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A special issue of Crystals (ISSN 2073-4352). This special issue belongs to the section "Crystalline Metals and Alloys".

Deadline for manuscript submissions: closed (31 March 2023) | Viewed by 16013

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Dr. Ronghai Wu

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School of Mechanics and Civil Architecture, Northwestern Polytechnical University, Xi’an 710072, China
Interests: dislocation dynamics; phase-field simulation; constitutive modeling; molecular dynamics; superalloys
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Xiaoxiang Wu

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School of Iron and Steel, Soochow University, Suzhou 215137, China
Interests: superalloy; TEM; defects; mechanical properties
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Superalloys demonstrate a series of superior properties at high temperatures. Due to their ‘super’ performance, they have sustained wide applications in aircraft, industrial gas turbines, oil equipment, and vehicles for over seven decades. As we move through the third decade of the twenty-first century, higher requirements are put forward for the temperature bearing capacity of aircraft segments. Directional structures and single crystal technology have already extended the useful temperature range of superalloys concurrent with the introduction of thermal barrier coating and gas cooling design. Nevertheless, microstructure stability and the mechanical properties of materials during long-term service at elevated temperatures are still facing great challenges, such as (i) decomposition of primary carbides; (ii) precipitation and inhibition of σ, δ, η, μ, and Laves TCP phases; (iii) depletion and coarsening of g¢ phase; (iv) occurrence of trace elements in alloys and the related consequences. These challenges together with the call for future demands necessitate a Special Issue to review recent progress upon which new developments can be built. The present Special Issue on Advances in Superalloys may become a status report summarizing the progress achieved in the past few years. During the time of COVID-19, we hope that not only researchers but also our community can benefit from the outcomes of this Special Issue.

Dr. Ronghai Wu
Prof. Dr. Xiaoxiang Wu
Guest Editors

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Keywords

superalloys
microstructure stability
carbides
TCP phase
γ′ depletion and coarsening
element occurrence

Published Papers (10 papers)

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Research

12 pages, 32644 KiB

Open AccessArticle

Influence of Strain Amplitude on Low-Cycle Fatigue Behaviors of a Fourth-Generation Ni-Based Single-Crystal Superalloy at 980 °C

by Pengfei Wang, Xinbao Zhao, Quanzhao Yue, Wanshun Xia, Qingqing Ding, Hongbin Bei, Yuefeng Gu, Yuefei Zhang and Ze Zhang

Crystals 2023, 13(4), 686; https://doi.org/10.3390/cryst13040686 - 17 Apr 2023

Viewed by 1483

Abstract

Total strain-control, low-cycle fatigue experiments of a fourth-generation Ni-based single-crystal superalloy were performed at 980 °C. Scanning electron microscopy and transmission electron microscopy are employed to determine fracture morphologies and dislocation characteristics of the samples. As the strain amplitude increased from 0.6 to 1.0%, the cyclic stress and plastic strain per cycle increased, the cyclic lifetime decreased, more interfacial dislocation networks were formed, and the formation rate accelerated. Cyclic hardening is associated with the reaction of accumulated dislocations and dislocation networks, which hinder the movement of dislocations. The presence of interfacial dislocations reduces the lattice mismatch between the γ and γ′ phases, and the presence of dislocation networks that absorb mobile dislocations results in cyclic softening. At a strain amplitude of 1.0%, the reaction of a high density of dislocations results in initial cyclic hardening, and the dislocation cutting into the γ′ phase is one of the reasons for cyclic softening. The crack initiation site changed from a near-surface defect to a surface defect when the strain amplitude increased from 0.6 to 0.8 to 1.0%. The number of secondary cracks initiated from the micropores decreased during the growth stage as the strain amplitude increased. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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11 pages, 3956 KiB

Open AccessArticle

Microstructural Evolution and Tensile Properties of a Corrosion-Resistant Ni-Based Superalloys Used for Industrial Gas Turbines

by Zhenhuan Gao, Shikun Li, Gang Liu, Zhao Shang, Dazhuo Song, Gongxian Yang, Juntao Zou and Shuhua Liang

Crystals 2023, 13(4), 669; https://doi.org/10.3390/cryst13040669 - 12 Apr 2023

Cited by 4 | Viewed by 1348

Abstract

As an important mechanical property, tensile behavior has been regarded as an indicator for the creep and thermal mechanical fatigue properties of Ni-based superalloys. The tensile property of Ni-based superalloys is closely related to the amount, size, and distribution of

γ^{'}

-phase [...] Read more.

γ^{'}

-phase and carbides. To further clarify the tensile deformation mechanism of the CM247LC alloy, this study investigated its solidification characteristics and directionally-solidified and heat-treated microstructure. The dependence of tensile properties on the varied temperature ranging between 650 and 950 °C is discussed in detail. It was found that the deformation mechanism at 650 °C is dominated by the shearing of dislocations into the

γ^{'}

-precipitates to form the superlattice stacking faults. At 800 °C, the K-W lock leads to the anomalous yield effects. At 950 °C, the deformation mechanism is dominated by the dislocations bypassing the

γ^{'}

-precipiates. The results provide a comprehensive understanding of the CM247LC alloy and are beneficial for the development of corrosion-resistant Ni-based superalloys. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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15 pages, 8443 KiB

Open AccessArticle

Thickness Effects on Oxidation Behavior and Consequent γ’ Degradation of a High-Al Ni-Based Single Crystal Superalloy

by Haigen Zhao, Wenqi Guo, Wenyue Zhao, Yi Ru, Junwu Wang, Yanling Pei, Shengkai Gong and Shusuo Li

Crystals 2023, 13(2), 234; https://doi.org/10.3390/cryst13020234 - 29 Jan 2023

Cited by 1 | Viewed by 1080

Abstract

High temperature oxidation is considered to play an essential role in the thickness debit effect on the creep rupture life of Ni-based single crystal (SC) superalloys. In order to clarify thickness effects, thin-walled specimens of different thickness (t = 0.1, 0.3, 1.0 mm) were prepared. Cyclic and isothermal oxidation tests of a high-Al Ni-based SC superalloy IC21 were carried out at 900 °C and 1100 °C in order to study the thickness effects on the oxidation behavior and consequent microstructural degradation. Thin-walled specimens of S01-N exhibited a good oxidation resistance when the protective scale-forming elements were enough in the matrix. Specimen thickness tuned the oxidation kinetics by changing the spallation behavior but had few influences on the thermodynamics. The easier stress relief via creep deformation in thin specimens is the main reason behind this phenomenon. Moreover, the obvious temperature effects on the multilayer oxide scales caused the different thickness-related spallation behaviors. Weaker microstructural degradation appeared and was further mitigated by reduced specimen thickness. Sufficient Al content is considered to be indispensable for the formation of protective α-Al₂O₃ scale with less degradation of matrix during the high temperature oxidation of thin-walled Ni-based SC superalloy castings. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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10 pages, 6829 KiB

Open AccessEditor’s ChoiceArticle

Thickness Debit Effect in Creep Performance of a Ni₃Al-Based Single-Crystal Superalloy with [001] Orientation

by Shuangqi Zhang, Guoquan Ma, Haibo Wang, Wenqi Guo, Haigen Zhao, Yong Shang, Yanling Pei, Shusuo Li and Shengkai Gong

Crystals 2023, 13(2), 200; https://doi.org/10.3390/cryst13020200 - 22 Jan 2023

Cited by 5 | Viewed by 1318

Abstract

With the complexity of the structure of aero-engine turbine blades, the blade wall thickness continues to decrease. It is found that when the blade wall thickness decreases to a certain extent, its mechanical properties will decline significantly. It is extremely important to study this phenomenon of a significant decline in mechanical properties caused by wall thickness. In this paper, the creep behavior of a second-generation Ni₃Al-based single crystal superalloy with different wall thicknesses and [001] orientation at 980 °C/220 MPa has been studied and compared with the creep life of Φ4 round bar. The experimental results show that the second orientation and the surface affected zone are not the main reasons for the reduction of the life of thin-walled samples under this experimental condition. By analyzing the fracture morphology and deformed microstructure of thin-walled samples with different thicknesses, it is found that the thickness debit effect of the single crystal alloy occurs since the effective stress area of the alloy changes due to internal defects and surface affected zone during the creep process. For thicker samples, the creep life of the alloy can be extended by making the samples undergo certain plastic deformation through better plastic deformation coordination ability, while for thinner samples, the plastic coordination ability is poor, and the ability to extend the creep life through plastic deformation is also weaker when the effective stress area of the alloy changes, which leads to the thinner samples being more prone to fracture. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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11 pages, 5468 KiB

Open AccessArticle

Topologically Closed Packed Phase and Its Interaction with Dislocation Movement in Ni–Based Superalloy during High–Temperature Creep

by Wenqi Guo, Haigen Zhao, Yi Ru, Yanling Pei, Junwu Wang, Qiaomu Liu, Xuehang Li, Haibo Wang, Shuangqi Zhang, Shengkai Gong and Shusuo Li

Crystals 2022, 12(10), 1446; https://doi.org/10.3390/cryst12101446 - 13 Oct 2022

Cited by 1 | Viewed by 1684

Abstract

In superalloys, topologically close–packed (TCP) phases, which contain refractory elements, usually significantly influence the mechanical properties. The current work investigates the structure and composition of the TCP phase in an Al–Mo–rich Ni–based single crystal superalloy. It is shown that after 40 h of thermal exposure, a large number of strip–like TCP phases are formed, which are enriched in Mo and Re. The structure of the TCP phase is identified as the tetragonal σ phase with the lattice parameter a being 0.93 nm and c being 0.50 nm. During the creep process, the single crystal tilts obviously and leads to orientation variation from <1 1 0> direction. Two groups of dislocations are observed in the deformed sample. One group contains straight dislocation lines and another group contains dislocation networks. The interaction between TCP phase and dislocation in the single crystal superalloy is studied to reveal the effect of the TCP phase on the deformation behavior. During creep, the σ phase hinders the dislocation movement, which may contribute to the propagation of the cracks and the final fracture. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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11 pages, 3154 KiB

Open AccessFeature PaperArticle

Temperature Effects on the Deformation Mechanisms in a Ni-Co-Based Superalloys

by Rui Hu, Jiaheng Zhao, Cheng Yang, Jinhui Du, Xian Luo, Zhongnan Bi and Bin Gan

Crystals 2022, 12(10), 1409; https://doi.org/10.3390/cryst12101409 - 05 Oct 2022

Cited by 4 | Viewed by 1372

Abstract

The tensile properties of a Ni-Co-based superalloy were investigated from room temperature to 900 °C. From 25 to 650 °C, the yield strength and tensile strength of the alloy decreased slightly, while the elongation decreased sharply. From 760 °C to 900 °C, the yield strength and tensile strength were greatly reduced, while the elongation also had a low value. With the increase in temperature, the deformation mechanism transformed from anti-phase boundary shearing to stacking fault shearing, and then from deformation twinning to Orowan bypassing, respectively. Deformation twins were generated in the deformed alloy with high-density stacking faults and they can contribute to the high strength. The alloy in this study has good mechanical properties and hot working characteristics below 760 °C and can be used as a turbine disk, turbine blade, combustion chamber, and other aircraft structural parts. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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21 pages, 11840 KiB

Open AccessArticle

The Effect of Cooling Rate from Solution Treatment on γ′ Reprecipitates and Creep Behaviors of a Ni-Based Superalloy Single-Crystal Casting

by Jiapeng Huang, Cheng Ai, Yi Ru, Yong Shang, Yanling Pei, Shusuo Li, Shengkai Gong and Heng Zhang

Crystals 2022, 12(9), 1235; https://doi.org/10.3390/cryst12091235 - 01 Sep 2022

Cited by 2 | Viewed by 1463

Abstract

Slowing down the coarsening of the γ′ phase and suppressing the precipitation of the topologically close-packed (TCP) phase is crucial for optimizing the creep properties of Ni-based single crystal superalloys, which are affected by the solution treatment history. In the present study, the effect of cooling rate on the morphology, size and lattice misfit of γ′ reprecipitates after solution treatment, as well as the microstructural evolution (e.g., the coarsening of γ′ reprecipitate and precipitation of the TCP phase) and creep properties of samples under different cooling rates at 1100 °C were investigated. The findings suggested that as the cooling rate increasing, the size of γ′ reprecipitates decreased, while the morphology transformed from polygonal to cubic. Additionally, faster cooling rate, on the one hand, the lower the degree of lattice misfit of γ/γ′ phases, which is beneficial to slow down the coarsening of the γ′ phase; on the other hand, the supersaturation of the γ-phase was reduced, i.e., the Mo and Re contents in the γ matrix were lower compared to the slow-cooling sample, which led to a lower TCP phase area fraction during thermal exposure at 1100 °C. It is crucial that the creep life of the alloy significantly improved by increasing the cooling rate after solution treatment; this is facilitated by the formation of rafts from finer cubic γ′ phase and reduction in the TCP phase as a source of microcracks. In conclusion, the findings of this study provide new insights into suppressing the precipitation of the TCP phase and optimizing alloy heat treatment processes to improve creep properties. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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19 pages, 56941 KiB

Open AccessEditor’s ChoiceArticle

Effect of Abnormal Grains on the Mechanical Properties of FGH96 Solid-State Diffusion Bonding Joint

by Jingqing Zhang, Yong Shang, Qiaomu Liu, Junwu Wang, Yanling Pei, Shusuo Li and Shengkai Gong

Crystals 2022, 12(8), 1017; https://doi.org/10.3390/cryst12081017 - 22 Jul 2022

Viewed by 2077

Abstract

The hollow twin-web disk is designed to improve the thrust-to-weight ratio of the aero engine, where the welding joint microstructures determine the disk’s mechanical properties. This study aimed to elucidate the effect of abnormal grains formed in the welding region on the mechanical properties of FGH96 solid-state diffusion bonding joints. Digital image correlation using images captured by scanning electron microscopy (SEM-DIC) and electron backscattering diffraction (EBSD) technologies were applied. The results show that abnormally large grains (2.5 times that of the matrix), with preferred orientation in the bonding region, were detrimental to the joint mechanical properties. The yield and tensile strengths were 995.85 MPa and 1456.67 MPa, respectively, and the corresponding relative (ratio to the matrix) ones were 92.54% and 88.81%. After modifying the bonding process, the grain size in the bonding region was tailored to close to that of the matrix, and considerable twin boundaries (TBs) formed, leading to the relative tensile and yield strength reaching 98.86% and 99.37%. Furthermore, the failure mode changed to intragranular type from intergranular type. It demonstrates that tailoring the newborn grain size, introducing TBs inside, and eliminating preferred orientation during the welding process can be an efficient way to improve the joint mechanical properties. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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12 pages, 4237 KiB

Open AccessArticle

Microscopic Phase-Field Simulation of γ′ Precipitation in Ni-Based Binary Alloys Coupled with CALPHAD Method

by Zhenzhi Liu, Yan Zhao, Xuyu Zhang, Xiao-Gang Lu, Chuanjun Wang and Yu Zhang

Crystals 2022, 12(7), 971; https://doi.org/10.3390/cryst12070971 - 12 Jul 2022

Cited by 1 | Viewed by 1235

Abstract

In the present work, the first (1st) and second (2nd) nearest-neighbor interaction energies are calculated by coupling the microscopic phase-field kinetic model with the calculation of phase diagrams (CALPHAD) method. The morphological evolution of the γ′ precipitate and the variation of its atomic ordering parameter for Ni–X (X = Al, Fe, Mn, Pt, or Si) alloys during aging are studied. The simulation results predict different occupation preferences for solute and solvent atoms in the γ′ phase, i.e., solute atoms are inclined to occupy the corner sites and solvent atoms tend to occupy the face sites. In order to understand the precipitation process of the γ′ phase systematically, the ordering and clustering behaviors of solute atoms are analyzed. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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16 pages, 5038 KiB

Open AccessArticle

Study of the Creep Behavior of Nickel-Based Single Crystal Superalloy Micro Specimens with Dimensional Effects

by Wangjiao Xiong, Xing Ai, Jianfang Wang, Quanzhong Wang, Yanyun Zhao, Haiyan Zhu, Hao Cheng and Sheng Zhang

Crystals 2022, 12(5), 592; https://doi.org/10.3390/cryst12050592 - 22 Apr 2022

Cited by 3 | Viewed by 1886

Abstract

Nickel-based single-crystal superalloys are widely used in aeroengine hot-end components, owing to their unique crystal structure and outstanding high-temperature mechanical properties. In the present study, round rod specimens of different sizes were subjected to high temperature creep tests at 980 °C/300 MPa of a second-generation nickel-based single crystal superalloy. The effect of size on the creep behavior of nickel-based single-crystal superalloys was studied with reference to the creep curves and microstructure morphologies. Creep interruption tests of 3-mm micro-round rod specimens were performed for 30, 60, and 90 h until creep fracture occurred. It was found that for nickel-based single crystal superalloys, the smaller the diameter of the specimen, the longer its creep life. Furthermore, the creep fracture morphology showed obvious creep cavitation in the fracture region. The law of organization evolution was used to analyze the rafting phenomena during the creep process. A typical “N”-type drifting strip structure was found during the creep process. Meanwhile, the width of the γ-phase channel increases continuously with creep, and the rate of change of the width of the matrix phase was fastest at the earliest stage of creep, slowing significantly during the middle and late stages of creep with the completion and appearance the rafting phenomenon. Full article

(This article belongs to the Special Issue Experiments and Simulations of Superalloys)

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