Gamma (FCC)/Gamma-Prime (L12) Co-Based Superalloys II: Alloy Development
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

Tuesday 2:00 PM
February 28, 2017
Room: Pacific 14
Location: Marriott Marquis Hotel

Session Chair: Alessandro Mottura, University of Birmingham; Wei Xiong, University of Pittsburgh

2:00 PM  Invited
γ’-strengthened Co-Base Alloys – Development and Challenges: Akane Suzuki1; 1GE Global Research
    γ’ strengthening in cobalt-base alloys offers opportunities to develop a new class of high temperature alloys that can potentially replace conventional cobalt-base alloys which mainly rely on solid-solution strengthening. In order to identify alloy compositions that possess a balance of high temperature mechanical capability and environmental resistance, alloy composition spaces were explored using the diffusion-multiple approach. Key properties and processibility of γ+γ’ Co-base alloys, as well as challenges in developing the alloys, will be discussed.

2:30 PM  Invited
An Update on Cobalt Based Co-Mo-Al –X Alloys with γ-γ’ Microstructure: Effect of Alloying Additions, Mechanical Properties and Interaction with Different Environments: Kamanio Chattopadhyay1; Dipankar Banerjee1; Abhshek Singh1; Rajarshri Banerjee2; Surendra Makineni1; Nitin Bellari2; Abhishek Sharma2; Praful Pandey2; Saurabh Das2; 1Indian Institute of Science; 2University of North Texas
    We have recently shown that cobalt alloys with a basic composition of Co-10Al-5Mo with quaternary additions of Nb or Ta can, after heat treatment can generate ’ microstructure similar to nickel-based superalloys with no penalty on density. The present paper updates the current development in our group on the effect of alloying additions and their effect on the stability of the microstructure after long-term exposure at different temperatures. This also includes physical and mechanical properties of these alloys such as thermal, electrical and magnetic properties and the creep properties. The suitability of these alloys at different environments is also being studied and the results available will be presented. Finally, we shall, on the basis of the available experimental data, evaluate the possible application domain of this class of alloys and the further challenges that need to be overcome in taking this class of alloys further.

3:00 PM  
Integrated Computational Materials Engineering of Co Bushing Alloy: Ida Berglund1; James Saal1; Jason Sebastian1; David Snyder1; Clay Houser1; Dana Frankel1; Nicholas Hatcher1; Gregory Olson1; 1QuesTek Innovations
    To address the need for replacing Be-containing alloys, Integrated Computational Materials Engineering (ICME) tools have been applied to the design of L12 precipitate-strengthened high-strength wear-resistant Co-base alloy. Thermodynamic and kinetic databases were optimized and used in the design. Heat treatment optimi¬zation enabled more efficient homogenization and a reduction in the anneal time. Property optimization centered on achieving higher yield strength in the Co alloy through simulation-driven thermal process optimization. To reduce the isothermal temper time to peak strength, an experimental aging study was coupled with precipitation simulations to identify novel temper schemes, including a two-step process. Two intermediate scale prototypes have been produced and prepared for testing, including sub-scale bushing testing, with preliminary data confirming high potential for the alloy as a CuBe replacement.

3:20 PM  
The Microstructure and Hardness of Ni-Co-Al-Ti-Cr Quinary Alloys: Katerina Christofidou1; Nicholas Jones1; Roxana Flacau2; Mark Hardy3; Howard Stone1; 1University of Cambridge; 2Canadian Neutron Beam Center; 3Rolls-Royce plc
    Elevated concentrations of Co and Ti in conventional Ni-based superalloys have been identified as beneficial to alloy properties, conferring improved strength and stability over conventional superalloy compositions. However, the metallurgical origins of this behaviour remain unknown and further studies are required to facilitate our understanding. To address this need, a series of 27 alloys from the Ni-Co-Al-Ti-Cr system have been examined following super-solvus homogenisation heat treatments using neutron diffraction, DSC and SEM. In addition, the phase stability of these compositions following exposures at 800˚C for 1000 hours has been determined with regards to the susceptibility of the alloys towards the formation of deleterious phases. Consequently, the effects of varying the Ni:Co, Al:Ti ratios and Cr concentration on the microstructural evolution and hardness of the material were assessed, and thermodynamic modelling was used to complement the results. This work was supported by Rolls-Royce plc and the EPSRC under EP/H022309/1 and EP/H500375/1.

3:40 PM Break

4:00 PM  
Thermodynamics and Kinetics of L12-containing Co-base Superalloys from First-Principles: Robert Rhein1; Tresa Pollock1; Anton Van der Ven1; 1University of California Santa Barbara
    L12-containing Co-base superalloys have been shown to have high-temperature creep properties that are comparable to second generation Ni-base superalloys. Thermomechanical stability of the L12 precipitate phase is critical in the design of these alloys. Electronic structure calculations have been used as the foundation for the construction of a finite temperature thermodynamic description of the Co-Al- W system from first principles. A cluster expansion technique has been used to fit the formation energies of several unique configurations of fcc, bcc, and hcp Co-Al- W structures. Grand canonical Monte Carlo simulations have been performed on these cluster expansions in order to assess the effects of configurational entropy at finite temperature. Vibrational entropy of selected structures has also been calculated. Free energy surfaces incorporating these entropic effects have been constructed and the implications for high-temperature phase stability and kinetics will be discussed.

4:20 PM  
Thermodynamic Database for the Co-Al-W-Ni-Ti-Ta-Cr Superalloy System: Peisheng Wang1; Wei Xiong2; Oleg Kontsevoi1; Ursula Kattner3; Carelyn Campbell3; Eric Lass3; Gregory Olson1; 1Northwestern University; 2University of Pittsburgh; 3National Institute of Standards and Technology
    Co-base alloys attract attention due to their high solvus temperature, superior hot corrosion resistance, better weldability and thermal fatigue resistance compared to Ni-based superalloys. Limiting the application of traditional Co-based alloys is the low strength at high temperatures. In our previous work, we performed a comprehensive CALPHAD assessment of the Co-Al-W system. The ternary gamma’ phase was described as a metastable phase utilizing results from density functional theory (DFT) calculations. To stabilize the gamma’ phase and improve the properties of the Co-Al-W based superalloys, additional components (Ni, Ti, Ta, Cr) are added. In the present work, a thermodynamic database for the Co-Al-W-Ni-Ti-Ta-Cr superalloy system is constructed, aiming to include all binaries, key ternary (including but not limited to Co-Al-X, Co-W-X), and important quaternary systems. Comparisons between the calculated results and experimental data for various multi-component Co-Al-W-X based superalloys are made demonstrating the fidelity of the new Co superalloy database.

4:40 PM  
Calphad Design of Co-based Gamma-prime-strengthened Superalloys: Eric Lass1; 1National Institute of Standards and Technology
    This work explores the effectiveness of current Calphad-based tools in describing thermodynamic and kinetic behavior of Co-based γ’-strengthened superalloys, including phase equilibria, diffusion and microstructural evolution, and various thermophysical properties such as molar volume and thermal expansion. Compositions with desirable properties are identified and tested experimentally to evaluate the predictive capabilities of the present generation of Calphad databases. Current thermodynamic databases of multicomponent Co-based alloys demonstrate a reasonable ability to predict composition regions where deleterious phases may not form. However, shortcomings arise in properly identifying the γ’ solvus temperatures and volume fractions, as well as solidus and liquidus temperatures. Current efforts to build the next generation of Calphad-based thermodynamic and atomic mobility modeling tools are discussed; along with incorporating essential thermophysical properties, including temperature-dependent molar volume and elastic property descriptions.