Design of Ni-base superalloys and MCrAlY coatings from first-principles and computational thermodynamics

Open Access
Author:
Liu, Xuan
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 09, 2015
Committee Members:
  • Zi Kui Liu, Dissertation Advisor
  • Zi Kui Liu, Committee Chair
  • Long Qing Chen, Committee Member
  • Vincent Henry Crespi, Committee Member
  • William George Noid, Committee Member
  • Brian Gleeson, Special Member
Keywords:
  • Superalloy
  • MCrAlY
  • CALPHAD
  • Nb-Re
  • First-Principles
Abstract:
This thesis explores the thermodynamics of Ni-base superalloys and metallic coatings used in the protection of these alloys. Due to the large design space in composition and temperature for many new superalloys and coatings, trial-and-error methodologies of the past prove ineffective and costly. All mechanical or corrosion properties for superalloys depend heavily on the processing and the resulting phases. Multi-component alloys often contain a gamut of phases, many of which are actually detrimental to the alloy. This problem is exacerbated with the addition of refractory transition metals such as Nb, Re, and W to improve mechanical properties. However, computational phase diagram studies for the constituent systems have proven successful for the prediction of detrimental phases in superalloys. Unfortunately, some of the constituent systems have not been examined in detail, one of which is the Nb-Re binary system. First, a thermodynamic description of the Nb–Re binary system is developed by means of the CALculation of PHAse Diagrams (CALPHAD) method supplemented by first-principles calculations based on density functional theory (DFT) and experimental data in the literature. In addition to terminal solution phases in the Nb-Re system, there are two intermetallic phases, sigma (σ) and chi (χ), all modeled with sublattice models. Special quasi-random structures (SQS) are employed to mimic the random mixing of the bcc, hcp, and fcc solid solution phases from first-principles. Finite temperature thermodynamic properties of end-members and dilute mixing in each sublattice of the complex σ and χ phases are predicted from first-principles calculations and the Debye-Grüneisen model. The calculated phase diagram agrees well with selected experimental phase equilibrium data in the literature. The utility of the Debye-Grüneisen model is then investigated with respect to its fitting parameter known as the scaling factor, and it is found that the prediction of finite-temperature properties can be improved by modification of this factor. This scaling factor is studied using bcc, fcc, hcp systems and the Mg-Zn binary system due to the abundance of thermodynamic data. Predicted Debye temperatures (Θ_D), using a calculated scaling factor, show good agreement with experiments and improvements over the scaling factor derived by Moruzzi et al. Finite-temperature thermodynamic properties of intermetallics are investigated to show the efficiency and improved accuracy of the calculated scaling factor. However, for the intermetallic Mg2Zn11, the Debye-Grüneisen model cannot account for anomalous lattice dynamics at low temperatures. The calculated scaling factor is then used throughout the present work for finite-temperature predictions. Another missing piece of the literature includes the thermodynamics of Al-Co-Cr-Ni bond coat system used in the protection of superalloys. First, it is found that there is an incomplete description of the crucial ternary Al-Co-Cr subsystem. As a result, the phase relations and thermodynamic properties of this system are investigated using first-principles calculations based on DFT and phase-equilibria experiments that led to X-ray diffraction (XRD) and electron probe micro-analysis (EPMA) measurements. A thermodynamic description is developed by means of the CALPHAD method using experimental and computational data from the present work and the literature. Emphasis is placed on modeling the bcc-A2, B2, fcc-γ, and σ phases in the temperature range of 1173 to 1623 K. Liquid, bcc-A2 and fcc-γ phases are modeled using substitutional solution descriptions. First-principles SQS calculations predict a large bcc-A2 (disordered)/B2 (ordered) miscibility gap, in agreement with experiments. A partitioning model is then used for the A2/B2 phase to effectively describe the order-disorder transitions. The critically assessed thermodynamic description describes all phase equilibria data well. A2/B2 transitions are also shown to agree well with previous experimental findings. In order to build a correct quaternary description of the Al-Co-Cr-Ni system, additional ternaries must also be modeled to combine with the Al-Co-Cr system. The phase relations and thermodynamic properties of the Al-Co-Ni and Co-Cr-Ni ternary alloys are investigated using first-principles calculations based on DFT. Thermodynamic descriptions are developed by means of the CALPHAD method using experimental and computational data from the present work and the literature. In addition to the phases found in the Al-Co-Cr system, the ordered fcc-γ phase, L12-γ’, is taken into account. A partitioning model is used again for the A2/B2 phase to effectively describe the order-disorder transitions when coupled with the Al-Co-Cr system. The thermodynamic description describes all phase equilibria data well. The Al-Co-Cr, Al-Co-Ni, and Co-Cr-Ni systems are combined with the Al-Cr-Ni system from Dupin et al. [1] to produce a complete Al-Co-Cr-Ni thermodynamic model. Predictions are shown to agree well with experimental alloys from collaborators. It is shown that by critically assessing each of the ternary subsystems, a full quaternary description can be developed successfully with minimal use of excess thermodynamic parameters. This thesis also studies the stability of interfaces that form in Ni-base superalloys during microstructure evolution processes. Understanding the coarsening of superalloy microstructures during processing or service requires the knowledge of the stabilities of solid-solid interfaces. As a result, the technologically important {100} coherent γ/γ’ interface is investigated using first-principles calculations. The change in the interfacial energy of the Ni-Al γ/γ’ coherent interface when a ternary element is segregated near the interface is studied. Binary calculations are performed to validate previous results; it is predicted that the Ni-Al γ/γ’ interfacial energy is 19 mJ/m2 at 0 K which is in agreement with the literature. When a ternary element is added into the system and partitions to the γ phase, the interfacial energy can vary between 4-28 mJ/m2. It is found that the additions of the Mo, Re, and W elements can decrease the interfacial energy significantly to 4-5 mJ/m2 while the additions of the Ru and Pt elements can increase it to 25-28 mJ/m2. When elements reside in γ, magnetic ordering contributions at the interface are shown to have the greatest effect on the change in the interfacial energy. Ferromagnetic elements Co and Fe are shown to have little effect on the interfacial energy as they do not disrupt magnetic spin across the interface. All ternary additions to γ’ are shown to increase the interfacial energy except for Pt, which decreases it to 16 mJ/m2. Otherwise, ternary additions in γ’ can vary the interfacial energies between 20-37 mJ/m2. Overall, a fundamental understanding of the thermodynamics relevant to Ni-base superalloys and coatings is developed using first-principles and CALPHAD approaches.