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Superalloys–Currents Trends in Development of Their Microstructure and Properties

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Manufacturing Processes and Systems".

Deadline for manuscript submissions: closed (31 December 2021) | Viewed by 41906

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Prof. Dr. Jan Sieniawski

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Department of Materials Science, Faculty of Mechanical Engineering and Euronautics, Rzeszów University of Technology, W. Pola 2 St., 35-959 Rzeszów, Poland
Interests: superalloys; single-crystals; crystallization
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Dear Colleagues,

Superalloys are metallic alloys (nickel-, cobalt- and iron-based) capable of being used at high temperatures, often in excess of 0.7 of their absolute melting temperature. Since the 1950s, their development has led to significant increases in operating temperatures—from about 750 to 1050 ºC. This was possible by modification of their chemical compositions (e.g., addition of Rare Earth elements) and manufacturing processes (investment casting, single-crystal production). The efficiency of this approach seems to have been much lower in last two or three decades, and surface treatment methods became the primary means for increasing hot corrosion resistance of structural elements made of superalloys. Independent of surface engineering achievements, the role of substrate materials is incontestable, especially in terms of the assurance of adequate creep resistance.

The main role of alloying additions in superalloys is to develop thermally-stable microstructures containing optimal volume fractions of phase constituents, e.g., γ’ hardening precipitates in nickel-based superalloys. Another tool in the microstructure development process is heat treatment, mainly precipitation strengthening. Proper selection of conditions requires knowledge of phase transformations kinetics, which requires the application of advanced material examination methods. An important achievement in the field of superalloy casting was the application of directional solidification. It is generally accepted that single-crystal casts exhibit much better creep resistance compared with polycrystalline ones. In the case of turbine blades, it is crucial to know how to obtain the optimal crystal orientation and evaluate its performance.

The scope of this forthcoming Special Issue will focus on recent innovative and pioneering works in the field of metallurgy and processing, structure and microstructure examination, and the development of the operational properties of superalloys.

I invite our colleagues to submit a manuscript to this Special Issue, which can be in the form of a full research paper, communication, or review.

Prof. Jan Sieniawski
Guest Editor

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Keywords

superalloys
directional solidification
single-crystals
microstructure characterization
creep resistance

Published Papers (13 papers)

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Research

17 pages, 9044 KiB

Open AccessArticle

Microstructural Investigations of Ni-Based Superalloys by Directional Solidification Quenching Technique

by Tobias Wittenzellner, Shieren Sumarli, Helge Schaar, Fu Wang, Dexin Ma and Andreas Bührig-Polaczek

Materials 2020, 13(19), 4265; https://0-doi-org.brum.beds.ac.uk/10.3390/ma13194265 - 24 Sep 2020

Cited by 9 | Viewed by 2191

Abstract

The improvement of the mechanical properties of Ni-based superalloys is achieved in most cases by modifying the chemical composition. Besides that, the processing can be modified to optimize the as-cast microstructure with regard to the mechanical properties. In this context, the present study highlights the solidification mechanism of several Ni-based superalloys by conducting experiments using a modified, laboratory-scale Bridgman-Stockbarger furnace. In that context, the single-crystal rods are partially melted, directionally solidified and quenched sequentially. Several characterization methods are applied to further analyze the influence of the alloying elements and the variation of the withdrawal rate on the as-cast microstructure. Four stages of solidification are distinguished whereby the morphology observed in the different stages mainly depends on the cooling rate and the local concentration of the carbide forming elements. The effect of carbide precipitation and the effect on the as-cast microstructure is investigated by employing energy dispersive X-ray spectrometry (EDX) and electron backscatter diffraction (EBSD) analysis techniques. A local polycrystalline structure is observed in the single-crystal system as consequence of the influence of the carbon content and the cooling rate. The present work aims to develop strategies to suppress the formation of the polycrystalline structure to maintain the single-crystal microstructure. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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14 pages, 15141 KiB

Open AccessArticle

Characterization of γ′ Precipitates in Cast Ni-Based Superalloy and Their Behaviour at High-Homologous Temperatures Studied by TEM and in Situ XRD

by Łukasz Rakoczy, Ondrej Milkovič, Bogdan Rutkowski, Rafał Cygan, Małgorzata Grudzień-Rakoczy, František Kromka and Anna Zielińska-Lipiec

Materials 2020, 13(10), 2397; https://0-doi-org.brum.beds.ac.uk/10.3390/ma13102397 - 22 May 2020

Cited by 22 | Viewed by 4223

Abstract

In situ X-ray diffraction and transmission electron microscopy has been used to investigate René 108 Ni-based superalloy after short-term annealing at high-homologous temperatures. Current work is focused on characterisation of γ′ precipitates, their volume fraction, evolution of the lattice parameter of γ and γ′ phases and misfit parameter of γ′ in the matrix. Material in the initial condition is characterised by a high-volume fraction (over 63%) of γ′ precipitates. Irregular distribution of alloying elements was observed. Matrix channels were strongly enriched in Cr, Co, W and Mo, whereas precipitates contain large amount of Al, Ti, Ta and Hf. Exposure to high-homologous temperatures in the range 1100–1250 °C led to the dissolution of the precipitates, which influenced the change of lattice parameter of both γ and γ′ phases. The lattice parameter of the matrix continuously grew during holding at high temperatures, which had a dominant influence on the more negative misfit coefficient. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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21 pages, 18858 KiB

Open AccessArticle

Fabrication and Characterization of the Newly Developed Superalloys Based on Inconel 740

by Małgorzata Grudzień-Rakoczy, Łukasz Rakoczy, Rafał Cygan, František Kromka, Zenon Pirowski and Ondrej Milkovič

Materials 2020, 13(10), 2362; https://0-doi-org.brum.beds.ac.uk/10.3390/ma13102362 - 21 May 2020

Cited by 13 | Viewed by 2480

Abstract

The chemical composition of standard Inconel 740 superalloy was modified by changes in the Al/Ti ratio (0.7, 1.5, 3.4) and addition of Ta (2.0, 3.0, 4.0%). Remelted Inconel 740 (A0) and nine variants with various chemical compositions were fabricated by lost-wax casting. The microstructure, microsegregation, phase transformation temperatures, thermal expansion coefficients and hardness of the superalloys were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, differential scanning calorimetry, dilatometry and Vickers measurements. Typical dendritic microstructure was revealed with microsegregation of the alloying elements. Segregation coefficient kⁱ for Ti, Nb and Ta did not exceed unity, and so precipitates enriched mainly in these elements were found in interdendritic spaces. The Nb-rich blocky precipitates, MC carbides, MN nitrides, oxides, and fine γ’ was in all modified castings. Presence of other microstructural features, such as Ti-rich needles, eutectic γ-γ’ islands, small Al-rich and Cr-rich precipitates depended on the casting composition. The lowest solidus and liquidus temperatures were observed in superalloys with a high Al/Ti ratio. Consequently, in A7–A9 variants, the solidification range did not exceed 100 °C. In the A0 variant the difference between liquidus and solidus temperature was 138 °C. Hardness of all modified superalloys was at least 50% higher than for the remelted Inconel 740 (209 HV10). Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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13 pages, 3950 KiB

Open AccessArticle

Variation of Crystal Orientation and Dendrite Array Generated in the Root of SX Turbine Blades

by Włodzimierz Bogdanowicz, Jacek Krawczyk, Robert Paszkowski and Jan Sieniawski

Materials 2019, 12(24), 4126; https://0-doi-org.brum.beds.ac.uk/10.3390/ma12244126 - 09 Dec 2019

Cited by 7 | Viewed by 2119

Abstract

The variation of the crystal orientation and the dendrite array generated in the root of the single-crystalline (SX) turbine blades made of CMSX-4 superalloy were studied. The blades with an axial orientation of the [001] type were solidified by the industrial Bridgman technique using a spiral selector at a withdrawal rate of 3 mm/min. The analysis of the crystal orientation and dendrite arrangement was carried out using scanning electron microscopy, X-ray diffraction topography, and Laue diffraction. It was found that the lateral growth of such secondary dendrite arms, which are defined as “leading” and grow in the root at first, is related to the rotation of their crystal lattice, which is the reason for creation of the low-angle boundary (LAB) type defects. The primary crystal orientation of the selector extension (SE) area determines the areas and directions of the lateral growth of the leading arms. Additionally, it was found that in the SE areas of the root, near the connection with the selector, the spatial distribution of the [001]γ′ crystallographic direction has a complex wave-like character and may be related to the shape of the crystallization front. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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14 pages, 5363 KiB

Open AccessArticle

The Effect of Withdrawal Rate on Crystal Structure Perfection, Microstructure and Creep Resistance of Single Crystal Castings Made of CMSX-4 Nickel-Based Superalloy

by Kamil Gancarczyk, Maciej Zubko, Aneta Hanc-Kuczkowska, Barbara Kościelniak, Robert Albrecht, Dariusz Szeliga, Maciej Motyka, Waldemar Ziaja and Jan Sieniawski

Materials 2019, 12(20), 3422; https://0-doi-org.brum.beds.ac.uk/10.3390/ma12203422 - 19 Oct 2019

Cited by 13 | Viewed by 2973

Abstract

This study focuses on the evaluation of the crystal structure perfection in the single crystal made of CMSX-4 nickel superalloy and its effect on creep resistance. Single crystal castings were manufactured by directional solidification process at the withdrawal rate of 1, 3, 5 and 7 mm/min. Light (LM) and electron (SEM, TEM) microscopy, X-ray diffraction and Mossbauer spectroscopy were used for evaluation of the microstructure and crystal structure perfection. Castings were also subjected to creep tests. The best creep resistance was obtained for the casting manufactured at the withdrawal rate of 3 mm/min, characterized by the highest crystal structure perfection compared to the other castings examined. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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13 pages, 3386 KiB

Open AccessArticle

Primary Crystal Orientation of the Thin-Walled Area of Single-Crystalline Turbine Blade Airfoils

by Włodzimierz Bogdanowicz, Jacek Krawczyk, Robert Paszkowski and Jan Sieniawski

Materials 2019, 12(17), 2699; https://0-doi-org.brum.beds.ac.uk/10.3390/ma12172699 - 23 Aug 2019

Cited by 11 | Viewed by 2681

Abstract

The thin-walled airfoil areas of as-cast single-crystalline turbine blades made of CMSX-4 superalloy were studied. The blades were produced by the industrial Bridgman technique at withdrawal rates of 2, 3 and 4 mm/min. The angle between the [001] crystallographic direction and blade axis, related to the primary orientation, was defined by the Ω-scan X-ray diffraction method at points on the camber line located near the tip of an airfoil and at points of a line located in parallel and near the trailing edge. Additionally, primary crystal orientation was determined by Laue diffraction at the selected points of an airfoil. The influence of mould wall inclination on the primary crystal orientation of the thin-walled areas is discussed. The effect of change in the [001] crystallographic direction, named as “force directing”, was considered with regard to the arrangement of primary dendrite arms in relation to the trailing edge and the camber line. It was stated that when the distance between the mould walls is less than the critical value of about 1.5 mm the “force directing” increases as the distance between the walls of the mould decreases. The effect may be controlled by selecting an appropriate secondary orientation using a seed crystal in the blade production process. The model of dendrite interaction with the mould walls, including bending and “deflection”, was proposed. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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11 pages, 4944 KiB

Open AccessArticle

Precipitates in Additively Manufactured Inconel 625 Superalloy

by Beata Dubiel and Jan Sieniawski

Materials 2019, 12(7), 1144; https://0-doi-org.brum.beds.ac.uk/10.3390/ma12071144 - 08 Apr 2019

Cited by 46 | Viewed by 5591

Abstract

Laser-based additive manufacturing processes are increasingly used for fabricating components made of nickel-based superalloys. The microstructure development, and in particular the precipitation of secondary phases, is of great importance for the properties of additively manufactured nickel-based superalloys. This paper summarizes the literature data on the microstructure of Inconel 625 superalloy manufactured using laser-based powder-bed fusion and directed energy deposition processes, with particular emphasis on the phase identification of precipitates. The microstructure of Inconel 625 manufactured by laser-based directed energy deposition in as-built condition is investigated by means of light microscopy and transmission electron microscopy. Phase analysis of precipitates is performed by the combination of selected area electron diffraction and microanalysis of chemical composition. Precipitates present in the interdendritic areas of as-built Inconel 625 are identified as MC and M₂₃C₆ carbides as well as the Laves phase. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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22 pages, 7039 KiB

Open AccessArticle

Application of Inner Radiation Baffles in the Bridgman Process for Flattening the Temperature Profile and Controlling the Columnar Grain Structure of Directionally Solidified Ni-Based Superalloys

by Dariusz Szeliga, Waldemar Ziaja, Maciej Motyka, Krzysztof Kubiak and Jan Sieniawski

Materials 2019, 12(6), 935; https://0-doi-org.brum.beds.ac.uk/10.3390/ma12060935 - 21 Mar 2019

Cited by 11 | Viewed by 3514

Abstract

The technique of flattening the temperature profile and controlling the formation of both the dendritic microstructure and grain structure in the directional solidification of nickel-based superalloy casting, using the novel inner radiation baffles (IRBs) in the Bridgman process, is presented in this paper. These baffles matched to the shape of mold and were placed horizontally along its height at various distances from the casting base. The plate castings of CMSX-4 superalloy were fabricated without and with the use of IRBs, withdrawing the mold at the rate of 6 mm/min from the heating to the cooling area of the industrial Bridgman furnace. Thermal analysis of the directional solidification of castings was carried out using the ProCAST software for a process where the various designs of the radiation baffle were applied. The results of the solidification conditions, the shape of liquidus and solidus isotherms, and grain structure obtained for the IRBs were compared with those reached for the standard ring-shaped (AERB) or perfectly adjusted (PARB) radiation baffles. The use of IRB resulted in flattening of the temperature distribution and decrease of the curvature of liquidus and solidus isotherms, as well as an increase of temperature gradient and cooling rate, compared with the process where AERB was only used. Consequently, primary dendrite arm spacing (PDAS) reached similar values across the width of casting and equaled to approximately 370 μm, reducing its average value by 26%, compared with the standard process. The change in predicted axial temperature gradient in casting was not found when thermophysical properties of molybdenum IRBs were used. The increase in graphite IRBs number in mold from seven to 14 caused the reduction of inhomogeneity of axial temperature gradient along the casting height. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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15 pages, 4809 KiB

Open AccessArticle

Defect Creation in the Root of Single-Crystalline Turbine Blades Made of Ni-Based Superalloy

by Jacek Krawczyk, Robert Paszkowski, Włodzimierz Bogdanowicz, Aneta Hanc-Kuczkowska, Jan Sieniawski and Bartosz Terlecki

Materials 2019, 12(6), 870; https://0-doi-org.brum.beds.ac.uk/10.3390/ma12060870 - 15 Mar 2019

Cited by 12 | Viewed by 2626

Abstract

An analysis of the defects in the vicinity of the selector–root connection plane occurring during the creation of single-crystalline turbine blades made of CMSX-6 Ni-based superalloy was performed. X-ray diffraction topography, scanning electron microscopy, and positron annihilation lifetime spectroscopy were used. Comparing the area of undisturbed axial growth of dendrites to the area of lateral growth concluded that the low-angle boundaries-like (LAB-like) defects were created in the root as a result of unsteady-state lateral growth of some secondary dendrite arms in layers of the root located directly at the selector–root connection plane. Additional macroscopic low-angle boundaries (LABs) with higher misorientation angles were created as a result of concave curvatures of liquidus isotherm in platform-like regions near selector–root connections. Two kinds of vacancy-type defects, mono-vacancies and vacancy clusters, were determined in relation to the LABs and LAB-like defects. Only mono-vacancies appeared in the areas of undisturbed axial growth. Reasons for the creation of macroscopic LABs and LAB-like defects, and their relationships with vacancy-type defects were discussed. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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8 pages, 3164 KiB

Open AccessArticle

Grain Selection in a High-Efficiency 2D Grain Selector During Casting of Single-Crystal Superalloys

by Xintao Zhu, Fu Wang, Dexin Ma and Andreas Bührig-Polaczek

Materials 2019, 12(5), 789; https://0-doi-org.brum.beds.ac.uk/10.3390/ma12050789 - 07 Mar 2019

Cited by 7 | Viewed by 2821

Abstract

Using electron backscattered diffraction techniques (EBSD) and optical microscopy (OM), the grain selection and competitive growth in a new-designed high-efficiency two-dimensional (2D) selector during solidification of Ni-based single-crystal (SX) superalloys have been investigated with emphasis on the geometry of the selector part in this article. It is found that the efficiency of the grain selector depends greatly on the thickness and eccentric distance of the selector part. When the thickness is smaller than 3 mm, a single grain can be selected. After reducing this value, the grain selector becomes more effective. When the eccentric distance is larger than 8 mm, one grain can be selected. As the eccentric distance increases, the selector’s efficiency is optimized. Recommendations for optimizing the geometry of the selector part are provided. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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9 pages, 5555 KiB

Open AccessArticle

Development of a High-Efficiency Z-Form Selector for Single Crystal Blades and Corresponding Grain Selection Mechanism

by Xintao Zhu, Fu Wang, Dexin Ma and Andreas Bührig-Polaczek

Materials 2019, 12(5), 780; https://0-doi-org.brum.beds.ac.uk/10.3390/ma12050780 - 07 Mar 2019

Cited by 9 | Viewed by 3168

Abstract

Single crystal (SX) is widely used in modern turbine blades to improve the creep fracture, fatigue, oxidation, and coating properties of the turbine, so that the turbine engine has excellent performance and durability. In this paper, the single crystal super alloy MM247LC is used as the research material. The evolution of grain structure in a two-dimensional grain selector was studied by directional experiments, and the mechanism of grain selection in the two-dimensional channel during directional solidification was clarified. In order to optimize the production process of single crystal turbine blades, the effects of the geometrical structure of a Z-type separator (i.e., wire diameter and take-off angle) on the crystal orientation, microstructure, and grain efficiency of blades were discussed. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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9 pages, 3482 KiB

Open AccessArticle

Effects of Withdrawal Rate on the Microstructure of Directionally Solidified GH4720Li Superalloys

by Jinglong Qu, Shufeng Yang, Zhengyang Chen, Jingshe Li, Anping Dong and Yu Gu

Materials 2019, 12(5), 771; https://0-doi-org.brum.beds.ac.uk/10.3390/ma12050771 - 06 Mar 2019

Cited by 1 | Viewed by 2549

Abstract

Increasing the ingot size of GH4720Li superalloys makes it difficult to control their microstructure, and the withdrawal rate is an important factor in controlling and refining the microstructure of GH4720Li superalloys. In this study, GH4720Li superalloy samples were prepared via Bridgman-type directional solidification with different withdrawal rates. The morphology and average size of the dendrites in the stable growth zone during directional solidification in each sample, morphology and average size of the γ’ phases, and microsegregation of each alloying element were analyzed using optical microscopy, Photoshop, Image Pro Plus, field emission scanning electron microscopy, and electron probe microanalysis. Increasing the withdrawal rate significantly helped in refining the superalloy microstructure; the average secondary dendrite arm spacing decreased from 133 to 79 µm, whereas the average sizes of the γ’ phases in the dendrite arms and the interdendritic regions decreased from 1.02 and 2.15 µm to 0.69 and 1.26 µm, respectively. Moreover, the γ’ phase distribution became more uniform. The microsegregation of Al, Ti, Cr, and Co decreased with the increase in the withdrawal rate; the segregation coefficients of Al, Cr, and Co approached 1 at higher withdrawal rates, whereas that of Ti remained above 2.2 at all the withdrawal rates. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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18 pages, 6335 KiB

Open AccessArticle

Effect of Long-Term High Temperature Oxidation on the Coking Behavior of Ni-Cr Superalloys

by Stamatis A. Sarris, Manjunath Patil, Kim Verbeken, Marie-Françoise Reyniers and Kevin M. Van Geem

Materials 2018, 11(10), 1899; https://0-doi-org.brum.beds.ac.uk/10.3390/ma11101899 - 04 Oct 2018

Cited by 15 | Viewed by 2890

Abstract

The service time of an industrial cracker is strongly dependent on the long-term coking behavior and microstructure stability of the reactor coil alloy. Super alloys are known to withstand temperatures up to even 1400 K. In this work, several commercially available alloys have been first exposed to a long term oxidation at 1423 K for 500 h, so-called metallurgic aging. Subsequently, their coking behavior was evaluated in situ in a thermogravimetric setup under ethane steam cracking conditions (T_gasphase = 1173 K, P_tot = 0.1 MPa, X_C2H6 = 70%, continuous addition of 41 ppmw S/HC of DMDS, dilution δ = 0.33 kg_H2O/kg_HC) and compared with their unaged coking behavior. The tested samples were also examined using scanning electron microscopy and energy diffractive X-ray for surface and cross-section analysis. The alloys characterized by increased Cr-Ni content or the addition of Al showed improved stability against bulk oxidation and anti-coking behavior after application of metallurgic aging due to the formation of more stable oxides on the top surface. Full article

(This article belongs to the Special Issue Superalloys–Currents Trends in Development of Their Microstructure and Properties)

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