In Situ Fabrication of Ti-xNb Alloys by Conventional Powder Metallurgy

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript titled "In situ fabrication of Ti-xNb alloys by conventional powder metallurgy" describes production of Ti alloys for medical application using PM. The study shows the influence of Nb content on physical and mechanical properties of Ti matrix, and it follows changes in the Ti matrix during sintering with various Nb content. The results show that observed alloy could be potential substitute for commercially used Ti46Al4V alloy. Even though presented results are probably obtained at beginning of comprehensive study, they are valuable and useful for ongoing and future research on similar topic. Manuscript should be revised before being accepted for publication in Coatings. 

Comments:

- In the abstract authors should mention that investigated alloy is for implants.

- Introduction should be extended to cover all possible Ti phases, and their influence of elastic modulus. Also, some comparison between spark plasma sintering and used PM technique should be mentioned since SPS is the topic of the special issue. At the and of the Introduction, authors should add what is the novelty of their work.

- Units should be uniform, for example, use or seconds, or min or hours.

- In the Fig.7a unit is missing for the strain.

Comments on the Quality of English Language

Minor editing of English language is required, a few spelling errors are detected.

Author Response

The manuscript titled "In situ fabrication of Ti-xNb alloys by conventional powder metallurgy" describes production of Ti alloys for medical application using PM. The study shows the influence of Nb content on physical and mechanical properties of Ti matrix, and it follows changes in the Ti matrix during sintering with various Nb content. The results show that observed alloy could be potential substitute for commercially used Ti46Al4V alloy. Even though presented results are probably obtained at beginning of comprehensive study, they are valuable and useful for ongoing and future research on similar topic. Manuscript should be revised before being accepted for publication in Coatings. 

Comments:

- In the abstract authors should mention that investigated alloy is for implants.

Thanks for your comments. It has been added in the abstract

- Introduction should be extended to cover all possible Ti phases, and their influence of elastic modulus. Also, some comparison between spark plasma sintering and used PM technique should be mentioned since SPS is the topic of the special issue. At the and of the Introduction, authors should add what is the novelty of their work.

The different Ti phases will be added in introduction with its respectability references, and the comparative whit SPS is also added.

- Units should be uniform, for example, use or seconds, or min or hours.

Thanks for observation, unities were uniformed in the whole manuscript

- In the Fig.7a unit is missing for the strain.

The Strain it is a dimensionless value

Comments on the Quality of English Language

Minor editing of English language is required, a few spelling errors are detected.

The English was revised and some modifications were made.

Reviewer 2 Report

Comments and Suggestions for Authors

The study found that Niobium increases porosity and reduces the density of the titanium matrix after sintering. Nb stabilizes the β-Ti phase and promotes α'-Ti and α''-Ti phases. It decreases compressive strength but enhances ductility and lowers the Young’s modulus. Corrosion resistance depends more on the Ti phase than on Nb content, with 10% and 15% Nb samples showing the best resistance. These findings suggest that materials composed of non-toxic elements and produced via Powder Metallurgy (PM) techniques are promising for medical applications due to their favorable mechanical properties and corrosion resistance. This paper can shed light in the field. Please see the comments below for revision.

1.       In the introduction section, the standard terminology should be maintained. For example, on Page 2, Line 47, “low moduli” should be “low elastic moduli,” and on Page 2, Line 58, “Young’s values” should be “Young’s modulus”.

 

2.       In the introduction section, there is a lack of recent updates, it is recommended to highlight some recent literature on developing Ti alloys with the addition of Nb, Zr, Mo, and Ta for designing new biomedical titanium alloys with excellent mechanical properties and wear resistance. For example, “Microstructure and mechanical properties of Ti43Al6Nb alloys with different zirconium contents, 2023” and “Sliding and Fretting Wear Behavior of Biomedical Ultrafine-Grained TiNbZrTaFe/Si Alloys in Simulated Physiological Solution, 2024” and “Towards load-bearing biomedical titanium-based alloys: From essential requirements to future developments, 2024”, plus many relevant review articles in 2024 .

 

3.       In Section 2.1, the mixing of Ti-Nb should indicate it is in weight percentage.

 

4.       in Section 2.1, it mentions “the mixture was compacted uniaxially inside a cylinder steel mold with a load of 450 MPa”, what is the size for this steel mold? The “load of 450 MPa” indicates pressure?

 

5.       In Section 2.4, it states “the specimens with dimensions of 6 mm x 6 mm.” Please specify whether these specimens are cubes or cylinders and provide the relevant ASTM standard for choosing these dimensions.

 

6.       For the Section 3.1 Dilatometry analysis and relative density, the methodology is not provided for this analysis. For Figure 2, it would be preferable to represent the y-axis as axial shrinkage in percentage form.

 

7.       Additionally, the dilatometer analysis regarding the higher contraction with lower Nb content is confusing. From the figure, both pure Ti and Ti-15Nb samples exhibit similar axial shrinkage around 8%, whereas the Ti-5Nb sample shows 9.5% shrinkage. Considering Nb has a significantly higher melting temperature compared to Ti, and the sintering temperature is well below Ti’s melting point, the purpose of this analysis is unclear.

 

8.       On Page 6, Line 205, the statement is inappropriate, they are not similar “The α'-Ti and α''-Ti and β-Ti faces present low moduli similar to those of real human bones”.

 

9.       On Figure 5b, please relabel the which are α phase and β phase to improve clarity.

 

10.   In addition, the Figure 5c indicates the α'' phase. The authors should clarify how they distinguished between α'' and α' phases. It is unclear why the XRD analysis shows no α and α' phase for Ti-15Nb, it is supposed to be wrong XRD analysis. Also, what is the reason that the XRD does not detect Nb particles?

 

11.   On page 8, Line 254, it states “the α'' phase increases the elastic modulus of Ti-254 10Nb and Ti-15Nb alloys”, why α'' phase increase the elastic modulus more than α'? In terms of the compression ductility analysis, elastic modulus, and electrochemical behaviors of Ti-Nb and Ti-Mo alloys, the following papers can help authors enhance their understanding and discussion: such as “Selective laser melting of Ti–35Nb composite from elemental powder mixture: Microstructure, mechanical behavior and corrosion behavior, 2019 and other Ti-35Nb literatures, and “The effect of secondary phase on corrosion behaviors of the titanium-zirconium- molybdenum alloy, 2024”.

 

12.   On Page 9, Line 266, the statement “the sample with 10 wt% Nb comes from an anodic zone and goes toward negative potentials, indicating that little corrosion was generated”, why toward negative indicate little corrosion, little corrosion is good or bad? there are so many confusing discussions.

 

13.   There is a typo on Page 10, Line 275, “Ni-20Nb” should be “Ti-20Nb”. On Page 12, Line 331 “β-Tipaa”

 

14.   On Page 10, Line 278, the statement is not clear: “Ti-5Nb presented a maximum at frequencies between 1 and 100 Hz”.

 

15.   In Figure 9c, the capacitor is labeled as C2, but the description and in Table 2 uses C1?

 

16.   It is still not clear why the Ti-20Nb shows lower corrosion resistance compared to Ti-15Nb.

 

17.   On Page 12, Line 333, it states “after relatively accelerated cooling that occurs”, why does only this Ti-15Nb sample get accelerated cooling?

Author Response

The study found that Niobium increases porosity and reduces the density of the titanium matrix after sintering. Nb stabilizes the β-Ti phase and promotes α'-Ti and α''-Ti phases. It decreases compressive strength but enhances ductility and lowers the Young’s modulus. Corrosion resistance depends more on the Ti phase than on Nb content, with 10% and 15% Nb samples showing the best resistance. These findings suggest that materials composed of non-toxic elements and produced via Powder Metallurgy (PM) techniques are promising for medical applications due to their favorable mechanical properties and corrosion resistance. This paper can shed light in the field. Please see the comments below for revision.

1. In the introduction section, the standard terminology should be maintained. For example, on Page 2, Line 47, “low moduli” should be “low elastic moduli,” and on Page 2, Line 58, “Young’s values” should be “Young’s modulus”.

Thanks for the comments, the suggested changes were made

2. In the introduction section, there is a lack of recent updates, it is recommended to highlight some recent literature on developing Ti alloys with the addition of Nb, Zr, Mo, and Ta for designing new biomedical titanium alloys with excellent mechanical properties and wear resistance. For example, “Microstructure and mechanical properties of Ti43Al6Nb alloys with different zirconium contents, 2023” and “Sliding and Fretting Wear Behavior of Biomedical Ultrafine-Grained TiNbZrTaFe/Si Alloys in Simulated Physiological Solution, 2024” and “Towards load-bearing biomedical titanium-based alloys: From essential requirements to future developments, 2024”, plus many relevant review articles in 2024

Thanks for the recommendations, references were added in the appropriate section

3. In Section 2.1, the mixing of Ti-Nb should indicate it is.

Thanks for the observation, it was indicated that it is in weight percentage (wt.%).

4. in Section 2.1, it mentions “the mixture was compacted uniaxially inside a cylinder steel mold with a load of 450 MPa”, what is the size for this steel mold? The “load of 450 MPa” indicates pressure?

Certainly, the diameter of the die used is missing. Regarding the "load", it was a mistake because it is a pressure of 450 MPa. This was already updated.

5. In Section 2.4, it states “the specimens with dimensions of 6 mm x 6 mm.” Please specify whether these specimens are cubes or cylinders and provide the relevant ASTM standard for choosing these dimensions.

The dimensions of the samples used and the geometry were added

6. For the Section 3.1 Dilatometry analysis and relative density, the methodology is not provided for this analysis. For Figure 2, it would be preferable to represent the y-axis as axial shrinkage in percentage form.

Information about how the relative density was estimated is added in the 3.1 section.

7. Additionally, the dilatometer analysis regarding the higher contraction with lower Nb content is confusing. From the figure, both pure Ti and Ti-15Nb samples exhibit similar axial shrinkage around 8%, whereas the Ti-5Nb sample shows 9.5% shrinkage. Considering Nb has a significantly higher melting temperature compared to Ti, and the sintering temperature is well below Ti’s melting point, the purpose of this analysis is unclear.

The analysis of dilatometry helps to understand how the densification of the Ti-Nb system evolves in this case. Based on this behavior, it is possible to control residual porosity and the final microstructure. Additionally, information can be obtained to determine the transition temperatures of the different phases formed in the system, which can be useful for designing subsequent heat treatments after sintering.

8. On Page 6, Line 205, the statement is inappropriate, they are not similar “The α'-Ti and α''-Ti and β-Ti faces present low moduli similar to those of real human bones”.

Certainly, the present phases help to obtained low Young's moduli, it’s not similar to the human bones. Nevertheless, it lower than the Ti alloys fabricated by casting or SPS. 

9. On Figure 5b, please relabel the which are α phase and β phase to improve clarity.

Thanks for the observation, it was modified for clarity.

10. In addition, the Figure 5c indicates the α'' phase. The authors should clarify how they distinguished between α'' and α' phases. It is unclear why the XRD analysis shows no α and α' phase for Ti-15Nb, it is supposed to be wrong XRD analysis. Also, what is the reason that the XRD does not detect Nb particles?

The distinction between phases α'' and α' is explained on the corresponding section. It is stated that phase α' is distinguished by having an acicular arrangement of intercalated sheets of phase α and β, while phase α'' is presented in the form of disordered needles.

For XRD analysis, the samples from Ti-15Nb do not present the α and α' phases. There is reference indicating that Nb favors obtaining the β-Ti phase from 100°C. After cooling in systems with high percentages of β-Ti phase, the transformation to martensitic phases occurs mainly in the α''. Furthermore, due to the saturation achieved by β-Ti, it is not easily transformed into the α' phase. A brief explanation with references to this phenomenon will be added.

Finally, the reason why Nb is not detected in the diffraction pattern is due to sintering temperature. The source and brief explanation of this analysis was also be added.

11. On page 8, Line 254, it states “the α'' phase increases the elastic modulus of Ti-254 10Nb and Ti-15Nb alloys”, why α'' phase increase the elastic modulus more than α'? In terms of the compression ductility analysis, elastic modulus, and electrochemical behaviors of Ti-Nb and Ti-Mo alloys, the following papers can help authors enhance their understanding and discussion: such as “Selective laser melting of Ti–35Nb composite from elemental powder mixture: Microstructure, mechanical behavior and corrosion behavior, 2019 and other Ti-35Nb literatures, and “The effect of secondary phase on corrosion behaviors of the titanium-zirconium- molybdenum alloy, 2024”.

Thanks for the paper recommendation. It was explained based on those references, where the unsolved Nb particles have an important contribution to mechanical properties.

12. On Page 9, Line 266, the statement “the sample with 10 wt% Nb comes from an anodic zone and goes toward negative potentials, indicating that little corrosion was generated”, why toward negative indicate little corrosion, little corrosion is good or bad? there are so many confusing discussions.

Thanks for the observation, this sentence was rewritten to give greater clarity to the writing.

13. There is a typo on Page 10, Line 275, “Ni-20Nb” should be “Ti-20Nb”. On Page 12, Line 331 “β-Tipaa”

Thanks for the observation, that was modified.

14. On Page 10, Line 278, the statement is not clear: “Ti-5Nb presented a maximum at frequencies between 1 and 100 Hz”.

Thanks for the observation, it was corrected

15. In Figure 9c, the capacitor is labeled as C2, but the description and in Table 2 uses C1?

Thanks for the observation, it was corrected

16. It is still not clear why the Ti-20Nb shows lower corrosion resistance compared to Ti-15Nb.

The quantity of the β-Ti phase obtained in the samples, with 20 wt.% of Nb, would determine a better corrosion resistance, as per the studies reported. However, the residual porosity affects the corrosion behavior, making the material more susceptible to surface pore-related issues.

This explanation was rewritten to give greater clarity to the writing.

17. On Page 12, Line 333, it states “after relatively accelerated cooling that occurs”, why does only this Ti-15Nb sample get accelerated cooling?

It refers to all samples. All samples were fabricated with the same conditions.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

In the revisoin, the contents were revised according to the review comments. Howevever, a lot of referneces have incomplete biobliogrphical informaiton (with no Volume Number or Article Number). 

Author Response

Thank you to the reviewer

n the revisoin, the contents were revised according to the review comments. Howevever, a lot of referneces have incomplete biobliogrphical informaiton (with no Volume Number or Article Number). 

All references were updated and with the same format.