Effect of Dislocation Slip Mechanism under the Control of Oxygen Concentration in Alpha-Case on Strength and Ductility of TC4 Alloy

Round 1

Reviewer 1 Report

The authors present a seminal study on the influence of an oxide scale on the microstructure and mechanical properties of titanium TC4 alloy. They study very systematically the formation of the alpha-case under various heat treatment conditions and its influence on the mechanical properties. The authors discover a continous trend in embrittlement of the material, but a maximum in its strength. The microscopic mechanisms leading to this behavior are investigated and discussed in detail, such that the paper is expected to have a significant impact within the community.

The language of the paper is generally fine, but there is a relatively large number of grammar and spelling mistakes such that a thorough proof reading is strongly recommended.

Two small recommendations regarding the content of the paper:

(i) The authors should consider plotting the microhardness of the NPO sample in Fig. 5, to indicate whether this agrees with the microhardness at larger depths of the PO samples.

(ii) In Fig. 10 (c) and (d) it would support the orientation of the readership if the direction towards the surface was indicated.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In the manuscript, the authors report on the oxidation behavior of the TC4 alloy. The manuscript contains some interesting results, that may deserve the publication in metals after major revision. I recommend the publication of the manuscript after major revision. In the following I provide a list of my major points:

1. The authors do not explain their intention behind the central study of varying the pre-oxidation time. What is the hypotheses? What is the expected result of this study?
2. The hardness profiles shown in Figure 5 do not provide significantly different oxygen diffusion layer thicknesses for the different pre-oxidation treatment. Unfortunately, this seams to result in samples that are not really significantly different. This is not a good basis for the subsequent analysis of the strength of the overall material via tensile tests.
3. Therefore, the significance of the “peak-strength”, as shown in Figure 6, can be questioned. If the peak-strength is considered (or even proved?) to be significant, then it would be interesting to provide some hypotheses for a possible mechanistic origin of this peak in the strength.
4. In Figure 8 right panel: It would be more valuable to plot the three color-plots are with the same color range.
5. Line 25, line 345 and fig. 10c: It is not clear what is meant by “typical wave slip”. It is really hard to see something characteristic in the figure, please make it more clear. On which scientific work is the term of wave slip based?
6. Line 364: “... coherent fine precipitate α2.” What in figure 10 is regarded to be a fine coherent precipitate? I do not see a possible candidate.
7. Line 365: “ ... materials lead to the existence of antiphase domains (APD) and their related antiphase boundaries (APB) What in figure 10 is regarded to be an antiphase domain or boundary? I do not see possible candidates.

Minor orthographic issues:

1. line 333: ... obtained via by TEM

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This paper presents a study of the effect of oxygen concentration in alpha-case on the strength and ductility of TC4 alloy. Alpha-case samples with different concentrations were obtained by controlling oxygen diffusion process parameters, and strength and ductility were investigated. The results seem interesting. However, the manuscript is not well-organized and contains many mistakes or insufficient explanation of words, which makes it hard to read. Overall, the reviewer would suggest extensive revision.

The authors should clarify/correct the points listed below:

 

1. Notations should be unified through the manuscript:

TC4 alloy, Ti-6Al-4V alloy, or TC4 titanium alloy,

Original material, initial material, or NPO sample,

The numbering of the oxygen diffusion steps: (i, ii, iii), (1, 2, 3), or (I, II, III)

Oxygen diffusion sample or oxidized sample

The thickness of the alpha-case or alpha thickness

 

2. In line 127, “holding pressure of 10s” should be corrected to “holding period of 10s”.

 

3. In line 137, “X-ray diffraction spectrum” should be abbreviated and corrected to “XRD patterns”.

 

4. In line 174, “GDS” should be spelled out.

 

5. In lines 179-180, the explanation of normalization procedure seems not consistent with the description in lines 180-184.

 

6. In Figure 6(b), correspondence with the right or left y-axes should be clearly presented using arrows.

 

7. In the part of “3.4 Fracture behavior of TC4 alloy”, how the regions A and B, i.e. the crack initiation region and the crack propagation region can be distinguished is not clearly shown. Higher resolution images of the corresponding regions would be helpful.

 

8. The first paragraph of Discussion part, “4.1 Formation of alpha-case under different oxygen diffusion processes” is just a repeat of the previously mentioned results. Furthermore, the other contents of the “Discussion” part should be written in the “Results” part. Then, the reviewer suggests the authors organize these parts into a “Results and Discussion” part.

 

9. In Figure 8, monochromatic figures, which are not mentioned in the manuscript nor in the caption, are not necessary.

 

10. In lines 332-333, “FIB is employed to cut and deform the alpha-case.” should be corrected to “FIB is employed to cut the alpha-case.”

 

11. For the results of TEM observations, the geometry of slip direction and tensile load direction should be clearly presented. Otherwise, it is not obvious whether the observed dislocations are along the slip planes or not. In addition, in Figure 10(d), dislocation lines, which should be detected as dark lines, are not clearly observed. In order to characterize the Burgers vector of dislocations, dark-field TEM observations with two-beam conditions should be employed.

 

12. Figures 10(e) and (f) do not seem to be TEM images but they seem to be HAADF STEM images.

 

13. In lines 393-394, “the X-axis and the Y-axis” and “the Z-axis” should be a-axis and c-axis, respectively.

 

14. Figures12(c) and (d) are not FFT images but IFFT images obtained from masked FFT patterns. The authors should explain it clearly.

 

15. HRTEM images in Figures 12(a) and (b) do not have sufficient quality to judge the existence of dislocations. In addition, even if the dislocations exist, they must have different dislocation line direction from that observed in Figures 10(c) and (d): dislocations in Figure 10 are parallel to the TEM specimen, while those in Figure 12 are perpendicular to the TEM specimen. Overall, the results of TEM observations fail to convince the reader of the authors’ interpretation. Thus, the reviewer suggest the authors to exclude the part based on the results of TEM observations.

 

16. There are a number of grammatical and typographical errors throughout the manuscript: “only stable rutile can be was detected” in line 143, “diffraction peak 57.13b °” in line 144, “… the ample may have intergranular and cleavage transgranular fractures may have occurred.” in lines 245-246, “This is results in surface crack initiations.” in lines 292-293, “observed the crack propagation path is observed by SEM.” In lines 308-309, “In Figure 10a and b, show…” in line 334, “the plane slip has shown adverse effects effect on plasticity.” In lines 347-348, etc...

The manuscript would be improved by a thorough English language review.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

To my opinion, the revised manuscript is not sufficiently improved to be suited for the publication in metals. Sadly the authors did not take the chance to improve their manuscript based on my comments. Maybe, this is simply a misunderstanding. I still think, that the manuscript contains some interesting results, that may deserve the publication in metals after major revision. See below the list of my previous distraction points, followed with my reasoned opinion on the reaction of the authors. I can only recommend publication after successful major revision.

1. The authors do not explain their intention behind the central study of varying the pre-oxidation time. What is the hypotheses? What is the expected result of this study? Reaction from the authors: The authors explain their interesting intention to me: “The purpose of changing the pre-oxidation time is to form an oxide film with different oxygen content on the surface of the TC4 alloy”. To my opinion, the authors also provide an interesting working hypotheses to me: “As the pre-oxidation time increases, the oxygen content of the oxide film formed on the surface of the TC4 alloy increases.” However, the manuscript is left unchanged in this regard. My opinion on this reaction: inadequate!
2. The hardness profiles shown in Figure 5 do not provide significantly different oxygen diffusion layer thicknesses for the different pre-oxidation treatment. Unfortunately, this seams to result in samples that are not really significantly different. This is not a good basis for the subsequent analysis of the strength of the overall material via tensile tests. Reaction from the authors: Again the authors provide a lot of interesting information on their intention behind the study to me: It seams as they intended to produce samples with equally sized diffusion layers that just differ by their oxygen concentration. By the why, it would be interesting to read in the manuscript why. Accepting that, then in order to judge the significance of the difference between the samples, we have to have a look at Figure 4b instead of Figure 5. Unfortunately, Figure 4b incomplete in this regard, as the oxygen concentration of PO20 and PO40 are missing. Thus, based on this, only PO10 and PO60 could be considered as different. The manuscript has not been improved during the revision in this regard. My opinion on this reaction: inadequate!
3. Therefore, the significance of the “peak-strength”, as shown in Figure 6, can be questioned. If the peak-strength is considered (or even proved?) to be significant, then it would be interesting to provide some hypotheses for a possible mechanistic origin of this peak in the strength. Reaction from the authors: The authors explain to me, that they “believe that the strength and ductility of different samples are comparable” My opinion on this reaction: inadequate!
4. In Figure 8 right panel: It would be more valuable to plot the three color-plots are with the same color range. Reaction from the authors: The authors kindly explain to me that have their own opinion on this issue. My opinion on this reaction: acceptable!
5. Line 25, line 345 and fig. 10c: It is not clear what is meant by “typical wave slip”. It is really hard to see something characteristic in the figure, please make it more clear. On which scientific work is the term of wave slip based? Reaction from the authors: The authors explain to me, that wave slip is a “quoted name“, and some further words about this term. However, the manuscript is left unchanged in this regard. My opinion on this reaction: inadequate!
6. Line 364: “... coherent fine precipitate α2.” What in figure 10 is regarded to be a fine coherent precipitate? I do not see a possible candidate. Reaction from the authors: The statement, I previously stepped over, has been reformulated in the revised manuscript. However, now it reads as if the authors think that they have observed some “fine particles with diameter below 10 nm”. This sounds questionable to me. Beside this, again no citation on similar observations of other researchers is provided, although it is stated that this shall be a “typical feature”. Also negative is that the changes, that have been made in the revised manuscript due to the reviewers comments, seem to be not highlighted. My opinion on this reaction: inadequate!
7. Line 365: “ ... materials lead to the existence of antiphase domains (APD) and their related antiphase boundaries (APB) What in figure 10 is regarded to be an antiphase domain or boundary? I do not see possible candidates. Reaction from the authors: The authors explain to me, what an antiphase domains or boundaries is, which was not the question. The manuscript has not been improved during the revision in this regard. My opinion on this reaction: inadequate!

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comprehensively considering the authors’ replies to the reviewer’s comments, the reviewer does not recommend publication of this study.

 

The reviewer is content with the authors’ reply concerning the comments 1-5, 10, 13-17.

 

The authors reply to the comments 6-9 are acceptable, although the reviewer’s suggestions were not reflected in the revised manuscript.

 

As for the authors’ reply concerning the comment 11, the results of TEM observations lack the desired quality and are not enough to convince the reader of the dislocation morphology. The reviewer does not agree that TEM images in the manuscript are enough to reflect the point the authors want to express.

 

As for the authors’ reply concerning the comment 12, the authors insist Figures 10 (e) and (f) are obtained in the HAADF mode of TEM. However, HAADF is one of the mode in STEM and “HAADF mode of TEM” is impossible.

Author Response

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Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

The authors answered to all my questions and comments. The manuscript has been improved and I recommend the paper for publication.