Materials for High Temperature Applications: Next Generation Superalloys and Beyond: Ir Alloys and Next Generation Superalloys
Sponsored by: TMS Structural Materials Division, TMS: High Temperature Alloys Committee, TMS: Refractory Metals Committee
Program Organizers: Akane Suzuki, GE Global Research; Martin Heilmaier, Karlsruhe Institute of Technology (KIT); Pierre Sallot, Safran Tech; Stephen Coryell, Special Metals Corporation; Joseph Licavoli, NETL - Department of Energy; Govindarajan Muralidharan, Oak Ridge National Laboratory

Thursday 8:30 AM
March 2, 2017
Room: Balboa
Location: Marriott Marquis Hotel

Session Chair: Govindarajan Muralidharan, Oak Ridge National Lab; Stephen Coryell, Special Metals Corporation

8:30 AM  Invited
Weldability and Weld Properties in Iridium Alloys: Roger Miller1; George Ulrich1; Govindarajan Muralidharan1; 1Oak Ridge National Laboratory
     DOP-26 is an iridium alloy that balances mechanical properties and weldability and is the current alloy used to encapsulate fuels for radioisotope thermoelectric generators. The weldability of iridium alloys is limited by the thorium level due to hot cracking susceptibility with high energy density welding processes being more tolerant to higher levels of thorium. Similar to weldments in many other refractory metal systems, the high melting point and purity of DOP-26 results in weld microstructures that degrade high temperature weld properties. This talk will discuss the weldability of iridium alloys with respect to thorium levels and the techniques used to improve weldability and weld properties. Additionally, current research to potentially increase weld properties and methods of evaluating weld structures will be outlined.Research sponsored by the Office of Space and Defense Power Systems at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC for the U.S. Department of Energy.

9:00 AM  Invited
Oxidation Resistance of Aluminized Ir-based Refractory Alloys: Hideyuki Murakami1; Masahide Yamashina1; Kazuya Shimoda1; 1National Institute for Materials Science
    Recently, Ir has attracted much attention for the use under high temperature environments, such as engine plug-tips, space planes, etc. This is because Ir has a high melting point, good chemical stability and mechanical stabilities at high temperatures. In addition, Hf addition to Ir is reported to enhance high temperature strength, and Ir-3at%Hf alloy exhibited over 80MPa of 0.2% compressive flow stress, compared with 19.4MPa of pure Ir at 2223K. On the other hand, Ir forms volatile oxides above 1173K, which could limit the long-term application under oxidative atmosphere. Further understanding of oxidation kinetics in the case of Ir alloys is thus important. Surface treatments on such materials should also be considered for oxidation protection. In our research group, aluminizing on Ir based alloys were successfully conducted to protect them against oxidation. This presentation describes the microstructure of aluminized Ir-based refractory alloys and their oxidation resistance.

9:30 AM  
Effect of Trace Levels of Si on Grain Boundary Segregation in an Ir Alloy: Dean Pierce1; Govindarajan Muralidharan1; Lee Heatherly1; Cecil Carmichael1; George Ulrich1; 1Oak Ridge National Laboratory
    Iridium (Ir) alloy DOP-26 (Ir-0.3W-0.006Th-0.005Al wt.%) is used as a fuel cladding material in radioisotope thermoelectric generators for space applications owing in part to its excellent high temperature impact ductility. Previous work has shown that additions of 50 to 1500 wppm of silicon (Si) to DOP-26 resulted in grain boundary Si concentrations of up to ~60 times bulk levels, contributing to loss of impact ductility. The focus of the present work was to understand the effect of high temperature annealing treatments on Si segregation to grain boundaries in Ir-alloys with lower levels of Si (less than 50 wppm). Auger Electron Spectroscopy on in-situ fractured specimens was used to characterize grain boundary segregation. Effects of bulk Si levels and annealing temperatures on the observed Si segregation to grain boundaries will be discussed.

9:50 AM Break

10:10 AM  
Long Term Grain Growth Behavior of Ir-Alloy DOP-26*: Govindarajan Muralidharan1; Dean Pierce1; Ethan Fox1; Seth Lawson1; Cecil Carmichael1; Easo George2; George Ulrich1; 1Oak Ridge National Laboratory; 2Ruhr University Bochum
    DOP-26, an Ir-alloy has been successfully used as a fuel-cladding material in radioisotope thermoelectric generators. Grain size has a significant effect on the high temperature, high strain rate impact ductility of this alloy. Although it is known that Ir-Th intermetallic precipitates limit grain growth in this alloy, the relationship between precipitate sizes and limiting grain sizes has not been evaluated. Grain growth behavior was characterized in DOP-26 sheet specimens for times up to 10,000 hours at 1300 and 1400 ░C. Grain size, grain size distribution, and grain aspect ratio were measured as a function of time at temperature. This talk will outline the effect of temperature on the kinetics of grain growth along with relationships observed between limiting grain sizes and precipitate characteristics. Research sponsored by Office of Space and Defense Power Systems at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC for the U.S. Department of Energy.

10:30 AM  
Atom Probe Tomography Study of Sigma Phase in Long Term-thermally Exposed High Refractory Ni-based Superalloy: Stoichko Antonov1; Jiajie Huo2; Qiang Feng2; Dieter Isheim3; David Seidman3; Sammy Tin1; 1Illinois Institute of Technology; 2State Key Laboratory for Advanced Metals and Materials; 3Northwestern University Center for Atom Probe Tomography (NUCAPT)
    In polycrystalline Ni-base superalloys, grain boundary precipitation of secondary phases can be very important due to the effects they pose on the mechanical properties. Furthermore due to the nature of TCP phases, other properties such as oxidation and corrosion resistance can also be negatively affected. Long term thermal exposures on an experimental powder processed Ni-base superalloys containing elevated levels of Nb was completed to assess the precipitation kinetics and thermodynamic stability of Cr,Mo-rich sigma phase precipitates. Atom probe tomography was used to evaluate the composition profiles of the sigma and the surrounding γ’ phases, as well as to study any interfacial segregation and compositional gradients that might exist between the two phases. In addition, TEM was used to confirm the sigma crystal structure and orientation relationship with the matrix. Results of the study will be presented and compared to thermodynamic calculations of the sigma phase composition.

10:50 AM  
The Effect of Molybdenum on the Microstructure and Properties of Model Quinary Nickel-based Superalloys: Amy Goodfellow1; Enrique Galindo-Nava1; Nick Jones1; Mark Hardy2; Howard Stone1; 1University of Cambridge; 2Rolls Royce plc
    The efficient design of new nickel based superalloys to meet the service needs of future gas turbine engine components requires accurate descriptions of the factors that influence alloy properties. In the case of alloy strength, models exist that relate the relative contributions of grain size, solid-solution strengthening and precipitate hardening to alloy composition and microstructural parameters. However, given the complexity of commercial nickel-based superalloys, assessing the efficacy of these models can be challenging. In this study, a series of model quinary alloys has been investigated, with varying molybdenum content and thereby lattice misfit that changes from positive to negative across the series. Detailed characterisation of these alloys’ phase compositions and precipitate size distributions has enabled predictions to be made of their mechanical properties and these have been critically compared with those measured experimentally. This work was supported by Rolls-Royce plc and the EPSRC under EP/H022309/1 and EP/H500375/1.

11:10 AM  
Portevin-Le Chatelier Effect in a Ni-Co Based Superalloys with Different Gamma Prime Content: Chuanyong Cui1; 1Institute of Metal Research
    The gamma prime plays a critical role in improving the mechanical properties of Ni-base superalloys. The Portevin-Le Chatelier effect always appears in Ni-base superalloys. In order to investigate the influence of the gamma prime on the PLC effect, four superalloys with various gamma prime contents were designed. The results indicated that the components of the matrix are the same for all alloys. The ultimate strength increases while the elongation decreases with increasing gamma prime.Serration changes from type A to type B and to type C with raising temperature, decreasing strain rate or increasing gamma prime content.The bimodal amplitude distribution was found in type B serrations. The gamma prime was effective to move the PLC effect occurring region to the range of low temperatures and high strain rates.The serration amplitude increases with increasing gamma prime content at the same temperatures, which indicated that the gamma prime makes the DSA effect aggravated.

11:30 AM  
Nanosized TaC Precipitates for Strengthening High-Temperature Co-Re Based Alloys: Ralph Gilles1; Debashis Mukherji2; Pavel Strunz3; Lukas Karge1; Premysl Beran4; Armin Kriele5; Michael Hofmann1; Joachim Roesler2; 1TU Muenchen; 2TU Braunschweig; 3Nuclear Physics Institute of the CAS, ; 4Nuclear Physics Institute of the CAS,; 5Helmholtz Zentrum Geesthacht
    This contribution will present a promising candidate for gas turbine applications - the high tempertaure Co-Re alloy [1]. Co-Re alloys consist of a complex microstructure with various kinds of precipitates at different size scales. The nanosized fine spherical TaC (< 50 nm) are the most important for strengthening. Larger TaC are present as rounded (0.5 - 5 Ám) or as so called Chinese script (> 5 Ám) precipitates. In addition, large Cr2Re3 primary σ phase particles (a few Ám) and fine lamellar Cr23C6 precipitates (thickness < 100 nm) exist in the alloy. All these phases are embedded in a matrix structure of ε phase (hcp) which transforms allotropically at high temperature to γ phase (fcc). Samples were characterized using SEM, XRD, neutron diffraction and small-angle neutron scattering. [1] J. R÷sler et al., Adv. Eng. Mater. 9 (2007) 876-881; [2] R. Gilles et al., J. Appl. Cryst. (2016) in print.