Advancing Current and State-of-the-Art Application of Ni- and Co-based Superalloys: Co-based Superalloys – Structure & Properties
Sponsored by: TMS Structural Materials Division, TMS: High Temperature Alloys Committee, TMS: Corrosion and Environmental Effects Committee
Program Organizers: Chantal Sudbrack, National Energy Technology Laboratory; Mario Bochiechio, Pratt & Whitney; Kevin Bockenstedt, ATI Specialty Materials; Katerina Christofidou, University of Sheffield; James Coakley, Chromalloy; Martin Detrois, National Energy Technology Laboratory; Laura Dial, Ge Research; Bij-Na Kim; Victoria Miller, University of Florida; Kinga Unocic, Oak Ridge National Laboratory
Wednesday 8:30 AM
February 26, 2020
Room: 11B
Location: San Diego Convention Ctr
Session Chair: Kevin Bockenstedt, ATI Specialty Materials; Katerina Christofidou, University of Sheffield
8:30 AM Invited
Design of γ’-strengthened Co/Ni-based Superalloys: Eric Lass1; 1University of Tennessee, Knoxville
Co-based alloys strengthened by coherent L12-γ’ precipitates have been extensively studied in recent years as potential alternatives to Ni-based superalloys in high temperature applications. Properties such as increased melting temperature, decreased segregation during solidification, and improved environmental resistance offer potential benefits over Ni-based alloys in some applications. The significant research efforts have produced a significant amount of thermodynamic and phase equilibria data for Co-based alloy systems where little was previously known; which has allowed the development new of Calphad thermodynamic databases focused on Co-based alloys. This work applies computational thermodynamics to design new Co-based γ’-strengthened alloys with improved properties specific to particular applications such as single-crystals, cast and wrought alloys, or additive manufacturing. Microstructure and properties of a recent compositionally-optimized Co/Ni-based alloy are presented, and demonstrate an extremely promising combination of high creep resistance and oxidation resistance.
9:00 AM
Advancing Characterization of Co/Ni-based Superalloys with Statistics using Correlative Electron Diffraction and X-ray Spectroscopy: Tom McAuliffe1; Alex Foden1; Chris Bilsland1; Dafni Daskalaki Mountanou1; David Dye1; Thomas Britton1; 1Imperial College London
Understanding carbides in Co/Ni-based superalloys is important for high temperature creep and oxidation of next generation jet engines. We develop a new approach to classify carbide types in Co-based superalloys using maps which contain simultaneous energy dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) data that has been captured in the SEM. These signals are combined and analysed using principal component analysis (PCA) followed by rotation of the components using multivariant statistical analysis to make physical analysis of each label more meaningful. Each characteristic signal labels a region of common chemistry (EDS) and structure (EBSD) and amplifies signal to noise. We use this to clearly separate FCC Co/Ni-matrix, M23C6, M2C, M6C and MC type structures in the Co/Ni-based alloys. Ultimately, our robust characterisation of these phases will help us understand the role of each carbide type in the high temperature performance of these materials and enable alloy design.
9:20 AM
Grain Boundary Chemistry and Mechanical Properties of a Multicomponent Co-based L12–ordered Intermetallic Alloy: Francesca Long1; Sung II Baik1; Ding Wen Chung1; Fei Xue1; Eric Lass2; David Seidman1; David Dunand1; 1Northwestern University; 2The University of Tennessee
The chemistry, thermodynamics and mechanical properties of the L12-ordered Co3(Al,W) γ′ phase are crucial to the understanding of γ(fcc)/γ′(L12) cobalt-base superalloys. A single-phase γ′ alloy with composition Co-30Ni-11Al-5.5W-4Ti-2.5Ta-0.10B (at.%) and γ′ solvus temperature of 1268°C was recently identified using a Calphad-methodology. Scanning and transmission electron microscopy reveal that the single-phase microstructure is stable at 900 and 1000°C for 1000 h and at 1100°C for 168 h without other phases observed, resulting in a similar level of microhardness within grain interiors. Atom probe tomography of a grain boundary reveals depletion of Co, Ni, W and Ta and enrichment of Al and B. A marked yield stress anomaly is observed with yield strength increasing from ~300 to ~700 MPa as temperature increases from 20 to 800°C, stronger than L12-Co3(Al,W) and Ni3Al. The creep tests at 850°C show a power-law behavior with a stress exponent and an activation energy similar to single-phase L12-Ni3Al.
9:40 AM
Optimising the γ/γ' Microstructure and Increasing the High Temperature Strength of a Co-base Superalloy: Daniel Hausmann1; Cecilia Solís2; Lisa Freund1; Andre Heinemann2; Mathias Göken1; Ralph Gilles2; Steffen Neumeier1; 1Department Werkstoffwissenschaften WW1; 2Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II)
The newly developed polycrystalline Co-base-superalloy CoWAlloy2 provides a high potential for application as wrought alloy due to the large gap between solidus and γ'-solvus temperature along with a high γ'-volume fraction. The scope of this study was the improvement of the high temperature strength by optimizing the γ/γ'-microstructure and adjusting different annealing steps.The microstructure and mechanical properties were investigated by SEM, compression and hardness tests. In-situ high temperature SANS helped to understand the microstructural evolution during heat treatment. The size of γ'-particles increases with increasing annealing time and temperature of the first annealing step. As a result, the hardness of the alloy increases until a maximum after 4 h annealing is reached. The reason is an optimum γ'-particle size, which can be explained by the weak and strong pair-coupling model of dislocations. A second annealing step leads to a further increase of yield strength due to an increasing γ'-fraction.
10:00 AM Break
10:20 AM Invited
From Co-Al-W-alloys to Advanced CoNi-base Superalloys: Steffen Neumeier1; Mathias Göken1; 1University of Erlangen-Nürnberg
The discovery of a Co3(Al,W) phase in the Co-Al-W system more than a decade ago stimulated worldwide research on γ/γ' Co-base superalloys. However, the initial Co-Al-W-based alloys suffered from a poor oxidation resistance, low phase stability and high density. In this talk, an overview about the investigations on the effect of elements on the alloy system and the interdiffusion coefficients of transition metals in Co are presented. It is shown, how alloying elements affect the thermo-physical properties, microstructures, mechanical properties, oxidation resistance and diffusion, and how this resulted in the development of advanced γ/γ' CoNi-base superalloys with promising properties. The differences and similarities as well as advantages and disadvantages of these novel Co-base superalloys are compared to the established Ni-base superalloys and their future prospects are discussed.
10:50 AM
Very High Cycle Fatigue of a Polycrystalline Co-base Superalloy: Alice Cervellon1; Sean Murray1; Chris Torbet1; Tresa Pollock1; 1University of California Santa Barbara
Co-base superalloys have been extensively studied over the past decade, since γ/γ' microstructures with high volume fractions of the L12 phase were demonstrated in ternary Co-Al-W alloys. More complex compositions based on this system are promising for applications in turbine engines, as Co-base superalloys exhibit a higher melting temperature compared to Ni-base alloys. The high portion of Co also results in lower stacking fault energies, which directly affect the deformation mechanisms. The influence of stacking fault energy on creep deformation has been studied, but its influence on other properties such as very high cycle fatigue (VHCF) is not understood. The VHCF properties of the wrought Co-base superalloy CoNi-SB10+ were then studied in this work at ambient temperature using an ultrasonic fatigue machine. The crack initiation stage and the resultant fatigue life have been characterized and compared with Ni-base superalloys. The implication of the deformation mechanisms will be discussed.
11:10 AM
First-principles Study of Displacive-diffusive Phase Transformations during High Temperature Creep: from Ni- to Co-based Superalloys: Dongsheng Wen1; Sae Matsunaga1; Michael Titus1; 1Purdue University
Displacive-diffusive phase transformations that involve complex shearing, reordering, and diffusion mechanisms in the L12-γ’ phase during high temperature creep have recently been observed in Ni-, NiCo-, and Co-based superalloys. These phase transformations have subsequently been correlated with the high temperature mechanical properties of these alloys. While the driving forces in some alloys have been identified and limited experimental evidence for such phase transformations has been gathered, a wholistic understanding of these complex processes has yet to emerge. To better understand these processes, we have calculated the driving force for solute segregation to superlattice intrinsic stacking faults as a function of Co content utilizing first-principles density functional theory calculations. The driving force for solute segregation, which appears to be a precursor for phase transformations, increases as a function of Co content. These results and implications for future alloy design will be discussed.
11:30 AM
Deformation Mechanisms of Co-Cr-W-Ni Alloys at Ambient Temperature: Shaolou Wei1; Sabrina Hernandez1; Matt Bender2; Andy Martinez2; Cemal Tasan1; 1Massachusetts Institute of Technology; 2ATI Flat Rolled Products
Metallic alloys with exceptional capabilities at elevated temperatures have been persistently sought to facilitate the advancement of high-temperature structural components in various applications. With regard to this, Co-Cr-W-Ni alloys have stimulated broad research interest owing to their superior softening resistance together with excellent cold and hot workability. Hence, appreciable effort has been focused on examining and further optimizing their high-temperature performances, yet, leaving behind their deformation behavior at ambient temperature less understood. In the present study, we aim to clarify the room-temperature deformation micro-mechanisms of Co-Cr-W-Ni alloys with the aid of coupled scanning electron microscope (SEM)/electron backscatter diffraction (EBSD) technique. We will systematically address: (i) what kind of deformation modules presents in these alloys? (ii) what is the corresponding defect sub-structure evolution characteristic? and (iii) what micro-event plays the dominant role in damage nucleation?