Anomalous Work Hardening Behavior of a Single Crystalline Co-Base Superalloy

Round 1

Reviewer 1 Report

·       Add the EDS table with the compostion.

·       Show the novelty of the paper compared to the literature, however there is still much work on this topic.

·       Why you choose these materials?

·       In the Introduction section, the last paragraph should contain the scientific contribution and scientific hypotheses of your research. Complete, further elaborate the scientific contribution and scientific hypotheses of your research. Be explicit. In addition to the goal of the research (which was written), the novelty in the context of the scientific contribution should be pointed out. Scientific contributions should be written based on the shortcomings of previous research in the literature. In this way, the authors will better emphasize novelty and scientific soundness.

·       Analyze and discuss possibilities of practical application.

·       Complete the conclusions with the limitations of the proposed methodology. Also write future research.

·       Add a sentences after Figure 5.

·       Generally, the quality of the writing could be improved.

Author Response

• Add the EDS table with the compostion.

The EDS composition is included in line 86-87 as follows:

The experimental alloy ERBOCo-VF60 (nominal composition: Co-8.9Al-9.0W-2.3Ta (at.%), EDS: Co-8.2Al-9.4W-1.7Ta (at.%)) was […]

• Show the novelty of the paper compared to the literature, however there is still much work on this topic.

In line 73, we wrote that “A direct correlation between the stress-strain curves associated with this work hardening behavior and the evolution of the underlying defect structure is missing for the otherwise extensively investigated class of Co-Al-W-Ta alloys.” Additionally, the direct influence of both temperature and work-hardening on this anomalous work-hardening behavior has not been investigated up to now, which revealed the importance of segregation on the occurring deformation mechanisms and work-hardening behaviors.

Additionally, we added after the second paragraph in the introduction:

“Similarly, in Co-base superalloys, which have a positive lattice misfit and are deformed under compression, plastic deformation is initiated in the horizontal g matrix channels [28,29]. However, no distinctive microyielding stages have been reported in this alloy class up to now.“

• Why you choose these materials?

The Co-Al-W-Ta alloys are very suitable model alloys, since they can deform under the formation of stacking faults, which is correlated with this strong work hardening behavior. This massive formation of SF in this type of alloys was already found in a similar alloy by Suzuki et. al in 2008. However, while the class of Co-Al-W-Ta alloys has been investigated quite extensively, more detailed in-depth studies on the observed work-hardening behavior and the correlation with the underlying defect structures has not been conducted as of now.

• In the Introduction section, the last paragraph should contain the scientific contribution and scientific hypotheses of your research. Complete, further elaborate the scientific contribution and scientific hypotheses of your research. Be explicit. In addition to the goal of the research (which was written), the novelty in the context of the scientific contribution should be pointed out. Scientific contributions should be written based on the shortcomings of previous research in the literature. In this way, the authors will better emphasize novelty and scientific soundness.

In line 73, we wrote that “A direct correlation between the stress-strain curves associated with this work hardening behavior and the evolution of the underlying defect structure is missing for the otherwise extensively investigated class of Co-Al-W-Ta alloys.” Subsquently, we changed the sentence to “The goal of this study is to study the work hardening behavior of a single crystalline Co-Al-W-Ta superalloy via conventional transmission electron microscopy to rationalize the high work-hardening rates in Ta-containing Co-base superalloys.“

• Analyze and discuss possibilities of practical application.

We added the following lines in line 316:

“These investigations undoubtedly reveal the strengthening and softening effects of stacking fault shear and microtwinning, respectively. As shown by Smith et al. [19,34], microtwinning can be inhibited by local phase transformations along the stacking faults. Thus, the strengthening effect provided by stacking fault shearing might be used in future alloys, which simultaneously inhibit microtwinning and thus exhibit superior mechanical properties at high temperatures.”

• Complete the conclusions with the limitations of the proposed methodology. Also write future research.

As written in the journal guidelines, the conclusion section should summarize the main conclusions of this work, which is why we decided not to include the limitations of the proposed methodology and future research in this section.

• Add a sentences after Figure 5.

As the sentence in line 310-312 is the ending of the discussion, we prefer not to add another one after Figure 5, since it has been already introduced in line 235.

• Generally, the quality of the writing could be improved.

We modified the manuscript accordingly and hope the reviewer is satisfied.

Reviewer 2 Report

The manuscript is well written and the topic is of great interest to the scientific community. However, I would suggest the revision of the following points to improve its quality:

 

1. Concerning the Introduction and the cited references:

References [29-33] are only cited at the Discussion section and are never mentioned in the Introduction. Works with such relevance for the topic must be introduced at the beginning and be accompanied with an explanation on what the current manuscript brings of novelty with respect to those works.

 

2.      Materials and Methods:

First paragraph (lines 63-69) Reference [28] does not seem appropriate/relevant. Not same material and apparently not same process.

 

3.      Results:

3.1. Sentence at lines 97-99 should be re-written clearly.

3.2. Sentence at lines 99-101: “… at 850°C and 950°C, respectively, …”

3.3. Lines 107-111: please specify which values of strain-rate (10-3, 10-4 and/or 10-5 s-1) you are referring to when using the terms “higher strain-rates” and “lower strain-rates”.

3.4. Figure 1: There is no previous explanation on how the plotted variables were calculated, namely, the true stress (is it the Cauchy stress?), the work hardening rate (dσ/dεpl ? which increment size?) and the true plastic strain (calculated via Hooke’s law from the total strains?). Please add that information in the Materials and Methods section.

3.5. Sentence at lines 132-134 is not clear. Could you re-write it?

3.6. Plots of Figures 2, 3, 4 and 5 – No legend nor axes numbering. I am not sure about the intention of the “200 MPa, 1%” axis representation – is this supposed to replace the axes numbering?

 

4.      Discussion:

As already mentioned in point 1., relevant information from the literature should be provided in the introduction (as it is the case of the paragraph at lines 200-220, for example). Such works can be again cited in this section to justify/support the discussion of results presented in this manuscript. The discussion section is not supposed to be a literature review section. Please revise the Discussion and Introduction sections accordingly.

 

 

 

 

5.      Conclusions:

5.1.  First conclusion (line 318) must indicate it is for a temperature of 950°C.

5.2.  Third conclusion (line 330), for the lower temperature please remove the “less than or equal” symbol as only the 850°C temperature is presented in this work, not lower temperatures. The same for the higher temperature (1000°C, not ≥1000°C).

 

Finally, just a suggestion of a recent review article on the same topic that may make sense to cite:

Zhao, Y., Zhang, Y., Zhang, Y., Luo, Y., Tang, D., Liu, H., & Fu, H. (2021). Deformation behavior and creep properties of Co-Al-W-based superalloys: A review. Progress in Natural Science: Materials International, 31(5), 641-648.

 

Author Response

The manuscript is well written and the topic is of great interest to the scientific community. However, I would suggest the revision of the following points to improve its quality:

We want to thank the reviewer for the feedback and hope that we could answer the raised points satisfactorily.

1. Concerning the Introduction and the cited references:

References [29-33] are only cited at the Discussion section and are never mentioned in the Introduction. Works with such relevance for the topic must be introduced at the beginning and be accompanied with an explanation on what the current manuscript brings of novelty with respect to those works.

The mentioned references are now also cited in the Introduction. Additionally, we added the following sentences in the introduction after the microyielding paragraph: “Similarly, in Co-base superalloys, which have a positive lattice misfit and are deformed under compression, plastic deformation is initiated in the horizontal γ matrix channels [28,29]. However, no distinctive microyielding stages have been reported in this alloy class up to now.“

2. Materials and Methods:

First paragraph (lines 63-69) Reference [28] does not seem appropriate/relevant. Not same material and apparently not same process.

The used Bridgman furnace and experimental procedure was first introduced in [28] and remains the same for all alloys cast at our institutes since then. Hence, we would like to keep the reference here to show interested readers more details in regards to the precise parameters used for the casting via the Bridgman process.

3. Results:

3.1. Sentence at lines 97-99 should be re-written clearly.

We hope we could clear this sentence by re-writing it as follows:

At 850 °C and 950 °C, an increase in the work hardening rate is observed at approximately 1.5 and 2.0 % plastic strain, respectively (see also Figure 1c).

3.2. Sentence at lines 99-101: “… at 850°C and 950°C, respectively, …”

We changed the sentence accordingly.

3.3. Lines 107-111: please specify which values of strain-rate (10-3, 10-4 and/or 10-5 s-1) you are referring to when using the terms “higher strain-rates” and “lower strain-rates”.

We changed the sentence accordingly.

3.4. Figure 1: There is no previous explanation on how the plotted variables were calculated, namely, the true stress (is it the Cauchy stress?), the work hardening rate (dσ/dεpl ? which increment size?) and the true plastic strain (calculated via Hooke’s law from the total strains?). Please add that information in the Materials and Methods section.

We added the following part to the Materials and Methods section:

From the measured force and displacement data, the engineering stress  and strain  are calculated and converted to the true stress σ_t=σ(1+ε) and true strain ε_t=ln(1+ε). Subsequently, the elastic part of the stress-strain curve is fitted with a linear function and the resulting elastic true strain is subtracted from the true strain to obtain the true plastic strain ε_pl. By dividing the true stress by the true plastic strain, the work hardening rate Θ is determined.

3.5. Sentence at lines 132-134 is not clear. Could you re-write it?

We rewrote the sentence to:

When dislocations cut into the γ′ precipitates, the nature of the resulting stacking faults are mostly intrinsic as determined by the fringe contrast criterion in CDF images (Figure 2d).

3.6. Plots of Figures 2, 3, 4 and 5 – No legend nor axes numbering. I am not sure about the intention of the “200 MPa, 1%” axis representation – is this supposed to replace the axes numbering?

In order to make the figure as visually pleasing as possible, we removed the axis numbering and just included the 200 MPa, 1% for completeness. The reason for introducing the plots of figures 2-5 was just to visually aid the reader at which plastic strain the tests were interrupted and show the reproducibility with the results from Figure 1.

4. Discussion:

As already mentioned in point 1., relevant information from the literature should be provided in the introduction (as it is the case of the paragraph at lines 200-220, for example). Such works can be again cited in this section to justify/support the discussion of results presented in this manuscript. The discussion section is not supposed to be a literature review section. Please revise the Discussion and Introduction sections accordingly.

We want to thank the reviewer for this suggestion and removed this review from the discussion and included them in the introduction and changed the manuscript accordingly (see lines 47 to 67 and lines 227 to 230).

5. Conclusions:

5.1.  First conclusion (line 318) must indicate it is for a temperature of 950°C.

We added temperature and also strain-rate to this conclusion.

5.2.  Third conclusion (line 330), for the lower temperature please remove the “less than or equal” symbol as only the 850°C temperature is presented in this work, not lower temperatures. The same for the higher temperature (1000°C, not ≥1000°C).

We changed the conclusion accordingly.

Round 2

Reviewer 1 Report

The paper was improved.