Heat Treatment Induced Structural Transformations and High-Temperature Oxidation Behavior of Al₂₁Co₂₂Cr₂₂Fe₁₃Ni₂₂ High-Entropy Coatings Produced by Non-Vacuum Electron Beam Cladding

Round 1

Reviewer 1 Report

The manuscript titled “Heat treatment induced structural transformations and high-temperature oxidation behavior of Al21Co22Cr22Fe13Ni22 high-entropy coatings produced by non-vacuum electron beam cladding” focuses on important aspect of microstructural changes caused by annealing and oxidation in Al21Co22Cr22Fe13Ni22 coatings obtained by non-vacuum electron-beam cladding. The selected measurement methods are adequate to the problem. The results are described correctly and their interpretation is proper. I have practically no criticisms. I saw only two errors in the work that should have been corrected before publication:

-          In line 348. It should be “Figure 3” not “Figure X”

-          In line 343 Russian language appear.

Author Response

The authors gratefully thank the Reviewers and the Editors for their contributions to improving this manuscript and their detailed comments. We have considered all the comments and we agreed with all the recommendations. We hope that the paper can be accepted for publication in the present form. Our modifications are marked using a yellow color below and in the file of the revised manuscript “metals-2612024_revised”. (The corrections to the comment on English grammar are marked separately and can be seen using the review mode of Microsoft Word in the file “metals-2612024_revised_ write mode”).

Reviewer #1:

Comments and Suggestions for Authors:

 

The manuscript titled “Heat treatment induced structural transformations and high-temperature oxidation behavior of Al21Co22Cr22Fe13Ni22 high-entropy coatings produced by non-vacuum electron beam cladding” focuses on important aspect of microstructural changes caused by annealing and oxidation in Al21Co22Cr22Fe13Ni22 coatings obtained by non-vacuum electron-beam cladding. The selected measurement methods are adequate to the problem. The results are described correctly and their interpretation is proper. I have practically no criticisms. I saw only two errors in the work that should have been corrected before publication:

Comment from the authors:

The authors thank the reviewer for high appreciation of the manuscript.

Comment #1:

In line 348. It should be “Figure 3” not “Figure X”

 Response to the comment #1:

 We thank the Reviewer for thorough analysis of the text. The Figure number was corrected.

Comment #2:

In line 343 Russian language appear.

 Response to the comment #2:

We thank the Reviewer for the remark. The text was corrected as follows.

“However, after 50 h of annealing, the SXRD patterns showed weak B2 reflections (see the scaled area in Fig. 5a, pattern #2)”

Besides, Figure 5a was modified by including the scaled areas. This was done to show the weak reflections of B2 phase.

Reviewer 2 Report

The article is very interesting and well-written. It contains a great deal of information geared towards both surface engineering and HEA professionals. I have a few comments and observations on the publication text - which I have included below:

Please indicate clearly what is innovative in the publication, HEA rates alone are no longer an indicator of innovation. 

Line: 35.....resistance at elevated temperatures [1-9]. - Too many citations in one paragraph. Later parts are explained in detail, The properties presented are obvious, in fact it is book knowledge at this point, so it could even be written without citations. 

Line 91: The blocks of steel substrates had the sizes 100 × 50 × 10 mm3 . - Incorrect notation, mm3 is a volume dimension and here we have mm 

With the equipment, e.g. VSE VACUUMFURNACE, the manufacturer, city, and country should be given, and all microscopes should also be so described according to the requirements of the journal. 

Figure 8 has escaped and is after the conclusions.

There is a bit of a lack of EBSD studies that unambiguously identify the type of phase in this type of alloy. Are the authors able to add such studies to the paper?

Author Response

The authors gratefully thank the Reviewers and the Editors for their contributions to improving this manuscript and their detailed comments. We have considered all the comments and we agreed with all the recommendations. We hope that the paper can be accepted for publication in the present form. Our modifications are marked using a yellow color in the file with responses and in the file of the revised manuscript “metals-2612024_revised”. (The corrections to the comment on English grammar are marked separately and can be seen using the review mode of Microsoft Word in the file “metals-2612024_revised_ write mode”).

Reviewer #2:

Comments and Suggestions for Authors:

 Comment #1:

• The article is very interesting and well-written. It contains a great deal of information geared towards both surface engineering and HEA professionals. I have a few comments and observations on the publication text - which I have included below:

Response to comment #1:

The authors thank the Reviewer for high assessment of the manuscript.

 Comment #2:

• Please indicate clearly what is innovative in the publication, HEA rates alone are no longer an indicator of innovation. 

Response to comment #2:

The scientific novelty of the study was clarified in the introduction part as follows:

The literature analysis showed that structural evolution of Al-Co-Cr-Fe-Ni HEAs was studied in detail only for the equiatomic composition or for the Al_хCoCrFeNi system. This work examines the structural transformations of an Fe-depleted alloy of the Al-Co-Cr-Fe-Ni family and compares it with the equiatomic one.

The non-vacuum electron-beam cladding technique was used in this study to produce coatings. This technique is a promising high-performance method for the industrial production of coatings. Due to the high initial energy of electrons (1.4–2.2 MeV) and high beam power (up to 100 kW), this method allows applying coatings up to 2 mm thick in a single pass. However, the structure and phase transformations during annealing of AlCrCoFeNi HEA coatings obtained by non-vacuum electron beam cladding haven’t yet been considered in the scientific literature. Thus, in this work the effect of annealing on structure of Al-Cr-Co-Fe-Ni HEA coating obtained by non-vacuum electron-beam cladding on low-carbon steel was investigated for the first time.

The structure of material near the “coating-substrate” interface also changes during annealing. The reliability, durability, and adhesive properties of the coatings often depend on the structure of the interface. Therefore, one of the important tasks is to study the structural transformations at the interface between the Al-Cr-Co-Fe-Ni HEAs coating and the substrate. However, this issue is rarely considered in detail in publications on AlCrCoFeNi HEA coatings. Since one of the potential applications of Al-Cr-Co-Fe-Ni HEAs is protective heat resistant coatings, the structural changes during annealing should be considered in combination with the oxidation resistance of HEA coatings at the same conditions.

Thus, in this study, the structure of the sample before and after annealing was thoroughly characterized both in the coating and at the “coating-substrate” interface, and the oxidation behavior of the coatings were estimated.”

 Comment #3:

• Line: 35.....resistance at elevated temperatures [1-9]. - Too many citations in one paragraph. Later parts are explained in detail, The properties presented are obvious, in fact it is book knowledge at this point, so it could even be written without citations. 

 Response to comment #3:

We agree with the Reviewer, the described properties of AlCrCoFeNi-based НЕАs are widely known and are described in many books. We deleted the citations here.

Comment #4:

• Line 91: The blocks of steel substrates had the sizes 100 × 50 × 10 mm3 . - Incorrect notation, mm3 is a volume dimension and here we have mm 

Response to сomment #4:

We thank the Reviewer for the comment. The unit was corrected to “mm”.

Comment #5:

• With the equipment, e.g. VSE VACUUMFURNACE, the manufacturer, city, and country should be given, and all microscopes should also be so described according to the requirements of the journal. 

Response to comment #5:

The manufacturer, city, and country for each equipment were added to the text as follows:

“The cladding experiments were performed on an industrial electron accelerator ELV-8 (Institute of Nuclear Physics, Novosibirsk, Russian Federation)”

“The samples were annealed in a VSE VACUUMFURNACE (Vacuum Technologies and Equipment Ltd., Novosibirsk, Russian Federation)”

“Samples for metallographic studies were cut on a Struers Discotom-100 (Struers GmbH, Willich, Germany)”

“Structural observations were performed using a Carl Zeiss Axio Observer Z1m (Zeiss Microscopy, Munich, Germany) light microscope (LM), Carl Zeiss Sigma 300 and Carl Zeiss EVO 50 XVP (Zeiss Microscopy, Munich, Germany) scanning electron microscopes (SEM) equipped with an Oxford Instruments X-Act energy-dispersive X-ray (EDX) spectrometer (Oxford Instruments, Tubney Woods, Abingdon, UK).”

“To determine the phase composition, the samples were examined by synchrotron X-ray diffraction (SXRD) on synchrotron radiation facility VEPP-4 (Institute of Nuclear Physics, Novosibirsk, Russian Federation). For this purpose, the 0.5 mm thick specimens were cut from the coating on a Sodick AG400L EDM machine (Sodick Inc., Schaumburg, IL, USA).”

“The diffraction patterns were recorded on a Mar345 detector (3500 × 3500 pixel resolution, pixel size 200 × 200 μm²) (MarXperts GmbH, Norderstedt, Germany), the exposure time was 2 min.”

“Vickers microhardness was measured using a Wolpert Group 402MVD (Wolpert, Borgharenweg, Netherlands) machine.”

“The surface of the oxidized specimens was examined using a Carl Zeiss Sigma 300 (Zeiss Microscopy, Munich, Germany) SEM.”

 Comment #6:

• Figure 8 has escaped and is after the conclusions.

Response to comment #6:

Figure 8 was moved above and inserted after the first mention in the text.

 Comment #7:

• There is a bit of a lack of EBSD studies that unambiguously identify the type of phase in this type of alloy. Are the authors able to add such studies to the paper?

 

Response to comment #7:

We agree with the comment. EBSD analysis of the samples would help to confirm the conclusions concerning the type of phases unambiguously. Unfortunately, our SEM with EBSD installation is out of order for this moment. Anyway, based on optical microscopy, X-ray diffraction and EDX analysis, as well as comprehensive analysis of literature data, we have no doubt about the type of the phases and their localization. In future, we are planning to provide TEM and SAED investigations of the precipitations, which appeared during heating in the series of samples including those described in this study. These results will be published as a separate article.

For more convincingness, we added the following references in the section “3.1 Structure of the coating”, which confirm our observations and conclusions:

“A typical dendritic structure was observed in the coating after crystallization. The dendritic bodies consisted of the bcc phase, whereas the fcc phase was predominantly distributed in the interdendritic spaces in the shape of discontinuous thin interlayers with a thickness not exceeding 2 μm (Fig. 2a,b). The EDX analysis results of local dendritic and interdendritic zones are presented in table 3. It follows that the bcc phase was enriched in Al and Ni, while the fcc phase was enriched in Fe and Cr (EDX point # 1 and 2, Fig. 2b). Such dendritic structure and elements distribution in bcc and fcc phases are typical for as-cast Al-Co-Cr-Fe-Ni alloys [29–31].”

Author Response File: Author Response.pdf

Reviewer 3 Report

Ogneva et al. present results of the structural transformations in Al-Cr-Co-Fe-Ni-based high-entropy alloy coating obtained by non-vacuum electron-beam cladding on low-carbon steel, and evaluate the oxidation behavior of this coating at high temperatures. Overall, the idea received my attention and the methodology is technically sound. However, there are some specific issues the authors should address by making modifications before we can proceed and positive action can be taken.

 

 

1. I found three subsections enumerated as 2.1 in the manuscript: “2.1. Preparation of samples”, “2.1. Methods of the investigation” and “2.1. Structure of the coatings”. Two subsections enumerated as 3.3…

2. When you use an abbreviation in both the abstract and the text, define it in BOTH places upon first use. Further, after you define an abbreviation, use only the abbreviation. Do not alternate between spelling out the term and abbreviating it. i.e., EDX, SXRD…

3. The English language requires improvements. Spelling and grammatical errors exist in the manuscript. i.e., are shown in Figures X b,c; the inclusions in the dendrites took an elongated shape, them thickness was about 0.5 μm,… The coating zone where internal aluminum oxides and nitrides were formed, were… We recommend you ask a native English speaker to edit the paper or use an independent professional editor.

4. The authors study the oxidation behavior. Have the authors noticed some other studies related to this topic? i.e., [Electronic and optical properties of heterostructures based on transition metal dichalcogenides and graphene-like zinc oxide. Sci. Rep. 2018, 8, 12009, doi:10.1038/s41598-018-30614-3] and [MoS2/ZnO van der Waals heterostructure as a high-efficiency water splitting photocatalyst: a first-principles study. Phys. Chem. Chem. Phys. 2018, 20, 13394–13399, doi:10.1039/C8CP00808F]…

5. The conclusions section reads rather long and should be streamlined.

The English language requires improvements. Spelling and grammatical errors exist in the manuscript. i.e., are shown in Figures X b,c; the inclusions in the dendrites took an elongated shape, them thickness was about 0.5 μm,… The coating zone where internal aluminum oxides and nitrides were formed, were… We recommend you ask a native English speaker to edit the paper or use an independent professional editor.

Author Response

The authors gratefully thank the Reviewers and the Editors for their contributions to improving this manuscript and their detailed comments. We have considered all the comments and we agreed with all the recommendations. We hope that the paper can be accepted for publication in the present form. Our modifications are marked using a yellow color below and in the file of the revised manuscript “metals-2612024_revised”. (The corrections to the comment on English grammar are marked separately and can be seen using the review mode of Microsoft Word in the file “metals-2612024_revised_ write mode”).

Reviewer #3:

 Comment #1:

1. I found three subsections enumerated as 2.1 in the manuscript: “2.1. Preparation of samples”, “2.1. Methods of the investigation” and “2.1. Structure of the coatings”. Two subsections enumerated as 3.3…

Response to comment #1:

The mistakes in numeration were corrected.

Comment #2:

1. When you use an abbreviation in both the abstract and the text, define it in BOTH places upon first use. Further, after you define an abbreviation, use only the abbreviation. Do not alternate between spelling out the term and abbreviating it. i.e., EDX, SXRD…

Response to comment #2:

The order of abbreviations appearance was corrected according to the suggestion in the comment.

 Comment #3:

1. The English language requires improvements. Spelling and grammatical errors exist in the manuscript. i.e., are shown in Figures X b,c; the inclusions in the dendrites took an elongated shape, them thickness was about 0.5 μm,… The coating zone where internal aluminum oxides and nitrides were formed, were… We recommend you ask a native English speaker to edit the paper or use an independent professional editor.

Response to comment #3:

We thank the Reviewer for detailed analysis of the text, and we fully agree with this comment. The manuscript was checked and edited. Since there were a lot of corrections, we uploaded the additional file of the manuscript entitled “metals-2612024_revised_ review mode”, where all corrections on this comment can be seen in the review mode of Microsoft Word.

 Comment #4:

1. The authors study the oxidation behavior. Have the authors noticed some other studies related to this topic? i.e., [Electronic and optical properties of heterostructures based on transition metal dichalcogenides and graphene-like zinc oxide. Sci. Rep. 2018, 8, 12009, doi:10.1038/s41598-018-30614-3] and [MoS2/ZnO van der Waals heterostructure as a high-efficiency water splitting photocatalyst: a first-principles study. Phys. Chem. Chem. Phys. 2018, 20, 13394–13399, doi:10.1039/C8CP00808F]…

Response to comment #4:

The authors thank the Reviewer for the useful references. The references were added to the manuscript:

“The concept of high-entropy alloys (HEAs) was reported in 2004 for the first time by Yeh et al. [1], and, since then this idea have been developing by many researchers. Al-Cr-Co-Fe-Ni HEAs has attracted a lot of interest due to its unique set of properties, including, but not limited to, high wear resistance, strength, hardness, and oxidation resistance at elevated temperatures. Compared to other HEA systems, Al-Cr-Co-Fe-Ni alloys are characterized by a low weight gain during oxidation. The structure and composition of oxides plays an important role in performance characteristics of materials [8–11], including the oxidation behavior as well. In Al-Cr-Co-Fe-Ni alloys, the stable rhombohedral Cr₂O₃ and Al₂O₃ oxide films grow on the surface during oxidation and protect them from intensive interaction with air [2–7]. In this regard, one of the perspective applications of Al-Cr-Co-Fe-Ni alloys is heat-resistant coatings for protection the Fe-based structural alloys from oxidation at elevated temperatures.”

 Comment #5:

1. The conclusions section reads rather long and should be streamlined.

 Response to comment #5:

The conclusions section was streamlined according the suggestion. Also, a brief description of the future research direction for future work in this field were added to the conclusion in the last paragraph according to the advice of another Reviewer. As a result, the conclusions section was modified as follows:

“4. Conclusions

Bcc (A2 + B2) + fcc (A1) high-entropy coating Al₂₁Co₂₂Cr₂₂Fe₁₃Ni₂₂ with the fcc interlayer at the “coating-substrate” interface were formed on a low-carbon steel substrate using non-vacuum electron-beam cladding of powders. After annealing at 900 °C, elongated fcc particles and rounded particles of the σ-phase precipitated in the coating. In the fcc interlayer, annealing promoted the precipitation of nanoscale plates of the B2 bcc phase. The precipitation of fcc and σ-phase led to decrease in microhardness of the coating from 563 HV to 441 HV; however, microhardness level over cross-section of the coating became more uniform.

The oxidation of the coatings in air at 900 °C during 50 h promoted the formation of oxide films consisting of two sublayers. The top sublayer contained large rhomboid crystals of Cr₂O₃ and spinel crystals of a branched rose-like morphology. The lower sublayer mainly contained needle-like crystals of Al₂O₃.

Due to high corrosion resistance such alloys could replace more brittle intermetallic compounds as protective coatings for Fe-based alloys. For further investigations, the effect of composition on mechanical properties of the coatings should be considered. In addition, when cladding the powder mixtures with different compositions, the amount of Fe transferred from the substrate can vary, that is also an interesting issue for future studies.”

Author Response File: Author Response.pdf

Reviewer 4 Report

 

Overall, this study systematically explored the structural evolution and antioxidant properties of the non-vacuum electron beam coated Al-Cr-Fe-Ni high-entropy alloy coatings, resulting in a number of valuable experimental results and a informative data-contributing article. It is hoped that these suggestions will help the authors improve their thesis. Please keep trying and I look forward to reading the author's follow-up research.

1. The title and excerpt can clearly express the main contents and contributions of the thesis. However, some of the statements in the summary can be improved slightly to make them more explicit and concise. For example, the phrase "the preparation of FCC articles and sigma phases in the main area of the coasting warning led to a decision in the level of microhardness" could be replaced with "the preparation of FCC and sigma phasms prepared for the microhardness of the coasting."

2. The sample preparation and characterization methods of the experimental part are described in detail, which supports the analysis of subsequent results. However, some chapters are too detailed, and consideration could be given to streamlining some of the details that do not affect understanding to highlight the main points.

3. Results The whole structure of the analysis part is clear, and the text matches the image well. However, the presentation of individual images can be further improved, and the subplot notation in Figure 3 can be adjusted to match the text description for quick focus.

(1) There is an error in the serial number of the subheadings in the Results section.

(2) EDX of Figure 2b in Outcome 3.1 shows that BCC phase is rich in Al and Ni, while FCC phase is rich in Fe and Cr. Relevant literature can be cited for discussion.

（3）3.2. Section 3 annealing induced the formation of B2 phase particles along the required grain boundary of FCC. It is suggested to add TEM observations to confirm its chemical composition and discuss the drivers of this phase transition.

(4) The gold phase diagram in Figure 3 observed some distinct changes in organizational characteristics that can be mapped and discussed in depth in conjunction with the EDX results.

4. The phase transition mechanism of high-entropy alloys involved in this paper is complex, and the authors have carried out a lot of experimental characterization. However, there is room to strengthen the theoretical analysis.

5. The conclusion can be added to the potential application prospect of the alloy, increase the significance of the study, and provide a brief description of the future research direction for future work in this field.

 

 

 

 

 

Author Response

The authors gratefully thank the Reviewers and the Editors for their contributions to improving this manuscript and their detailed comments. We have considered all the comments and we agreed with all the recommendations. We hope that the paper can be accepted for publication in the present form. Our modifications are marked using a yellow color in the file with responses and in the file of the revised manuscript “metals-2612024_revised”. (The corrections to the comment on English grammar are marked separately and can be seen using the review mode of Microsoft Word in the file “metals-2612024_revised_ write mode”).

Comments and Suggestions for Authors:

 Overall, this study systematically explored the structural evolution and antioxidant properties of the non-vacuum electron beam coated Al-Cr-Fe-Ni high-entropy alloy coatings, resulting in a number of valuable experimental results and a informative data-contributing article. It is hoped that these suggestions will help the authors improve their thesis. Please keep trying and I look forward to reading the author's follow-up research.

 Response to reviewer:

The authors thank the Reviewer for high appreciation of the manuscript.

 Comment #1:

1. The title and excerpt can clearly express the main contents and contributions of the thesis. However, some of the statements in the summary can be improved slightly to make them more explicit and concise. For example, the phrase "the preparation of FCC articles and sigma phases in the main area of the coasting warning led to a decision in the level of microhardness" could be replaced with "the preparation of FCC and sigma phasms prepared for the microhardness of the coasting."

 Response to comment #1:

The summary was improved. Also, according to the similar suggestion of another Reviewer, the summary was rewritten and streamlined. As a result, all summary section, including the indicated phrase, was modified as follows:

“4. Conclusions

Bcc (A2 + B2) + fcc (A1) high-entropy coating Al₂₁Co₂₂Cr₂₂Fe₁₃Ni₂₂ with the fcc interlayer at the “coating-substrate” interface were formed on a low-carbon steel substrate using non-vacuum electron-beam cladding of powders. After annealing at 900 °C, elongated fcc particles and rounded particles of the σ-phase precipitated in the coating. In the fcc interlayer, annealing promoted the precipitation of nanoscale plates of the B2 bcc phase. The precipitation of fcc and σ-phase led to decrease in microhardness of the coating from 563 HV to 441 HV; however, microhardness level over cross-section of the coating became more uniform.

The oxidation of the coatings in air at 900 °C during 50 h promoted the formation of oxide films consisting of two sublayers. The top sublayer contained large rhomboid crystals of Cr₂O₃ and spinel crystals of a branched rose-like morphology. The lower sublayer mainly contained needle-like crystals of Al₂O₃.

Due to high corrosion resistance such alloys could replace more brittle intermetallic compounds as protective coatings for Fe-based alloys. For further investigations, the effect of composition on mechanical properties of the coatings should be considered. In addition, when cladding the powder mixtures with different compositions, the amount of Fe transferred from the substrate can vary, that is also an interesting issue for future studies.”

Comment #2:

2. The sample preparation and characterization methods of the experimental part are described in detail, which supports the analysis of subsequent results. However, some chapters are too detailed, and consideration could be given to streamlining some of the details that do not affect understanding to highlight the main points.

Response to comment #2:

We agree with the comment. Detailed sample preparation and characterization methods were shortened.

Comment #3:

3. Results The whole structure of the analysis part is clear, and the text matches the image well. However, the presentation of individual images can be further improved, and the subplot notation in Figure 3 can be adjusted to match the text description for quick focus

 Comment #3.1:

• There is an error in the serial number of the subheadings in the Results section.

Response to comment #3.1:

The numeration was corrected.

Comment #3.2:

• EDX of Figure 2b in Outcome 3.1 shows that BCC phase is rich in Al and Ni, while FCC phase is rich in Fe and Cr. Relevant literature can be cited for discussion.

Response to comment #3.2:

Generally, both Al–Ni rich ordered bcc (B2) and Fe–Cr rich disordered bcc (A2) exist in bcc-type AlCoCrFeNi HEA systems. The following comment was added and the relevant literature was cited:

“The EDX analysis results of local dendritic and interdendritic zones are presented in table 3. It follows that the bcc phase was enriched in Al and Ni, while the fcc phase was enriched in Fe and Cr (EDX point # 1 and 2, Fig. 2b). Such dendritic structure and elements distribution in bcc and fcc phases are typical for as-cast Al-Co-Cr-Fe-Ni alloys [29–31].”

Comment #3.3:

• 2. Section 3 annealing induced the formation of B2 phase particles along the required grain boundary of FCC. It is suggested to add TEM observations to confirm its chemical composition and discuss the drivers of this phase transition.

Response to comment #3.3:

We totally agree with the comment. In spite of the fact that we have no doubt about the type of phases precipitated during heating (this conclusion is done using a combination of light microscopy, X-ray diffraction and EDX analysis, as well as based on the literature data), TEM studies would help to confirm the conclusions unambiguously. Obviously, TEM observations would discover new circumstances and features of the structure, which would allow more deep discussion about driving mechanisms of this phase formation.  However, we believe that such labor-intensive analysis should be presented in a separate paper. We are planning to provide TEM and SAED investigations for the series of samples including those described in this study, and to publish the results as a separate article next year.

Anyway, in the literature data to which we referred, the precipitation of B2 nanoparticles in fcc matrix of  Al–Co–Cr–Fe–Ni HEAs were observed by TEM. We added this clarification to the text in the section “3.2.3. Structural transformations occurring at the “coating-substrate” interface”:

“In [35], the above calculations were confirmed experimentally by TEM, where the appearance of B2 particles in the fcc matrix of the Al_0.3CoCrFeNi alloy at 700 °C was shown. In studies of Gwalani et al. in [36], B2 particles also were detected by TEM when the alloy was heated above 800 °C. In [33], B2 particles were precipitated after 5 h of homogenization at 1250 °C. In all these works, B2 particles were also precipitated in the fcc of Al–Co–Cr–Fe–Ni HEAs mainly in the shape of nanosized plates, the dimensions of which depended on the temperature and duration of annealing.”

The explanation on the drivers of this phase transition were added in the section “3.2.3. Structural transformations occurring at the “coating-substrate” interface”:

“According to the thermodynamic calculations of the AlxCoCrFeNi phase diagrams [32–34], the fcc-phase field on the diagram is extending with increasing temperature despite the fact that Al plays a contradictory role as strong bcc stabilizer. With decreasing temperature below 1250 °C, the amount of Al in fcc-solid solution is decreasing and Ni-Al-enriched B2 solid solution are precipitated.”

Comment #3.4:

(4) The gold phase diagram in Figure 3 observed some distinct changes in organizational characteristics that can be mapped and discussed in depth in conjunction with the EDX results.

Response to comment #3.4:

We thoroughly analyzed the comment 3.4. We believe, that there is some mistake in this comment since our paper is not related to the gold phase diagram.

 Comment #4:

4. The phase transition mechanism of high-entropy alloys involved in this paper is complex, and the authors have carried out a lot of experimental characterization. However, there is room to strengthen the theoretical analysis.

Response to comment #4:

The theoretical analysis about the fcc and σ-phase formation in bcc in the section 3.2.1 was rewritten and supplemented as follows:

“When analyzing the transformations that occurred during annealing in the main part of the coating, it should be noted that according to the equilibrium phase diagrams the σ-phase is stable in Al-Co-Cr-Fe-Ni system [32–34]. The phase equilibria calculations for the Al–Co–Cr–Fe–Ni system predicted the formation of A1+A2+B2+σ-phase mixture in wide concentration range from 10 to 20 at. % of Al [26]. Wang et al. [35] confirmed the phase transition of A2+B2 bcc structure to fcc+σ+B2 structure in Al_0.9–Al_1.2 AlxCoCrFeNi alloys by differential scanning calorimetry. Strumza et al. [36] found that a fcc phase started to precipitate from the B2 bcc matrix at ~590 ℃ and the high-energy interphase at the fcc/bcc boundary led to σ-phase precipitation.

After electron-beam cladding the structure of the coating is in non-equilibrium state due to rapid heat removal from the melt pool towards the massive steel substrate. Besides, the dendritic liquation of the elements occurred during crystallization. When annealing, the alloy decomposed into a matrix with an increased Al and Ni content, and precipitations with higher Fe and Cr content. Therefore, when annealing of Al-Co-Cr-Fe-Ni HEAs, the fcc and σ-phase precipitates as a result of homogenization.

The most known studies on fcc and σ-phase precipitation during annealing are related to the equiatomic AlCoCrFeNi and Al_xCoCrFeNi alloy. In the preset research the amount of Fe was smaller. Besides, the annealing regimes in most analyzed works are different, that makes the comparison with other works complicated. Anyway, some researchers with close regimes of annealing can be compared with the present study. For instance, when studying an AlCoCrFeNi alloy after 18 h of annealing at 900 °C, Strumza et al. [36] revealed the appearance of elongated fcc particles and rounded σ-phase particles, where fcc and σ-phase were predominantly concentrated in the former interdendritic areas, and fine fcc precipitations were observed in the center of the former dendrites. However, the amount and sizes of precipitates in the equiatomic alloy in [36] was noticeably larger than in the present work because of the higher concentration of Fe. According to the calculated data in [31], a decrease in the amount of Fe in the AlCoCrFeNi leads to an increase in the B2 bcc and a decrease in fcc volume fraction.”

The theoretical analysis about the B2 precipitation in fcc sublayer was added to the section 3.2.3:

“According to the thermodynamic calculations of the AlxCoCrFeNi phase diagrams [32–34], the fcc-phase field on the diagram is increasing with increasing temperature despite the fact that Al plays a contradictory role as strong bcc strong stabilizer. With decrease in temperature below 1250 °C, the amount of Al in fcc-solid solution reduces and Ni-Al-enriched B2 phase precipitates.”

Comment #5:

5. The conclusion can be added to the potential application prospect of the alloy, increase the significance of the study, and provide a brief description of the future research direction for future work in this field.

 Response to comment #5:

The proposed information was added to the last paragraph of the conclusions:

“Due to high corrosion resistance such alloys could replace more brittle intermetallic compounds as protective coatings for Fe-based alloys. For further investigations, the effect of composition on mechanical properties of the coatings should be considered. In addition, when cladding the powder mixtures with different compositions, the amount of Fe transferred from the substrate can vary, that is also an interesting issue for future studies.”

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you very much for addressing my comments and taking the time to prepare a response, I have no further objections, I am forwarding the article to the editor, and I will accept it for publication.

best regards

reviewer