Thesis Abstracts 2002
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A Thermodynamic Model For Nickel Based Single Crystal Gas Turbine Alloys
By: Capt Kevin Jaansalu, PhD
Supervisor: Dr. W.T. Thompson
Abstract
A thermodynamic model has been developed to address the gamma, gamma prime, and liquid phase equilibrium for single crystal gas turbine alloys. The model is based on a critical review of the Ni-X (X = Al, Co, Cr, Hf, Mo, Nb, Re, Ru, Pt, Ta, Ti, W), Al-Y (Y = Co, Cr, Pt) and Co-Z (Z = Cr, Ta, Re, W) binary systems, where the observed binary phase diagrams are well reproduced with original and self-consistent thermodynamic models that incorporate measured thermodynamic properties. The ternary systems Al-Ni-X are reviewed with attention focussed on the gamma – gamma prime equilibrium. The gamma and liquid phases are modeled using a substitutional solution formalism, the gamma prime phase is modeled using a simple sub-lattice formalism that was originally developed for molten salt solutions. Calculated phase equilibrium compositions and phase transformation temperatures have been calculated that compare very well with data for two commercial alloys PWA 1484 and CMSX-10. From these comparisons, the importance of the constituent binary phase diagrams is revealed. Selected binary sections for these two alloys are calculated and reveal the influence of segregation on the gamma prime solvus temperature and possible effects on industrial heat-treatment processes. The phase models have been further exploited to address the precipitation of the topologically close packed phase and the formation of the secondary reaction zone in CMSX-10. On the basis of compositions in the SRZ measured by SEM/EDS, Gibbs energy for the precipitation of the TCP phase is calculated. Selected binary sections for the TCP phase solvus in CMSX-10 are calculated, showing that the solvus temperature is a strong function of the rhenium content. The model has been developed in such a way that it can be modified, broadened, and improved to address other industrial processes of interest to alloy producers and engine manufacturers.
