Review Reports - Structure and Tensile Strength of Pure Cu after High Pressure Torsion Extrusion

Round 1

Reviewer 1 Report

Cross-structure and tensile strength of pure copper after HPTE is reported. This subject is surely worth of studying and the authors approach seems to be interesting. Especially a gradient structure of commercially pure copper is important to get high strength and high elongation to failure after HPTE, and the authors prove them.

The question is, however, why do they considered only mechanical surface treatment, by grinding, and not electrochemical finishing of the samples? This would affect not only surface properties but mechanical behaviour as well.

In their paper, the authors write about cylindrical samples and the thread M10 would confirm that. If yes, so Fig. 2 b is wrong and should be corrected, because now it presents a flat sample (here: lacking lines connecting at the end of thread).

Moreover, English and many other faults and errors should be corrected and improved, as indicated by color markings in the Enclosure.

Comments for author File: Comments.pdf

Author Response

We are very thankful to the reviewer for his high estimate of our work and valuable comments.

According to the ASTM E 8/E 8M-08 “Standard test methods for tension testing of metallic materials” tensile testing the surface finishing should provide the roughness value of Rz 3.2 which is enough to exclude the effect of surface defects on the resulting engineering curve.

We are thankful to the reviewer for this comment and corrected the drawing.

Moreover, English and many other faults and errors should be corrected and improved, as indicated by color markings in the Enclosure.

Thanks a lot for the corrections in the English faults and errors. We have additionally revised the text of the paper and asked an English native speaker to help us with that.

Changes to the .docx document were tracked and could be displayed in the activated "All Markup" mode in Microsoft Word.

Reviewer 2 Report

Dear authors,

You did a good job with this manuscript. The investigation is sound, the topic interesting, the methods selected properly and the results are well presented. The extensive discussion contributes to the overall good merit of the manuscript. According to my understanding, the only part you could explain in more depth is the reasoning for the achieved higher levels of elongation.

Do you consider the variation of the grain size might have played a role in this? Especially the fine grained areas in contradiction to the ultra fine ones... And also the role of twins must be underlined for its contribution to the enhancement of ductility.

A minor comment, perhaps it would be useful if you would present the stress-strain distribution of the specimen during the extrusion. This would also assist you in explaining the reason for the variation in the grain size (fine grain versus ultra fine grain size).

As one important conclusion is the achievement of high fracture elongation, I suggest you to add a small paragraph explaining the reasons for this fact in the discussion.

Minor revision is suggested.

Sincerely,

The reviewer

Author Response

We appreciate the high estimate of our work and are very thankful to the reviewer for his comments.

According to my understanding, the only part you could explain in more depth is the reasoning for the achieved higher levels of elongation. Do you consider the variation of the grain size might have played a role in this? Especially the fine grained areas in contradiction to the ultra fine ones... And also the role of twins must be underlined for its contribution to the enhancement of ductility.

Here we would like to emphasize that HPTE processed Cu with gradient microstructure indeed demonstrated a large value of elongation to failure. However, as it was mentioned in the paper, the uniform elongation was rather limited, much smaller than that of Cu after SMAT [1]. It was mentioned on the page 15, line 566. Typically, SPD-processed materials demonstrate an extended post-necking strain, which is most likely related to the increased strain rate sensitivity of ultra-fine grained materials [2], [3], [4]. This is a well known phenomenon, and the investigation of the underlying mechanisms is over the scope of the present investigation. For a clarity, we have added one sentence on page …:

A rapid strengthening and a short uniform elongation range characterized the stress-elongation curve of copper after HPTE. A decrease of uniform elongation from ~16% in the annealed state to ~2% after the HPTE deformation is a typical behavior for the materials produced by SPD methods. However, after the deformation is localized, the elongation to fracture reaches 30%, which is two times higher than post-necking elongation of the annealed copper (Fig. 8 b). Such a behavior is typical for materials with ultrafine-grained microstructure [2], [3], [4] and most likely related to the increased strain rate sensitivity of such materials.

Yes, indeed the knowledge about the stress-strain distribution in the specimen during HPTE is important for the understanding of the microstructure formation. This analysis had been already performed in refs. [5] and [6] in application to copper, and we used these results for the discussion of the variation of the grain size in the specimen.

As one important conclusion is the achievement of high fracture elongation, I suggest you to add a small paragraph explaining the reasons for this fact in the discussion.

As mentioned above, we have added one sentence on page …:

Such a behavior is typical for materials with ultrafine-grained microstructure [2], [3], [4] and most likely related to the increased strain rate sensitivity of such materials.

References

[1] T.H. Fang, W.L. Li, N.R. Tao, K. Lu: Revealing Extraordinary Intrinsic Tensile Plasticity in Gradient Nano-Grained Copper. In: Science 331 (2011), S. 1587-1590.

[2] R. Valiev: Nanostructuring of metals by severe plastic deformation for advanced properties. In: Nature Materials 3 (2004), S. 511-516.

[3] M.A. Meyers, A. Mishra, D.J. Benson: Mechanical properties of nanocrystalline materials. In: Prog. Mater. Sci. 51 (2006), S. 427-556.

[4] Y.M. Wang, E. Ma: Strain hardening, strain rate sensitivity, and ductility of nanostructured metals. In: Materials Science and Engineering: A 375-377 (2004), S. 46-52.

[5] Y. Ivanisenko, R. Kulagin, V. Fedorov, A. Mazilkin, T. Scherer, B. Baretzky, H. Hahn: High Pressure Torsion Extrusion as a new severe plastic deformation process. In: Materials Science and Engineering: A 664 (2016), S. 247-256.

[6] R. Kulagin, Y. Beygelzimer, Y. Estrin, Y. Ivanisenko, B. Baretzky, H. Hahn: A Mathematical Model of Deformation under High Pressure Torsion Extrusion. In: Metals 9 (2019).

Round 2

Reviewer 1 Report

The revised version of the paper looks much better (e.g. Figure 2), and may be considered for publication.

However, there are still many faults and flaws which should be corrected, before edition. They have been marked in color, just see the lines: 83, 101 (wt%), 102 and 113 (separate deg C), 115, 139, 442, 544; 146 (20 kV), 203, 206; 166, 168; 211 (kG, not Kg!); 215 (full MINUS), 407, 408, Table 2; 298 (41%), 339, 401, 512; 314, 321, 328, 380, 555, 570; 375, 389.

Comments for author File: Comments.pdf