Microstructure, Phase Transformation, and Mechanical Properties of Ni-Ti-Hf-La Alloys

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The introduction section is too long. It is recommended to simplify the background and focus on the research objectives.
There are a few irregularities in the icon format, such as the need to capitalize the first letter of the horizontal axis' temper 'in Figure 2a.
The expression of comprehensive assessment in the conclusion lacks basis, and it is recommended to revise the wording.

Comments on the Quality of English Language

The language expression in this article is basically correct, the logical expression is smooth, and there are a few grammar errors and improper wording. It is recommended to check and modify it.

Author Response

Dear editor and reviewer:

Thank you very much for your reviewing.

We have submitted it as an attachment.

Thank you.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is interest. It is not accurate to say La does not dissolve in the matrix according to EDS because of it large size. Dissolution in matrix is not a simple concept based on the size only.  Please modify the statement. Additionally precipitates only contain 80at% of the La please provide an explanation as to where the rest is.  Authors call the La rich phase as precipitates. Can you please verify whether it forms during solidification from melt or during a heat treatment. If it is the first case then it is not precipitates but a second phase. 

Although La  is a hard rare earth element - this does not make sense. Does it typically have an hardening effect ? Why do you think the overall hardness comes down. Why does the elastic modulus increase. Generally solid solutions are required to change the elastic modulus is dependent on where the indent is made. If made on the La rich particles you get a different value to matrix. 

Please provide error bars for the hardness and Elastic modulus as well as DSC results for authors' work . Is the elastic modulus measured here representative of NiTiHf shape memory alloys?

 

Is is not clear why NiTi alloy with La is important. Understand the effect of Hf etc additions but not the Hf+La in terms of rational for the work done 

 

The last line of the conclusions has not been evidenced by the results 

Author Response

Dear editor and reviewer:

Thank you very much for your reviewing.

We have submitted it as an attachment.

Thank you.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The study is devoted to the study of the effect of adding the rare earth element lanthanum (0, 0.5, 1 and 2 at.%) on the microstructure, phase transformations and mechanical properties of high-temperature Ni₅₀Ti₃₀Hf₂₀₋ₓLaₓ alloys. The alloys were synthesized by arc melting with subsequent heat treatment at 850°C for 1 hour, after which they were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Brinell hardness and elastic modulus measurements by nanoindentation, as well as differential scanning calorimetry (DSC) to determine the phase transition temperatures. The results showed that lanthanum does not dissolve in the Ni-Ti-Hf matrix due to its large atomic radius, forming La-rich precipitates (80–82 at.% La) with sizes ranging from 6 μm (for 0.5 at.% La) to 9 μm (for 2 at.% La). The addition of lanthanum significantly reduced the martensitic transformation onset temperature (Mₛ) from 320°C (alloy without La) to 210°C (2 at.% La), while maintaining the single-stage nature of the B2↔B19′ transformation, which is explained by the suppression of the lattice rearrangement kinetics by La precipitates. In mechanical properties, a moderate decrease in hardness was observed from 456 HV (x=0) to ~420 HV (x=2), which the authors attribute to the probable formation of relatively soft intermetallic phases of lanthanum, while the elastic modulus demonstrates an increase of 15%, reaching 136 GPa at 2 at.% La, which may be associated with strengthening of grain boundaries and an increase in matrix rigidity. The authors concluded that controlled alloying with lanthanum allows for targeted regulation of the martensitic transformation temperature and rigidity of alloys for high-temperature applications, however, further studies of the shape memory effect and cyclic stability of modified compositions are necessary for practical implementation.

 

Comments on the essence of the presented material:
1. In Table 2, according to the work data, it is indicated that Ms of the Ni50Ti30Hf20 alloy has a value of 320 ℃, according to the data given in the same table from the literary source [17] for an alloy of similar composition, Ms is equal to 193.25 ℃. What is the reason for such a difference?
2. Lanthanum is a very active metal, prone to oxidation, so it is important to indicate in what environment the melting was carried out?
3. When interpreting the decrease in hardness (Fig. 3), the authors associate this effect exclusively with the influence of La itself, in particular, the formation of soft phases of La. However, in the alloy without La (Ni₅₀Ti₃₀Hf₂₀), the maximum hardness (456 HV) was recorded, which indicates a significant contribution of Hf to the strengthening of the matrix. Since in the Ni50Ti30Hf20-хLaх series the increase in La content is accompanied by a proportional decrease in the Hf fraction, the observed decrease in hardness can be primarily due to a decrease in the concentration of Hf, a known hardener of Ti-Ni alloys. The authors are advised to consider this alternative mechanism for the effect of composition on mechanical properties, for example, by analyzing the correlation between the Hf content and hardness.
4. Fig. 3 does not provide error bars for hardness and elastic modulus. Without them, it is not entirely correct to talk about a "slight decrease in hardness" (from 456 to 420 HV at x = 2).
5. The objective states: "We sought to determine the extent to which La doping can be used to tune transformation temperatures and mechanical performance, providing insight into the alloy design of next-generation high-temperature shape memory materials", but the conclusions do not provide quantitative recommendations on the composition.
6. The choice of 4 remelting cycles in an arc furnace is not explained. How does this affect homogenization?
7. "Studies have shown that increasing the Ni content in Ni-Ti-Hf alloys can enhance their mechanical properties". What mechanical properties exactly does the increased nickel content improve? What studies were used to draw this conclusion?
8. The ruler is not visible in Figure 1.
9. Figure 2a, there are no values ​​on the ordinate axis.
10. The title of Figure 2b is "(b) martensitic start temperature curve", but the figure shows the dependence of the martensitic transition start temperature on the La concentration.
11. "Table 2. Phase transformation temperatures". It is necessary to fully characterize Table 2.
12. Table 2 does not have a source for the Ni50Ti29Hf19Nd2 alloy.
13. The article also contains unsuccessful formulations "microstructural inhomogeneities", "counterintuitive behavior" and others.

Author Response

Dear editor and reviewer:

Thank you very much for your reviewing.

We have submitted it as an attachment.

Thank you.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is much easier to read and folow. Thank you for the corrections.

On pg 1 last paragraph  "These studies indicate that the... hardness increases".  Could you please add why hardness increases and whether  it is a good thing or bad thing.  This will help readers understand what is needed for designing a good shape memory alloys

In Figure1 please identify whether the imaging mode is SE or BSE.  I believe it is BSE but confirmation is needed.

Could you please plot errors bars on Figure 3. This makes it much easier to see the range and the effect 

Author Response

Dear editor and reviewer:

Thank you very much for your reviewing.

We have submitted it as an attachment.

Thank you.

Author Response File: Author Response.pdf