Prenucleation at the Liquid/Substrate Interface: An Overview

Round 1

Reviewer 1 Report

In the present paper the concept of prenucleation is discussed in terms of special structure formation in liquid, and this structure can be characterized as some layer structure with special crystallography or some 2D ordering in the layer.

It seems important to pay special attention to the condition of existence of such a structure. Either it is an equilibrium (quasi-eqilibrium) state for the system, or, it is dynamically stable structure, but in fact they are metastable or labile. In dependence on the answer of the question it seems reasonable to describe the general methods which allows to define the status, how to prove that our simulated system correspond to one or another  state.

It seems strange the choice of the system to demonstrate the effect of chemical interaction (chapter 3.4). If Al-Cd system is good example of positive energy of interaction, the choice of W and Ag seems not the best. Such an elements as Ti, Zr etc. corresponds to higher energy of interaction with Al and can demonstrate more strong effects. As it was shown in chapter 3.7 such systems were under investigations by different authors. If there is any data for prenucleation (or ordering) in these systems? Can this results be used in discussion in term of chemical interaction effect (chapter 3.4).

The background for discussion in terms of interfacial energy (chapter 4.3) is not described. Nowhere before we can see the results about interfacial energy calculation/estimation, but they appear in discussion, and it is looking strange. Without some results this discussion looks weak.

Author Response

We would like to thank all the reviewers for their constructive comments and helpful suggestions. Here is a summary of our response to their comments. 

Reviewer 1:

• “It seems important to pay special attention to the condition of existence of such a structure. Either it is an equilibrium (quasi-eqilibrium) state for the system, or, it is dynamically stable structure, but in fact they are metastable or labile. In dependence on the answer of the question it seems reasonable to describe the general methods which allows to define the status, how to prove that our simulated system correspond to one or another state.”

Response: Prenucleation describes the phenomenon of substrate-induced atomic ordering in the liquid adjacent to the liquid/substrate interface at temperatures above the nucleation temperature. The energy changes due to the formation of such atomic ordering is compensated by the reduction of interfacial energy of the liquid/substrate interface. Therefore, it is a dynamically stable structure at the liquid/substrate interface. It is important to realise that such substrate-induced atomic ordering at the liquid/substrate interface is not a bulk phase, but a local atomic arrangement. The interfacial ordered structure can only become metastable if it is a bulk solid (a new phase) at temperatures above the liquidus.

In this manuscript, we have described the general methods used in our research, including: local bond-order analysis, time-average atomic positions, atomic layering and in-plane atomic ordering, as summarized in Section 3.1. With all these methods combined, we believe that we can define adequately the status of the atoms at the interface.

• “It seems strange the choice of the system to demonstrate the effect of chemical interaction (chapter 3.4). If Al-Cd system is good example of positive energy of interaction, the choice of W and Ag seems not the best. Such an elements as Ti, Zr etc. corresponds to higher energy of interaction with Al and can demonstrate more strong effects. As it was shown in chapter 3.7 such systems were under investigations by different authors. If there is any data for prenucleation (or ordering) in these systems? Can these results be used in discussion in term of chemical interaction effect (chapter 3.4).”

Response: For an atomically flat substrate surface, prenucleation strongly depends on two factors: the lattice misfit between the solid and the substrate (the structural effect) and the chemical interaction between the liquid and the substrate (the chemical effect). In order to assess the effect of chemical interaction alone we need to keep the structural effect constant by choosing systems with smallest misfit. This is why we chose W and Ag, rather than Ti and Zr to demonstrate the chemical effect on prenucleation. However, we do agree with Reviewer 1 that Ti and Zr would have stronger chemical interaction with liquid Al than W and Ag. This has been reflected in the revised manuscript marked as red in Section 3.4.

• “The background for discussion in terms of interfacial energy (chapter 4.3) is not described. Nowhere before we can see the results about interfacial energy calculation/estimation, but they appear in discussion, and it is looking strange. Without some results this discussion looks weak”

Response: The background for discussion in terms of interfacial energy (Section 4.3) was provided in the Section 5.3 in Ref. [89] and Section 5.1 in Ref. [90], both of which are our papers published in the same Special Issue of metals. We will refer to Refs. [89,90] at the beginning of Section 4.3 in the revised manuscript to provide a better background for the readers to understand.

Reviewer 2 Report

The authors give a very good summary of the current state of the art of the nucleation problem (both computational and experimental), followed by the review of their own computational results. 

This is clearly a difficult problem and an open research field.   Authors propositions (pre-nucleation) is one of the competing approaches, so that the paper certainly deserves publication.  It is a significant contribution and is well written.

I can submit one suggestion for authors' consideration.  Part of the difficulty may be the ambiguity in mathematical description of interfaces, specifically the description of partially ordered states.  Perhaps authors would care to comment on the mathematical state of the art (in the Discussion section)? 

Author Response

Response to Reviewers 2’ comments

We would like to thank all the reviewers for their constructive comments and helpful suggestions. Here is a summary of our response to their comments.

 Reviewer 2:

• “I can submit one suggestion for authors' consideration.  Part of the difficulty may be the ambiguity in mathematical description of interfaces, specifically the description of partially ordered states.  Perhaps authors would care to comment on the mathematical state of the art (in the Discussion section)?”

Response: We agree with Reviewer 2 that “the difficulty may be the ambiguity in mathematical description of interfaces”. Currently, in the literature there is no mathematical description of atomic ordering at the liquid/substrate interface. In another paper in this special issue (Ref. 89) we used the fraction of solid atoms, either in the entire interface or in each atomic layer within the interface, to quantify the atomic ordering in the interface. This has allowed us to quantify the evolution of atomic ordering during both prenucleation and nucleation processes. More importantly, this has led to a mathematical description of various interfacial energies as a function of atomic ordering within the interfacial regions, the details of which will be described in another paper (to be submitted).