A THERMODYNAMICS BASED GUIDE TO ADDING HF TO A NI-SUPERALLOY TO IMPROVE OXIDATION RESISTANCE
Open Access
- Author:
- Ross, Austin Joseph
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 18, 2017
- Committee Members:
- Zi-Kui Liu, Dissertation Advisor/Co-Advisor
Long-Qing Chen, Committee Chair/Co-Chair
Hojong Kim, Committee Member
Ismaila Dabo, Committee Member
Vincent Crespi, Outside Member - Keywords:
- Oxidation
Superalloys
Bond-coat
HfO2
Hafnium
external scale
TGO - Abstract:
- This thesis focuses on the addition of the reactive element Hf to aid in the oxidation resistance of Ni-superalloys. Hf will improve the oxidation resistance of a given Al2O3 forming alloy when added to the alloy in small concentrations. However, when too much Hf is added, or rather when Hf is “over-doped”, the oxidation resistance begins to worsen again. To date there is no design criteria for how much Hf should be added. The traditional value is around 0.1 at.%. This thesis looks to establish thermodynamic criteria for the addition of Hf based on consideration for the reactions at the alloy/Al2O3 interface. The first part of this thesis is on the important Hf-Ni binary system. This system has been investigated several times in the literature but often using Hf sources contaminated by Zr. This work seeks to remodel the Hf-Ni system through the calculation of phase diagram (CALPHAD) approach based on first-principles calculations based in density functional theory (DFT) and additional experiments using high purity Hf. Diffusion couples in the Ni-rich portion of the Hf-Ni system were heat treated and analyzed at 1173.15, 1273.15 and 1373.15 K, respectively, to measure phase stability and Hf solubility in the fcc phase. The solubility observed in fcc Ni from Ni/Ni50Hf50 (at.%) diffusion couples is larger than that reported in previous experiments. These results are the only source fit to during modeling of the fcc solubility to avoid effects from Zr contamination. Data in the literature suggests that the high temperature crystal structure of the B33 NiHf phase is, in fact, the B2 structure. Because this phase has been observed to be in solid solution with the high temperature B2-NiTi, it has been modeled here as B2. Modeling of this phase was aided by first-principles calculations using special quasi-random structures (SQS). . Following this, the Ni-rich Al-Hf-Ni system and the Ni-rich Cr-Hf-Ni system are modeled to develop a Ni-rich model for the Al-Cr-Hf-Ni system. These models are based on experiments and first-principles calculations using density functional theory, both carried out in the present work. Thermodynamic models are developed with a focus on the fcc-γ, L12-γ', A2, B2, L21 and P62m-NiAlCr phases. Additional focus is placed on the η-Ni7Hf2 phase and the λ-Ni3Hf phase, both with solubility of Al, Cr and Ni based on available work in the literature. Isothermal heat treatments are performed on alloys in the Ni-rich Al-Hf-Ni system and the Ni-rich Cr-Hf-Ni system in the range of 1273.15 K to 1473.15 K using high purity Hf to avoid Zr contamination. Density functional theory calculations are performed to obtain formation enthalpies for stoichiometric phases and the mixing enthalpies in the fcc-γ, L12-γ', A2, and B2 phases using SQS. Finite temperature properties of some end-members in the γ', L21 and P62m-NiAlCr phases are obtained by first-principles calculations. The current models replicate the present experiments in the Ni-rich Al-Hf-Ni and Cr-Hf-Ni systems well and agree with many features from other experimental investigations in the literature. These thermodynamic descriptions are then combined with thermodynamic models of the Al-Cr-Ni system and other binary models from the literature to form the thermodynamic description of the quaternary system. Good agreement is found between quaternary experiments and the calculations from the present thermodynamic description. Over-doping is accompanied by precipitation of HfO2 in and beneath the external Al2O3 layer, i.e. the co-existence of these two oxides at the alloy/Al2O3 interface. This equilibrium can thus be used to determine the maximum Hf concentration which will result in HfO2 precipitation. This Hf concentration is termed the “Hf-tolerance” and calculated with the alloy/Al2O3/HfO2 equilibrium. The Al-Cr-Hf-Ni thermodynamic description is combined with the thermodynamic descriptions of Al2O3 and HfO2 to study this equilibrium in terms of the Hf and Al activities in the alloy. The calculated Hf-tolerance is compared with available observations in the literature and is in good agreement. It is shown that the γ' phase plays a key role in increasing the Hf-tolerance. Finally, the effects of other alloying elements on the Hf-tolerance is explored. In the present work interactions between Hf and other alloying elements in the fcc/L12 phase are obtained from DFT-based first-principles calculations. Since the ordered L12-Ni3Al (γ') phase is already understood to be important for oxidation resistance, only elements with a solubility in the L12 phase of greater than 5 at.% at 1000 °C are chosen for study. The calculations confirm that Pt, Pd, Ru, Rh, and Si are the most beneficial alloying elements as reported in the literature experimentally. Given this information, the Al-Cr-Hf-Ni-Pt-O system is modeled, and used to calculate the Hf-tolerance in the Ni-rich Al-Hf-Ni-Pt and Al-Cr-Hf-Ni-Pt systems. Results from these calculations agree well with experimental observations. In summary, this thesis provides a thermodynamic criterion on the maximum Hf contents on improving oxidation resistance of Al2O3 forming Ni-superalloys. This criterion is established by developing thermodynamic descriptions of several multicomponent systems including xxx using data from DFT-based first-principles calculations and experiments in the current work and the literature and by calculating the equilibrium between alloys/Al2O3/HfO2.