Effect of Different Precipitation Routes of Fe2Hf Laves Phase on the Creep Rate of 9Cr-Based Ferritic Alloys
Round 1
Reviewer 1 Report
The present paper is very valuable scientific contribution to knowledge on the effects of precipitation routes of Fe2Hf Laves phase particles on the creep rate of 9Cr based heat resistant ferritic alloys. These findings really open the novel precipitation routes to strengthen heat resistant ferritic steels.
In the description of figures 4 - 10 there is mistakenly a text: “Figures should be placed in the main text near to the first time they are 214 cited. A caption on a single line should be centered.”, which must be removed before publishing the article.
Author Response
Thank you for your valuable comments. We put captions for figures 4-10. The manuscript is revised according to three reviewers’ comments. You can find the revised parts in red.
Reviewer 2 Report
- It is in lack of details of specimen preparing for SEM observation. it would be good to provide more info about metal specimen surface grinding, chemical corrosion, etc.
- The experiment designation sounds not very convincing. For example, the microstructure before creep test is not mentioned, not investigated with phase composition. No TEM observation to these fine particles and so the evidence of Hf-rich particle is not strong.
- Hf-rich particles are not identified by EDS data supporting.
- Significant error was found at the captions for the figures 3 to 10. All the eight figures are sharing the same caption. More reticular, the caption is a description about how to write a cation and has no relation to any real figure.
- I failed to finish reading because of caption mission in most of the figures.
It is suggested to return it back to the authors and to resubmit as a new one by a complete checking, amending, and adding TEM data.
Author Response
Thank you for your valuable comments. We apologize to forget to put captions for figures 4-10. We revised our manuscript according to your comments, and you can find our responses below. Modified parts are shown in red in the revised manuscript. It would be nice if you would make a review again.
1. We added detailed descriptions of sample preparation for microstructure observation in experimental procedure part.
2. Microstructures for (before) creep tests are shown in Figure 1. We have characterized the microstructures, especially the Hf-rich fine particles, in the 0.33Hf alloy with TEM and STEM [7, 8], and we identified they are of C14 type Fe2Hf Laves phase. We modified description in introduction to clarify this point.
3. We added EDS Hf maps which correspond to the STEM bright field images shown in Figure 7.
4. We put captions for figures 4-10.
Reviewer 3 Report
Please check the captions of Figure 4-10.
Add an explanation of what is white dots in Fig 6, 8, 9. Is it pollution? Noise?
Need explanation why it was chosen these alloys.
Why Hf content was taken in these amounts?
What is LMP?
Why need to calculate Pz value? What does it shows?
How it was measured number density (Ns) and the area fraction (f)?
In Figures where are given microstructures should be added information about visible areas: to identify areas by arrows.
Author Response
Thank you for your valuable comments. We apologize to forget to put captions for figures 4-10. We revised our manuscript according to your comments, and you can find our responses below. Modified parts are shown in red in the revised manuscript. It would be nice if you would make a review again.
1. Why these alloys were chosen and the Hf contents were taken?
As explained in introduction, we recently found a new type of fine Hf-rich (Fe2Hf) phase particles in 9Cr based steel compositions, and revealed that the particles were precipitated through an interphase precipitation and are relatively stable at high temperatures compared to other Laves phase and carbide phases. This study is motivated by this finding since fine and stable second-phase particles are generally considered to be effective for creep resistance. We chose 0.33Hf to form fine Fe2Hf particles by interphase precipitation route, 0.09Hf to form fine Fe2Hf particles through conventional route (0.09Hf), and 0.03Hf as a reference to study their roles on the creep resistance.
2. What is LMP?
LMP is one of the temperature-time parameters, which is often used in our field to describe the equivalence of time when we like to compare length of time at different temperatures. We added an explanation in the caption of Figure 3.
3. Why need to calculate Pz?
We calculate Pz to compare the pinning force of particles which may act to retard the migration of interfaces. The higher Pz values of IP particles in the 0.33Hf alloy than those of CP particles in the 0.09Hf alloy at higher LMP conditions suggest the former particles would have greater ability to retard recovery and recrystallization by pinning subgrain and grain boundaries. We added some explanations in the manuscript line 207.
4. How the Ns and f were measured?
We use a free image analyzing software, and we added this explanation in the manuscript. You can find a detailed explanation in the manuscript lines 199-202.
5. Information of microstructures
We added explanations either within the micrograph or in the figure caption, hoping that this helps readers to understand the microstructures in the figures more easily. White dots are not pollution but fine Hf-rich particles!!
Round 2
Reviewer 2 Report
Please see the attached PDF document.
Comments for author File: 
Comments.pdf
Author Response
Thank you for your very valuable comments. We revised our manuscript according to your comments, and you can find our responses below. Modified parts are shown in red in the revised manuscript. It would be nice if you could accept our revision.
1. Abstract
We modified as the reviewer suggests.
2. Claim in introduction
We cited a reference. We found a mistake in the value of the ratio, and modified it accordingly.
3. Materials and methods
(1) We added test instrument type. We already explain necessary testing parameters.
(2) We added SEM type and voltage.
(3) We added TEM type and voltage.
4. Phase identification
As the reviewer claimed, it is true that we did not perform any diffraction study on the present samples, and we have newly performed XRD on samples before and after the creep tests. As we explained in the previous response, we already identified C14 type Fe2Hf phase is formed through the interphase precipitation. Newly performed XRD experiments detected diffraction peaks from C15 type Fe2Hf phase as well as C14/C36 Fe2Hf phase in the creep ruptured samples of the 0.33Hf alloy, which suggests that the C15 type Laves phase is formed or transformed from the C14 type Laves phase during the creep experiments. We added XRD data to show these results in the revised manuscript. Unfortunately, no clear diffraction peaks from the fine precipitates in the 0.09Hf alloy were detected. Although we believe the precipitates are of the same type as formed in the 0.33Hf according to a phase diagram principle, identification of the crystal structure type of the fine precipitates is beyond the scope of this paper.
5. Microstructure of the matrix
We added EBSD maps to show not only its crystal structure but also its microstructure.
6. Necessity of phase identification
We are afraid to say but we would not agree with this comment. The clear difference in the creep behaviors between the alloys is reasonably explained by the different precipitate routes and precipitate stability, as explained in the manuscript. We do not say that there are no effects of the crystal structure types on the creep behaviors, but we are sure that the obtained results cannot be interpreted by the difference in the crystal structure types only.
7. Effects of subgrains on creep
Thank you for this comment. It is generally known in the creep of ferritic/martensitic heat resistant steels that initial lath/block structures gradually recover to form cell structures with creep deformation. This recovery process is understood to soften the material and causes creep acceleration. That is the reason why we focus on the effect of fine precipitate particles on retarding the recovery processes by particle pinning effects. We added some sentences to explain this point in the manuscript.
8. Large Hf rich particles
We are not sure how the big Hf-containing particles affect gbs. We, however, find the amount and distribution of the particles are similar in the samples, and we therefore consider this factor is less important than the formation routes and the stability of the fine precipitates. We found the particles are not of Hf carbide but of Hf oxide, and thus we modified it.
9. Dislocation lines
We show evidences for the dislocation-particle interaction in the new Fig. 7(c) and in the new Fig. 8(a, b).
10. Discussion on interface coherency matter
We agree with this point and remove this part from the manuscript.
