Effect of Aluminum Ion Irradiation on Chemical and Phase Composition of Surface Layers of Rolled AISI 321 Stainless Steel

Round 1

Reviewer 1 Report

This manuscript investigated surface modification of a cold worked 321 austenitic stainless steel using low energy Al ion implantation. The quality and significance of the study is low. Austenitic stainless steels in either as-casted state or cold-worked state were already extensively studied under irradiation or ion implantation. It is not clear what is the new findings from the current work. In the conclusion session, the authors listed five bullet points but none of them is novel. In addition, there are so many grammatical errors in the text, and the figures were not nicely prepared making the entire manuscript very difficult to interpret. The experimental methods also need significant improvement. For example, the authors should use stopping and range of ions in matter (SRIM) program to predict the Al ion range in the materials and compare with their experimental results. Microscopy characterization at a higher spatial resolution (such as scanning electron microscopy) is highly recommended to resolve a cross-sectional layer with about 1 um thickness. 

Author Response

Please see the attached file

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors discussed the interesting phenomenon of phase change in stainless steel after irradiation.

1) I am afraid the English level of the current manuscript was not satisfying enough, and the paper needs more proof-reading as well. Just some examples:

• Abstract Line 18: What is A₂O₃? I guess it is Al₂O₃?
• Abstract Line 18: What is 30 mn? I guess it is 30 nm?
• Line 86: “investigation discussed about” may be “investigation discussed above”
• Line 104: “Al+”, the “+” needs to be superscripted
• Line 145: “which depth” should be “whose depth”
• Line 202: “Ar-implantation” may be “Al-implantation”
• Line 208: “obtain ed” should be “obtained” (no space)
• etc.

2) Some parts of the figures did not follow the rule of academic writing, for example:

• Figure 2: Since the figure is named as “Depth distribution”, the sentence “1 min tentative corresponds to 1 nm of depth” in line 174 needs to be added into the title of the figure. Otherwise the figure itself could be confusing.
• Figure 3 and 4: unit of “degree” is lost in the x axis.
• Figure 5: the meaning of “V%” in the y axis was not explained.
• Figure 6: The location of surface should be marked by a line. And why the magnification is different in figure b? Why the direction of the specimen are all different in a), b) and c)? It bring difficulty for comparison.

Due to the above reason, I am afraid it was quite difficult for me to comprehensively understand the story of the manuscript. I try to give some comments here:

3) Line 86-94: what is the objective of this study? The authors discussed many papers between line 48-85, but I think their findings are not directly related to the topic of this manuscript. The authors point outed that the deformation during specimen preparation may cause phase transformation. If this is the objective of the study, the following contents are desired:

• Was similar phase transformation previously observed in literature?
• What is the definition of “double-phase surface” (Line 94)? How do you tune the parameters to prepare a “double-phase surface”?
• Can thermal annealing after cold rolling cancel out the phase transformation? Thermal annealing is a common practice after cold rolling.
• Can electro-chemical polishing after mechanical polishing cancel out the phase transformation? Usually, electro-chemical polishing is  strongly recommended before irradiation experiments.
• Why the experiment is designed in this way? I mean, usually, a control group without deformation before irradiation is desired (maybe prepared by long-time electro-chemical polishing) to support your conclusion “presence of α phase in the initial structure can promote the intensive γ -> α transformation at ion irradiation.” You need to add some explanation to support your experiment design.

4) When discussing the depth of irradiation damage and implantation, it is recommended to run the SRIM code (http://www.srim.org/) to calculate the depth profile of irradiation damage and implanted ions.

5) As mentioned in figure 2, the peak of the Al concentration is ~20 nm. Then by SRIM code, the irradiation damage peak is usually less than 20 nm. The irradiation temperature is less than 50 degree C, so point defect diffusion was very limited. Then here are two questions:

• Why Vickers hardness test was selected? The deformation zone of Vickers hardness test is usually at the scale of mm (or even more), whereas your irradiation damage layer is ~20 nm thick. I suppose Vickers hardness cannot properly reflect the hardness change caused by irradiation. Nano-indentation may be a better choice (deformation zone at the scale of 10-1000 nm).
• Why the modified layer had a large thickness of ~ 1 um? (Line 251). Some scientific discussion is desired here.

6) Line 179: Why the phenomenon is associated with irradiation-induced segregation? To my understanding, irradiation-induced segregation means the segregation toward or away from “sinks” (e.g. surface, grain boundary, dislocation loops, void surface) is enhanced under irradiation. What is the sink here? And I think irradiation-induced segregation is usually not obvious at room temperature (T<50 degree C).

Author Response

Please see the attached file

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors investigated the effects of Al-ion irradiation on the increase in alpha phase near the surface of samples made of AISI 321 stainless steel. The component of article is well ordered, and the methods and results look appropriate. However, as a journal article, quantity of study is not enough. For example, parametric studies for conditions of unirradiated samples, such as percentage of initial alpha phase or chemical compositions of the material, will give some new aspects of your target phenomenon. Further studies are expected.

Author Response

Please see the attached file

Author Response File: Author Response.pdf

Reviewer 4 Report

The paper investigates the effects of Al irradiation on the surface properties of a cold rolled austenitic stainless steel (321).

The studied effect is interesting in the sense that it gives some depth of reasoning into what microstructural changes can be imposed on the material with a fast method such irradiation.

However, in the paper the methodology and the results that are presented are much lower in quality than the standard expectations. This inevitably results in the fact that the conclusions cannot be fully supported by the presented results. Below are the concerns of the reviewer that clarifies this point:

• The methods that are used are not adequately described. For instance the hardness measurement is talked about and the name of the equipment given however it is not understood how a micro-hardness test has been successfully used to observe effects happening in depths less than 1 micrometer.
• In addition the results of the microhardness tests, supposedly 10 per measurement point, before and after, are not presented. Why? Only vague values of 1.5 and 2.5 GPa are mentioned without any supporting evidence. 
• The measurement of the phase transformation to martensite has been carried out by XRD. In itself this method has difficulties resolving the amount of martensite quantitatively. Why not use micrography (optical/SEM) to determine this amount reliably and quantitatively. The procedure of conversion of XRD intensity results to actual martensite percentage is not given (and not trivial) and therefore not conclusive.
• The compounds that are found by XPS are only mentioned in text without any supporting data. Without seeing the actual data it is not possible to reproduce any of the conclusions that the researchers claim.
• The micrographs that are presented are of very low quality and do not help in any way support the researchers claims. Due to low quality the reviewer is not convinced that what is seen in fig 6.c is the modified layer which should be in the order of 50 nanometers which seems to be far less than the optical resolution of the equipment used. In figure 6b it is not clear what caused the difference close to the surface in the microstructure. It seems to be in the order of 40 microns which deviates from any mechanism that is mentioned in the paper related to irradiation.

Other remarks:

- In the abstract change mn -> nm

- lines 48,62 (and any other occurrence) remove "paper"

- last paragraph of introduction, what is "martensitic mechanism?". If it si relevant please explain.

- "alpha" is usually used to denote the ferrite phase, whereas "alpha prime" is martensite. It is also not clear in the paper to an unexperienced reader that alpha (bcc) refers to martensite.

- line 142: "sin" is not understood.

- typo in line 227.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The reviewer is not convinced by the authors' response. Although the authors tried to clarify part of the work, there are still several major flaws. The reviewer appreciates the SRIM calculation, but the authors didn’t present any dose vs. depth and ion range vs. depth plots in the revised manuscript. In addition, microhardness is not the best method to probe the hardness of a 54.4 nm irradiated layer. The authors mentioned there is a hardness increment without showing any hardness values. The results are not convincing at all. As the reviewer suggested in the previous report, microscopy at a higher spatial resolution is recommended to resolve a 1 um thick layer. At least, scanning electron microscopy coupled with electron dispersive X-ray analysis should be carried out on the irradiated sample from a cross-sectional view. Therefore, this work is still not recommended for publication.

Author Response

Please find the repley as attached file

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have revised following the comments

Author Response

Thank you for the positive desision

Reviewer 4 Report

• The results of the microhardness tests are still not presented. With only vague average values of 1.5 and 2.5 GPa, the reliability and reproducibility of these measurements are very questionable. Two of the three references that are cited for microhardness measurement do not give any supporting evidence in this manner as well.
• The XRD method that is cited is also developed by the authors and in the reference, there is also no convincing data to show the measurement accuracy but only talks about the algorithm. Without proven information on the accuracy of this method any conclusion derived from this measurement cannot be supported.
• The use of micrography (optical/SEM) is a much more common method due to its reliability especially in quantifying fcc vs bcc phases in the microstructure. 
• The micrographs shown are still the same ones with slightly higher resolution. It is the optical resolution that matters in this case which is clearly not sufficient in supporting any of the claims based on these images. Image in 6b definitely shows a different phase that is found up to a depth of 40 um which is much larger than the region of influence that the authors claim due to irradiation (< 2um).

Author Response

Please find the reply as attached file

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

The authors have addressed my concerns properly. I now recommend it for publication as its current version. 

Author Response

Thanks for your positive decision.

Reviewer 4 Report

The explanation of the procedure of XRD phase determination adds value to the presented results and also gives a measure of the error to the readers.

Please check the typing quality of the equations, there seems to be a resolution error.

The addition of nano-hardness results definitely has improved what the authors wanted to show, that is the effect of irradiation on the mechanical properties as a function of depth of penetration. 

The addition of SEM imaging is appreciated as it gives a more clear picture of what is 'not' happening. That is in this scale there is no difference before and after irradiation. I would not agree with the suggestion that the transition between the zones is smoother. With limited imaging it is not conclusive however it is ok that the authors may make this observation.

- In this regard, I would recommend consideration of removing the optical imagery since the quality of the images are quite low.

- Please check once again for language mistakes as I have seen a few while reading.

Author Response

Thanks for your positive comments. Please see the reply as attached file.

Author Response File: Author Response.pdf