Microstructural Evolution of Wrought-Nickel-Based Superalloy GH4169

Round 1

Reviewer 1 Report (New Reviewer)

The idea is interesting, however, it is not clear where the innovation in the current contribution lies. Although the introduction is relevant, in general, the elaboration of the previously reported research is missing. That is, the authors cited the previous works but failed to outline the main findings or shortcomings of those works. The experimental and characterization methods are generally appropriate, although clarification of a few details should be presented. The results and discussions are clear, but some additional discussions should be given for readers to comprehend the reported results. Last but not least, the language editing is required throughout the manuscript. Nevertheless, the following comments are intended to help the authors:

Abstract

·      First sentence, the following part should be corrected as “…to show the fine precipitates and their morphologies.”

·      The following part should be corrected as “The obtained microstructures can vary…”

·      The significance of the main findings from the reported study should be given at the end of this section. Additionally, what the reported research brings to the field of study should be mentioned.

Introduction

·      It feels like there is no connection between the sentences while explaining the development and application of the superalloy GH4169 given in the first paragraph.

·      Please insert appropriate references at the end of the sentence that ends with “…for specific service conditions.” (Lane 34)

·      Overall, the introduction is relevant. Sufficient information about the previous investigations is presented, with the exception of few, for readers to follow the present study. However, in general, elaboration of the previously reported research is missing. That is, the authors cited the previous works but failed to outline the main findings or shortcomings of those works.  

Experiments and Methods

·      The homogenization and the hot forging temperatures / times are chosen based on what? There should be some reasons of choosing the temperature as well as time with respect to microstructural development. Please insert appropriate reference here. 

·      Unless the authors have conducted all the works, please cite the works given in Table 2.

·      Although the scope of the submitted work is about the microstructural evolution and the reaction of the material against different etching to reveal the microstructure, mechanical tests, even a simple hardness test, will be beneficial to correlate the material’s mechanical properties with the obtained microstructures, that is, the second phases. For example, a group at ARL investigated the microstructural evolution of Fe-based alloy during severe plastic deformation and reported that the second phases of different size and compositions had major influences in the high temperature mechanical properties (please take a look at the following paper; High temperature mechanical properties and microstructures of thermally stabilized Fe-based alloys synthesized by mechanical alloying followed by hot extrusion).

Results

·      How did the authors distinguish the inclusions, Laves phases, and the MCs from each other, given in Figure 1, without applying a proper chemical analysis? Also, is it possible to have even smaller second phases that are beyond the detection limit by conventional OM? If so, how they might affect the microstructural evolution and the resultant properties? (also see the suggested reference given above)

·      How do the authors know that the Laves phase is brittle without conducting any mechanical properties or even referencing the literature?

·      Lane 221, please change “cabides” to carbides.

·      Last sentence of page 7; “The most typical multilayer composite structure of MCs is clearly observed in Figure 5, surrounded by a large number of acicular nanoprecipitated phases.” It is questionable how the authors determined the second phases with smaller than 100 nm in diameter. 

·      The corresponding EDS compositional analysis should be provided for the phases given in Figure 6. 

·      It is not clear if the results given under 3.1 belong to the as-cast or homogenized microstructures? What is the difference with respect to microstructural features such as grain size and second phase formations? How did the authors determine that the applied homogenization temperature and time were enough for the studied material? 

·      “When the deformation is small and the deformation temperature is too low or the deformation temperature is too high, the stress-strain mismatch will result in a mixed grain structure.” The authors have not conducted hot forging at different temperature, yet, how did they come up with such conclusions? 

Discussion

·      Effect of second phases formed along the grain boundaries should be discussed with respect to thermal stability and high temperature mechanical properties of the investigated alloy. Dr. Koch’group at NCSU and Dr. Schuh group at MIT have published many papers on that matter (For example: stability of nanocrystalline metals: the role of grain boundary chemistry and structure).

Conclusions

·      A summary of what has been done is given in the conclusion. It lacks the main points like what the reported research brings to the literature and what is its contribution. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

It is really a well written paper dealing with a a material which is today to be considered also low CO2 emission production. My only concern is about the lacking of calling to such topic.

I suggest to modify the sentence at line 29 as following:

"The material was designed for 29 aviation and aerospace power systems, land-based gas turbines, and oil & gas fields  [4,6], fuel cells application, also as an alternative to high quality stainless steels [7-8]

I suggest as references 7 and 8 the following:

7. Corrosion behavior of high temperature fuel cells: issues for materials selection, Metalurgija, 58(3-4), 347-351, 2019

8. Recrystallization and grain growth of super austenitic stainless steel AISI 904L: a multivariate regression approach, Metals, 12(2), 200, 2022

Author Response

Response: We are very grateful for the reviewer's professional comments. In response to the literature suggested by the reviewers, we have supplemented the description of the application of “, fuel cells application, also as an alternative to high quality stainless steels [6-7]”, please see page 1 (line 32-33).

References

6. Sharma, D. K.; Filipponi, M.; Schino, A. D.; Rossi, F.; Castaldi, J. P., Corrosion behaviour of high temperature fuel cells: Issues for materials selection. Metalurgija 2019, 58, 347-351.
7. Stornelli, G.; Gaggiotti, M.; Mancini, S.; Napoli, G.; Rocchi, C.; Tirasso, C.; Di Schino, A., Recrystallization and Grain Growth of AISI 904L Super-Austenitic Stainless Steel: A Multivariate Regression Approach. Metals-Basel 2022, 12, (2), 200.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report (New Reviewer)

All comments fully addressed. Acceptance is recommended.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The authors' efforts to significantly improve the microstructure of the cast Ni-based superalloy can be greatly appreciated. Unfortunately, the paper is written in a rather incomprehensible way, where it is difficult to understand what was actually done and what results were achieved. For example, it is not clear whether heat treatment,  what was proposed after DSC analysis, was also applied before forging, and whether there are any results from this heat treatment.

It is clear from the results that the coarse Laves phase is not present after forging, but the coarse MCs remain, even though they should have been removed by the heat treatment. It is also not clear what phases represent the coarse inclusions that, like the MCs, initiate dynamic recrystallization.

The paper is also not at all concerned with the fine precipitating phases after forging, which are critical for strengthening.

I assume that in the revised version, the authors will either add the information requested in my comments or at least give a proper justification for not adding it to the article.

The article requires major revision.

Author Response

Response to Reviewer 1 Comments

We would like to thank the reviewer 1 providing many professional comments in our manuscript, entitled “Microstructural evolution of wrought Nickel-based superalloy GH4169”. We have revised our manuscript according to these comments. Changes made are highlighted in red in the marked-up manuscript. The detailed responses are shown as follows (highlighted in red). We hope it will satisfy the concerns about the manuscript.

Reviewer comments:

Point 1: The authors' efforts to significantly improve the microstructure of the cast Ni-based superalloy can be greatly appreciated. Unfortunately, the paper is written in a rather incomprehensible way, where it is difficult to understand what was actually done and what results were achieved. For example, it is not clear whether heat treatment, what was proposed after DSC analysis, was also applied before forging, and whether there are any results from this heat treatment.

Response: We thank the reviewer for the recommendation. The parameters of the homogenization heat treatment before forging have been added in detail in the article (line 99), and the heat treatment schedule is generally determined by the DSC thermal analysis curve of as-cast GH4169. The homogenization temperature of the first and second stages is 20-30℃ lower than Laves phase and MC incipient melting temperature, respectively. Combined with the references[1-3], Laves phase can be eliminated or redissolved after homogenization at 1160℃×24h+1200℃×24h (line 363).

[1] Sohrabi, M. J.; Mirzadeh, H.; Rafiei, M., Solidification behavior and Laves phase dissolution during homogenization heat treatment of Inconel 718 superalloy. Vacuum 2018, 154, 235-243.

[2] Mitchell, A., Primary Carbides in Alloy 718. 2012; Vol. 1, pp 161-167.

[3] Miao, Z. J.; Shan, A. D.; Wu, Y. B.; Lu, J.; Hu, Y.; Liu, J. L.; Song, H. W., Effects of P and B addition on as-cast microstructure and homogenization parameter of Inconel 718 alloy. T Nonferr Metal Soc 2012, 22, (2), 318-323.

Point 2: It is clear from the results that the coarse Laves phase is not present after forging, but the coarse MCs remain, even though they should have been removed by the heat treatment. It is also not clear what phases represent the coarse inclusions that, like the MCs, initiate dynamic recrystallization.

Response: We thank the reviewer for this useful comment. Some studies have shown that “The precipitation temperature of MC is about 1252℃” (line 71), and the present study showed that “the initial melting temperature of MC is =1233.7℃” (line 269). “Therefore, the heat treatment temperature is much lower than the initial melting temperature of MC, so that MC rarely dissolve into the matrix. The main second phases represent like inclusions (Al₂O₃), carbides (MC, TiC), and nitrides (TiN), initiate dynamic recrystallization, second phases in Figure 7(b) can explain “the DRX mechanism of particle stimulated nucleation (PSN)” (line 292). See References [3] and [4].

[3] Miao, Z. J.; Shan, A. D.; Wu, Y. B.; Lu, J.; Hu, Y.; Liu, J. L.; Song, H. W., Effects of P and B addition on as-cast microstructure and homogenization parameter of Inconel 718 alloy. T Nonferr Metal Soc 2012, 22, (2), 318-323.

[4] Moretti, M. A.; Dalai, B.; Åkerström, P.; Arvieu, C.; Jacquin, D.; Lacoste, E.; Lindgren, L.-E., High Strain Rate Deformation Behavior and Recrystallization of Alloy 718. Metallurgical and Materials Transactions A 2021, 52, (12), 5243-5257.

Point 3: The paper is also not at all concerned with the fine precipitating phases after forging, which are critical for strengthening.

Response: We thank the reviewer for this professional comment. The fine precipitating phases after forging mainly include such as γ’, γ” and δ phases, and minor secondary phases such as Laves phases and carbides and/or nitrides. The γ” phase is generally considered as Ni₃Nb with a tetragonal unit cell (space group: I4/mmm). But in the presence of constituent elements in alloy 718, Ni₃(Nb, Ti, Al) has also been proposed as the general chemical formula for the γ” phase since this phase can retain other elements too. Similarly, γ’ phase has been given a general chemical formula of Ni₃(Ti, Al, Nb) opposed to Ni₃(Al,Ti) or Ni₃Al with a cubic unit cell (space group: Pm-3m). The δ-Ni₃Nb phase has an orthorhombic unit cell with a space group: Pmmn. See References [5-9]. The influence of fine precipitating phases on the properties of the alloy is particularly important, which we will systematically study in the future.

[5] Silva, C.; Song, M.; Leonard, K.; Wang, M.; Was, G.; Busby, J., Characterization of alloy 718 subjected to different thermomechanical treatments. Mat Sci Eng a-Struct 2017, 691, 195-202.

[6] Reed, R. C., The Superalloys Fundamentals and Applications. The Superalloys: Fundamentals and Applications 2006, 1-372.

[7] Silva, C.; Song, M.; Leonard, K.; Wang, M.; Was, G.; Busby, J., Characterization of alloy 718 subjected to different thermomechanical treatments. Mat Sci Eng a-Struct 2017, 691, 195-202.

[8] Jaeger, J. D.; Solas, D.; Baudin, T.; Fandeur, O.; Schmitt, J. H.; Rey, C., INCONEL 718 Single and Multipass Modelling of Hot Forging. John Wiley & Sons, Inc.: 2012; pp 663-672

[9] Zenk, C. H.; Feng, L.; McAllister, D.; Wang, Y.; Mills, M. J., Shearing mechanisms of co-precipitates in IN718. Acta Mater 2021, 220.

I assume that in the revised version, the authors will either add the information requested in my comments or at least give a proper justification for not adding it to the article.

Author Response File: Author Response.pdf

Reviewer 2 Report

The article describes the microstructural evolution of wrought Nickel-based superalloy. It is interesting, however, some issues demand further investigations.

1. Introduction: „The purpose of this research work is to quickly and conveniently characterize the microstructure...” The purpose of the article should be rewritten since the paper should tackle the scientific problem not only characterize the microstructure e.g. Optimizing the microstructure of superalloy by adjusting the size and shape of carbides to slow down the initiation and propagation of microcracks.

2. Experiments and methods:

   Please give details about SEM observations e.g. voltage, SE/BSE mode

   Please give details about EDS analysis e.g. voltage

3. What is the accuracy of the particle size measuremets from the light microscope? It is rather not measured with the accuracy of 0.01 microm.

4. Fig.4 Please identify peaks in the EDS spectrum - white arrow:Laves

5. Please be careful with the words structure/microstructure like line 323 the bulk structure.

6. The phase analysis should be enriched with TEM diffraction e.g. referring to M23C6 carbides.

7.  Table 3: Please verify/comment on the given carbon content and its accuracy. Carbon may also come from the contamination.

8. Discussion: should be imroved since it is a summary but it should discuss the mentioned scientific problem.

Author Response

Response to Reviewer 2 Comments

We would like to thank the reviewer 2 providing many professional comments in our manuscript, entitled “Microstructural evolution of wrought Nickel-based superalloy GH4169”. We have revised our manuscript according to these comments. Changes made are highlighted in red in the marked-up manuscript. The detailed responses are shown as follows (highlighted in red). We hope it will satisfy the concerns about the manuscript.

Reviewer comments:

The article describes the microstructural evolution of wrought Nickel-based superalloy. It is interesting, however, some issues demand further investigations.

Response: We thank the reviewer for the recommendation.

Point 1: Introduction: “The purpose of this research work is to quickly and conveniently characterize the microstructure...” The purpose of the article should be rewritten since the paper should tackle the scientific problem not only characterize the microstructure e.g. Optimizing the microstructure of superalloy by adjusting the size and shape of carbides to slow down the initiation and propagation of microcracks.

Response: We are very grateful to the reviewer's professional comments. We have supplemented the introduction “The microstructure of superalloys can also be optimized by controlling the size and shape of second phases such as carbides and inclusions to slow down the initiation and propagation of microcracks” (line 89).

Point 2: Experiments and methods:

Please give details about SEM observations e.g. voltage, SE/BSE mode

Please give details about EDS analysis e.g. voltage.

Response: We thank the reviewer for professional comments. We have given details about SEM observations (line 122) and EDS analysis (line 127) in the revised version.

Point 3: What is the accuracy of the particle size measurements from the light microscope? It is rather not measured with the accuracy of 0.01 micron.

Response: We have supplemented “The average grain size and particle size measurements from the light microscope were calculated using Image J software with the accuracy of 0.01 micron (line 124)”.

Point 4: Fig.4 Please identify peaks in the EDS spectrum - white arrow: Laves.

Response: We thank the reviewer for this serious error. We have modified the white arrow to Point B in Figure 4.

Point 5: Please be careful with the words structure/microstructure like line 323 the bulk structure

Response: We thank the reviewer for this professional comment. We have changed “and the bulk structure is more prone to fracture” to “The larger the size of the particles, the more prone to fracture (line 327)”.

Point 6: The phase analysis should be enriched with TEM diffraction e.g. referring to M₂₃C₆ carbides.

Response: We thank the reviewer for this professional comment. In Fig. 1, the M₂₃C₆ carbides are mostly located on grain boundaries. See reference [1].

Fig. 1. SEM images of all samples after double hit deformation at 900 ℃ and 0.1 s⁻¹.

In reference [2], M₂₃C₆ Carbide with the Lattice Parameter of a = 1.094 nm. The details are as follows:

Fig. 2 (d) are corresponding EDS spectrums of the globular particles in (b)

Fig. 3 (a) TEM bright-field micrograph of the specimen (b) corresponding EDS. Lα and Kα peaks of the elements Mo, Nb, and Cr are indicated (c), (d) selected area diffraction (SAD) pattern of the particle in the [001] and [012] zone axes

These are the results of others' research, and this issue is important for us to study systematically in the future.

[1] Paramo-Kanetas, P. J.; Ozturk, U.; Calvo, J.; Guerrero-Mata, M. P.; Zamora-Antunano, M. A.; Cabrera-Marrero, J. M., Analysis of strain-induced precipitates by delta-processing in Inconel 718 superalloy. Mater Charact 2021, 173.

[2] Aghajani, A.; Tewes, J.; Parsa, A. B.; Hoffmann, T.; Kostka, A.; Kloewer, J., Identification of Mo-Rich M₂₃C₆ Carbides in Alloy 718. Metall Mater Trans A 2016, 47a, (9), 4382-4392.

Point 7: Table 3: Please verify/comment on the given carbon content and its accuracy. Carbon may also come from the contamination.

Response: We thank the reviewer for this professional comment. The content of carbon in Table 3 is mainly for the semi-quantitative analysis of MC. Currently carbon does not come from the contamination. The mapping distributions in Figure 4 confirm the existance of carbon. The analysis of carbon is also presented in the same way in the references [3].

[3] Firoz, R.; Basantia, S. K.; Khutia, N.; Bar, H. N.; Sivaprasad, S.; Murthy, G. V. S., Effect of microstructural constituents on mechanical properties and fracture toughness of Inconel 718 with anomalous deformation behavior at 650 degrees C. J Alloy Compd 2020, 845.

Point 8: Discussion: should be improved since it is a summary but it should discuss the mentioned scientific problem.

Response: We thank the reviewer for this professional comment. We have made additional changes in “Discussion”(line 377-388).

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors did not improved the paper very much, but sufficiently.  I must do only one comment:

The authors should better distinguish between melting a phase and its dissolution in the solid state. After correcting this in the text, the paper can be published.

Author Response

The authors did not improved the paper very much, but sufficiently. I must do only one comment:

The authors should better distinguish between melting a phase and its dissolution in the solid state. After correcting this in the text, the paper can be published.

Response: We thank the reviewer for this useful comment. We have changed “melting” to “dissolving” at the corresponding position in the text.

Reviewer 2 Report

1.       The average grain size and particle size measurements from the light microscope were calculated using Image J software with the accuracy of 0.01 micron (line 124). The accuracy should be related with the resolution of the applied microscope. What is the resolution of your light microscope? Is it 0.01 micron? it is rather impossible.

2.       If carbon does not come from the contamination, please explain how the surface of samples was prepared for the investigations.

In Fig 4. In the EDS spectrum point 3 carbon is labelled, however it is presence is not mentioned in the table. How is it  possible?

3.       Fig.4 Please identify peaks in the EDS spectrum, the first peaks are still not labelled.

4.       The phase analysis should be enriched with TEM diffraction e.g. referring to M₂₃C₆ carbides. If it is not added, please remove the part referring to their presence.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

The EDS peaks should be fully identified.

The explanation of the carbon content is inconsistent.

The explanation sent by the Authors shows that the analysis was not performed in the professional way.

Author Response

The EDS peaks should be fully identified.

The explanation of the carbon content is inconsistent.

The explanation sent by the Authors shows that the analysis was not performed in the professional way.

Response: We thank the reviewer for this useful comment. We have read a lot more literature, and in response to the professional comments of the reviewers, For the two problems that the EDS spectra peak is not marked and the carbon content is inconsistent, we have deleted these inaccurate data.

Round 4

Reviewer 2 Report

The inaccurate data should be further analysed, not simply deleted.