Gamma (FCC)/Gamma-Prime (L12) Co-Based Superalloys II: Microstructural Evolution
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 8:30 AM
February 28, 2017
Room: Pacific 14
Location: Marriott Marquis Hotel

Session Chair: Eric Lass, NIST; TBD TBD, TBD

8:30 AM Introductory Comments

8:35 AM  Keynote
Coarsening Kinetics and Elemental Partitioning of (f.c.c.) Gamma Plus (L12) Gamma-prime-strengthened Co-base Superalloys: Daniel Sauza1; Peter Bocchini1; James Coakley1; Eric Lass2; David Dunand1; David Seidman3; 1Northwestern University; 2National Institute of Standards and Technology (NIST); 3Northwestern University Center for Atom Probe Tomography (NUCAPT)
    The emergence of Co-base superalloys with a microstructure consisting of (L12) gamma-prime precipitates in a (f.c.c.) gamma matrix presents an exciting opportunity for the development of a new class of high-temperature structural alloys. Optimized Ni-base superalloys, which contain an analogous microstructure, may contain 10-12 elemental additions, necessitating a thorough understanding of alloying effects on the Co-Al-W system. 3D picosecond ultraviolet laser-assisted local-electrode atom-probe (LEAP) tomography and scanning electron microscopy are utilized to investigate the temporal evolution and elemental partitioning behavior of several model Co-Al-W-X alloys. The coarsening kinetics of the (L12) gamma-prime precipitates are determined via multivariate regression analyses from the experimental measurements, and the resulting temporal exponents show good agreement with the predictions of Lifshitz-Slyozov-Wagner and Philippe-Voorhees models. LEAP tomography is utilized to investigate the elemental partitioning behavior of a Co-30Ni-7Al-2W-3Mo-2Nb-1Ta at.% alloy with a high (L12) gamma-prime solvus temperature (1227 °C) and small W-content for reduced bulk density.

9:15 AM  Invited
On the Role of the Base Elements Co and Ni in γ'-hardened Superalloys: Steffen Neumeier1; Christopher Zenk2; Nicklas Volz2; Timur Halvaci2; Mathias Göken2; 1Friedrich-Alexander-Universität Erlangen-Nürnberg ; 2Friedrich-Alexander-Universität Erlangen-Nürnberg
    γ/γ'-Co-base superalloys share many similarities with Ni-base superalloys, but significant differences exist. To illustrate how alloy properties change, when Co gradually substitutes Ni, two series of Ni-Co-Al-W-(Cr) alloys with and without 8 at.% Cr and otherwise constant contents of Al and W were investigated with various techniques, such as electron and atom probe microscopy and neutron diffraction. The segregation tendency of elements on the dendrite scale increases with increasing Ni-content. All alloys form a γ/γ'-microstructure. The change of the partitioning behavior of elements and the change of the lattice misfit from negative on the Ni to positive values on the Co-side affects the γ'-morphology. The γ'-solvus temperature is strongly decreasing, but the γ'-fraction is slightly increasing towards the Co-side. This indicates that γ/γ'-Co-base superalloys have a great potential as wrought alloys. The creep strength changes significantly with varying Co/Ni content, however the tendency depends strongly on the presence of Cr.

9:45 AM  
Properties of γ'-phase in L12-precipitation Hardened Co-base Alloys with Different W-content: Yuzhi Li1; Uwe Lorenz1; Steffen Neumeier2; Andreas Schreyer3; Andreas Stark1; Li Wang1; Florian Pyczak1; 1Helmholtz-Zentrum-Geesthacht; 2Friedrich-Alexander Universität Erlangen-Nürnberg; 3European Spallation Source ERIC
    While Co-Al-W alloys hardened by a L12-type γ'-phase show microstructural similarities to Ni-base superalloys, they differ in many significant details. For a knowledge based development of alloys derived from the Co-Al-W alloy system, it is necessary to investigate and understand these unique features especially of the γ'-hardening phase. Here results about a range of properties in Co-Al-W alloys with different W contents are shown with an emphasis on the long term stability of the γ'-phase, the lattice mismatch, creep properties and plastic deformation mechanisms during creep. It is found that the γ'-phase is metastable and transforms to D019 Co3W after extended annealing. The lattice mismatch is positive up to the γ'-solvus temperature, which influences the development of the microstructure during high temperature deformation. During creep cutting of the γ'-precipitates by partial dislocations generating extended planar faults is a common feature already in the early stages of creep.

10:05 AM Break

10:25 AM  Invited
Structural Stability of L12 and TCP Phases in Co-based Superalloys: Thomas Hammerschmidt1; Arthur Bialon1; Jörg Koßmann1; Ralf Drautz1; 1ICAMS, Ruhr-Universität Bochum
    The further development of Co-based superalloys requires to optimise the γ/γ' microstructure and to suppress the precipitation of topologically close-packed (TCP) phases. Both optimisation targets depend directly on the local chemical composition. In order to gain an understanding of the formation of L12 and TCP phases, we apply structure maps that relate the structural stability of a compound to its chemical composition. The alloying elements enter in terms of the number of valence electrons, the atomic volume and the electro-negativity. By combination with casting experiments and microstructure analysis, we demonstrate that a structure map for intermetallic phases correctly predicts TCP-phase precipitates from experimentally determined local chemical compositions in Co-based superalloys. From the stability window of the L12 phase in a structure map for sp-d valent compounds we can determine modifications of the chemical composition of Co3(Al,W) that are likely to form a γ/γ' microstructure.

10:55 AM  
Elemental Partitioning Behaviour in Ni-Co-Al-Ti-Cr Alloys : Sioned Llewelyn1; Katerina Christofidou1; Vicente Araullo-Peters2; Nick Jones1; Emmanuelle Marquis2; Mark Hardy3; Howard Stone1; 1University of Cambridge; 2University of Michigan; 3Rolls-Royce plc
    The relative concentrations of alloying elements in Ni-based superalloys have a marked effect on elemental partitioning behaviour within the principally γ-γ' microstructure, thus determining the properties conferred by the alloying additions. A recently developed class of high Co-Ti superalloys, exhibiting superior high temperature strength over conventional superalloys, contradicts established theory regarding the effects of alloying with Co. An improved understanding of the phase equilibria in such alloys is critical if optimised compositions are to be designed. This study uses three-dimensional atom-probe tomography to examine the effects of varying the Ni:Co ratio on elemental partitioning in alloys based on the Ni-Co-Al-Ti-Cr system at 800°C. In addition, phase compositions are correlated with those obtained using thermodynamic modelling to determine the extent to which computational tools can reliably guide future alloy design. This work was supported by Rolls-Royce plc, the EPSRC under EP/H022309/1 and EP/M005607/1 and the University of Michigan College of Engineering.

11:15 AM  
Modeling Precipitate Coarsening in Cobalt-based Superalloys: Andrea Jokisaari1; Shahab Naghavi1; Peisheng Wang2; Wei Xiong1; Kil-Won Moon2; Christopher Wolverton1; Ursula Kattner2; Careyln Campbell2; Peter Voorhees1; Olle Heinonen3; 1Northwestern University; 2National Institute of Standards and Technology; 3Argonne National Laboratory
    Given the novel nature of γ/γ' cobalt-based superalloys, extensive research is underway to understand their microstructural evolution under service conditions. Advances in numerical frameworks, greater computational resources, and a strong collaboration with other researchers allow us to study precipitate evolution in this novel material system. Here, we present a mesoscale model of γ' precipitate coarsening in cobalt-based superalloys that is informed by atomistic and experimental data. The effect of elastic stiffnesses of the γ and γ' phases, γ/γ' misfit strain, interfacial energy, applied stress, and CALPHAD-based thermodynamic and kinetic information are incorporated to predict microstructural evolution. Three-dimensional simulations were performed to study the effect of temperature, particle size, applied stress, and spatial distribution on precipitate morphology and spacing. This approach should help shorten the development cycle of new γ/γ' cobalt-based superalloys by predicting microstructure evolution before performing time-consuming experimental testing of new compositions.

11:35 AM  
Gammaprime Precipitation in Model CoAlW Alloys: Ahmad Azzam1; Frederic Danoix1; Annie Hauet1; Didier Locq2; Pierre Caron2; Didier Blavette1; 1Normandy Université - CNRS; 2Onera
    Cobalt-based superalloys are promising materials for high temperature applications, especially for advanced turbine engines. They derive their potential from a high volume fraction of ordered Co3(AlW) L12 gammaprime precipitates embedded in a disordered face centered cubic (FCC) matrix. Understanding the formation of gammaprime precipitates from supersaturated matrix is therefore critical for controlling and tailoring the final microstructures. The aim of this work is to study the transformation paths in model ternary CoAlW alloys at the sub nanometer scale using Atom Probe Tomography (APT) and Transmission and Scanning Electron Microscopies (TEM, SEM). We will first focus on characterizing the nano gammaprime particles already present in the as quenched condition. The temporal evolution of average size of the γ’ precipitates (growth and coarsening) as a function of aging time at 900°C will then be studied experimentally with particular emphasis on phase compositions, in order to assess the currently available phase diagrams.