Atomistic Simulations and Experimental Investigations of the Diffusion Behavior of Steel/ZCuPb20Sn5 Bimetals

Round 1

Reviewer 1 Report

The manuscript “Atomistic simulations and experimental investigations of the diffusion behavior of steel/ZCuPb20Sn5 bimetals” presents an interesting study combining experimental and theoretical methodologies to investigate microscopic properties in bimetallic composites, more specifically in the interface between two metals. The manuscript is well structured, but some language improvement is needed, so the authors are asked to seek help from a native speaker.

Considering that the experimental methods are outside my field of expertise, I would only comment the simulation methodology and results.

The methods seem to be adequate, but some details are lacking, such as the chosen ensemble, the eventual presence of barostats, thermostats or even the mention of the time length of the simulation.

The authors should explain in more detail the puzzling result that Cu atoms have higher diffusion coefficient than Fe, but diffuse less in the Fe bulk.

Are the mean square displacements calculated as an ensemble average over all atoms or over the interface atoms?

Regarding the radial distribution function, it would be interesting to investigate this quantity at different simulation times, showing the evolution of the RDF at the interface as a function of time.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

In this work the bimetallic composite carbon steel/ZCuPb20Sn5 is investigated regarding its diffusion behavior.

Noticeably, the manuscript reports the result of authors effort in combining theoretical (atomistic simulations) as well as experimental studies: this is always appreciated and provides a deeper view into the system under investigation.

This combination of the two approaches results in a complete study of the system, including molecular dynamic simulation together with the casting process, the microstructural and microhardness characterization as well as the study of the mechanical properties.

A brief discussion provides likely rationalization of the dominant diffusion mechanism.

I see 5 weaknesses.

• abstract: it is merely a list of things done by the authors: please build a story and synthesize it. writing a good abstract is not easy, this effort must be taken by the authors
• conclusions: points are effective but very inelegant: keep them but please rewrite the full story in single and brief conclusions section
• chapter 2.4 “Microstructural characterization” vs 3.3 “Microstructure characterization”: almost identical titles for different sections create confusion…
• use of English language here and there sounds quite poor: please improve it
• the importance of this study in the field and the potential impact of the result are poorly or barely described along the manuscript: authors should take the effort to valorize and clearly communicate the relevance of their results

Upon fixing the these five points, the manuscript can be suitable for publication in Coatings.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, experiments and semi-empirical molecular dynamics simulations are employed to investigate the properties of steel/copper interfaces. The ms is quite well written and presented, and the results are of interest to the readers of coatings. Unfortunately, there are major flaws that prevent me from recommending publications.

While I am not able to judge on the experimental part of the work, the theory and modeling part contains several problems. The authors should carefully address all issues listed below. I may reconsider after proper revision.

Major problems

1. This statement is totally misleading:

“To insure the misfit of the interface as close as possible to the experimental ratio of the lattice constants, the number of atoms in interface copper layer was chosen as 30×15×4=1800 82 and in one iron layer as 38×19×2=1444, since 38/30=19/15=1.27”

The lattice parameter of copper and iron remains the same irrespective of how many times you replicate your cells! The misfit is given by the ratio of the lattice constants only, NOT by the number of replicated cells!

2. The simulation methodology is missing several information. The section method should contain all information relevant to be able to reproduce the simulations. Which temperatures were used to the simulations? How many timesteps at each temperature? Which thermostat or, more in general, which ensembles have been employed to sample the configurational space?

3. The authors incorrectly say that the EAM potential was used. The potential used is, in fact, the modified embedded atom method (MEAM), as stated in the original work of Lee (ref. 36 in ms). Please correct.

4. What structures were used for Cu and Fe? I assume that Cu is fcc. How about Fe? Have the authors considered that Fe undergoes various crystal lattice and magnetic transitions with temperature? fcc Fe is paramagnetic (the authors use a simulation temperature of ≈1500 K, which corresponds to this phase in the Fe phase diagram). The parameters of the MEAM model potential in Lee’s paper have been primarily fitted to the properties of bcc ferromagnetic Fe at 0 kelvin. The authors should not expect that their MEAM model is perfectly reliable at finite temperatures and with different magnetic state. There is even the risk that their structure is dynamically unstable in the fcc phase due to model forces being fitted to the ferromagnetic state (see for example [1]).

5. Related to my previous comment, the authors need to inform readers that MD based on empirical model potential is not always reliable. I suggest adding the following in the methods section (after the authors have introduced the MEAM model):

“Although the MEAM model provides realistic description of the dynamics of solid surfaces [2, 3] and bulk [4, 5], the predicted properties need to be verified by first-principles MD simulations (for relatively small supercell sizes) at different temperatures [6-8]”

[1] F. Kormann, A. Dick, B. Grabowski, T. Hickel, J. Neugebauer, Atomic forces at finite magnetic temperatures: Phonons in paramagnetic iron, Physical Review B 85 (2012).

[2] D. Edström et al., Effects of incident N atom kinetic energy on TiN/TiN (001) film growth dynamics: A molecular dynamics investigation, Journal of Applied Physics 121 (2017) 025302.

[3] D. Edstrom et al., Ti and N adatom descent pathways to the terrace from atop two-dimensional TiN/TiN(001) islands, Thin Solid Films 558 (2014) 37.

[4] W.-S. Ko, B. Grabowski, J. Neugebauer, Development and application of a Ni-Ti interatomic potential with high predictive accuracy of the martensitic phase transition, Physical Review B 92 (2015).

[5] S.B. Maisel, W.S. Ko, J.L. Zhang, B. Grabowski, J. Neugebauer, Thermomechanical response of NiTi shape-memory nanoprecipitates in TiV alloys, Physical Review Materials 1 (2017).

[6] D.G. Sangiovanni, A.B. Mei, D. Edström, L. Hultman, V. Chirita, I. Petrov, J.E. Greene, Effects of surface vibrations on interlayer mass transport: Ab initio molecular dynamics investigation of Ti adatom descent pathways and rates from TiN/TiN (001) islands, Physical Review B 97 (2018) 035406.

[7] D.G. Sangiovanni, D. Edström, L. Hultman, I. Petrov, J.E. Greene, V. Chirita, Ti adatom diffusion on TiN(001): Ab initio and classical molecular dynamics simulations, Surface Science 627 (2014) 34.

[8] G.A. Almyras et al., Semi-Empirical Force-Field Model for the Ti1-xAlxN (0 ≤ x ≤ 1) System, Materials (Basel, Switzerland) 12 (2019) 215.

 

Minor problems

a. The method is named “molecular dynamics”, not “molecular dynamic”. Please correct throughout the text.

b. Line 52: the sentence is missing a verb “Fe-Cu bimetals always used in high voltage and power complex environments”

c. odd choice to have “simulation methodology” (section 2.1) as subsection of “experimental procedures (Section 2).”

I suggest changing the header of Section 2 to something general like “Methods”

d. Line 75-76: “…the inter-atomic potentials in describing the potential energy of atoms play a pivotal function, an embedded …” Add a full stop between “function” and “an embedded”.

e. Line 162 reads: “In addition, the distance of Fe atoms diffuse into Cu bulk is larger than that of Cu atoms diffuse into Fe bulk”. Can you please give some values in the text? How much is this distance and during how long time were these distances reached?

f. In LAMMPS, there are different implementation of the MEAM potential. Not all of them correspond to Lee’s implementation. The authors should specify which “ibar” parameters (which picks a specific Gamma function of the background density) have been used for Cu and Fe in the library file, and which “ialloy” parameter was used in the MEAM interaction (Cu-Fe) parameter file. If I remember correctly, Lee’s implementation of the MEAM formalism is based on ibar=3 and ialloy=2.

g. Do the authors judge that a timestep integration of 2 fs during MD is sufficiently short? Have they checked that the total energy during NVE dynamics is conserved with a timestep of 2 fs for the duration of, let’s say, at least 1 – 2 ns?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

The ms has sufficiently improved during revision.