Effects of Structural Radiation Disorder in the Near-Surface Layer of Alloys Based on NbTiVZr Compounds Depending on the Variation of Alloy Components

Review Report Form 1

1. In line 92, the phrase 'background radiation' usually refers to naturally occurring radiation, which might cause some misunderstanding. I believe the author wants to emphasize the radiation environment in a nuclear reactor.

The authors thank the reviewer for this remark; in fact, what was meant was the impact of fission fragments of nuclear fuel and gamma radiation. The text of the article has been supplemented with new data.

2. In lines 145-147, the author states that there is a reduction in crystalline size. Does this imply an increase in density for NbTiVZr or NbTiV alloys? If the data is available, please provide the densities of the synthesized alloys.

The authors thank the reviewer for this comment; the density values are presented in Table 1. These values were determined according to changes in structural parameters.

3. In the introduction part, Dr. Yanwen Zhang's group has focused extensively on investigating the mechanism of HEA radiation resistance. When the author mentions that many researchers have studied the radiation resistance of HEA alloys in lines 49-52, I recommend citing her paper 'Role of electronic energy loss on defect production and interface stability: Comparison between ceramic materials and high-entropy alloys, Current Opinion in Solid State and Materials Science, Volume 26, Issue 4, 2022, 101001.'

The authors thank the reviewer for this comment and for providing such a significant work devoted to the study of radiation damage and the influence of electronic losses on changes in the properties of materials. This work is cited in the text of the article.

4. In the materials and methods section 2.2, regarding the irradiation of alloys with heavy ions to simulate radiation exposure comparable to that of fission fragments of nuclear fuel, please provide more details about the irradiation experiment setting. For example, where was such ion irradiation produced? What is the model of the equipment? The author mentioned that the energies span from 150 to 230 MeV. Does this mean there are other irradiation energies between 150 and 230 MeV? The flux is crucial information for irradiation experiments. What is the flux of the irradiation? How were the energy losses and irradiation depths calculated? Were the values simulated by SRIM or other calculation methods?

The authors thank the reviewer for this comment. 

The samples were irradiated at the DC-60 heavy ion accelerator (Institute of Nuclear Physics of the Ministry of Energy of the Republic of Kazakhstan, Astana, Kazakhstan). Heavy ion irradiation was performed under the following parameters. For Kr¹⁵⁺ ions, the training energy was chosen to be 150 MeV; for Xe²³⁺ ions, the irradiation energy was chosen to be 230 MeV.

The choice of the type of heavy ions, specifically Kr¹⁵⁺, Xe²³⁺ is based on their capacity to simulate the mechanisms of radiation-induced damage similar to fission fragments, comparable to reactor tests, which made it possible to evaluate the possibilities of using the selected alloy compositions as reactor materials. It is worth to note that the use of these types of ions makes it possible to simulate radiation damage and the kinetics of their accumulation in the near-surface layer of alloys, about 10 – 15 μm thick, which is most susceptible to external influences both in the case of reactor tests and mechanical influences. To estimate the magnitude of energy losses, as well as the maximum travel depth of ions in the alloys, calculations were carried out using the SRIM Pro 2013 program code. Regarding the assessment of ionization losses with respect to variations in alloy components, the values of dE/dx_electron are in the range of 15 to 18 keV/nm for Kr¹⁵⁺ ion irradiation and 20 to 25 keV/nm for Xe²³⁺ ion irradiation. Additionally, the values of dE/dx_nuclear are approximately 0.3 to 0.6 keV/nm for Kr¹⁵⁺ ions and 0.5 to 0.7 keV/nm for Xe²³⁺ ions. In this context, the maximum ion penetration depth into the material is estimated to be around 11 to 12 µm when irradiated with Kr¹⁵⁺ ions and 15 to 16 µm for Xe²³⁺ ions.

5. In line 210, the author states 'the concentration of defective regions within the damaged layer was calculated.' How was the defective concentration calculated? Could the author provide the detailed calculation method or a reference regarding this calculation?

The authors thank the reviewer for this comment.  The swelling value was ascertained by evaluating the alteration in the crystal lattice volume before and after irradiation, and this evaluation depended on the irradiation fluence. The deformation factor was determined by examining the changes in the crystal lattice parameters before and after irradiation. When calculating the swelling values and the concentration of defective inclusions in the specimens, the penetration depth was considered by measuring X-ray diffraction patterns and their subsequent analysis.

6. For figure 2, please use a log scale for the x-axis for better visualization

The authors thank the reviewer for this comment; corrections have been made to the figure.

7. Regarding Xe ion charge status, on page 7, the status is 'Xe22+' and for the rest of the text, the status is 'Xe23+'. Please correct this typo and revise the article.

The authors thank the reviewer for this comment; corrections have been made to the text of the article.

8. In lines 275-277, the sentence 'Examination of the collected data reveals a more pronounced structural distortion and an accumulation of deformation-related structural distortions in the alloy specimens exposed to heavy Xe22+ ions.' The reason is evident since Xe ions are heavier and carry more energy than Kr ions.

The authors agree with this reviewer's remark regarding the fact that structural distortions are more intense when irradiated with heavy Xe²³⁺ ions. The following text has also been added to the text of the article.

When considering the application of these alloys as structural materials subjected to ionizing radiation, especially irradiation with heavy ions similar in energy to nuclear fuel fission fragments, understanding the kinetics of radiation damage accumulation and crystal structure deformation is a crucial factor in assessing their potential and possible uses. The acquired data reveals that altering the alloy's composition results in an improved resistance to consequences like deformation swelling, which arises from the accumulation of radiation-induced damage in the surface layer. This rise in stability and resistance to radiation swelling is due to the following factors. Firstly, a change in the amount of components in the alloys leads to the emergence of additional inter-boundary effects associated with the formation of smaller grains, which in turn leads to an increase in dislocation density, a change in which leads to strengthening and increased resistance to swelling. Also, the presence of interboundary effects associated with grain sizes leads to the appearance of additional defect sinks, which leads to an increase in the number of annihilated point defects and vacancies at the sink boundary. The review Zhang, Z., et.al. [30] provides a thorough description of the effect of grain boundaries and dislocation strengthening in high-entropy alloys. The findings and explanations within this review are consistent and elucidate the structural modifications observed in this study. These changes are linked to an enhanced resistance to radiation swelling, which correlates with a rise in the number of components in the alloy. Furthermore, as indicated in Xia S. Q. et.al. [31], altering the type of high-entropy alloy results in improved resistance to radiation embrittlement due to the specific crystal structure and its remarkable stability against external influences. The established relationships between changes in dislocation density and the extent of structural disorder (swelling) reveal the beneficial impact of dislocation strengthening. This effect is linked to the reduction in grain size and, consequently, the emergence of numerous interboundary influences that impede the migration of vacancy and point defects. These interboundary effects also create additional hindrances to the deformation distortion of the crystal structure.

9. In line 327, the author states that 'the dotted lines in the figures indicate the hardness values for the original samples not subjected to irradiation.' However, it is clear to me that there is a trend of decreasing hardness as fluence increases.

The authors thank the reviewer for this comment. These lines were added to clearly demonstrate changes in hardness values in comparison with the initial value, indicating softening of the alloys.

Review Report Form 1

1.       Section 3.1 Comparative analysis of changes in structural disorder of a damaged alloy layer upon irradiation with heavy ions: It is worth adding a detailed discussion on the implications of the observed structural changes. For example, how these changes may affect the performance of the material in practical applications e.g. in the context of nuclear reactors. Moreover based on results presented in Figure 4 discussion about relationship between dislocation density and resistance to swelling should be expended by describing the mechanisms or causality in depth. Providing a clearer explanation of why higher dislocation density leads to less swelling would enhance the understanding of the findings.

The authors thank the reviewer for this comment and such a high assessment of the submitted work. The following description of the observed effects and their relationship to each other was added to the text of the article.

When considering the application of these alloys as structural materials subjected to ionizing radiation, especially irradiation with heavy ions similar in energy to nuclear fuel fission fragments, understanding the kinetics of radiation damage accumulation and crystal structure deformation is a crucial factor in assessing their potential and possible uses. The acquired data reveals that altering the alloy's composition results in an improved resistance to consequences like deformation swelling, which arises from the accumulation of radiation-induced damage in the surface layer. This rise in stability and resistance to radiation swelling is due to the following factors. Firstly, a change in the amount of components in the alloys leads to the emergence of additional inter-boundary effects associated with the formation of smaller grains, which in turn leads to an increase in dislocation density, a change in which leads to strengthening and increased resistance to swelling. Also, the presence of interboundary effects associated with grain sizes leads to the appearance of additional defect sinks, which leads to an increase in the number of annihilated point defects and vacancies at the sink boundary. The review Zhang, Z., et.al. [30] provides a thorough description of the effect of grain boundaries and dislocation strengthening in high-entropy alloys. The findings and explanations within this review are consistent and elucidate the structural modifications observed in this study. These changes are linked to an enhanced resistance to radiation swelling, which correlates with a rise in the number of components in the alloy. Furthermore, as indicated in Xia S. Q. et.al. [31], altering the type of high-entropy alloy results in improved resistance to radiation embrittlement due to the specific crystal structure and its remarkable stability against external influences. The established relationships between changes in dislocation density and the extent of structural disorder (swelling) reveal the beneficial impact of dislocation strengthening. This effect is linked to the reduction in grain size and, consequently, the emergence of numerous interboundary influences that impede the migration of vacancy and point defects. These interboundary effects also create additional hindrances to the distortion of the crystal structure during deformation.

2.       Section 3.2 Effect of irradiation with heavy ions on changes in the strength properties of alloys: Based on the results presented relationship between alloy composition, dislocation density, and changes in hardness the explanation of why certain alloys exhibit greater resistance to softening should be added.

The authors thank the reviewer for this comment and such a high assessment of the submitted work. The following description of the observed effects and their relationship to each other was added to the text of the article.

The hardening effect observed for NbTiV and NbTiVZr alloys is due to their increased resistance to radiation-induced swelling associated with the accumulation of radiation damage (point defects, vacancies, primary knocked-out atoms). In this case, the hardening effect observed for these alloys with increasing irradiation fluence (as well as with changing the type of ions during irradiation) is in good agreement with several experimental works [32,33], in which this hardening is explained by dislocation strengthening. In the case of a high dislocation density, as well as the presence of interboundary effects (associated with small grain sizes), the propagation of microcracks in the structure under external loads is difficult, which leads to an increase in resistance to embrittlement and destruction of strength properties. Notably, as illustrated in Figure 6, the most pronounced hardening effect is observed in NbTiVZr alloys. These alloys possess an equiatomic distribution of elements in their structure, which contributes to an elevated resistance to embrittlement and destruction, as elaborated in references [30-32].

3.       Section 3.3 3.3. Evaluation results of the studied alloy samples for thermal heating resistance: A more comprehensive explanation of why certain alloys exhibit better thermal stability would improve the interpretation. Moreover, The text briefly mentions the importance of thermal stability for alloy performance, but it could expand on this by discussing real-world applications and implications.

The authors thank the reviewer for this comment and such a high assessment of the submitted work. The following description of the observed effects and their relationship to each other was added to the text of the article.

In the case of using these alloys as structural materials for high-temperature nuclear reactors, which have the greatest prospects for the development of the nuclear industry in the coming decades, the stability of the alloys and the preservation of the stability of structural and strength properties under high-temperature operating conditions is one of the key factors. Under high-temperature operating conditions (700 – 1000°C), the crystal lattice of the alloy undergoes additional changes associated with an increase in the intensity and amplitude of thermal vibrations of atoms, which, together with the accumulation of radiation damage, can lead to accelerated degradation and embrittlement of the damaged layer. In this case, accelerated degradation can result in a decline in strength characteristics, which will adversely affect the resistance to external mechanical influences.

The heightened resistance to thermal degradation over prolonged periods exhibited by NbTiV and NbTiVZr alloys can be attributed to their structural characteristics, which stem from the equiatomic distribution of elements within the structure. This distribution reduces the thermal oscillations of atoms within the crystal lattice, consequently resulting in an enhancement of the softening resistance.

Review Report Form 2

1. When describing the choice of irradiation conditions, authors should focus on the reasons for choosing these conditions and the type of ions for irradiation.

The authors thank the reviewer for this comment and such a high assessment of the submitted work. The following description of the observed effects and their relationship to each other was added to the text of the article.

The choice of the type of heavy ions, specifically Kr¹⁵⁺, Xe²³⁺ is based on their capacity to simulate the mechanisms of radiation-induced damage similar to fission fragments, comparable to reactor tests, which made it possible to evaluate the possibilities of using the selected alloy compositions as reactor materials. It is worth to note that the use of these types of ions makes it possible to simulate radiation damage and the kinetics of their accumulation in the near-surface layer of alloys, about 10 – 15 μm thick, which is most susceptible to external influences both in the case of reactor tests and mechanical influences.

2. The authors should explain exactly how they calculated the amount of swelling and changes in the defective fraction, whether the thickness of the samples during irradiation was taken into account, and how exactly these values were determined.

The authors thank the reviewer for this comment and such a high assessment of the submitted work. The following description of the observed effects and their relationship to each other was added to the text of the article.

The swelling value was ascertained by evaluating the alteration in the crystal lattice volume before and after irradiation, and this evaluation depended on the irradiation fluence. The deformation factor was determined by examining the changes in the crystal lattice parameters before and after irradiation. When calculating the swelling values and the concentration of defective inclusions in the specimens, the penetration depth was considered by measuring X-ray diffraction patterns and their subsequent analysis.

3. The authors should determine exactly how the experiments to determine the hardness of the samples were carried out and also provide a comparison of changes in hardness in the case of changing irradiation conditions (type of ions).

The authors thank the reviewer for this comment and such a high assessment of the submitted work. The following description of the observed effects and their relationship to each other was added to the text of the article.

The hardness of the samples was measured using the indentation method, and the measurements were carried out in the form of serial tests over the entire area of the sample in order to determine the uniformity and isotropy of the observed changes associated with softening.

4. How the degree of softening of alloys exposed to irradiation was calculated.

The authors thank the reviewer for this comment and such a high assessment of the submitted work. The following description of the observed effects and their relationship to each other was added to the text of the article.

The softening degree was assessed by comparing the obtained data on the hardness of the samples in the initial state with irradiated data, followed by conversion to a percentage.

5. In conclusion, a number of suggestions for further research in this direction should be given. Note that recently  structural-radiation disorder was studied in detail in several functional ceramics by thermal annealing and prominent effects on radiation fluence and type of radiation were demonstrated: 

Kotomin, E.,  et al (2018). Anomalous kinetics of diffusion-controlled defect annealing in irradiated ionic solids. The Journal of Physical Chemistry A, 122(1), 28-32.

Ananchenko, D. V., et al. (2020). Radiation-induced defects in sapphire single crystals irradiated by a pulsed ion beam. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 466, 1-7.

The authors thank the reviewer for this comment and such a high assessment of the submitted work. The following description of the observed effects and their relationship to each other was added to the text of the article.

In the future, based on the results obtained and a comparative analysis of the resistance of various types of alloys to radiation damage and their evolution under conditions of changing irradiation fluences, studies will be carried out aimed at determining the stability of alloy samples to radiation damage under irradiation conditions as close as possible to real operating conditions (irradiation at high temperatures, as well as irradiation with neutrons). The data obtained during the new experiments, as well as their comparison with the work already carried out, will make it possible to predict the potential for the use of these alloys as structural materials in nuclear energy.