Modification of Cantor High Entropy Alloy by the Addition of Mo and Nb: Microstructure Evaluation, Nanoindentation-Based Mechanical Properties, and Sliding Wear Response Assessment

Round 1

Reviewer 1 Report

This investigation presents the modification of cantor alloy by the introduction of Mo and Nb elements. The microstructure, creep properties by nanoindentation examination, and sliding wear properties were studied systematically. It is recommended to address the following comments to further consider its publication.

• Introduction: The introduction is too verbose and should be shortened. Although the excellent properties of Cantor alloys, there are opportunities for optimization and further research. The authors should throw light on the effects of alloying elements on microstructure and properties in previous works. Cite the relevant works briefly and explain clearly what has been done and what should be further investigated, and describe logically the prior motivation of this work.
• Phase prediction parametric models: It is unnecessary for this context to be separated, some phase prediction models should be briefly expanded in the results and discussion.
• Experimental procedure: Why not directly use high-purity bulk materials by the arc-melting method instead of metallic powders to smelt after tableting? The microstructure results show the existence of the oxide phase, please explain how oxygen was introduced into the specimen during preparation and give the oxygen actual content. What is the effect of oxide on relative microstructure and properties? Please comment about these issues.
• Give the actual compositions of the alloys.
• In figure 1, different phases were identified in the XRD pattern. However, these XRD results should be labeled by different crystal planes and should be combined into one figure, the subfigures are too large. Please explain the correctness of the FCC and BCC phase diffraction angle identification in detail.
• The lack of characterization and analysis of dislocations in nanoindentation creep process studies to better elucidate the creep deformation mechanism.
• Figures and tables should be presented independently. For example, the figure 3 and figure 4, the EDS mapping figures should be cropped and beautified in the paper instead of the original results. Meanwhile, the labels in the figures are too small and are difficult to see labels. Thus, please these figures need to be revised.
• The format of figures and tables need to be unified, respectively.
• These pictures are too simple. Figure 6 and figure 7, from figure 8 to figure 11 should be presented in the form of combined graphs for better discussion and analysis. Thus, these figures need to be revised,
• In references: The reference format for citation is not uniform, please revise according to the citation requirements to be resubmitted.

Author Response

In the attached file, you can find detailed response to your comments 

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper investigates the effect of Mo and Nb additions on the classic Cantor (FeCoCrMnNi) isoatomic high entropy alloy, the mechanical properties and microstructure evolutions are studied thoroughly. Due to the additions, the modulus of elasticity and microhardness are increased. The data and results are detailed and reliable. Thus, this paper is suggested to be published in Journal of Alloy. However, there some minor revisions are better to be done as follows:

1. The experimental procedure should be clearer, like the temperature of the melting, the pressure of a hydraulic press and so on.
2. What is the sample size of dynamic indentation tests and wear test?
3. For the table, it is better to use three lines table.
4. The figure descriptions should be checked.
5. The equation number should be in order.
6. The conclusions are better not to be separated into sections.

Author Response

in the attached file, you can find detailed response to your comments 

Author Response File: Author Response.pdf

Reviewer 3 Report

In the manuscript, the authors studied the effects of Nb and Mo additions on the microstructure, mechanical properties of Cantor alloys. The results show that Mo and Nb additions favour the formation of secondary phases, leading to an increase the hardness, modulus, creep resistance and sliding wear resistance relative to the classic Cantor alloy. Overall this manuscript is very good written and the topic matches well the scope and aims of the journal of Alloys, MPDI. There are a few points that the authors may want to address to make the manuscript easier to follow,

Major concerns:

1) In some statistics datasets, the standard deviations are missing. Figure 4, the authors reported the chemical position using EDX point analysis. Please add the errors or standard deviations to show the chemical variations. In table 3,  the values of stress component, strain sensitivity etc are averaged from the 10 indent datasets. Please also include the errors or standard deviations. This would be helpful to show the variations within the materials.

2) Section 4.3.1 Calculation frame. The authors have provided a detailed procedure to calculate important parameters for creep analysis using nanoindentation. However, in the derivations, some parameter notations are causing confusion. For example, is the λ in equation (6), (7) and (9) stands for the same parameter?  

3) Conclusions, 5.2 Nanoindentation, the authors concluded the hardness increase for alloys with Mo and Nb additions is due to the presence of new hard phases. As the authors reported in the previous sections, Mo and Nb can also lead to increased lattice distortions. The lattice distortions can play solid-solution like strengthening mechanisms.  So the role of lattice distortions may also need to be included in the conclusion part.

4) Since the Cantor alloy and those with Mo and Nb additions are relatively new. Could the author provide some comments on the comparison of creep resistance between the cantor alloys and tradition alloys? A good reference can be found in Fig 9 in (https://doi.org/10.1016/j.jallcom.2021.161038 ).

5) Could the author provide some EBSD data. The EBSD data will be helpful to understand the phase distributions in Figure 1 and Figure 2.   

Minors

1) Page 5, Line 200. Ti getter?

2) Figure 1, axis titles and unit need to be included.

3) It’s interesting to see the as-cast materials did not show any dendritic structures during the solidification process, Figure 2. According to the descriptions in the experimental section, the authors get the homogeneity through multiple flipping and remelting process.  Could the authors provide more details (e.g. temperature, time) on the flipping and remelting process? This can help other researchers to reproduce materials of similar microstructure.

4) Table 1, could the authors add a few sentences to induct the color legend for ΔHf. The size of table 1 should be adjusted to fit the page size.

5) I will suggest the authors to use the more standard decimal point “.” instead of “,” .

6) Figure 6 and 7, which sample(s) is these data from?

7) Figure 14 and 15, the EDX spectrum are difficult to read. The authors may want to crop the spectrum and only show the energy range of 0-10 keV.

Author Response

In the attached file, you can find detailed response to your comments 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

It is incorrect to place the sample too long time before the experiment to cause oxides. Before the XRD test, the samples should be re-polished to eliminate the surface effect, and the authors should re-test the XRD to illustrate the issue.

Author Response

Dear reviewer, the authors do recognise that this is an importatn issue. As such in order to clarify the situation, a sample was metallographically re - processed and a standard XRD run was conducted, under the same testing conditions as in the previous cases. It was observed that the oxide phases were eliminated. The Figure 1 was accordingly modified and attached in the new form