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Crystallographic and compositional analysis of impurity phase U2MoSi2C in UMo alloys
Journal of Nuclear Materials 519 (2019) 287e291
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Crystallographic and compositional analysis of impurity phase U2MoSi2C in UMo alloys Libor Kovarik a, *, 1, Arun Devaraj b, **, 1, Curt Lavender c, Vineet Joshi c a
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, 99354, United States Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA, United States c Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, United States b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Analyzed the structure of a new impurity phase U2MoSi2C in UMo alloys by electron diffraction. HRSTEM and APT results helped decipher the atomic scale structure of unit cell and composition of U2MoSi2C. Ab-initio calculations aided in refining the lattice parameter of the new phase. Significance of the U2MoSi2C phase on UMo microstructural evolution discussed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2018 Received in revised form 23 February 2019 Accepted 22 March 2019 Available online 28 March 2019
Impurity phases in metallic nuclear materials can critically influence the microstructural evolution and mechanical properties, making it crucial to understand their structure, composition, and distribution. Using transmission electron microscopy, atom probe tomography, and ab initio calculations, we provide the atomic-scale crystallographic structural and compositional analysis of an impurity phase, U2MoSi2C, in an important nuclear fuel Ue10Mo alloy. We identify this phase as having tetragonal symmetry with lattice parameters of a ¼ 6.67 Å and c ¼ 4.33 Å, and space group P4/mbm (No.127). Ab initio calculations were performed to verify structural stability and to perform atom position refinement. © 2019 Published by Elsevier B.V.
1. Introduction In nuclear materials, it is critical to develop a detailed, atomicscale understanding of the structure and composition of impurity phases to accurately infer their influence on phase transformation
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Kovarik), [email protected] (A. Devaraj). 1 The first two authors contributed equally. https://doi.org/10.1016/j.jnucmat.2019.03.044 0022-3115/© 2019 Published by Elsevier B.V.
mechanisms during processing as well as under irradiation in reactors [1,2]. Aligned to this goal, in this work, we focus on studying the phase-forming properties of impurity element Si in model Ue10Mo alloys, which are considered a promising candidate for replacing highly enriched U fuels, mainly to reduce nuclear proliferation, while retaining the same performance for the reactors [3,4]. The interest in UMo alloys as nuclear fuels stems mostly from the possibility of stabilizing the microstructure in g-UMo (bodycentered cubic) phase at a relatively low level of Mo alloying, which exhibits advantageous mechanical properties in an irradiation environment [5]. The transformation mechanism of g-UMo phase
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by either eutectoid (a-U þ g0 -U2Mo), or discontinuous precipitation (a-U þ Mo enriched g-UMo) has been the focus of a number of recent studies [1,6e10]. The Ue10Mo alloys have been shown to contain multiple impurity elements at microalloying levels [11]. From our recent work, the C and Si impurities were identified as the most prominent, influencing the structure of grain boundaries [1]. The structure of grain boundaries is modified either by solute segregation, or alternatively by formation of wetting U2MoSi2C grain boundary phases influencing the kinetics of discontinuous precipitation of gUMo during subeutectoid annealing [1]. The wetting U2MoSi2C phase represents a crystallographically unknown quaternary phase that precipitates in UeMo alloys. Devaraj et al. [1] conducted a compositional analysis of this impurity U2MoSi2C phase, but full structural and crystallographic analysis, including space group and derivations of atom positions, is yet to be revealed. In this work, we address the crystallographic nature and composition of U2MoSi2C phase using electron diffraction, high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), atom probe tomography (APT) and ab initio structural refinement. The complementarity of TEM, APT, and ab initio methods provides powerful means for crystallographic analysis of embedded nanoscale phases, which may not be otherwise studied with conventional x-ray diffraction techniques [12,13]. 1.1. Experimental procedure The presently studied alloy of U-10 wt% Mo (U-21.61 at% Mo) was fabricated at the Y12 National Security Complex by melting and casting in graphite molds. The cast microstructure was homogenized at 900 C for 48 h or at 1000 C for 16 h, and further annealed at the subeutectoid temperature of 500 C. Detailed description of the heat treatments can be found in our previous work [1]. The samples for crystallographic analysis were prepared with an FEI Quanta 3D focused ion beam instrument. Structural and crystallographic analysis was performed with a probe-corrected FEI Titan 80e300 operated at 300 kV. The observations were performed using STEM with a HAADF detector. The probe convergence angle was 17.8 mrad, and the inner detection angle on the HAADF detector was three times higher than the probe convergence angle. Simulation of HAADF images was performed with a computer code developed by E. Kirkland [14]. The calculations were performed with microscope parameters that closely correspond to the experimental conditions (E ¼ 300 kV, cs ¼ 5 mm, convergence angle 18 mrad). The simulated images were convoluted with a Gaussian of full width at half maximum (FWHM) ¼ 0.08 nm to account for spatial incoherence of the imaging system. Structural refinement was performed with density functional theory calculations using the Vienna Ab initio Simulation Package (VASP) [15,16]. The projector augmented wave method was used in combination with generalized gradient approximation for the exchange correlation potential. All reported calculations were performed with 520 eV energy cutoff. Laser assisted APT analysis was conducted using a CAMECA LEAP 4000XHR APT system equipped with a 355 nm UV laser, using 100 pJ laser energy, while the specimen was kept at 45 K and the evaporation rate was maintained at 0.005 atoms/pulse. The APT results were reconstructed using interactive visualization and analysis software (IVAS 3.8). 2. Results and discussion The U2MoSi2C forms within the homogenized microstructure of UeMo alloys. The morphology of U2MoSi2C phase varies depending on the homogenization temperature [1]. For low temperature homogenization treatments of 900 C, the U2MoSi2C phase adopts
blocky/round morphology, and it often forms at interphase boundaries of g-UMo/UC, or as isolated faceted precipitates along grain boundaries. For higher temperature homogenization treatments of 1000 C, the U2MoSi2C phase is a thin, intergranular wetting phase at g-UMo grain boundaries. An example of U2MoSi2C precipitate phases from 900 C and 1000 C homogenization treatments is shown in Fig. 1(a and b). The crystal symmetry and lattice parameters of U2MoSi2C were analyzed using selected area diffraction. The experimental observations from multiple precipitates and along several zone axes enabled us to reconstruct the full 3D reciprocal lattice. The key lowindexed selected area electron diffraction patterns are shown in Fig. 1. The observations reveal the presence of a single zone with fourfold symmetry (labeled the D1 zone), which suggests that the U2MoSi2C phase possess tetragonal symmetry. The assignment of tetragonal symmetry is supported by the symmetry and the angular relationship between the obtained diffraction patterns. Full self-consistent indexing of the diffraction patterns is provided for the key zones presented in Fig. 1. The lattice parameters of U2MoSi2C phase is found to be: a ¼ 6.7 Å, c ¼ 4.3 Å. Fig. 2 shows the APT result of a U2MoSi2C precipitate and adjacent g-UMo matrix. The precipitate phase composition was estimated to be approximately U ¼ 33%, Si ¼ 33%, Mo ¼ 16%, and C ¼ 16% (at.%), with several minor impurity elements, which lead us to predict the U2MoSi2C stoichiometry [1]. A 3 nm 4 nm 20 nm section of the APT data for U2MoSi2C phase was extracted and is shown in Fig. 2(b), which highlights the observation of parallel planes separated by a distance of 0.43 nm. This observation was quantitatively analyzed using spatial distribution mapping (SDM) along the z-axis of the extracted volume. The ripples in the SDM correspond to the observed periodicity in the APT reconstruction, which corresponds to 0.43 nm. Interestingly, this matches very well the c-axis of the U2MoSi2C phase structure, as can be seen from the inset of the structural model. Careful analysis of the structural model reveals that the midplane along the c-axis is the only plane where U atoms reside, and these will have a periodicity corresponding to the c-axis of the structure. Because APT spatial resolution is 0.2 nm along the z-axis, it is clear that APT results could distinguish these U lattice planes in the reconstruction, which helps with independent validation of the structural finding from STEM imaging and diffraction. The combined results from electron diffraction and APT indicate that U2MoSi2C represents a quaternary silicide carbide phase. According to the Inorganic Crystal Structure Database, there are no known quaternary UMo silicide carbide phases, nor any related isostructural polymorphs in ternary UeSieC, UeMoeC, or UeMoeSi alloy systems. Examination of compositionally related uranium silicide carbides, including U20Si16C3 and U3Si2C2 [17], as well as the molybdenum silicide carbide Mo4$8Si3C0.6 [18], show no structural relationship, indicating an independent and novel status of this phase. To further investigate the structural and crystallographic nature of U2MoSi2C, with the goal to obtain full crystallographic description, we captured a series of atomic-level HAADF images. HAADF imaging provides directly interpretable and quantifiable atomic level contrast, which scales as Z~1.7 [19]. The contrast in U2MoSi2C is expected to be dominated by heavy uranium and molybdenum atoms. If resolved independently, the contrast from U is expected to be ~4 higher than Mo, ~25 higher than Si, and ~100 higher than C. HAADF observations from low-index zones of [001], [011], [100], and [310] are shown in Fig. 3. The observation from [001] is particularly important, as it enables resolution of the structure along the highest symmetry fourfold axis. Two types of sites were identified based on the relative contrast, and these two sites can be
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Fig. 1. (a,b) HAADF images of U2MoSi2C along from homogenization at 900C and 1000C. c) Diffraction analysis of the newly identified U2MoSi2C from different zones.
Fig. 2. (a) APT reconstruction of U2MoSi2C precipitate and surrounding g-UMo matrix. (b) A section of APT data from the U2MoSi2C phase showing APT resolving lattice planes along C axis with 0.433 nm spacing (c) spatial distribution map plotted using U, Mo, Si and C along Z axis of the precipitate showing the 0.433 nm spacing. The crystal structure of U2MoSi2C is shown as inset.
consistently interpreted in terms of the projected potential of U and Mo atoms. The sites with relatively strong intensities (highlighted by squares) are interpreted as U sites. There are four of these sites per unit cell. The low intensity sites, which are on the fourfold rotational axis, can be interpreted as Mo atoms. The sites are marked at the origin and face-centered position of the cell, as highlighted in Fig. 3(a). Assuming the periodicity along the projected direction of c ¼ 4.3 Å, the identified atomic sites correspond to one atom of U and one of Mo per unit cell. Observations from [100] and [310] zones, both of which are orthogonal with respect to the fourfold zone axis, are shown in
Fig. 3(b and c). The sites with strong intensities are consistent with U contrast, while the low intensity sites can be interpreted as Mo. Both U and Mo are identified in independent projections, and more importantly, on adjacent (002) planes, which provides an important constraint for determining the relative positions of these atomic species. Additionally, we observed that, in both projections, the high intensity U sites are streaked, suggesting that U is not projected into a single atomic column, but instead corresponds to two closely spaced columns. For the [100] orientation in Fig. 3(b), the two closely spaced U columns can be resolved in a few regions of the image, with the separation of ~1.2 Å. On the other hand, the
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Fig. 3. HAADF STEM observation of U2Mo2SiC phase along low index zone axis.
separation of U sites in [310] orientation was not resolved. Further information regarding the projected positions of U and Mo atoms was obtained from the [101] orientation, which represents another important orientation where U and Mo sites can be independently resolved. As shown in Fig. 3(d), the high intensity columns indicate projection of U, while the diffuse intensity between adjacent U atoms indicates the presence of Mo. Given the constraints from the electron diffraction, APT, and HAADF observations, we propose that U2MoSi2C phase has tetragonal crystal structure, as shown in Fig. 4. The atomic sites for U and Mo were determined directly from HAADF observations. On the other hand, the atomic sites for Si and C were determined indirectly based on evaluation of the coordination environment around U and Mo of the tetragonal cell. To be consistent with the APT results, we expect the structure to contain two carbon and four silicon atoms. The two carbon atoms can be assigned to octahedral sites defined
by four U and two Mo neighbors. The resulting C bonding environment, in terms of coordination number and atomic distances, is analogous to the octahedral bonding in UC phase. The four silicon atoms, on the other hand, can be assigned to sites coordinated by nine atoms. This bonding environment includes six coordinating U atoms with the bond distance of 2.856 Åe2.956 Å, two Mo coordinating atoms with the bond distance of 2.614 Å, and one Si with the bond distance of 2.47 Å. The structure optimization of U2MoSi2C was performed employing the conjugate gradient structure optimization algorithm as implemented in VASP. The results from the structural refinement are reported in Table .1 The structure was found to have tetragonal symmetry, with lattice parameters of a ¼ 6.67 Å and c ¼ 4.33 Å, and belongs to space group P4/mbm (No.127). This is fully consistent with the electron diffraction measurements in Fig. 1. Importantly, we find that the refined atomic sites for U and Mo fully reproduce
Fig. 4. (a). Crystal structure of the newly identified silicide carbide phase. 4(bee) HAADF simulations of [001], [100], [310] and [110] zones.
L. Kovarik et al. / Journal of Nuclear Materials 519 (2019) 287e291 Table 1 Crystallographic description of newly identified U2MoSi2C phase, space group P4/ mbm (No.127). The lattice parameters, a ¼ 6.67 Å, c ¼ 4.33 Å and atom positions were refined using ab-initio methods.
C Mo Si U
Occ.
Mul. Wyck.
Symmetry
x
y
z
1 1 1 1
2a 2b 4g 4h
4/m 4/m m2m m2m
0 0 0.869 0.171
0 0 0.369 0.329
0.5 0 0 0.5
291
The relevance of this work is discussed in the context of estimating the volume fraction of U2MoSi2C phase and its influence on discontinuous precipitation kinetics in the UMo alloys. This study highlights the potential for discovery of new phases in UMo alloys using high resolution characterization, especially with introduction of different impurity elements and different thermomechanical treatments. Acknowledgments
the atomic level HAADF observations, and that the refined atomic positions for Si and C are consistent with our initial assignment. The consistency of HAADF simulations with the experimentally observed zones is shown in Fig. 4(b,c,d,e). The structural and compositional details of the impurity phase U2MoSi2C provides the necessary basis for quantitatively estimating the volume fraction of this phase [20]. This is especially important in the context of understanding the rate of g-UMo discontinuous precipitation, because the extent of the thin wetting layer of U2MoSi2C precipitation along g-UMo grain boundaries has been proposed as controlling the rate of discontinuous precipitation [1]. The ability of U2MoSi2C to act as sink for other impurity elements is yet another topic that should be further explored in relationship to phase stability and properties of UMo alloys. Partitioning of Al, B, P, Ni Ca, and Ti into U2MoSi2C has been observed [1]. Additionally, Ni and Al were observed to segregate at the g-UMo and U2MoSi2C interphase, forming complexion structures. All of these factors are expected to have direct implications for the fuel quality, and thus the irradiation performance in the reactor. This detailed structural and compositional analysis of U2MoSi2C phase will also serve as a baseline for possible investigation of changes to the structure of this impurity phase if the concentration of other dissolved impurity elements (such as Si, C, Al, B, P, Ni, Ca, and Ti) or new impurities, such as Cr or Co, in UMo alloys is increased significantly by changes in chemistry of feedstock materials. Such studies will also be necessary in developing a comprehensive understanding of the influence of impurity elements on microstructural evolution of UMo alloys as a function of various thermomechanical treatments, which will directly affect the fuel quality and its irradiation performance in reactor. 3. Conclusions Electron diffraction, STEM HAADF analysis, APT, and ab initio calculations were used to study the crystal structure and composition of U2MoSi2C phase. The work enabled full crystallographic description of U2MoSi2C phase, which was found to have tetragonal symmetry with lattice parameters of a ¼ 6.67 Å and c ¼ 4.33 Å, and space group P4/mbm (No.127). Ab initio calculations were used to obtain atom position refinement and to verify structural stability.
The work was supported by the U.S. Department of Energy (DOE) National Nuclear Security Administration under Contract DEAC05-76RL01830. The research was performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jnucmat.2019.03.044. References [1] A. Devaraj, L. Kovarik, E. Kautz, B. Arey, S. Jana, C. Lavender, V. Joshi, Acta Mater. 151 (2018) 181e190. [2] A. Devaraj, E. Kautz, L. Kovarik, S. Jana, N. Overman, C. Lavender, V.V. Joshi, Scripta Mater. 156 (2018) 70e74. [3] D.E. Burkes, T. Hartmann, R. Prabhakaran, J.-F. Jue, J. Alloy. Comp. 479 (2009) 140e147. [4] J.L. Snelgrove, G.L. Hofman, M.K. Meyer, C.L. Trybus, T.C. Wiencek, Nucl. Eng. Des. 178 (1997) 119e126. [5] D.D. Keiser, S.L. Hayes, M.K. Meyer, C.R. Clark, JOM-US 55 (2003) 55e58. [6] S. Saubert, R. Jungwirth, T. Zweifel, M. Hofmann, M. Hoelzel, W. Petry, J. Appl. Crystallogr. 49 (2016) 1e11, https://doi.org/10.1107/S1600576716005744. , M. Dzevenko, L. Havela, A. Warren, [7] I. Tkach, N.T.H. Kim-Ngan, S. Maskova C. Stitt, T. Scott, J. Alloy. Comp. 534 (2012) 101e109. [8] S. Jana, A. Devaraj, L. Kovarik, B. Arey, L. Sweet, T. Varga, C. Lavender, V. Joshi, J. Alloy. Comp. 723 (2017) 757e771. [9] S. Neogy, M.T. Saify, S.K. Jha, D. Srivastava, G.K. Dey, Phil. Mag. 95 (2015) 2866e2884. [10] B.-S. Seong, C.-H. Lee, J.-S. Lee, H.-S. Shim, J.-H. Lee, K.H. Kim, C.K. Kim, V. Em, J. Nucl. Mater. 277 (2000) 274e279. [11] S. Jana, L. Sweet, D. Neal, A. Schemer-Kohrn, C. Lavender, V. Joshi, J. Nucl. Mater. 509 (2018) 318e329. [12] L. Kovarik, F. Yang, A. Garg, D. Diercks, M. Kaufman, R.D. Noebe, M.J. Mills, Acta Mater. 58 (2010) 4660e4673. [13] F. Yang, D.R. Coughlin, P.J. Phillips, L. Yang, A. Devaraj, L. Kovarik, R.D. Noebe, M.J. Mills, Acta Mater. 61 (2013) 3335e3346. [14] E.J. Kirkland, Advanced Computing in Electron Microscopy, Springer, 2010. [15] G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15e50. [16] G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169. €ttgen, D. Kaczorowski, W. Jeitschko, J. Mater. Chem. 3 (1993) 253e258. [17] R. Po [18] E. Parthe, W. Jeitschko, V. Sadagopan, Acta Crystallogr. 19 (1965) 1031e1037. [19] P. Hartel, H. Rose, C. Dinges, Ultramicroscopy 63 (1996) 93e114. [20] E. Kautz, A. Devaraj, L. Kovarik, C. Lavender, V.V. Joshi, Effect of Silicon in U10Mo Alloy, PNNL-26790, Pacific Northwest National Laboratory, Richland, WA (United States), 2017.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Crystallographic and compositional analysis of impurity phase U2MoSi2C in UMo alloys Libor Kovarik a, *, 1, Arun Devaraj b, **, 1, Curt Lavender c, Vineet Joshi c a
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA, 99354, United States Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA, United States c Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, United States b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Analyzed the structure of a new impurity phase U2MoSi2C in UMo alloys by electron diffraction. HRSTEM and APT results helped decipher the atomic scale structure of unit cell and composition of U2MoSi2C. Ab-initio calculations aided in refining the lattice parameter of the new phase. Significance of the U2MoSi2C phase on UMo microstructural evolution discussed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 December 2018 Received in revised form 23 February 2019 Accepted 22 March 2019 Available online 28 March 2019
Impurity phases in metallic nuclear materials can critically influence the microstructural evolution and mechanical properties, making it crucial to understand their structure, composition, and distribution. Using transmission electron microscopy, atom probe tomography, and ab initio calculations, we provide the atomic-scale crystallographic structural and compositional analysis of an impurity phase, U2MoSi2C, in an important nuclear fuel Ue10Mo alloy. We identify this phase as having tetragonal symmetry with lattice parameters of a ¼ 6.67 Å and c ¼ 4.33 Å, and space group P4/mbm (No.127). Ab initio calculations were performed to verify structural stability and to perform atom position refinement. © 2019 Published by Elsevier B.V.
1. Introduction In nuclear materials, it is critical to develop a detailed, atomicscale understanding of the structure and composition of impurity phases to accurately infer their influence on phase transformation
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Kovarik), [email protected] (A. Devaraj). 1 The first two authors contributed equally. https://doi.org/10.1016/j.jnucmat.2019.03.044 0022-3115/© 2019 Published by Elsevier B.V.
mechanisms during processing as well as under irradiation in reactors [1,2]. Aligned to this goal, in this work, we focus on studying the phase-forming properties of impurity element Si in model Ue10Mo alloys, which are considered a promising candidate for replacing highly enriched U fuels, mainly to reduce nuclear proliferation, while retaining the same performance for the reactors [3,4]. The interest in UMo alloys as nuclear fuels stems mostly from the possibility of stabilizing the microstructure in g-UMo (bodycentered cubic) phase at a relatively low level of Mo alloying, which exhibits advantageous mechanical properties in an irradiation environment [5]. The transformation mechanism of g-UMo phase
288
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by either eutectoid (a-U þ g0 -U2Mo), or discontinuous precipitation (a-U þ Mo enriched g-UMo) has been the focus of a number of recent studies [1,6e10]. The Ue10Mo alloys have been shown to contain multiple impurity elements at microalloying levels [11]. From our recent work, the C and Si impurities were identified as the most prominent, influencing the structure of grain boundaries [1]. The structure of grain boundaries is modified either by solute segregation, or alternatively by formation of wetting U2MoSi2C grain boundary phases influencing the kinetics of discontinuous precipitation of gUMo during subeutectoid annealing [1]. The wetting U2MoSi2C phase represents a crystallographically unknown quaternary phase that precipitates in UeMo alloys. Devaraj et al. [1] conducted a compositional analysis of this impurity U2MoSi2C phase, but full structural and crystallographic analysis, including space group and derivations of atom positions, is yet to be revealed. In this work, we address the crystallographic nature and composition of U2MoSi2C phase using electron diffraction, high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM), atom probe tomography (APT) and ab initio structural refinement. The complementarity of TEM, APT, and ab initio methods provides powerful means for crystallographic analysis of embedded nanoscale phases, which may not be otherwise studied with conventional x-ray diffraction techniques [12,13]. 1.1. Experimental procedure The presently studied alloy of U-10 wt% Mo (U-21.61 at% Mo) was fabricated at the Y12 National Security Complex by melting and casting in graphite molds. The cast microstructure was homogenized at 900 C for 48 h or at 1000 C for 16 h, and further annealed at the subeutectoid temperature of 500 C. Detailed description of the heat treatments can be found in our previous work [1]. The samples for crystallographic analysis were prepared with an FEI Quanta 3D focused ion beam instrument. Structural and crystallographic analysis was performed with a probe-corrected FEI Titan 80e300 operated at 300 kV. The observations were performed using STEM with a HAADF detector. The probe convergence angle was 17.8 mrad, and the inner detection angle on the HAADF detector was three times higher than the probe convergence angle. Simulation of HAADF images was performed with a computer code developed by E. Kirkland [14]. The calculations were performed with microscope parameters that closely correspond to the experimental conditions (E ¼ 300 kV, cs ¼ 5 mm, convergence angle 18 mrad). The simulated images were convoluted with a Gaussian of full width at half maximum (FWHM) ¼ 0.08 nm to account for spatial incoherence of the imaging system. Structural refinement was performed with density functional theory calculations using the Vienna Ab initio Simulation Package (VASP) [15,16]. The projector augmented wave method was used in combination with generalized gradient approximation for the exchange correlation potential. All reported calculations were performed with 520 eV energy cutoff. Laser assisted APT analysis was conducted using a CAMECA LEAP 4000XHR APT system equipped with a 355 nm UV laser, using 100 pJ laser energy, while the specimen was kept at 45 K and the evaporation rate was maintained at 0.005 atoms/pulse. The APT results were reconstructed using interactive visualization and analysis software (IVAS 3.8). 2. Results and discussion The U2MoSi2C forms within the homogenized microstructure of UeMo alloys. The morphology of U2MoSi2C phase varies depending on the homogenization temperature [1]. For low temperature homogenization treatments of 900 C, the U2MoSi2C phase adopts
blocky/round morphology, and it often forms at interphase boundaries of g-UMo/UC, or as isolated faceted precipitates along grain boundaries. For higher temperature homogenization treatments of 1000 C, the U2MoSi2C phase is a thin, intergranular wetting phase at g-UMo grain boundaries. An example of U2MoSi2C precipitate phases from 900 C and 1000 C homogenization treatments is shown in Fig. 1(a and b). The crystal symmetry and lattice parameters of U2MoSi2C were analyzed using selected area diffraction. The experimental observations from multiple precipitates and along several zone axes enabled us to reconstruct the full 3D reciprocal lattice. The key lowindexed selected area electron diffraction patterns are shown in Fig. 1. The observations reveal the presence of a single zone with fourfold symmetry (labeled the D1 zone), which suggests that the U2MoSi2C phase possess tetragonal symmetry. The assignment of tetragonal symmetry is supported by the symmetry and the angular relationship between the obtained diffraction patterns. Full self-consistent indexing of the diffraction patterns is provided for the key zones presented in Fig. 1. The lattice parameters of U2MoSi2C phase is found to be: a ¼ 6.7 Å, c ¼ 4.3 Å. Fig. 2 shows the APT result of a U2MoSi2C precipitate and adjacent g-UMo matrix. The precipitate phase composition was estimated to be approximately U ¼ 33%, Si ¼ 33%, Mo ¼ 16%, and C ¼ 16% (at.%), with several minor impurity elements, which lead us to predict the U2MoSi2C stoichiometry [1]. A 3 nm 4 nm 20 nm section of the APT data for U2MoSi2C phase was extracted and is shown in Fig. 2(b), which highlights the observation of parallel planes separated by a distance of 0.43 nm. This observation was quantitatively analyzed using spatial distribution mapping (SDM) along the z-axis of the extracted volume. The ripples in the SDM correspond to the observed periodicity in the APT reconstruction, which corresponds to 0.43 nm. Interestingly, this matches very well the c-axis of the U2MoSi2C phase structure, as can be seen from the inset of the structural model. Careful analysis of the structural model reveals that the midplane along the c-axis is the only plane where U atoms reside, and these will have a periodicity corresponding to the c-axis of the structure. Because APT spatial resolution is 0.2 nm along the z-axis, it is clear that APT results could distinguish these U lattice planes in the reconstruction, which helps with independent validation of the structural finding from STEM imaging and diffraction. The combined results from electron diffraction and APT indicate that U2MoSi2C represents a quaternary silicide carbide phase. According to the Inorganic Crystal Structure Database, there are no known quaternary UMo silicide carbide phases, nor any related isostructural polymorphs in ternary UeSieC, UeMoeC, or UeMoeSi alloy systems. Examination of compositionally related uranium silicide carbides, including U20Si16C3 and U3Si2C2 [17], as well as the molybdenum silicide carbide Mo4$8Si3C0.6 [18], show no structural relationship, indicating an independent and novel status of this phase. To further investigate the structural and crystallographic nature of U2MoSi2C, with the goal to obtain full crystallographic description, we captured a series of atomic-level HAADF images. HAADF imaging provides directly interpretable and quantifiable atomic level contrast, which scales as Z~1.7 [19]. The contrast in U2MoSi2C is expected to be dominated by heavy uranium and molybdenum atoms. If resolved independently, the contrast from U is expected to be ~4 higher than Mo, ~25 higher than Si, and ~100 higher than C. HAADF observations from low-index zones of [001], [011], [100], and [310] are shown in Fig. 3. The observation from [001] is particularly important, as it enables resolution of the structure along the highest symmetry fourfold axis. Two types of sites were identified based on the relative contrast, and these two sites can be
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Fig. 1. (a,b) HAADF images of U2MoSi2C along from homogenization at 900C and 1000C. c) Diffraction analysis of the newly identified U2MoSi2C from different zones.
Fig. 2. (a) APT reconstruction of U2MoSi2C precipitate and surrounding g-UMo matrix. (b) A section of APT data from the U2MoSi2C phase showing APT resolving lattice planes along C axis with 0.433 nm spacing (c) spatial distribution map plotted using U, Mo, Si and C along Z axis of the precipitate showing the 0.433 nm spacing. The crystal structure of U2MoSi2C is shown as inset.
consistently interpreted in terms of the projected potential of U and Mo atoms. The sites with relatively strong intensities (highlighted by squares) are interpreted as U sites. There are four of these sites per unit cell. The low intensity sites, which are on the fourfold rotational axis, can be interpreted as Mo atoms. The sites are marked at the origin and face-centered position of the cell, as highlighted in Fig. 3(a). Assuming the periodicity along the projected direction of c ¼ 4.3 Å, the identified atomic sites correspond to one atom of U and one of Mo per unit cell. Observations from [100] and [310] zones, both of which are orthogonal with respect to the fourfold zone axis, are shown in
Fig. 3(b and c). The sites with strong intensities are consistent with U contrast, while the low intensity sites can be interpreted as Mo. Both U and Mo are identified in independent projections, and more importantly, on adjacent (002) planes, which provides an important constraint for determining the relative positions of these atomic species. Additionally, we observed that, in both projections, the high intensity U sites are streaked, suggesting that U is not projected into a single atomic column, but instead corresponds to two closely spaced columns. For the [100] orientation in Fig. 3(b), the two closely spaced U columns can be resolved in a few regions of the image, with the separation of ~1.2 Å. On the other hand, the
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Fig. 3. HAADF STEM observation of U2Mo2SiC phase along low index zone axis.
separation of U sites in [310] orientation was not resolved. Further information regarding the projected positions of U and Mo atoms was obtained from the [101] orientation, which represents another important orientation where U and Mo sites can be independently resolved. As shown in Fig. 3(d), the high intensity columns indicate projection of U, while the diffuse intensity between adjacent U atoms indicates the presence of Mo. Given the constraints from the electron diffraction, APT, and HAADF observations, we propose that U2MoSi2C phase has tetragonal crystal structure, as shown in Fig. 4. The atomic sites for U and Mo were determined directly from HAADF observations. On the other hand, the atomic sites for Si and C were determined indirectly based on evaluation of the coordination environment around U and Mo of the tetragonal cell. To be consistent with the APT results, we expect the structure to contain two carbon and four silicon atoms. The two carbon atoms can be assigned to octahedral sites defined
by four U and two Mo neighbors. The resulting C bonding environment, in terms of coordination number and atomic distances, is analogous to the octahedral bonding in UC phase. The four silicon atoms, on the other hand, can be assigned to sites coordinated by nine atoms. This bonding environment includes six coordinating U atoms with the bond distance of 2.856 Åe2.956 Å, two Mo coordinating atoms with the bond distance of 2.614 Å, and one Si with the bond distance of 2.47 Å. The structure optimization of U2MoSi2C was performed employing the conjugate gradient structure optimization algorithm as implemented in VASP. The results from the structural refinement are reported in Table .1 The structure was found to have tetragonal symmetry, with lattice parameters of a ¼ 6.67 Å and c ¼ 4.33 Å, and belongs to space group P4/mbm (No.127). This is fully consistent with the electron diffraction measurements in Fig. 1. Importantly, we find that the refined atomic sites for U and Mo fully reproduce
Fig. 4. (a). Crystal structure of the newly identified silicide carbide phase. 4(bee) HAADF simulations of [001], [100], [310] and [110] zones.
L. Kovarik et al. / Journal of Nuclear Materials 519 (2019) 287e291 Table 1 Crystallographic description of newly identified U2MoSi2C phase, space group P4/ mbm (No.127). The lattice parameters, a ¼ 6.67 Å, c ¼ 4.33 Å and atom positions were refined using ab-initio methods.
C Mo Si U
Occ.
Mul. Wyck.
Symmetry
x
y
z
1 1 1 1
2a 2b 4g 4h
4/m 4/m m2m m2m
0 0 0.869 0.171
0 0 0.369 0.329
0.5 0 0 0.5
291
The relevance of this work is discussed in the context of estimating the volume fraction of U2MoSi2C phase and its influence on discontinuous precipitation kinetics in the UMo alloys. This study highlights the potential for discovery of new phases in UMo alloys using high resolution characterization, especially with introduction of different impurity elements and different thermomechanical treatments. Acknowledgments
the atomic level HAADF observations, and that the refined atomic positions for Si and C are consistent with our initial assignment. The consistency of HAADF simulations with the experimentally observed zones is shown in Fig. 4(b,c,d,e). The structural and compositional details of the impurity phase U2MoSi2C provides the necessary basis for quantitatively estimating the volume fraction of this phase [20]. This is especially important in the context of understanding the rate of g-UMo discontinuous precipitation, because the extent of the thin wetting layer of U2MoSi2C precipitation along g-UMo grain boundaries has been proposed as controlling the rate of discontinuous precipitation [1]. The ability of U2MoSi2C to act as sink for other impurity elements is yet another topic that should be further explored in relationship to phase stability and properties of UMo alloys. Partitioning of Al, B, P, Ni Ca, and Ti into U2MoSi2C has been observed [1]. Additionally, Ni and Al were observed to segregate at the g-UMo and U2MoSi2C interphase, forming complexion structures. All of these factors are expected to have direct implications for the fuel quality, and thus the irradiation performance in the reactor. This detailed structural and compositional analysis of U2MoSi2C phase will also serve as a baseline for possible investigation of changes to the structure of this impurity phase if the concentration of other dissolved impurity elements (such as Si, C, Al, B, P, Ni, Ca, and Ti) or new impurities, such as Cr or Co, in UMo alloys is increased significantly by changes in chemistry of feedstock materials. Such studies will also be necessary in developing a comprehensive understanding of the influence of impurity elements on microstructural evolution of UMo alloys as a function of various thermomechanical treatments, which will directly affect the fuel quality and its irradiation performance in reactor. 3. Conclusions Electron diffraction, STEM HAADF analysis, APT, and ab initio calculations were used to study the crystal structure and composition of U2MoSi2C phase. The work enabled full crystallographic description of U2MoSi2C phase, which was found to have tetragonal symmetry with lattice parameters of a ¼ 6.67 Å and c ¼ 4.33 Å, and space group P4/mbm (No.127). Ab initio calculations were used to obtain atom position refinement and to verify structural stability.
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