Effect of Heat Treatment Temperature on the Microstructure and Mechanical Properties of Cu_0.3Cr₂Fe₂Ni₃Mn₂Nb_x High-Entropy Alloys

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In my opinion, the article is logically and empirically well designed, the experimental study and the findings obtained are described and explained in detail. However, there are some deficiencies, especially in the "Introduction" section, which I have mentioned below, I think these sections should be revised again.

1)The introduction presents a disparate array of studies and findings, lacking clear transitions. This makes it challenging to grasp the overall context and purpose of the introduction.

2) Before delving into specific studies and findings, start with a broader overview of high entropy alloys and their importance.

3) State the importance of Cu0.3Cr2Fe2Ni3Mn2Nbx alloy and where it is used.

4)It is important to ensure that the introduction is structured in a way that effectively highlights the significance and relevance of the current research, rather than merely providing an exhaustive review of related studies.

5) Check the sentence in line 81 "However, studies on the stability of Laves phase after high temperature heat treatment." which should be corrected.

6) In Figure 1 use the Theta symbol 

 

Comments on the Quality of English Language

There are some simple spelling mistakes, you should read the article thoroughly and correct them.

Author Response

Coatings

Editorial Office

Dear editors,

Thanks for your great efforts and reviewers’ thoughtful comments, all of which have been incorporated into the revised manuscript. Detailed descriptions are listed as follows and marked by highlighted in the revised manuscript (with marked).

Reviewer 1

Comment 1: In my opinion, the article is logically and empirically well designed, the experimental study and the findings obtained are described and explained in detail. However, there are some deficiencies, especially in the "Introduction" section, which I have mentioned below, I think these sections should be revised again. 1) The introduction presents a disparate array of studies and findings, lacking clear transitions. This makes it challenging to grasp the overall context and purpose of the introduction. 2) Before delving into specific studies and findings, start with a broader overview of high entropy alloys and their importance. 3) State the importance of Cu_0.3Cr₂Fe₂Ni₃Mn₂Nb_x alloy and where it is used. 4) It is important to ensure that the introduction is structured in a way that effectively highlights the significance and relevance of the current research, rather than merely providing an exhaustive review of related studies. 5) Check the sentence in line 81 "However, studies on the stability of Laves phase after high temperature heat treatment." which should be corrected.

Response 1: Thank you very much for the valuable revision suggestions provided by the reviewer, which will greatly promote the scientific and logical nature of this article. According to the reviewer's comments, the five main questions are focused on the introduction section. Therefore, based on these five comments, we have made key revisions to the introduction section. The modified content is as follows. At the same time, relevant references were supplemented and adjusted.

Yeh [1] and Cantor [2] first proposed the concept of high entropy alloy (HEA) in 2004. The so-called high entropy alloy refers to a material composed of four or more metal elements, with an atomic percentage between 5% and 35%, and contains simple solid solution phases. Due to the mixing of multiple main elements, HEAs exhibit typical high entropy effects, lattice distortion effects, slow diffusion effects, and cocktail effects [3]. These four typical effects result in HEAs exhibiting typical disordered FCC and BCC phases, ordered FCC and BCC phases, and HCP phases in terms of microstructure [4-12]. In addition, due to the presence of special elements (such as Ti, Nb, C, Si, and so on.), HEAs often form crystal structures dominated by solid solution phases, with Laves, s, h, R, and m phases as secondary topological arrangement phases [13-17]. The rich and diverse crystal structure types in HEAs give them performance characteristics comparable to traditional alloys, such as high hardness, high strength, good plasticity, wear resistance, corrosion resistance, oxidation resistance, and good soft magnetic properties. Therefore, they have good application prospects in both structural and functional materials.

The CuCr₂Fe₂Ni₂Mn₂ alloy with a single FCC structure exhibited better corrosion resistance than traditional 304S stainless steel in both 1M H₂SO₄ solution and 30 wt.% HNO₃ solution [18]. To further improve the corrosion resistance of the alloy, Ren et al. designed Nb_xCu_0.3Cr₂Fe₂Ni₃Mn₂ HEAs by reducing the Cu content and increasing the Ni content while introducing corrosion-resistant Nb element. The phase structure of the alloy changed from a single FCC phase to FCC+Laves phase, and the degree of Cu segregation was reduced. The corrosion resistance was significantly improved in 10 wt.% and 30 wt.% HNO₃ solutions [19]. However, the strength of Cu_0.3Cr₂Fe₂Ni₃Mn₂ HEA with a single FCC structure is relatively low, which limits its application. Therefore, how to improve the strength of this alloy is a valuable research work. Many researchers use the method of adding solid solution strengthening elements to improve the strength of a single FCC HEA. As the Ti content increased, the FCC phase content decreased and the BCC and HCP phase content increased in the CrFeNiNb_0.1Ti_x HEAs. The hardness of the alloy increased from 512 HV to 867 HV, and the yield strength of the alloy increased from 1250 MPa to 2223 MPa [20]. As the Al content increased, the phase structure of Al_xFeMnNiCrCu_0.5 HEAs changed from dual FCC phase to dual BCC phase, and the alloy hardness increased from 216 HV to 518 HV [21]. Qin et al. [22] added Nb element to CoCrCuFeNi HEA, forming FCC+Laves phase, which increased its compressive yield strength by 2.9 times. After introducing non-metallic elements Si and C into Fe_2.5CoNiCu HEA, the solid solution strengthening effect was increased, significantly improving the strength and hardness of the HEAs [23]. These studies indicate that introducing alloying elements with larger or smaller atomic radii into HEAs can lead to significant lattice distortion effects, resulting in stronger solid solution strengthening effects, which are beneficial for improving mechanical properties.

In addition, appropriate heat treatment of HEAs can induce phase transformation without changing the chemical composition of the alloy to improve its properties. At the same time, it can eliminate defects such as component segregation, cold cracking shrinkage, and large internal stress caused by the as cast process. Therefore, the influence of heat treatment technology on the microstructure and properties of HEAs has become one of the current research hotspots. Sun et al. [24] conducted long-term aging of Cr_0.8FeMn_1.3Ni_1.3 HEA at 300 °C, 500 °C, and 700 °C and found that after aging at 500 °C for 1500 h, the alloy underwent complex phase decomposition, producing a tetragonal σ-FeCr phase, L10 NiMn phase, and a small amount of Cr-rich BCC and Fe-rich FCC phases. After aging at 700 °C for 1500 h, the alloy precipitated a Cr-rich BCC phase, forming a dual phase microstructure, increasing the tensile strength, and decreasing ductility. The study by Yin et al. [25] on the high-temperature aging behavior of Fe₄₅Ni₂₅Cr₂₅Mo₅ HEA showed that the alloy exhibited a high aging hardening effect at temperatures of up to 900 °C. Moreover, owing to the precipitation of monoclinic Cr_5.5Mo_1.5Fe_6.5Ni_1.5 type σ phases, peak aging of 48 h was achieved at 900 °C, leading to a substantial improvement in compression performance. Chen et al. [26] found that as-cast Al_0.3CrFe_1.5MnNi_0.5 HEA was composed of BCC and FCC phases. After aging at 600 °C for 100 h, a hard intermetallic compound ρ phase (Cr₅Fe₆Mn₈ phase) precipitated in the alloy, achieving a high age hardening effect and microhardness of up to 850 HV. Ren et al.'s [27] aging study on CuCr₂Fe₂NiMn alloy showed that after aging at 800 °C for 12 h, the microhardness of the alloy increased from 334 HV in the as-cast state to 450 HV. After aging at 950 °C and 1100 °C for 12 h, the microhardness decreased to 180 HV. The age hardening of the alloy is attributed to the precipitation of hard intermetallic compound ρ phase from the metastable BCC phase at lower temperatures (600-800 °C), which significantly increases the hardness of the alloy. After further increasing the aging temperature to 950 °C, the ρ phase partially decomposes and completely decomposes at 1100 °C, resulting in age softening of the alloy. It can be seen that appropriate heat treatment temperature or holding time can produce the precipitation of second phases or intermetallic compounds, and can also promote tissue refinement, thereby improving the mechanical properties of HEAs. Therefore, in this work, different contents of Nb element were added to Cu_0.3Cr₂Fe₂Mn₂Ni₃ HEA, and Cu_0.3Cr₂Fe₂Mn₂Ni₃Nb_x HEAs were prepared by vacuum arc furnace. Further heat treatment was performed on Cu_0.3Cr₂Fe₂Mn₂Ni₃Nb_x HEAs at different temperatures to investigate the effect of heat treatment temperature on the microstructure and mechanical properties of the alloy system, providing experimental support and theoretical reference for the application of the alloy system in high-temperature structural materials.

 

Comment 2: In Figure 1 use the Theta symbol.

Response 2: According to Comment 2, the Theta symbol has been added to Figure 1.

 

Comment 3: There are some simple spelling mistakes, you should read the article thoroughly and correct them.

Response 2: According to Comment 3, we conducted a grammar check on the entire text and corrected relevant grammar errors.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper, the authors develop a five and six-component metal alloy that contains Cu, Cr, Ge, Ni, Mn and may or may not contain Nb. When the alloy does not contain Nb, it adopts the FCC phase. The addition of Nb converts into the leaves phase (in the form of rods/particles), which increases with the addition of Nb. Heat treatment converts the alloys into BCC phase, which starts to break down with the increase in temperature when Nb is present (Nb0.2 and Nb0.4). The latter two alloy compounds show the highest hardness. However, the harness of all materials deteriorates for temperatures above 800 °C. All compounds are characterised based on XRD and EDS.

Overall, the paper is well-structured. Methodologically, I do not have main concerns. However, the authors could have explained more carefully how the work fits into coatings research. MDPI has another journal, Alloys, where this paper would be more suitable. In the context of coatings, the authors need to explain and demonstrate the utility of this new 6-metal alloy as a functional coating.

In the introduction, the authors have reviewed research works that include alloys of multiple metals. On the one hand, it is understandable that as the alloy complexity increases, there may be an extension in its realm of functional properties. But with six metals present, the ally chemical space is very large and finding any good application may feel like serendipity. The literature has many papers where someone has definitively tried to explore some special combination, and thus it is not a strong reason why now trying this new combination of metals. The authors need to further justify their selection of these materials and explain how this research helps to navigate towards other alloys that may have improved properties or target custom applications.

The binary mixing enthalpies do not seem relevant to this work as the authors deal with multicomponent alloys. It would be better if they could ask someone to calculate a few supercells containing and reflecting on the present component mixture. 

Finally, the authors write "Data Availability Statement: All the raw/processed data required to reproduce these findings are available and can be requested from the authors." This is not a permanent solution. If the data is indeed accessible and there, please just upload it as a supporting information to this manuscript.

Comments on the Quality of English Language

Line 21: "strengthening of the Laves phase and the solid solution strengthening of the BCC phase" should be "strengthening of the Laves phase, and the solid solution strengthening of the BCC phase."

Line 73: "phase, produing an increase in its yield strength and fracture strength" should be "phase, producing an increase in its yield strength and fracture strength."

Line 210: "In the as-cas In the as-castt" should be "In the as-cast." One as-cast is given extra.

Line 215: "the binary mixing enthalpies of Cr-Fe and Cr-Mn are −1 and 2 kJ/mol" should be "the binary mixing enthalpies of Cr-Fe and Cr-Mn are −1 kJ/mol and 2 kJ/mol."

 

Line 223: "where more Laves phases precipitates" should be "where more Laves phases precipitate."

Author Response

Coatings

Editorial Office

Dear editors,

Thanks for your great efforts and reviewers’ thoughtful comments, all of which have been incorporated into the revised manuscript. Detailed descriptions are listed as follows and marked by highlighted in the revised manuscript (with marked).

Reviewer 2

Comment 1: Overall, the paper is well-structured. Methodologically, I do not have main concerns. However, the authors could have explained more carefully how the work fits into coatings research. MDPI has another journal, Alloys, where this paper would be more suitable. In the context of coatings, the authors need to explain and demonstrate the utility of this new 6-metal alloy as a functional coating. 

Response 1: Thank you for the reviewer's comments. The Coatings journal invited us via email on June 21st to submit our manuscript to the special issue of "Research and Application of High-Entropy Alloys". The research content of this article is the effect of heat treatment temperature on the microstructure and mechanical properties of high-entropy alloys, which meets the requirements of this special issue for submissions. The Cu_0.3Cr₂Fe₂Ni₃Mn₂Nb_x high-entropy alloy exhibits excellent corrosion resistance at a Nb content of 0.4 mol, and its mechanical properties can be further improved by heat treatment. Our research shows that the alloy still has good microhardness and compressive strength after high-temperature heat treatment at 900 ^oC. Therefore, this alloy is expected to serve as a high-temperature and corrosion-resistant coating material.

 

Comment 2: In the introduction, the authors have reviewed research works that include alloys of multiple metals. On the one hand, it is understandable that as the alloy complexity increases, there may be an extension in its realm of functional properties. But with six metals present, the ally chemical space is very large and finding any good application may feel like serendipity. The literature has many papers where someone has definitively tried to explore some special combination, and thus it is not a strong reason why now trying this new combination of metals. The authors need to further justify their selection of these materials and explain how this research helps to navigate towards other alloys that may have improved properties or target custom applications. 

Response 2: Thank you for the reviewer's comments. The development of high-entropy alloys has only been more than 20 years, and compared with traditional alloys, the research time is relatively short, and the depth and breadth of research content are insufficient. The design of traditional alloys has binary or ternary phase diagrams as references, but there is no mature theory for the design of high-entropy alloys. People tend to use equimolar ratios of multi-element high-entropy alloys as a basis, or add other alloying elements, or change the relative content of certain elements to study the influence of alloying elements on the microstructure and properties of high-entropy alloys. We previously studied the structure and properties of CuCrFeMnNi high-entropy alloy system and found that CuCr₂Fe₂Ni₂Mn₂ alloy with a single FCC phase has better corrosion resistance than traditional 304S stainless steel in 1M H₂SO₄ solution and 30 wt.% HNO₃ solution. To further improve the corrosion resistance of the alloy, the Nb_xCu_0.3Cr₂Fe₂Ni₃Mn₂ high-entropy alloys were designed by reducing the Cu content and increasing the Ni content, while introducing the corrosion-resistant Nb element. The phase structure of the alloys changed from a single FCC phase to FCC+Laves phase, and the segregation degree of Cu element was reduced. The corrosion resistance was significantly improved in 10% and 30% HNO₃ solutions. This article further explores the influence of heat treatment temperature on the microstructure and mechanical properties of high-entropy alloys based on previous work, aiming to provide experimental support and theoretical reference for the application of this alloy system in high-temperature structural materials.

 

Comment 3: The binary mixing enthalpies do not seem relevant to this work as the authors deal with multicomponent alloys. It would be better if they could ask someone to calculate a few supercells containing and reflecting on the present component mixture.

Response 3: Thank you for the reviewer's comments. During the preparation process of multi-component high-entropy alloys, multiple metal elements are mixed with each other to form solid solutions or mixed structures of solid solutions and intermetallic compounds. Both as-cast and sintered alloys exhibit a certain degree of compositional segregation. Existing research has shown that the formation of phase structure and compositional segregation in high-entropy alloys are mainly related to mixing enthalpy of the elements. The segregation phenomenon of constituent elements in high-entropy alloys can be reasonably explained by utilizing the binary mixing enthalpy between elements. In this study, the high-entropy alloy after high-temperature heat treatment exhibited a microstructure different from that of the as-cast state, especially the formation of BCC phase, which is also related to the binary mixing enthalpy between elements. The calculation of mixing enthalpy of super monomers in high-entropy alloys should be helpful for understanding the phase structure of as-cast high-entropy alloys, but we are not very familiar with this and have not calculated it. We hope to explore it in future research.

 

Comment 4: Finally, the authors write "Data Availability Statement: All the raw/processed data required to reproduce these findings are available and can be requested from the authors." This is not a permanent solution. If the data is indeed accessible and there, please just upload it as a supporting information to this manuscript.

Response 4: Thank you for the reviewer's comments. Based on the suggestions of the reviewers, we have uploaded the relevant phase, microstructure, and performance testing data of this article to the manuscript submission system for reference by the reviewers and readers.

 

Comment 5: Line 21: "strengthening of the Laves phase and the solid solution strengthening of the BCC phase" should be "strengthening of the Laves phase, and the solid solution strengthening of the BCC phase."

Response 5: The above sentence has been modified according to Comment 5.

 

Comment 6: Line 73: "phase, produing an increase in its yield strength and fracture strength" should be "phase, producing an increase in its yield strength and fracture strength."

Response 6: The spelling of the incorrect words in the above sentence has been corrected.

 

Comment 7: Line 210: "In the as-cas In the as-castt" should be "In the as-cast." One as-cast is given extra.

Response 7: The spelling of the incorrect words in the above sentence has been corrected.

 

Comment 8: Line 215: "the binary mixing enthalpies of Cr-Fe and Cr-Mn are −1 and 2 kJ/mol" should be "the binary mixing enthalpies of Cr-Fe and Cr-Mn are −1 kJ/mol and 2 kJ/mol."

 Response 8: The above sentence has been modified according to Comment 8.

 

Comment 9: Line 223: "where more Laves phases precipitates" should be "where more Laves phases precipitate."

Response 9: The grammar errors in the above sentence have been corrected.

 

Author Response File: Author Response.pdf