Influence of Processing Routes to Enhance the Mechanical Properties of Mg–6Zn–1Y–3.5CeMM (wt.%) Alloy
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this paper, the microstructure and mechanical properties of the Mg-6Zn-1Y-3.5CeMM (wt.%) alloy processed by extrusion at 400℃ of as-cast ingots (ACE alloy) or atomized powders (PME alloy) were investigated. This alloy exhibits superior mechanical properties over other Mg-materials while retaining acceptable ductility. This paper is well-written. However, there are still some questions needing a minor revision before it can be accepted.
1. Please standardize the format of tables and images. For example, Table 1, Table 2, Figure 1, etc.
2. The stress-strain curve features are not obvious.
3. The introduction mainly outlines the influence of alloy composition on mechanical properties. However, there are lacks of the influence of processing route on mechanical properties.
4. To ensure that your writing is innovative, please cite articles published within the last five years.
5. Please explain in detail the influence of the second phase and processing route on mechanical properties.
Author Response
"Please see the attachment."
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsIn this paper the influence of processing routes to enhance the mechanical properties of Mg-6Zn-1Y-3.5CeMM (wt.%) alloy is studied. The microstructure and mechanical properties of the magnesium alloy processed by extrusion were investigated. This is a useful study for the material sciences and future application of this alloys. The article contains conclusions about the nature of the second phases and mechanism of deformation. However, the introduction is very short for a scientific article. It would be very good if the authors added the purpose and objectives of the study and explained in more detail why rare earth metals are added to the alloy. Why CeMM?
Below there are my notes and questions for you and it should be corrected for the publication.
Line 12: “…W-phase (Mg3Zn3Y2)….” Please change the numbers in the spelling to lower case. Check the spelling throughout the article.
Line 30: “…[1-5]their low…” should be changed to “…[1-5] their low…” Please add a space.
Line 51: It is necessary to indicate whether mass or atomic percentages were used. Why is the accuracy of composition determination up to hundredths of a percent? What equipment was used to determine the composition? What is the error in determining the composition?
Line 73: The X-ray technique need specify the measurement step and exposure time.
Line 78: ‘10 KN’ should be changed to ‘10 kN’.
Line 85: It is very strange that section ‘3.1. Microstructural characterization’ begins with a figure 1. The article should first describe the figure and only after the reference to the figure in the text should the figure be presented.
Line 182: Figure 3 needs to be improved. Intensity is measured in arbitrary units, and it can be abbreviated arb. units or just (a.u.) and the numbers near the y-axis should be removed. Also, indices for each reflection should be added to the XRD patterns.
Line 425: In the table 5 the labels ‘CAST’ and ‘PM’ are shifted to the left, may be better to shift it to the center of the ACE and PME characteristics. Throughout the article it is necessary to use the same spelling for as-cast and PM samples. Please write ACE or as-cast everywhere.
Line 429: 182 KJ/mol should be changed to kJ/mol. Please do this throughout the article.
Line 471: In the conclusions you wrote that the yield stress is higher 37%, and in the abstract, you wrote that it increased by 40%. So how much did the yield stress increase? I don't understand what values ​​are being compared. Please add the values of ​​ the yield stress to the conclusions.
Author Response
"Please see the attachment."
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe conducted study is of significant relevance to both pure science and engineering, offering valuable insights into how the freedom to manipulate feedstock microstructure creates opportunities for the optimization of manufactured part properties. The contribution is of high quality, reflecting a thorough investigation by the authors. Nevertheless, to further enhance the manuscript's overall quality, some corrections are recommended. Detailed suggestions for improving both the scientific content and the writing style of the paper are provided below.
1. In the Abstract, the author wrote: ‘The microstructure and mechanical properties of the Mg-6Zn-1Y-3.5CeMM (wt.%) alloy processed by extrusion at 400 °C of as-cast ingots (ACE alloy) or atomized powders (PME alloy), were investigated’. It is better to indicate the ‘cold-compacted atomized powder’ since the extrusion of the atomized powder sounds confusing.
2. In the Abstract, the statement, ‘Grain size refinement is the main hardening contribution on PME alloy while second phase particles entail the major strengthening contribution on ACE alloy’, gives a false signal to the reader in the sense that the grain refinement in PME was sourced from mechanical loading during extrusion. However, in the Results and Discussion, the pining effect originated from the presence of the second phase inhibiting the grain growth was introduced as the main responsible for this observation, which appears the same source of hardening as to that of ACE. Please rewrite this sentence to reflect your observation in a more understandable manner.
3. In the Abstract, the last two sentences could have been written in a more attention-grabbing way to deliver the main conclusion of your study, the principal reason leading to the mechanical properties differences in the two investigated processing routes. This, according to your study, could primarily be the microstructural differences, like the homogenous distribution of the second phase in PME, smaller grain size due to the nature of gas atomization compared to casting, and the load transfer differences.
4. In the Introduction, the author wrote ‘Notwithstanding the good properties of the magnesium alloys [1-5] their low corrosion resistance, poor ductility and insufficient strength at room and elevated temperatures make necessary to develop new magnesium alloys to overcome these drawbacks’. The term, ‘good properties’, is so generic. Please indicate what properties you were referring to.
5. In the Introduction, please provide references for ‘One strategy to improve the mechanical properties of magnesium alloys is modifying the composition through the addition of appropriate amounts of different elements to the magnesium matrix. In this regard, recently developed magnesium alloys belonging to the Mg-Zn-Y and Mg-Zn-RE families with rare earth additions are promising for increasing mechanical strength’.
6. In the Introduction, the term, ‘on the other hand’, in ‘On the other hand, the mechanical properties can also be improved by changes in their microstructure, which can be achieved by controlling thermomechanical processing [ 16-20]’ seems redundant as the sentence follows the same message provided in the previous sentence.
7. In the Introduction, please define the abbreviation, ‘CeMM’, before use.
8. For the paper with 18 pages long, half a page for the Introduction is pretty small reflecting a poor review of the state-of-the-art. For example, discussing about the applications requiring that specific Mg alloy and the effect of rare earth alloying elements on the mechanical properties, which were repeatedly discussed in the Results and Discussion, would be appreciated.
9. The main goal of the study written at the end of the Introduction is better to be rewritten since, according to your study, the main goal sounds to be a comparative investigation of different processing routes rather than an improvement of one established manufacturing technique for the alloy investigated.
10. In the Materials and Methods, page 2, the author indicated the tensile measurement was performed for both ACE and PME only in a direction parallel to the extrusion direction: ‘Cylindrical samples of diameter 6 mm and gauge length of 10 mm were machined from the extruded bars with the longer dimension parallel to the extrusion direction’. Since in the Results and Discussion, a significant influence of the second phase distribution behavior on the mechanical properties was addressed, it would be interesting to show tensile strength results also for the direction perpendicular to the extrusion direction, highlighting potentially better isotropic mechanical properties of the PME.
11. On page 3, the author wrote: ‘During the solidification process, after the formation of the magnesium dendrites, the remaining liquid, enriched in all the alloying elements, is disposed at the interdendritic space where second phases are finally located’. Please elaborate on this part. As the microsegregation is time and cooling rate dependent, it would be relevant to indicate what cooling rate was chosen for the solidification process. Did you measure the influence of the cooling rate on the composition of the T phase and W phase? If yes, please mention the results and any correlation you found.
12. On page 5, at the end of the page, the author wrote: ‘The higher concentration of alloying elements in solid solution in the Mg matrix of the PME material favours subsequent precipitation during the extrusion process, which is accompanied by the coarsening of the precipitates’. Please provide the causes leading to this observation. It seems the higher cooling rate during the gas atomization resulted in a rather diffusionless solidification. Thus, a question arises: In the PME, did you see any contribution to strengthening from the non-participated fraction whose presence in the Mg matrix distorts the lattice, and consequently, inhibits the dislocations movement?
13. On page 7, the conclusion drawn before the start of section ‘3.2. Mechanical characterization’ is confusing. The author wrote: ‘On the contrary, as in the case of the PME alloy, the main effect of fine second phases dispersed in the magnesium matrix is impeding grain growth since they act as effective obstacles for boundary migration [36]. Thus, recrystallized grains without preferential orientation contribute to weaken the overall texture’. While the elevated pinning effect from the finer second phase in the PME inhibited complete recrystallization and preferential orientation along the basal and prismatic planes, the author reported higher intensity in the texture analysis in the pole figures for the PME than that of ACE. Could you explain why is that?
14. On page 8, please merge the sentence in the middle of the page with the last paragraph of the page 7. It is not a good writing style to make a paragraph from one sentence occupying only one line. This mistake was repeated on pages 10, 11, and 15.
15. The values in Table 3 on page 8 do not have error margins showing the reproducibility and consistency of measurements. Could you provide error values for each measurement? Moreover, the elongation to failure reported for the PME for the temperatures of 300°C and 350°C does not follow the same decreasing trend reported for the previous temperature ranges in the list. Was that a measurement error? Or do you have any explanation for that observation?
16. On page 9, at the end of the first paragraph, the author wrote: ‘The effect of the coarse second-phase particles, mostly T-phase, results in increasing the yield strength of the alloy and hardening the material in the early stages of plastic deformation’. Please explain why the T phase contributes the most to the strengthening. Was it due to the higher volume fraction of the T phase in the interdendritic region? Or was it because of the lattice distortion due to the presence of Ce having larger lattice parameter, which resulted in the formation of the compressive/tensile region, repelling dislocations?
17. The second paragraph on page 9 starts with: ‘Both alloys show a multitude of cavities characteristic of a ductile fracture, containing embedded second phase particles’. In the SEM images shown in Figure 2, it is not clear if there were any shrinkage cavities during solidification originating from different thermal expansion coefficients of the matrix and the second phase. Did you observe such cavities apart from the cavities formed from ductile fractures? If yes, did you study the load transfer regimes at the interfaces of cavities and their influences on the mechanical properties of the alloy investigated?
18. On page 10, the first paragraph, the author provided the following statement as the reason why the elongation to failure was higher for the PME: ‘On the other hand, in the case of PME alloy, second phases are homogeneously dispersed not only at grain boundaries, but also inside the magnesium grains (see Figure 1c). Because of this homogeneous dispersion, all the magnesium matrix could transfer some load to the particles’. This explanation sounds reasonable but it seems that the provided description probably was not the only reason for the observed effect. A mechanically-induced lattice deformation is typically associated with displacing atoms whose presence before applying the stress inhibits dislocation movements. Thus, the higher ductility for the PME most probably comes from a homogenous distribution of the finer second phase leading to both efficient load transfer from the matrix to the harder second phase particles and the removal of dislocation movement barriers by displacing the alloying atoms/second phase.
19. On page 11, for equation 6, the author wrote: ‘In any case, the contribution due to Orowan mechanism is almost negligible, below 10%, for both materials’. Please explain why the contribution from the Orowan mechanism to strengthening is negligible. The effective strengthening by the Orowan mechanism is commonly dependent on the size of the second phase and the distance from each other. While the explanation for the latter was provided later in the text by the author, the question is: did you observe any dependency on the former on the Orowan mechanism strengthening?
20. On page 12 below Figure 7, the statement ‘For the ACE alloy, the major contribution to strengthening is given by the reinforcement of the second phases, mainly due to its larger grain size reached after extrusion at 400 °C (6.8 µm) and therefore the lower contribution of grain size reinforcement’ is confusing as the grain refinement strengthening also showed a significant share of 43.6% compared to that of the hard second phase (49.7%). Since there was less than a 7% difference and no error margin was provided by the author for their calculations, it is better to rewrite this sentence in a more justifiable manner.
21. At the end of page 12, the effect of the grain size on the strengthening was linked to the pinning effect for dislocations which is correct for the first number of dislocations but as the density of dislocations at the grain boundaries increases, the newly-formed dislocations that approach the grain boundary feel the spring back effect from the previously-piled up dislocations pushing new dislocations toward the center of the grains. Please rewrite this sentence to better explain what is actually happening inside the grains.
22. On page 13, in the first paragraph, the author wrote: ‘Even though it would be expected higher hardening associated with the higher volume fraction of fine precipitates present in the magnesium matrix for the powder metallurgy alloy, maintenance at the relatively high extrusion temperature of 400 °C causes particle coarsening (mainly of MgZn composition), with an average size around 125 nm, and, consequently, the increase of interparticle distance’. Please elaborate on this part. An observed increase in particles distance probably came from the Ostwald ripening phenomenon where the finer particles gradually dissolve to the coarse ones due to the increase in temperature, which results in coarser particles and an increase in their distance.
23. On page 15, in paragraph 6, the author wrote: ‘It is interesting to note that GBS tends to operate at progressively higher strain rates with increasing the temperature’. At high temperatures where the grain boundary density decreases due to the grain coarsening, it is expected that the grain boundary sliding (GBS) is activated at lower stress levels due to lower friction of grain boundaries. Could you explain why in your case a higher strain rate was needed at high temperature to activate the grain boundary sliding? Was it due to the grain boundary precipitates? If yes, please indicate for which manufacturing routes (ACE or PME) you made this observation.
24. In the Conclusion, the term, ‘nature’, in the first bullet point is so generic. Please clearly mention what properties you are referring to as ‘nature’.
25. Please provide, at the end of the Conclusion, the suggestions for further research you feel they could contribute to the science in the context you investigated (e.g., the influence of the W phase on what you studied).
26. The results of the study is highly interesting for those exploring the optimization of feedstock properties for sustainable manufacturing in industrial symbiosis contexts where the minimization of wastes is among the major challenges. Since the PME route resulted in better mechanical properties at the expense of increasing the manufacturing efforts (e.g., the need for the gas atomization), the evaluation of both manufacturing routes in the form of a comparative life cycle assessment would be highly appreciated. If such an analysis was not performed, please add this at the end of the Conclusion as a suggestion for further study.
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Author Response
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