Fluid Flow and Defect Formation in the Three-Dimensional Dendritic Structure of Nickel-Based Single Crystals
Authors
Open AccessArticle
- First Online:
DOI: 10.1007/s11661-011-0823-8
Abstract
Fluid flow within the dendritic structure at the solid–liquid interface in nickel-based superalloys has been studied in two directionally solidified alloy systems. Millimeter-scale, three-dimensional (3D) datasets of dendritic structure have been collected by serial sectioning, and the reconstructed mushy zones have been used as domains for fluid-flow modeling. Flow permeability and the influence of dendritic structure on flow patterns have been investigated. Permeability analyses indicate that the cross flow normal to the withdrawal direction limits the development of flow instabilities. Local Rayleigh numbers calculated using the permeabilities extracted from the 3D dataset are higher than predicted by conventional empirical calculations in the regions of the mushy zone that are prone to the onset of convective instabilities. The ability to measure dendrite surface area in 3D volumes permit improved prediction of permeability as well.
1 Introduction
Directional solidification provides control of the orientation and growth of columnar grain and single-crystal materials.[1,2] This advancement in processing has yielded a significant improvement in the high-temperature creep and fatigue properties of nickel-based superalloys.[1–4] Unfortunately, depending on alloy and process conditions, directional solidification may result in the formation of grain defects such as misoriented, high-angle grain boundaries and/or freckle chains.[4–7] The frequency of such defects has been shown to be related directly to slower cooling rates that characteristically produce coarser dendritic structures and larger interdendritic porosity.[7] Advanced alloys with higher levels of refractory alloying elements and large, more geometrically complex cast components are even more prone to defect formation. Thus, accurate solidification models that can predict the conditions under which these defects form as well as provide input to alloy and component design can clearly provide substantial benefits.[5,6,8–16]
The density gradient in the liquid is denoted by the term (Δρ/ρ _{o}), g is the acceleration because of gravity, α is thermal diffusivity, ν is kinematic viscosity, and \( \bar{K} \) is the average permeability. Among the aforementioned examples, permeability is the only factor derived typically by means other than direct assessment from the chemistry and thermodynamics of the system. Permeability can vary by orders of magnitude within the mushy zone,[17–21] so the details of the dendritic structure can influence the Rayleigh number strongly. Permeability has typically been defined empirically with consideration of flow through porous media.[17,20–28] More recently, computational analyses of permeability have been performed with more accurate representations of the dendritic structure, albeit with relatively small volumes considered.[18,19,29–31] Other difficulties associated with assessment of the Rayleigh number in superalloy solidification include (1) accurate prediction of solute density gradients in the mushy zone,[32,33] (2) selection of an optimal length scale that encompasses the scale of convective potential accurately,[12,13] and (3) accurate knowledge of thermal diffusivity and kinematic viscosity in multicomponent alloy systems.
The current investigation uses experimentally derived, 3D dendritic networks removed from the solidification front for simulation of isothermal, noncompressible fluid flow for direct permeability assessment. The relatively large volume of the reconstructed dendritic structure, with micrometer-scale resolution, permits the analysis of flow properties across multiple primary and secondary dendrites and from dendrite tips to deep within the mushy zone. Two different alloys with substantially different dendritic structure have been studied. The relationship of dendritic structure to interfacial surface area (ISA), flow tortuosity (T _{avg.}), and resulting permeability (K) are examined, and the implications for the development of convective instabilities are addressed.
2 Experimental method and computational approach
3 Results
3.1 Permeability Analysis
Summary of René N4 Flow Simulations and Calculated Permeabilities by Case
Case |
Cell Volume (mm^{3}) |
Inlet Volume Fraction Liquid |
Velocity_{min − max} (mm/s) |
Pressure Differential (Pa) |
Length over Pressure Drop (×10^{–4} m) |
Volumetric Flow Rate (×10^{–12} m^{3}/s) |
Permeability (×10^{–11} m^{2}) | |
---|---|---|---|---|---|---|---|---|
Vertical Flow |
N4-Y1 |
2.18 × 1.64 × 0.98 |
0.08 |
0.0532 – 1.06 |
7.7 |
2.4 |
19 |
0.11 |
N4-Y2 |
2.18 × 1.65 × 0.84 |
0.10 |
0.0527 – 1.05 |
5.4 |
1.5 |
19 |
0.09 | |
N4-Y3 |
2.18 × 1.64 × 0.57 |
0.22 |
0.0537 – 1.07 |
5.2 |
1.5 |
170 |
1.3 | |
N4-Y4 |
2.18 × 1.65 × 0.48 |
0.32 |
0.0536 – 1.07 |
3.8 |
0.74 |
160 |
1.4 | |
N4-Y5 |
2.18 × 1.65 × 0.40 |
0.43 |
0.0538 – 1.08 |
5.5 |
0.55 |
240 |
1.8 | |
N4-Y6 |
2.18 × 1.65 × 0.35 |
0.60 |
0.0535 – 1.07 |
3.8 |
0.51 |
390 |
3.7 |
Case |
Cell Volume (mm^{3}) |
Volume Fraction Liquid |
Velocity_{min – max} (mm/s) |
Pressure Differential (Pa) |
Length over Pressure Drop (× 10^{–4}m) |
Volumetric Flow Rate (×10^{–12} m^{3}/s) |
Permeability (×10^{–11} m^{2}) | |
---|---|---|---|---|---|---|---|---|
Cross Flow |
N4-X1 |
2.16 × 1.62 × 0.03 |
0.42 |
0.00509 – 0.102 |
1.6 |
20 |
0.07 |
4.5 |
N4-X2 |
2.16 × 1.62 × 0.02 |
0.56 |
0.00520 – 0.104 |
0.63 |
20 |
0.18 |
21 | |
N4-X3 |
2.16 × 1.62 × 0.01 |
0.73 |
0.00512 – 0.102 |
0.21 |
20 |
0.26 |
183 | |
N4-X4 |
2.16 × 1.62 × 0.01 |
0.89 |
0.00502 – 0.100 |
0.079 |
20 |
0.59 |
659 |
Summary of Ni-Al-W Ternary Flow Simulations and Calculated Permeabilities by Case
Case |
Cell Volume (mm^{3}) |
Inlet Volume Fraction Liquid |
Velocity_{min − max} (mm/s) |
Pressure Differential (Pa) |
Length over Pressure Drop (×10^{–4} m) |
Volumetric Flow Rate (×10^{–12} m^{3}/s) |
Permeability (×10^{–11} m^{2}) | |
---|---|---|---|---|---|---|---|---|
Vertical Flow |
NAW-Y1 |
2.59 × 1.63 × 3.60 |
0.10 |
0.051 − 1.03 |
3.01 |
3.00 |
53.2 |
5.20 |
NAW-Y2 |
2.59 × 1.64 × 3.00 |
0.20 |
0.050 − 1.00 |
1.55 |
4.30 |
69.9 |
9.33 | |
NAW-Y3 |
2.59 × 1.64 × 2.50 |
0.30 |
0.051 − 1.01 |
1.82 |
4.00 |
200.5 |
13.7 | |
NAW-Y4 |
2.59 × 1.64 × 1.90 |
0.40 |
0.052 − 1.03 |
1.07 |
3.00 |
254.4 |
19.4 | |
NAW-Y5 |
2.59 × 1.64 × 1.20 |
0.50 |
0.051 − 1.02 |
3.57 |
1.40 |
436.3 |
36.5 | |
NAW-Y6a |
2.59 × 1.64 × 0.96 |
0.60 |
0.051 − 1.02 |
1.12 |
0.49 |
691.3 |
52.9 | |
NAW-Y6b |
1.29 × 1.64 × 0.74 |
0.60 |
0.051 − 1.02 |
1.47 |
0.50 |
334.8 |
41.7 |
Case |
Cell Volume (μm^{3}) |
Volume Fraction Liquid |
Velocity Range (min − max) m/s |
Pressure Differential (Pa) |
Length over Pressure Drop (× 10^{–4}m) |
Volumetric Flow Rate (×10^{–12} m^{3}/s) |
Permeability (×10^{–11} m^{2}) | |
---|---|---|---|---|---|---|---|---|
Cross Flow |
NAW-X1 |
1.34 × 1.63 × 0.47 |
0.25 |
0.0052 − 0.103 |
.922 |
10.7 |
0.106 |
0.43 |
NAW-X2 |
2.60 × 1.63 × 0.67 |
0.35 |
0.0054 − 0.107 |
2.11 |
22.6 |
0.105 |
0.12 | |
NAW-X3 |
2.60 × 1.61 × 1.42 |
0.45 |
0.0051 − 0.101 |
.438 |
22.3 |
0.598 |
3.46 | |
NAW-X4 |
2.60 × 1.63 × 0.28 |
0.55 |
0.0054 − 0.108 |
.206 |
19.9 |
0.132 |
2.99 | |
NAW-X5 |
2.60 × 1.63 × 0.13 |
0.65 |
0.0050 − 0.100 |
.425 |
20.0 |
0.264 |
4.21 | |
NAW-X6 |
1.30 × 1.63 × 0.08 |
0.75 |
0.0052 − 0.104 |
.203 |
11.4 |
1.42 |
27.0 | |
NAW-X7 |
1.30 × 1.62 × 0.10 |
0.85 |
0.0053 − 0.107 |
.107 |
11.7 |
2.42 |
74.0 |
Because permeability can be anisotropic based on the environment of flow, K _{ y } for parallel flow and K _{ x } for cross flow were independently calculated, and are listed as functions of inlet volume fraction for vertical flow cases and total volume fraction for cross flow instances. The calculated permeabilities and related values are summarized for each flow case in Table I for René N4 and Table II for Ni-Al-W, respectively.
The error bars in Figures 5 and 6 represent the local variation in solid fraction for each flow cell. Error bars for vertical flow permeabilities (K _{ y }) are one sided as the cross-sectional areas used in these permeability calculations occur at the inlets and yield the maximum fraction solid within each vertical flow cell.
For René N4, Heinrich-Poirier Blake-Kozeny models, K _{ BK(HP)}, coincide well with directly assessed permeabilities across all vertical flow simulations with the exception of a minor deviation at 0.8 f _{S}. K _{ BK(HP)} models also describe cross flow results well at high solid fraction but underestimate permeability in the range 0.4 < f _{S} < 0.6. Conversely in this range, the Kozeny-Carmen model appears to accurately describe the cross flow permeability as determined by simulation. This would suggest that the interfacial surface area per unit volume has a significant influence on permeability near 0.50 f _{S} in cross flow for the experimentally observed morphology in René N4.
For Ni-Al-W, cross-flow permeabilities (K _{ x }) show reasonable correlation with K _{ BK(HP)} prediction with departures at 0.15f _{S,} 0.55f _{S}, and 0.65f _{S} by a factor of ± 5. The failure of the K _{ BK(HP)} criterion to predict permeability in these structures is likely due to large constrictions existing in cross flow resulting in the localization of relatively high velocity flow to select channels, Figure 4(b). Conversely, vertical flow permeabilities (K _{ y }) are higher than all K _{ BK(HP)} by a factor of 15 to 20. Interestingly, in comparison to Kozeny-Carman predictions with measured S _{V}, K _{ y } are higher by a maximum factor of five across all fractions solid with convergence between simulation results and K _{ KC } prediction at 0.4f _{S}.
3.2 Dendrite Morphology Considerations
Flow Path Ratios, ISA Per Unit Volume & Permeability by Case for René N4 Simulations
Case |
Inlet Volume Fraction Liquid |
T_{avg. } {± Var.} |
Specific S _{v} (×10^{4} m^{–1}) |
Permeability (×10^{–11} m^{2}) | |
---|---|---|---|---|---|
Vertical Flow |
N4-Y1 |
0.08 |
2.29 ± 0.83 |
58.1 |
0.11 |
N4-Y2 |
0.10 |
2.56 ± 1.4 |
32.6 |
0.09 | |
N4-Y3 |
0.22 |
1.61 ± 0.33 |
57.0 |
1.3 | |
N4-Y4 |
0.32 |
1.57 ± 0.63 |
30.0 |
1.4 | |
N4-Y5 |
0.43 |
1.67 ± 0.63 |
19.4 |
1.8 | |
N4-Y6 |
0.60 |
1.51 ± 0.32 |
14.7 |
3.7 |
Case |
Volume Fraction Liquid |
T_{avg. } {± Var.} |
Specific S _{v} (×10^{4} m^{–1}) |
Permeability (×10^{–11} m^{2}) | |
---|---|---|---|---|---|
Cross Flow |
N4-X1 |
0.42 |
1.26 ± 0.22 |
12.0 |
4.5 |
N4-X2 |
0.56 |
1.39 ± 0.01 |
9.99 |
21 | |
N4-X3 |
0.73 |
1.21 ± 0.04 |
4.41 |
183 | |
N4-X4 |
0.89 |
1.13 ± 0.003 |
2.23 |
659 |
Flow Path Ratios, ISA Per Unit Volume and Permeability by Case for Ni-Al-W Simulations
Case |
Inlet Volume Fraction Liquid |
T _{avg. } {± Var} |
Specific S _{v} (×10^{4} m^{–1}) |
Permeability (×10^{–11} m^{2}) | |
---|---|---|---|---|---|
Vertical Flow |
NAW-Y1 |
0.10 |
1.30 ± 0.23 |
18.6 |
5.20 |
NAW-Y2 |
0.20 |
1.18 ± 0.05 |
49.4 |
9.33 | |
NAW-Y3 |
0.30 |
1.16 ± 0.03 |
57.5 |
13.7 | |
NAW-Y4 |
0.40 |
1.25 ± 0.07 |
47.4 |
19.4 | |
NAW-Y5 |
0.50 |
1.19 ± 0.10 |
20.5 |
36.5 | |
NAW-Y6a |
0.60 |
1.24 ± 0.17 |
7.37 |
52.9 | |
NAW-Y6b |
0.60 |
1.27 ± 0.13 |
3.73 |
41.7 |
Case |
Volume Fraction Liquid |
T _{avg. } {± Var.} |
Specific S _{v} (×10^{4} m^{–1}) |
Permeability (×10^{–11} m^{2}) | |
---|---|---|---|---|---|
Cross Flow |
NAW-X1 |
0.25 |
1.51 ± 0.11 |
26.3 |
0.43 |
NAW-X2 |
0.35 |
1.19 ± 0.01 |
89.0 |
0.12 | |
NAW-X3 |
0.45 |
1.32 ± 0.03 |
66.2 |
3.46 | |
NAW-X4 |
0.55 |
1.24 ± 0.01 |
41.8 |
2.99 | |
NAW-X5 |
0.65 |
1.10 ± 0.01 |
16.7 |
4.21 | |
NAW-X6 |
0.75 |
1.24 ± 0.02 |
4.41 |
27.0 | |
NAW-X7 |
0.85 |
1.07 ± 0.01 |
3.55 |
74.0 |
It should be reiterated that vertical flow cells in Ni-Al-W simulations exhibit liquid domains of linear channels upward through the mushy zone, Figure 4(a), resulting in substantially higher permeabilities in vertical flow compared to the René N4 material. Consistent with this, the interfacial surface area per unit volume peaks much lower in the Ni-Al-W material as cross-sectional morphologies remain somewhat consistent throughout the height causing maximum ISA to coincide closely with 50 pct fraction solid (Figure 7).
3.3 Rayleigh Number Determination
Using these temperature-composition relationships, a solute gradient profile based on the liquidus to solidus temperature difference (ΔT _{liquidus–solidus}) and experimentally determined thermal gradients were established. This allows for the estimation of temperature-specific densities as a function of the mushy zone height to be computed. Ra_{ h } calculations using these profiles will be denoted as axial ΔT. Alternatively, a height-based thermal profile was determined by fitting the ΔT _{liquidus–solidus} difference to the reconstruction heights. Because both reconstruction heights are shorter than predicted by the product of the withdrawal rate and axial thermal gradient present during casting, significant nonaxial heat extraction during casting is a likely explanation for the shortened mushy zone heights. This is probable as samples used in this investigation were extracted from within 1–10 mm of the castings walls. For this reason, the second set of discrete local Ra_{ h } calculations will be denoted as nonaxial ΔT. Both thermal profiles are then used to assess the local compositional variation and the resultant density difference at specific heights allowing for calculation of a local, height-specific Rayleigh number, Ra_{ h }. Both axial ΔT and nonaxial ΔT profiles are combined with the directly assessed 3D dendritic permeabilities for the determination of an axial ΔT Ra_{ h } and a nonaxial ΔT Ra_{ h } prediction. These results are compared with the conventional Ra_{ h }, which has been determined from the permeability approximation of Eq. [7] combined with the CALPHAD-assessed composition variations.
Overall, the Rayleigh numbers are higher for the Ni-Al-W material, compared with René N4. This finding suggests that convective instabilities will form more easily in the Ni-Al-W material not only because of the solute density inversion, but also because of the higher permeabilities below 0.4 fraction solid. However, because the cross-flow permeability at f _{L} = 0.6 is lower than the vertical permeability in the Ni-Al-W material, the Rayleigh number may not provide an accurate prediction if the flow instabilities originate deep in the mushy zone. Conversely, cross-flow permeabilities across all fraction solid are higher than vertical permeabilities in René N4; therefore, the Rayleigh number should predict convective instabilities rather reasonably.
The analysis herein indicates maximum Rayleigh number in each system are strongly dependent on cross flow permeabilities and occur in the vicinity of 0.1 solid fraction. Additionally, Ra_{ h } curves exhibit a rapid drop after their peak and small values for all fraction solids greater than 0.4. Finally, it should be mentioned that the lower limit in direct permeability assessments of vertical flow in this study also occurs at 0.4 fraction solid because of a lack of contiguous paths of vertical liquid domains over an appreciable length of dendritic structure. However, the analysis based on the Rayleigh number offered in this article indicates that full descriptions of vertical flow at values less than 0.4 fraction solid may not be entirely necessary as critical Ra_{ h } values occur in the vicinity of 0.1–0.2 fraction solid because of cross permeabilities. Furthermore, recent empirical models have suggested cross-flow permeabilities as the limiting flow type[38,45] in the lower ranges of fraction solid as well.
The calculations reported here also suggest a mechanism for the preferential location of freckles at mold walls and outer boundaries of castings, independent of geometry. The differences in Ra_{ h } affected by variation in thermal profile between axial ΔT and nonaxial ΔT (Figures 13 and 14) illustrate this point. Over the exact same dendritic network, significant lateral heat extraction increases the local Rayleigh number dramatically because of a shortening of the mushy zone, forcing the density gradient to occur in a more constrained volume of liquid.
4 Discussion
It has been demonstrated that direct assessments of permeability based on actual dendritic structure in the mushy zone can be used for improved assessment of permeability and Rayleigh number (Ra_{ h }). In this study, 3D datasets of material at the solid–liquid interface in directionally solidified René N4 and a model Ni-6.5Al-9.5W (wt pct) ternary were used to assess vertical and cross-flow permeabilities directly over the mushy zone. The Rayleigh numbers calculated with experimentally determined permeabilities are higher than those determined with conventional permeability approximations.[11,14] Additionally, it has been shown that permeability scales inversely with interfacial surface area independent of the degree of flow path tortuosity. As a result, the dimensionless liquid ratio \( f_{\text{L}}^{3} /\left( {1 - f_{\text{L}} } \right)^{ 2} \) combined with accurate measures of the interfacial surface area can yield more accurate estimates for permeability.
In the systems investigated here, Rayleigh numbers do not exceed a critical value of 1.0 with conventional or 3D dendritic array permeabilities, suggesting instabilities should not occur in either alloy. However, because freckles have been observed in these castings (Figure 1(c)), it is apparent that other physical quantities in the Rayleigh analysis other than permeability, as was focused on here, are not determined adequately. The density gradient (Δρ/ρ _{o}), kinematic viscosity (α), and the thermal diffusivity (ν) are likely sources of error and require additional investigation. With regard to liquid density gradients, there may be errors in the thermodynamic assessments, and currently, insufficient information is available on the liquid density as a function of temperature. Partial molar volume density calculations have been shown to be accurate to within 5 pct[32,33] for certain alloys; however, a larger error may arise because of uncertainty in the prediction of liquid compositions themselves. The accuracy of phase calculation software is limited by the database used and can vary substantially for unexplored compositions as well as in select commercial nickel-based superalloys with typically three or more elements. Because (Δρ/ρ _{o}) is directly proportional to Ra_{ h }, a variation in density gradient of an order of a magnitude would affect the local Rayleigh number by the same amount. Alternatively, the kinematic viscosity and thermal diffusivity are maintained as constants and are, as a product, considered equal to the value 5 × 10^{–12} m^{4}/s^{2}.[11,13,14] Because this consistent approximation is used by various investigators, no discrepancies between the results of this and other investigations are introduced by its use. However, it is unlikely that kinematic viscosity, thermal diffusivity, or their product remain constant over the entire liquidus to solidus temperature range in most alloys, including those presented in this study. The absence of variation with temperature or composition is likely to have an uncertainty on the order of ± 5 pct for each quantity. Because (αν) is inversely proportional to Ra_{ h }, a 25 pct increase in the product of the thermal diffusivity and kinematic viscosity would produce a 25 pct decrease in the local Rayleigh number or vice versa. It is, therefore, also reasonable to assert that the greatest amount of error in the current Rayleigh calculations likely reside in either the lack of variation in the quantity (αν) or the assessment of the actual density gradient across the liquidus to solidus temperature range.
5 Conclusions
- 1.
Permeability varies inversely with interfacial surface area per unit volume (S_{V}) for vertical and cross-flow with slightly less sensitivity than the Kozeny-Carmen model.
- 2.
The commercial alloy René N4 has a lower propensity for freckling than the simple Ni-Al-W ternary alloy because of lower permeability in the mushy zone.
- 3.
Tortuosity decreases in vertical flow correlate with increases in permeability up to a value of 10 × 10^{–11}m^{2}. In cross flow, tortuosity is largely unchanged at all permeabilities as flow paths in cross flow are largely dictated by the fourfold dendritic symmetry of secondary arms regardless of volume fraction present.
- 4.
Direct permeability assessments show maximum Ra_{ h } values occur in the vicinity of 0.1 fraction solid for the two alloys investigated. Critical Ra_{ h } values are a factor of two to three times higher than conventional Ra_{ h } estimates using the average permeability approximation.
- 5.
Accurate knowledge of interfacial surface area improves permeability prediction in both vertical and cross flow within directionally solidified dendritic structures. When direct assessment is not practical, the following relation reasonably estimates permeability for the alloys considered in this investigation where C _{1} = 10.7 and n = 0.5:
- 6.
Simulations show higher driving forces for freckle formation are present when significant lateral heat extraction exists during directional solidification. In many cases, this condition exists at or near mold walls, where increases in local Rayleigh number occur as a result. This suggests the mechanism for the appearance of freckles at and along outer walls of castings is related to such lateral heat extraction.
Acknowledgments
Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The authors are grateful for the technical assistance of C. Torbet, UCSB, as well as support from T. Van Vranken and L. Graham of PCC Airfoils, Inc. The authors have also benefited from useful discussions with C. Beckermann, D. Poirier, P. Voorhees, and J. Dvorkin. The authors also thank C. Woodward, D. Trinkle, and Mark Asta for ab initio MD liquidus density calculations. The financial support of the AFOSR MEANS-II Program Grant No. FA9550-05-1-0104 is acknowledged and K. Thornton also acknowledges the support of NSF under Grant No. 0746424.