Superalloys 2024: Interactive Session B: Disk Alloy Mechanical Behavior
Program Organizers: Jonathan Cormier, ENSMA - Institut Pprime - UPR CNRS 3346
Monday 7:10 PM
September 9, 2024
Room: Winterberry
Location: Seven Springs Mountain Resort
B-1: Effect of Grain Boundary Serrations on Creep Deformation of Udimet-720Li Superalloy: Tso-Wei Chen1; Bo-Chen Wu2; Hideyuki Murakami3; Yoshiaki Toda3; An-Chou Yeh1; Yung-Chang Kang4; 1National Tsing Hua University; 2S-Tech Corporation; 3National Institute for Materials Science; 4Gloria Material Technology Corporation
This article investigates grain boundary serration and its effects on high-temperature creep behavior of Udimet-720Li. Grain boundary serration is induced by controlled cooling during heat treatments, with continuous and discontinuous precipitation of γ′ phase identified as the competing mechanisms affecting serration formation. Continuous precipitation of coarse γ′ particles pin grain boundaries and leads to a slight serration termed the continuous precipitation type (type-I) boundary, while discontinuous reaction forming cellular γ/γ′ behind the mobile grain boundary causes larger serration known as the discontinuous precipitation type (Type-II) boundary. Samples with straight (STB), Type-I (SRB-1), Type-II (SRB-2) grain boundaries were produced, and creep behaviors under 700 °C/700MPa were investigated.
Cancelled
B-2: Effect of Non-fully Recrystallized Grain Structures on the Fatigue Behavior of a Wrought γ-γ' Ni-base Superalloy: Linhan Li1; Ji Zhang1; Ran Duan1; Kangkang Liu1; Qiang Tian1; Wenyun Zhang1; Zhongmin Shen1; Beijiang Zhang1; 1China Iron & Steel Research Institute Group
The effect of non-fully recrystallized (NFRX) microstructures on the fatigue performance of γ-γ' Ni-base superalloy GH4065A was investigated by comparing the fatigue properties of specimens with a homogeneous fine-grained structure and a heterogeneous grain structure containing many NFRX regions. The fatigue life was characterized by performing strain-controlled fatigue tests at 500 ° with a maximum strain amplitude Δεmax/2 ranging from 0.8 % to 0.45 %. In order to study the fatigue slip, crack initiation and propagation behaviors associated with the NFRX structures, interrupted stress-controlled fatigue tests of dog-bone shape specimens were carried out at 400 °, employing two loading levels with the maximum stress amplitude Δσmax above and below the yield strength, respectively. The NFRX grain structures were characterized and classified into two distinct types: unrecrystallized (URX) and partially recrystallized (PRX). Their effect on the slip bands formation, crack initiation and propagation were investigated and compared with that of the fully recrystallized (FRX) structure. The superior resistance of NFRX structures to the fatigue damage, particularly restricting slip-induced crack initiation and inclusion-induced crack propagation, makes the FRX regions accumulate most of the fatigue damage, resulting in unnoticeable variation in fatigue life.
B-3: How Can the Non-metallic Inclusions Distribution Lead to an Anisotropy in the Fatigue Life Durability of Forged γ/γ' Ni-based Disks Alloys?: Adèle Govaere1; Moubine Al-Kotob1; Xavier Baudequin1; Caitline Lasne1; Romain Lambert1; Jonathan Leblanc1; Arnaud Longuet1; Nicolas Sutter1; Alexia Wu1; Jonathan Cormier2; Azdine Nait-Ali2; Malo Prié2; 1Safran Aircraft Engines; 2Institut Pprime, ISAE-ENSMA
The impact of non-metallic inclusions (NMIs) and the forging process on the low-cycle fatigue (LCF) durability of a γ/γ'' superalloy is investigated. X-ray synchrotron tomography was performed to characterize the NMIs populations. Low cycle fatigue was tested at 400 °C on specimens taken in various orientations on forged disks using the ring rolling process or the die forging one. The results show anisotropic tensile and fatigue properties for the ring rolled disks, especially in radial and axial directions. On the other hand, no reduction of the lifespan was observed for the die forged ones. Fractographic observations and a macroscopic approach in modelling of a particle during forging were conducted. It showed that abnormal low lifetimes are connected to crack initiation at NMIs. However, it is not a sufficient condition. The length of NMIs stringers and their orientation in the disk are important parameters. The forging process has a strong impact on the plastic flow and the initial state of the particles. Indeed, ring rolling promotes decohesion at the interface between the inclusion and the matrix. The crack initiation time and the first stages of the propagation to the matrix highly contribute to low lifespan. A better understanding of the detrimental configurations of NMIs would allow optimized forging processes.
B-4: Improvement in Mechanical Properties of Ni-base Superalloy Alloy247 Using Hot Forging: Shintaro Yoshimoto1; Yuhi Mori1; Masahiro Hayashi1; Takashi Shibayama2; Takeshi Izumi2; Shinya Imano2; 1Honda R&D Co.,Ltd.; 2Mitsubishi Heavy Industries Ltd.,
To improve the low-cycle fatigue (LCF) strength of Alloy247, refinement of the alloy microstructure through the forging process has been trialed and the mechanical properties and microstructure of the forged alloy were compared with that of the conventional casted alloy. By utilizing the MH (Mitsubishi Hitachi) process, an ingot of Alloy247 with high volume fraction of γ' phase was successfully forged without cracking. In contrast with cast Alloy 247, forged Alloy247 exhibited uniform microstructure with γ' precipitates, the grain size was significantly reduced from 10mm in the cast alloy to 200 mm in the forged alloy, and the eutectic structure was eliminated completely. Results obtained from high temperature tensile testing showed remarkable improvement in elongation which led to higher ultimate tensile strength (UTS) values. Owing to the fine microstructure, forged Alloy247 showed 15 times longer LCF life than that of the cast alloy although creep strength slightly decreased in low stress regions. Significant improvement in LCF life of Alloy247, which has intrinsically high creep strength, allows for tuning of the mechanical properties based on requirements and broadens the application of the alloy to hot section parts.
B-5: Low-cycle Fatigue Performance and Associated Deformation Mechanisms of HAYNES 244 Alloy and Waspaloy: Michael Fahrmann1; Michael Titus2; Thomas Mann1; 1Haynes Intl.; 2Purdue University
One key performance attribute of cases and seals in advanced gas turbine engines is resistance to thermal fatigue, which is dictated by strains associated with thermal cycling of the component and the material’s intrinsic resistance to fatigue crack initiation and propagation. Frequently isothermal strain-controlled low-cycle fatigue (LCF) testing is utilized to assess the candidate alloys’ intrinsic fatigue capabilities. In addition, a low coefficient of thermal expansion (CTE) is desirable since the CTE largely controls, for a given thermal cycle, the magnitude of the thermally induced strains. The primary aim of this study was to compare the LCF performance of recently developed low CTE, high strength HAYNES® 244® alloy to that of Waspaloy, a legacy case alloy, from room temperature to 1400 °F (760 °C). A secondary aim was to shed light on the active deformation and damage mechanisms in these alloys. We observed that both alloys exhibited comparable fatigue lives, within experimental scatter, but the 244 alloy deformed by deformation twinning, whereas Waspaloy deformed by dislocation slip and precipitate shearing and looping.
Cancelled
B-6: Microstructure and Mechanical Properties of a Novel γ′-strengthened Multi-component CoNi-based Wrought Superalloy: Li Huiwei1; Zhang Xiaorui2; Lu Song2; Zhuang Xiaoli2; Li Longfei2; Wen Xinli1; Feng Qiang2; 1Beijing Beiye Functional Materials Corporation; 2University of Science and Technology Beijing
To reveal the deformation behavior of a novel ã′-strengthened multi-component CoNi-based wrought superalloy, the tensile properties from room temperature to 850 °C and creep property under 725 °C/630 MPa were studied. The results show that after hot forging and sub-solvus solution plus two-step aging heat treatment, the grain size of the investigated alloy is uniform, accompanied by the bimodal distribution of ã′ precipitates. Elements such as Co, Cr, and Mo segregate within the ã phase, whereas Al, W, Ni, Ti, and Ta exhibited segregation in the ã′ precipitate. In comparison with typical wrought superalloys U720Li and Waspaloy, the investigated alloy exhibits enhanced yield strength at 750 ℃~850 °C, which may be associated with the alloy's larger grain size and the increase in the intermediate temperature flow stress anomaly inherent to Co-based alloys. Meanwhile, the creep property of the investigated alloy surpasses those of alloys U720Li and Waspaloy, which is primarily attributed to its larger grain size and relatively higher ã' volume fraction. Additionally, the lower stacking fault energy of the alloy facilitates the formation of stacking faults and micro-twins, further enhancing its creep property. The findings of this study will hold importance for the further optimization of CoNi-based wrought superalloys.
B-7: Stress-induced Acceleration of the Growth of δ-Phase Precipitates in Ni-based Superalloy GH4169 (IN718) and Its Effect on Acceleration of Intergranular Cracking Under Creep Loading at Elevated Temperature: Ayumi Nakayama1; Run-Zi Wang1; Ken Suzuki1; Hideo Miura1; 1Tohoku University
In this study, intermittent creep tests and EBSD (Electron Backscatter diffraction) analyses were conducted on GH4169 (IN718) at 800°C to elucidate the change mechanism of its micro texture and the degradation mechanism of the strength of grain boundaries under mechanical loading at elevated temperature. Under creep loading, the growth of δ-phase (Ni3Nb) precipitates was found to be accelerated around grain boundaries. This growth of δ-phase precipitates caused the disappearance of the solute strengthened phase in the surrounded grains and, thus, the effective strength of grains decreased. Even though the precipitation around grain boundaries increased their effective strength at first, however, it started to decrease with time due to the acceleration of the growth and accumulation of vacancies and dislocations around the interface between the precipitates and the matrix. This acceleration was attributed to the large lattice mismatch between the precipitate and the matrix, and the structural singularity around the edges of the needle-shape precipitates. Finally, intergranular cracking was accelerated. The degradation of the strength of the material was validated by using a micro tensile test using a scanning electron microscope. The acceleration process was successfully explained by the concept of the stress-induced acceleration of diffusion, by applying the modified Arrhenius equation. The growth time of δ-phase precipitates around grain boundaries was also predicted quantitatively.
B-8: The Effect of Direct Current Heating on the Creep Behaviour of Polycrystalline Ni-based Superalloys: Ryo Sasaki1; Satoshi Utada2; Yuanbo Tang2; Carlos Nunes3; Roger Reed2; 1Proterial; 2University of Oxford; 3University of São Paulo
This study discusses the effect of the direct current heating (DCH) on the creep behaviour of Ni-based superalloys, in an attempt to converge on best practice and also reliability. Particular features are the use of an electro-thermal mechanical testing (ETMT) system which has allowed for direct comparison of the DCH heating method with the more conventional resistance furnace heating (FH) approach. In addition, particular attention is paid to a testing protocol to confirm the accuracy of temperature measurement during DCH. Waspaloy, Alloy713C and Alloy246 are used in different geometrical configurations, thus enabling us to establish the difference in creep behaviours caused by altering the sample sizes and the heating methods. The creep rupture lives of the materials tested by the DCH method were consistently shorter than that tested by the FH method. Microstructural investigation revealed that there is no influence of heating method on the coarsening rate of the fine γ′ precipitates and on the thickness of the oxide layer in Waspaloy during creep test at 732 °C. Moreover, the presence of an axial temperature gradient did not influence the creep life unduly. The radial temperature gradient as estimated by an analytical model has affected to some extent. It is hypothesised that the diffusivity is increased by the direct current, and the dislocation climb is facilitated in the DCH testing for Waspaloy.
B-9: The PLC Effect in the Absence of Long-range Cottrell Atmospheres in RR1000: Bradley Rowlands1; James Miller1; Lewis Owen2; Howard Stone1; Wenwen Song3; Xiao Shen3; Enrique Galindo-Nava4; Cathie Rae1; 1University of Cambridge; 2University of Sheffield; 3RWTH Aachen University; 4University College London
The Portevin-Le Chatelier (PLC) effect is widely attributed to the partitioning of interstitial C atoms around dislocation cores in mild steels, to form Cottrell atmospheres. However, limited empirical evidence exists to clarify how similar mechanisms extend to complex multicomponent alloy systems such as Ni-based superalloys. The presence of the PLC effect was demonstrated in coarse-grained RR1000 from 200 °C – 550 °C through digital image correlation (DIC). The effect was also demonstrated in simple binary Ni-(20-25)Cr (wt. %) alloys, suggesting that the microstructural complexity of superalloys is not key to elucidating the atomistic origins. Probe-corrected scanning transmission electron microscopy (STEM)-energy dispersive X-ray spectroscopy and atom probe tomography were performed on interrupted specimens displaying the PLC effect. No evidence of long-range solute enrichment was observed in the vicinity of dislocation cores. Hence, alternative origins for the effect were considered; evidence for short-range order was assessed in Ni-Cr powder through neutron diffraction. A preference was observed for particular local configurations of Ni and Cr atoms within a unit cell, resembling, but not identical to, those in long-range ordered Ni2Cr. High resolution STEM imaging demonstrated the presence of nanodomains in Ni-Cr and RR1000 containing diffraction contrast effects resembling superlattice fringes. STEM image simulations based on refined unit cell structures demonstrated that such features may relate to the local lattice distortion in short-range ordered (SRO) domains. The results do not support the presence of long-range Cottrell atmospheres within the temperature range of the PLC effect, but instead of SRO, an alternate hypothesised origin of the PLC effect.
B-10: Microstructure-based Modeling of Temperature-dependent Yield Strength in Polycrystalline Ni-base Superalloys: Moritz Muller1; Howard Stone2; Enrique Galindo-Nava3; Felix Schleifer1; Michael Fleck1; Uwe Glatzel1; 1University of Bayreuth; 2University of Cambridge; 3University of College London
A new model for the temperature-dependent yield strength of polycrystalline Ni-base superalloys at high strain rates (10-3 to 10-4s-1 ) featuring multimodal precipitate microstructures is presented. It extends existing mean field models based on the summation of particle strengthening and other strengthening contributions, which were originally created to describe behavior at room temperature. Temperature dependence is introduced for each contribution using existing models or datasets from the literature while maintaining physical validity and avoiding unnecessary fitting parameters. Further, the expected change in the precipitate distributions at high temperatures is achieved using CALPHAD. By considering such microstructural changes, the model can predict the yield strength from room temperature to beyond the solvus temperature of the precipitates. Due to the strategy for combining different strengthening contributions, the model can also predict the active deformation modes at any temperature and can be extended easily to incorporate additional deformation mechanisms. The model’s sensitivity to microstructural parameters and its strengths and weaknesses are also discussed. Application of the model to three commercial alloys shows good agreement over the whole temperature range.