Gamma (FCC)/Gamma-Prime (L12) Co-Based Superalloys II: Mechanical Behavior II
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

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

Session Chair: Qiang Feng, University of Science and Technology Beijing; Eric Lass, NIST

8:30 AM  Invited
Creep Behavior in a γ′ Strengthened Co-Al-W-base Single Crystal Superalloys: Song Lu1; Haijing Zhou1; Fei Xue1; Wendao Li1; William Yi Wang2; Zi-Kui Liu3; Qiang Feng1; 1University of Science & Technology Beijing; 2Northwestern Polytechnical University; 3The Pennsylvania State University
    The discovery of a stable γ/γ′ two phase field in the ternary Co-Al-W system at 900 ºC opened new pathways for developing novel cobalt-based superalloys. However, the lack of basic creep data and fundamental understanding of creep mechanisms consequently restrict the engineering applications of this class of superalloys. The investigations on creep behaviors of a novel Co-Al-W-base single crystal superalloy were carried out at 900 ºC and 1000 ºC. The γ/γ′ microstructural evolution and sub-microstructure, including dislocations, stacking faults and anti-phase boundaries (APB) have been identified at different creep stages. In terms of bonding charge density and electron work function, the bonding structures and strengths of the stacking faults and the anti-phase boundaries (APB) of Co3(Al, W) have been investigated by first-principles calculations. Theoretical calculation and experimental results are integrated and analyzed, and they will be helpful for fundamental understanding of creep mechanisms in γ/γ′ two-phase superalloys.

9:00 AM  
Dislocation Interactions during High-temperature Creep and Yield of Polycrystalline Co-Ni-Al-W-based Superalloys and L12 γ’ Phases: Vassili Vorontsov1; Caroline Taylor1; Henry Chan1; Paul Mulvey1; Mark Hardy2; David Dye1; 1Imperial College London; 2Rolls-Royce plc
    Using conventional and high-resolution TEM/STEM, we investigate the active plastic deformation mechanisms in polycrystalline Co-Ni-Al-W superalloys during creep and yield at elevated temperatures. The effects of chemical composition and material processing on the factors which define the mechanical properties are considered in detail. In addition to the microstructural factors, the compositional effects on the energetics of planar defects, such as antiphase boundaries and stacking faults, are also evaluated. This includes both the contribution from the γ’ forming elements as well as the role of the cobalt to nickel ratio. Our study considers prototype multi-component systems, as well as ternary and quaternary model alloys and isolated L12 γ’ phases. Finally, we discuss the implications of our findings for the development of mathematical models for prediction and optimisation of high-temperature mechanical properties.

9:20 AM  
Double Minimum Creep of a Ta-containing Single Crystal Co-base Superalloy: Fei Xue1; Christopher Zenk1; Steffen Neumeier1; Mathias Göken1; 1Friedrich-Alexander-Universität Erlangen-Nürnberg
    Tantalum is one of the most effective alloying elements to improve the mechanical properties of γ′-hardened Co-base superalloys. To further study how Ta affects the lattice misfit and creep behavior, high temperature neutron diffraction experiments and creep tests at 950°C were performed on a single crystalline Co-Al-W-Ta alloy. The diffraction experiments show that a positive misfit with large magnitude exists even at 950°C, leading to pronounced rafting during creep. The creep curves feature two creep rate minima and interrupted creep tests at different strains were conducted to study the microstructural evolution and deformation mechanisms. It is shown that rafting starts before the first creep minimum, dislocations cut the rafts and elements segregate to the stacking faults with ongoing creep deformation. The creep performance of several Co-Al-W-Ta alloys with different chemical compositions from the γ-γ′ tie line was additionally investigated to understand the connection between γ′ volume fraction and creep behavior.

9:40 AM  
The Influence of Cr in γ' Strengthened Co-base Superalloys: Christopher Zenk1; Ivan Povstugar2; Steffen Neumeier1; Mathias Göken1; 1FAU Erlangen-Nürnberg; 2MPIE Düsseldorf
    It will be shown that the addition of Cr to Co-Al-W-alloys strongly affects the γ′ morphology and increases the γ′ volume fraction. APT shows that Cr partitions to the γ phase and Neutron diffraction reveals that this leads to a decrease of the γ/γ′ lattice misfit. The higher γ′ volume fraction, however, does not improve the mechanical properties. On the contrary, Cr additions reduce the creep strength. This can be partly explained by a change of the other elements’ partitioning behaviour. Additionally, a ternary low density γ′ strengthened Co-Ti-Cr alloy will be presented. The lattice misfit is sufficiently low to form a regular γ/γ′ microstructure. The γ′ volume fraction is above 60 % and the solvus temperature above 1100°C. The yield strength shows a distinct increase above 600°C and surpasses the one of a Co-Al-W-alloy.

10:00 AM Break

10:20 AM  
Analyzing the Tension/Compression Asymmetry in Creep Deformed Single Crystal Co-base Superalloys: Malte Lenz1; Yolita Eggeler1; Christopher Zenk1; Steffen Neumeier1; Mathias Göken1; Philip Wollgramm1; Gunther Eggeler2; Erdmann Spiecker1; 1FAU Erlangen-Nürnberg; 2Ruhr-Uni Bochum
     In the present work a new Co-based superalloy (ERBOCo-1) was creep deformed in tension and compression under a stress of 400 MPa along the [001]-direction at 850 °C. Tests were interrupted at 0.3 % and 5 % plastic strain and show a pronounced tension/compression asymmetry regarding the total creep times and the minimal creep rates. To obtain a better understanding of the underlying fundamental mechanisms CTEM, HRTEM and LACBED investigations have been carried out. In the early stages of creep, EM analysis reveals different predominant deformation mechanisms for tensile and compressive testing. In tension a planar fault configuration consisting of a superlattice intrinsic stacking fault enclosed by anti-phase boundaries is formed by glide of two loosely coupled superpartial dislocations.In contrast, compressively deformed samples exhibit pronounced channel dislocation networks and SF in γ’-precipitates and occasionally γ-channels, indicating glide of single partials. I acknowledge DFG&CENEM for financial support.

10:40 AM  
The Grain Boundary Pinning Effect of the μ-phase in Polycrystalline L12 Hardened Co-base Superalloys: Lisa Freund1; Steffen Neumeier1; Mathias Göken1; 1Friedrich-Alexander-Universität Erlangen-Nürnberg
    In polycrystalline Ni-base superalloys, the pinning of grain boundaries by precipitates during recrystallization is used to obtain a homogeneous grain structure with fine grains. Such a microstructure provides good low cycle fatigue properties and high temperature strength. In order to use this effect for γ/γʹ Co-base superalloys, a polycrystalline cast-and-wrought alloy was produced, containing a significant amount of Mo to intentionally introduce a grain boundary pinning third phase. Diffraction results and TEM-EDS identified this phase as μ-phase (Co,Ni,Cr)7(W,Mo)6. Since the μ-phase and the γʹ-phase are competing for W, a lower γ’-volume fraction is formed in comparison to Mo-free alloys. In-situ high energy X-ray diffraction experiments confirm the stability of the grain structure up to 1050°C and therefore the grain boundary pinning effect of this phase. The homogeneous distribution of small grains results in a higher strength compared to a polycrystalline γ/γʹ Co-base superalloy without grain boundary pinning phases.

11:00 AM  
Solute-vacancy Binding Energies and Diffusion Rates in fcc Cobalt: A First-principles Database: Shahab Naghavi1; Vinay Hegde1; Chris Wolverton1; 1Northwestern University
    Understanding solute-vacancy interactions and diffusion of solutes is crucial for the design of novel cobalt-based superalloys. Despite their importance, relatively little experimental and theoretical data is available for the above properties. Here, we present accurate first-principles calculations of solute-vacancy (SV) binding energies and diffusion rates of various solutes in fcc cobalt. Using density functional theory (DFT), we compile a database of SV and diffusivities of all 3d, 4d and 5d transition metals in fcc cobalt. Our calculations reveal a systematic relationship between solute-vacancy binding energies and solute diffusion with d-occupancy of the solute, explained by the d-band model. Both the properties are the largest for elements at the far-right and far-left of the d-block of transition metals, and decrease toward the middle of the series. We also find that magnetism has a profound effect on the diffusion rates, whenever applicable.

11:20 AM  
Influence of Replacement of Ta by Nb in a γ/γʹ- structure Co Base Superalloys: Alex Costa1; Marcus Salgado2; Eder Lopes3; Carlos Nunes2; Andre Tschiptschin4; 1LNNano-CNPEM; 2The Engineering School of Lorena (EEL-USP); 3Faculty of Mechanical Engineering of University of Campinas; 4Metallurgical and Materials Department of University of Sao Paulo
    The microstructural effects of replacement of Ta by Nb in Co-40Ni-3Ta superalloy with γ/γʹ structure was studied. In the as-cast condition, the Co-40Ni-3Nb alloy showed a dendritic γ-Co grain structure. Inside the γ-Co grains fine γʹ spheroidal precipitates with 15 to 20 nm radius, were identified. Primary precipitates of µ - Co7W6 phase and a Chinese script γ+NbC eutectic structure were found in the interdendritic regions. The alloy was heat-treated by two steps (1250 ºC/ 72 h and 1000 ºC/ 168 h) that resulted in the precipitation of µ - Co7W6, with different morphologies and sizes, embedded in the γ/γʹ structure. The Nb modified alloy showed greater microstructural heterogeneities. Coarsening of the µ - Co7W6 was also observed when ageing was performed at 1250 ºC.

11:40 AM Concluding Comments