Optimizing Boron Content for Controlled Boride Formation in Fe–Ni–Cr–Cu–Si–B–C Alloy: A CALPHAD-Guided Experimental Study

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents a combined CALPHAD–experimental investigation of the effect of boron content on phase formation in a Fe–Ni–Cr–Cu–Si–B–C alloy system. The topic is relevant to the design of wear-resistant Fe-based alloys and coatings; however, the manuscript in its current form suffers from significant scientific, methodological, and presentation shortcomings that limit its contribution. While the authors demonstrate the use of thermodynamic calculations together with microstructural characterization, the novelty is insufficiently established, the experimental validation remains largely qualitative, and several conclusions are not adequately supported by quantitative evidence. Therefore, I recommend Major Revision before the manuscript can be considered for publication. Below are detailed suggestions for enhancing the clarity, impact, and overall quality of the manuscript.

• What is the principal scientific novelty of this study beyond the application of established CALPHAD modeling and conventional experimental characterization techniques to a specific Fe–Ni–Cr–Cu–Si–B–C alloy system?
• How does the present work advance the current understanding of boride formation mechanisms in Fe-based multicomponent alloys compared with previously published studies?
• Why is the investigated material discussed within the framework of high-entropy alloys (HEAs)? Can the authors provide calculations of configurational entropy, atomic size mismatch, and other relevant HEA parameters to justify this classification?
• What specific knowledge gap in the existing literature does this study address?
• Why was boron selected as the only optimization variable while the concentrations of the other alloying elements remained unchanged?
• Were the CALPHAD calculations performed under equilibrium or non-equilibrium conditions? If non-equilibrium conditions are relevant to the actual processing route, how were they considered?
• Why was the TTFe database selected for the thermodynamic calculations, and what are its limitations for predicting boride-rich multicomponent systems?
• Were any sensitivity analyses performed to evaluate the reliability of the thermodynamic predictions?
• What objective criterion was used to conclude that 4 wt.% boron is the optimal composition?
• Is the optimization based solely on maximizing Fe₂B content, or were other factors such as hardness, toughness, adhesion, and wear resistance considered?
• How would boron concentrations above 5 wt.% or below 2 wt.% affect phase evolution and alloy performance?
• What is the scientific basis for selecting 638 °C as the optimal processing temperature?
• Was the predicted optimum temperature experimentally validated through heat-treatment experiments?
• How sensitive are the predicted phase fractions to small variations in temperature around 638 °C?
• Can the authors quantitatively compare the CALPHAD-predicted phase fractions with experimentally measured phase fractions?
• What is the percentage error between predicted and experimentally observed phase compositions?
• How were the phases identified in the SEM micrographs shown in Figure 5?
• Can the authors provide point EDS/WDS analyses for the individual phases identified as borides, carbides, and silicides?
• What evidence supports the assignment of the gray regions to borides and the dark regions to carbides?
• Were higher-magnification SEM images obtained to verify the morphology and distribution of the secondary phases?
• Given the significant peak overlap in the XRD pattern, why was Rietveld refinement not performed?
• Can the authors provide quantitative phase analysis from the XRD data?
• What direct experimental evidence confirms the formation of Fe₂B, which is the primary focus of this study?
• How do the experimentally identified phases compare with the phases predicted by CALPHAD calculations?
• Are there any phases predicted thermodynamically that were not detected experimentally? If so, how can this discrepancy be explained?
• Conversely, were any experimentally observed phases absent from the thermodynamic predictions?
• What is the relationship between the calculated Fe₂B fraction and the measured hardness values?
• Which specific phases contribute most significantly to the observed hardness increase?
• Can the authors quantitatively correlate hardness evolution with phase composition?
• How many hardness measurements were performed, and what were the corresponding standard deviations?
• Were the reported adhesion measurements statistically analyzed?
• What was the failure mode during adhesion testing: cohesive failure within the coating, adhesive failure at the interface, or substrate failure?
• How does the measured adhesion strength compare with that of commercial self-fluxing coatings?
• Since the alloy is proposed for wear-resistant applications, why were wear tests not conducted?
• How can the claimed improvement in wear resistance be verified without tribological testing?
• Were coating porosity, cracks, oxide inclusions, or other defects quantified?
• What effect do such defects have on the measured mechanical properties?
• How reproducible are the coating deposition and characterization results?
• How many independent samples were fabricated and characterized?
• What are the limitations of the proposed CALPHAD–experimental design methodology?
• Can the proposed alloy-design strategy be extended to other Fe-based multicomponent alloy systems?
• How does the performance of the developed alloy compare with commercially available Fe-based and Ni-based wear-resistant coatings?
• What are the practical industrial advantages of the proposed alloy relative to existing coating materials?
• What future work is necessary to validate the alloy for real engineering applications?
• Can the authors provide a more critical discussion of the limitations, uncertainties, and assumptions associated with both the thermodynamic calculations and the experimental validation?
• How does the present study contribute to the broader field of computational alloy design beyond this specific alloy composition?
• Can the authors clarify whether the observed microstructure corresponds to an equilibrium condition predicted by CALPHAD or to a kinetically constrained non-equilibrium microstructure resulting from the actual processing route?
• In view of the largely qualitative nature of the experimental validation, how do the authors substantiate the claim that the CALPHAD predictions have been successfully validated?
• While the manuscript is generally well-written, there are areas where sentence structure could be improved for clarity. Thus, proofreading is necessary to correct major grammatical errors and to streamline complex sentences would improve readability.

 

Comments on the Quality of English Language

While the manuscript is generally well-written, there are areas where sentence structure could be improved for clarity. Thus, proofreading is necessary to correct major grammatical errors and to streamline complex sentences would improve readability.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The present manuscript describes a combined approach using CALPHAD thermodynamic modelling and experimental characterization to investigate the effect of boron content on the formation of boride phases in the Fe–Ni–Cr–Cu–Si–B–C alloy system. The topic is timely and relevant to the development of wear-resistant coatings and multicomponent Fe-based alloys. The authors identify an optimal boron content (~4 wt.%) and an optimal processing temperature (638 °C) for achieving a stable multiphase microstructure with a significant fraction of the Fe₂B phase.

However, the optimal boron content is determined solely by CALPHAD calculations, whereas the experimental characterization is performed for only a single alloy composition. This limitation should be clearly stated in the Abstract. The current wording is somewhat misleading, as it may give the impression that several compositions were experimentally investigated.

The manuscript is not consistent in its terminology. The investigated material is referred to both as a high-entropy alloy (HEA) and as an Fe-based alloy, although the alloy contains a higher Cu content than Fe. The terminology should be unified throughout the manuscript, or the classification should be properly justified.

The description of the experimental methods is inconsistent. X-ray diffraction (XRD) is not mentioned in the Abstract despite being an important characterization technique. Furthermore, the Experimental section states that wavelength-dispersive spectroscopy (WDS) was employed, whereas the Results section refers to EDS data. This inconsistency should be resolved. In particular, the determination of boron and carbon by EDS is problematic due to the limited accuracy of this technique for light elements.

The Experimental section should specify the chemical form in which boron was introduced into the powder mixture. In addition, were the contributions of carbon and silicon originating from the liquid glass binder included in the nominal alloy composition and thermodynamic calculations?

Since oxygen was used as the spraying gas, was the presence of oxide phases investigated or detected in the deposited coatings?

In Figure 2, it should be more clearly emphasized that the presented phase fractions are CALPHAD predictions rather than experimentally verified results. In addition, presenting the results as four separate plots instead of two combined figures would considerably improve readability.

The prediction of three coexisting liquid phases is unusual and deserves a more detailed discussion. In this respect, experimental thermal analysis (DSC or DTA) would provide valuable validation of the calculated phase transformations and should be considered or at least discussed as a limitation of the present study.

The scale bars in the SEM micrographs should be enlarged or otherwise made more visible.

The elemental maps appear blurred. Presenting them in a monochromatic colour scale may improve their clarity and facilitate interpretation.

Figure 8 requires a more detailed description. What do the individual data points and their error bars represent? Furthermore, does connecting the points with lines have any physical meaning, or would another type of graphical representation be more appropriate?

Finally, the presence and morphology of fine precipitates cannot be conclusively demonstrated by SEM alone. If the authors wish to discuss nanoscale precipitates, additional characterization by transmission electron microscopy (TEM) or another high-resolution technique (e.g., small-angle scattering) would be necessary.

The claimed agreement between CALPHAD predictions and experimental observations should be moderated. The presented SEM, EDS/WDS, and XRD results provide only qualitative evidence of the predicted phases. Without quantitative phase analysis (e.g., Rietveld refinement of the XRD data), it is not possible to conclude that the thermodynamic predictions have been fully validated.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Optimizing Boron Content for Controlled Fe₂B Formation in 2 Fe–Ni–Cr–Cu–Si–B–C Alloy: A Combined CALPHAD and Experimental Study is very interestig paper! Minor improvements are required.

Line 14, 15: A combined thermodynamic and experimental study of the Fe–Ni–Cr–Cu–Si–B–C alloy  system was conducted to evaluate the effect of boron on phase composition and microstructural evolution (in what temperature interval?)

Line 18: Vertical phase-diagram sections were used to assess phase stability over a wide temperature range (such as…)

Line 124: The prepared powder was deposited onto steel 45 substrates using gas-flame spraying (which type of powder?)

Line 170: The effect of boron content on phase composition was investigated (in what range?)

Line 206, 207: Conversely, the reduction of boride content at elevated temperatures (in what range?) indicates a transition toward a  more homogeneous matrix-dominated structure.

Line 263: The re-precipitation of borides at this temperature leads to the formation of a stable multiphase structure (what is chemical composition of this multiphase structure?)

Conclusion:

Line 397, 398: The results highlight the critical role of boron in tailoring phase composition  and microstructure, indicating that the studied alloy system (Fe–Ni–Cr–Cu–Si–B–C??) is a promising candidate for wear-resistant applications.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The current form of the manuscript is accepted for publication. The authors have addressed all my comments and made a significant contribution to the field, and their findings will significantly interest the readers of metals Journal.

Comments for author File: Comments.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The revised manuscript has been significantly improved, and I appreciate the authors' efforts in addressing the reviewers' comments. The Abstract is now clearer regarding the experimentally investigated alloy, the terminology has been largely unified, and the description of the experimental methods has been corrected. I also appreciate that the authors moderated their statements concerning the agreement between CALPHAD predictions and experimental observations.