Gamma (FCC)/Gamma-Prime (L12) Co-Based Superalloys II: Mechanical Behavior I
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 2:00 PM
March 1, 2017
Room: Pacific 14
Location: Marriott Marquis Hotel

Session Chair: Michael Titus, Purdue University; David Dunand, Northwestern University

2:00 PM  Invited
Mechanical Properties of Co-based Superalloys with FCC+L12 Two-phase Microstructures: Haruyuki Inui1; Norihiko Okamoto1; 1Kyoto University
    The creep properties of ‘Co-base superalloys’ consisting of FCC solid-solution based on Co and L12-(Co3(Al,W) cuboidal precipitates have recently been reported to be not so high, being only comparable to that of the Ni-base superalloy of the 1 st generation. There may be many factors for this such as the low γ’-solvus temperature and the low CSF (complex stacking fault) energy of the γ’ phase, both of which are related to the phase stability of the L12 phase. We believe that the insufficient high-temperature mechanical properties of Co-based superalloys are due to the low strength of the constituent L12 compound at high temperatures, which comes from the very low CSF energy, as is evident from the four-fold dissociation of <110> dislocations. We will discuss some strategies for strengthening Co-based alloys with L12 cuboidal precipitates in the lights of our experimental findings.

2:30 PM  Invited
Mechanical Behavior of Polycrystalline (L12)gamma-prime-strengthened Co-base Superalloys: Peter Bocchini1; Daniel Sauza1; James Coakley1; Qinyuan Liu1; David Seidman2; David Dunand1; 1Northwestern University; 2Northwestern University Center for Atom Probe Tomography (NUCAPT)
    Conventional Ni-base and emerging Co-base superalloys are both characterized by large volume fractions of (L12) gamma-prime precipitates embedded in a (f.c.c.) gamma matrix. This unique microstructure confers excellent creep strength as well as the so-called flow-stress anomaly, by which the yield strength increases with temperature, exhibiting a maximum between 700-825 °C. However, polycrystalline Co-Al-W suffers from intergranular cracking, and the anomalous flow stress response of (L12) Co3(Al, W) is reduced compared to (L12) Ni3Al. We present an overview of our work improving these properties through alloying additions: B improves the creep strength of Co-Al-W when in sufficient amounts to promote W-borides, and the creep strength of ‘W-free’ Co-Ni-Al-Mo-Nb is comparable to ternary Co-Al-W with reduced density. Ti and Ni additions dramatically improve the anomalous flow stress response, and the creep strength of Co-5.6Al-5.8W-6.6Ti-0.12B at.% is comparable to Co-9Al-9W-0.12B at.% despite lower precipitate volume fraction and lack of borides along GB.

3:00 PM  
Planar Defect Formation in the γ' Phase during High Temperature Creep in Single Crystal CoNi-base Superalloys: Yolita Eggeler1; Julian Müller1; Mike Titus2; Akane Suzuki3; Tresa Pollock4; Erdmann Spiecker1; 1Friedrich Alexander Universität Erlangen-Nürnberg; 2Purdue Universtity; 3GE Global Research Center; 4University of California Santa Barbara
    The structure and formation mechanisms of extended planar defects in the γ/γꞌ microstructure of tensile creep deformed CoNi-base single crystal superalloys have been analyzed in detail using complementary electron microscopy methods. A total crystallographic slip of 1/2[-1-12] leaves superlattice intrinsic stacking faults (SISF) embedded in antiphase boundaries (APB), which are separated by a 1/6[-1-12] dislocation loop contained inside of the γꞌ precipitates. A characteristic APB/SISF/APB configuration along contiguous γꞌ precipitates remains after dislocation slip. Energy dispersive X-ray spectroscopy (EDXS) provides evidence for pronounced segregation at both types of planar defects. This newly-identified deformation mechanism emphasizes the important role that planar defect energies and solute segregation have in determining the active deformation mode. Methods for characterizing this complex deformation mode will be presented, and implications for alloy design will be discussed.

3:20 PM  
Load Transfer between Phases during Deformation of Superalloys: James Coakley1; Eric Lass2; David Seidman3; Howard Stone1; David Dunand3; 1University of Cambridge; 2National Institute of Standards and Technology; 3Northwestern University
    In-situ neutron diffraction studies are performed to characterize the micromechanical deformation occurring during tensile creep of single-crystal nickel-based superalloys with negative lattice parameter misfits and cobalt-based superalloys with positive lattice parameter misfits. The loading responses of the matrix gamma phase and the precipitate gamma-prime phase are distinct, and the creep deformation of rafted microstructures is compared to non-rafted microstructures in both cobalt-based and nickel-based superalloys.

3:40 PM Break

4:00 PM  Invited
Deformation Microstructures of L12 Ordered Intermetallic Phases in Ni-, Co- and Co-Ni-base Superalloys: Duchao Lv1; Robert Rhein2; Michael Titus2; Tresa Pollock2; Yunzhi Wang1; 1The Ohio State University; 2University of California, Santa Barbara
    The gamma’ intermetallic phase (L12, cubic) is the primary strengthening phase in nickel-base, cobalt–base and cobalt-nickel-based superalloys. In this study, the deformation mechanisms in these three alloys are investigated at the elementary defect level by using a combination of ab initio calculations and phase field simulations. In particular, the composition-dependent generalized-stacking-fault energy surfaces of the gamma and gamma’ phases, as determined from ab initio calculations, and available experimental data are used in the phase field simulations. Sophisticated deformation pathways leading to various planar defects including antiphase boundary + superlattice intrinsic stacking fault and stacking fault ribbon configurations are predicted as a function of the alloy composition. The predicted stacking fault configurations are consistent with recent TEM observations. The approach can be used to study plastic deformation of other intermetallic phases, and the detailed deformation mechanisms uncovered could be incorporated in constitutive microstructure-property relationships in advanced crystal plasticity modeling.

4:30 PM  
Superlattice Intrinsic Stacking Fault Energies and Solute Segregation to Planar Defects in Co-based Superalloys: Michael Titus1; Robert Rhein2; Alessandro Mottura3; Min-Hua Chen2; Anton Van der Ven2; Tresa Pollock2; 1Purdue University; 2University of California Santa Barbara; 3University of Birmingham
    Co-based superalloys strengthened by the γ’-(L12) phase exhibit comparable and, in some cases, superior high temperature creep resistance to 1st-generation Ni-based superalloys. Despite the comparable creep resistance between Co- and Ni-based superalloys, the high temperature creep deformation modes are markedly different: the γ’ phase in Ni-based superalloys is typically sheared via coupled a/2<110> matrix dislocations, whereas the γ’ phase in Co-based superalloys is sheared via Shockley superpartial a/3<112> dislocations, which leave superlattice intrinsic stacking faults (SISF) behind in their wake. We have calculated the SISF energies over a broad range of compositions via density functional theory (DFT), including both Co3(Al,W)- and Co3(Al,Mo)- based alloys. Incorporating vibrational free energy, we have calculated the temperature-dependent SISF energies. The SISF energies will be linked to the observed solute segregation behavior at SISFs, and implications for improving high temperature Co-based superalloy strength will be discussed.

4:50 PM  
Solid Solution Strengthening of Co3(Al, TM) L12 Phase: An Integrated First-principles Calculations and Experimental Study: William Yi Wang1; Bin Gan1; Fei Xue2; Shun-Li Shang3; Yi Wang3; HongChao Kou1; JinShan Li1; Xi-Dong Hui2; Qiang Feng2; Zi-Kui Liu3; 1Northwestern Polytechnical University; 2University of Science and Technology Beijing; 3The Pennsylvania State University
    In this work, the solutes strengthening of Co3(Al, TM) are investigated by the integrated first-principles calculations and experimental study. Here, the solute atoms (TM) include Hf, Mo, Re, Ru, Ta, Ti, V, W and Y. In terms of bonding charge density and electron work function, the bond structures and strengths of the Co3(Al, TM) phase and its solute-containing (001) anti-phase boundary (APB) reveal the complex electron environment induced by the variation of the lattice distortion around the fault layers. It is understood that with the segregation of TM at (001) APB, the bond strength around the fault layers are improved by the electron redistribution. The improved local bonding strength, contributing to an enhanced tensile strength and hardness, is further validated by the instrumented nanoindentation. This work reveals the electronic and the atomic basis for the solid solution strengthening mechanism in Co3(Al, TM) L12 Phase.

5:10 PM  
Multi-scale Modelling of High-temperature Deformation Mechanisms in Co-Al-W-based Superalloys: Hikmatyar Hasan1; David Dye1; Peter Haynes1; Vassili Vorontsov1; 1Imperial College London
    The chemical ordering present in the γ' intermetallic phase precipitates of Co-Al-W-based superalloys gives rise to complex dislocation configurations. These can feature a variety of possible planar fault structures, which have associated surface energies. In order to accurately model this complexity, we have calculated Gamma-surfaces for Co-Al-W superalloys using the Density Functional Theory, as implemented in CASTEP. Also known as Generalised Stacking Fault energies, these 2D energy surfaces describe the energy cost of associated local atomic displacements at the dislocation core. These ab initio data were incorporated into a Phase Field Dislocation Dynamics model to investigate the meso-scale interactions of the dislocations with the microstructure of the alloys over a range of loading conditions. The phase field approach has also been extended to investigate the effects of solute atom segregation to the site of the stacking faults during high-temperature creep and the resulting influence on the deformation resistance.