Microstructural and Cavitation Erosion Behavior of the CuAlNi Shape Memory Alloy

Round 1

Reviewer 1 Report

In this work, the authors try to reveal the effect of microstructure of CuAlNi shape memory alloy on its cavitation erosion resistance. There exist some drawbacks and we hope that the author can perfect them before publication. The main problems are listed as follows:

1. “Continuous casting of the bar was carried out with a speed of 320 mm/min”. Does the speed of 320 mm/min refer to cooling rate or cast rate? If it refers to cast speed, we think that it is impossible to obtain a smaller crystal size of the cast one than that of the solution annealed one.
2. Why is there the sentence “The gap between the surface of the cold spray coating and the transformer probe was 0.5 mm.” in line 99? This work focused on the shape memory alloy rather than the cold spray coating!
3. Please give more experimental details, for instance, the testing solution, the test temperature, the cavitation erosion sample size, and so on.
4. In the “Results and discussion” section, it is not clear of Figures 1, 2, 4 and 5, and these figures must be re-obtained with high quality.
5. Error bars should be added in the mass loss plots.
6. Figure 7 is the AFM image but not the SEM EDS results. Also, Figures 8 and 9 are also confused.
7. The results of roughness indicated that the annealed SMA had a lower roughness after 420 min cavitation erosion compared with the cast one. Usually, a low roughness indicates a higher resistance to cavitation erosion implying that the former had better cavitation erosion than the latter, which was in contradiction with the results of mass loss. In other words, the reported results in this work were not self-consistent.
8. A deep discussion on the difference in microstructure after varied heat treatments and the effect of microstructure on the cavitation erosion resistance should be added in this work, or it is too shallow.
9. Some irregular formats should be corrected, such as double spaces in the article.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer, thank you very much for your valuable comments and invested time. Attached you can find PDF-file with our answers and new improvement according to your comments.

Author Response File: Author Response.pdf

Reviewer 2 Report

Paper investigates the cavitation erosion of Cu-based shape memory alloy. My comments on the paper:

1) The cited literature is old and out of date. On the 27 cited items only 4 pos. are younger / newer than 5 years. The weakness of this study is a carelessly conducted literature review, as such an approach by the Authors may indicate that the research topic discussed by them is out of date.
2) An unreasonable practice has been observed, erroneously / deceptively giving the impression of a large amount of literature cited. And so, for example, verse 80 in one sentence quoted as many as 14 items. literature - [12-25]. What is this behavior supposed to do?
3) In family 2.2. In the "Cavitation test", first of all the key / main parameters of the test should be mentioned. On the other hand, the reference in verse 98-99 to literature [18,19,23,24] is in this case incorrect, because in these positions there are some differences in the experiments carried out or insufficiently described. In this case, one should indicate specifically one literature item according to which the cavitation test procedure was adopted. In addition, there was no justification for the duration of the cavitation duration, i.e. why the cavitation test was terminated after 420 min.
4) The greatest omission is the failure to provide in the text information on the size of the sample / tests used, among others during cavitation tests or hardness measurements. Maybe because the presented data from Fig. 3 indicate that only one sample from two material groups was used for the tests. Were there repetitions? What is the credibility of the obtained results?
5) There is no description of eroded surface morphology in the research results (in the results section). Only slight mentions of optical microscopy and SEM were observed in the text, referring only to the reference area and after 420 min.
6) In short, the results of the cavitation tests are "bland" and very brief / poor. One could attempt a broader analysis - for example the determination of the erosion rate or CER (cavitation erosion resistance).
7) One of the biggest shortcomings of the work is the lack of scientific discussion of the results. Not to mention the fact that in Gen. The authors of "Results and discussion" cite only two items of literature 26 and 27.
8) There is a noticeable range of laboratory tools used to describe the issues / phenomena discussed. The study of the structure was limited to microscopic evaluation only (and only for ref samples and 420 mins), and a basic XRD analysis could be useful here. Cavitation issues were also narrowed down to a minimum (mass loss measurement). The scientific aspect was missing in the whole work - no discussion and a broader analysis of the results, and yet the title of the authors included the word - "behavior"

9) There are many up-to-date papers published recently in the MDPI-journal (Metals, Materials, Coatings) regarding to the "cavitation erosion topic" and I believe should be cited in this paper to presents the current research-trends. Please check the literature.

The object of the research is poorly investigated, hence the recommendation to reject.

Author Response

Dear Reviewer, thank you very much for your valuable comments and invested time. Attached you can find our PDF file with our answers and new improvement.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

1. The discussion on the difference in microstructure after varied heat treatments and the effect of microstructure on the cavitation erosion resistance needs to be improved although the authors have made revision. However, we think it is simple.
2. It seem not appropriate for the error bars with the same length.

Author Response

Dear Reviewer,

thank you very much for your invested time and valuables comments.

We are sending our answers in attached documents on 2 pages!

Author Response File: Author Response.pdf

Reviewer 2 Report

I am partly satisfied by your improvements. I suggest adding some description likewise mark the interesting features or explain the changes in surface development in the figures containing the OM and SEM images. Now, they are difficult to read.

The scale bar in figs. 4 and 5 are difficult to read. 

Also, the cobalt-based alloys and Co- and Ni-based cermets are up-to-date anti-cavitation research object. Therefore I suggest you refer in the introduction to the following papers:  
https://doi.org/10.1016/j.triboint.2021.106954
https://doi.org/10.3390/ma14092324
https://doi.org/:10.3390/coatings8090307

The discussion section could be strengthened by comparing the obtained results with the literature data given for other materials, tested in the same conditions.

The paper contains typos and should be formatted according to the journal requirements - I hope it will be done during the next review round. 

Author Response

Dear Reviewer,

thank you very much for your valuable comments and invested time.

Attached I am sending our answers in Word file on 3 pages.

Author Response File: Author Response.pdf