Superalloys 2021: Tuesday Interactive Session on Disk Alloy Behavior
Program Organizers: Sammy Tin, University of Arizona; Christopher O'Brien, ATI Specialty Materials; Justin Clews, Pratt & Whitney; Jonathan Cormier, ENSMA - Institut Pprime - UPR CNRS 3346; Qiang Feng, University of Science and Technology Beijing; Mark Hardy, Rolls-Royce Plc; John Marcin, Collins Aerospace; Akane Suzuki, GE Aerospace Research
Tuesday 3:10 PM
September 14, 2021
Room: Poster Area
Location: Virtual Event
Is the Carbon Content Really an Issue for the LCF Durability of Forged γ/γ' Ni-based Disk Alloys?: Adele Govaere1; Anne-Laure Rouffié2; Jean-Michel Franchet2; Daniel Galy2; Alexandre Devaux3; Coraline Crozet3; Paraskevas Kontis4; Patrick Villechaise5; Jonathan Cormier5; 1Ensma / Pprime Institute - Safran Tech; 2Safran Tech; 3Aubert & Duval; 4Max Planck Institute; 5Pprime Institute
The Non Metallic Inclusions (NMIs) population of two versions of AD730™ (the standard one and a high carbon doped one) alloy were studied in terms of oxidation and low-cycle fatigue behavior at 450°C and 700°C. Cracking of NMIs was observed in the early stages of oxidation without any applied load, even at intermediate temperature. It is assisted by volume expansion and thermal mismatch between the NMIs and the surrounding g/g′ matrix. It was observed that the cracking took place in the Nb-rich part of the inclusions. However, the presence of pre-cracked inclusions does not have a detrimental effect on the fatigue lifetimes. The mechanical behaviors and mechanisms were investigated. Moreover, addition of carbon may lead to a debit in LCF life depending on the loading direction with respect to the NMIs cluster alignments. The observation of the fracture surfaces showed that no cracks initiated within inclusions in the high carbon content material despite a much higher density of NMIs. The inclusions and grain size distributions, the particles alignment orientations as well as the environment strongly contribute to the crack initiation mechanisms.
High Temperature Dwell Fatigue Crack Growth in Cold-worked and Direct-aged 718Plus ™: Andrew Merrison1; Hangyue Li1; Wei Li2; Paul Bowen1; 1University of Birmingham; 2Rolls-Royce
The high temperature dwell fatigue crack growth behaviors of 30% cold-drawn and direct aged ATI 718PlusTM were studied. This was compared to an annealed and direct aged variant of the same material. This was in order to rationalize the effects of cold work. Tests were conducted at 650°C using different dwell times, in air and in relative vacuum, at R=0.1. It was found that the cold worked and direct aged condition produced slower dwell fatigue crack growth rates in all tests. This has been linked to the larger elongated grain morphology and the dislocation structure retained after cold working. Increased crack tortuosity and roughness-induced crack closure is considered to enhance the cycle-dependent crack growth properties. Superior resistance to environmentally-assisted intergranular failure is related to greater crack tip stress relaxation and more homogeneous grain boundary deformation.
Contribution of Primary γ' Precipitates in the Deformation Creep Mechanisms in the Ni-based Polycrystalline AD730™ Superalloy: Florence Pettinari-Sturmel1; Muriel Hantcherli1; Winnie Vultos1; Cécile Marcelot1; Bénédicte Warot-Fonrose1; Maud Tisseyre1; Joël Douin1; Patrick Villechaise1; Jonathan Cormier1; 1CEMES - Université de Toulouse
TEM characterization of the deformation micromechanisms in the case of AD730TM disk superalloy have been performed in order to identify the relevant parameters controlling its creep behavior at 700 °C under 600 MPa or 850 MPa. The creep behavior has been investigated for different microstructures resulting from different heat treatments: a coarse grain (CG) and a fine grain microstructures (FG). The specific influence of the primary g' precipitates, which are only present in the fine grain microstructure, is of main focus. TEM observations indicate that, in the first stage of the creep deformation, primary g' precipitates may act as dislocation sources. The stability of this phase was confirmed using samples aged at 850 °C for several hundreds of hours. TEM spectroscopy has been used to characterize the local chemical composition after aging. A clear evolution of these primary g' precipitates has been evidenced and a dissolution of the secondary g' precipitates during aging. The presence of these primary g' precipitates induces a strong localization of the deformation. Its detrimental effect on the creep properties in the case of polycrystalline Ni-based superalloy at high temperature may be concluded.
Impact of Coarse γ’ Phase on Recrystallization Modeling in new Ni-based Superalloy M647: Nishimoto Takashi1; Takuma Okajima1; Kenta Yamashita1; Qiaofu Zhang2; Jiadong Gong2; Greg Olson2; 1Daido Steel.Co; 2QuseTek Innovations
M647, a Ni-based superalloy with excellent mechanical properties and good hot deformability, was recently developed for application in airplane engine disks. In airplane engines, fine-grained superalloys are required to improve high-temperature fatigue properties. Methods to control fine grains have been extensively studied, including a refined method of applying the pinning effect of a coarse ã’ phase. However, previous reports focused on the mechanism of nucleation and abnormal grain growth, and reports on modeling to predict grain size are rare, making it difficult to optimize the forging process. The present study proposes a modeling method of M647 recrystallization with a coarse ã’ phase and compares modeling and experimental results. Recrystallization was experimentally observed in M647 with a coarse ã’ phase originating from the grain boundaries of the prior ã phase. Moreover, recrystallization is promoted by heating at a high temperature, applying a high strain, and maintaining the heat for a long duration. Grain growth is restricted by the pinning effect of the coarse ã′ phase. The area fraction of the coarse ã′ phase changed with the heating temperature, and the ã′ grain size increased with heating. Electron backscatter diffraction analysis shows that the kernel average misorientation increased with increasing forging temperature. These trends indicate that the pinning and driving forces of recrystallization fluctuate continuously during forging and reheating. The microstructure is predicted by applying Avrami-type equations, but the accuracy is insufficient because the fluctuation effects are not considered.
Development of a Prediction Model and Process-microstructure-property Database on Forging and Heat Treatment of Superalloy 720Li: Nobufumi Ueshima1; Chuya Aoki2; Toshio Osada3; Satoko Horikoshi4; Akira Yanagida4; Hideyuki Murakami3; Toshiki Ishida2; Yoko Yamabe-Mitarai3; Katsunari Oikawa1; Nobuki Yukawa5; Jun Yanagimoto6; 1Tohoku University; 2Hitachi Metals, LTD.; 3National Institute for Materials Science (NIMS); 4Tokyo Denki University; 5Nagoya University; 6The University of Tokyo
A process-microstructure-property database on forging and heat treatment of superalloy 720Li was established by high precision large-scale 1,500 ton forging simulator and laboratory-scale forging simulator. The database was utilized to determine the parameters of flow stress, microstructure and strength prediction models. The models were integrated to CAE software to predict process-microstructure-property relationships. In the integrated model, the stress, strain and temperature distributions and their temporal development are calculated by using flow stress model and thermophysical properties. The calculated stress, strain and temperature data are inputted into the microstructure model. The microstructure model considers grain growth, recrystallization and precipitation of ã' and calculates the temporal evolution of microstructural features. The strength model considers solution, grain boundary and precipitation strengthening and calculates high-temperature 0.2% tensile proof stress, which is related to creep and low cycle fatigue properties, from the calculated microstructural features. The integrated model successfully predicted the load, microstructure and strength distribution of a prototype forging experiment conducted by the Hitachi Metals 6,000 ton forging machine. The integrated model is a promising tool to design the forging and heat-treatment process of the alloy.
Phase-field Modeling of γ' and γ" Precipitate Size Evolution during Heat Treatment of Ni-base Superalloys: Felix Schleifer1; Michael Fleck1; Markus Holzinger1; Yueh-Yu Lin1; Uwe Glatzel1; 1University of Bayreuth
The goal of an aging heat treatment in Ni-base superalloys is to control the volume fraction and size of the strengthening precipitates. We predict the precipitate size evolution of primary L12 and D022 precipitates in the industrially relevant alloys CMSX 4, René80 and IN718 using a one-dimensional phase-field model with an artificial Gibbs-Thomson driving force considering the shape of non-spherical precipitates. In comparison to the classical theory of precipitate ripening, the presented phase-field model considers all alloying elements, non-isothermal cooling and heating stages and off-equilibrium volume fraction. We implicitly consider elastic effects between precipitate and fcc solid solution matrix by adjusting the mobility parameters and considering size dependent non-spherical D022 precipitate shapes. Literature data reveals the strong predictive potential of this multi-scale method for integrated computational materials engineering. We identify individual aging stages during which precipitate growth is dominated by either ripening, or precipitation from the supersaturated matrix phase.
Role of Non-metallic Inclusions and Twins on the Variability in Fatigue Life in Alloy 718 Nickel Base Superalloy: Damien Texier1; Jean-Charles Stinville2; Marie-Agathe Charpagne2; Zhe Chen2; Valéry Valle3; Patrick Villechaise3; Tresa Pollock2; Jonathan Cormier3; 1Institut Clément Ader - UMR CNRS 5312; 2University of California, Santa Barbara; 3Institut Pprime - UPR CNRS 3346
Non-metallic inclusions (NMIs) and slip bands parallel to and slightly offset from twin boundaries are observed to be preferential sites for fatigue crack nucleation in wrought superalloys. Potential interactions between NMI cracking and slip activity within neighboring grains or at twin boundaries were investigated under monotonic tensile loading (up to 1.3 % total strain) at room temperature. High resolution- and Heaviside-digital image correlation measurements were performed during interrupted tensile loading to identify strain localization, associated slip systems, and damage initiation. Different mechanisms and scenarios were identified: (1) Microplasticity generally starts at twin boundaries even at stresses as low as 70 % of the macroscopic yield strength, (2) Transgranular slip activity intensively develops above the macroscopic yield stress, (3) Intense slip activity develops near and parallel to 21 % of the twin boundaries intercepting NMIs, (4) 7 % of the twin boundaries intercepting NMIs lead to slip-assisted NMI cracking, (5) No transgranular slip activity participates in NMI cracking, (6) The fraction of cracked NMIs progressively increases with the load, and (7) Within the NMIs that initiated cracks, 67 % cracked below 90 % of the macroscopic yield strength without the presence of slip activity in the neighboring grains. While slip assisted-NMI cracking was evidenced in the present study, most NMI cracking is due to strain incompatibility between NMIs and neighboring grains at the high end of the elastic regime without slip interaction.
Effect of Nb Alloying Addition on Local Phase Transformation at Microtwin Boundaries in Nickel Based Superalloys: Ashton Egan1; You Rao1; Babu Viswanathan1; Timothy Smith2; Maryam Ghazisaeidi1; Sammy Tin3; Michael Mills1; 1Ohio State University; 2NASA Glenn Research Center; 3University of Arizona
This work investigates two nominally similar polycrystalline alloys, with a subtle difference in Nb content, intended to elucidate its effect on local phase transformation strengthening during high temperature creep. Tests were conducted at 750 °C and 600 MPa to target the creep regime dominated by superlattice intrinsic and extrinsic stacking faults, as well as microtwinning. Alloy A, with higher Nb and lower Al, was found to be superior in creep strength to Alloy B, with lower Nb and higher Al, as well as previously investigated ME3 and LSHR. Atomic resolution scanning transmission electron microscopy and energy dispersive spectroscopy found that this increased creep strength was due to a novel local phase transformation occurring along microtwin boundary interfaces as a result of the Nb increase. Complementary density functional theory calculations helped to confirm that this was ÷ phase formation. It is hypothesized that this transformation was the cause of the increased creep strength exhibited by Alloy A.
Atomic Structure and Chemical Composition of Planar Fault Structures in Co-base Superalloys: Malte Lenz1; Mingjian Wu1; Junyang He2; Surendra Makineni2; Baptiste Gault2; Dierk Raabe2; Steffen Neumeier3; Erdmann Spiecker1; 1Institute of Micro- and Nanostructure Research; 2Max-Planck-Institut für Eisenforschung GmbH; 3Institute for General Materials Properties
We report atomic structures and chemical compositions of defects associated to planar faults in a creep deformed Co-base superalloy and discuss their formation and contribution to plastic deformation. The multinary single crystalline Co-base superalloy was creep deformed under tension along -direction at 850 °C and 400 MPa. The creep microstructure comprises a high density of planar defects. Solute segregation to superlattice intrinsic stacking faults (SISF) is characterized via EDXS analysis of a statistically relevant number of faults and compared at different creep stages. The amount of solute segregation shows negligible difference at different creep stages indicating that segregation directly occurs during planar fault formation and does not significantly evolve afterwards. Based on the observation and analysis of Frank partial dislocations with Burgers vectors terminating SISF, we discuss a new route to SISF formation via dislocation climb. Additionally, two more complex fault structures are analyzed and potential formation mechanisms are discussed. The first of these structures is a terminating end of an SISF where an partial dislocation splits up into two closely spaced partials separated by an SESF. The second structure consists of two parallel SISFs connected by an anti-phase boundary (APB). All deformation mechanisms described in this study show an involvement of solute segregation directly affecting formation and propagation of creep defects by changing planar fault energies and chemical environments of dislocations. Solute segregation is therefore expected to be key to future alloy design by enabling control of creep deformation mechanisms in specific temperature and stress regimes.
Characteristic Flow Behavior of γ+γʹ Duplex and Its Significant Applications in Hot Working Process of Superalloys: Beijiang Zhang1; 1China Iron & Steel Research Inst Group
The characteristic flow behavior of the ã+ã′ duplex structure during the thermomechanical processing (TMP) of highly alloyed disc alloys was comprehensively investigated. Based on the TMP of the ã+ã′ duplex, a series of promising techniques for the billet conversion and microstructure customization can be developed. The superplasticity of ã+ã′ duplex was confirmed by tensile tests, and the optimum combination of temperature and strain rate was determined. For a certain alloy, the maximum elongation up to more than 1000% can be achieved at the temperature of equilibrium ã′ solvus minus 100 ℃. Near-net-shape forgings can be produced within this temperature region. During conversion of the VAR ingots, a proper TMP can trigger a discontinuous precipitation instead of the dynamic strain aging effect which usually leads to a drastic degradation of plasticity. Therefore, the processing window of ingot conversion can be greatly broadened. Producing the dual microstructure disc forging using TMP from a ã+ã′ duplex billet exhibits the inherent advantages over standard heat treatment techniques. The grain size of coarse-grained region can be continuously modulated by controlling the local plastic strain, and the location of the interface between the coarse-grained region and fine-grained region can be precisely determined by controlling the strain distribution. The multi-cycle TMP techniques can play a constructive role in the conversion of the as-HIP billets of P/M material. By producing the ã+ã′ microduplex, prior particle boundaries (PPBs) and inclusions can be efficiently broken up during TMP. As a result, the hot working ductility and the quality of the ultrasonic-inspections can be greatly improved.
Abnormal Grain Growth in the Presence of Grain Boundary Pinning Precipitates: Michael Fahrmann1; David Metzler2; 1Haynes International, Inc.; 2Haynes International
The occurrence of abnormally large overgrown grains upon sub-solvus annealing of certain hot-worked structures has been reported for several cast & wrought and powder metallurgy superalloys. These overgrown grains typically feature a high density of annealing twins and a precipitate distribution similar to the one in the adjacent fine-grained areas. While these special hot working conditions can be established experimentally for a given alloy, it is desirable to provide some general guidance for the avoidance of these features. Following a recently published framework by Bozzolo’s group in France, i.e., Zener pinning pressure versus stored energy driving force, abnormal grain growth (AGG) maps were generated for two commercial HAYNES alloys. Thermo-mechanical processing paths resulting in AGG in these alloys are studied in detail. The generality and limitations of aforementioned framework are discussed.
Deformation Mechanisms of γ’ and γ” Co-precipitates in IN718: Longsheng Feng1; Donald McAllister1; Christopher Zenk1; Michael Mills1; Yunzhi Wang1; 1The Ohio State University
Alloy 718 is widely used in gas turbine engines. Even though recent studies have been focusing on the unique deformation mechanisms of the tetragonal ã phase as compared to those of the cubic ã′ phase, ã′ and ã′′ often co-precipitate and form composite particles. The deformation mechanisms of these composite particles have not been investigated in detail. In this work, we use a combination of ab initio and microscopic phase field methods to study shearing of a dual-lobed type ã′′/ã′/ã′′ composite particle by various dislocations. Complicated fault structures within both ã′ and ã′′ phases are predicted and some of them have been observed in the experiment. The difficulty associated with experimental characterization of the fault structures in the co-precipitates is also discussed.