Transformations of the Microstructure and Phase Compositions of Titanium Alloys during Ultrasonic Impact Treatment. Part I. Commercially Pure Titanium

Round 1

Reviewer 1 Report

The work paid attention to the experimental and theoretical studies enabled to reveal patterns of surface roughening and the microstructure refinement in the surface layer of pure titanium during ultrasonic impact treatment. Some valuable conclusions have been achieved, but some questions need to be answered.

1. Author mentioned that inside the uppermost surface layer, a sub-microcrystalline structure of the α phase with grain sizes of less than 200 nm was observed 134 (Figure 2a; Figures 3a b). Reviewer wants some descriptions about the characteristics of the α phase.

2. What kind of steady phase in room temperature is CP-Ti? Single-phase? If not, how do you achieve the average grain size was 70 μm?

3. How do you achieve the results in Fig.5?

4. What is the load definition in the molecular dynamics simulation?

5. In fig.7 BCC and HCP crystal structures were considered. Is there BCC in commercially pure titanium?

Author Response

We greatly appreciate the reviewer’s time for careful reading of the manuscript, helpful suggestions and valuable comments. All the changes made in the revised manuscript were highlighted by a yellow marker. The point-to-point responses to the reviewer’s comments are listed as following:

1. Author mentioned that inside the uppermost surface layer, a sub-microcrystalline structure of the α phase with grain sizes of less than 200 nm was observed 134 (Figure 2a; Figures 3a b). Reviewer wants some descriptions about the characteristics of the α

Thank You for this relevant comment. The relevant information has been added to the manuscript.

TEM bright-and dark-field images of a phase grain were inserted into the manuscript as Figure 2 to clarify the microstructure of the sample.

 

2. What kind of steady phase in room temperature is CP-Ti? Single-phase? If not, how do you achieve the average grain size was 70 μm?

Commercially pure titanium is s single phase alloy. The microstructure of as-cast CP-Ti sample mainly consists of polycrystalline grains with a size of 10-100 µm.

3. How do you achieve the results in Fig.5?

The simulation of the process was carried out by successive triple indentations of a spherical indenter into an initially defect-free titanium crystallite in the form of a parallelepiped. The details of simulation are described in the paragraph 2 “Materials and Methods”.

4. What is the load definition in the molecular dynamics simulation?

The indenter action was realized through the force field in the shape of sphere with a radius of 6.5 nm. Load was directed from the indenter center to atoms within this sphere. The load value was calculated by the F=-〖k(R-r)〗^2 ratio, where k was the indenter stiffness coefficient; R was the sphere radius; r was distance between  the centers of the indenter and atoms. The indenter was moved along the Z axis at a fixed depth of 3.5 nm with a constant indentation rate of 15 m/s. The indentation depth was chosen so that the maximum load on the indenter did not exceed 12 GPa. The information has been added to the “Materials and methods” section.

5. In fig.7 BCC and HCP crystal structures were considered. Is there BCC in commercially pure titanium?

CP-Ti exhibits a hexagonal close-packed (HCP) crystal structure (α-phase) at room temperature and a body-centered cubic structure (β-phase) above 882°C.  At the present work, the hcp-bcc local transformations during the UIT processing under room temperature were demonstrated using molecular dynamics simulation. The later manifest itself as the formation of a non-equilibrium cluster phase with bcc order at the a phase sub-grain boundaries which are characterized by a high degree of crystal lattice distortion.

Reviewer 2 Report

Nice paper combining experimental and theoretical approaches to the studied problem. I have only few formal items:

Fig. 1: it is not clear what is the difference between Fig. (a) and (b). It should be better explained.

Figs. 2, 3 and line 134: Are the objects in Fig. () really grains? Are they not rather dislocation cells/subgrains? In the later case, I recommend not to speak about „grain size“ but rather about „crystallite size“.

lines 161, 169: for the dislocation density, I would recommend to use the SI unit, m^-2.

Figs. 7b,c: replace BBC by BCC please.

Author Response

Thank You for attentive reading and valuable comments. They have allowed to improve the manuscript a lot. Please find the replies below. We have also made some minor changes in the manuscript according to Your comments and suggestions.

1. 1: it is not clear what is the difference between Fig. (a) and (b). It should be better explained.

Figs. 1(a) and 1(b) demonstrate the surface morphology of the CP-Ti sample studied using optical profilometer and atomic force microscope. The larger scan size of optical image allowed us to reveal the repeated semicircular pile-ups while the detailed AFM image demonstrates the fine structure of the pile-up. This information was added to the manuscript.

2. Figs. 2, 3 and line 134: Are the objects in Fig. () really grains? Are they not rather dislocation cells/subgrains? In the later case, I recommend not to speak about „grain size“ but rather about „crystallite size“.

Thanks for this important comment. Indeed, the microstructure of the uppermost surface layer consists of a phase grain/subgrain substructure. TEM bright-and dark-field images of a phase grain were inserted into the manuscript as Figure 2 to clarify the microstructure of the sample.

3. lines 161, 169: for the dislocation density, I would recommend to use the SI unit, m^-2.

Thank You for this suggestion. The relevant information has been corrected.

4. Figs. 7b,c: replace BBC by BCC please.

You are correct! The abbreviation has been revised.

Reviewer 3 Report

Comments for the authors are in the attachment.

Comments for author File: Comments.pdf

Author Response

We appreciate reviewer very much for the kind comments and recommendations of our manuscript. We made some minor changes in the manuscript according to most of the comments and suggestions of other reviewers. All the modifications were marked in yellow in the revised manuscript.

1. There are errors in the text of the manuscript that need to be corrected.

Thank You for the careful reading. The mistake has been corrected.

2. It is necessary to describe in more detail the procedure for preparing samples for investigation by the method of transmission electron microscopy (TEM).

Thanks for this important comment. The required information has been added to the new version of the Introduction.

3. It is necessary to characterize in more detail the effect of UST on the mechanical properties of the samples under investigations.

This information was added to the manuscript.

4. It is recommended to slightly expand the list of publications for a more detailed presentation of existing approaches and ideas about the problem being solved.

Thanks for this valuable remark. The corresponding information has been added to the text.

5. It is proposed to expand the text of the abstract in order to describe in more detail the approach used and the obtained results.

Thank You for this suggestion. The information has been added to the “abstract” section.