Gamma (FCC)/Gamma-Prime (L12) Co-Based Superalloys II: Processing and Environmental Resistance
Sponsored by: TMS Functional Materials Division, TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: High Temperature Alloys Committee, TMS: Integrated Computational Materials Engineering Committee, TMS: Phase Transformations Committee
Program Organizers: Eric Lass, National Institute of Standards and Technology; Qiang Feng, University of Science and Technology Beijing; Alessandro Moturra, University of Birmingham; Chantal Sudbrack, NASA Glenn Research Center; Michael Titus, Purdue University; Wei Xiong, Northwestern University

Wednesday 8:30 AM
March 1, 2017
Room: Pacific 14
Location: Marriott Marquis Hotel

Session Chair: Chantal Sudbrack, NASA Glenn Research Center; David Dye, Imperial College

8:30 AM  Keynote
Developing Polycrystalline Ni-Co Rich Alloys, Strengthened by Co3AlW L12 Gamma Prime Precipitates for High Temperature Applications: David Dye1; Farah Ismail1; Trevor Lindley1; Paul Mulvey1; Richard Chater1; Ioannis Bantounas1; Barbara Shollock2; Mark Hardy3; 1Imperial College; 2The University of Warwick; 3Rolls-Royce plc
    Since Sato et al reported the occurrence of a stable L12 phase (Co3AlW) in the Co-Al-W ternary system, there has been considerable interest in developing Co-based alloys for high temperature applications. Much activity has focused on single crystal aerofoil alloys, given the high melting temperatures and their reduced elemental segregation, which potentially reduces convective instabilities and therefore, freckle formation. The low solvus temperature has required significant Ni, Al, Ta and Ti additions, suggesting a potential for lower temperature polycrystalline applications. This presentation considers the design and manufacturing requirements for Ni-Co rich polycrystalline alloys in static and rotating applications, provides some insights from current research and suggests areas for future study. Given the temperature requirements, oxidation will be a particular concern, particularly in the context of fatigue, and therefore alloying effects on oxidation will be a particular focus. We acknowledge the EPSRC/Rolls-Royce Strategic Partnership for funding (EP/M005607/1, EP/H022309/1).

9:10 AM  Invited
Novel Cast and Wrought γ/γ’ Cobalt Base Superalloys - Creep Properties, Deformation Mechanisms, and Oxidation: Mathias Göken1; Lisa Freund1; Steffen Neumeier1; 1Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
    γ/γ’ Co–base superalloys have become an intensely investigated material in the last few years. They are regarded as potential candidates that could replace Ni–base superalloys in some high temperature and high strength applications. These new Co-base superalloys are especially interesting candidates for polycrystalline wrought alloys, since they possess a quite low γ’ solvus temperature, which in combination with the high solidus temperature offers a wide processing and forging window. Promising new multinary γ’ hardened Co–base superalloys have been developed that can compete with the commercially available Ni–base wrought superalloys Udimet720Li and Waspaloy in terms of mechanical properties and oxidation resistance. The creep strength of the new Co-alloys is significantly better than that of the Ni-alloys. Investigations of the deformation mechanisms show that microtwinning plays an important role during creep. Oxidation tests show that the oxidation resistance is comparable or even better than that of commercial Ni-base superalloys.

9:40 AM  
Supersolvus Thermomechanical Processing of Cast Co–Base Superalloys: Donald Weaver1; Katelun Wertz1; S. Lee Semiatin1; Rajiv Shivpuri2; Stephen Niezgoda2; Michael Mills2; 1Air Force Research Laboratory; 2The Ohio State University
    Two–phase, nickel–base superalloys strengthened by high volume fractions of the L12 (γ’) phase are prone to poor hot workability in the coarse–grain, cast condition and are thus typically synthesized via a powder–metallurgy route. By contrast, recently–developed γ–γ’alloys based on the Co–Al–W ternary exhibit a relatively large difference between the solidus and γ’–solvus temperatures that may enable less expensive ingot–metallurgy processing. To this end, plastic flow and microstructure evolution during supersolvus forging of a cast cobalt–base superalloy were investigated to develop fundamental insight into hot–working behavior and to define a processing window to obtain wrought product with a refined gamma grain size. Specifically, isothermal, hot compression tests were conducted at a variety of supersolvus temperatures and strain rates followed by quantitative metallography. The effect of process variables on recrystallization kinetics and recrystallized grain size will also be discussed.

10:00 AM Break

10:20 AM  Invited
Coating Systems for New Cobalt Base Single Crystals: Wesley Jackson1; Mike Titus1; Tresa Pollock1; Matt Begley1; 1University of California Santa Barbara
    New cobalt-base alloys containing L12 strengthening precipitates show promise in their single crystal form for high temperature turbine applications. As with Ni-base alloys, it is expected that multilayered coating systems compatible with the single crystal substrate will be required for thermal and environmental protection. Elevated temperature structure evolution in MCrAlY bond coatings applied to Co-base single crystals has been investigated. To assess the compatibility of MCrAlY + 7YSZ thermal barrier coatings, the thermal expansion behavior of the Co-base single crystals has been characterized. Stresses and energy release rates driving interfacial cracking have been analyzed in comparison to Ni-base systems. The driving force for coating failure is sensitive to the composition of the Co-base alloy. Strategies for reducing the driving forces for delamination will be discussed.

10:50 AM  
Role of Two Phase Microstructure during Early Stages of High Temperature Oxidation of Co-base Superalloys: Martin Weiser1; Sannakaisa Virtanen1; 1University of Erlangen-Nuernberg (FAU)
     Elementary mechanisms of scale formation on Co-base superalloys are relatively unexplored. Although this class of high temperature materials reveals a microstructure similar to Ni-base superalloys, the nature of base element changes the segregation behavior and the diffusion velocities of alloying elements. Since both properties are essential for the formation of protective scales the inferior oxidation performances become explainable.A series of polycrystalline model alloys of the quaternary system Co-xAl-yW-zTa (x = 8.8-8.9 at%, y = 4.0-9.0 at%, z = 0.5-2.3 at%) with varying γ′ - volume fraction was chosen to systematically investigate the oxidation properties of the individual phases at temperatures up to 900°C. The composition of phases stayed unaltered. Besides classical thermogravimetry, the change of surface topography was monitored by in-situ experiments in a SEM equipped with a heating stage. The layering sequences of the scales were investigated and compared by SEM and further high resolution surface analysis methods.

11:10 AM  
Influence of Alloy Composition on Oxide Scale Formation in Novel Co-Based γ-γ’ Superalloys: Colin Stewart1; Akane Suzuki2; Tresa Pollock1; Carlos Levi1; 1University of California Santa Barbara; 2GE Global Research
    A set of combinatorial libraries have been generated across Co-Ni-W-Al-Cr space to develop an understanding of compositional effects on intrinsic oxidation resistance at 1100°C in Co-based alloys relevant to γ-γ’ superalloys. Photo-stimulated luminescence spectroscopy is employed to rapidly assess the presence of α-Al2O3 , and further characterization is accomplished with SEM/EDX. Domains of composition where α-Al2O3 forms are outlined, and insights from the onset of external Al2O3 formation and the presence of non-protective oxides are discussed.

11:30 AM  
A High-throughput Search for New Ternary Superalloys: Chandramouli Nyshadham1; Corey Oses2; Jocob Hansen1; Ichiro Takeuchi3; Stefano Curtarolo2; Gus Hart1; 1Brigham Young University Provo Utah; 2Duke University; 3University of Maryland, College Park
     In 2006, a novel cobalt-based superalloy was discovered[1]. Inspired by this unexpected discovery, we performed a first-principles search through 2224 ternary metallic systems of the form X3[A0.5, B0.5], where X = Ni, Co, or Fe, and [A, B] = metals. We found 115 systems that have a smaller decomposition energy and a lower formation enthalpy than the Co3(Al, W) superalloy. They have a stable two-phase equilibrium with the host matrix within the concentration range 0< x <1 (X3[Ax, B1-x]) and have a relative lattice mismatch with the host matrix of less than 7%. These new candidates, narrowed from thousands of possibilities, suggest experimental exploration for new superalloys. Of these 115 systems, 42 have no reported phase diagrams. Based on cost, experimental difficulty, and toxicity, we list[2] the top 10 candidates for further study. [1] Sato et al., Science 2006; 312 (5770):90-1. [2] Nyshadham et al., arXiv:1603.05967 (2016).

11:50 AM  
Phase Stability, Element Partitioning and Atomic Site Location in Co-9Al-9W-2X Alloys: Li Wang1; Michael Oehring1; Uwe Lorenz1; Andreas Stark1; Florian Pyczak1; 1Helmholtz-Zentrum Geesthacht
    In the Co–Al–W system the γ’–L12 phase can be stabilized by the addition of W and other alloying elements. In this study alloy samples with nominal compositions Co-9Al-9W-2X (X=Ti, Nb, V, Ta, Cr, Ni and Mo, at. %) were produced by arc-melting. The partitioning behavior of the alloying elements in γ and γ’ phases were investigated in dependence on temperature by energy-dispersive X-ray spectroscopy in the transmission electron microscopy in quenched specimens. In order to analyze the element occupation at atomic sites, atom location by channeling enhanced microanalysis technique was applied. The results from annealing experiments show that after 5000 hours at 900 °C, B2–CoAl and D019–Co3W phases form in all alloys except Co-9Al-9W-2V. The amount, shape and type of these phases depend on the alloying element addition. Details about the partitioning behavior and the atomic site occupation of alloying elements in these alloys will be discussed in the presentation.