Microstructure and Mechanical Properties of Sintered and Heat-Treated HfNbTaTiZr High Entropy Alloy

Round 1

Reviewer 1 Report

Dear Authors, the paper was overall well organized and several analyses and measurements were performed, but in the abstract the aim was missing.

In my opinion, the introduction was too general and quite obvious without a focus on the specific properties and process of the choosen alloy.

In the conclusion the difference in process parameter to obtain the microstructure and mechanical properties were not summarized.

 

Author Response

Comment:

In my opinion, the introduction was too general and quite obvious without a focus on the specific properties and process of the chosen alloy.

Response

We have added some information about the Hf NbTaTiZr alloy and its properties into the Introduction chapter

Comment

In the conclusion the difference in process parameter to obtain the microstructure and mechanical properties were not summarized.

Response:

We do not fully understand this remark. If the sintering time is mentioned, we guess that it was concluded in Conclusion No.1

Reviewer 2 Report

The manuscript is clearly organized and written (except for some misspellings, see below). The experiments are well described (except for grain size and porosity, see below).

Conclusions on microstructure (presence of bcc, bcc2 and hcp in various conditions), grain size evolution, hardness and compressive strength are in line with the experimental results. However, the third conclusion should be moderated because of the relatively high oxygen content.

Comments

The authors shall specify how the grain sizes have been measured (from EBSD images (automatic output)? From SEM images (intercept line method?)?).

The authors shall also specify the size of the fields (... µm x ... µm) used to measure the porosity and the typical size of the pores (line 122)

Eq. 1 is written too late in the text, 5 pages after Figure 9, where it is first mentioned (in the caption). I would write it between Figure 9 and Figure 10.

In lines 307-310, the authors refer to their previous paper [42] to give the oxygen content or arc-melted alloy, but in this article the oxygen content is not mentioned.

line 251: “was  reported by many author” => add references

line 261-262: “diffusivity of Ta ... is smaller than that of other used elements was reported previously” => add an order of magnitude

 

Hereafter some errors to be corrected

Note that in the text, many “a/an” (cf. line 332) and “the” (cf. line 293) are missing (that I have not corrected). The manuscript should be read and corrected by a person speaking fluently a good English.

Abstract:

line 22: “in the microstructure” => “in the bcc microstructure”

line 23: “during annealing 1200°C/1h” => “during annealing (1200°C/1h)”

 

1 . Introduction

line 35: “construction materials” => “structural materials”

line 39: “will exhibit” => “will exhibit a”

line 46: “its relation effects” => “its related effects

line 72-73: “and it is difficult (and expensive) to obtain it” => “and difficult (and expensive) to obtain”

 

2. Materials and Methods

line 85: check if there are not two spaces before “400 MPa”: if yes, delete one

line 86: check if there are not two spaces before “10^-3 Pa”: if yes, delete one

line 94: “back scattered electron mode” => “back-scattered electron mode”

line 112: in “mm2”, “2” should be a superscript

 

3. Results

line 118: “of processed specimens was described first following by the mechanical properties which are supposed to correlate with microstructure” => “of the processed specimens is (or will be) described first, followed by the presentation of the mechanical properties which are supposed to correlate with the microstructure”

line 132: “The matrix has chemical composition” => “The matrix has a chemical composition”

line 159: “rich in – Hf and Zr” => “rich in Hf and Zr”

line 160: check if there are not two spaces before “XRD”: if yes, delete one

line 162: “both, in the vicinity” => “both in the vicinity”

line 174: ““64”h specimen” => ““64h” specimen”

line 180: “as the fine ones.According” => “as the fine ones. According”

line 181: check if there are not two spaces before “matrix{110}” and “and”: if yes, delete one

line 181: “matrix” and “hcp” in the relationships sould be written as subscripts

line 184-185: “In contrast to samples after sintering” => “In contrast to as-sintered samples”

line 188: “(Table2).” => “(Table 2).”

line 190: “proved,” => “proved”

line 207: check if there are not two spaces before “(Figure 10)”: if yes, delete one

line 214: “ascribed” => “is ascribed”

 

4. Discussion

line 250: “The porosity decreased (Figure 1) due to diminishing the free surface energy” => “The porosity decreased (Figure 1) due to the reduction of the free surface energy”

line 260: “conglomerates” => “agglomerates”

line 263-264: : there is a verb missing in this sentence.“ascribed”. Or maybe “consisting” should be corrected to “consisted”?

line 266: “phases phases” => “phases”

line 281: I do not understand what “event” means here

line 289: from here on, replace “slow cooling rate” with “low cooling rate” (several occurences). The cooling is slow but the cooling rate is low

line 293: “In present work” => “In the present work”

line 303-304: “On the other the hcp” => “On the other hand the hcp”

line 327: “in accordance the stabilizing effect” => “in accordance with the stabilizing effect”

line 329: “that higher oxygen” => “that the higher oxygen”

line 331: “(~ 830°C/wt. % of O)” => “(with a slope of ~ 830°C/wt. % of O)”

line 332 “plays important role” => “plays an important role”

 

Figure captions

line 144: “back scattrd electron” => “back-scattered electron”

line 145: "aeas" => "areas"

line 183: “(a) sample “ 64h”;” => “(a) sample “64h”;”

line 205: “plotted by points” => “plotted by circles”

Author Response

Reviewer 2

Comment:

Conclusions on microstructure (presence of bcc, bcc2 and hcp in various conditions), grain size evolution, hardness and compressive strength are in line with the experimental results. However, the third conclusion should be moderated because of the relatively high oxygen content.

Response:

We agree with this comment. We have modified the Conclusion No.3 by adding the remark about the oxygen content.

Comment:

The authors shall specify how the grain sizes have been measured (from EBSD images (automatic output)? From SEM images (intercept line method?)?).

The authors shall also specify the size of the fields (... µm x ... µm) used to measure the porosity and the typical size of the pores (line 122)

Response:

We have added some information to the chapter „Materials and Methods“. The grain size was also compared with EBSD results and was in good agreement. However the EBSD was done on smaller area on the specimens and therefore we assumed the line intercept methods would be more reliable

Comment:

Eq. 1 is written too late in the text, 5 pages after Figure 9, where it is first mentioned (in the caption). I would write it between Figure 9 and Figure 10.

Response:

We agree. However we have decided to move the equation into the Discussion section into which, we believe, it belongs. We also didn’t want to use two figures showing the same information. On the other hand the grain size should be in the section „Resutls“. So that is the reason of such gap between the figure and the equation. We have at least added remark into figure caption.

Comment:

In lines 307-310, the authors refer to their previous paper [42] to give the oxygen content or arc-melted alloy, but in this article the oxygen content is not mentioned.

Response:

We are sorry for such inconvenience. We were convinced that we had mentioned the oxygen content in our previous work. We changed the references by the works where the oxygen content was measured and mentioned. It was not on this exact alloy, but on titanium alloys contain in similar elements and prepared in the same furnace from the same pure metals (if they were in given alloys)

Comment:

line 251: “was  reported by many author” => add references

Response:

Some references have been added.

Comment:

line 261-262: “diffusivity of Ta ... is smaller than that of other used elements was reported previously” => add an order of magnitude.

Response:

We have added some information of self diffusivity. Moreover in cited reference more information can be also found by the reader.

 

 

 

We tried to do our best to improve the manuscript along to reviewer‘s comments and we are really grateful for detailed and valuable review.

 

Reviewer 3 Report

  Logically constructed research result Sintering has greatly improved the microstructure of the alloy. My question is, where does oxygen come from when it is heat treated in an inert atmosphere? What I miss in the summary is why it's good decomposition of single phase bcc to bcc2 and hcp?

Author Response

Comment:

My question is, where does oxygen come from when it is heat treated in an inert atmosphere?

Response:

This was mentioned in the discussion. Although annealed in a vacuum, some residual oxygen is always present in the furnace. During quite long time (up to 64 hours) and high annealing temperature (1400°C) the specimen may act as a gatter as it consist of elements with high afinity to oxygen

Comment

What I miss in the summary is why it's good decomposition of single phase bcc to bcc2 and hcp?

Response:

Well this is good question. We did not mention that the decomposition is good. Some authors claim that one phase bcc solid solution is desired. really it gives good results of mechanical properties (ref.20). On the other hand in some cases two phase microstructure could also be desired (or better say metastability of the alloy (Li et al. Nature 534 (2016), 227-230). The advantage of bcc2 (of hcp) could be also its thermal stability (comparing with bcc) for elevated temperature application. o we did not want to make just one statement. We wanted to show that the stability of phases was shifted in our alloy (mainly due to increased oxygen) and leave to the reader how to use it.