Preparation of High Entropy Alloys Without Pre-Alloying, Using Laser Melt Deposition (LMD) Technique

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

coatings-3416663

 

In this work the authors explored the fabrication of an equimolar high entropy CoCrFeNi alloy using laser metal deposition technique on an austenitic stainless steel substrate. They mixed elemental metal powders in a planetary ball mill and directly deposited them to study the effect of the number of layers on alloy composition and substrate mixing. The findings from the experiments showed a significant reduction in concentration across the first four layers, with the mixing of substrates impacting the composition. This research showcases the viability of creating multilayer high-entropy alloys from elemental powders, simultaneously addressing the challenges associated with maintaining compositional stability.

1.      It is essential for authors to explain the reasoning behind their decision regarding the number of layers.

2.      Authors should elucidate the effect that the number of layers has on concentration.

3.      I propose that the authors conduct a study on the stability of this material.

4.      The authors can improve the conclusion section.

5.      Hardness was assessed by the authors for both single and ten-layer configurations. They are required to clarify and substantiate the effect of layer count on hardness.

6.      The authors did not take into account the relationship between temperature and hardness.

In conclusion, my opinion this work is important. This paper can be approved upon the implementation of minor revisions.

Author Response

Dear Editor and Reviewer,

Thank you for the opportunity to submit our manuscript to Coatings, and for the detailed and constructive feedback provided by the reviewer. We greatly appreciate the time and effort invested in evaluating our work. In this response letter, we address all the comments raised by the reviewer and outline the revisions made to the manuscript accordingly.

1. Reviewer’s comment:

“It is essential for authors to explain the reasoning behind their decision regarding the number of layers.”

Response:

Thank you for highlighting this important point. We have provided a detailed explanation of the reasoning behind our decision regarding the number of layers in the manuscript. The updated text can be found in lines 53–59. We hope this clarification addresses your concern.

2. Reviewer’s comment:

“Authors should elucidate the effect that the number of layers has on concentration.”

Response:

Thank you for your valuable comment. We have elaborated on the relationship between the number of layers and concentration in the manuscript. The revised text can be found in lines 137–139 and 149–155. We hope these changes provide the necessary clarification.

3. Reviewer’s comment:

“I propose that the authors conduct a study on the stability of this material.”

Response:

Thank you for your suggestion. We agree that investigating the stability of this material is an important aspect that could further enrich our findings. However, the scope of the current study does not allow us to include an in-depth stability analysis currently. We plan to address this issue in a future research project.

 

 

4. Reviewer’s comment:

“The authors can improve the conclusion section.”

Response:

Thank you for pointing this out. In response, we have revised and condensed the Conclusions section, aiming to more clearly highlight the novelties of our study. We hope that the updated version now effectively communicates the key findings and the unique contributions of our work.

5. Reviewer’s comment:

“Hardness was assessed by the authors for both single and ten-layer configurations. They are required to clarify and substantiate the effect of layer count on hardness.”

Response:

Thank you for your thoughtful comment. We have clarified and substantiated the effect of layer count on hardness in the manuscript. The revised text can be found in lines 175–186. We hope these updates properly address your concern.

6. Reviewer’s comment:

“The authors did not take into account the relationship between temperature and hardness.”

Response:

Thank you for bringing this to our attention. We acknowledge that the relationship between temperature and hardness was not examined in our current study. While this aspect was beyond the scope of our work, we agree that it represents an important avenue for future research. We plan to investigate this relationship in more detail in our subsequent studies.

 

Thank you again for the reviewers' insightful comments, which have significantly improved the quality of the manuscript. I hope that the revisions and clarifications provided in this response letter meet your expectations. Should you have any further questions or concerns, please do not hesitate to contact me.

Reviewer 2 Report

Comments and Suggestions for Authors

In conclusion, the article requires some additions and comments. After these, let us say, minor corrections, I recommend the paper for publication.

Comments for author File: Comments.pdf

Author Response

Dear Editor and Reviewer,

Thank you for the opportunity to submit our manuscript to Coatings, and for the detailed and constructive feedback provided by the reviewer. We greatly appreciate the time and effort invested in evaluating our work. In this response letter, We address all the comments raised by the reviewer and outline the revisions made to the manuscript accordingly.

1. Reviewer’s comment:

“In the light of SEM-EDS measurements the intended HEA composition, is not achieved in the first layer, thus the HEA solid solution with a coherent crystal structure is not formed by 10 layers. Moreover, the EDS measurement shows a 2.5 mm wide transition layer. Authors conclude that significant substrate mixing was observed in the initial layer. In consequence this mixing led to a dilution effect. Moreover, as successive layers were deposited, the influence of substrate mixing diminished up to the 5th to 6th layers, where the desired elemental distribution was achieved. Why is there no experimental evidence of this effect in the paper?”

Response:

As the build-up layers are deposited, mixing occurs at approximately the same rate and volume per layer. As a result after about 4 layers, where the mixing volumen do not contained the "pure" substrate, the mixing contained only materials of substrate from  previously deposited layers. As a consequence, the Fe fraction from the substrate is significantly reduced, as confirmed by EDX measurements.

1. Reviewer’s comment:

“According to the authors, repeated thermal cycles, inherent to the deposition of multiple layers, led to a slight softening of the material. This effect is well known from literature, but it is not known on what basis the authors of the work draw such conclusions. There is no description of how this was observed?”

Response:

Thank you for pointing this out. In response, we have revised the Conclusions section.

 

 

1. Reviewer’s comment:

“Several effects are well known from the literature, for example both 316L steel substrate and CoCrFeNi HEA have the same FCC crystal structure (along with similar lattice constants). Furthermore, no residual stresses were detected at the interface. This is not so surprising since the structure and periodicity of the crystal lattice are remarkably similar, have lattice distortions been observed in this group of compounds? Please comment.”

Response:

Thank you for your thoughtful comment. Indeed, it is well documented in the literature that both 316L steel substrate and CoCrFeNi HEA have the same FCC crystal structure with similar lattice constants. The substrate change was deliberate. Our aim was to have a material with the same crystal structure and a minimal difference in lattice constant. We are currently at the beginning of our experimental with LMD in this type of HEA, so we have not yet experienced a significantly higher lattice strain rate than occurs in principle in HEA.

1. Reviewer’s comment:

“The similar, well-known effect of Marangoni flow, which creates strong convection currents and distributes alloying elements evenly to achieve better grain structure and mechanical properties, is nothing new. What exactly constitutes new qualitative information regarding this class of compounds?”

Response:

We fully agree that the Marangoni flow is not a new phenomenon in itself. However, in our case, we wanted to demonstrate how Marangoni flow during direct HEA production from elemental powders (not prealloyed) affects the microstructure and chemical composition of multilayer films. For alloys made from elemental powders, proper melt stirring and mixing can be critical in achieving a homogeneous composition. Although Marangoni flow itself is not new, we believe that the presented correlation with the HEA system constructed from elemental powders provides a new perspective for understanding the phenomenon.

1. Reviewer’s comment:

“The EBSD images clearly indicate that the grain morphology is predominantly columnar and how does it relate to the postulated (Figure 5 (b)) hardness? Please describe it clearly. Especially since for some time now, an enigmatic term has been used in this class of compounds: the cumulative thermal effects of the deposition process.”

Response:

Thanks for your comment. During the lay-ups, the test specimen was placed on a grid specimen holder, with no other thermal dissipation. As a consequence, the treated layer had the same initial temperature from about the second layer construction on the test specimen, thus inducing epitaxial growth of the newly formed grains.

The hardness values, with the exception of the boundary transition and the top layer, are basically almost identical in the built-up layer, indicating identical hardness values for a longitudinal grain. In the orientation map of the longitudinal grains, no significant misorientation within the same grain is observed, consistent with identical hardness values within the grains.

In addition, when observing the hardness test in more detail, we can also observe a periodicity in the hardness values, which has also been observed in similar work [1]. This type of periodicity may be consistent with the depth of the mixed volume created. To investigate this further, we plan to use more microhardness measurements with smaller step sizes.  

1. Reviewer’s comment:

“In summary, presented here studies demonstrate the feasibility of producing multilayer HEAs directly from elemental powders using LMD. These studies lead to an understanding of the compositional stability and microstructural evolution in additively manufactured HEAs. Either way, the question arises, what is so novel about this communication? Pease explain.”

Response:

Thank you for your remarks again. Based on the best of our knowledge, HEA coatings produced by the LMD technique have only been produced from HEA alloy powders. The novelty of this manuscript is that it demonstrates the feasibility and practicality of producing HEA coatings from elemental powders.

 

Thank you again for the reviewers' insightful comments, which have significantly improved the quality of the manuscript. I hope that the revisions and clarifications provided in this response letter meet your expectations. Should you have any further questions or concerns, please do not hesitate to contact me.

Reviewer 3 Report

Comments and Suggestions for Authors

It is an interesting paper about the preparation of equimolar CoCrFeNi alloy by laser melt deposition, with special reference to the deposition of ten alloy layers. The paper is well written and organized and the cited references are appropriate. Only minor revisions are required prior to publication.

1.      Figure 2b seems to be not particularly informative about the layers characteristics and it is not discussed in the text.

2.      I think that the meaning of Figure 4 is not sufficiently clear. It should be discussed in detail in the text. 

3.      The conclusions should be reported in a more concise way, clearly evidencing the novelties of the paper. Conclusions should not be a summary of the paper content.

 

 

Author Response

Dear Editor and Reviewer,

Thank you for the opportunity to submit our manuscript to Coatings, and for the detailed and constructive feedback provided by the reviewer. We greatly appreciate the time and effort invested in evaluating our work. In this response letter, we address all the comments raised by the reviewer and outline the revisions made to the manuscript accordingly.

1. Reviewer’s comment:

“Figure 2b seems to be not particularly informative about the layers characteristics and it is not discussed in the text.”

Response:

Thank you for your comment. Taking this comment into account, we have added a reference to Figure 2(b) in lines 118-120. We will keep that figure for the purpose of demonstrating the absence of porosity.

2. Reviewer’s comment:

“I think that the meaning of Figure 4 is not sufficiently clear. It should be discussed in detail in the text.”

Response:

Thank you for pointing out this issue. We have revised the description of Figure 4 and updated the corresponding discussion in the manuscript. The revised text can be found in lines 158–169. We hope that these changes make the figure's meaning clearer and more aligned with the work's context.

3. Reviewer’s comment:

“The conclusions should be reported in a more concise way, clearly evidencing the novelties of the paper.”

Response:

Thank you for pointing this out. In response, we have revised and condensed the Conclusions section, aiming to more clearly highlight the novelties of our study. We hope that the updated version now effectively communicates the key findings and the unique contributions of our work.

 

 

 

Thank you again for the reviewers' insightful comments, which have significantly improved the quality of the manuscript. I hope that the revisions and clarifications provided in this response letter meet your expectations. Should you have any further questions or concerns, please do not hesitate to contact me.