Ultrafine-Grained Tungsten Heavy Alloy Prepared by High-Pressure Spark Plasma Sintering

Round 1

Reviewer 1 Report

This paper deals with the thermoelectric properties of MgSnO by changing the amount of Sn.

The results of conductivity and Seebeck coefficients could be useful to researchers.

However, authors have to measure thermal conductivity to confirm the thermoelectric property exactly. 

In this paper, adding Sn increased the electrical conductivity so that the Seebeck coefficient could increase naturally.

Increasing the electrical conductivity can increase the thermal conductivity, and then the thermoelectric property (figure of merit) could decrease.

In addition, the authors have to discuss the reason for increasing the electrical conductivity by adding Sn in depth.

That is the essential information in this paper.

 

The most important thing is all samples must sinter fully to densify, and then all of the properties should be measured.

I cannot find the sample information to measure the properties in the paper. I think the authors estimate the properties of just powders.

In the current state, authors must inform densities of all samples and apply them to electrical and thermal conductivities.

Author Response

Dear reviewer：

  your comments are not directed at our manuscript. 

Reviewer 2 Report

I read the article entitled “Ultra-fine grained tungsten heavy alloy prepared by high-pressure spark plasma sintering” which is interesting one, and having sound contributions. The authors have focussed to develop ultra-fine grained heavy tungsten alloy via spark plasma sintering. They have achieved high density products (> 98%) with improved mechanical properties in terms of hardness. I can recommend this article to be published in your esteemed journal. However, I have observed the following comments which the authors have to address it.

1.      The reasons for improved density and high hardness due to present consolidation method is to be incorporated at the end of abstract. This is missing.

2.      Further, in the middle of abstract, the authors have to mentioned the variation of sintering temperature ranges

3.      Reference is to be cited for the equation number 1 and 2

4.      Some more literature related to high pressure SPS are to be incorporated. Further, the main objectives of the present work are not clear. It has to be re-written at the end of introduction part

5.      What is the vacuum pressure used in the ball mill vial? How it was set?

6.      On what basis, the sintering temperature range from 800 to 1300 degree was selected? Usually, WHA will sintered by liquid phase sintering around 1500 degree. Need to be addressed

7.      From XRD results of Figure 2, minor binding phase was observed at 800 degree whereas it was not observed in further increasing of temperature. Why? Proper explanation is to be addressed

8.      How SEM fracture surface morphology was carried out? Have the authors conducted any tensile test?

9.      Explanation/discussion related to Figure 3 and Figure 4 cannot be acceptable. The tungsten will not melt in the selected temperature range. In this case, how grain can be observed from fracture surfaces. Fracture surfaces, we can observe the size of dimples or W particles pull-out. It has to be checked.

10.   Error bar is to be introduced in Figure 5. How the grain size was measured from fracture results? This is not clear

11.   Error bar is also to be introduced in the hardness results of Figure 8 and Table 3.  

Author Response

Dear Reviewer,

Thanks for the careful reading and scientific comments. Following are our reply to you.

 

1. The reasons for improved density and high hardness due to present consolidation method is to be incorporated at the end of abstract. This is missing.
2. Further, in the middle of abstract, the authors have to mentioned the variation of sintering temperature ranges

Response:

Thanks for your kind suggestion! These two parts of the description have been added into the revised manuscript.

 

3. Reference is to be cited for the equation number 1 and 2

Response:

Sorry for the negligence! The references have been added into the revised manuscript for equation 1 and equation 2.

 

4. Some more literature related to high pressure SPS are to be incorporated. Further, the main objectives of the present work are not clear. It has to be re-written at the end of introduction part

Response:

The corresponding parts have been revised in the manuscript.

 

5. What is the vacuum pressure used in the ball mill vial? How it was set?

Response:

The circulating water multi-purpose vacuum pump was used to control the vacuum degree with a vacuometer value of -0.09. Ethanol was used in the ball mill process for mixing powders.

 

6. On what basis, the sintering temperature range from 800 to 1300 degree was selected? Usually, WHA will sintered by liquid phase sintering around 1500 degree. Need to be addressed

Response:

According to existing literature reports, the sintering temperature of tungsten alloys by SPS is about 1100 ℃. In this paper, high pressure (>100MPa) sintering was used for fabricate tungsten alloys for the first time. We mentioned to explore the relationship between pressure and sintering temperature, so a wider temperature range was selected in the present work.

 

7. From XRD results of Figure 2, minor binding phase was observed at 800 degree whereas it was not observed in further increasing of temperature. Why? Proper explanation is to be addressed

Response:

Thanks for the carefully reading! The binding phase also existed at higher sintering temperatures, which have not pointed in the XRD patterns. We just have pointed the first time it appeared at low temperature.

 

8. How SEM fracture surface morphology was carried out? Have the authors conducted any tensile test?

Response:

Due to the small size of high pressure sintered samples, the bending strength test cannot be carried out under standard conditions. The fracture surfaces of the samples were obtained by breaking with a universal testing machine, which is an easy and also common method to observe the fracture mode for as-sintered materials.

 

9. Explanation/discussion related to Figure 3 and Figure 4 cannot be acceptable. The tungsten will not melt in the selected temperature range. In this case, how grain can be observed from fracture surfaces. Fracture surfaces, we can observe the size of dimples or W particles pull-out. It has to be checked.

Response:

We agree that the selected temperature range did not reach the melting point of tungsten. In most situation, sintering high melting point metal or ceramics is a method to densify a powder compact by eliminating pores at temperature ~70% of melting point. The grain boundary migration cause grain growth besides contributing to densification. In this present work, that means grain boundary never disappear below melting point. The fracture mode in Figure 3 and 4 is intergranular fracture. So the grain can be observed, which is similar to ceramics.

 

10. Error bar is to be introduced in Figure 5. How the grain size was measured from fracture results? This is not clear.
11. Error bar is also to be introduced in the hardness results of Figure 8 and Table 3.

Response:

Thanks for the kind suggestion! The error bars for the data in the present work have been added in Figure 5, Figure 8 and Table 3 in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper describes the properties of spark plasma sintered ultra-fine grained tungsten alloys. There are some points in the article that should be corrected:

1-    In the title, the dot (.) before the “high” should be deleted.

2-    Page 2, line 45, the Hall-Petch equation describes the relation between strength and grain size, but the authors have replaced hardness with strength in this equation. Therefore, it is necessary to modify the equation.

3-    Simultaneous use of nanoparticles such as Y₂O₃ and SPS is also suitable solutions to enhance the properties of tungsten base alloys. It is appropriate to pay attention to these references as well:

https://doi.org/10.1016/j.jallcom.2016.11.364

https://doi.org/10.1016/j.jre.2017.04.007

https://doi.org/10.1016/j.ijrmhm.2021.105728

 

4-    At the end of the introduction section, innovation and the distinction of the research with other similar researches should be specified.

5-    Page 2, line 71, Why did author choose the 93W-5.6Ni-1.4Fe (wt.%) composition? What is the exact application of this alloy? It should be explained in the introduction.

6-    Lines 71-72, how were the ball milling conditions chosen?

7-    According to Figure 1(b), the morphology and shape of the powders are not clear. The morphology of the raw powders must be determined by proper SEM image.

8-    Figure 2 should be moved after the relevant explanations. Also, Figs. 5, 6, 7, and 8.

9-    Basically, is it expected that liquid state sintering will occur or not? At what temperatures? Shouldn't we expect some phases to form due to the occurrence of liquid phase sintering? Authors should explain the occurrence of liquid and solid state sintering in each of the temperatures and pressures.

10- According to table 3, some alloys have a different composition compared to the alloy used in this research, which can affect the properties, for example:

Ref. [14] 90W–7Ni–3Fe

Ref. [15] W-6Ni-2Fe-2Co (W90), W-12Ni-4Fe-4Co (W80) and W-18Ni-6Fe-6Co (W70)

Ref. [34] 95 wt% W, 3.5 wt% Ni and 1.5 wt% Fe

Ref. [36] W–2Mo–7Ni–3Fe

Ref. [20] W–7Ni–3Fe

 

The authors should check this and explain on what basis they chose the alloys in Table 3.

 

 

Author Response

Dear Reviewer,

Thanks for the careful reading and scientific comments. Following are our reply to the you.

1. In the title, the dot (.) before the “high” should be deleted.

Response:

Thanks for your kind suggestion. We are so sorry for the negligence! The mistake has been corrected in the revised manuscript.

 

2. Page 2, line 45, the Hall-Petch equation describes the relation between strength and grain size, but the authors have replaced hardness with strength in this equation. Therefore, it is necessary to modify the equation.

Response:

Hall-Petch equation describes the relation between grain size and yield stress (σ), which is related to strength and hardness as well. We have added a reference “R. W. Rice, C. C. Wu, F. Boichelt, J. Am. Ceram. Soc. 1994, 77, 2539.”

 

3. Simultaneous use of nanoparticles such as Y₂O₃ and SPS is also suitable solutions to enhance the properties of tungsten base alloys. It is appropriate to pay attention to these references as well:

https://doi.org/10.1016/j.jallcom.2016.11.364

https://doi.org/10.1016/j.jre.2017.04.007

https://doi.org/10.1016/j.ijrmhm.2021.105728

Response:

Thanks for your advice. We have added the suggested literatures into the references in the revised manuscript.

 

4. At the end of the introduction section, innovation and the distinction of the research with other similar researches should be specified.

Response:

The article has been revised. Thank you for your suggestion.

 

5. Page 2, line 71, Why did author choose the 93W-5.6Ni-1.4Fe (wt.%) composition? What is the exact application of this alloy? It should be explained in the introduction.

Response:

According to Li’s work (Ref.18) and many other available literatures, 93W-5.6Ni-1.4Fe heavy alloys could possess high mechanical properties. However, it’s still a challenge to fabricate high relative density and fine grain bulks, which is expected to show enhanced performance.

 

6. Lines 71-72, how were the ball milling conditions chosen?

Response:

Thanks for the question! Actually we have tried many milling conditions based on previous work involved in ball milling. And the selected one favor to the homogenous mixing.

 

7. According to Figure 1(b), the morphology and shape of the powders are not clear. The morphology of the raw powders must be determined by proper SEM image.

Response:

Figure 1b just shows the morphology of the mixture after ball milling, which is not the initial raw powder. We are sorry we could not support other images. If the reviewer thinks it unsuitable here, we could eliminate it. 

 

 

8. Figure 2 should be moved after the relevant explanations. Also, Figs. 5, 6, 7, and 8.

Response:

Thank you so much for the kind suggestion! The previous order is just for composing the main text and figures. It has been modified according to your comment in the revised manuscript.

 

9. Basically, is it expected that liquid state sintering will occur or not? At what temperatures? Shouldn't we expect some phases to form due to the occurrence of liquid phase sintering? Authors should explain the occurrence of liquid and solid state sintering in each of the temperatures and pressures.

Response:

The selected temperature range did not reach the melting point of tungsten. In most situation, sintering high melting point metal or ceramics is a method to densify a powder compact by eliminating pores at temperature ~70% of melting point, which is a solid-state process. The grain boundary migration cause grain growth besides contributing to densification. In this present work, high sintering pressure is considered to support additional driving force for the sintering and lead to the low-temperature densification, which effectively limited grain growth and enhance the hardness.

 

10. According to table 3, some alloys have a different composition compared to the alloy used in this research, which can affect the properties, for example:

Ref. [14] 90W–7Ni–3Fe

Ref. [15] W-6Ni-2Fe-2Co (W90), W-12Ni-4Fe-4Co (W80) and W-18Ni-6Fe-6Co (W70)

Ref. [34] 95 wt% W, 3.5 wt% Ni and 1.5 wt% Fe

Ref. [36] W–2Mo–7Ni–3Fe

Ref. [20] W–7Ni–3Fe

The authors should check this and explain on what basis they chose the alloys in Table 3.

Response:

Thanks for the good question! Here we just want to compare the W-based alloy properties between our work and available literatures with or without the same composition. The present work supports a promising method to fabricate harder W alloy with finer grain size at lower sintering temperature.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have addressed reviewer's comments and the manuscript is now suitable for publication.

Reviewer 2 Report

The revised manuscript was prepared well and hence, I am recommending to accept it

Reviewer 3 Report

The authors tried to apply the desired corrections in the revised version as much as possible. According to the provided explanations and correction of the article, my suggestion is acceptance.