Research on homogeneous nucleation and microstructure evolution of aluminium alloy melt

In this paper, based on the embedded atom method (EAM) potential, molecular dynamics simulations of the solidification process of Al–4 at.%Cu alloy is carried out. The Al–Cu alloy melt is placed at different temperatures for isothermal solidification, and each stage of the entire solidification process is tracked, including homogeneous nucleation, nucleus growth, grain coarsening and microstructure evolution. In the nucleation stage, the transition from high temperature to low temperature manifests a change from spontaneous nucleation mode to divergent nucleation mode. The critical nucleation temperature of the Al–Cu alloy is determined to be about 0.42 Tm (Tm is the melting point of Al–4 at.%Cu) by calculating the nucleation rate and the crystal nucleus density. In the nucleus growth stage, two ways of growing up are observed, that is, a large crystal nucleus will absorb a smaller heterogeneous crystal nucleus, and two very close crystal nuclei will merge. In the microstructure evolution of the isothermally solidified Al–Cu alloy, it is emerged that the interior of all nanocrystalline grains are long-period stacking structure composed of face centred cubic (FCC) and hexagonal close-packed (HCP). These details provide important information for the production of Al–Cu binary alloy nano-polycrystalline products.


1.
Can the authors provide the fraction of Cu in the grains/nuclei? 2.
The statement "… mainly focused on the analysis of the solidification structure under rapid solidification conditions..." is dubious, since the phase diagram is based on equilibrium state. The study for slow solidification had been done long-long time ago. 3.
Please define abbreviations for the first-time use. 4.
The authors need to clarify the statement "the research on the homogeneous nucleation mechanism of Al-Cu binary alloy is still inconclusive." 5.
The study was focused on the nucleation in a layer structure. The authors need to provide the boundary conditions along the thickness direction. Note that the periodic condition is no good along the thickness direction. 6.
According to Fig. 3, there exists an incubation period. The authors please discuss the temperature effect on the incubation period. 7.
On Fig. 5a, pleas use the density of grains? Otherwise, the figure is meaningless. 9.
On Fig. 5b, please compare the results to the CNT theory. 10.
The authors please provide the geometry of the nuclei. Are the nuclei resembled to sphere? If not, Eqs. (2) and (40 need to be modified.

11.
Eq. (6) should be D^n-D_0^n=kt with n being 2 or 3. The numerical values 0f 1/0.05 is too large. The authors need to check their calculations.

Decision letter (RSOS-210501.R0)
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Dear Dr Zhan:
Title: Research on homogeneous nucleation and microstructure evolution of aluminum alloy melt Manuscript ID: RSOS-210501 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The editor assigned to your manuscript has now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Subject Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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1. The authors need to discuss more on content of two growth modes for elaborating nucleus growth stage.
2. The terminologies like FCC, HCP need to be written in full form and should be incorporated in the manuscript body.
3. The authors should include more references with respect to isothermal solidification, and periodic stacking. 4. The authors should write melting temperature in Kelvin (K) and not to use Tm in the abstract. If the authors would like to use Tm in the abstract, then Tm meaning needs to be mentioned, so that it will clear to the large readers.
5. The authors need to incorporate more discussion points with respect to Fig.11, for crystal nucleus growth and grain coarsening 5.
Reviewer: 2 Comments to the Author(s) This is a paper focusing on the MD simulation of the homogeneous nucleation of growth of crystal in a film structure during solidification. The authors observed the presence of both FCC and HCP phases and analyzed the nucleation rate and growth behavior. They compared the simulation results with the classical nucleation theory and the crystal growth theory.
The following is a list of comments.
1. Can the authors provide the fraction of Cu in the grains/nuclei? 2. The statement "… mainly focused on the analysis of the solidification structure under rapid solidification conditions..." is dubious, since the phase diagram is based on equilibrium state. The study for slow solidification had been done long-long time ago. 3. Please define abbreviations for the first-time use. 4. The authors need to clarify the statement "the research on the homogeneous nucleation mechanism of Al-Cu binary alloy is still inconclusive." 5. The study was focused on the nucleation in a layer structure. The authors need to provide the boundary conditions along the thickness direction. Note that the periodic condition is no good along the thickness direction. 6. According to Fig. 3, there exists an incubation period. The authors please discuss the temperature effect on the incubation period. 7. Please add scale to Figs. 3 and 4. 8. On Fig. 5a, pleas use the density of grains? Otherwise, the figure is meaningless. 9. On Fig. 5b, please compare the results to the CNT theory. 10. The authors please provide the geometry of the nuclei. Are the nuclei resembled to sphere? If not, Eqs. (2) and (40 need to be modified. 11. Eq. (6) should be D^n-D_0^n=kt with n being 2 or 3. The numerical values 0f 1/0.05 is too large. The authors need to check their calculations.

Decision letter (RSOS-210501.R1)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear Dr Zhan:
Title: Research on homogeneous nucleation and microstructure evolution of aluminum alloy melt Manuscript ID: RSOS-210501.R1 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.
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Thank you for your fine contribution. On behalf of the Editors of Royal Society Open Science and the Royal Society of Chemistry, I look forward to your continued contributions to the Journal. Thank you very much for your letter and for the reviewers' comments concerning our manuscript entitled "Research on homogeneous nucleation and microstructure evolution of aluminum alloy melt" (ID: RSOS-210501). Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied the valuable comments from you and reviewers carefully, and tried our best to revise the manuscript. Revised portion are marked in red in the paper. The main corrections in the paper and the responds to the reviewer's comments are as flowing: Responds to the reviewer's comments:

Reviewer 1
Thank you very much for your evaluation of our paper and your valuable comments! We have carefully studied your comments and answered your comments. Hope you are satisfied with our responses! Comment 1: The authors need to discuss more on content of two growth modes for elaborating nucleus growth stage.

Response:
We are very grateful to you for your valuable suggestions. We have conducted an in-depth analysis of the nucleus growth methods, and further discussed the reasons for the two growth methods.(For details, see the red part in the first paragraph of section 4.4.)

Comment 2:
The terminologies like FCC, HCP need to be written in full form and should be incorporated in the manuscript body.
Response: Thank you for your instructive suggestions. We have adopted the full form in the manuscript body. Comment 4: The authors should write melting temperature in Kelvin (K) and not to use Tm in the abstract.
If the authors would like to use Tm in the abstract, then Tm meaning needs to be mentioned, so that it will clear to the large readers.
Response: Thank you for your valuable comment. We have added an explanation of Tm in the abstract.

Comment 5:
The authors need to incorporate more discussion points with respect to Fig.11, for crystal nucleus growth and grain coarsening.

Response:
We are very grateful to you for your valuable suggestions. We have already conducted an in-depth discussion on Fig.11. For example, in the process of grain growth, amorphous atoms are gradually replaced by FCC and HCP, which increases the degree of crystallization.

Reviewer 2
Thank you very much for your evaluation of our paper and your valuable comments! We have carefully studied your comments and answered your comments. Hope you are satisfied with our responses! Comment 1: Can the authors provide the fraction of Cu in the grains/nuclei? Response: Thank you for your instructive suggestion. We have provided in the manuscript the distribution of Cu inside the grains and the percentage of Cu change with time at the grain boundaries during solidification at different isothermal temperatures. (See section 4.6 for details) Comment 2: The statement "… mainly focused on the analysis of the solidification structure under rapid solidification conditions..." is dubious, since the phase diagram is based on equilibrium state. The study for slow solidification had been done long-long time ago.
Response: Thank you for your careful reading of our manuscript. For this sentence, we may not be accurate enough in expression. We have revised this sentence in the manuscript to "So far, experimental studies on the nucleation and growth behavior of aluminum alloys have mainly adopted optical microscopy (OM), scanning morphology characterization, electron microscopy (SEM) and in-situ transmission electron