Mössbauer and X-ray Studies of Radiation-Induced Processes in Nb–Zr Alloys Implanted with 57Fe Ions

The effect of implanting 57Fe ions on the crystal structure of Nb–Zr alloys has been studied using Mössbauer spectroscopy on 57Fe nuclei and X-ray diffraction. As a result of implantation, a metastable structure was formed in the Nb–Zr alloy. The XRD data indicated a decrease in the crystal lattice parameter of niobium; that is, there was a compression of the niobium planes when implanted with iron ions. Mössbauer spectroscopy revealed three states of iron. The singlet indicated a supersaturated Nb(Fe) solid solution; the doublets characterized the diffusion migration of atomic planes and crystallization of voids. It was shown that the values of the isomer shifts in all three states did not depend on the implantation energy, which indicates the invariance of the electron density on the 57Fe nuclei in the studied samples. The resonance lines of the Mössbauer spectra were significantly broadened, which is typical for materials with low crystallinity and a metastable structure that is stable at room temperature. The paper discusses the mechanism of radiation-induced and thermal transformations in the Nb–Zr alloy, which leads to the formation of a stable well-crystallized structure. A Fe2Nb intermetallic compound and the Nb(Fe) solid solution formed in its near-surface layer, while Nb(Zr) remained in the bulk.


Introduction
Developing structural materials for operating and prospective nuclear plants is a very complex scientific and technical task [1][2][3].
Niobium alloys started to be developed in the early 1950s when it became necessary to create high-temperature materials for new technologies (rocket science, aviation and space technology, nuclear energy, etc.). Niobium alloys are characterized by good technological properties, a relatively low density, low thermal neutron capture cross-section and good resistance in contact with liquid metal coolants. Almost all niobium alloys have high hardness and electrical conductivity. These alloys are widely used as high-temperature fuel cells. They have good stability against mechanical and radiation damage. Zr additives are introduced into Nb-based alloys to increase strength and improve oxidation resistance and compatibility with liquid metals [4,5]. Only 1% of Zr in the alloy composition is sufficient to provide flexibility and toughness, making it suitable for the production of the thinnest wires and foils.
Nb-Zr alloys, used in modern nuclear reactors, contain a niobium concentration from 2% to 10% by weight. The composition of these alloys is carefully controlled to ensure optimum properties for certain types of nuclear reactor applications, including good mechanical strength, corrosion resistance and low neutron absorption. Zr-2.5Nb alloys are used in pressurized water reactors (PWRs) and boiling water reactors (BWRs); Zr-2Nb alloys are utilized in CANDU reactors; and Zr-3Nb alloys are used in research reactors. Zr-Nb alloys are also used as cladding materials for light water reactors (LWRs) [6].
components. Thus, in [34], during the study of a layered Be-Fe-Be system, it was established that irradiation with He ions resulted in the transition of some Fe atoms from a magnetically ordered to a paramagnetic state (the effect of radiation-induced segregation in the zone of primary knocked-out atoms). Such processes were also observed in [35], and similar results were obtained in [28].
The purpose of this research is to study the radiation-induced processes in 2.1Zr alloys implanted with ions of the 57 Fe Mössbauer atom and to obtain data on the effect of the zone of influence of these ions on the structural-phase state of the implanted samples. The elemental composition of the alloy chosen for this research belongs to the insufficiently studied niobium angle of the state diagram of the Nb-Zr binary system.

Materials and Methods
The samples for our study were prepared from Nb and Nb-Zr alloy plates rolled to an effective thickness (20 µm). The resulting foils were subjected to homogenizing annealing at a temperature of 800 • C in a vacuum pressure of 5 × 10 −7 mm Hg for 2 h.
The implantation of high-energy singly charged 57 Fe ions with an energy of 1 MeV into Nb (sample A0) and 57 Fe ions with energies of 1, 0.35 and 0.25 MeV into the Nb-Zr alloy (samples A1, A2, A3, respectively) with a fluence of 5 × 10 16 ion/cm 2 and an ion beam current of~100 nA was performed using the heavy ion accelerator UKP-2-1 at the Institute of Nuclear Physics (Almaty, Kazakhstan). The target chamber was additionally equipped with a magnetic discharge pump, which made it possible to obtain a vacuum pressure of~1.5 × 10 −7 mm Hg and to minimize the rate of carbon film burning on the target during implantation. The target temperature under the selected irradiation modes did not exceed 60 • C.
The elemental composition of the studied samples was determined using a scanning electron microscope JEOLJSM-06610 (Tokyo, Japan) equipped with an IncaX-act energy dispersive analyzer with a resolution of 3 nm at an accelerating voltage of 0.3-30 kV and accumulation time of 10 min. Before implantation, the Nb-Zr alloy contained 97.9% Nb and 2.1% Zr.
The changes in the crystal structure of the studied materials before and after implantation were recorded using a diffractometer Bruker D8 ADVANCE (Karlsruhe, Germany) with a Cu-K α emitter in the Bragg-Brentano geometry. Crystalline phases were identified using the file cabinet ASTM and TCPDS.
The SRIM-2008 program [36,37] in Quick Calculation mode was used to assess the degree of ion beam impact on the crystal lattice of the implanted materials. The average projective range of ions and the number of displacements per atom (dpa) were determined. The concentration of implanted 57 Fe ions in the target and the concentration of vacancies and knocked-out atoms in the matrix were calculated; the number of backscattered 57 Fe ions was estimated. With a decrease in the implantation energy, the number of displacements per atom and 57 Fe concentration increased ( Figure 1).
The electronic environment of 57 Fe atoms after completion of the implantation process was determined using Mössbauer spectroscopy in transmission geometry (MS) mode and in the electron channel backscattering mode (CEMS). Mössbauer studies were performed using a MS-1104Em spectrometer at room temperature. 57 Co in a chromium matrix served as the source of γ-quanta; gamma radiation was recorded using a resonant scintillation detector. Conversion electrons were recorded using a gas-proportional counter with a He + 8% CH 4 mixture. The Mössbauer spectra were processed through model interpretation using the SpectrRelax program [38]. The electronic environment of 57 Fe atoms after completion of the implantation process was determined using Mössbauer spectroscopy in transmission geometry (MS) mode and in the electron channel backscattering mode (CEMS). Mössbauer studies were performed using a MS-1104Em spectrometer at room temperature. 57 Co in a chromium matrix served as the source of γ-quanta; gamma radiation was recorded using a resonant scintillation detector. Conversion electrons were recorded using a gas-proportional counter with a He + 8% CH4 mixture. The Mössbauer spectra were processed through model interpretation using the SpectrRelax program [38]. Figure 2 shows the MS spectrum of Nb implanted with 57 Fe ions with a fluence of 5 × 10 16 ion/cm 2 and 1 MeV energy. The sample spectrum consisted of a singlet and a doublet. Similar results were obtained earlier when 57 Fe ions were implanted into one-component materials (Ta, Mo, Al, Zr) [20,32,39,40]. Isomer shifts and quadrupole splittings are typical and specific for these metals with 57 Fe probe atoms. Thus, it can be stated with a high probability that in the spectrum of 57 Fe implanted in Nb, the singlet with an isomer shift of δ = −0.026 mm/s is inherent in the implants localized at the sites of the cubic crystal lattice of the target [30,32] in a volume limited by the projective range of the ions. It is known that elements of the 3d group have a low solubility when alloyed with Nb (at 600 °C, the solubility of Fe in Nb is only 0.1 at.%). It should be assumed that the observed singlet can be interpreted as a solid solution of Fe substitution in Nb-Nb(Fe). Under conditions of intensive implantation, the limiting solubility can increase significantly. Indeed, in the implanted Nb layer (3.591 × 10 −5 cm), 5 × 10 16 57 Fe ions were localized, 46% of which were in a solid solution Nb(Fe) (see Table 1). In addition to the singlet, the spectrum contained a doublet with an isomer shift of δ = −0.18 mm/s and a quadrupole splitting of Δ = 0.45 mm/s. The singlet and the doublet were defined by 57 Fe in the paramagnetic state. This indicates that the implant was atomically scattered over the target volume and did not have Mössbauer atoms in the nearest environment. The ions of 57 Fe in the singlet and doublet had a negative isomer shift and, as a consequence, a higher electron density relative to metallic iron.  Figure 2 shows the MS spectrum of Nb implanted with 57 Fe ions with a fluence of 5 × 10 16 ion/cm 2 and 1 MeV energy. The sample spectrum consisted of a singlet and a doublet. Similar results were obtained earlier when 57 Fe ions were implanted into onecomponent materials (Ta, Mo, Al, Zr) [20,32,39,40]. Isomer shifts and quadrupole splittings are typical and specific for these metals with 57 Fe probe atoms. Thus, it can be stated with a high probability that in the spectrum of 57 Fe implanted in Nb, the singlet with an isomer shift of δ = −0.026 mm/s is inherent in the implants localized at the sites of the cubic crystal lattice of the target [30,32] in a volume limited by the projective range of the ions. It is known that elements of the 3d group have a low solubility when alloyed with Nb (at 600 • C, the solubility of Fe in Nb is only 0.1 at.%). It should be assumed that the observed singlet can be interpreted as a solid solution of Fe substitution in Nb-Nb(Fe). Under conditions of intensive implantation, the limiting solubility can increase significantly. Indeed, in the implanted Nb layer (3.591 × 10 −5 cm), 5 × 10 16 57 Fe ions were localized, 46% of which were in a solid solution Nb(Fe) (see Table 1). In addition to the singlet, the spectrum contained a doublet with an isomer shift of δ = −0.18 mm/s and a quadrupole splitting of ∆ = 0.45 mm/s. The singlet and the doublet were defined by 57 Fe in the paramagnetic state. This indicates that the implant was atomically scattered over the target volume and did not have Mössbauer atoms in the nearest environment. The ions of 57 Fe in the singlet and doublet had a negative isomer shift and, as a consequence, a higher electron density relative to metallic iron. Figure 3 shows the MS spectra of the Nb-Zr alloy subjected to implantation of 57 Fe ions with a fluence of 5 × 10 16 ion/cm 2 and energies of 1, 0.35 and 0.25 MeV. The MS spectra were processed with sufficient quality using two doublets and a singlet; the Mössbauer parameters are provided in Table 1.  Figure 3 shows the MS spectra of the Nb-Zr alloy subjected to implantation of 57 Fe ions with a fluence of 5 × 10 16 ion/cm 2 and energies of 1, 0.35 and 0.25 MeV. The MS spectra were processed with sufficient quality using two doublets and a singlet; the Mössbauer parameters are provided in Table 1.     Relative transmission, % The distribution of 57 Fe ions (N i ) over the individual phases of the samples ( Table 2) was calculated according to the MS results. The following formula was used for the calculation:

Results
where D is the fluence; S is the area of the target under the beam; Y is the backscattering coefficient; I i is the relative spectral area of the i phase.
It should be noted that at the chosen energies, the backscattering coefficient Y obtained from the SRIM program is very small, so this parameter was not taken into account in further discussions. The number of backscattered 57 Fe ions increased with decreasing implant energy from 2.05 × 10 14 at 1.0 MeV energy (Y = 0.004) to 7.45 × 10 14 at 0.25 MeV energy (Y = 0.015). Analysis of the Mössbauer spectra suggests that the 57 Fe ions in the samples (Singlet and Doublet 1) had a charge density on the nucleus of +4 with an electronic configuration of the outer shells of 3s 2 3p 6 3d 4 4s 0 . The isomer shift of the Singlet was less than that of α-Fe. Consequently, the electron density on the 57 Fe nucleus, a substitutional impurity in the Nb crystal lattice, was greater. The isomer shift of Doublet 1 was also smaller than that of the α-Fe and Singlet, which is associated with a strong disordering of the crystal lattice, disorder, partial amorphization and the growth of the electron density on the 57 Fe nucleus. It is known that the isomer shift is determined by the electron density on the nucleus and is directly related to the nature of interaction in the crystal. The change in compounds allows us to obtain information about the redistribution of electrons between the atoms of the components and the change in the strength of interatomic bonds in the crystal lattice. The charge density on the nucleus is only created by s-electrons. Even though d-electrons have zero density in the nucleus, their shielding effect on s-electrons contributes to the total electron density. The growth of d-electrons density in an atom reduces the charge density on the nucleus and leads to a positive isomer shift relative to α-Fe. The isomer shift of Doublet 2 was greater than that of α-Fe. The value of the isomer shift and quadrupole splitting of this phase indicates that Fe had a charge density of +3, and the electron configuration of the outer shells was 3s 2 3p 6 3d 5 4s 0 . The implant was localized in disordered amorphous areas of the target and had a reduced charge electron density. This is possible due to the shielding of the outer 3s-electrons by additional electrons of the d-shell. The relative area of Doublet 2 was small at high implantation energies and increased with decreasing implantation energy.
Since the diffusion coefficients of the alloy components are different, the corresponding atomic fluxes are also different. Of the two counter streams, the one that is directed from the crystal with a lower melting point is larger. Thus, in a region with high flux, vacancies are formed at the lattice sites. It is worth mentioning that in the process of mutual diffusion, vacancies can appear and disappear without a trace on the sinks of dislocations, cracks, pores, etc. Similar phenomena cannot occur with atoms, as their numbers are constant. Consequently, as a result of diffusion processes, not only does relocation of plane atoms occur, but also the growth of voids. Two types of sinks are formed: formation of the vacancies (voids) and relocation of the atomic planes [41].
The amorphous structure was more evident in the CEMS spectra of the samples implanted with 57 Fe ions with lower energies. The highest implant concentration was observed at these energies (see Figure 1), which causes a decrease in crystallinity and an increase in amorphism of the Nb-Zr alloy. Figure 4 shows the CEMS spectra of the Nb-Zr alloy samples subjected to implantation of 57 Fe ions with 0.25, 0.35 and 1 MeV energies; the Mössbauer parameters are indicated in Table 3. An increase in the resonance lines widths indicates a highly amorphous structure.
numbers are constant. Consequently, as a result of diffusion processes, not only does relocation of plane atoms occur, but also the growth of voids. Two types of sinks are formed: formation of the vacancies (voids) and relocation of the atomic planes [41].
The amorphous structure was more evident in the CEMS spectra of the samples implanted with 57 Fe ions with lower energies. The highest implant concentration was observed at these energies (see Figure 1), which causes a decrease in crystallinity and an increase in amorphism of the Nb-Zr alloy. Figure 4 shows the CEMS spectra of the Nb-Zr alloy samples subjected to implantation of 57 Fe ions with 0.25, 0.35 and 1 MeV energies; the Mössbauer parameters are indicated in Table 3. An increase in the resonance lines widths indicates a highly amorphous structure.   The elemental composition of the samples after implantation of 57 Fe ions was determined using SEM. The analysis was performed on 10 points and was calculated as an average value. Sample A2 contained 96.89% Nb, 1.58% Zr and 1.53% Fe; sample A3 contained 96.65% Nb, 1.61% Zr and 1.74% Fe.
The changes in the crystal structure of the studied materials before and after the implantation of 57 Fe ions were identified using XRD. X-ray diffraction patterns of Nb (JCPDS card No. 35-0789) and Nb-Zr alloy samples before irradiation were represented by three reflections. However, the reflections in the Nb-Zr alloy were shifted towards smaller angles. This means that Zr as an impurity was localized at the sites of the Nb crystal lattice. The atomic radius of this impurity (1.60) was larger than that of Nb (1.43). This led to an increase in the lattice parameter of the Nb-Zr alloy (a = 3.315 Å) in comparison with the lattice parameter of Nb (a = 3.301 Å).
The XRD pattern of the Nb-Zr alloy before and after implantation of 57 Fe ions with energies of 1, 0.35 and 0.25 MeV are shown in Figure 5. After irradiation, the reflections in the Nb-Zr alloy were shifted towards larger angles, indicating an out-of-plane compressive strain [18]. It can be concluded that after irradiation, the system tends to relax, i.e., the atomic planes tend to move to be similar to Nb; a similar behavior was observed after Cu irradiation of Zr/Nb [6,17]. In the zone of primary knocked-out atoms, there was a depletion of zirconium and niobium atoms and an enrichment of iron ions, i.e., radiationinduced segregation occurs in the region of the solid solution Fe in the Nb-Zr alloy. This led to a decrease in the crystal lattice parameter (see Table 4). 57 Fe localized in the cubic Nb crystal lattice corresponded to a singlet in the MS spectrum. Broadened resonance lines of the doublets, detected by nuclear gamma-resonance spectroscopy, are typical for materials with low crystallinity and high amorphism. Such phases are not amenable to XRD, but their presence is noticeable on the background track of the X-ray diffraction patterns. The broadening and asymmetry of the diffraction peaks of the A3 sample can be explained by the higher amorphism of the irradiated sample. This is associated with a lower projective range of 57 Fe ions, which leads to an increase in the concentration of the implant in the surface layer.  In this paper, the interpretation of the obtained experimental results was based on the mechanisms of the Kirkendall and Frenkel effects. Diffusion migration of atomic planes from crystal to crystal is reflected in Doublet 1; crystallization of the void, along the periphery of which the knocked-out atoms are located, is reflected in Doublet 2. These mechanisms were previously used to explain the segregation of atoms in displacement cascades in the zone of primary knocked-out atoms [28,34,35]. In the zone of primary knocked-out atoms, strongly heated regions are formed (thermal peaks), which can be compared with atomic plasma. When this plasma cools at a rapid rate (10 10 -10 15 K/s), it hardens with a transition of the irradiated medium into a metastable amorphous state.
As noted above, the most sensitive effect of irradiation was displayed in the Mössbauer spectra of the Nb-Zr alloy sample after implantation with 57 Fe ions with 0.25 MeV energy. Therefore, the crystallization kinetics of this sample was studied. After irradiation, Sample A3 was subjected to successive four-hour isochronous annealings in a vacuum pressure of 5 × 10 −7 mm Hg at temperatures up to 1000 °C with a step of 200 °C. The most typical MS and CEM spectra of this sample are shown in Figure 6; the parameters of hyperfine interactions are indicated in Table 5. It can be seen that the spectra of the annealed sample sharply changed in structure. Doublet 2 had completely disappeared. An explanation for this can be found using the inverse Kirkendall effect.
The vacancy flow causes a depletion of the sink region in components with a high diffusion rate (Fe) and enrichment in components with a lower diffusion coefficient (Zr). The intensity of the singlet in the MS spectrum of sample A3 before annealing was 45%,  In this paper, the interpretation of the obtained experimental results was based on the mechanisms of the Kirkendall and Frenkel effects. Diffusion migration of atomic planes from crystal to crystal is reflected in Doublet 1; crystallization of the void, along the periphery of which the knocked-out atoms are located, is reflected in Doublet 2. These mechanisms were previously used to explain the segregation of atoms in displacement cascades in the zone of primary knocked-out atoms [28,34,35]. In the zone of primary knocked-out atoms, strongly heated regions are formed (thermal peaks), which can be compared with atomic plasma. When this plasma cools at a rapid rate (10 10 -10 15 K/s), it hardens with a transition of the irradiated medium into a metastable amorphous state.
As noted above, the most sensitive effect of irradiation was displayed in the Mössbauer spectra of the Nb-Zr alloy sample after implantation with 57 Fe ions with 0.25 MeV energy. Therefore, the crystallization kinetics of this sample was studied. After irradiation, Sample A3 was subjected to successive four-hour isochronous annealings in a vacuum pressure of 5 × 10 −7 mm Hg at temperatures up to 1000 • C with a step of 200 • C. The most typical MS and CEM spectra of this sample are shown in Figure 6; the parameters of hyperfine interactions are indicated in Table 5. It can be seen that the spectra of the annealed sample sharply changed in structure. Doublet 2 had completely disappeared. An explanation for this can be found using the inverse Kirkendall effect. similar phase is also nucleated in the region of the crystal that does not contain extended defects [41]. This is associated with the fact that the regions remaining at the site of the atom-atom collision cascade passage has a structure typical of the point defect distribution-a zone depleted in atoms in the center with interstitial atoms located along the periphery. Moreover, a situation is possible when the depleted zone will be replenished with atoms predominantly of the same type. Fe combines with the matrix element (Nb), forming the Fe2Nb intermetallic compound, the Mössbauer parameters of which are similar to those of Doublet 1 and the intermetallic compound obtained by mechanical alloying [42,43]. The zirconium atoms return to their primary state, forming the Nb(Zr) solid solution. This was confirmed by the increase in the crystal lattice parameter of the annealed Nb-Zr alloy sample (a = 3.317 Å).  A layered system was produced as a result of the studies of the alloy subjected to ion implantation and subsequent thermal annealing. According to the Mössbauer spectroscopy results, it was established that the layer to a depth of 1 × 10 −5 cm was represented by Fe2Nb intermetallic compound. Further, down to the depth of the possible projective range of 57 Fe ions, the layer was characterized by a binary structure consisting of the Fe2Nb intermetallic compound and the Nb(Fe) solid solution with a limiting solubility of 0.1 at.% Fe. According to the XRD data, the basis of the sample was the Nb(Zr) solid solution.

Conclusions
The Nb-Zr alloy was implanted with 57 Fe ions with 1, 0.35 and 0.25 MeV energies and a fluence of 5 × 10 16 ion/cm 2 . The electronic state of 57 Fe and the crystal structure of the implanted samples were determined using Mössbauer spectroscopy on 57 Fe nuclei and X-ray diffraction. As a result of implantation, a metastable structure was formed in the  The vacancy flow causes a depletion of the sink region in components with a high diffusion rate (Fe) and enrichment in components with a lower diffusion coefficient (Zr). The intensity of the singlet in the MS spectrum of sample A3 before annealing was 45%, and in the CEMS spectrum, it was 32%. Sample annealing at 1000 • C led to a decrease in intensity (up to 23%) in the MS spectrum and to the complete disappearance of this component in the CEMS spectrum. The explanation for this fact lies in the results of the recrystallization annealing of Nb(Fe). There are two mechanisms involved in the implantation process. As a result of radiation-induced segregation, a new phase is released on the surface and on the defects in the crystal lattice (dislocations, pores, grain boundaries, etc.) of the irradiated material. In addition, during radiation-induced separation, a similar phase is also nucleated in the region of the crystal that does not contain extended defects [41]. This is associated with the fact that the regions remaining at the site of the atom-atom collision cascade passage has a structure typical of the point defect distribution-a zone depleted in atoms in the center with interstitial atoms located along the periphery. Moreover, a situation is possible when the depleted zone will be replenished with atoms predominantly of the same type. Fe combines with the matrix element (Nb), forming the Fe 2 Nb intermetallic compound, the Mössbauer parameters of which are similar to those of Doublet 1 and the intermetallic compound obtained by mechanical alloying [42,43]. The zirconium atoms return to their primary state, forming the Nb(Zr) solid solution. This was confirmed by the increase in the crystal lattice parameter of the annealed Nb-Zr alloy sample (a = 3.317 Å).
A layered system was produced as a result of the studies of the alloy subjected to ion implantation and subsequent thermal annealing. According to the Mössbauer spectroscopy results, it was established that the layer to a depth of 1 × 10 −5 cm was represented by Fe 2 Nb intermetallic compound. Further, down to the depth of the possible projective range of 57 Fe ions, the layer was characterized by a binary structure consisting of the Fe 2 Nb intermetallic compound and the Nb(Fe) solid solution with a limiting solubility of 0.1 at.% Fe. According to the XRD data, the basis of the sample was the Nb(Zr) solid solution.

Conclusions
The Nb-Zr alloy was implanted with 57 Fe ions with 1, 0.35 and 0.25 MeV energies and a fluence of 5 × 10 16 ion/cm 2 . The electronic state of 57 Fe and the crystal structure of the implanted samples were determined using Mössbauer spectroscopy on 57 Fe nuclei and X-ray diffraction. As a result of implantation, a metastable structure was formed in the Nb-Zr alloy. The XRD data indicated a decrease in the crystal lattice parameter of niobium; that is, there is a compression of the niobium planes implanted with iron ions. MS and CEMS spectroscopy revealed three states of iron. The singlet indicated a supersaturated Nb(Fe) solid solution; Doublet 1 characterized the diffusion migration of atomic planes; Doublet 2 indicated the crystallization of voids. It was shown that the values of the isomer shifts in all three states did not depend on the implantation energy, which indicates the invariance of the electron density on the 57 Fe nuclei in the studied samples. The resonance lines of the Mössbauer spectra were significantly broadened, which is typical for materials with a low crystallinity and metastable structure that is stable at room temperature. The amorphous structure was more evident in the CEMS spectra of the samples implanted with low-energy 57 Fe ions. A stable structure was obtained by annealing the implanted alloy at 1000 • C. It consisted of a highly crystallized Nb(Fe) solid solution with a limiting solubility of 0.1 at.% Fe and a well-crystallized Fe 2 Nb intermetallic compound.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.