Effects of Second-Phase Particles on Microstructure Evolution in Mg-2Zn Based Magnesium Alloys during Annealing Treatment

Round 1

Reviewer 1 Report

The paper “Effects of second phase particles on microstructure evolution in magnesium alloys during annealing treatment” describes the static recrystallization behavior of two magnesium alloys (Mg-2Zn and Mg-2Zn-1Al-0.2Ca-0.2Gd-0.2Mn). The authors analyzed the hardness decreasing and structure evolution during the annealing after the hot deformation. The paper is lack of the scientific novelty because the processes of the particle stimulated nucleation and limited by the fine particles of the grain growth in magnesium alloys were investigated a lot of times previously (https://doi.org/10.1016/j.actamat.2009.02.032, https://doi.org/10.1016/j.scriptamat.2017.02.039, etc.). The paper cannot be accepted for publication accordingly following reasons also:

1. It is known that the first process during annealing is recovery. During the recovery, the dislocation density may significantly decrease without the appearance of the new recrystallized grains. The authors found two-stage static recrystallization. It is not correct. Initial decreasing in hardness in Fig. 1a is due to static recovery processing. The time is needed to form the new grains' nuclei. The acceleration of the hardness decreasing on the curves is the beginning of the static recrystallization. So, the fractional softening in Vickers hardness, which authors determined by Eq. (2) cannot be used as a volume fraction of the recrystallized grains. That’s why Avrami exponents in Table 2 has the wrong values.

2. The resolution of the EBSD maps is too low to analyze the initial stages of the recrystallization. It is hard to obtain useful information from Fig. 6a. The step-size during the EBSD scanning has the same size as the nuclei size.

3.  

   Some minor corrections also should be made in the manuscript:

   - The authors describe in the manuscript that three alloys were investigated. However, only two different compositions of alloys were prepared. The second alloy was investigated in two different states. So, the text of the manuscript should be changed.

   - The Materials and experimental procedure part is very pure. Authors should provide in details the procedures of the hot deformation (temperature, number of passes, etc.) and XRD and EBSD scanning (step-size, exposition time, etc.)

   - The error bars should be added to the hardness plots. It is hard to determine the significance of any changes in the curves.

   - The errors should be added to the grain size values.

Author Response

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Reviewer 2 Report

This work is interesting and can be accepted after revision for some important points indicated as follows: The authors need to answer to these comments point by point.

1. What is the starting point of this work; I mean from where you start your point of research. This should be clear in your manuscript.
2. Where is the relation between this work and other previous works of the same subject? I mean from where they end where you start.
3. The main objective of this work should be more clear with the most applicable materials and engineering application used by this work.
4. The methodology should be revised carefully with more details to the readers to easily follow up on this interesting work.

Author Response

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Reviewer 3 Report

Dear authors,

The manuscript is generally well written, but there are some major problems.

However, from my point of view I miss the novel or innovative character of this work and the explanation why Mg-2Zn was compared to the Mg-2Zn alloy with 4 additional alloy additions or why only one alloy was TRC.

There is no information about which alloys or which alloy system has been investigated; this should be added either in the title or at least in the abstract.

It is not possible for me to interpret the microstructure results. My first impression was that the microstructures were dehydrated corrosion layers. The microstriutures are strongly over etched and show different results in the investigations with EBSD (e.g. Fig. 5 (a)5 s IC-Z2 alloy ~3 grains over 25 µm) and SEM in BSE mode (Fig. 4 (a) 5 s IC-Z2 alloy ~10 grains over 50 µm) for the grain size at the same condition.

I recommend to improve the preparation technique and for the grain size itself the light microscope will give a better impression of the grain structure. A good etching solution is therefore published in “Kree, V., Bohlen, J., Letzig, D., Kainer, K.U.;The metallographical examination of magnesium alloys; (2004) Praktische Metallographie/Practical Metallography, 41 (5), pp. 233-246.”.

There is a major problem with the preparation of the microstructure, so I will not go into the results and the discussion. An interpretation of invalid results is in my opinion a waste of time, so I would only continue here after an improvement in the form of a better examination of the microstructure.

In some places it is written that an annealing treatment is performed, e.g.  “to eliminate internal stress” or “recovery and static recrystallization (SRX)”, but this is incorrect for me, the correct formulation “stress relief heat treatment or stress relief annealing” or “recrystallisation annealing or recrystallisation treatment” should be used. It sounds to me that every annealing has this goal, but annealing treatment is the umbrella term for heat treatment.

A few small remarks:

1. check for the distance between unit and values
2. use the corrected format for a phase like Al₈Mn₅

Many thanks and I look forward to receive your modified manuscript.

Author Response

Dear Reviewer:

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Thank you very much for their suggestions and help

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The authors

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Reviewer 4 Report

The reviewed manuscript focuses on important issues in magnesium alloys and will be interesting for readers. A very interesting research methodology was used in the work and the obtained results are very interesting and quite well described. The work is undoubtedly very valuable and I strongly suggest it publish.

I have only a few remarks mainly concerning the editing mistakes detailed below.

Line 77: The ingots of Z2 and ZA21 alloys were casting by conventional casting methods and defined as IC-Z2 and IC-ZA21 magnesium alloys. - What were the dimensions of that ingot?

Line 185: The relationship between second phase particles and the SRX process is still controversial [36]. - Why is the reference number in red?

Line 192: Figure 6. Local magnified IPF figures and misorientation variation. TRC-ZA21-10s alloy (a) local magnified IPF; (b) misorientation variation; TRC-ZA21-3600s alloy (c) local magnified IPF; (d) misorientation variation. – (c) and (b) descriptions are swapped.

Line 306: in descriptions in figure 7 should be “GOS” instead “OS”

Line 309: Figure 7. variation of… - Figure 7. Variation of…

Lines 348-351: Polar plots in Figure 8 are almost invisible.

Author Response

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The authors

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Round 2

Reviewer 1 Report

Dear Authors, thank you for your responses to my comments. But, there are still some remarks on the manuscript.

1. PSN mechanism should be written instead of SPN mechanism in lines 620-621.
2. Usually, the JMAK model is expressed as: X=1-exp(-βtⁿ). And please explain what is the coefficient k in equation (3)?

Author Response

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Reviewer 2 Report

The authors did all required corrections and modifications. The manuscript can be accepted in its present form.

Author Response

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Reviewer 3 Report

Dear authors,

still recommend to improve the preparation technique. However, the SEM image needs the information of the mode used (SE or BSE) and should be adjusted at least with brightness and contrast to improve the recognizability and compareably to the SEM image. 

Regarding your feedback:”… https://doi.org/10.1016/j.msea.2016.07.049 and https://doi.org/10.1016/j.msea.2019.138332. Orthophosphoric acid solution for eletro-polished can be seen in https://doi.org/10.1016/j.msea.2018.15.113 and https://doi.org/10.1016/j.matchar.2017.09.003.” The first and the last publication do not use two-step etching with nitric acid solution (10% HNO3+90% alcohol). I have no access to the second publication and the third DOI is wrong and does not work.

Take a look at: “Hort N., Floss V., Gavras S., Wiese G., Tolnai D. (2019) Metallography of Mg Alloys. In: Joshi V., Jordon J., Orlov D., Neelameggham N. (eds) Magnesium Technology 2019. The Minerals, Metals & Materials Series. Springer, Cham https://link.springer.com/chapter/10.1007%2F978-3-030-05789-3_40 “.

Regarding: “Due to the closed lab for a long time considering some force majeure factors (Covid-19), we cannot complete the SEM and EBSD test in the near future.” Covid-19 should not be the reason for not good results in a publication. However, I know that you are all under pressure to publish something, but the brightness and contrast can be changed without a lab. In addition, word can do it or other open source software like GIMP etc.

Major remarks:

1. “The second phase particles with coarse size can be observed in IC-Z2 and IC-ZA21 alloys, …” With the current preparations I am not sure whether it is a particle as a second phase from the alloy or some oxides. It is not written what kind of mode was used for these SEM investigations. In case of SE it is an oxide for me and BSE it may be a second phase, but I strongly recommend to perform energy dispersive X-ray spectroscopy on this particle.
2. “Moreover, second phase particles with fine size were also found in IC-ZA21 alloy.” I have problems to see this in figure 3c. After printing, the brightness in fig. 3c will continue to decrease. Therefore a figure 3d with a higher magnification will show the distribution, shape and position of the particles better. I would make this mandatory (even if I am not convinced of the preparation and quality of these images). In Fig. 4 (i) to (l), no particles are visible at the same magnification, which is somewhat surprising if the work discusses the effect of these particles.
3. For me the conclusion that the particles act as nucleation with a larger size can only be an isolated effect in the neighbourhood of the particles and cannot explain effects in the entire material. If the particles are smaller and have the right distribution, they can block or hold the grain growth, as might be the case in TRC-ZA21. However, there is no error or standard deviation from the results in Fig. 1, or how many measurements per point have been made, so the statistics are either too low or non-existent. Therefore, it is not 100% reliable whether there is a significant difference between IC-ZA21 alloy and TRC-ZA21 alloy.I would suggest to add the error bars or the standard deviation.
4. “The microstructure information included inverse pole figure (IPF), kernel average misorientation (KAM), grain boundary figures and distribution of misorientation angles. The” Indicate: What's what in the subfigures? Even in the description in Fig. 5 it is not self-explanatory for people without an EBSD background..
5. I spoke with an expert about etching and then EBSD on Mg. He said that EBSD on etched Mg can produce two major defects: 1. most etchings produce a small oxide layer and interfere with elastic scattering. Depending on thickness, only the oxide reflections are measured or the Mg signal is disturbed. 2. etching will produce an uneven surface, which can lead to a misinterpretation of the scattered signal by the software, since a flat surface is used for the correction algorithms in the calculation.
6. There is still the question if this is real “…, it also indicated that nucleation stage of the SRX process was contributed to weakened basal texture.” Because the size and distribution of the particles or simply the presence of Gd as a rare earth metal influence the texture. As shown for Gd in particular in many publications (as “Stanford, N., and Barnett, M. R. (2008). The origin of “rare earth” texture development in extruded Mg-based alloys and its effect on tensile ductility. Sci. Eng. A. 496, 399–408. doi: 10.1016/j.msea.2008.05.045”, “Stanford, N., Atwell, D., and Barnett, M. R. (2010). The effect of Gd on the recrystallisation, texture and deformation behaviour of magnesium-based alloys. Acta Mater. 58, 6773–6783. doi: 10.1016/j.actamat.2010.09.003”, “Kim, S. H., Jung, J. G., You, B. S., and Park, S. H. (2017). Microstructure and texture variation with Gd addition in extruded magnesium. J. Alloys Compd. 695, 344–350. doi: 10.1016/j.jallcom.2016.10.179”, “Harmuth J, Wiese B, Bohlen J, Ebel T and Willumeit-Römer R (2019) Wide Range Mechanical Customization of Mg-Gd Alloys With Low Degradation Rates by Extrusion. Front. Mater. 6:201. doi: 10.3389/fmats.2019.00201”). So I miss at least one statement about another effect that will influence the texture in the presence of Gd in a Mg alloy. This effect does not require the presence of particles that will only have Gd as atoms that will diffuse to the dislocations.

A few small remarks:

1. As in the first round!!! Check for the distance between unit and values! 60 s and not 60s!!
2. As in the first round!!! Use the corrected format for a phase like Al8Mn5. Even if the editor changes this, it is the author's job to correct any error, wherever it comes from!
3. ...conventional casting methods … is for me to general. Was it a steel mold or sand etc. and was it preheated?
4. Add some more information about the twin roll setup with some parameter and information about the twin roller himself.
5. Add the mode for the SEM investigation. BSE or SE?
6. ... After 1h salt bath annealing treatment, … 3600 s as used throughout the manuscript..
7. Indicate the rolling direction in or for the figures 3, 4, 5 and 6 (a) and (b).
8. Where to serves the equation 4? If it is not used, it can be deleted.

Author Response

Dear Reviewer:

Please see the attachment.

Thank you very much for their suggestions and help

Best regards!

The authors

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Round 3

Reviewer 3 Report

Dear authors,

I'm sorry, but I have only one last wish: Add the description of the energy dispersive X-ray spectroscopy detector in section 2. Microstructure observation.

Then I accept the publication

I thank you for your effort in incorporating most of my annoying suggestions.

Kindly

Author Response

Dear Reviewer:

Please see the attachment.

Thank you very much for their suggestions and help

Best regards!

The authors

Author Response File: Author Response.pdf