Laser-Ignited Self-Propagating Sintering of AlCrFeNiSi High-Entropy Alloys: An Improved Technique for Preparing High-Entropy Alloys

Round 1

Reviewer 1 Report

Dear authors,

 

Thank you for addressing my previous comments.

The manuscript still needs some substantial editing and proofreading. The abstract and conclusion are affected by incomprehension arising from bad English and typos. I am not going to address them, though, because I believe a good read through prior to submission would have removed many repetition arising from lack of attention (for example, the presence of the same reference twice, as 3 and 4).

1.       Line 169. Since the technique consists of fast heating and fast cooling (i.e. quenching), you are probably not at the equilibrium. The alloy might be in a martensitic state (which is quite likely, considering that you have ball-milled it first). This is not a HEA, unless it can be proved by annealing that the structure is indeed stable with T and no crystallization of Si occurs. I believe you should address this issue in future works, and call the specimen in the current manuscript “a multicomponent alloy”.

2.       Line 24. As stated previously, there seem to be some confusion regarding the definition of “cocktail effect”. I reiterate the suggestion of reading the relevant literature which appeared in the last two years, where it is clearly stated that “cocktail effect” is not a mechanism, but rather a way of describing situations in which the ending result is superior to the simple additions of its components. I particularly suggest reading the review from Miracle and coworkers.

3.       (line 25-26) The sentence is unclear. Which effects are you considering?

4.       (Line 53-62) While the paragraphs written by the authors in response to my previous comment add some value to the introduction, they do not properly address Spark Plasma Sintering. Indeed, there is no mention of SPS at all: the manuscript provides the advantages of laser-ignited sintering, but they are mixed up with the ones of other techniques. In general, the section is affected by lack of clarity.

5.       Line 85-86. As previously stated, the ball-milling process should be reported with r.p.r. and ball-to-powder ratio.

6.       Line 106. The Greek letter for the theoretical density is missing.

7.       Line 123-127. The authors claim to have addressed my previous comments, but nothing has changed from the previous version. While I have certainly understood the intentions of the authors, they are not taking into consideration the recent literature (see the review from Miracle et al.). I am not arguing against the definition of configurational entropy, or its importance in the description of HEAs, but against the statement regarding the Gibbs Law. Gibbs Law is not a predictive tool, meaning that it cannot be used to support the formation (or not) of a certain phase. It simply states that, at a certain pressure T and P, with a specific number of components, the maximum number of phases in the system equals n+1. This is in no way in contradiction with the notion of HEAs, because Gibbs Law does not state that the number of phases when mixing diverse components has to be n+1 (in other words, it comprises the possibility of it being less than n+1).

8.       Figure 2. The non-indexed peaks in the XRD pattern should be indexed, because their presence jeopardize the premise of the entire manuscript (i.e. that the authors have achieved a duplex FCC-BCC phase with no intermetallics). EDS has lower sensitivity in determining phases present in small quantities, therefore it cannot be considered as enough prove.

9.       Line 148-149, line 155. The authors claim to have corrected the word “tissue” referred to microstructure, but it is still there.

10.   Line 160. It should be specified if EDS results come from point analysis (how many?) or surface maps.

11.   In all graphs and tables, error bars should be provided for data to have any scientific meaning.

12.   Line 165. The authors claim to have a honeycomb-like structure and answered my comment as: “the pore in the alloy is fine and uniformly distributed, and the structure is honeycomb-like, similar in shape. As an image metaphor, we would like to explain it to the author in order to be more vivid”. Nevertheless, the definition of a honeycomb is that of an array of hollow cells formed between thin vertical walls. That is not the case for the reported alloys, therefore the use of the word is incorrect.

Comments for author File: Comments.pdf

Author Response

Dear reviewer:

I'm glad to receive your suggestion. For this article, we have carefully analyzed your suggestion. According to this article, we have made corresponding amendments and explanations. We try our best to modify and add the content of the article. I sincerely hope you can understand our efforts in this article.

Your valuable advice will be of great help to us in our future research. Thank you for your help and recognition of this article.

Have a nice day

Li Zhu

 

1. Line 169. Since the technique consists of fast heating and fast cooling (i.e. quenching), you are probably not at the equilibrium. The alloy might be in a martensitic state (which is quite likely, considering that you have ball-milled it first). This is not a HEA, unless it can be proved by annealing that the structure is indeed stable with T and no crystallization of Si occurs. I believe you should address this issue in future works, and call the specimen in the current manuscript “a multicomponent alloy”.

Response: According to the suggestion , we agree with you very much. In this paper, we call specimens "multicomponent alloys".

2.  Line 24. As stated previously, there seem to be some confusion regarding the definition of “cocktail effect”. I reiterate the suggestion of reading the relevant literature which appeared in the last two years, where it is clearly stated that “cocktail effect” is not a mechanism, but rather a way of describing situations in which the ending result is superior to the simple additions of its components. I particularly suggest reading the review from Miracle and coworkers.

3. (line 25-26) The sentence is unclear. Which effects are you considering?

Response: According to the suggestions 2 and 3, we revised the content of the article. As follows:

High entropy alloys consists of at least five elements, according to the atomic fraction is greater than 5% and not more than 35% of composition. Due to the high-entropy effect, lattice distortion effect, hysteretic diffusion effect and cocktail effect among alloying elements, high-entropy alloys show excellent characteristics different from other alloys in mechanical properties, high temperature properties and magnetic properties.

4. (Line 53-62) While the paragraphs written by the authors in response to my previous comment add some value to the introduction, they do not properly address Spark Plasma Sintering. Indeed, there is no mention of SPS at all: the manuscript provides the advantages of laser-ignited sintering, but they are mixed up with the ones of other techniques. In general, the section is affected by lack of clarity.

Response: According to the suggestion , we reinterpreted it in the article.

5. Line 85-86. As previously stated, the ball-milling process should be reported with r.p.r. and ball-to-powder ratio.

Response: According to the suggestion , we supplement the ball milling data in section 3.

6. Line 106. The Greek letter for the theoretical density is missing.

Response: According to the suggestion , we revised it in the article.

7. Line 123-127. The authors claim to have addressed my previous comments, but nothing has changed from the previous version. While I have certainly understood the intentions of the authors, they are not taking into consideration the recent literature (see the review from Miracle et al.). I am not arguing against the definition of configurational entropy, or its importance in the description of HEAs, but against the statement regarding the Gibbs Law. Gibbs Law is not a predictive tool, meaning that it cannot be used to support the formation (or not) of a certain phase. It simply states that, at a certain pressure T and P, with a specific number of components, the maximum number of phases in the system equals n+1. This is in no way in contradiction with the notion of HEAs, because Gibbs Law does not state that the number of phases when mixing diverse components has to be n+1 (in other words, it comprises the possibility of it being less than n+1).

Response: According to the suggestion , We have revised Section 4.1 this time. We understand your suggestion, and we do not use Gibbs law as a predictive tool entirely. It is only a reflection of the effect of high-entropy alloys. In this paper, we support the content of high entropy alloys. This amendment is expected to be accepted by you.

8. Figure 2. The non-indexed peaks in the XRD pattern should be indexed, because their presence jeopardize the premise of the entire manuscript (i.e. that the authors have achieved a duplex FCC-BCC phase with no intermetallics). EDS has lower sensitivity in determining phases present in small quantities, therefore it cannot be considered as enough prove.

Response: According to the suggestion , We have carefully revised the labeling of XRD graphs, and the way in which we label the peaks can fully express the meaning expressed in the content. With regard to EDS analysis, we combine XRD analysis, EDS analysis and morphology to support what we have expressed in this paper. Their combination is equivalent to a closed-loop structure that can adequately support and prove the problems expressed in this paper.

We understand your suggestion and hope to accept the structure and description of the content in our article.

9. Line 148-149, line 155. The authors claim to have corrected the word “tissue” referred to microstructure, but it is still there.

Response: According to the suggestion , we revised it in the article.

10.  Line 160. It should be specified if EDS results come from point analysis (how many?) or surface maps.

Response:According to the suggestion , five points were analyzed in EDS atlas and explained and analyzed in this paper.

11. In all graphs and tables, error bars should be provided for data to have any scientific meaning.

Response: According to the suggestion , we think this is a good suggestion. We take part of the error into account in the process of the experiment, so we try to avoid the error in the experiment as much as possible.

In sections 4.3 and 4.4, we provide standard deviations of alloy density, alloy porosity, hardness and wear rate, respectively. Ensure that the data is of scientific significance.

12.  Line 165. The authors claim to have a honeycomb-like structure and answered my comment as: “the pore in the alloy is fine and uniformly distributed, and the structure is honeycomb-like, similar in shape. As an image metaphor, we would like to explain it to the author in order to be more vivid”. Nevertheless, the definition of a honeycomb is that of an array of hollow cells formed between thin vertical walls. That is not the case for the reported alloys, therefore the use of the word is incorrect.

Response: According to the suggestion , we have revised this sentence of the article

Reviewer 2 Report

Authors addressed all reviewer's comments.

Author Response

Dear reviewer:

I'm glad to receive your suggestion. For this article, we have carefully analyzed your suggestion. According to this article, we have made corresponding amendments and explanations. I'm honored to be recognized by you for this article.

Your valuable advice will be of great help to us in our future research. Thank you for your help and recognition of this article.

Have a nice day

Li Zhu

Round 2

Reviewer 1 Report

Thank you addressing the issues. Please take better care in your next work.

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

In the present manuscript, authors give a study of HEAs prepared using self-propageting synthesis. It is potentially important topic and will attract attantion of the community. Nevertheless, to be more readable text should be structured in a better way. Authors should wright discussion section and extend results and discussion section to compare their data with data obtained for alloys prepared using alternative techniques. Self-propagating reaction is relatively rare technique and has complex nature. I suggest to give more information about actual starting materials and chemical processes which are behind. At least chemical reactions as well as thermodinamic calculations for the reaction should be performed to support preparatory technique.

There are several minor suggestions:

Table 1 has a little information. information given in Table 1 can be given as a text or in table 2.

Authors should also refer to Table 2 as well as give in its capture more information, how numbers were obtained and are they correspond to nominal composition or final product.

Line 76. "hystological and morphology" looks a bit from biology, I guess "microstrusture" would be better

I suggest to give exact final analytical composition for all samples as well as support Fig 3 with EDX elemental composition maps.

Conclusions: authors claim porosity of all samples nevertheless, the nature of porosity was not discussed; I suggest to measure porosity directly using porosimetry to be able to understand the its nature.

Reviewer 2 Report

The paper titled “Laser-ignited Self-Propagating Sintering of AlCrFeNiSi High-Entropy Alloys: An Improved Technique for Preparing High-Entropy Alloys” reports the synthesis of five samples of the family AlCrFeNiSix by means of sintering and laser melting. The specimens are reported to be single or duplex-phase and porous. Their mechanical properties (hardness and wear resistance), discussed with respect to increasing Si content, are related by the authors to the appearance of the secondary FCC phase.

 

The article shows several shortcomings.

-          No citation to the extensive literature of High-Entropy Alloys are given in the introduction. I later realized that some references are given at the end of the paper, but they do not appear in the main text.

-          The papers reported in the Bibliography do not cover the relevant literature on the topic. For example, other works focusing on the addition of Si to HEAs are not discussed (e.g. “Phase evolution of CoCrCuFeNiSix high-entropy alloys prepared by mechanical alloying and spark plasma sintering”)

-          The similarity and differences between the reported process and the most commonly used Spark Plasma Sintering should be discussed (or at least addressed).

-          Section 4.1 is very confusing in its discussion of Gibb’s Law. The sentence “Due to the high entropy effect, only simple bulk BCC and FCC phases are formed in the microstructure of AlCrFeNiSi alloy, and brittle intermetallic compounds are not preferred. The phase number is far less than six.” implies the use of Gibb’s Law as a predictive toll – whereas in fact the reported equation (in which it should be specified that pressure is constant) simply provides the maximum number of phases in a multicomponent system. Also, it should be specified that the “cocktail effect” per se is not enough to inhibit the precipitation of intermetallics. Other factors are equally important – and should be taken into consideration when discussing the formation criteria of HEAs (see for example “Hume-Rothery for HEA classification and Self-Organizing Map for phases and properties prediction”).

-          The entire paper is based on the premise that the Si-containing HEA is single phase (BCC) and turns into duplex-phase (FCC+BCC). However, the XRD from Figure 2 shows some small un-indexed peaks (maybe a hexagonal phase?) which could relate to the presence of Si-based intermetallics. Also, the paper claims that elements are homogeneously dispersed in the matrix, but no EDX maps are provided: indeed, the elemental composition of the FCC phase (C, D and E in Figure 3) is not reported in Table 3. As such, the author’s claims do not seem to be supported by scientific evidence.

-          No standard deviation or uncertainty of the data is reported in the paper. Figure 5 should display error bars, and since the experimental section stated that mechanical properties data were taken as an average of 5 measurements, the standard deviation should be provided.

-          The density and porosity results are discussed in length. This is not necessary and quite confusing, because the two properties are directly related. It would be better to incorporate the data reported in Table 4 into Figure 5 and discuss the change of properties with respect to porosity.

-          At lines 153-156 data from previous experiments are discussed: the references for these should be provided.

-          Lines 140-144 present a somewhat counterintuitive concept, and should be elaborated on more deeply.

The experimental section lacks details on instruments, metal purity and providers.

-          When the ball-milling is described, ball-to-powder ratio and rpm should be provided.

-          Since planetary ball-milling is a high-energy technique, it is incorrect to consider it as simply mixing. Indeed, the ball-milled powders should be checked with PXRD to see if alloying has already occurred, or if the lines corresponding to each metal are still present. See for example: “A Novel Low-Density, High-Hardness, High-entropy Alloy with Close-packed Single-phase Nanocrystalline Structures”.

-          The XRD procedure should indicate if the analysis was done in reflection or transmission geometry, on which instrument, and with which software were the lines indexed.

-          EDX should report if the composition results were taken from maps (if so, of which dimensions) or single points (how many?), and ensure statistical variation.

-          At line 95, it is stated that the wear is obtained through calculation – but the formula and corresponding error are not provided.

The article is affected by a general lack of clarity due to poor English. Extensive proof-reading should address repetitions, grammar and spelling mistakes. Some of the most striking ones:

-          The porosity of some specimens is labeled “honeycomb”. This is unsuitable because the pores are not located within a fixed, ordered geometry.

-          Line 42: inconsistences in naming alloys.

-          Line 76: the term “histological” refers to animal of human tissue. It is inappropriate in this context.

-          Line 118:  the metallic structure should not be referred to as “tissue”, as it is not a living organism.

-          Line 122: The meaning of “energy spectrum” in this context is unclear.

-          Line 175: Rather than “lattice aberration”, maybe “lattice distortion”?