg 2 Si and the solidification velocity, V, fits the equation λ 1.93 V = 7.1 x 10 -17 G -0.414 where G is the temperature gradient ahead of the solid-liquid interface in the melt, which is in K/m, λ is in m and V is in m/s.

Alignment of the TiAl/Ti3Al lamellar microstructure in TiAl alloys by directional solidification

Abstract: By using an appropriately oriented seed from the TiAl–Si system, the TiAl/Ti 3 Al lamellar microstructure was aligned parallel to the growth direction for ingots having compositions of Ti–47Al, Ti–43Al–3Si, and Ti–46.5Al–3Nb–0.5Si (at.%). Directional solidification was performed in a floating zone furnace at growth rates between 5 and 40 mm h −1 . In addition, some ingots were directionally solidified in 16 mm diameter ceramic crucibles. Evidence for the solidification of the β -phase can be observed for some Ti–47Al ingots grown at 40 mm h −1 and different possible growth morphologies are discussed to explain the observed microstructures. Preliminary results from room temperature tensile tests and high temperature compression tests are also presented.

The effect of electromagnetic stirring on macrostructure and macrosegregation in the aluminium alloy 7150

Abstract: Ingots of aluminium alloy 7150 (Al-Zn-Mg-Cu) were unidirectionally solidified vertically downwards under conditions of natural convection and electromagnetic stirring of the bulk liquid. Increased fluid flow in the bulk liquid was found to promote the formation of equiaxed grains and hence the columnar-equiaxed transition (CET). Increased fluid flow was also associated with an increase in macrosegregation of the alloying elements. The extent of segregation was greatest for Cu and least for Zn. The experimental observations suggest that macrosegregation in the ingots was caused by the displacement of solute-enriched liquid from the mushy zone as a result of the electromagnetically stirred bulk liquid being driven into the solidification front.

Prospects of new planar optical waveguides based on eutectic microcomposites of insulating crystals: The ZrO2(c)-CaZrO3 erbium doped system

Abstract: A new approach to produce structured optical materials is described. The method relies on the capability of growing eutectic crystals of wide optical band gap materials by directional solidification procedures. The laser float zone technique was used to produce ordered arrays of alternate lamellae with thickness of the order of microns of erbium doped CaZrO3 and calcia stabilized zirconia single crystals. The later, having a higher refractive index, exhibited planar waveguiding effects as it has been proved experimentally. The possibility of producing waveguides from eutectic crystals promises the fabrication of hundreds of planar waveguides integrated into a crystal grown at a speed of several tenths of cm/h.

Interface pinning in high tc-high jc nd1+xba2-xcu3oy directionally solidified in air

Abstract: Single domain bars of NdBa2Cu3O7/Nd4Ba2Cu2O10 (Nd123/422) superconducting composites have been fabricated in air by using a directional solidification technique. Tc values of 96.5 K can be obtained after an Ar heat treatment and further oxygenation thus showing that reduced oxygen partial pressure during solidification is not a fundamental requirement to prepare high-Tc Nd123 bulk superconductors. The temperature dependence of the critical currents point to a limited efficiency of the Nd123/422 interfaces to pin vortices. High resolution electron microscopy observations clearly reveal smooth Nd123/422 interfaces thus suggesting that interface pinning may not be as relevant as compared to Y123/211 composites.

Effect of unidirectional solidification rate on Jc for current lead

Abstract: The dependence of transport J c value on the traveling rate and the nominal composition was investigated by taking different nominal compositions of YBa 2 Cu 3 O 6+ x (Y123) + n mol% Y 2 BaCuO 5 (Y211) ( n = 10, 20, 30, 40 and 50) with addition of 0.5 wt% of Pt on samples processed by different unidirectional solidification rates, namely 1, 3 and 5 mm/h. The highest J c was found in the sample with 30% Y211 addition by the higher traveling rate adjusted to prevent the formation of polycrystals. In this φ 1.56 × 60 mm sample the values of transport I c and J c were 1370 A and 71 700 A/cm 2 , respectively, which were obtained by the conventional DC four-probe method with criterion of 1 μV/cm at 77 K and self-field.

Interlamellar residual stresses in single grains of NiO-ZrO2(cubic) directionally solidified eutectics

Abstract: Phase-to-phase residual stresses due to thermal expansion mismatch were measured in lamellar NiO-ZrO{sub 2} (cubic) directionally solidified eutectics (DSEs). The triaxial strain tensors for both phases were measured using single-crystal X-ray diffractometry techniques on isolated grains of the DSE. From the strain tensors, the stress tensors were calculated, taking into account the full elastic anisotropy of the phases. The resulting stress tensors indicated that very large compressive stresses accumulated in ZrO{sub 2} whereas large tensile stresses were amassed in NiO parallel to the lamellae during the solidification process. The large magnitudes of the stresses indicated that the interfaces between the lamellae were very well bonded and did not facilitate slip or other stress-relieving processes.

An analytical model for optimal directional solidification using liquid metal cooling

Abstract: In what follows, a model is developed that describes the optimal processing parameters for directional solidification using liquid metal cooling (LMC). The model considers a sample with a flat geometry and, as a first approximation, can be used to treat the flat sections of a turbine blade. The model predicts (1) the optimal withdrawal rate of the casting from the hot zone, (2) the temperature gradient in the liquid at the solidification interface, and (3) the temperature profile along the length of the casting. The model is then used to perform a sensitivity analysis of the LMC process. Cooling bath temperature, baffle thickness, shell thickness, and shell thermal conductivity are shown to have a strong influence on system performance.