Effects of Substitution of Y with Yb and Ce on the Microstructures and Mechanical Properties of Mg_88.5Zn₅Y_6.5

Round 1

Reviewer 1 Report

I suggest that the article includes full tensile curves, not only tensile strength values. There weren't too many samples in the experiment, so it can be done. A tensile curve gives a more complete picture of the mechanical properties of a material than a single parameter such as tensile strength. If it would not be possible to supplement the article with tensile diagrams, at least the values of Young's modulus for individual alloys can be included.

Author Response

Response to Reviewer’s Comments

 

Effects of substitution of Y with Yb and Ce on microstructures and mechanical properties of Mg88.5Zn5Y6.5

by H. Liao, T. Lee, J. Song, J. Kim*, and F. Pan* submitted to Metals (metals-1024719)

 

The authors appreciate the peer-review for our manuscript given by the reviewer and editor. We have carefully examined the reviewer’s valuable comments and revised the entire manuscript on the basis of them. Our responses to the reviewer’s comments are given in blue letters. The added/changed sentences are marked in highlights in the revised manuscript.

 

Reviewer

the article includes full tensile curves, not only tensile strength values. If it would not be possible to supplement the article with tensile diagrams, at least the values of Young's modulus for individual alloys can be included.

In fact, the tensile curves is not important. We study the effect of Ce/Y and Yb/Y ratios on the mechanical properties of Mg88.5Zn5Y(6.5-x)Rex alloys. The major is the change rule by variation of element type, and variation diagram (Fig. 6 and Fig. 7) is much more distinct to find the rule than normal tensile curves. So it will be repetition if the variation diagram and tensile curves appear in one paper together. That’s why we choose variation diagram rather than tensile curves.

For Young’s modulus, it’s not important for this paper. In fact, the Young’s modulus is almost same according to the tensile curves.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper. original contribution, is clearly presented. The methods used for the microstructure analysis are described in detail. The results are new and very important. References are adequate.

Comments: 1] Did you estimate the grain size distribution?

2] Did you estimate the effect of grain size on the yield strength? 

Author Response

Response to Reviewer’s Comments

 

Effects of substitution of Y with Yb and Ce on microstructures and mechanical properties of Mg88.5Zn5Y6.5

by H. Liao, T. Lee, J. Song, J. Kim*, and F. Pan* submitted to Metals (metals-1024719)

 

The authors appreciate the peer-review for our manuscript given by the reviewer and editor. We have carefully examined the reviewer’s valuable comments and revised the entire manuscript on the basis of them. Our responses to the reviewer’s comments are given in red letters. The added/changed sentences are marked in highlights in the revised manuscript.

 

Reviewer

1. Did you estimate the grain size distribution?
2. Did you estimate the effect of grain size on the yield strength?

    We have check the grain size, and find the average grain size is 1.5 μm. The grain size distribution is almost same in all alloys. So we think the effect of grain size on the yield strength is very small. And the result excluded the effect of grain size on the strength, so we can focus on study the second phases to the effect of mechanical properties. The paper have made corresponding adjustment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript deals with the influence of changes in the Ce/Y and Yb/Y ratios on the microstructure and mechanical properties of Mg88.5Zn5Y(6.5-x)REx alloys. The results prove clearly how the replacement of Y by Ce or Yb affects the nature and the volume fraction of the phases existing in the alloy. In opinion, of this reviewer the results are new in the case of alloys with Yb, but similar studies have been also done with other rare earth elements. Such studies should be revised in the introduction section.

The manuscript also demonstrates that the strength of the alloys is given by the LPSO-phase rather than for the presence of other intermetallic phases. This result is not new and it should be mentioned in the discussion that such results agree with results reported in the literature.

Other aspect not clear is the influence of Ce and Yb refining the grain size of the extruded alloys. It is stated that refining is associated with solute drag induced by Ce and Yb atoms in the magnesium matrix. However, in the case of Ce, this element is totally immiscible in magnesium, so all the Ce should be forming intermetallic compounds. Thus, it seems harder to assume that Ce atoms could prevent grain growth during extrusion. In fact, when the alloys are subject to thermal treatment there is a considerable grain growth. It seems more reasonable other mechanism proposed in the literature. According to "Influence of Y/CeMM ratio on the microstructure and mechanical properties of Mg95Zn2(Y,CeMM)3 alloys, Intermetallics 2012, 196" second phases assist the refinement of the microstructure during the extrusion process through a particulate stimulated nucleation mechanism. As found in the present study, the volume fraction of fine grains is increased as increases the Y concentration in the alloy. These authors considered that an increase in the volume fraction of LPSO results in a significant stress transferred from the magnesium matrix towards the LPSO-phase, much higher than that assumed by the intermetallic MgRE phases. When these phases are broken, the stress goes back to the magnesium matrix, being much higher when the volume fraction of the LPSO-phase is increased. Therefore, the driven force for inducing recrystallization is promoted because the strain energy stored by the magnesium matrix is maximized when the volume fraction of LPSO-phase increases.

Other point that need to be addressed is that regarding kinking. It is true that kinking is a deformation mechanism characteristic of the LPSO-phase but only when the LPSO-phase is under compression. In addition, most of Mg-alloys containing the LPSO-phase combine high strength with excellent ductility, especially in the 100-250°C temperature range, so these alloys cannot be considered high strength alloys with low ductility. Elongations between 20-30 % are often reported for these kinds of alloys.

Author Response

Response to Reviewer’s Comments

 

Effects of substitution of Y with Yb and Ce on microstructures and mechanical properties of Mg88.5Zn5Y6.5

by H. Liao, T. Lee, J. Song, J. Kim*, and F. Pan* submitted to Metals (metals-1024719)

 

The authors appreciate the peer-review for our manuscript given by the reviewer and editor. We have carefully examined the reviewer’s valuable comments and revised the entire manuscript on the basis of them. Our responses to the reviewer’s comments are given in red letters. The added/changed sentences are marked in highlights in the revised manuscript.

 

Reviewer

The similar studies should be revised in the introduction section.

Kim et al. reported that each RE element exerted a unique influence on the volume fractions of the LPSO phases and intermetallic compounds. As a result of the replacement of Y by different RE element in Mg97Zn1Y2 alloys, the alloy with Ce and La exhibits higher mechanical properties with the exist of LPSO phases, but the alloys with Nd and Sm get lower strength because of the disappear with LPSO phases.

Thanks for your valuable opinion. We find a manuscript with similar studies with other rare earth elements. And have revised in the introduction section.

 

This result is not new and it should be mentioned in the discussion that such results agree with results reported in the literature.

Multiple researchers have reported that alloys containing LPSO phases exhibit low elongation in addition to an extremely high strength.

Thanks for your suggestion. We find some paper to agree with the results. And have revised in the paper.

 

Other aspect not clear is the influence of Ce and Yb refining the grain size of the extruded alloys. It is stated that refining is associated with solute drag induced by Ce and Yb atoms in the magnesium matrix. However, in the case of Ce, this element is totally immiscible in magnesium, so all the Ce should be forming intermetallic compounds. Thus, it seems harder to assume that Ce atoms could prevent grain growth during extrusion.

According to IPF maps (Figure 9), the average grain size of Mg88.5Zn5Y6.5 and Mg88.5Zn5Y5.0Ce1.5 alloys both are 1.5 μm. In fact, the average grain size of all alloys are almost same, and this result excluded the effect of grain size on the strength, so the major factor to effect of mechanical properties is the second phases..

As shown in Figure 9, the worked Mg grains that existed in Mg88.5Zn5Y5Ce1.5 and Mg88.5Zn5Y6.5 alloys, and disappeared after the heat treatment at 400 °C for 1 h. This result indicated the presence of deformed storage energy inside the grains after extrusion. The heat treatment enabled the deformed storage energy to become the driving force for recrystallization nucleation. This induced the transformation of the worked Mg grains to recrystallized grains through a particulate stimulated nucleation mechanism. As well known, the compound have higher thermal stability than LPSO phases. Therefore, Mg88.5Zn5Y5Ce1.5 possessed fine grains and more steady microstructures at high temperatures..

Thanks for your suggestion. We agree with your opinion, and have revised in the paper.

 

Other point that need to be addressed is that regarding kinking. It is true that kinking is a deformation mechanism characteristic of the LPSO-phase but only when the LPSO-phase is under compression. In addition, most of Mg-alloys containing the LPSO-phase combine high strength with excellent ductility.

Thanks for your opinion. We know that some alloys with LPSO phases have high strength and excellent ductility, but it’s just part of alloys. For these alloys in our paper, the higher volume fraction of LPSO leads to higher strength and lower ductility. So this rule is fit these alloying system.

Kink deformation is surely produced during the LPSO-phase is under compression, but the orientation of LPSO phases is not only one. It means that part of LPSO phase is under compress stress and other is under tensile stress when the alloys is under tensile test. So there is also part of kink deformation happened during the tensile process.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The authors have not understood some of the comments made by this reviewer and they have not modified the text in the line marked by this reviewer.

It is obvious that recrystallization is due to strain energy stored within the magnesium matrix. The main point is that the magnitude of strain energy depends on the nature of the phases present in the alloy as ell as their volume fraction, as commented in the first review report. This depends on the Y/RE ratio of the alloy, as demonstrated in the literature (see the reference given in the first review report)

The second point regards the fact that alloys containing the LPSO are inherently brittle. This is not true. Of course, if the strength is raised up to 400 or 500 MPa, the ductility is significantly reduced, but still the elongations are 1-2 %. Magnesium alloys containing exclusively intermetallic Mg-RE compounds are much more brittle, even without ductility for volume fractions much lowers than  magnesium alloys containing the LPSO phase. Consequently, the statement wrote in the manuscript should be removed or explained properly in the line expressed by this reviewer.

Author Response

Response to Reviewer’s Comments

 

Effects of substitution of Y with Yb and Ce on microstructures and mechanical properties of Mg88.5Zn5Y6.5

by H. Liao, T. Lee, J. Song, J. Kim*, and F. Pan* submitted to Metals (metals-1024719)

 

The authors appreciate the peer-review for our manuscript given by the reviewer and editor. We have carefully examined the reviewer’s valuable comments and revised the entire manuscript on the basis of them. Our responses to the reviewer’s comments are given in blue letters. The added/changed sentences are marked in highlights in the revised manuscript.

 

Reviewer

It is obvious that recrystallization is due to strain energy stored within the magnesium matrix. The main point is that the magnitude of strain energy depends on the nature of the phases present in the alloy as ell as their volume fraction, as commented in the first review report.

As shown in Figure 9, the worked Mg grains that existed in Mg88.5Zn5Y5Ce1.5 and Mg88.5Zn5Y6.5 alloys, and disappeared after the heat treatment at 400 °C for 1 h. This result indicated the presence of deformed storage energy inside the grains after extrusion. The heat treatment enabled the deformed storage energy to become the driving force for recrystallization nucleation. This induced the transformation of the worked Mg grains to recrystallized grains through a particulate stimulated nucleation mechanism. The high volume fraction of LPSO phases results in a significant stress transferred from Mg matrix to LPSO phases which much higher than compounds [28]. Therefore, Mg88.5Zn5Y5Ce1.5 alloy with low LPSO phases have much more strain energy to induce recrystallization than MgZn5Y6.5 alloy, and leads to finer grain size after heat treatment.

Thanks for your suggestion. We have read the paper and your comments one more time. We have understood and revise the paper.

 

The second point regards the fact that alloys containing the LPSO are inherently brittle. This is not true.

There was a marked increase in the TYS, with the increase in the content of the LPSO phase. The LPSO phases exhibit a unique mode of plastic deformation, known as kink deformation [24]. The deformation models of the LPSO phases and α-Mg are significantly different. Therefore, the deformation of the LPSO phases in the grains is incompatible with that of α-Mg. Consequently, the alloys containing the LPSO phases exhibit high strength [25]. Multiple researchers have reported that alloys containing LPSO phases exhibit an extremely high strength [26,27]. The variation in the mechanical properties was primarily attributed to the volume fraction of the LPSO phases.

Thanks for your valuable opinion. We analyse the related data and manuscript, and think you are right that the poor ductility is not caused by LPSO. And the related statement have been revised.

 

Author Response File: Author Response.pdf

Round 3

Reviewer 3 Report

Aafter the changes the manuscript is suitable for publishing