Mechanical Properties and the Microstructure of β Ti-35Nb-10Ta-xFe Alloys Obtained by Powder Metallurgy for Biomedical Applications

Round  1

Reviewer 1 Report

Line 23, 148, 149: it appears that this sentence includes an incomplete comparison (..better, smaller).

Have been studied the oxygen content of Fe powder?

Is the lack of diffusion in the tested samples observed only on the basis of SEM images?

The EBSD technique could be used to identify the isothermal metastable ω phase.

Why the Vickers microhardness remained constant, when the porosity increases and the density is reduce?

Please correct:

Line 18: ...hardness test...

Line 70: The bending of powders...

Line 138: No description to figure 2.

Author Response

The answers to the comments and suggestions made by the reviewer are attached.

Only indicate that the language has been reviewed by a native translator specialized in engineering.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper deals with powder metallurgical production of beta-Ti-alloys for biomedical implants by blended elemental powder technology. The basic composition Ti-35Nb-10Ta is modified by adding different amounts of Fe to further stabilization of beta-phase by usage of a comparably cheap element. The idea of this study is to investigate the effect of this addition on sintering behavior, microstructure and mechanical properties (Young’s modulus, compression strength, Vickers hardness).

Principally, the study is of interest, because the problem of cost-efficient fabrication of beta-titanium alloys with high mechanical properties is still not solved. However, even if the study itself was performed quite well, I have some concerns about the paper. Thus, in the present state I cannot recommend publishing.

One main concern is choice of experimental parameters. While a variation of Fe content between zero and 4.5% makes sense, a variation of the sintering temperature from 1250 °C to 1300 °C is too narrow to compare influence of Fe-content and sintering temperature as it is done by the authors. It is clearly shown that the microstructure is not homogeneous after sintering. For me, it makes only limited sense to deduce information from these samples. Furthermore, Fe is added instead of replacing Nb and/or Ta. This leads to a continuously decreasing percentage of titanium, meaning that rather different alloys are compared.

This is my second concern. The results are hardly comparable in some cases, because the samples differ not only in composition and sintering temperature, but also in porosity. In the following I try to clarify my concerns on some examples and also mention some smaller points.

- Lines 66 to 70: Please, mention also the geometry of the powders (spherical, irregular,…)

- Fig 2 shows clearly the inhomogenous microstructure after sintering at 1250 °C. Thus, Fig 1 only reveals a rough tendency, nothing more, and the values in table 1 are results on samples which not represent the alloys, at least not at 1250 °C. Fig 3 d-f shows only small parts of the microstructure, however, even at 1300 °C the microstructure appears not really homogeneous. I am wondering in particular about the Ta particles which should diffuse very slowly. They are small but should be seen in this magnification. The authors do not comment on Ta.

- Line 113. I am wondering where the high amount of oxygen comes from. From the powders? Or the processing?

- In Table 1 “.” has to be used in the alloy names as decimal separator.

- Line 127. There seems to be a mistake. Alpha-Ti volume fraction should not “came close to 55%”. I assume not solved Nb is meant.

- Sentence in lines 128 to 130 is not clear to me. What does “became more important” mean here? Please, rephrase the sentence.

- Caption of Figure 2 is missing.

- Line 144. In Table 1 no amount of Nb particles is shown.

- Figures 2, 3 and 4 are of rather poor quality with low contrast. Maybe the authors can enhance the quality.

- Maybe the authors can explain more what we learn from Figure 4. For me the images show high noise and just reveal that beta-stabilizers are found preferentially in the beta-phase which is of no surprise. Because the phases are not clearly expressed in the EDS maps this could be further hint that the microstructure is not in equilibrium. And again: why are the samples sintered at 1250 °C chosen for this investigation?

- Line 180/181. Please, explain more clearly about the grain growth, the sentence is not clear to me. Maybe better to do it the discussion part.

- Paragraph 179 to 195. I like to recommend to avoid text just describing and repeating numbers in tables. Just mention tendencies you like the reader to notice.

- Line 193 to 195: I don’t understand this sentence. If this difference between 5.5. and 5.6 (please, use either one decimal or two, but not sometimes this, sometimes that. In the table two decimals are named as relevant) is a dfference, also other alloys show a variation in density.

- Table 2 Samples with more than about 5% porosity have also some open pores influencing the Archimedes measurement. This should be discussed.

- Line 218/219. Increase in grain size decreases generally the strength.

- Discussion: in general this chapter is too descriptive. Too few mechanisms are explained. E.g. line 254: why does Fe influence the grain size? What is the mechanism? What about the change of the beta-transus? It is nowhere discussed but could have a severe influence, e.g. on grain size.

- Sentences like 259/260 are more or less trivial and should be removed.

- I have concerns with Figure 6. It implies that grain size is influenced by the amount of alpha phase. But both are determined at samples differing in composition, sintering parameters, porosity, sintering behavior. I cannot reveal any useful information from this. It is a spurious correlation, I fear.

- Line 266. If porosity wouldn’t be correlated to density in this range of total porosities one of the measurements would be wrong. So, either the sentence is trivial or it should be stated that this proves that principally the measurements were okay.

- Lines 272 to 275. Again just repetition of the tables content, no discussion.

- Lines 276ff. I doubt that the authors proved the omega phase in their samples influence the mechanical properties in the way mentioned. Young’s modulus is strongly influenced by alloy composition and porosity. In the paper it is not shown that the different samples reveal different amount of omega phase, so it cannot be used to explain differences in the Young’s modulus or hardness. It would be very helpful to have arc-melted samples with homogenous microstructure and no porosity as comparison. The whole paragraph is somewhat unclear and should be rephrased.

- Line 327. What does this sentence mean, “more” compared to what? If it means low contents of Fe are more effective than high ones, this is a contradiction to the linear dependence stated before.

- Lines 329ff. Again, what means “more”. Both, Fe and sintering temperature show comparable changes in grain size. But temperature range is too narrow. What will happen at 1400 °C?

- Lines 333ff, The clear influence of porosity on Young’s modulus and strength is not discussed or mentioned.

Author Response

The answers to the comments and suggestions made by the reviewer are attached.

Author Response File: Author Response.pdf

Round  2

Reviewer 2 Report

Thank you for the improvement of the paper. I don't agree in all cases but for me the content is interesting and I now recommend publication.