Thermal Stability of the Precipitates in Dilute Al-Er-Zr/Hf Alloys at Elevated Temperature
Round 1
Reviewer 1 Report
The manuscript is devoted to the study of the influence of the temporal evolution of microhardness and the size of Al3(Er,Zr/Hf) precipitates in Al-Er-Zr/Hf alloys during aging at 450 and 500 ℃. As a main result, the authors show that the addition of the Hf element is useful for increasing the coarsening resistance and improving the thermal stability of the alloys.
I have the following comments on the manuscript:
1. Introduction. Very weak introduction. The authors reviewed a very small number of references. Over the past few years, a large number of papers have been published on the effect of small additions of zirconium and erbium on the microstructure and mechanical properties of aluminum alloys.
2. Material and methods. “The composition of Al-0.045Er-0.08Zr, Al-0.045Er-0.18Hf and Al-0.045Er-0.1Hf-0.08Zr (at. %) alloys were prepared by ingot metallurgy.” What is the size of the ingot? Moreover, why did the authors not analyze the cast microstructure of the ingot? This is important, since to ensure the accuracy of the experiment, it is necessary that all alloying elements be dissolved in the aluminum solid solution.
3. Figure 1. On the graph, the alloy contains 0.1 hafnium, while in the method it is indicated that the alloy contains 0.18 hafnium.
4. Figure 3. It is necessary to add units of measure for the precipitation coarsening rate.
Author Response
Dear reviewer:
Thank you for your comments and kindly recommendations. We have revised our manuscript according to your recommendations and requirements, and all the changes have been denoted by yellow. The change-list is as follows.
Reviewer's comments:
Concern 1：
Introduction. Very weak introduction. The authors reviewed a very small number of references. Over the past few years, a large number of papers have been published on the effect of small additions of zirconium and erbium on the microstructure and mechanical properties of aluminum alloys.
Response:
We have followed the suggestions of the reviewer, supplement the introduction part, and increase some new cited references in the revised manuscript.
Concern2：
Material and methods. “The composition of Al-0.045Er-0.08Zr, Al-0.045Er-0.18Hf and Al-0.045Er-0.1Hf-0.08Zr (at. %) alloys were prepared by ingot metallurgy.” What is the size of the ingot? Moreover, why did the authors not analyze the cast microstructure of the ingot? This is important, since to ensure the accuracy of the experiment, it is necessary that all alloying elements be dissolved in the aluminum solid solution.
Response:
We have followed the suggestions of the reviewer, and briefly described the size of the ingot (...the size of the ingot is about 30×90×150mm3...), and analyze the as-homogenized microstructure of the ingot in the revised manuscript.
...The as-homogenized and peak hardness state of Al-Er-Hf alloy was selected for microstructure analysis, due to the relatively high composition. some residual lamellar eutectic structures are located on the grain boundaries in the as-homogenized, as shown in Fig 2a. A round-shaped eutectic structure (AlErHf ternary), which size is approximately 1μm, can been seen in Fig. 2c. A high number density of homogeneously distributed nearly spherical precipitates can be seen in Fig. 2d, which is the micrographs of Al-Er-Hf alloy isothermal annealed to the peak hardness...
Concern3：
Figure 1. On the graph, the alloy contains 0.1 hafnium, while in the method it is indicated that the alloy contains 0.18 hafnium.
Response:
In this article, the experiment alloys is Al-0.04Er-0.08Zr, Al-0.04Er-0.18Hf and Al-0.04Er-0.08Zr-0.1Hf (at. %).
Concern4：
Figure 3. It is necessary to add units of measure for the precipitation coarsening rate.
Response:
We have added the units of measure for the precipitation coarsening rate in the revised manuscript.
Kind regards
Hao Wu
E-mail: [email protected]
Reviewer 2 Report
I don't think aging is the correct term for the heat treatment you experimentally applied. Aging is used for the heat treatment process at lower temperatures. For your example, I recommend using annealing.
Chapter 2 does not clearly describe all investigation procedures. What does "isothermal aging to the peak aged state" mean? Which device did you use for the microhardness measurements?
The title of Figure 2 is unclear, what does "after peak aging" mean?
Author Response
Dear reviewer:
Thank you for your comments and kindly recommendations. We have revised our manuscript according to your recommendations and requirements, and all the changes have been denoted by yellow. The change-list is as follows.
Reviewer's comments:
Concern 1：
I don't think aging is the correct term for the heat treatment you experimentally applied. Aging is used for the heat treatment process at lower temperatures. For your example, I recommend using annealing.
Response:
We have followed the suggestions of the reviewer, and replaced aging with annealing in the revised manuscript.
Concern2：
Chapter 2 does not clearly describe all investigation procedures. What does "isothermal aging to the peak aged state" mean? Which device did you use for the microhardness measurements?
Response:
We have followed the suggestions of the reviewer, we describe "isothermal aging to the peak aged state" in the part of experiment method, and added the device for the microhardness measurements.
Concern3：
The title of Figure 2 is unclear, what does "after peak aging" mean?
Response:
We have followed the suggestions of the reviewer, and revised the the title of Figure 2.
Kind regards
Hao Wu
E-mail: [email protected]
Author Response File: 
Author Response.pdf
Reviewer 3 Report
The authors of the paper "Thermal stability of the precipitates in dilute Al-Er-Zr/Hf alloys at elevated temperature" have investigated the growth kinetics of the particles during annealing at high temperatures. The authors have shown significant inhibition of the growth by the Hf atoms. The paper is well written. The authors have made a large experimental work. However, the scientific value of the manuscript should be enlarged. It is recommended to modify the manuscript accordingly following comments:
1. The Introduction part is too poor and should be significantly increased. Most of the cited references are too old. It is recommended to consider new papers devoted to low-alloyed aluminium alloys with rare-earth elements (e.g. works of Barkov R. et al).
2. It is not correct to compare the alloys with different atomic concentrations of Zr+Hf elements. The reason for the significant difference in the size of the particles in the Al-0.045Er-0.08Zr and other alloys is that the concentration of (Zr+Hf) in this alloy is more than twice lower than in the Al-0.045Er-0.18Hf and Al-0.045Er-0.1Hf-44 0.08Zr alloys. It is recommended to investigate the alloy Al-0.045Er-0.18Zr or find appropriate references for correct comparison.
3. The initial microstructure of the alloys (as cast, as homogenized, and after peak ageing) should be added to the manuscript.
4. It is recommended to add SAED to TEM images to approve the crystalline structure of the particles and make an EDX analysis to determine their chemical composition.
5. The authors should construct the Orowan-type model for the dependence of the hardness and volume fraction and particle size. It significantly improves the scientific part of the paper.
6. The growth model of the particles should be also added to the manuscript.
Author Response
Dear reviewer:
Thank you for your comments and kindly recommendations. We have revised our manuscript according to your recommendations and requirements, and all the changes have been denoted by yellow. The change-list is as follows.
Reviewer's comments:
Concern 1：
The Introduction part is too poor and should be significantly increased. Most of the cited references are too old. It is recommended to consider new papers devoted to low-alloyed aluminium alloys with rare-earth elements (e.g. works of Barkov R. et al).Response:
We have followed the suggestions of the reviewer, supplement the introduction part, and increase some new cited references in the revised manuscript.
Concern2：
It is not correct to compare the alloys with different atomic concentrations of Zr+Hf elements. The reason for the significant difference in the size of the particles in the Al-0.045Er-0.08Zr and other alloys is that the concentration of (Zr+Hf) in this alloy is more than twice lower than in the Al-0.045Er-0.18Hf and Al-0.045Er-0.1Hf-44 0.08Zr alloys. It is recommended to investigate the alloy Al-0.045Er-0.18Zr or find appropriate references for correct comparison.
Response:
In this article, the experiment alloys is Al-0.04Er-0.08Zr, Al-0.04Er-0.18Hf and Al-0.04Er-0.08Zr-0.1Hf (at. %). 0.08at.%Zr ≈ 0.27wt.%Zr and 0.18at.% ≈ 0.6wt.%Zr, which far exceeds the maximum solid solubility of Zr in Al matrix, about 0.28wt.%.
We compare the alloys with different atomic concentrations of Zr+Hf elements, the main purpose is to study the influence of different elements on the coarsening resistance of alloys.
Concern3：
The initial microstructure of the alloys (as cast, as homogenized, and after peak ageing) should be added to the manuscript.
Response:
We have followed the suggestions of the reviewer, and analyze the as-homogenized and peak ageing microstructure of the ingot and in the revised manuscript.
...The as-homogenized and peak hardness state of Al-Er-Hf alloy was selected for microstructure analysis, due to the relatively high composition. some residual lamellar eutectic structures are located on the grain boundaries in the as-homogenized, as shown in Fig 2a. A round-shaped eutectic structure (AlErHf ternary), which size is approximately 1μm, can been seen in Fig. 2c. A high number density of homogeneously distributed nearly spherical precipitates can be seen in Fig. 2d, which is the micrographs of Al-Er-Hf alloy isothermal annealed to the peak hardness...
Concern4：
It is recommended to add SAED to TEM images to approve the crystalline structure of the particles and make an EDX analysis to determine their chemical composition.
Response:
We have followed the suggestions of the reviewer, added SAED to TEM images of precipitates in Fig. 3 in the revised manuscript.
Concern5：
The authors should construct the Orowan-type model for the dependence of the hardness and volume fraction and particle size. It significantly improves the scientific part of the paper.
Response:
We have followed the suggestions of the reviewer, described the dependence of the hardness and volume fraction and particle size with the help of the Orowan-type model in the revised manuscript.
...Hardness is directly related to volume fraction and particle size. In our case, the Orowan bypass mechanism should be responsible for the particle-induced strengthening, the strength is directly proportional to volume fraction and inversely proportional to particle size, and the volume fraction proportional to number density of particles. With the increase of annealing temperature and time, the number density of precipitate decreases and the particle size increases...
Concern6：
The growth model of the particles should be also added to the manuscript.
Response:
We have followed the suggestions of the reviewer, added the coarsening model to describe the coarsening kinetics and mechanism in the revised manuscript.
Lifshitz, Slyozov and Wagner proposed a classic coarsening model controlled by volume diffusion (LSW model), which can predict the coarsening kinetics of the alloys, According to the LSW coarsening model, the average particle size and coarsening time should conform to the following relationship [18, 19]:
(1)
Where is a coarsening rate, is the average particle size at , and is any time at or after the initiation of quasi-stationary state coarsening.
Kind regards
Hao Wu
E-mail: [email protected]
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have significantly revised and improved the manuscript in accordance with the comments. I think the manuscript may be accepted for publication.
Reviewer 3 Report
The authors have answered previous comments and significantly improved the manuscript. The paper may be accepted for publication.
