Oxidation Resistance of γ-TiAl Based Alloys Modified by C, Si and Y₂O₃ Microdopants

Round 1

Reviewer 1 Report

This paper studies the oxidation kinetics of TNM-B1 alloy doped with a small amount of carbon, silicon and yttrium oxide at high temperature, and the alloy modified with 1.5Y2O3 has the best oxidation resistance at 1100 °C. The Y2Ti2O7 phase formed in the oxide layer has low oxygen permeability and high temperature stability. In addition, an intermediate layer rich in niobium and molybdenum is formed at the oxide layer/substrate interface. This intermediate layer ensures the thermal stability of the alloy by preventing the diffusion of oxygen into the bulk material and diffusion of titanium and aluminum to the surface.

Author Response

We would like to thank the reviewer for thorough reading of the article and its appreciation.

Reviewer 2 Report

The present study reports “Oxidation resistance of γ-TiAl based TNM-B1 alloys modified by C, Si and Y2O3 microdopants”. To make this paper publishable the authors need to consider following comments:

-Title & Abstract: need to explain the abbreviations which is used in this section, e.g. TNM-B1 alloy or SHS is unknown for readers here. In first impression, there is a confusion do you study heat resistance or oxidation resistance? Also, TNM-B1 is an γ-TiAl based alloy anyway, why need to mention it in title?

-Introduction: in the last paragraph need to be clarify what is Ti-Al-Nb-Mo-B composition in %, e.g. Ti–43Al–4Nb–1Mo–0.1B; also, how much is this “small concentrations of C, Si, and Y2O3” you mentioned.

-Materials and Methods: it’s good to have two sub-sections for this part; in the present form, devices and materials are mixed together.

-Results and Discussion: it’s better to find a better name/expression for “The initial sample's structure”, maybe make a name/number for them which is more common. Please replace the Figures 2 and 6 with higher resolution micrographs. It’s not so clear in present form and the named SEM and XRD has ability of better photos definitely.

Figure 7 needs to have a measurement stuff and/or write the dimension/diameter in the caption of figure. Why the lateral side is not rounded? Is it polishing issue?

After consideration of aforementioned comments, I will check it again. 

Author Response

Dear Reviewer,

we wholeheartedly thank you for your thorough analysis of our manuscript and efforts to improve it. Your comments definitely helped us to make our paper better.

Please find the answers to your comments below.

Our corrections are highlighted in yellow.

Comment 1

-Title & Abstract: need to explain the abbreviations which is used in this section, e.g. TNM-B1 alloy or SHS is unknown for readers here. In first impression, there is a confusion do you study heat resistance or oxidation resistance? Also, TNM-B1 is an γ-TiAl based alloy anyway, why need to mention it in title?

Response to comment 1

In this work we studied oxidation resistance. Term “heat resistance” was replaced with “oxidation resistance” everywhere. Regarding the “TNM-B1” term, the idea was to mention the class of studied TiAl alloy without focusing on a particular composition. Although we agree with the comment, the class of the alloy is not necessary in the title.

Comment 2

-Introduction: in the last paragraph need to be clarify what is Ti-Al-Nb-Mo-B composition in %, e.g. Ti–43Al–4Nb–1Mo–0.1B; also, how much is this “small concentrations of C, Si, and Y2O3” you mentioned.

Response to comment 2

We added information about alloy composition in the text.

(This work aimed to study the oxidation resistance of Ti–43Al–4Nb–1Mo–0.1B alloy, doped with small (0.2-1.5 wt.%) concentrations of C, Si, and Y2O3.)

Comment 3

-Materials and Methods: it’s good to have two sub-sections for this part; in the present form, devices and materials are mixed together.

Response to Comment 3

According to your recommendation, we made a subdivision of Section 2.

Comment 4

-Results and Discussion: it’s better to find a better name/expression for “The initial sample's structure”, maybe make a name/number for them which is more common. Please replace the Figures 2 and 6 with higher resolution micrographs. It’s not so clear in present form and the named SEM and XRD has ability of better photos definitely.

Response to Comment 4

We named Section 3.1 “The structure and phase composition of as-HIPed samples” to highlight, that samples before oxidation annealing are meant.

All investigated alloys were labeled with concentration and type of the additive (0.2C, 1.5Si and 1.5Y₂O₃). It is placed in the end of Section 2.1.

Also we replaced Figures 2 and 6 to sharper ones.

Comment 5

Figure 7 needs to have a measurement stuff and/or write the dimension/diameter in the caption of figure. Why the lateral side is not rounded? Is it polishing issue?

Response to Comment 5

We made a correction in Section 2.2, related to specimen height:

Oxidation specimens had cylindrical shape with height h=4 mm and diameter d=6 mm.

A certain irregularity of shape at the lateral side comes from polishing with 2400 mesh SiC paper. This operation was necessary to clean the surface out from copper after electric discharge cutting.

Figure 7 was supplemented with scale bar.

Reviewer 3 Report

Dear author, 

This manuscript presents the Oxidation resistance of gamma-TiAl based TNM-B1 alloys modified by C, Si and Y2O3 microdopants.

The technical content is important to reduce the oxidation of TiAl alloy during high temperature use and the manuscript has merit publication.

However, the method and the explanation of result is not kind to the readers. The author needs to amend it regarding the following points.

(1) The author needs to explain their originality in detail.

      Refer to the related literature papers and describe the position of this     

      manuscript among them.

(2) Description of experimental method is poor regarding Materials and 

      methods. Explain the aim of each experinment and the point to the

      measurement of hig temperature oxidation behavior (regarding

      composition of materials and resultant microstructures.

(3) The description of experimental result is not kind for understanding.

      Regarding XRD spectrum, explain the relationship with fabrication

      methods and resultant microstructure which govern high temperature

       oxidation behavior.

       What is the definitive difference of 0.2C, 1.5Si and 1.5Y2O3?

       The relationship the composition of gamma-TiAl  and thermal treatment

        is not well-described. This manuscript is not kind for readers who does

        not specific knowledge.

       Regarding Figure 10, what is delamination and/or trace of oxidation?

       Explanation is not well described.

(4)   Overall, the content is suitable, but there is less explanation kind for                readers. The reviewer want the author to brush up the technical                      contents more clearly. 

Author Response

Dear Reviewer,

we wholeheartedly thank you for your thorough analysis of our manuscript and efforts to improve it. Your comments definitely helped us to make our paper better.

Please find the answers to your comments below.

Our corrections are highlighted in blue.

Comment 1

The author needs to explain their originality in detail. Refer to the related literature papers and describe the position of this manuscript among them.

Response to Comment 1

The introduction was supplemented by analysis of current works, devoted to study of oxidation resistance of TiAl based alloys, doped with C, Si and Y2O3, and it uncovers the position of our work among them.

TiAl-based alloys, doped with C, Si, and Y₂O₃, proved their superior high temperature strength and creep resistance, yet some incomplete data on their oxidation resistance can be found. The protection mechanism utilized by carbon addition consists in impeding either intracrystalline diffusion by dissolved carbon atoms [26] or intergranular diffusion of oxygen by TiC carbide nanoparticles or Ti₂AlC and Ti₃AlC₂ MAX-phases [27, 28]. Silicon is considered as the most beneficial element for improving the oxidation resistance of TiAl-based alloys, since it forms SiO₂ oxide with low oxygen solubility, and also suppresses the growth of rutile TiO₂ in the oxide scale. However, it should be noted that the most outstanding results were obtained at sufficiently high Si concentrations (5–13%) [29], which lie far beyond the known limits for effective strength and creep resistance improvement. To achieve a combination of high mechanical properties and oxidation resistance, it is advisable to protect the surface of TiAl-based alloys with coatings Ti_xSi_y [30, 31], TiAlSiN [32, 33], Al-Si [34-36], and others, obtained by magnetron sputtering and ion implantation. The effect of Y/Y₂O₃ additives on the oxidation resistance of TiAl-based alloys is well covered. In [21, 37, 38], the positive effect of Y₂O₃ in the oxidized layer is explained by the fact that these particles block the grain boundary migration of oxygen atoms, as well as the formation of a stronger oxidized layer due to the pinning of TiO₂ and Al₂O₃ layers with Y-containing phases.

The potential of each of the above mentioned additives for improving oxidation resistance has been proven by many works. However, there is lack of data on their comparison with each other, which is possible only when using alloys with identical chemical composition and microstructure.

Comment 2

Description of experimental method is poor regarding Materials and methods. Explain the aim of each experinment and the point to the measurement of high temperature oxidation behavior (regarding composition of materials and resultant microstructures).

Response to Comment 2

We described the experimental methods in more detail. Specifically, we added information about samples consolidation using HIP, TEM investigations, FIB for lamella preparing. Also we explained the chosen temperatures of oxidation annealing and the aim of additional experiment on annealing Y₂O₃ with TiO₂ and Al2O₃. The corresponding corrections are embedded in the text.

HEBM was carried out in order to obtain the uniform distribution of components in powder mixture in an argon atmosphere in an Activator-4M planetary ball mill (Activator, Russia) with steel grinding media and ball to powder mass ratio of 15:1. The SHS method was used to obtain the required phase composition of the powder.

Compact samples were obtained by hot isostatic pressing (HIP) of powder mixtures in titanium capsules at a temperature of 1250 °C for 2 hours. No special heat treatment was carried out after HIP in order to keep equiaxed microstructure of alloys.

Oxidation experiments were carried out in a muffle furnace with an air atmosphere for 30 hours at 800 °C and 1100 °C. The first temperature was chosen as it is typical for operating of TiAl-based turbine units – the main application field of TiAl-based alloys. Oxidation experiments at 1100 °C were meant to demonstrate the potential of C, Si and Y₂O₃ modified alloys for use in more extreme conditions and to make the difference in oxidation kinetics more apparent.

An additional experiment on oxidation annealing of samples, consisting of Y₂O₃ particles and main oxide components (Al₂O₃ and TiO₂), was carried out at temperature 1100 °C. It allowed to reveal the Y₂O₃ phase stability under oxidation conditions and interaction direction in the presence of Al₂O₃ and TiO₂.

The fine structure of oxide scale after annealing was analyzed by transmission elec-tron microscopy (TEM) using JEM 2100 microscope (Jeol, Japan), equipped with X-MAX 80 EDX detector (Oxford Instruments, UK). A lamella with 5×3 µm dimensions was produced using a focused ion beam technique using Scios dual-beam microscope (FEI, USA).

Comment 3

The description of experimental result is not kind for understanding. Regarding XRD spectrum, explain the relationship with fabrication methods and resultant microstructure which govern high temperature oxidation behavior. What is the definitive difference of 0.2C, 1.5Si and 1.5Y2O3? The relationship the composition of gamma-TiAl  and thermal treatment is not well-described. This manuscript is not kind for readers who does not specific knowledge. Regarding Figure 10, what is delamination and/or trace of oxidation? Explanation is not well described.

Response to Comment 3

We added brief explanation of the structure features in the beginning of Section 3. The studied alloys really have microstructure, which is not typical for as-cast TiAl-alloys due to low temperatures of consolidation and no heat treatment after it.

The investigated TiAl alloys had an irregular two-phase structure and consisted of grains of the γ-TiAl and α₂-Ti₃Al. Figure 2 shows images of typical structures of the samples with each dopant after HIP. The microstructure of the studied alloys is not typical for common TiAl-based alloys, produced by casting (leading to formation of duplex or lamellar structure). They inherited the microstructure of powder particles after HEBM and SHS [39]. It was preserved due to consolidation at low temperature, below β-Ti field in the Al-Ti phase diagram, and lack of special heat treatment.

The definition 0.2C, 1.5Si and 1.5Y₂O₃ is added in the Section 2.1. 0.2C stands for Ti–43Al–4Nb–1Mo–0.1B with 0.2 wt. % of carbon; 1.5Si – the same alloy with 1.5% of Si and 1.5Y₂O₃ – the same alloy with 1.5% of Y₂O₃.

Also we mentioned, that no heat treatment was carried out for the studied alloys, to preserve the two-phase near-globular microstructure of alloys. The corresponding comment was added in the Section 2.2.

Comment 4

Overall, the content is suitable, but there is less explanation kind for readers. The reviewer want the author to brush up the technical contents more clearly.

Response to Comment 4

We hope, that introduced revisions made the content more clear. The main changes concerned the motivation of the study, explanation of the experiments and comments on the structure of the as-HIPed samples, which is not typical for common cast TiAl-based alloys.

Round 2

Reviewer 2 Report

The comments are answered and the manuscript is improved. 

Reviewer 3 Report

Dear author, 

This manuscript presents oxidation resistance of gamma-TiAl based TNM-B1 alloys modified by C, Si and Y2O3 microdopants.

Oxidation resistance of TiAl alloys is dramatically improved by the element addition and the underlying mechanism is elucidated.

The revised manuscript evidences the improvement clearly.

In my opinion, this manuscript can be accepted in the present form.