Mechanical Properties and Microstructure of W-6Ni-4Co Alloy by a Two-Step Sintering Process
Round 1
Reviewer 1 Report
This paper deals with a problem of the effect of two-step sintering process on mechanical properties and microstructure of W-6Ni-4Co alloys.
The paper is clearly written. The title, abstract and conclusion are within the paper aim. The results are carefully discussed. English is rather good. The paper is scientifically sound and not misleading. The work is relevant and the quality of presentation meets the requirements.
Nevertheless, there are several questions on the text.
1. It is not stated to what size the powders W, Co, Ni are crushed by using a planetary ball mill.
2. How many samples were investigated in the work?
3. The method of calculating the average grain size of W-6Ni-4Co should be transferred to the "experimental procedure".
4. When specifying the average grain size, a three-digit number after the point (2.326 μm) is used. How important is it? Is it possible to round the number to the tenth (for example, 2.3 μm). Specify the measure of inaccuracy.
The paper may be attached to the publication after minor revision.
Author Response
Q1: It is not stated to what size the powders W, Co, Ni are crushed by using a
planetary ball mill.
Answer: Thank for the reviewer pointed out the question. We have added the testing methods and results for particle size “The average size of ball milled powders was performed using a laser granularity analyzer (Rise-2008, Beijing shang de tong science and technology CO., LTD).” and “The average size of ball milled powders for 10 h and 15 h was 6.4 μm. and 7.5 μm.” and “The average size of ball milled powders for 20 h and 25 h was 5.3 μm and 4.0 μm.” (Please see the 70th and 71th rows in Page 2, and the 96th and 99th rows in Page 3.)
Q2: How many samples were investigated in the work?
Answer: Thank for the reviewer proposed the question. We have added the experimental procedure “and five samples under each sintering process were used for structure and property analysis” (Please see the 68th and 69th rows in Page 2.)
Q3: The method of calculating the average grain size of W-6Ni-4Co should be transferred to the "experimental procedure".
Answer: Thank for the reviewer proposed the question. We have added the experimental procedure “The average grain size of sintered samples was also calculated using grid method.” (Please see the 75th row in Page 2.)
Q4: When specifying the average grain size, a three-digit number after the point (2.326 μm) is used. How important is it? Is it possible to round the number to the tenth (for example, 2.3 μm). Specify the measure of inaccuracy.
Answer: Thank for the reviewer proposed the question. We have changed the average grain size of 2.326, 2.835, 2.948, 3.544, 3.308, 3.917 and 3.866 μm to 2.3, 2.8, 2.9, 3.5, 3.9 and 3.9 μm. (Please see the 150th, 154th and 156th rows in Page 5, and the 159th and 162th rows in Page 6.)
Moreover, we have not edited the English language of the paper roundly because of short duration. If the English language of the paper is necessary to modify for improve its quality. We will please a professional English editing company for editing the English language of the paper.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this study, the relationship between sintering condition and mechanical properties of W-6Ni-4Co alloy has been demonstrated well in terms of microstructural evolution. On the other hand, there are several revisions needed as listed below.
The meaning of first step in the two-step sintering process is not clear. It seems that the similar microstructure can be obtained by simple sintering at 1450 or 1500 degree C. The benefit and necessity of this first step should be explained.
What do you mean by “double arborization” in line 85? This should be described in a simpler way.
The microstructure and mechanical properties should be compared to other W-Ni-Co alloys and discussed in detail in the text to show the novelty and advantage of this study.
Author Response
Q1: The meaning of first step in the two-step sintering process is not clear. It seems that the similar microstructure can be obtained by simple sintering at 1450 or 1500 degree C. The benefit and necessity of this first step should be explained.
Answer: Thank for the reviewer pointed out the question. We have added “indicating obvious characteristic of solid phase sintering due to the lower sintering temperature below liquid phase sintering of 1453 ℃” and “It can be seen the average grain size of alloys by two-step sintering at the temperature of 1500 ℃ is obviously smaller than that of reference [21] by liquid phase sintering at 1500 ℃. This is because dissolution and precipitation in liquid phase and physical collapse of W during liquid phase sintering would be prevented by skeleton structure resulting from the first step of solid phase sintering” (Please see the 151th-153th rows in Page 5, and the 165th- 169th rows in Page 6.)
Q2: What do you mean by “double arborization” in line 85? This should be described in a simpler way.
Answer: Thank for the reviewer proposed the question. We have changed “double arborization” to “arborization and arborization”(Please see the 92th row in Page 3.)
Q3: The microstructure and mechanical properties should be compared to other W-Ni-Co alloys and discussed in detail in the text to show the novelty and advantage of this study.
Answer: Thank for the reviewer proposed the question. We have added “more than that of alloys by one-step sintering at 1450 ℃ in references [9] and [18] because of refine grain strengthening and network strengthening effect of W” (Please see the 197th-199th rows in Page 7.)
Author Response File: Author Response.pdf
Reviewer 3 Report
Manuscript ID: metals-519510
Mechanical properties and microstructure of W-6Ni-3 4Co alloy by a two-step sintering process
This manuscript is focused on the influence of two-step process sintering on the microstructure and mechanical behaviour of hard W-6Ni-3 4Co alloy.
It suggests several remarks.
1- The amount of each present phase in the as-obtained alloy was inferred from X-ray diffraction patters by measuring the surface area of important peak for each phase (W, NiW and Co3W). This method is approximate and must be replaced by a method such as MAUD based on Rietveld refinement. Examining the table 3, the percentages obtained are random and do not seem to follow a regular evolution according to the temperature or the time. These percentages should be recalculated using more accurate method suggested (MAUD or similar program)
2- The SEM images given in the manuscript cannot allow an accurate discussion on the influence of the heat treatment on the grain size and microstructure. Images with higher magnification should be provided and discussed. Furthermore the mean size of grain should be determined by means of image Tool program for instance (size distribution curve)
3- The discussion of the mechanical behaviour (HV, sigma 0.2……) should be based on the evolution of such properties in function of the microstructure and mainly the grain size. So curve with mechanical property versus grain size will be helpful.
4- The manuscript suffers from a lack of discussion and comparison of the results obtained here with those reported in the literature. The manuscript looks like a description of the results without any perspective. References hereafter will help : Dinçer et al. International Journal of Refractory Metals and Hard Materials, Volume 50, May 2015, Pages 106-112; Chen et al. JALCOM, 731 (2018) 78-83 and Chen et al. JALCOM, 711 (2017) 488-494
Author Response
Q1: The amount of each present phase in the as-obtained alloy was inferred from X-ray diffraction patters by measuring the surface area of important peak for each phase (W, NiW and Co3W). This method is approximate and must be replaced by a method such as MAUD based on Rietveld refinement. Examining the table 3, the percentages obtained are random and do not seem to follow a regular evolution according to the temperature or the time. These percentages should be recalculated using more accurate method suggested (MAUD or similar program)
Answer: Thank for the reviewer proposed the question. The aim of calculation for phase volume fraction is to qualitatively analyze microstructure evolution of alloys. Therefore, the phase volume fraction calculated by diffraction peak area ratio is enough to analyze the microstructure evolution of alloys.
Q2: The SEM images given in the manuscript cannot allow an accurate discussion on the influence of the heat treatment on the grain size and microstructure. Images with higher magnification should be provided and discussed. Furthermore the mean size of grain should be determined by means of image Tool program for instance (size distribution curve)
Answer: Thank for the reviewer proposed the question. Images with higher magnification have been obtained during experimental procedure. By contrast, the lower magnification was more suitable for observing distribution of phases and shape of W-rich phase. The grid method was used for qualitatively analyzing the change trend of average grain size in alloys.
Q3: The discussion of the mechanical behaviour (HV, sigma 0.2……) should be based on the evolution of such properties in function of the microstructure and mainly the grain size. So curve with mechanical property versus grain size will be helpful.
Answer: Thank for the reviewer proposed the question. We have added the relationship of yield strength and average grain size “Obviously, the mechanical properties of W-6Ni-4Co alloys by different sintering process have a close relationship with the average grain size of alloys. In order to understand the relationship between mechanical properties and average grain size of alloys more directly, the change curves of yield strength and average grain size of alloys are shown in figure 8, presenting the opposite trend of mechanical properties and average grain size. ” (Please see the 211th-215th rows in Page 7) , and Figure 8 in Page 8.
Q4: The manuscript suffers from a lack of discussion and comparison of the results obtained here with those reported in the literature. The manuscript looks like a description of the results without any perspective. References hereafter will help: Dinçer et al. International Journal of Refractory Metals and Hard Materials, Volume 50, May 2015, Pages 106-112; Chen et al. JALCOM, 731 (2018) 78-83 and Chen et al. JALCOM, 711 (2017) 488-494
Answer: Thank for the reviewer proposed the question. Dinçer et al. investigated the effects of composition and sintering temperature on the microstructural characteristics of liquid phase sintered 90W-Ni-Co alloys. The results showed that the coarsening process was mainly governed by Ostwald ripening mechanisms in the liquid state. The calculated activation energies were found to be in rather close agreement to the literature data given for W–Ni–Fe alloys, indicating that grain coarsening in W–Ni–Co and W–Ni–Fe alloys most probably takes place through similar diffusional processes. Chen et al. investigated the effects of Ni / Co ratio and secondary ball milling method on the characteristics and sintering behavior of W-Ni-Co tungsten heavy alloy. The results showed that the high Ni/Co ratio facilitates the process of solid phase sintering and promotes a fine-grained structure. The mechanisms of the transition between the solid and liquid states during sintering could be associated with the amount of binder phase and its composition, tungsten grain size, deformation strain and sintering conditions; The use of the secondary ball milling promoted the formation of solid phase during sintering. A microstructure composed of the fine and uniformly distributed g(Ni,Co) binder phase with a large amount of twinned structures was obtained, which could be related to an increase of mechanical properties.
We have quoted the reference 21 from Dinçer et al. and references 9, 17 and 18 from Chen et al. We have added “Dinçer et al. [21] presented that W grains could exhibit physical collapse under gravity during liquid phase sintering, which leads to the agglomeration of W grains and exuding of the liquid phase due to the large density differences between W grains and the binder matrix phase.” and “The results were the similar as that of reference 18.” (Please see the 45th-48th lines in Page 2 and 99th rows in Page 3)
Moreover, we have not edited the English language of the paper roundly because of short duration. If the English language of the paper is necessary to modify for improve its quality. We will please a professional English editing company for editing the English language of the paper.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The resubmitted manuscript has been revised almost accordingly. But in line 90 on p.3, do you mean that the original shape of W powder is polyhedron, and those of Ni and Co are arborization? If so, the sentences should be rewritten like I mentioned, for example.
Author Response
Q1: The resubmitted manuscript has been revised almost accordingly. But in line 90 on p.3, do you mean that the original shape of W powder is polyhedron, and those of Ni and Co are arborization? If so, the sentences should be rewritten like I mentioned, for example.
Answer: Thank for the reviewer proposed the question. We have changed “The original shapes of W, Ni and Co are polyhedron, arborization and arborization” to “The original shape of W powder is polyhedron, and those of Ni and Co are arborization”.
Reviewer 3 Report
Overall, the authors gave satisfactory answers and the manuscript was improved. So the manuscript can now be accepted
Author Response
Comments and Suggestions for Authors
Overall, the authors gave satisfactory answers and the manuscript was improved. So the manuscript can now be accepted
