Modeling solidification of turbine blades using theoretical phase relationships |
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Authors: | Shuang-Lin Chen W Oldfield Y Austin Chang M K Thomas |
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Affiliation: | (1) Department of Materials Science and Engineering, University of Wisconsin, 53706 Madison, WI;(2) Materials and Computer Simulation Inc., 27944 Hertford, NC;(3) Naval Air Warfare Center, 18974 Warminster, PA |
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Abstract: | The solidification of hot-stage turbine blades made from René N4 nickel-base superalloy has been modeled to show the morphology
of porosity and the local changes in solute concentration. The key task of the present study was the calculation of the solid-liquid
phase equilibria of this 9-component nickel-base superalloy from the thermodynamic values of these phases. The Gibbs energies
of the solid and liquid phases were obtained from those of the 36 binaries using the Muggianu and Kohler methods of extrapolation.
The phase equilibrium data were then used to compute the change in fraction solid with temperature, initially using the complete
mixing approximation (Scheil equation). The predicted freezing range was somewhat longer than measured. A modified Scheil
equation was derived assuming incomplete mixing. Assuming 60 pct mixing of the solute, the calculated freezing range agreed
with experiments. Fraction solid temperature allowed the detailed morphology of the“mushy”zone to be predicted. Using measured
dendrite spacings and assuming the crystals to grow in a cubic array, the shape of the crystals and, consequently, the size
of the liquid channels were predicted as a function of position. Hence, computation of the rate of fluid flow in the channels
(from the known changes of temperature with time) allowed the pore morphology to be inferred. |
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