Effect of V and Ti on the Oxidation Resistance of WMoTaNb Refractory High-Entropy Alloy at High Temperatures

Round 1

Reviewer 1 Report

The paper describes the effect of the isothermal oxidation on three refractory high entropy alloys. In general, the investigated subject is very interesting. However, due to basic errors in measurement methodology and interpretation of obtained results, as well as a lack of essential new knowledge, this paper cannot be recommended for publication.

Detailed remarks:

1. The studied samples are very small and consequently the undesirable edge effect is large.
2. The time it takes to reach the reaction temperature is too long (about 1.5 h), which causes a relatively thick scale to form before the isothermal measurements begin. Consequently, the mass changes of the samples in the initial stage of oxidation cannot be regarded as occurring under isothermal conditions.
3. The work does not describe how the complete oxidation was determined, which is not a simple task in the case of simultaneous processes of scale growth and evaporation.
4. Equation (1) has been well known for about one hundred years, thus relatively new references [14,15] do not seem reasonable.
5. Interpretation of the kinetic results based on a comparison of K values is completely incorrect, because the “n” parameters in Eq. 1 are different. Such an interpretation should rather be performed on the basis of the slope of lines in Fig. 2.
6. Determining the vaporization rate of scales would be valuable for interpreting the oxidation behavior of alloys.
7. It is well known that oxidation resistance of refractory metals is poor. Thus, the results obtained in this work are not surprising and do not deliver new essential knowledge.

 

Summing up all these remarks, I suggest that this paper be rejected.

Author Response

Response to Reviewer 1 Comments

 

Point 1: The studied samples are very small and consequently the undesirable edge effect is large.

 

Response 1: Thermo gravimetric system (TG) is widely used to investigate the oxidation process of alloys for obtaining continuous mass gain curves and avoiding the impact of thermal shock in repeated weighing. Although the size of samples is limited by TG, the comparative study of samples with uniform size is reasonable. The effect of edge can be mitigated to an acceptable level by preparing samples without edge defect.

 

Point 2: The time it takes to reach the reaction temperature is too long (about 1.5 h), which causes a relatively thick scale to form before the isothermal measurements begin. Consequently, the mass changes of the samples in the initial stage of oxidation cannot be regarded as occurring under isothermal conditions.

 

Response 2: The initial heating process can’t be avoided in the experiments using TG. However, the program of heating process is fixed, which ensures the uniform isothermal oxidation experiments and makes the comparative study reliable.

 

Point 3: The work does not describe how the complete oxidation was determined, which is not a simple task in the case of simultaneous processes of scale growth and evaporation

 

Response 3: According to our experimental results, when the mass gain keeps going down after the prolonged oxidation, the sample is completely oxidized. The descending mass gain is due to the ending oxidation increase and the volatilization of some oxides. This state is suggested to be completely oxidized, and has been reported in the reference (F. Müller, et al. Corrosion Science 159 (2019) 108161). We have added this determination into the revised manuscript.

 

Point 4: Equation (1) has been well known for about one hundred years, thus relatively new references [14,15] do not seem reasonable.

 

Response 4: We have removed the related references.

 

Point 5: Interpretation of the kinetic results based on a comparison of K values is completely incorrect, because the “n” parameters in Eq. 1 are different. Such an interpretation should rather be performed on the basis of the slope of lines in Fig. 2.

 

Response 5: Thank you very much for the reviewer’s professional advice. The comparison of K values based on the different n values is incorrect. We have deleted the corresponding discussion. And according to the advice of reviewer, we have selected the slopes of initial linear stage to explain the mass gain of forming passivating oxide layer and obtained the same conclusion.

 

Point 6: Determining the vaporization rate of scales would be valuable for interpreting the oxidation behavior of alloys.

 

Response 6: There is the volatile oxidation in the oxidation process of refractory alloys containing Mo, V and W, which leads to the power law oxidation kinetics rather than parabolic law. Determining the vaporization rate of scales is valuable for interpreting the oxidation behavior of alloys. However, the accurate collection of volatiles is difficult and corresponding study is rarely reported. Thus, the researches on the oxidation of refractory alloys are depended on the mass gain curves, phases and microstructure of scales.

 

Point 7: It is well known that oxidation resistance of refractory metals is poor. Thus, the results obtained in this work are not surprising and do not deliver new essential knowledge.

 

Response 7: Three refractory high entropy alloys studied in this paper possess excellent mechanical properties at elevated temperatures, making them extremely promising high-temperature structural materials. However, the oxidation resistance at elevated temperatures must to be considered before the application. This work provides the references of oxidation resistance of three RHEAs for the application at elevated temperatures. V addition aggravates the volatility of V₂O₅, MoO₃ and WO₃, and leads to the disastrous internal oxidation. Ti addition reduces the mass gain of forming the full coverage of passivating scale and prolongs the passivation duration of alloys.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript reports on the effect of V and Ti on the oxidation resistance of WMoTaNb RHEA. It is an interesting manuscript presenting the results in an acceptable way. However, there are some critical issues which need to be addressed before further processing.

• Introduction can be improved by citing the most recent development on the oxidation resistance of RHEAs. For example these articles should be cited:
• Ostovari Moghaddam et al., High temperature oxidation resistance of W-containing high entropy alloys, (https://doi.org/10.1016/j.jallcom.2021.162733).
• Liet al., Enhanced oxidation resistance of MoTaTiCrAl high entropy alloys by removal of Al, (https://doi.org/10.1007/s40843-020-1332-2).
• Gorr et al., A new strategy to intrinsically protect refractory metal based alloys at ultrahigh temperatures, (https://doi.org/10.1016/j.corsci.2020.108475).

 

• The authors attributed the XRD peak broadening to the grain refinement effect of V. So, why there is no peak broadening in WMoTaNbTi which has even a lower grain size. The peak broadening may be also due to the lattice strain. Please check your results.

 

• What do the authors mean with complete oxidation? Does it mean that the samples reach a passivation state and are protected against further oxidation, or it means that the samples are already completely oxidized? If it is passivation state, so it seems that WMoTaNbV rapidly reaches the passivation state and exhibits better oxidation resistance (lower mass gain compared to WMoTaNbTi). Please clarify this in page 3.

 

• It is stated that “It can be found from the fitted equations that the oxidation coefficient K decreases with Ti addition and increases with V addition, which indicates that the mass gain of forming passivating oxide layer is reduced by Ti addition and increased by V addition”. However, from Fig. 2 it is clear that WMoTaNbTi shows the maximum weight gain. Why?

 

• In Fig. 3, why the XRD pattern of WMoTaNbV oxidized for 4h is not shown?

 

• The thickness of oxide layer in WMoTaNbV is almost 8 times higher than that in WMoTaNbTi RHEA. This should be reasonably translated to a high mass gain and low oxidation resistance. However, Fig.2 shows lower mass gain for WMoTaNbV compared to WMoTaNbTi. The authors should repeat this test (oxidation kinetics) to assure the accuracy of their data.

 

• It is inferred from the SEM/EDS results that the parent WMoTaNb alloy has the best oxidation resistance among all the samples. This in some way consists with Fig. 2. However, the discussion about oxidation resistance of the samples is misleading. This should be clearly addressed in the revised version.

 

• I suggest the authors to improve the discussion about oxidation resistance of the samples by comparing their results with RHEA and transition HEAs, such as (among others):
• Lo et al., An oxidation resistant refractory high entropy alloy protected by CrTaO4-based oxide, (https://doi.org/10.1038/s41598-019-43819-x)
• Moghaddam et al., High temperature oxidation resistance of Al0.25CoCrFeNiMn and Al0.45CoCrFeNiSi0.45 high entropy alloys (https://doi.org/10.1016/j.vacuum.2021.110412).

 

• The conclusion need to be rewritten considering the above comment.

 

Author Response

Response to Reviewer 2 Comments

 

Point 1: Introduction can be improved by citing the most recent development on the oxidation resistance of RHEAs. For example these articles should be cited:

• Ostovari Moghaddam et al., High temperature oxidation resistance of W-containing high entropy alloys, (https://doi.org/10.1016/j.jallcom.2021.162733).
• Liet al., Enhanced oxidation resistance of MoTaTiCrAl high entropy alloys by removal of Al, (https://doi.org/10.1007/s40843-020-1332-2).

• Gorr et al., A new strategy to intrinsically protect refractory metal based alloys at ultrahigh temperatures, (https://doi.org/10.1016/j.corsci.2020.108475)

 

Response 1: Thank you very much for the reviewer’s professional advice. These works about the oxidation resistance of RHEAs are impressive and we have cited them in the revised manuscript.

 

Point 2: The authors attributed the XRD peak broadening to the grain refinement effect of V. So, why there is no peak broadening in WMoTaNbTi which has even a lower grain size. The peak broadening may be also due to the lattice strain. Please check your results.

 

Response 2: Thank you very much for the reviewer’s professional advice. We have checked the XRD results again and agree with your view. The XRD peak broadening should be due to the lattice strain caused by lattice distortion, which is frequently found in solid solution structure of HEAs.

 

Point 3: What do the authors mean with complete oxidation? Does it mean that the samples reach a passivation state and are protected against further oxidation, or it means that the samples are already completely oxidized? If it is passivation state, so it seems that WMoTaNbV rapidly reaches the passivation state and exhibits better oxidation resistance (lower mass gain compared to WMoTaNbTi). Please clarify this in page 3.

 

Response 3: Regarding the suggestion about explaining complete oxidation, we think it is really a very good advice. We suggest that the sample is completely oxidized, when the mass gain keeps going down after the prolonged oxidation. The descending mass gain is due to the ending oxidation increase and the volatilization of some oxides. This state is suggested to be completely oxidized, and has been reported in the reference (F. Müller, et al. Corrosion Science 159 (2019) 108161). We have added this determination into the revised manuscript.

 

Point 4: It is stated that “It can be found from the fitted equations that the oxidation coefficient K decreases with Ti addition and increases with V addition, which indicates that the mass gain of forming passivating oxide layer is reduced by Ti addition and increased by V addition”. However, from Fig. 2 it is clear that WMoTaNbTi shows the maximum weight gain. Why?

 

Response 4: It can be seen in Fig. 2 that Ti-containing RHEA has the minimum mass gain at initial linear stage, which is the mass gain of forming passivating oxide layer. Meanwhile, V-containing RHEA needs the maximum weight gain to form passivating oxide layer. These conclusions can be obtained from the fitted equations. The mass gain of RHEAs at the further stage is due to the growth rate of the passivating scale, which is related to the oxidation exponent (n). The maximum final weight gain of WMoTaNbTi is due to the higher n value, indicating the faster growth rate of the passivating scale. It is in agreement with the thickness of scales in Fig. 4f. The diffusion rate of oxygen in the passivating scale of WMoTaNbTi is higher that that of WMoTaNb, which leads to the disastrous internal oxidation.

 

Point 5: In Fig. 3, why the XRD pattern of WMoTaNbV oxidized for 4h is not shown?

 

Response 5: Because the sample was hideously deformed after 4 h oxidation and unfit for XRD test. On the other hand, as WMoTaNbV is oxidized sufficiently at the initial 0.5h, the surface XRD after further oxidation does not make more sense.

 

Point 6: The thickness of oxide layer in WMoTaNbV is almost 8 times higher than that in WMoTaNbTi RHEA. This should be reasonably translated to a high mass gain and low oxidation resistance. However, Fig.2 shows lower mass gain for WMoTaNbV compared to WMoTaNbTi. The authors should repeat this test (oxidation kinetics) to assure the accuracy of their data.

 

Response 6: The final mass gain of WMoTaNbV is less than expectations from the thickness of oxide layer, which is due to the volatile oxidation of V, Mo and W. V addition aggravates the volatility of V₂O₅, MoO₃ and WO₃, and leads to the disastrous internal oxidation, which results in the higher thickness of oxide layer.

 

Point 7: It is inferred from the SEM/EDS results that the parent WMoTaNb alloy has the best oxidation resistance among all the samples. This in some way consists with Fig. 2. However, the discussion about oxidation resistance of the samples is misleading. This should be clearly addressed in the revised version.

 

Response 7: As the reviewer proposed, the SEM/EDS results and oxidation kinetic indicate the best oxidation resistance of WMoTaNb RHEA. We have clearly addressed it in the revised manuscript.

 

Point 8: I suggest the authors to improve the discussion about oxidation resistance of the samples by comparing their results with RHEA and transition HEAs, such as (among others):

• Lo et al., An oxidation resistant refractory high entropy alloy protected by CrTaO4-based oxide, (https://doi.org/10.1038/s41598-019-43819-x)

• Moghaddam et al., High temperature oxidation resistance of Al0.25CoCrFeNiMn and Al0.45CoCrFeNiSi0.45 high entropy alloys (https://doi.org/10.1016/j.vacuum.2021.110412)

 

Response 8: Thank you very much for the reviewer’s professional advice. Comparing the oxidation behavior of WMoTaNb RHEA with that of other HEAs is essential, we have added the corresponding discussion in the revised manuscript and cited related references.

 

Point 9: The conclusion need to be rewritten considering the above comment.

 

Response 9: We have rewritten the conclusion in the revised manuscript based on the revised discussion.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

This is a useful experimental study aimed at solving an urgent problem and I think it deserves publication after some addition of the text:

In my opinion, the authors could expand the introduction and the number of references in it at the expense of the data presented in the doi: 10.3390/ma14102595 review. This review contains references to works devoted to the oxidation of high-entropy alloys with tungsten.
In addition, I would recommend taking into account the data from work doi: 10.1016/j.jallcom.2021.162733, devoted to the oxidation of such alloys, as well as, possibly, works doi: 10.1016/j.ijrmhm.2021.105608 and doi: 10.1007/s11837-021-04847-z about the use of these and similar alloys. In general, I would recommend that the authors pay more attention to the justification for the need to conduct their research.

2. Judging by the data in Table 1, the compositions selected by the authors are not completely equimolar. Why? Perhaps the initial charge was equimolar. But why are the alloys different? If some elements are more than others, then some of the elements, the content of which is less, disappeared somewhere. Where? I think this needs to be explained in the text.

3. For the designation of indices in chemical formulas, I recommend using smaller numbers.

4. Line 85. "Fig. 2a". There are no letters in Figure 2.

Author Response

Response to Reviewer 3 Comments

 

Point 1: In my opinion, the authors could expand the introduction and the number of references in it at the expense of the data presented in the doi: 10.3390/ma14102595 review. This review contains references to works devoted to the oxidation of high-entropy alloys with tungsten.

In addition, I would recommend taking into account the data from work doi: 10.1016/j.jallcom.2021.162733, devoted to the oxidation of such alloys, as well as, possibly, works doi: 10.1016/j.ijrmhm.2021.105608 and doi: 10.1007/s11837-021-04847-z about the use of these and similar alloys. In general, I would recommend that the authors pay more attention to the justification for the need to conduct their research.

 

Response 1: Thank you very much for the reviewer’s professional advice. We have improved the introduction and cited the corresponding references

 

Point 2: Judging by the data in Table 1, the compositions selected by the authors are not completely equimolar. Why? Perhaps the initial charge was equimolar. But why are the alloys different? If some elements are more than others, then some of the elements, the content of which is less, disappeared somewhere. Where? I think this needs to be explained in the text.

 

Response 2: The discrepancy between experimental composition of as cast RHEAs and normal composition is due to the different saturated vapor pressure of elements. In the studied RHEAs, W has the maximum saturated vapor pressure, Ti and V have the lesser values. The difference of saturated vapor pressure leads to the different volatilization in the repeated smelting process and results in the discrepancy between final content and initial content. This phenomenon has also been reported in the study of WMoTaNb RHEA by Senkov(O.N. Senkov, Intermetallics, 2011, 698-706). We also have added the corresponding explanation in the revised manuscript.

 

Point 3: For the designation of indices in chemical formulas, I recommend using smaller numbers.

 

Response 3: Thank you very much for the reviewer’s kindly remind. We have corrected the mistakes and checked the whole paper to avoid similar mistake.

 

Point 4: Line 85. "Fig. 2a". There are no letters in Figure 2.

 

Response 4: Thank you very much for the reviewer’s kindly remind. We have corrected the error and checked the whole paper.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Point 1:

Despite the fact that TG systems are widely used, not all systems are equally suitable for carrying out this type of research as described in the publication. In this case, the samples should preferably have a round or rectangular cross-section with a face area of at least 1 cm2 and a thickness of no more than 1 mm. In the opposite case, the influence of sample edges on mass changes is unacceptably high. This was shown for the first time in works carried out in the 60s and 70s of the last century.

 

Point 2:

The problem of choosing the right procedure for starting the oxidation process was also the subject of extensive research in the middle of the last century. It has been shown that it is best to insert the sample into the reaction chamber after the desired reaction temperature has been obtained or to heat it very quickly so that it can reach the reaction temperature in about 1 minute. Of course, not all TG devices are capable of realizing this procedure and therefore should not be used for this type of experiment.

 

Point 3:

Generally, the following sentence is not correct "The descending mass gain is due to the ending oxidation increase and the volatilization of some oxides. This state is suggested to be completely oxidized..."., because the total mass loss denotes only that the rate of volatilization is greater than sample mass gains, as a result of scale thickening. Even if the mass decrease occurs, the remains of the metallic core can be present in the sample. Thus, the best proof of the completely oxidation state is the lack of a metal core in the cross-section of investigated sample and such a photo should be presented in the paper.

 

Point 6:

When a sample is completely oxidized, its mass should decrease according to the linear rate law. Consequently, parameters of such a dependence can be determined from the presented data.

 

Point 7:

I understand the authors' explanations, which does not change the fact that the results obtained in this work are not surprising and do not deliver new essential knowledge. Therefore, I would suggest to add information to the paper that the potential application of the investigated alloys at high temperatures will require the use of protective coatings, and such studies are already planned or in progress.

Author Response

Response to Reviewer 1 Comments

 

Point 1: Despite the fact that TG systems are widely used, not all systems are equally suitable for carrying out this type of research as described in the publication. In this case, the samples should preferably have a round or rectangular cross-section with a face area of at least 1 cm2 and a thickness of no more than 1 mm. In the opposite case, the influence of sample edges on mass changes is unacceptably high. This was shown for the first time in works carried out in the 60s and 70s of the last century.

 

Response 1: According to our studies on the oxidation of RHEAs, the effect of edge can be mitigated to an acceptable level by preparing samples without edge defect. The experiment is commonly used in the study of the oxidation behaviors of RHEAs. Some of the literature is as follows：

1. Waseem et al. have studied the oxidation behavior of Al₅Cr₂₅Nb₂₅Ti₂₅Zr_12.5, Al₁₂Cr₂₄-Mo₄Nb₂₄Ti₂₄Zr₁₂, and Al_11.5Cr₂₃Mo₈Nb₂₃-Ti₂₃Zr_11.5 RHEAs by TGA at 1000 °C (with dimensions of 5 mm × 5 mm × 5 mm). [O.A. Waseem et al. Journal of alloys and compounds 828 (2020) 154427]
2. C. Lo et al. have studied the oxidation behaviors of Cr-17.6Al-20.3Mo-15.2Nb-2.9Si-13.4Ta-5.4Ti at 1200, 1300 and 1400°C by using TAG (with dimensions of 6 × 3 × 2 mm³). [K.C. Lo et al. Intermetallics 119 (2020) 106711]
3. Müller et al. also have studied the oxidation mechanism of a series of RHEAs: TaMoCrTiAl, NbMoCrTiAl, NbMoCrAl and TaMoCrAl at 1000 °C by TGA (with dimensions of 5 mm × 5 mm × 2 mm). [F. Müller, et al. Corrosion Science 159 (2019) 108161]
4. Gorr et al. have studied the oxidation behavior of W–Mo–Cr–Ti–Al, Nb–Mo–Cr–Ti–Al and Ta–Mo–Cr–Ti–Al at 1000 and 1100°C by TGA (with dimensions of 6 mm × 6 mm × 2 mm). [B. Gorr et al. Oxidation of metals 88 (2017) 339-349]

 

Point 2: The problem of choosing the right procedure for starting the oxidation process was also the subject of extensive research in the middle of the last century. It has been shown that it is best to insert the sample into the reaction chamber after the desired reaction temperature has been obtained or to heat it very quickly so that it can reach the reaction temperature in about 1 minute. Of course, not all TG devices are capable of realizing this procedure and therefore should not be used for this type of experiment.

 

Response 2: In the last reply, we admit that the initial heating process can’t be avoided in the experiments using TG. However, the program of heating process is fixed, which ensures the uniform isothermal oxidation experiments and makes the comparative study reliable. The problem proposed by the reviewer exists in numerous studies on the oxidation behavior of alloys using TG, and was not questioned in these published papers. In addition, the oxidation experiments using muffle furnace may cause the problem of spalling and cracking of scales, which has the serious and uncontrollable impact on the oxidation behavior. So, although the method of TG has the disadvantage, it is reliable in the study on the oxidation behavior of RHEAs.

 

Point 3: Generally, the following sentence is not correct "The descending mass gain is due to the ending oxidation increase and the volatilization of some oxides. This state is suggested to be completely oxidized..."., because the total mass loss denotes only that the rate of volatilization is greater than sample mass gains, as a result of scale thickening. Even if the mass decrease occurs, the remains of the metallic core can be present in the sample. Thus, the best proof of the completely oxidation state is the lack of a metal core in the cross-section of investigated sample and such a photo should be presented in the paper.

 

Response 3: In our study, the samples were completely oxidized due to the cracking. It can be seen in Fig. 1 (the photos of oxidized samples) that the core of samples is also oxidized.

 

 

Fig. 1 The photos of samples oxidized at TGA system (a, d) WMoTaNb, (b, e) WMoTaNbTi and (c, f) WMoTaNbV

 

Point 6: When a sample is completely oxidized, its mass should decrease according to the linear rate law. Consequently, parameters of such a dependence can be determined from the presented data.

 

Response 4: When the sample is completely oxidized, its mass change should come from the volatilization of some oxides, such as V2O5, MoO3 and WO3. The volatilization rate of these oxides depends to their distribution. At the initial stage, the volatilization rate of oxides on the surface is high. At the further stage, the volatile oxides on the surface become less and the volatilization rate of oxides in the internal scales are much lower. The same phenomenon is found in the published research (seen in Fig. 2). [F. Müller, et al. Corrosion Science 159 (2019) 108161].

 

Fig. 2 Specific mass change as a function of time for TaMoCrTiAl and TaMoCrAl (a.) and NbMoCrTiAl and NbMoCrAl (b.) during isothermal exposure to air at 900-1100 °C

 

Point 7: I understand the authors' explanations, which does not change the fact that the results obtained in this work are not surprising and do not deliver new essential knowledge. Therefore, I would suggest to add information to the paper that the potential application of the investigated alloys at high temperatures will require the use of protective coatings, and such studies are already planned or in progress.

 

Response 5: Thank you very much for the reviewer’s professional advice. The comparison of K values based on the different n values is incorrect. We have deleted the corresponding discussion. And according to the advice of reviewer, we have selected the slopes of initial linear stage to explain the mass gain of forming passivating oxide layer and obtained the same conclusion.

 

Point 6: Determining the vaporization rate of scales would be valuable for interpreting the oxidation behavior of alloys.

 

Response 6: There is the volatile oxidation in the oxidation process of refractory alloys containing Mo, V and W, which leads to the power law oxidation kinetics rather than parabolic law. Determining the vaporization rate of scales is valuable for interpreting the oxidation behavior of alloys. However, the accurate collection of volatiles is difficult and corresponding study is rarely reported. Thus, the researches on the oxidation of refractory alloys are depended on the mass gain curves, phases and microstructure of scales.

 

Point 7: It is well known that oxidation resistance of refractory metals is poor. Thus, the results obtained in this work are not surprising and do not deliver new essential knowledge.

 

Response 7: The results obtain in this paper indicate that the oxidation resistance of WMoTaNb, WMoTaNbTi and WMoTaNbV is poor at high temperature. In our current works, the effect of Si and Y addition aiming at the improvement of oxidation resistance of WMoTaNb RHEA is studied. In addition, preparing micro-arc oxidation (MAO) and plasma spraying coatings on the surface of WMoTaNb RHEA to improve the oxidation resistance will also be investigated in our future works.

Author Response File: Author Response.pdf

Reviewer 2 Report

The quality of the revised paper has been improved, but it is still not suitable for publication. Further improvement is needed:

 

1. The equilibrium vapor pressure of W should be lower than those of Ti and V, which means lower evaporation of W compared to other elements.

 

2. Points 3 and 4 of the first review:

Point 3: What do the authors mean with complete oxidation? Does it mean that the samples reach a passivation state and are protected against further oxidation, or it means that the samples are already completely oxidized? If it is passivation state, so it seems that WMoTaNbV rapidly reaches the passivation state and exhibits better oxidation resistance (lower mass gain compared to WMoTaNbTi). Please clarify this in page 3.

Authors Response 3: Regarding the suggestion about explaining complete oxidation, we think it is really a very good advice. We suggest that the sample is completely oxidized, when the mass gain keeps going down after the prolonged oxidation. The descending mass gain is due to the ending oxidation increase and the volatilization of some oxides. This state is suggested to be completely oxidized, and has been reported in the reference (F. Müller, et al. Corrosion Science 159 (2019) 108161). We have added this determination into the revised manuscript.

 

 Point 4: It is stated that “It can be found from the fitted equations that the oxidation coefficient K decreases with Ti addition and increases with V addition, which indicates that the mass gain of forming passivating oxide layer is reduced by Ti addition and increased by V addition”. However, from Fig. 2 it is clear that WMoTaNbTi shows the maximum weight gain. Why?

Response 4: It can be seen in Fig. 2 that Ti-containing RHEA has the minimum mass gain at initial linear stage, which is the mass gain of forming passivating oxide layer. Meanwhile, V-containing RHEA needs the maximum weight gain to form passivating oxide layer. These conclusions can be obtained from the fitted equations. The mass gain of RHEAs at the further stage is due to the growth rate of the passivating scale, which is related to the oxidation exponent (n). The maximum final weight gain of WMoTaNbTi is due to the higher n value, indicating the faster growth rate of the passivating scale. It is in agreement with the thickness of scales in Fig. 4f. The diffusion rate of oxygen in the passivating scale of WMoTaNbTi is higher that that of WMoTaNb, which leads to the disastrous internal oxidation.

 

Reviewer reply:

If all the samples are completely oxidized, then there is no passivation. Passivation means the formation of a protective layer which hinder further oxidation of the sample and provides protection. However, here no protective layer is formed and all the samples are completely oxidized. So, first, passivation should be removed from the manuscript and the discussion need to be accordingly rewritten by selecting better terminology.

 

3. Point 6: The thickness of oxide layer in WMoTaNbV is almost 8 times higher than that in WMoTaNbTi RHEA. This should be reasonably translated to a high mass gain and low oxidation resistance. However, Fig.2 shows lower mass gain for WMoTaNbV compared to WMoTaNbTi. The authors should repeat this test (oxidation kinetics) to assure the accuracy of their data.

Response 6: The final mass gain of WMoTaNbV is less than expectations from the thickness of oxide layer, which is due to the volatile oxidation of V, Mo and W. V addition aggravates the volatility of V2O5, MoO3 and WO3, and leads to the disastrous internal oxidation, which results in the higher thickness of oxide layer.

Reviewer reply:

The reason for lower mass gain of WMoTaNbV compared to other samples (oxidation of volatile V, Mo and W oxides) need to be added to the manuscript.

 

4. Based on the above comments, the discussion need to be improved. Some examples are as follow:

• This sentences should be clarified ” The total mass gain of WMoTaNbTi RHEA after complete oxidation is higher than those of other RHEAs, which should be due to the reduction of elements forming volatile oxides.” What do the authors mean with “reduction of elements forming volatile oxides”. Is it chemical reduction? How?
• It is stated that “It is obvious that the PBR value of TiO2 is significantly less than those of other oxides, which can effectively relief the interstress of scales.” As the authors suggested, a smaller PBR typically results in protection against oxidation and superior oxidation resistance. However, this is not consistent with the inferior oxidation resistance of WMoTaNbTi compared to WMoTaNb.
• This is not clear: “The descending mass gain is due to the ending oxidation increase and…”. Generally, the English need to be improved.
• Reference [17] does not discuss the oxidation resistance of HEAs.

Author Response

Response to Reviewer 2 Comments

Point 1: The equilibrium vapor pressure of W should be lower than those of Ti and V, which means lower evaporation of W compared to other elements.

 

Response 1: Thank you very much for the reviewer’s kindly remind. The equilibrium vapor pressure of W is lower than those of Ti and V, leading to the lower evaporation of W compared to other elements. We have corrected the mistake in the revised manuscript.

 

Point 2: If all the samples are completely oxidized, then there is no passivation. Passivation means the formation of a protective layer which hinder further oxidation of the sample and provides protection. However, here no protective layer is formed and all the samples are completely oxidized. So, first, passivation should be removed from the manuscript and the discussion need to be accordingly rewritten by selecting better terminology.

 

Response 2: It is known that passivation is a process of forming oxide layer, which reduces the oxidation rate of alloy. And the growth rates of passivating layers in different alloys are different, due to the different diffusion rates of oxygen in these layers. It can also be reflected in the oxidation exponent (n) of power-law oxidation kinetic. The less value of n indicates the slower growth rate and more effective protective layer. when n = 1, the power law equates the linear law behavior; when n = 0.5, it complies with the parabolic law oxidation; when n =1/3, it conforms to the cubic law. However, the passivating layer has the failure time. In our work, the passivating layer will lose efficacy after a certain time of oxidation, owing to the cracking caused by the unbearable internal stress. Although Ti addition results in the serious internal oxidation, it prolongs the passivation duration of alloys. Because the PBR value of TiO2 is significantly less than those of other oxides, which can effectively relief the interstress of scales.

 

Point 3: The reason for lower mass gain of WMoTaNbV compared to other samples (oxidation of volatile V, Mo and W oxides) need to be added to the manuscript.

 

Response 3: Thank you very much for the reviewer’s professional advice. We have added this part in the revised manuscript.

 

Point 4: Based on the above comments, the discussion need to be improved. Some examples are as follow:

This sentences should be clarified ” The total mass gain of WMoTaNbTi RHEA after complete oxidation is higher than those of other RHEAs, which should be due to the reduction of elements forming volatile oxides.” What do the authors mean with “reduction of elements forming volatile oxides”. Is it chemical reduction? How?

 

Response: We are sorry for the misleading expression and have modified in the revised manuscript. The less volatile oxides attributes to the less W and Mo content, which is caused by the Ti addition.

 

It is stated that “It is obvious that the PBR value of TiO₂ is significantly less than those of other oxides, which can effectively relief the interstress of scales.” As the authors suggested, a smaller PBR typically results in protection against oxidation and superior oxidation resistance. However, this is not consistent with the inferior oxidation resistance of WMoTaNbTi compared to WMoTaNb.

 

Response: The smaller value of PBR will lower the interstress of scales and delay the time of cracking, which determines the effective time of passivating layer. However, the oxidation resistance depends on both the growth rate and the effective time of passivating layer. The addition of Ti increased the diffusion rate of oxygen in the scale, resulting in the serious internal oxidation and more mass gain.

 

This is not clear: “The descending mass gain is due to the ending oxidation increase and…”. Generally, the English need to be improved.

 

Response: Thank you very much for the reviewer’s kindly remind. We have improved the expression in the revised manuscript.

 

Reference [17] does not discuss the oxidation resistance of HEAs.

 

Response: Thank you very much for the reviewer’s kindly remind. We have corrected the mistake in the revised manuscript.

Author Response File: Author Response.pdf

Reviewer 3 Report

In my opinion, the authors have done serious work to improve the text of the paper and in this form (however, after final editing!) it can be recommended for publication.

Author Response

Thank you for your careful review on our paper.

Round 3

Reviewer 2 Report

The authors provided acceptable explanations for my comments, and the quality of the manuscript is improved. I recommend publication after minor revision:

1- "In our current works, the effect of Si and Y addition...." should be changed to the"In our current works, the effect of V and Ti addition...."

1- "should attribute to the oxidation of volatile V, Mo and W oxides." Should be changed to the "should attribute to the evaporation of volatile V, Mo and W oxides."

 

Author Response

Response to Reviewer 2 Comments

 

Point 1: The authors provided acceptable explanations for my comments, and the quality of the manuscript is improved. I recommend publication after minor revision:

• "In our current works, the effect of Si and Y addition...." should be changed to the"In our current works, the effect of V and Ti addition...."

Response: Thank you very much for the reviewer’s kindly remind. We are sorry for the misleading expression. This part is suggested by another reviewer and introduces our studies already planned or in progress to improve the oxidation resistance of WMoTaNb RHEAs. We have modified this sentence in the revised manuscript

1- "should attribute to the oxidation of volatile V, Mo and W oxides." Should be changed to the "should attribute to the evaporation of volatile V, Mo and W oxides."

Response: Thank you very much for the reviewer’s professional advice. We have improved the expression in the revised manuscript.

Author Response File: Author Response.pdf