A Study on the Damping Capacities of Mg–Zn–Y-Based Alloys with Lamellar Long Period Stacking Ordered Phases by Preparation Process

Round 1

Reviewer 1 Report

The paper presents experiments documenting the effect of different heat treatment on microstructure and mechanical and damping properties of Mg-Zn-Y. The topic and obtained results are interesting and the article can be considered for publication in Metals.

Before possible publication I recommend to improve the article in the following points:

• p.1, l.31: LPSO – Long-Period Stacking Order
• p.1, l.32: can can
• 3.1. Microstructure: I recommend to improve the description of the observed microstructures, e.g. it is confusing to use four different terms (grains, matrix, block phase, LPSO phase) in the description of two-phase structure.
• p.2, l.82: Figure 2d is missing.
• Figure 1: What is the meaning of the red circles inside the Figure? Increase the scale labels.
• Figure 3: Block LPSO or Bulk LPSO?
• Figure 3: TEM bright – fifield (field)
• p.5, l.132: damping alloy of the alloy
• p.5, l.135: strain should be 3x10^{-4}
• Figure 6: What was the “pre-study” sample?
• p.7, l.176: F_{B}

English language should be improved.

Author Response

1. 1, l.31: LPSO – Long-Period Stacking Order.

In recent years, the long-period stacking ordering (LPSO) phase in magnesium alloys has been a research hotspot [7-9].

2. 1, l.32: can can.

A large number of studies have shown that LPSO phase can strengthen magnesium alloys[10,11].

3. 1. Microstructure: I recommend to improve the description of the observed microstructures, e.g. it is confusing to use four different terms (grains, matrix, block phase, LPSO phase) in the description of two-phase structure.

Figure 1 shows SEM images of the Mg-Zn-Y alloys. In Figure 1a, the main phase in sample I is block phase along the grain boundaries, some white bright spot-like second phases gather on the grain boundary. After a 10 h heating process at 673 K, the grain size in Sample II is almost unchanged, and numerous lamellar phases are lined up in the matrix (Figure 1b, Figure 3a). For the air-cooling experiment in Figure 1c, due to the slow cooling rate, the grains of the alloy are quite coarse, about 100μm, the second phase is networked at the grain boundary, and other second phase structures are almost invisible in the grain. After the heat treatment, the grain size of Sample IV also remains unchanged, a small amount of the second phase can be seen in the matrix, but the lamellar phase in the matrix is less obvious than that in Sample II.  

4. 2, l.82: Figure 2d is missing.

In Figure 2b, the lamellar phase is obviously sparse and arranged neatly in the matrix.

5. Figure 1: What is the meaning of the red circles inside the Figure? Increase the scale labels.

In order to facilitate the understanding of the article, we have removed the circle.

6. Figure 3: Block LPSO or Bulk LPSO?

Sample I shows the block LPSO phase.

7. Figure 3: TEM bright – fifield (field)

Figure 3. TEM bright-field images and corresponding selected area electron diffraction (SAED) of Mg-Zn-Y alloys.

8. 5, l.132: damping alloy of the alloy

    The damping of the alloy is divided into two stages.

9. 5, l.135: strain should be 3x10^{-4}

Sample III has a relatively high damping capacity, and an existing critical strain (ε_cr) is at 3 × 10^-4.

10. Figure 6: What was the “pre-study” sample?

“pre-study” represents the mechanical and damping properties of rod-shape LPSO phase obtained from previous research.

Compared to alloy containing rod-shape LPSO phase^[20], which is represented by pre-study in Figure 6. The result shows sample IV sacrifice part mechanics for higher damping, it also presents excellent comprehensive performance as a damping alloy.

11. 7, l.176: F_{B}

In the formula, ρ is expressed as mobile dislocation density, L_N represents the average length between adjacent strong pinning points, and L_C represent the average length between adjacent the weak pinning point, and the binding force between the dislocation and the weak pinning point can be represented by F_B, E is the elastic modulus.

Author Response File: Author Response.pdf

Reviewer 2 Report

1. Line 36. The word 'mechanics' has been used at different places in the introduction in an interchangeable manner. I seek to confirm if the usage of word is accurate.
2. Line 57. Describe the morphology of pure Mg and Zn used in this study. Include the chemical composition of pure Mg, Zn and Mg-Y alloy with the impurities.
3. Line 61. I suggest the authors create a table for the nomenclature of these samples and the working conditions for better readability.
4. Line 61. K/S, 'S' has to be lowercase.
5. Line 66. Mention the ASTM standards.
6. Line 82. Change to Figure 2b.
7. Figure 1. The arrow and the circle are misleading. Remove the circle.
8. Line 94. Close parentheses.
9. Figure 4. Suggestion: The order in the figure can be yield strength followed by ultimate strength and elongation. 
10. The damping properties are explained in detail. More emphasis can be laid on the mechanical properties in the discussion part.
11. Did the authors observe twin lamellae formation as twinning and crack propagation are competing factors in the damping mechanism. Please comment.   

Author Response

1. Line 36. The word 'mechanics' has been used at different places in the introduction in an interchangeable manner. I seek to confirm if the usage of word is accurate.

The use of 'mechanics' here is correct.

2. Line 57. Describe the morphology of pure Mg and Zn used in this study. Include the chemical composition of pure Mg, Zn and Mg-Y alloy with the impurities.

Lump pure Mg, granular pure Zn, and master alloy with a composition ratio of Mg-30wt%Y obtained by smelting were used to make the Mg_95.3Zn₂Y_2.7 alloys.

3. Line 61. I suggest the authors create a table for the nomenclature of these samples and the working conditions for better readability.

A table is added to describe the status information of each sample

Table 1. Two groups of experimental alloy sample numbers and corresponding the status information of each sample

4. Line 61. K/S, 'S' has to be lowercase.

Corrections have been made in the manuscript.

5. Line 66. Mention the ASTM standards.

The relevant test follows the Chinese standards and does not contain ASTM standards.

6. Line 82. Change to Figure 2b.

In Figure 2b, the lamellar phase is obviously sparse and arranged neatly in the matrix.

7. Figure 1. The arrow and the circle are misleading. Remove the circle.

In order to facilitate the understanding of the article, we have removed the circle.

8. Line 94. Close parentheses.

Corrections have been made in the manuscript.

9. Figure 4. Suggestion: The order in the figure can be yield strength followed by ultimate strength and elongation.

 The order here is not considered important, its purpose is to compare a series of properties of the four alloys.

10. The damping properties are explained in detail. More emphasis can be laid on the mechanical properties in the discussion part.

Figure 5 shows the amplitude-dependent damping performance of the Mg_95.3Zn₂Y_2.7 alloys. The damping of the alloy is divided into two stages. In the low strain stage, the damping of the four samples is low and there is no significant difference. When reaching a high strain region, Samples I and II show extremely low damping. In addition, the damping value of the air-cooled specimen III and specimen IV increased at a speed significantly higher with the increase of strain than the damping value of the water-cooled specimen. And the damping values of the two are close, the Q^-1 value is about 0.015. Sample III has a relatively high damping capacity, and an existing critical strain (ε_cr) is at 3 × 10^-4. After the heat treatment, the damping of Sample IV increases greatly when the strain exceeds the critical strain (ε_cr). The damping value reaches 0.110 at ε = 10⁻³; this value is twice that for Sample III.

Compared to alloy containing rod-shape LPSO phase^[20], which is represented by pre-study in Figure 6. The tensile strength is reduced from 210MPa to 160MPa, but the yield strength is increased from 100MPa to 110MPa, and the damping value Q^-1 is increased from 0.048 to 0.11. The result shows sample IV sacrifice part mechanics for higher damping, it also presents excellent comprehensive performance as a damping alloy.

11. Did the authors observe twin lamellae formation as twinning and crack propagation are competing factors in the damping mechanism. Please comment.  

No twin layer was observed in this study.

Author Response File: Author Response.pdf

Reviewer 3 Report

The work presented a single alloy processed by two routes plus following heat treatments. Therefore the title must be adapted.

The description of microstructural results is not clear nor sound for later conclusions on dumping properties, namely:

• explain contrast and phases for each case I to IV.
• in Fig 1a fine bright contrast particles are present but not mentioned
• in Fig 1b specify the contrast as it seems SE instead of Backscattered as the others. Also there are bright contrast particles (more than in Fig1a), are they coming from sample preparation?
• points A and B belong both to sites in micrographs of AIR cooled material, therefore I don't understand your affirmation right afterwards that "quickly cooled samples contain more solid solution atoms than others".
• the contrast in Fig2a for region D is not clear whereas there is lamellar phase. Also D presents twice the at% vs A and B region (not similar as you  state)
• please adapt the contrast of SAED patterns in Fig 3 for prints
• sample II is not reported in TEM results nor mentioned the type of LPSO as it may seem lamellar within Mg dendrites
• please give the vol fraction of LPSO phases and also further background (references) or evidence (your results) of 14H "growth" during heat treatment

Regarding mechanical properties it is important to mention the asymmetry tension/compression in these materials. Also it is very important to discern in your discussion the effect of grain size refinement vs LSPO content/type (because you only mention the change for III and IV with same GS).

• in Fig 6 please give some information about the reference taken )i.e. fabrication, grain size, LPSO,)
• conclusion about LPSO about spacing is not sufficiently sustained, please implement the discussion.

English and other details must be revised, for example tensile rate to strain rate, long-cycle to long period stacking ordered, the description of all parameters in equations such as C1,C2 and b,

References to be implemented regarding LPSO phases and their evolution under HT and loading, notably missing:

Zhu, Acta Mater 2010

Egusa, Acta Mater 2012

Yamasaki Scripta Mater 2005

Nie Met Mat Trans A 2012

Onorbe, Scripta Mater 2011

Shi, Materials 2019

Author Response

1. The work presented a single alloy processed by two routes plus following heat treatments. Therefore the title must be adapted.

The title corrected to “A study on the the damping capacities of Mg–Zn–Y-based alloys with lamellar long period stacking ordering phases by preparation process.”

2. The description of microstructural results is not clear nor sound for later conclusions on dumping properties, namely:explain contrast and phases for each case I to IV.

Figure 1 shows SEM images of the Mg-Zn-Y alloys. In Figure 1a, the main phase in sample I is block phase along the grain boundaries, some white bright spot-like second phases gather on the grain boundary. After a 10 h heating process at 673 K, the grain size in Sample II is almost unchanged, and numerous lamellar phases are lined up in the matrix (Figure 1b, Figure 3a). For the air-cooling experiment in Figure 1c, due to the slow cooling rate, the grains of the alloy are quite coarse, about 100μm, the second phase is networked at the grain boundary, and other second phase structures are almost invisible in the grain. After the heat treatment, the grain size of Sample IV also remains unchanged, a small amount of the second phase can be seen in the matrix, but the lamellar phase in the matrix is less obvious than that in Sample II.  

3. In Fig 1a fine bright contrast particles are present but not mentioned.

It is shown in Figure 1a that the main phase in sample I is block phase along the grain boundaries, some white bright spot-like second phases gather on the grain boundary.

4. In Fig 1b specify the contrast as it seems SE instead of Backscattered as the others. Also there are bright contrast particles (more than in Fig1a), are they coming from sample preparation?

The Fig 1b have been unified into BSE as the others. After heat treatment, it was found that the white and bright second phase in the crystal increased after heat treatment. The white and bright second phase (rare-earth-rich region) of the grain boundary is transferred into the grain.

Figure 1. SEM images of Mg-Zn-Y alloys obtained under different conditions, (b) sample II

5. Points A and B belong both to sites in micrographs of AIR cooled material, therefore I don't understand your affirmation right afterwards that "quickly cooled samples contain more solid solution atoms than others".

In the process of writing, typesetting pictures caused this misunderstanding. We have made corrections to the sample area pointed to by point A and point B. Point A refers to the water-cooled sample and point B refers to the air-cooled sample.

6. The contrast in Fig2a for region D is not clear whereas there is lamellar phase. Also D presents twice the at% vs A and B region (not similar as you state)

In the process of writing, typesetting pictures caused this misunderstanding. The experimental results are mainly the comparison of water cooling and air cooling under different processes.The results of points D and F are similar to points A and B of the two cast alloys.

7. Please adapt the contrast of SAED patterns in Fig 3 for prints.

Figure 3 with clear contrast has been replaced in the manuscript.

8. Sample II is not reported in TEM results nor mentioned the type of LPSO as it may seem lamellar within Mg dendrites.

The innovation of this study is to compare the relative damping performance of LPSO with different density by comparing the as-cast and heat-treated samples of air-cooled samples. Taking the water-cooled sample II as a reference for comparison, it is found that the air-cooled samples can significantly improve the damping performance. Under the same composition and heat treatment conditions, the characteristics of LPSO obtained should be similar. At the same time, the equipment resources are limited, and TEM testing cannot be performed on all samples.

9. Please give the vol fraction of LPSO phases and also further background (references) or evidence (your results) of 14H "growth" during heat treatmentRegarding mechanical properties it is important to mention the asymmetry tension/compression in these materials. Also it is very important to discern in your discussion the effect of grain size refinement vs LSPO content/type (because you only mention the change for III and IV with same GS).

The as-cast sample does not contain the second phase in the crystal. After heat treatment, the LPSO phase can be observed in the matrix (EDS, TEM) . In addition, in samples with more solid-solution atoms, LPSO precipitates densely, while samples with less solid-solution atoms precipitate a more sparse second phase. The estimated value is as follows.

Figure 2. Volume Fraction and Average grain size of Mg-Zn-Y alloys obtained under different conditions: (a) sample I, (b) sample II, (c) sample III, (c) sample IV.

10. In Fig 6 please give some information about the reference taken )i.e. fabrication, grain size, LPSO,)?

A table is added to describe the status information of each sample

Table 1. Two groups of experimental alloy sample numbers and corresponding the status information of each sample

11.Conclusion about LPSO about spacing is not sufficiently sustained, please implement the discussion. English and other details must be revised, for example tensile rate to strain rate, long-cycle to long period stacking ordered, the description of all parameters in equations such as C1,C2 and b?

What we want to express is the influence of the density of the LPSO phase within the crystal on the damping characteristics, which can be obtained through microscopic photos. The average distance between the second phases is judged macroscopically by the density of the LPSO phase per unit area. The sentence has been adjusted to eliminate ambiguity.

C₁ is mainly related to the dislocation density ρ, the average dislocation segment length between strong pinning points L_N, and the average distance between weak pinning points Lc. C₂ reflects inversely proportional to the average distance Lc between weak pinning points. b is the dislocation Burt vector.

11. References to be implemented regarding LPSO phases and their evolution under HT and loading, notably missing:

Related literature has been referenced and cited in the manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I am satisfied with the current version and have no further comments.

Author Response

Thank you for your recognition of our work, we will do more meaningful research in the future work