Mechanism and Kinetics of t’ decomposition

1. Phase Evolution of Metastable Tetragonal Zirconia upon HT Aging Mechanism and Kinetics of t’ decomposition The ultimate temperature capability of thermal barrier oxides based on tetragonal ZrO2 solid solutions, which are typically metastable when cyclic durability is optimized, depends on the time required for the structure to become transformable to monoclinic. Synchrotron and TEM studies on the aging of PVD 7YSZ have shed considerable light on the mechanism of phase evolution, whose understanding was inadequate to build predictive models. It was found that the t’ phase decomposes at a short fraction of the life into a nano-scale structure of coherent Y-rich domains, interspersed with similarly small domains of Y-lean t-phase. The structure evolves to lamellar arrays that coarsen over time; the onset of transformability is linked to the coarsening kinetics. Phase field modeling has been initiated to set the foundation for a model with improved predictive capability. Evolution of the tetragonal+cubic structure arising from the dcomposition of t’ (Krogstad et al, to be published). Preliminary phase field results of the decomposition of t’ showing the evolution of lamellar structures as the compositional differences evolve (Sheng et al, GE-GRC)

2. ND Probes to Monitor HT Aging of Metastable Tetragonal Zirconia Monitoring Aging by Raman Spectroscopy The phase evolution of metastable tetragonal zirconia-yttria solid solutions upon aging ultimate limits the longevity and/or temperature capability of thermal barrier coatings in gas turbine applications. The evolution of the position and width of the Raman lines for these materials with annealing for different times at different temperatures has been monitored. It is found that the kinetics are properly captured using a Larson-Miller/Hollomon-Jaffee type parameter. This normalization allows the shift and sharpening of the Raman peaks of tetragonal zirconias having different stabilizer concentration and produced by different methods with different microstructures all to be described by a common curve. The observed Raman shifts are consistent with evolution into a coherent mixture of tetragonal and cubic phases. Raman peak positions of tetragonal zirconia with various compositions and microstructures as a function of Larson-Miller parameter (Limarga et al). The sign of the slopes is the same as when a hydrostatic stress is applied to tetragonal zirconia suggesting that the metastable tetragonal phase undergoes a coherent phase transformation into a mixture of stable tetragonal and cubic phases, in agreement with the TEM observations of Krogstad et al.

3. Exploiting Phase Evolution for Novel Concepts in Bond Coat Design Combinatorial approaches to bond coat chemistry Studies on bond coat compositions identified by combinatorial approaches have shown that the potential substitution of Pd for Pt appears to slow rumpling (bottom right). This has been ascribed to the evolution of the soft β phase into a more creep resistant L12 (γ’) phase at the surface due to oxidation and interdiffusion with the substrate. The observations suggest a new approach to bond coat design, wherein the chemical composition would be tailored to enable alumina formation but evolve relatively quickly to the more creep resistant γ’ phase. Combinatorial Exploration of Bond Coat Compositions Surface rumpling after 800 cycles of oxidation of in two Ni-Cr-Hf alloys with 5% addition of either Pt or Pd.. RMS of the surface indicate rumpling in the latter (Pd) is significantly lower than that of Pt Cyclic oxidation of β-NiAl alloys with/without addition of PGMs (Pd and Pt). The differences in mass gain appear similar or somewhat larger for Pd relative to Pt, but are accentuated at longer times

4. Interdiffusion of Pt Group Metals in NiAl:FP Calculations and Experimental Results NiAl is the foundation for a family of coatings for oxidation protection of superalloys. The goal of this research is to understand the role of alloying additions on interdiffusion with the substrate, which often limits system durability. DFT and KMC are combined to predict the relative populations of defects contributing to difusion and the corresponding migration barriers. The framework is applied to calculate the relative mobilities of PGMs on Ni sites (preferred occupancy) and the expected effects on diffusivity. Critical differences are predicted among the PGMs, which are confirmed by experimental studies on NiAl/(Ni,PGM)Al difusion couples. Diffusion of PGM in dilute solutions within NiAl. Triple point defect mechanism (left) with calculated migration barriers for the different steps (middle), ranked from lowest to highest. (Van der Ven et al.) Note consistency with experimental results from Pollock et al. (right).

5. Broader Impacts International Activities GS Jessica Krogstad spent 7 weeks at University of Sydney doing TEM and LEAP studies with Dr. Julie Cairney. GS Elizabeth Clark spent 10 weeks at University of Tokyo working on synthesis and characterization of oxide films. GSs Kendra Grant and Elisa Zaleski attended a 2 week International School on Glass Formers and Glasses at IISc Bangalore. Prof. Barbara Albert (TU Darmstadt) and UG/MS interns Maren Lepple (U. Stuttgart) and Emeric Veron-Tocquet (ENS Cachan) spent several months each at UCSB collaborating with Levi and his students. An international memorial conference honoring Prof. Evans held at UCSB in Sept. 2010. Levi and Pollock were members of organizing committee. UCSB GS Erin Donohue performing Jet Engine Thermal Simulation (JETS) tests while on an internship at GE Global Research. UCSB GS Jessica Krogstad (right) mentoring UG intern Maren Lepple on thermal processing of novel thermal barrier oxides. Industrial Interactions Participation of GE in the Goali program continues to be exemplary, with GE personnel actively participating in mentoring students and providing both material and intellectual support. GS Erin Donohue spent 10 weeks at GE Global Research in summer 2010 working on toughness testing of coatings and probing the delamination mechanisms of TBCs using the Jet Engine Thermal Simulation test. GS Jessica Krogstad performed synchrotron studies at BNL and ANL and continues to work closely with GE on aging studies in TBCs. The PIs and their students continue to have substantial interactions on high temperature coatings with other gas turbine companies, notably Pratt & Whitney, Honeywell, Siemens, Alcoa-Howmet and Rolls Royce.