High-Energy Ball Milling and Spark Plasma Sintering of the CoCrFeNiAl High-Entropy Alloy

Round 1

Reviewer 1 Report

The authors present the results of a high energy ball milling followed by plasma sintering of CoCrFeNiAl alloy. The experiment and the results are pretty much straight forward.

Although the manuscript is sound in terms of its physical metallurgy aspect, it would be a complete study if the authors plotted the hardness Pg 7, lines 192 on a graph to show how it varied with composition. Also, did the authors see a difference in hardness between the white and grey spots on Figure 2a and similar phases in the study?

Secondly, Figure 7 may be better if shown in color. 

Can the authors quantify the porosity and show a relation between composition and or sintering temperature.

Why does the peak at ~85 degrees disappear in the x-ray chart shown in Fig 3? 

Please include good review articles such as Corrosion, Erosion and Wear Behavior of Complex Concentrated Alloys: A Review published in this journal which would serve as a good pointer to new readers. 

Author Response

Reviewer #1.

1. The authors present the results of a high energy ball milling followed by plasma sintering of CoCrFeNiAl alloy. The experiment and the results are pretty much straight forward.

Response:

We agree, thank you.

2. Although the manuscript is sound in terms of its physical metallurgy aspect, it would be a complete study if the authors plotted the hardness Pg 7, lines 192 on a graph to show how it varied with composition. Also, did the authors see a difference in hardness between the white and grey spots on Figure 2a and similar phases in the study?

Response:

Thank you for this comment. Following this recommendation, we plotted the hardness data and presented them as bar graphs in a Supplementary file. In addition, SEM pictures of the Hv prints have been presented in the same file. The proper additions and corrections were made in the manuscript – page 7, lines 196-199.

As it was stated in the manuscript (page 3, lines 102-104), white and grey phases on Figure 2a have different chemical compositions. The inclusions of the white phase are residual Fe that did not dissolve into multicomponent matrix after 30 min of activation. Size of the inclusions (about 1 micron) is too small for microhardness measurements. Anyway, these residual inclusions disappear after 90 min of mechanical treatment.

3. Secondly, Figure 7 may be better if shown in color. 

Response:

Unfortunately, our SEM-EDS device gave us only black and white images and X-ray maps. Taking this into account, we did not use superimposed images and presented X-ray mapping separately for every element.

4. Can the authors quantify the porosity and show a relation between composition and or sintering temperature.

Response:

Thank you for this question. We have measured porosity on the cross-section SEM (BEI) images, measured physical density by weighting samples, and calculated theoretical densities of the FCC and BCC phases by results of XRD analysis. All these data are now presented in the resized manuscript – see page 5 lines 162-164 and page 8 lines 206-215.

5. Why does the peak at ~85 degrees disappear in the x-ray chart shown in Fig 3? 

Response:

If you mean peaks of Fe, Al and Cr in the initial mixture, they disappear due to dissolution of all these metals into HEA matrix. However, if you mean the peak 200 of the BCC phase (also ~85degrees), our explanation is the following. This peak has intensity 5 times less than intensity of the 110 BCC peak. During mechanical milling, intensities of both peaks decreases, because of (a) broadening of all peaks due to decreasing crystalline sizes, and (b) formation of FCC phase (intensity splitted between two phases). As a result, intensity of the relatively weak 200 BCC peak becomes comparable with noise (background).  

6. Please include good review articles such as Corrosion, Erosion and Wear Behavior of Complex Concentrated Alloys: A Review published in this journal which would serve as a good pointer to new readers. 

Response:

Thank you for the useful advice. We’ve made a reference for this interesting review article as no. [3], with re-numbering of other references and attending text on page 1, lines 23-25.

Reviewer 2 Report

Many similar investigations of NiCoFeCrAlx HEA can be found in the literature. These are thoroughly summarized in the Introduction. The novelty of this article is simply that it uses (i)  high energy ball milling (HEBM) and (ii) consolidation of the powder product by spark plasma sintering (SPS). The study is focusing on microstructure, (elemental and phase composition) as determined by SEM-EDS and XRD techniques. Actually is concentrating on equimolar nominal composition NiCoFeCrAl prepared by HEBM for different time and consolidated by SPS at two different temperatures (800 and 1000^oC).

Concerning the experimental part:

1. Please check whether it is a simple grammar mistake writing 1.5-3.5 micrometers for the powder size of Iron (see line 59) instead of more realistic 15-35 micrometers. Below 3 micrometers the iron powder is more and more pyrophoric, see, for example, P. Evans, W. Borland & P. G. Mardon (1976) Pyrophoricity of Fine Metal Powders, Powder Metallurgy, 19:1, 17-21, DOI:10.1179/pom.1976.19.1.17. As you have performed the milling in air (see line 61), there is a real danger of oxidation of Fe before the solid state reaction between the components. Not forgotten the passivation oxide layer on the surface of micron sized Fe powder which falsifies the planned proportion of the components.
2. The ball to powder weight proportion is not given. This data is necessary to decide whether the milling is a real high-energy ball milling.
3. The details of the sample preparation for SEM –EDS examination are not given. SEM images in backscattered electrons strongly depend on the quality of sample surface preparation.

Concerning the results and discussion part:

We have got a lot of pictures and tables on the microstructure, but, unfortunately, we did not know more about the disputed BCC –FCC  phase transformations taking place during annealing at 1073K/50 h (see Ref.11 of present paper) or during the SPS sintering at similar temperatures.  Although disputed (see Songge Yang etal. „Revisit the VEC rule in high entropy alloys (HEAs) with high-throughput CALPHAD approach and its applications for material design-A case study with Al_Co_Cr_Fe_Ni system ”  Acta Materialia 192 (2020) 11-19 ) the rule of VEC introduced by Guo et al. [S. Guo, C. Ng, J. Lu, C. Liu, Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys, J. Appl. Phys. 109 (2011) 103505] should be discussed in this paper in order to understand the phase stability of this important HEA.

The valence electron concentration (VEC) of this equimolar HEA is 7.2, just at the higher limit of BCC region. The smallest reduction of Cr (due to the Cr7C3 carbide formation by reduction of the Carbon content of WC balls or by contamination with carbon from press-mould) or of Al (due to the oxidation of the Al in the SPS process carried out under 20 Pa, see line 67 of the present manuscript) would increase the VEC above 7.2 into the double phase region. 15-20 Pa applied in this work correspond to a pre-vacuum only; the oxidation at 800-1000 ^oC of SPS cannot be excluded.

4. Please, comment on the applicability of the VEC rule in the case of equimolar NiCoFeCrAl HEA prepared by HEBM and SPS.
5. Please check the oxygen content of the sample after 30 min milling and 90 min milling in air. (A simple gravimetric check of oxidation would be also informative). Can you repeat the HEBM under Ar protective atmosphere?
6. Eventually revisit the Conclusion part.

Author Response

Reviewer #2.

Many similar investigations of NiCoFeCrAlx HEA can be found in the literature. These are thoroughly summarized in the Introduction. The novelty of this article is simply that it uses (i)  high energy ball milling (HEBM) and (ii) consolidation of the powder product by spark plasma sintering (SPS). The study is focusing on microstructure, (elemental and phase composition) as determined by SEM-EDS and XRD techniques. Actually is concentrating on equimolar nominal composition NiCoFeCrAl prepared by HEBM for different time and consolidated by SPS at two different temperatures (800 and 1000^oC).

Response:

Yes, we agree with this comment. Please note that this paper is submitted to the Special Issue of “Metals” that focuses on HEBM + SPS.

Concerning the experimental part:

1. Please check whether it is a simple grammar mistake writing 1.5-3.5 micrometers for the powder size of Iron (see line 59) instead of more realistic 15-35 micrometers. Below 3 micrometers the iron powder is more and more pyrophoric, see, for example, P. Evans, W. Borland & P. G. Mardon (1976) Pyrophoricity of Fine Metal Powders, Powder Metallurgy, 19:1, 17-21, DOI:10.1179/pom.1976.19.1.17. As you have performed the milling in air (see line 61), there is a real danger of oxidation of Fe before the solid state reaction between the components. Not forgotten the passivation oxide layer on the surface of micron sized Fe powder which falsifies the planned proportion of the components.

Response:

Thank you for alarming, however, there is no mistake on the size of the Fe particles. We used commercially available iron powder R-10 (Russian standard, produced by carbonyl method) that commonly has particle sizes within the range 2 – 4 microns. Our SEM analysis confirmed that the powder consists of such small spherical particles; some of them form agglomerates. Our experience has shown that this powder is not pyrophoric at room temperature. Probably, this is due to passivating oxide film that may already exist on the surface of the powder particles (note that the used Fe powder contained 1.2% of oxygen). We did not make special research on this matter; however, we have measured temperature of the powder self-ignition in air atmosphere, which is 260°C.

Concerning the possibility of oxidation during milling. The milling jar volume is 150 cm³. Subtracting volume of the milling balls (200 g) and powders (10 g), we have 124 cm³ of air at 1 atm. The jar is closed hermetically, therefore, the metals powders can react only with the oxygen containing inside the jar. Mass of this oxygen is 37 mg, which can give 0.37 wt.% in case if the metals absorb all oxygen from the air. This is not enough for burning. The proper information is added to the manuscript – page 2, lines 65-67.

2. The ball to powder weight proportion is not given. This data is necessary to decide whether the milling is a real high-energy ball milling.

Response:

A balls to powder mass ratio was 20:1, - this information is added to the manuscript at page 2, line 64.

3. The details of the sample preparation for SEM –EDS examination are not given. SEM images in backscattered electrons strongly depend on the quality of sample surface preparation.

Response:

The required information is added – page 2 lines 75-78.

Concerning the results and discussion part:

We have got a lot of pictures and tables on the microstructure, but, unfortunately, we did not know more about the disputed BCC –FCC  phase transformations taking place during annealing at 1073K/50 h (see Ref.11 of present paper) or during the SPS sintering at similar temperatures.  Although disputed (see Songge Yang etal. „Revisit the VEC rule in high entropy alloys (HEAs) with high-throughput CALPHAD approach and its applications for material design-A case study with Al_Co_Cr_Fe_Ni system ”  Acta Materialia 192 (2020) 11-19 ) the rule of VEC introduced by Guo et al. [S. Guo, C. Ng, J. Lu, C. Liu, Effect of valence electron concentration on stability of fcc or bcc phase in high entropy alloys, J. Appl. Phys. 109 (2011) 103505] should be discussed in this paper in order to understand the phase stability of this important HEA.

The valence electron concentration (VEC) of this equimolar HEA is 7.2, just at the higher limit of BCC region. The smallest reduction of Cr (due to the Cr7C3 carbide formation by reduction of the Carbon content of WC balls or by contamination with carbon from press-mould) or of Al (due to the oxidation of the Al in the SPS process carried out under 20 Pa, see line 67 of the present manuscript) would increase the VEC above 7.2 into the double phase region. 15-20 Pa applied in this work correspond to a pre-vacuum only; the oxidation at 800-1000 ^oC of SPS cannot be excluded.

Response:

The problem of phases stability in HEAs, described by reviewer, is very interesting and important; however, it is slightly out of the frames of this work. Our paper is submitted to the Special Issue titled “High Energy Ball Milling and Consolidation of Nanocomposite Powders”. For this reason, the paper focuses on HEBM-synthesis and SPS-consolidation, including influence of these processes on structure and phase composition. We plan to discuss more carefully phase selection and thermal stability of the CoCrFeNiAl HEA, produced by HEBM, in the next article, basing on other results (high-temperature XRD, etc.). Nevertheless, following Reviewer’s recommendations, we have added some discussion comments and references, including the two suggested by Reviewer, in the revised manuscript (refs [19]-[21]).

4. Please, comment on the applicability of the VEC rule in the case of equimolar NiCoFeCrAl HEA prepared by HEBM and SPS.

Response:

This discussion has been added at page 8-9, lines 218-243.

5. Please check the oxygen content of the sample after 30 min milling and 90 min milling in air. (A simple gravimetric check of oxidation would be also informative). Can you repeat the HEBM under Ar protective atmosphere?

Response:

An oxygen content in the as-synthesized HEA powder is about 3 at.% (line 181). Our comments concerning possibility of oxidation during HEBM were presented above. Unfortunately, gravimetric check of oxidation cannot help in this case, because total amount of the gaseous oxygen (37 mg) inside the jar is very small, much smaller than mass of the metals stocked at the jar wall and ball surface.

HEBM in Ar atmosphere causes significant altering of the alloying regime. It was shown (see ref. [18] and article A.S.Rogachev, N.F.Shkodich, S.G.Vadchenko, F.Baras, R. Chassagnon, N.V.Sachkova, O.D.Boyarchenko Reactivity of mechanically activated powder blends: Role of micro and nano structures. International Journal of Self-propagating High-temperature Synthesis, 2013, vol. 22, № 4. рр. 210-216), that dry friction and cold welding plays very important role in HEBM. Ar atmosphere accelerate welding and retard shear deformation. This is a matter of another research.

6. Eventually revisit the Conclusion part.

Response:

The additions made concerning VEC rule have now summarized in Conclusion.

Reviewer 3 Report

Although  the manuscript was improved by adding the text in yellow, some open questions remained:

1. Please check whether it is a simple grammar mistake writing 1.5-3.5 micrometers for the powder size of Iron (see line 59) instead of more realistic 15-35 micrometers. Below 3 micrometers the iron powder is more and more pyrophoric, see, for example, P. Evans, W. Borland & P. G. Mardon (1976) Pyrophoricity of Fine Metal Powders, Powder Metallurgy, 19:1, 17-21, DOI:10.1179/pom.1976.19.1.17. The passivation oxide layer on the surface of micron sized Fe  and Al powder  falsifies the planned proportion of the components.
2. Please check the oxygen content of the sample after 30 min milling and 90 min milling in air. (A simple gravimetric check of oxidation would be also informative). It is rcommended to handle the jar in glove box under protective atmosphere

Author Response

Please see the attached file for figure!

Responces to New Reviewer’s comments.

Reviewer #3

Although  the manuscript was improved by adding the text in yellow, some open questions remained:

1. Please check whether it is a simple grammar mistake writing 1.5-3.5 micrometers for the powder size of Iron (see line 59) instead of more realistic 15-35 micrometers. Below 3 micrometers the iron powder is more and more pyrophoric, see, for example, P. Evans, W. Borland & P. G. Mardon (1976) Pyrophoricity of Fine Metal Powders, Powder Metallurgy, 19:1, 17-21, DOI:10.1179/pom.1976.19.1.17. The passivation oxide layer on the surface of micron sized Fe and Al powder  falsifies the planned proportion of the components.

Response:

Thank you for alarming, however, there is no grammar mistake on the size of the Fe particles. We used commercially available iron powder R-10 (Russian standard, produced by carbonyl method) that commonly has particle sizes within the range 2 – 4 microns. Our SEM analysis confirmed that the powder consists of such small spherical particles (mainly in the range 1.5 – 3.5 micron); some of them form agglomerates. After your comment, we have double-check the size of the Fe particles with SEM (see the picture in the doc file):

Fe powder used in the work (FIGURE)

The SEM results have shown that, although most of the particles are smaller than 4 micron, some largest particles reach 6 microns. Thus, the following change was made in the manuscript: “Fe (>97%, 1.5–6.0 mm)”.

 Our experience has shown that this powder is not pyrophoric at room temperature. Probably, this is due to passivating oxide film that may already exist on the surface of the powder particles (note that the used Fe powder contained 1.2% of oxygen – according to its commercial passport). We did not make special research on this matter; however, we have measured temperature of the powder self-ignition in air atmosphere, which is 260°C. We have also added this statement to the manuscript.

2. Please check the oxygen content of the sample after 30 min milling and 90 min milling in air. (A simple gravimetric check of oxidation would be also informative). It is recommended to handle the jar in glove box under protective atmosphere

Response:

An oxygen content in the as-synthesized HEA powder is about 3 at.% (line 181). EDS analysis of the initial Fe powder, presented in the figure above, shown average content of oxygen 3.7 at.%, while local measurements in different particles gave from 2.8 at.% to 7.0 at.%. The EDS analysis of oxygen in the alloy gives only semi-quantitative data (see lines 182-183). More reliable chemical analysis gave 3.0 at.% for the sample milled 90 min. Unfortunately, we do not have results of the chemical analysis for the sample milled 30 min (chemical analysis is a long procedure), however, good agreement between the oxygen content in the initial powder and alloyed powder after 90 min HEBM shows that the oxidation during milling is negligible (lines 65-67). This conclusion is also confirmed by the following calculation.  

The milling jar volume is 150 cm3. Subtracting volume of the milling balls (200 g) and powders (10 g), we have 124 cm3 of air at 1 atm inside the jar. The jar is closed hermetically, therefore, the metals powders can react only with the oxygen containing inside the jar. Mass of this oxygen is 37 mg, which can give 0.37 wt.% in case if the metals absorb all oxygen from the air. This small amount cannot be determined by gravimetric check, because total amount of the gaseous oxygen (37 mg) that can be (theoretically) absorbed by the powders, is much smaller than mass of the metals stocked at the jar wall and ball surface. The proper information is added to the manuscript – page 2, lines 65-67.

We agree that it is better to handle the jar and powders in glove box under Ar atmosphere. We do this with really pyrophoric powders. However, the HEA powder is not pyrophoric, and later or earlier we have to expose it at air, because SEM, EDS, chemical analyses, SPS consolidation and other tests cannot be proceeded inside the glove box. Also, the high entropy alloys are intended to work in oxidation and other aggressive media.

Author Response File: Author Response.pdf