Tungsten Matrix Composite Reinforced with CoCrFeMnNi High-Entropy Alloy: Impact of Processing Routes on Microstructure and Mechanical Properties

Round 1

Reviewer 1 Report

The subject matter of the manuscript is novel and interesting. The manuscript itself is well written. I have no comments, except that it was a pleasure to review it.

Author Response

Thanks for the comments.

Reviewer 2 Report

The reported work represents really incremental results and much more need to be improved if reconsideration is still appropriate.

(1) no chemical analysis of the SPS samples was given.

(2) role of the sigma phase, likely brittle, was not mentioned.

(3) unclear, without a proof, if the SPS sample is wholely dense- packed.

(4) compression test results were tabulated, but flexural strength was not given in the Table.

(5) need to show indentation image from hardness test.

Author Response

We thank the reviewer for his7her comments and constructive criticism. We have now modified the manuscript accordingly. Hope the modified version may be accepted for publication.

Comment: (1) No chemical analysis of the SPS samples was given.

Since all the samples were of the same composition (original powder remains the same) and only difference is the consolidation process, we have not focused on the chemical analysis. This is because we anticipate the difference in consolidation process may not induce a difference in the chemical composition of the samples.

Comment: (2) Role of the sigma phase, likely brittle, was not mentioned.

Yes, we agree with the reviewer. WHHEA with more Cr-Mn rich oxide phase fraction a show reduced compressive strength, as the sigma phase may act as the site for crack initiation. Its roles is included in the results and discussion. 

Comment: (3) unclear, without a proof, if the SPS sample is wholly dense- packed.

The relative density of the WHHEA_S, WHHEA_M and WHHEA_C were found to be 96.3%, 96.1% and 94.8% respectively and reduction in the density could be attributable to the Cr-Mn rich oxide phase formation.

Comment: (4) Compression test results were tabulated, but flexural strength was not given in the Table.

WHHEA_S made by SPS is limited to only 20 mm diameter and ~ 7 mm height for the given temperature and pressure. So, bend test for SPSed WHHEA have not carried out (due to size restriction) and is mentioned in the results and discussion part.

Comment: (5) Need to show indentation image from hardness test.

The Vickers hardness indentation images were included in the revised version of the manuscript.

Reviewer 3 Report

The article presents some new results, it is quite well written and original. However, there is still a lot to be improved, especially more results is needed. 

1 - "'Since the HEAs have superior mechanical properties than low 74 melting elements like Co, Cu, Fe, Ni" This claim is wrong, the HEA you used has a melting point very close to that of Ni and Co and even lower than pure Fe. Low melting point elements are usually considered to be metal like Al and Mg etc...

2- The use of PCA often leads to significant contamination, by the formation of carbides and oxides such as in this case: (authors are advised to cite https://doi.org/10.3390/e21040363 and https://doi.org/10.1016/j.scriptamat.2018.07.034). You must observed oxides, what about carbide particles? PCA containing C should lead to their formation. Please, include oxides and carbides in all analysis and discuss this problem as well as their effect on mechanical properties (Cr-Mn-rich phase). 

3- So the powder were not milled in Ar atmosphere - only in PCA?

4- You do not present any information regarding the geometry of the bending and compression tests, strain rate, measurement setup etc. it has to be added to the manuscript for repeatability. 

5- Did you observe any porosity? Present the results

6- Show SEM images of the microstructures with much higher magnifications to really show the microstructural features. Also, mark different phases on every image - what is Mn-Cr oxide?.

7- Investigate more and show the nature of the HEA-W interphase. How strong is the interphase bonding? How much did the elements intermix? What is the chemistry? - showing line EDs analysis would be advisable.  

8- Indicate the respective phases on the fractography image. Also, show the fracture surfaces of all samples and discuss the differences, please. 

9- please show the point EDS analysis of mentioned Cr-Mn-rich. Is it oxide? - or it may be brittle sigma phase. If it is oxide - call it oxide everywhere (it is ok to show it), the results have to be completely transparent.

Author Response

We thank the reviewer for his/her constructive comments and criticism. We have modified the manuscript according to the comments and hope the revised version may be suitable for publication. 

Comment: (1) Since the HEAs have superior mechanical properties than low melting elements like Co, Cu, Fe, Ni" This claim is wrong, the HEA you used has a melting point very close to that of Ni and Co and even lower than pure Fe. Low melting point elements are usually considered to be metal like Al and Mg etc...

We thank the reviewer for his comments. Yes, the melting point of the CoCrFeMnNi-HEA used in this study is approximately 1450°C nearly equivalent to the alloying elements in the alloy, hence we have modified the sentence in the revised version of the manuscript.

Comment: (2) The use of PCA often leads to significant contamination, by the formation of carbides and oxides such as in this case: (authors are advised to cite https://doi.org/10.3390/e21040363 and https://doi.org/10.1016/j.scriptamat.2018.07.034). You must observed oxides, what about carbide particles? PCA containing C should lead to their formation. Please, include oxides and carbides in all analysis and discuss this problem as well as their effect on mechanical properties (Cr-Mn-rich phase). 

Thank for the suggestions. We have observed Cr-Mn rich phase as the metallic oxide that formed during the processing of the alloy and these oxides act as the crack initiation sites during compressive loading, which incurs brittleness in the alloy. The discussion part is modified and is highlighted.

Comment (3) So the powders were not milled in Ar atmosphere - only in PCA?

Yes, powder milling was done with PCA only.

Comment: (4) You do not present any information regarding the geometry of the bending and compression tests, strain rate, measurement setup etc. it has to be added to the manuscript for repeatability

The sample specification used for bend test is 1.5 x 5 x 40 mm and strain rate is 0.1/s was included.

For compression test, the diameter to height ratio was kept as constant 1:1.1, i.e., for WHHEA_C and WHHEA_M, sample specification of diameter 8 mm and height 8.8 mm was used and for the WHHEAS, the sample diameter and height of 5 mm and 5.5 mm was used due to smaller sample size. The compression test was carried out at a cross-head speed of 2 mm/s.

Comment: (5) Did you observe any porosity? Present the results

The relative density of the WHHEA_S, WHHEA_M and WHHEA_C were found to be 96.3%, 96.1% and 94.8% respectively and reduction in the density could be attributable to the Cr-Mn rich oxide phase formation.

Comment: (6) Show SEM images of the microstructures with much higher magnifications to really show the microstructural features. Also, mark different phases on every image - What is Mn-Cr oxide?

SEM images of all three WHHEA samples were given at higher magnification and different phases in the alloy were marked on it.

Comment (7) Investigate more and show the nature of the HEA-W interphase. How strong is the interphase bonding? How much did the elements intermix? What is the chemistry? - showing line EDs analysis would be advisable.

The EDS line analysis across the different phases were included as Figure 5, and the interphase bonding characteristics were discussed in terms of bonding strength between the phases.

Comment (8) Indicate the respective phases on the fractography image. Also, show the fracture surfaces of all samples and discuss the differences, please.

Fractographs of the fracture surface of bend tested WHHEA_M were added in Fig. 8 and the difference in their features were discussed.

Comment (9) Please show the point EDS analysis of mentioned Cr-Mn-rich. Is it oxide? - or it may be brittle sigma phase. If it is oxide - call it oxide everywhere (it is ok to show it), the results have to be completely transparent

The EDS analysis of the Cr-Mn rich phase is given in the Table. 2. It is indeed an oxide and is used in the entire manuscript.

Round 2

Reviewer 2 Report

ok

Author Response

We thanks the reviewer for his comments and the acceptance for publishing this piece of work.

Reviewer 3 Report

The article has been significantly improved, from the last version. However, the authors failed to address several of the original raised points. The article has to be improved still. Also, if I ask the question, it is not OK to answer it only in response to reviewers. Every point has to be addressed and included IN THE NEW VERSION OF MANUSCRIPT.

Here are the points to correct:

1- I asked about the carbides before. Yet, the authors did not even mention them in the manuscript, nor in their answer. I am asking again: PCA containing C should lead to carbide formation in the microstructure. Did you observe them? Put the results to the manuscript and discuss it there.

2- I asked the question "Did you observe any porosity? Present the results"

the authors answered in response to my comment. PLEASE, INCLUDE THE RESULTS REGARDING THE POROSITY IN THE MANUSCRIPT. Also, discuss the influence of porosity on mechanical properties. 

3- What is the reason to present images of indentations? I do not see any point. Explain it in the manuscript.

4- You were supposed to use the right name for the Mn-Cr oxide. Yet, I see in the conclusion the use of Cr-Mn rich phase "High heating rate and shorter holding time in sintering have resulted in a significant reduction of W-grain size and reduced volume fraction of Cr-Mn rich phase in WHHEA."

If it is an oxide, it has to be called OXIDE to not confuse the readers.

Improve the clarity of the paper!

5- The results of EDS line scan are good, but they can be presented more nicely and professional. Please, redo the graph in some other software. Also, what are the numbers after elements supposed to mean? I mean this CrK:23, CoK:22 etc.. Together with the results, including the SEM micrograph from which the line was taken and mark it in the image. 

Author Response

Tungsten matrix composite reinforced with CoCrFeMnNi-high-entropy alloy: Impact of processing routes on microstructure and mechanical properties

 Responses for Reviewer 3 comments:

We thanks the reviewer for his/her comments and constructive criticism. We have modified the manuscript accordingly.

1- I asked about the carbides before. Yet, the authors did not even mention them in the manuscript, nor in their answer. I am asking again: PCA containing C should lead to carbide formation in the microstructure. Did you observe them? Put the results to the manuscript and discuss it there.

We thank the reviewer for raising the issue again. However, we have not observed any carbon or carbide related peaks in our samples (from both XRD patterns and SEM images). It may be below the deductible limits of the XRD pattern and since we do not observe distinct carbide peaks, we rule out the presence of carbide formation during milling. It may be because of the milling conditions, where gentle milling was carried out. The same is included in the manuscript as well.

2- I asked the question "Did you observe any porosity? Present the results"

the authors answered in response to my comment. PLEASE, INCLUDE THE RESULTS REGARDING THE POROSITY IN THE MANUSCRIPT. Also, discuss the influence of porosity on mechanical properties. 

The relative density of WHHEA_S, WHHEA_M and WHHEA_C were found to be 96.3%, 96.1% and 94.8% respectively. The density of the WHHEA is found indirectly proportional to the fraction of Cr-Mn rich oxide phase. Hence, the reduction in densification may be attributed to the Cr-Mn rich oxides formed during the consolidation process.

3- What is the reason to present images of indentations? I do not see any point. Explain it in the manuscript.

Indentation images were added as suggested by reviewer -2. Necessary statements were included in the revised version of the manuscript.

4- You were supposed to use the right name for the Mn-Cr oxide. Yet, I see in the conclusion the use of Cr-Mn rich phase "High heating rate and shorter holding time in sintering have resulted in a significant reduction of W-grain size and reduced volume fraction of Cr-Mn rich phase in WHHEA."

If it is an oxide, it has to be called OXIDE to not confuse the readers.

Improve the clarity of the paper!

As suggested by the reviewer uniform term Cr-Mn rich oxide is used.

5- The results of EDS line scan are good, but they can be presented more nicely and professional. Please, redo the graph in some other software. Also, what are the numbers after elements supposed to mean? I mean this CrK:23, CoK:22 etc.. Together with the results, including the SEM micrograph from which the line was taken and mark it in the image. 

As suggested by the reviewer, we have modified the Figure in the revised version of the manuscript.