Evolution of Microstructure and Mechanical Properties of Steam Generator Material After Long-Term Operation in Nuclear Power Plant

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript investigates the evolution of microstructure and mechanical properties of WWER 440 steam generator steel (22K) after prolonged operational exposure in a nuclear power plant. Experimental analysis using optical microscopy and transmission electron microscopy demonstrated minor changes in carbide density and particle distribution with increasing operation time, indicating microstructural stability. Mechanical properties evaluated through Small Punch Tests (SPT) revealed minimal variations in tensile strength, yield strength, and ductile-to-brittle transition temperature (DBTT), confirming the material's sustained integrity and suitability for long-term nuclear applications.

 

1. Line 20: “transmission electron microscopy transmission electron microscopy” Comment: Remove the repetition; it should be “transmission electron microscopy.”
2. Line 22-23: “with the prolongation of operation time of the steam generator” Comment: “with the prolonged operational time of the steam generator.”
3. Line 26: “It was revealed minimal changes” Comment: “Minimal changes were revealed.”
4. Line 36: “The long-term operation (LTO) allows” Comment: Remove “The” to be concise: “Long-term operation (LTO) allows.”
5. Line 38: “which protect environment via minimizing CO2 emissions” Comment: “which protects the environment by minimizing CO2 emissions.”
6. Line 40: “but there is no fixed technical limit to the life of the reactors.” Comment: “but there is no fixed technical limit to reactor lifetimes.”
7. Line 43: “primary circuit components and its construction steel” Comment: “components and their construction steels.”
8. Line 46: “effective, reliable, and LTO behind the projected lifetime” Comment: “effective, reliable, and capable of LTO beyond the projected lifetime.”
9. Line 52: “The main mechanical loading of all individual parts are the influence” Comment: “The main mechanical loads affecting all individual parts are due to high pressure.”
10. Line 54: “This process led to fatigue damage” Comment: Should be in present tense: “This process leads to fatigue damage.”
11. Line 112: “For taking samples as part of the temperature aging.” Comment: “Samples were taken as part of the temperature aging.”
12. Line 120: “critical point of material evaluation has just cut and prepared specimens” Comment: “A critical aspect of material evaluation involves cutting and preparing specimens.”
13. Line 132: “samples to obtain semi-finished specimens are carried out by.” Comment: “samples to obtain semi-finished specimens was carried out using.”
14. Line 150: “Special testing rigs, depending on the testing temperature, are used.” Comment: “Depending on testing temperatures, special testing rigs are utilized.”
15. Line 152-153: “subsequently polished with abrasive papers up to fine grinding P1200” Comment: “subsequently polished with abrasive paper up to P1200 grit.”
16. Line 217: “significant changes in the microstructure caused by long-term operation are not observed” Comment: “No significant microstructural changes due to long-term operation were observed.”
17. Line 224: “Matrix is formed by polyhedral ferrite with small quasi-globular particles.” Comment: “The matrix consists of polyhedral ferrite containing small quasi-globular particles.”
18. Line 246: “especially in the size class of 100 to 200 nm” Comment: “particularly in the size range of 100 to 200 nm.”
19. Line 285-286: “Over time, irradiation-induced hardening can lead to material degradation, increasing the risk of failure in critical components, so regular monitoring and evaluation.” Comment: “Over time, irradiation-induced hardening may degrade materials, increasing the failure risk in critical components; thus, regular monitoring.”
20. Line 277-278: “were tested done at room temperature” Comment: “were conducted at room temperature.”
21. Line 292: “Mechanical properties measured by SPT test” Comment: “Mechanical properties measured via SPT.”
22. Line 321: “In order to study the effect of long-term operation of the on steel 22K were analyzed samples” Comment: “To study the effects of long-term operation, samples of steel 22K were analyzed.”
23. Line 325-326: “The presented data presented demonstrate” Comment: “The data demonstrate.”
24. Line 332: “Only for tensile strength at room temperature, the values displayed greater variation compared to years of operation.” Comment: Clearer wording: “Only tensile strength at room temperature showed considerable variation with years of operation.”
25. Line 335: “As it was expected, the longterm operation increases.” Comment: “As expected, long-term operation increases.”
26. Lines 56-57: “while it is mainly related to the nitrogen content and the so-called surface-active impurities” Comment: Provide clarification or justification about why nitrogen content specifically impacts thermal aging, as it may not be obvious to all readers.
27. Lines 60-62: “Nevertheless, this degradation mechanism of materials is an increasingly discussed issue, also because more than 63% of power reactors have been in operation for more than 30 years.” Comment: Consider specifying why exactly 30 years is significant in the context of thermal aging and whether it is related directly to observed degradation thresholds or merely an operational milestone.
28. Lines 154-156: “The polynomial function, because of compliance correction measurements, is introduced into system.” Comment: Clearly explain why and how the polynomial function is specifically used for compliance correction in the context of SPT, since the reader might find it ambiguous.
29. Lines 168-171: “However, in order to increase the accuracy of the SPT results, a specific correlation relationship for each tested material and testing machine can be used, as was the case for SPT tests under the SSP.” Comment: Clarify logically why a general correlation coefficient (βRm, βRe) from the standard is considered sufficient or insufficient, thus requiring a specific correlation.
30. Lines 217-218: “Based on the results obtained, it can be concluded that significant changes in the microstructure caused by long-term operation are not observed using LOM.” Comment: The absence of observable changes via LOM logically suggests limitations in this technique; it might be beneficial to explicitly state that more sensitive techniques (e.g., TEM) are necessary for such analysis.

Author Response

1. Line 20: “transmission electron microscopy transmission electron microscopy” Comment: Remove the repetition; it should be “transmission electron microscopy.”

Response 1: Thank you for your comment. The abstract was edited.

1. Line 22-23: “with the prolongation of operation time of the steam generator” Comment: “with the prolonged operational time of the steam generator.”

Response 2: Edited. Thank you very much for all the corrections to my English. I sincerely appreciate it.

3. Line 26: “It was revealed minimal changes” Comment:“Minimal changes were revealed.”

Response 3: Edited.

1. Line 36: “The long-term operation (LTO) allows” Comment: Remove “The” to be concise: “Long-term operation (LTO) allows.”

Response 4: Edited.

1. Line 38: “which protect environment via minimizing CO2 emissions” Comment: “which protects the environment by minimizing CO2 emissions.”

Response 5: Edited.

1. Line 40: “but there is no fixed technical limit to the life of the reactors.” Comment: “but there is no fixed technical limit to reactor lifetimes.”

Response 6: Edited.

1. Line 43: “primary circuit components and its construction steel” Comment: “components and their construction steels.”

Response 7: Edited.

1. Line 46: “effective, reliable, and LTO behind the projected lifetime” Comment: “effective, reliable, and capable of LTO beyond the projected lifetime.”

Response 8: Edited.

1. Line 52: “The main mechanical loading of all individual parts are the influence” Comment: “The main mechanical loads affecting all individual parts are due to high pressure.”

Response 9: Edited.

1. Line 54: “This process led to fatigue damage” Comment: Should be in present tense: “This process leads to fatigue damage.”

Response 10: Edited.

1. Line 112: “For taking samples as part of the temperature aging.” Comment: “Samples were taken as part of the temperature aging.”

Response 11: Edited.

1. Line 120: “critical point of material evaluation has just cut and prepared specimens” Comment: “A critical aspect of material evaluation involves cutting and preparing specimens.”

Response 12: Edited.

1. Line 132: “samples to obtain semi-finished specimens are carried out by.” Comment: “samples to obtain semi-finished specimens was carried out using.”

Response 13: Edited.

1. Line 150: “Special testing rigs, depending on the testing temperature, are used.” Comment: “Depending on testing temperatures, special testing rigs are utilized.”

Response 14: Edited.

1. Line 152-153: “subsequently polished with abrasive papers up to fine grinding P1200” Comment: “subsequently polished with abrasive paper up to P1200 grit.”

Response 15: Edited.

1. Line 217: “significant changes in the microstructure caused by long-term operation are not observed” Comment: “No significant microstructural changes due to long-term operation were observed.”

Response 16: Edited.

1. Line 224: “Matrix is formed by polyhedral ferrite with small quasi-globular particles.” Comment: “The matrix consists of polyhedral ferrite containing small quasi-globular particles.”

Response 17: Edited.

1. Line 246: “especially in the size class of 100 to 200 nm” Comment: “particularly in the size range of 100 to 200 nm.”

Response 18: Edited.

1. Line 285-286: “Over time, irradiation-induced hardening can lead to material degradation, increasing the risk of failure in critical components, so regular monitoring and evaluation.” Comment: “Over time, irradiation-induced hardening may degrade materials, increasing the failure risk in critical components; thus, regular monitoring.”

Response 19: Edited.

1. Line 277-278: “were tested done at room temperature” Comment: “were conducted at room temperature.”

Response 20: Edited.

1. Line 292: “Mechanical properties measured by SPT test” Comment: “Mechanical properties measured via SPT.”

Response 21: Edited.

1. Line 321: “In order to study the effect of long-term operation of the on steel 22K were analyzed samples” Comment: “To study the effects of long-term operation, samples of steel 22K were analyzed.”

Response 22: Edited.

1. Line 325-326: “The presented data presented demonstrate” Comment: “The data demonstrate.”

Response 23: Edited.

1. Line 332: “Only for tensile strength at room temperature, the values displayed greater variation compared to years of operation.” Comment: Clearer wording: “Only tensile strength at room temperature showed considerable variation with years of operation.”

Response 24: Edited.

1. Line 335: “As it was expected, the longterm operation increases.” Comment: “As expected, long-term operation increases.”

Response 25: Edited.

1. Lines 56-57: “while it is mainly related to the nitrogen content and the so-called surface-active impurities” Comment: Provide clarification or justification about why nitrogen content specifically impacts thermal aging, as it may not be obvious to all readers.

Response: Text was edited.

1. Lines 60-62: “Nevertheless, this degradation mechanism of materials is an increasingly discussed issue, also because more than 63% of power reactors have been in operation for more than 30 years.” Comment: Consider specifying why exactly 30 years is significant in the context of thermal aging and whether it is related directly to observed degradation thresholds or merely an operational milestone.

Response 27: The text explaining why 30 years is a significant milestone was added. (page no. 2, line 66)

1. Lines 154-156: “The polynomial function, because of compliance correction measurements, is introduced into system.” Comment: Clearly explain why and how the polynomial function is specifically used for compliance correction in the context of SPT, since the reader might find it ambiguous.

Response 28: Thank you for pointing this out. An explanation has been added to the text. (page no. 5, line 159)

1. Lines 168-171: “However, in order to increase the accuracy of the SPT results, a specific correlation relationship for each tested material and testing machine can be used, as was the case for SPT tests under the SSP.” Comment: Clarify logically why a general correlation coefficient (βRm, βRe) from the standard is considered sufficient or insufficient, thus requiring a specific correlation.

Response 29: We added a text with a logical explanation. (page no. 6, line187)

1. Lines 217-218: “Based on the results obtained, it can be concluded that significant changes in the microstructure caused by long-term operation are not observed using LOM.” Comment: The absence of observable changes via LOM logically suggests limitations in this technique; it might be beneficial to explicitly state that more sensitive techniques (e.g., TEM) are necessary for such analysis.

Response 30: We agree with this comment. We added clarification (page no. 8, line 255)

Reviewer 2 Report

Comments and Suggestions for Authors

This study is important as it helps establish the microstructural and mechanical stability of this grade of steel over a long period of operation. The findings can support future material selection and design studies for steel used in next-generation energy plants.

This work has potential for publication, but major revisions are required. I have clearly outlined the necessary corrections in the PDF of the article. 

Comments for author File: Comments.pdf

Author Response

Comments 1: This excerpt should be placed in the Introduction section rather than in the Abstract. The Abstract should clearly state what was done, how it was done, and the key results without extraneous statements.

Response 1: Thank you for your comment. The abstract was edited.

Comments 2: This is a long sentence. It should be broken down into three shorter sentences for clarity. This applies to other long sentences in the article

Response 2: Thank you. You are right. We broken down the sentence.

Comments 3 : Not clear what you mean here. Clarify please.

Response 3: Edited.

Comments 4 : I know what you mean, but try to make the sentence clearer.

Response 4 : Accepted. We have expanded the text to clarify this. (page 2, line 53)

Comments 5 : Use your abbreviation NPP here

Response 5 : Accepted.

Comments 6 : That is not correct; there are significant publications in this area. However, this paper is informative.

Response 6 : It has been found that only a few publications are available regarding the evaluation of components currently in operation in NPP

Comments 7 : Long sentence, break it down and make it clearer.

Response 7: Accepted.

Comments 8 : Use NNPs only. This has been defined previously

Response 8 : Accepted.

Comments 9 : No previous statement to warrant use of 'also' here.

Response 9: Thank you for pointing this out.

Comments 10 :  This is a long sentence. It should be broken down into three shorter sentences for clarity. This applies to other long sentences in this article.

Response 10: Accepted. We have broken down all the long sentences.

Comments 11 : Summarizes? You should be presenting your study and not the summary.

Response 11: Thanky you. Edited.

Comments 12 :  Use this grade naming in the paper title

Response 12: You are right, it would be better. But we are afraid to change the title of the article due to formal issues. the mention of gost 22 k is right in the first sentence of the abstract

Comments 13: Make the statement clearer

Response 13: Thank you. Edited.

Comments 14 : Rewrite, and make it concise.

Response 14: Thank you. We have rewritten the text to make it clearer.

Comments 15 : State the part of the vessel that was cut. Inner or outer surface of the RPV?

Response 15: Your comment is well founded. It is the outer surface. (pages 4, line 125/135)

Comments 16 : Introduce what it is used for before explaining the basis.

Response 16: With all due respect, we described the SPT test as it is usually described at ASTM conferences

Comments 17 : State only what you did with the tool. Further explanation and referencing articles is not necessary this section

Response 17: Although the idea was well-intentioned, we respectfully described the SPT test briefly to make it clear to the reader.

Comments 18 : What do you mean by this? This is not a review section.

Response 18: Thank you. We expanded the text with a polynomial formula

Comments 19: Please state clearly what you did - and not telling the reader more accurate means that is not explicit in the article.

Response 19: Thank you for the good warning. We edited the text. (page 6, line 182)

Comments 20: This is one of the main parts of your study - so why the use of supplemented here? Also, are you monitoring or examining?

Response 20: Thank you. Edited.

Comments 21 : Not clear. Rewrite.

Response 21: Accepted. We have rewritten the text

Comments 22: What is secondary excluded particles?

Response 22: The text was rewritten.. (page 6, line 225)

Comments 23: This is not a material..

Response 24: Thank you. Edited.

Comments 25: What do you mean by polyhedral morphology? State the phases present and clearly identify them?

Response 25: Thank you for your kind comments. The text in this chapter has been written more clearly.

Comments 26 :  How did you know this? I cannot see any grain measurement or example of how it was measured here.

Response 26: Thank you for your kind comments. The text in this chapter has been written more clearly.

Comments 27: Please show the acicular ferrite on the micrograph - to convince the reader of its presence.

Response 27: We edited the image

Comments 28, 29, 30: These microstructural comparisons must be clear for this study to be meaningful.  

Response28, 29, 30: Thank you. We have rewritten the description for TEM.

Comments 31,32,33:   This figure is not useful to the reader, as the numbers are not legible.

Response 31,32,33:  In our opinion, Figures 7c and 7d are suitable. Figure 7c presents a

standardly evaluated diffraction pattern obtained by electron diffraction. It

complements the bright and dark field image and provides phase identification

of the particles that were observed in the ferritic grains. Figure 7d is an

 image that characterizes the EDS spectrum of particles in the ferritic

matrix, which is again complementary information about these particles. They

support the results of the chemical stability of 22K steel with respect to

long-term operation.

Comments 34: Describe what you observed before making conclusion.

Response 34: An explanation has been added to the text.

Comments 35 : What do you mean by structural parameters? Are you referring to particle size measurements based on equation 3 -6 above?

Response 35: Thank you. We have clarified the text. (page 10, line 300)

Comments 36: Describe the data trend in the table before making conclusions.

Response 36: Accpeted.

Comments 37: This should be in the Method Section and not in Result section.

Response 37: Thank you. we have edited the text as is commonly used in the SPT section

Comments 38: Is this your result or typical trend? Pls be clear here.

Response 38: Edited.

Comments 39: Move this figure to Method Section.

Response 39: Thank you. The figure has been moved.

Comments 40: Is this figure your result or typical trend of your data? Anything that is not the result you obtained should not be in this section.

Response 40: Edited.

Comments 41: This test temperage range was not described in your Method Section

Response 41: The SPT standard does not specify a precise temperature range for the test.

Comments 42 : Why use the phrase 'an example of'? Is this your result or a general trend? You need to clearly state your own data, present the corresponding plot or analysis, description of the trend, and conclusion. The use of vague terms like 'example' and 'typical' makes the study appear less reliable.

Response 42: Thank you very much for this comment. We have edited the text.

Comments 43: You have not explained to your reader what DBTT means in your figure.

Response 43: We added an explanation of DBTT to the text. (page 13, line 361)

Comments 44: Your conclusion cannot be relied upon as the evidence is unclear.

Response 44: We appreciate your detailed review of the text. We have carefully revised it and made all necessary corrections to the best of our ability.

Reviewer 3 Report

Comments and Suggestions for Authors

1) In the Abstract the Authors should place some of the most important exact results obtained in the research which will clearly highlight paper novelty. At the moment, the Abstract is too general and mainly descriptive.

2) Section 2. Materials and Methods – in this Section the Authors should present the most important parameters (accuracy and precision, for example) related to each used measurement device. The measurement procedure is correctly explained, measurement equipment is mentioned – but any specifications related to the used measurement devices are missing. It should also be explained and discussed which is overall measurement accuracy and precision and which influence measurement equipment accuracy and precision has on the obtained and presented results.

3) Figures 6 and 7 should be properly explained and discussed. At the moment, the Authors have presented results only, any exact explanation, discussion, relation between the obtained results and any details are completely missing.

4) Figure 9 should be removed – it shows the same results as presented in Table 5. From Table 5 is clear trend presented in Figure 9, so it is really not necessary to show the same results twice – once in a form of a Table and once more in a form of a Figure.

5) Table 6 should be explained in much more detail.

6) Throughout the paper, the specific and exact explanations should be used, the explanations and discussions should not be general and overall (as many of them are at the moment).

7) Figure 12 should be removed because it shows the same data as previously presented in Table 7. There is no reason for the presentation of the same results twice.

8) Throughout the paper can be found various obvious and typing mistakes – all of them should be removed during the revision process. For example, Line 322 – the dot should be placed at the end of the sentence (after word “operation”). This is just one example, there are many more similar obvious and typing mistakes throughout the paper.

9) Section 4. Conclusions – in the Conclusions should be added the most important exact results obtained in the research (as in the Abstract). Also, at the end of the Conclusions should be added and explained guidelines in further research.

10) At the moment, this paper looks like technical report, not as an Original scientific paper. The Authors should much better highlight and explain which scientific novelties are obtained and which the main contributions of this paper to the specific research field are.

11) List of the References – in this List are dominantly used references which are not recent (older than 5 or 10 years). In the List of the References the Authors should add much more recent literature (not older than 5 years) related to the observed research field.

12) English is good and understandable, but it can and should be improved throughout the paper.

 

Final remarks: This is an interesting paper, but it requires deep, detail and proper revision (according to the above comments).

Comments on the Quality of English Language

English is good and understandable, but it can and should be improved throughout the paper.

Author Response

• In the Abstract the Authors should place some of the most important exact results obtained in the research which will clearly highlight paper novelty. At the moment, the Abstract is too general and mainly descriptive.

Response 1: Thank you for your comment. The abstract was edited.

• Section 2. Materials and Methods – in this Section the Authors should present the most important parameters (accuracy and precision, for example) related to each used measurement device. The measurement procedure is correctly explained, measurement equipment is mentioned – but any specifications related to the used measurement devices are missing. It should also be explained and discussed which is overall measurement accuracy and precision and which influence measurement equipment accuracy and precision has on the obtained and presented results.

Response 2: We have added text regarding the compressibility of TEM and . Energy-dispersive X-ray spectroscopy. In the case of SPT, the measurement accuracy is determined by the standard.

• Figures 6 and 7 should be properly explained and discussed. At the moment, the Authors have presented results only, any exact explanation, discussion, relation between the obtained results and any details are completely missing.

Response 3: Thank you for pointing this out. The text was expanded to include a discussion. (page no. 8, line 249)

• Figure 9 should be removed – it shows the same results as presented in Table 5. From Table 5 is clear trend presented in Figure 9, so it is really not necessary to show the same results twice – once in a form of a Table and once more in a form of a Figure.

Response 4: Your comment is well-founded, however, with all due respect, we believe that the figures complement the results and provide a clear graphical overview of the results.

• Table 6 should be explained in much more detail.

Response 5: An explanation has been added to the text. (page no. 11, 305)

• Throughout the paper, the specific and exact explanations should be used, the explanation and discussions should not be general and overall (as many of them are at the moment).

Response 6: Thank you very much for this comment. We have edited the text.

 

• Figure 12 should be removed because it shows the same data as previously presented in Table 7. There is no reason for the presentation of the same results twice.

Response 7: as in the case of comment 4, we value your opinion but we believe that the image is also a good complement to the results.

• Throughout the paper can be found various obvious and typing mistakes – all of them should be removed during the revision process. For example, Line 322 – the dot should be placed at the end of the sentence (after word “operation”). This is just one example, there are many more similar obvious and typing mistakes throughout the paper.

Response 8: Thank you for reading our manuscript in detail. We have corrected all these typing mistakes.

• Section 4. Conclusions – in the Conclusions should be added the most important exact results obtained in the research (as in the Abstract). Also, at the end of the Conclusions should be added and explained guidelines in further research.

Response 9: A text has been added to the conclusions, including a link to an international project that will expand our research.

10) At the moment, this paper looks like technical report, not as an Original scientific paper. The Authors should much better highlight and explain which scientific novelties are obtained and which the main contributions of this paper to the specific research field are.

Response 10: We are grateful for this observation and have addressed it in the revised manuscript. We have carefully revised it and made all necessary corrections to the best of our ability.

11) List of the References – in this List are dominantly used references which are not recent (older than 5 or 10 years). In the List of the References the Authors should add much more recent literature (not older than 5 years) related to the observed research field.

Response 11: When editing comments, we used scientific articles that met the these criteria

12) English is good and understandable, but it can and should be improved throughout the paper.

Response 12: Thank you for this comment, we agree and have corrected the level of English throughout the text.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Agreed with the revisions made by the authors. 

Author Response

Thank you for your valuable feedback. Your reviews have significantly contributed to improving the quality of our paper.

Reviewer 2 Report

Comments and Suggestions for Authors

Your detailed response to comments is appreciated.

Author Response

Thank you for your valuable feedback. Your reviews have significantly contributed to improving the quality of our paper.

Reviewer 3 Report

Comments and Suggestions for Authors

The Authors have performed the majority of modifications and improvements during the revision process. However, some elements mentioned in my review are not properly performed. I will refer to my previous comments:

COMMENTS 4 and 7 - I cannot agree with the Authors that it is reasonable to present the same results twice - once in a form of a Table, second time in a form of a Figure. I still believe that Figures 10 and 12 should be removed because they present the same results as previously shown in Table 5 and Table 7. The Authors and I will agree to disagree on this matter - I will suggest to the Editor(s) to decide what will be performed in this matter.

COMMENTS 5 and 6 - the Authors have added some additional explanations in the paper, but that explanations are dominantly minor or practically the same as in the first paper version. The explanations should be much more precise than at the moment - there are no notable improvements in the paper related to the results explanation.

 

Therefore, I believe that this paper still requires revision and modifications (this time minor ones).

Author Response

Comments 4 and 7:  I cannot agree with the Authors that it is reasonable to present the same results twice - once in a form of a Table, second time in a form of a Figure. I still believe that Figures 10 and 12 should be removed because they present the same results as previously shown in Table 5 and Table 7. The Authors and I will agree to disagree on this matter - I will suggest to the Editor(s) to decide what will be performed in this matter.  

Response: We appreciate your opinion. The image was originally included in the article to visually illustrate the small differences between the values obtained in our work. However we respect your viewpoint and the Figure 12 has been removed.

Comments 5 and 6: the Authors have added some additional explanations in the paper, but that explanations are dominantly minor or practically the same as in the first paper version. The explanations should be much more precise than at the moment - there are no notable improvements in the paper related to the results explanation.

Response:  We have tried to explain our results more precisely so that they can be published, because we believe they are interesting. For more extensive analyses on operating materials, there are ongoing projects that focus on the long-term operation of nuclear power plants. We believe that after the completion of the projects, more extensive results will be shared, which will also be included in material databases.