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Inferring radiation-induced microstructural evolution in single-crystal niobium through changes in thermal transport
Journal of Nuclear Materials 523 (2019) 378e382
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Inferring radiation-induced microstructural evolution in single-crystal niobium through changes in thermal transport Sara E. Ferry a, Cody A. Dennett a, Kevin B. Woller b, Michael P. Short a, * a b
Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
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Irradiated Nb was non-destructively analyzed by transient grating spectroscopy (TGS). Thermal diffusivity dropped by 4x at 0.001 dpa, recovering halfway by 0.01 dpa. These measurements were linked to point defect formation and subsequent clustering. TGS-measured thermal diffusivity can track radiation induced microstructural changes.
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Article history: Received 25 January 2019 Received in revised form 6 May 2019 Accepted 6 June 2019 Available online 8 June 2019
Ion beams enable accelerated radiation exposure experiments, but traditional destructive postirradiation examination techniques remain time-consuming. Here, measurements of thermal diffusivity are used to monitor microstructural changes in single crystal niobium irradiated with Si3þ ions up to 1.9 dpa (5.1 1015 ions/cm2). Changes in thermal transport are correlated to defect generation and clustering through dose-microstructure relationships. These measurements are made using transient grating spectroscopy (TGS), a multi-modal, non-destructive, rapid characterization technique. This work demonstrates that in this low-dose point defect clustering regime, direct measurements of thermal properties are a powerful tool for understanding irradiation-induced microstructure evolution. © 2019 Elsevier B.V. All rights reserved.
Keywords: Refractory metals Point defects Irradiation Transient grating spectroscopy (TGS) Thermal diffusivity
Nuclear materials in a reactor core are exposed to an aggressive thermomechanical environment and high radiation fluxes on a timescale of years to decades. Examples of radiation-induced changes that occur on these timescales include embrittlement of reactor pressure vessel (RPV) steels [1], spinodal decomposition of delta ferrite in cast austenitic stainless steel core barrels [2], void swelling [3,4], and thermomechanical property degradation [5].
* Corresponding author. E-mail address: [email protected] (M.P. Short). https://doi.org/10.1016/j.jnucmat.2019.06.015 0022-3115/© 2019 Elsevier B.V. All rights reserved.
Materials must be engineered to withstand these degradation modes to ensure safe and reliable system performance. Using direct neutron exposures to test candidate materials during the design process is slow, costly, and often presents activation hazards. As a result, ion irradiation studies that emulate neutron damage have become increasingly important to radiation materials science: they are cheaper, faster, and more accessible [6,7]. Traditional methods of post irradiation examination (PIE), such as transmission electron microscopy (TEM) and mechanical testing, have not kept pace with the order-of-magnitude decrease in
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exposure time offered by ion irradiation. Before ion irradiation can lead to rapid innovation in material design, faster methods of radiation damage characterization must be developed. To this end, there is significant interest in developing multi-modal, nondestructive evaluation (NDE) techniques for the study of nuclear materials. Few existing NDE methods are multi-modal, meaning that they are capable of returning multiple material properties from a single measurement. Single-mode NDE methods used in previous studies include traditional contact ultrasonics to measure density and porosity evolution [8], time-domain thermal reflectance (TDTR) measurements of thermal conductivity to study radiation defect cluster evolution [9e11], and non-linear ultrasound to study embrittlement in RPV steels [12,13]. These methods share the goal of using material properties and performance as a indirect indicator of microstructural evolution. One multi-modal NDE method that is being increasingly utilized for this purpose is transient grating spectroscopy (TGS) [14,15]. TGS optically induces and monitors 1-D periodic excitations in temperature and surface displacement on a sample. Analysis of the resulting signal returns thermal and elastic material properties, both of which are known to change due to the effects of radiation [16e20]. This methodology is particularly powerful because the length scale of the excitation can be matched to the depth of the ion-induced damage profile (typically confined to the first few micrometers into the sample). This allows measurements to be made only within the radiation-damaged surface layer [18,20], so the results are not affected by the undamaged bulk and are therefore more representative of the uniform radiation damage incurred by neutrons. TGS has recently been demonstrated to be a useful tool for monitoring radiation-induced microstructural evolution. For example, Hofmann et al. characterized the effects of helium implantation in tungsten by measuring the thermal and elastic properties of the samples with TGS [16,17], while Dennett et al. characterized high-dose effects of self-ion irradiation in copper by measuring elastic property changes with TGS [18]. The TGS method's ability to make rapid, non-contact measurements make it well-suited for in situ use with an ion beam. To this end, a TGS system was recently implemented as an in situ, real-time, multimodal property monitoring system for ion-irradiated materials [21]. However, there are still few examples in which TGS is used to characterize microstructural evolution at low doses (less than 0.01 dpa). In this study, we explore this low-dose defect accumulation regime in ion irradiated single-crystal niobium and show that thermal diffusivity is a strong, indirect indicator of microstructural evolution. We choose a simple, single-crystal, single-element test case to separate the effects of irradiation-induced segregation or grain evolution common in alloys and polycrystals. We find that for this damage regime, thermal diffusivity measurements are strongly correlated with the development and evolution of point defect creation and the onset of defect clustering. In doing so, this work demonstrates that the effects of radiation-induced evolution may be tracked from a pristine to a highly-damaged state through at least one of the property measurements available through TGS. Single crystal niobium samples 1 mm thick were cut from a 5 mm diameter, {011}-oriented rod using a low-speed saw equipped with a diamond blade. Sample orientation was confirmed via X-ray diffraction to be within ±2+ of the given index. The samples were hand-polished using diamond paper of progressively finer grit (30, 15, 9, 6, 3, 1, 0.5, and 0.1 mm) from South Bay Technologies. Two samples were compressively cold worked in a pellet press, one to 1500 lbs. and the other to 2000 lbs. force, to investigate if imposing a native dislocation network prior to irradiation affects the evolution of thermal transport properties. Previously, the effects of small defect cluster interactions with dislocations have
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been noted using this methodology in the radiation-induced evolution of acoustic velocities [18]. A third and final sample was left unpressed as a control. Dislocation densities were characterized by measuring Bragg peak width with a Bruker D8 High-Resolution X-Ray Diffractometer (HRXRD) equipped with a Ge(022) incident beam monochromator. Material defects, such as the dislocations induced by cold work, are known to cause Bragg peak broadening [22e25]. Sample Bragg peak widths were correlated to dislocation densities via a method outlined in Ref. [25], based on methods described in Refs. [26,27]. Samples were determined to have dislocation densities of 9.7 109 cm2 (control sample), 7.8 1011 cm2 (sample pressed to 1500 lbs. force), and 5.1 1011 cm2 (sample pressed to 2000 lbs. force). Each sample was progressively irradiated at room temperature (25+C, or T =Tmelt ¼ 0:11) to 0.006, 0.02, 0.06, 0.6, and 1.9 displacements per atom (dpa) with the CLASS (Cambridge Laboratory for Accelerator-based Surface Science) General Ionex 1.7 MV tandem ion accelerator at MIT using 5.3 MeV Si3þ ions at an average beam current of 100 nA. The beam passed through a circular aperture 8 mm in diameter, resulting in a Gaussian profile corresponding to a flux of 6.9 1011 Si3þ ions/cm2s at the sample center and a flux of 4.4 1011 Si3þ ions/cm2s at the sample edge. A lateral average of the ion flux 1.5 mm about the sample center was used in the dpa calculations, as this corresponds to the sample region in which TGS measurements were made [28]. SRIM was then used to determine radiation damage as a function of depth, which was used to determine the average radiation damage incurred in the region probed by TGS [29]. Fig. 1 shows schematics of the beam profile and the into-sample damage profile. A TGS measurement begins by crossing pulsed laser beams (the “pump” beams) on the sample surface to create an interference pattern. The resultant spatially periodic heating pattern generates standing acoustic waves (SAWs) on the sample surface due to thermal expansion. The SAWs decay before the next pumping pulse occurs. To measure the oscillation frequency and decay rate of the periodic excitations in surface displacement (caused by the SAWs) and reflectivity (caused by a periodic surface temperature profile), these excitations are used as a diffraction grating for a quasicontinuous wave (CW) probing laser. By monitoring the first order diffraction of this probing laser from the material excitations on a fast avalanche Si photodiode, the dynamics of these excitations can be observed and material properties extracted. Post-exposure TGS experiments were carried out at a nominal grating spacing of 5.5 mm, which corresponds to a thermal probing
Fig. 1. Left: The Gaussian ion beam profile masked by the 8 mm aperture as incident on the sample. (b) SRIM simulation of the radiation damage profile of by 5.3 MeV Si3þ ions into Nb, showing that the TGS thermal probe depth is matched to the range of the Si3þ ions.
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where A, B, and C are fitting constants, q is the excitation
wavevector (q ¼ 2p =L, where L is the excitation wavelength), a is the thermal diffusivity, b describes the ratio of displacement and reflectivity contributions to the signal, f is the SAW frequency, q is the acoustic phase, and t is an acoustic decay constant [33]. These constants and parameters are obtained from each TGS signal using the parameter extraction method described in Ref. [33]. Fig. 3 shows the measured thermal diffusivity of niobium after each irradiation. At the lowest irradiation dose of 0.006 dpa, thermal diffusivity decreases in all samples by more than 70%. At 0.02 dpa, thermal diffusivity recovers to about 50% of its pre-irradiation value, at which point it appears to stabilize even after additional irradiation. All three samples exhibit similar behavior, and the thermal diffusivity response of cold worked niobium to irradiation was not appreciably different from the response of the control sample, despite differences in initial dislocation density. Our hypothesis to explain the reduction in thermal diffusivity observed at 0.006 dpa is attributed to the isolated point defects that comprise the majority of radiation damage at low doses [36]. Point defects are efficient scatterers of free electrons, which are the primary conductors of heat in metals, and so thermal diffusivity sharply decreases when the point defect concentration increases [37,38]. As dose increases, point defects cluster into larger mesoscale defects like voids and dislocation loops, which form due to overlapping radiation damage cascades despite the low irradiation temperature (293 K), and these voids and loops become defect sinks [36]. The previously uniformly dispersed point defect population is now bound in extended defects, which subtend a far smaller fraction of the material's volume. Electron scattering decreases as a result, and so thermal diffusivity recovers to approximately 50% of its initial value (see 0.02 dpa in Fig. 3). The equilibrium concentration of point defects remaining in the bulk, in conjunction with the mesoscale defects that developed with increasing dose, results in this significant reduction of thermal diffusivity (see 0.02e1.9 dpa in Fig. 3) in comparison to the unirradiated state. Loomis and Gerber previously showed that irradiated niobium undergoes “microstructural cleanup” as dose is increased - e.g., increased homogenization of the bulk due to absorption of highconcentration, low-volume point defects at low-concentration, high-volume defect sinks. TEM images of neutron-irradiated niobium to similar doses at comparable temperature (50+C, or T =Tmelt ¼ 0:12), shown in Fig. 4, show how single-crystal niobium with varying oxygen impurity concentrations were affected by
Fig. 2. Example TGS measurement collected from single-crystal Nb following irradiation to 1.9 dpa. Characteristic SAW oscillations are superimposed on the thermal decay profile. The thermal decay profile is fitted during analysis to extract thermal diffusivity (red dashed line). The full model also fits the SAW oscillations, but only its thermal component is depicted here (see Ref. [33]). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. TGS-measured thermal diffusivity of irradiated, single-crystal niobium as a function of dpa. The thermal diffusivity is observed to drop by more than 70% after irradiation to 0.006 dpa. It rises to 50% of its initial value after further irradiation to 0.02 dpa. Error bars show the 1s standard deviation in the data. Data points are artificially spaced away from each other in x at each dose for easier interpretation of the overlapping data. The dotted lines point to the actual dose for all data in each box.
depth of about 1.75 mm into the surface [30,31]. SRIM results indicated that 5.3 MeV Si3þ ions into pure niobium have a range of 1.5 mm, so this probing depth captures the entirety of the damaged surface layer without sampling the undamaged material beneath it (see Fig. 1). The SRIM damage profile was used to calculate an average applied dose rate of 1.6 104 dpa/s in the 0e1.75 mm region [28]. Thermal properties were measured via TGS using the same dual-heterodyne system and facility parameters described in Ref. [32]. Signals were analyzed via the methods described in Refs. [19,33]. Fig. 2 shows a typical TGS signal from the non-cold worked Nb sample following its final exposure to a total dose of 1.9 dpa. The fast oscillations correspond to SAWs, the frequency of which can be easily extracted by performing a Fourier transform on the signal [30,33,34]. Oscillation amplitude decreases as the SAWs propagate out of the area interrogated by the probing laser. Energy loss in the form of acoustic damping may also occur due to the presence of lattice defects in the sample [35]. The signal dissipates to background as the thermal grating decays due to heat diffusion. The signal is amplified, and noise reduced, via the methods described in Ref. [32]. The true grating spacing was determined by obtaining a TGS signal from a well-characterized single-crystal tungsten calibration sample at the beginning of each measurement session. Each measurement consisted of an average of 30,000 individual laser shots measured at a repetition rate of 1 kHz. These traces were averaged to create the signal used for analysis. Between 80 and 400 measurements were made on each sample, at each progressive dose level, to obtain adequate statistics. Samples were rotated and/or moved laterally and/or vertically between each measurement, with measurements concentrated within 1.5 mm from the sample center. Analysis of these measurements showed that TGS-measured thermal diffusivity did not vary significantly with lateral or rotational surface position. Given this determination, only eight measurements were made at the 1.9 dpa level for each sample. The composite TGS response as measured on a pair of Si avalanche photodiodes is given as
pffiffiffiffiffi b IP ðtÞ ¼ A erfc q at pffiffi exp q2 at t þ B sinð2pft þ qÞexpðt=tÞ þ C;
(1)
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Fig. 4. TEM micrographs of single-crystal Nb irradiated with neutrons at 50+C as functions of neutron fluence in n =cm2 and oxygen content [39]. Numerous small defects are observed at low doses, clustering into fewer, larger ones with continued irradiation. The highest fluence corresponds roughly to when we observed thermal diffusivity saturation in our experiments, assuming a conversion of 2 1021 n= cm2 per dpa [41]. Figure reproduced from Ref. [39] with permission from Elsevier.
radiation [39]. The oxygen impurities were mobile during irradiation. As irradiation dose increased, the number of radiationinduced defects increased, trapping the oxygen impurities. Increased dose “cleaned up” the microstructure: defects and impurities concentrated in clusters (the dark spots in the TEM images), leading to an overall reduction in the concentration of point defects in the bulk. Both Loomis and Gerber [39] and Dutta et al. [40] show similar behavior for irradiated single-crystal and polycrystalline niobium, respectively, in the dose response of defect populations. TEM micrographs in both studies show an initial buildup of very small black dots believed to be small vacancy clusters. As the dose increased, defects were found to have migrated to larger defect sinks. It has been proposed in Ref. [40] that interstitial point defects diffuse more quickly to sinks, leaving behind a greater concentration of vacancy point defects. These then form the clusters that collapse to dislocation loops, which, in turn, become defect sinks. The presence of these dislocation loops were confirmed via TEM in that study. The TEM results from Refs. [39,40] and the dislocation density measurements from Ref. [40] suggest that similar microstructural evolution has occurred in the niobium in this study. In order to confirm this hypothesis, this study should be repeated, and TGS measurements should be followed by TEM examination to confirm the microstructural characterization predicted by the TGS results. This process can be hastened by irradiating five individual niobium samples to the doses of interest, as the data presented in Fig. 3 indicates that inter-sample differences in thermal diffusivity are negligible at each dose. Niobium is an electronic heat conductor, like many materials used for in-core applications. Therefore, it is a useful test case for proving the utility of TGS as an NDE technique for characterizing the thermal properties of nuclear materials, and how they change with radiation exposure. As shown in this work, the thermal diffusivity measurements also provide insight into radiation-
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induced microstructural evolution. TGS thus quickly and nondestructively provides both a direct measurement of a useful engineering property and insight into likely radiation defect population changes. This work demonstrated that TGS can be used to detect radiation-induced changes in thermal diffusivity in an ionirradiated, single-crystal metal, and that those changes can be used to infer the evolution of radiation defect populations without resorting to traditional PIE methods. In particular, TGS can be used to pinpoint the onset dose for mesoscale defect clustering. For the case of single-crystal niobium, this dose occurs between 0.006 and 0.02 dpa. Future studies should repeat this study using in situ TGS to provide finer resolution of the onset of defect clustering as measured by thermal diffusivity recovery, both on niobium and on other materials [21]. The authors thank Dr. Charles Settens for his assistance with XRD analysis. This work was supported in part by the MIT-SUTD International Design Center (IDC). C.A.D. acknowledges support from the DOE NNSA Stewardship Science Graduate Fellowship under cooperative Agreement No. DE-NA-0003864. M.P.S. acknowledges funding from the U.S. Nuclear Regulatory Commission's MIT Nuclear Education Faculty Development Program under Grant No. NRC-HQ-84-15-G-0045. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-14-19807. Data Availability Statement: The TGS signal processing code, along with all raw and processed data used in the creation of this manuscript, can be found in the GitHub repository for this study [28]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jnucmat.2019.06.015. References [1] N. Soneda, Irradiation Embrittlement of Reactor Pressure Vessels (RPVs) in Nuclear Power Plants, Elsevier, 2014. [2] Z.B. Li, W.-Y. Lo, Y.R. Chen, J. Pakarinen, Y.Q. Wu, T. Allen, Y. Yang, Irradiation response of delta ferrite in as-cast and thermally aged cast stainless steel, J. Nucl. Mater. 466 (2015) 201e207. [3] F.A. Garner, L. Shao, M.B. Toloczko, S.A. Maloy, V.N. Voyevodin, Use of self-ion bombardment to study void swelling in advanced radiation-resistant alloys, in: 17th Int. Symp. Conf. On Environmental Degradation of Materials in Nuclear Power Systems, Ottawa, Canada, 2015. [4] S. Zinkle, G. Was, Materials challenges in nuclear energy, Acta Mater. 61 (3) (2013) 735e758. [5] B.D. Wirth, M.J. Caturla, T.D. de la Rubia, T. Khraishi, H. Zbib, Mechanical property degradation in irradiated materials: a multiscale modeling approach, Nucl. Instrum. Methods Phys. Res. B 180 (1e4) (2001) 23e31. [6] G.S. Was, Z. Jiao, E. Getto, K. Sun, A.M. Monterrosa, S.A. Maloy, O. Anderoglu, B.H. Sencer, M. Hackett, Emulation of reactor irradiation damage using ion beams, Scripta Mater. 88 (2014) 33e36. [7] S.J. Zinkle, L.L. Snead, Opportunities and limitations for ion beams in radiation effects studies: bridging critical gaps between charged particle and neutron irradiations, Scripta Mater. 143 (2018) 154e160. [8] J. Etoh, M. Sagisaka, T. Matsunaga, Y. Isobe, F.A. Garner, P.D. Freyer, Y. Huang, J.M.K. Wiezorek, T. Okita, Development of a nondestructive inspection method for irradiation-induced microstructural evolution of thick 304 stainless steel blocks, J. Nucl. Mater. 440 (1e3) (2013) 500e507. [9] J. Pakarinen, M. Khafizov, L. He, C. Wetteland, J. Gan, A.T. Nelson, D.H. Hurley, A. El-Azab, T.R. Allen, Microstructure changes and thermal conductivity reduction in uo2 following 3.9 MeV He 2þ ion irradiation, J. Nucl. Mater. 454 (1) (2014) 283e289. [10] M. Khafizov, C. Yablinsky, T. Allen, D. Hurley, Measurement of thermal conductivity in proton irradiated silicon, Nucl. Instrum. Meth. Phys. Res. B 325 (2014) 11e14. [11] R. Cheaito, C. Gorham, A. Misra, K. Hattar, P. Hopkins, Thermal conductivity measurements via time-domain thermoreflectance for the characterization of radiation induced damage, J. Mater. Res. 30 (9) (2015) 1403e1412. [12] K.H. Matlack, J.J. Wall, J.-Y. Kim, Jianmin Qu, L.J. Jacobs, H.-W. Viehrig, Evaluation of radiation damage using nonlinear ultrasound, J. Appl. Phys. 111 (5)
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Inferring radiation-induced microstructural evolution in single-crystal niobium through changes in thermal transport Sara E. Ferry a, Cody A. Dennett a, Kevin B. Woller b, Michael P. Short a, * a b
Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
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
Irradiated Nb was non-destructively analyzed by transient grating spectroscopy (TGS). Thermal diffusivity dropped by 4x at 0.001 dpa, recovering halfway by 0.01 dpa. These measurements were linked to point defect formation and subsequent clustering. TGS-measured thermal diffusivity can track radiation induced microstructural changes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 January 2019 Received in revised form 6 May 2019 Accepted 6 June 2019 Available online 8 June 2019
Ion beams enable accelerated radiation exposure experiments, but traditional destructive postirradiation examination techniques remain time-consuming. Here, measurements of thermal diffusivity are used to monitor microstructural changes in single crystal niobium irradiated with Si3þ ions up to 1.9 dpa (5.1 1015 ions/cm2). Changes in thermal transport are correlated to defect generation and clustering through dose-microstructure relationships. These measurements are made using transient grating spectroscopy (TGS), a multi-modal, non-destructive, rapid characterization technique. This work demonstrates that in this low-dose point defect clustering regime, direct measurements of thermal properties are a powerful tool for understanding irradiation-induced microstructure evolution. © 2019 Elsevier B.V. All rights reserved.
Keywords: Refractory metals Point defects Irradiation Transient grating spectroscopy (TGS) Thermal diffusivity
Nuclear materials in a reactor core are exposed to an aggressive thermomechanical environment and high radiation fluxes on a timescale of years to decades. Examples of radiation-induced changes that occur on these timescales include embrittlement of reactor pressure vessel (RPV) steels [1], spinodal decomposition of delta ferrite in cast austenitic stainless steel core barrels [2], void swelling [3,4], and thermomechanical property degradation [5].
* Corresponding author. E-mail address: [email protected] (M.P. Short). https://doi.org/10.1016/j.jnucmat.2019.06.015 0022-3115/© 2019 Elsevier B.V. All rights reserved.
Materials must be engineered to withstand these degradation modes to ensure safe and reliable system performance. Using direct neutron exposures to test candidate materials during the design process is slow, costly, and often presents activation hazards. As a result, ion irradiation studies that emulate neutron damage have become increasingly important to radiation materials science: they are cheaper, faster, and more accessible [6,7]. Traditional methods of post irradiation examination (PIE), such as transmission electron microscopy (TEM) and mechanical testing, have not kept pace with the order-of-magnitude decrease in
S.E. Ferry et al. / Journal of Nuclear Materials 523 (2019) 378e382
exposure time offered by ion irradiation. Before ion irradiation can lead to rapid innovation in material design, faster methods of radiation damage characterization must be developed. To this end, there is significant interest in developing multi-modal, nondestructive evaluation (NDE) techniques for the study of nuclear materials. Few existing NDE methods are multi-modal, meaning that they are capable of returning multiple material properties from a single measurement. Single-mode NDE methods used in previous studies include traditional contact ultrasonics to measure density and porosity evolution [8], time-domain thermal reflectance (TDTR) measurements of thermal conductivity to study radiation defect cluster evolution [9e11], and non-linear ultrasound to study embrittlement in RPV steels [12,13]. These methods share the goal of using material properties and performance as a indirect indicator of microstructural evolution. One multi-modal NDE method that is being increasingly utilized for this purpose is transient grating spectroscopy (TGS) [14,15]. TGS optically induces and monitors 1-D periodic excitations in temperature and surface displacement on a sample. Analysis of the resulting signal returns thermal and elastic material properties, both of which are known to change due to the effects of radiation [16e20]. This methodology is particularly powerful because the length scale of the excitation can be matched to the depth of the ion-induced damage profile (typically confined to the first few micrometers into the sample). This allows measurements to be made only within the radiation-damaged surface layer [18,20], so the results are not affected by the undamaged bulk and are therefore more representative of the uniform radiation damage incurred by neutrons. TGS has recently been demonstrated to be a useful tool for monitoring radiation-induced microstructural evolution. For example, Hofmann et al. characterized the effects of helium implantation in tungsten by measuring the thermal and elastic properties of the samples with TGS [16,17], while Dennett et al. characterized high-dose effects of self-ion irradiation in copper by measuring elastic property changes with TGS [18]. The TGS method's ability to make rapid, non-contact measurements make it well-suited for in situ use with an ion beam. To this end, a TGS system was recently implemented as an in situ, real-time, multimodal property monitoring system for ion-irradiated materials [21]. However, there are still few examples in which TGS is used to characterize microstructural evolution at low doses (less than 0.01 dpa). In this study, we explore this low-dose defect accumulation regime in ion irradiated single-crystal niobium and show that thermal diffusivity is a strong, indirect indicator of microstructural evolution. We choose a simple, single-crystal, single-element test case to separate the effects of irradiation-induced segregation or grain evolution common in alloys and polycrystals. We find that for this damage regime, thermal diffusivity measurements are strongly correlated with the development and evolution of point defect creation and the onset of defect clustering. In doing so, this work demonstrates that the effects of radiation-induced evolution may be tracked from a pristine to a highly-damaged state through at least one of the property measurements available through TGS. Single crystal niobium samples 1 mm thick were cut from a 5 mm diameter, {011}-oriented rod using a low-speed saw equipped with a diamond blade. Sample orientation was confirmed via X-ray diffraction to be within ±2+ of the given index. The samples were hand-polished using diamond paper of progressively finer grit (30, 15, 9, 6, 3, 1, 0.5, and 0.1 mm) from South Bay Technologies. Two samples were compressively cold worked in a pellet press, one to 1500 lbs. and the other to 2000 lbs. force, to investigate if imposing a native dislocation network prior to irradiation affects the evolution of thermal transport properties. Previously, the effects of small defect cluster interactions with dislocations have
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been noted using this methodology in the radiation-induced evolution of acoustic velocities [18]. A third and final sample was left unpressed as a control. Dislocation densities were characterized by measuring Bragg peak width with a Bruker D8 High-Resolution X-Ray Diffractometer (HRXRD) equipped with a Ge(022) incident beam monochromator. Material defects, such as the dislocations induced by cold work, are known to cause Bragg peak broadening [22e25]. Sample Bragg peak widths were correlated to dislocation densities via a method outlined in Ref. [25], based on methods described in Refs. [26,27]. Samples were determined to have dislocation densities of 9.7 109 cm2 (control sample), 7.8 1011 cm2 (sample pressed to 1500 lbs. force), and 5.1 1011 cm2 (sample pressed to 2000 lbs. force). Each sample was progressively irradiated at room temperature (25+C, or T =Tmelt ¼ 0:11) to 0.006, 0.02, 0.06, 0.6, and 1.9 displacements per atom (dpa) with the CLASS (Cambridge Laboratory for Accelerator-based Surface Science) General Ionex 1.7 MV tandem ion accelerator at MIT using 5.3 MeV Si3þ ions at an average beam current of 100 nA. The beam passed through a circular aperture 8 mm in diameter, resulting in a Gaussian profile corresponding to a flux of 6.9 1011 Si3þ ions/cm2s at the sample center and a flux of 4.4 1011 Si3þ ions/cm2s at the sample edge. A lateral average of the ion flux 1.5 mm about the sample center was used in the dpa calculations, as this corresponds to the sample region in which TGS measurements were made [28]. SRIM was then used to determine radiation damage as a function of depth, which was used to determine the average radiation damage incurred in the region probed by TGS [29]. Fig. 1 shows schematics of the beam profile and the into-sample damage profile. A TGS measurement begins by crossing pulsed laser beams (the “pump” beams) on the sample surface to create an interference pattern. The resultant spatially periodic heating pattern generates standing acoustic waves (SAWs) on the sample surface due to thermal expansion. The SAWs decay before the next pumping pulse occurs. To measure the oscillation frequency and decay rate of the periodic excitations in surface displacement (caused by the SAWs) and reflectivity (caused by a periodic surface temperature profile), these excitations are used as a diffraction grating for a quasicontinuous wave (CW) probing laser. By monitoring the first order diffraction of this probing laser from the material excitations on a fast avalanche Si photodiode, the dynamics of these excitations can be observed and material properties extracted. Post-exposure TGS experiments were carried out at a nominal grating spacing of 5.5 mm, which corresponds to a thermal probing
Fig. 1. Left: The Gaussian ion beam profile masked by the 8 mm aperture as incident on the sample. (b) SRIM simulation of the radiation damage profile of by 5.3 MeV Si3þ ions into Nb, showing that the TGS thermal probe depth is matched to the range of the Si3þ ions.
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where A, B, and C are fitting constants, q is the excitation
wavevector (q ¼ 2p =L, where L is the excitation wavelength), a is the thermal diffusivity, b describes the ratio of displacement and reflectivity contributions to the signal, f is the SAW frequency, q is the acoustic phase, and t is an acoustic decay constant [33]. These constants and parameters are obtained from each TGS signal using the parameter extraction method described in Ref. [33]. Fig. 3 shows the measured thermal diffusivity of niobium after each irradiation. At the lowest irradiation dose of 0.006 dpa, thermal diffusivity decreases in all samples by more than 70%. At 0.02 dpa, thermal diffusivity recovers to about 50% of its pre-irradiation value, at which point it appears to stabilize even after additional irradiation. All three samples exhibit similar behavior, and the thermal diffusivity response of cold worked niobium to irradiation was not appreciably different from the response of the control sample, despite differences in initial dislocation density. Our hypothesis to explain the reduction in thermal diffusivity observed at 0.006 dpa is attributed to the isolated point defects that comprise the majority of radiation damage at low doses [36]. Point defects are efficient scatterers of free electrons, which are the primary conductors of heat in metals, and so thermal diffusivity sharply decreases when the point defect concentration increases [37,38]. As dose increases, point defects cluster into larger mesoscale defects like voids and dislocation loops, which form due to overlapping radiation damage cascades despite the low irradiation temperature (293 K), and these voids and loops become defect sinks [36]. The previously uniformly dispersed point defect population is now bound in extended defects, which subtend a far smaller fraction of the material's volume. Electron scattering decreases as a result, and so thermal diffusivity recovers to approximately 50% of its initial value (see 0.02 dpa in Fig. 3). The equilibrium concentration of point defects remaining in the bulk, in conjunction with the mesoscale defects that developed with increasing dose, results in this significant reduction of thermal diffusivity (see 0.02e1.9 dpa in Fig. 3) in comparison to the unirradiated state. Loomis and Gerber previously showed that irradiated niobium undergoes “microstructural cleanup” as dose is increased - e.g., increased homogenization of the bulk due to absorption of highconcentration, low-volume point defects at low-concentration, high-volume defect sinks. TEM images of neutron-irradiated niobium to similar doses at comparable temperature (50+C, or T =Tmelt ¼ 0:12), shown in Fig. 4, show how single-crystal niobium with varying oxygen impurity concentrations were affected by
Fig. 2. Example TGS measurement collected from single-crystal Nb following irradiation to 1.9 dpa. Characteristic SAW oscillations are superimposed on the thermal decay profile. The thermal decay profile is fitted during analysis to extract thermal diffusivity (red dashed line). The full model also fits the SAW oscillations, but only its thermal component is depicted here (see Ref. [33]). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. TGS-measured thermal diffusivity of irradiated, single-crystal niobium as a function of dpa. The thermal diffusivity is observed to drop by more than 70% after irradiation to 0.006 dpa. It rises to 50% of its initial value after further irradiation to 0.02 dpa. Error bars show the 1s standard deviation in the data. Data points are artificially spaced away from each other in x at each dose for easier interpretation of the overlapping data. The dotted lines point to the actual dose for all data in each box.
depth of about 1.75 mm into the surface [30,31]. SRIM results indicated that 5.3 MeV Si3þ ions into pure niobium have a range of 1.5 mm, so this probing depth captures the entirety of the damaged surface layer without sampling the undamaged material beneath it (see Fig. 1). The SRIM damage profile was used to calculate an average applied dose rate of 1.6 104 dpa/s in the 0e1.75 mm region [28]. Thermal properties were measured via TGS using the same dual-heterodyne system and facility parameters described in Ref. [32]. Signals were analyzed via the methods described in Refs. [19,33]. Fig. 2 shows a typical TGS signal from the non-cold worked Nb sample following its final exposure to a total dose of 1.9 dpa. The fast oscillations correspond to SAWs, the frequency of which can be easily extracted by performing a Fourier transform on the signal [30,33,34]. Oscillation amplitude decreases as the SAWs propagate out of the area interrogated by the probing laser. Energy loss in the form of acoustic damping may also occur due to the presence of lattice defects in the sample [35]. The signal dissipates to background as the thermal grating decays due to heat diffusion. The signal is amplified, and noise reduced, via the methods described in Ref. [32]. The true grating spacing was determined by obtaining a TGS signal from a well-characterized single-crystal tungsten calibration sample at the beginning of each measurement session. Each measurement consisted of an average of 30,000 individual laser shots measured at a repetition rate of 1 kHz. These traces were averaged to create the signal used for analysis. Between 80 and 400 measurements were made on each sample, at each progressive dose level, to obtain adequate statistics. Samples were rotated and/or moved laterally and/or vertically between each measurement, with measurements concentrated within 1.5 mm from the sample center. Analysis of these measurements showed that TGS-measured thermal diffusivity did not vary significantly with lateral or rotational surface position. Given this determination, only eight measurements were made at the 1.9 dpa level for each sample. The composite TGS response as measured on a pair of Si avalanche photodiodes is given as
pffiffiffiffiffi b IP ðtÞ ¼ A erfc q at pffiffi exp q2 at t þ B sinð2pft þ qÞexpðt=tÞ þ C;
(1)
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Fig. 4. TEM micrographs of single-crystal Nb irradiated with neutrons at 50+C as functions of neutron fluence in n =cm2 and oxygen content [39]. Numerous small defects are observed at low doses, clustering into fewer, larger ones with continued irradiation. The highest fluence corresponds roughly to when we observed thermal diffusivity saturation in our experiments, assuming a conversion of 2 1021 n= cm2 per dpa [41]. Figure reproduced from Ref. [39] with permission from Elsevier.
radiation [39]. The oxygen impurities were mobile during irradiation. As irradiation dose increased, the number of radiationinduced defects increased, trapping the oxygen impurities. Increased dose “cleaned up” the microstructure: defects and impurities concentrated in clusters (the dark spots in the TEM images), leading to an overall reduction in the concentration of point defects in the bulk. Both Loomis and Gerber [39] and Dutta et al. [40] show similar behavior for irradiated single-crystal and polycrystalline niobium, respectively, in the dose response of defect populations. TEM micrographs in both studies show an initial buildup of very small black dots believed to be small vacancy clusters. As the dose increased, defects were found to have migrated to larger defect sinks. It has been proposed in Ref. [40] that interstitial point defects diffuse more quickly to sinks, leaving behind a greater concentration of vacancy point defects. These then form the clusters that collapse to dislocation loops, which, in turn, become defect sinks. The presence of these dislocation loops were confirmed via TEM in that study. The TEM results from Refs. [39,40] and the dislocation density measurements from Ref. [40] suggest that similar microstructural evolution has occurred in the niobium in this study. In order to confirm this hypothesis, this study should be repeated, and TGS measurements should be followed by TEM examination to confirm the microstructural characterization predicted by the TGS results. This process can be hastened by irradiating five individual niobium samples to the doses of interest, as the data presented in Fig. 3 indicates that inter-sample differences in thermal diffusivity are negligible at each dose. Niobium is an electronic heat conductor, like many materials used for in-core applications. Therefore, it is a useful test case for proving the utility of TGS as an NDE technique for characterizing the thermal properties of nuclear materials, and how they change with radiation exposure. As shown in this work, the thermal diffusivity measurements also provide insight into radiation-
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induced microstructural evolution. TGS thus quickly and nondestructively provides both a direct measurement of a useful engineering property and insight into likely radiation defect population changes. This work demonstrated that TGS can be used to detect radiation-induced changes in thermal diffusivity in an ionirradiated, single-crystal metal, and that those changes can be used to infer the evolution of radiation defect populations without resorting to traditional PIE methods. In particular, TGS can be used to pinpoint the onset dose for mesoscale defect clustering. For the case of single-crystal niobium, this dose occurs between 0.006 and 0.02 dpa. Future studies should repeat this study using in situ TGS to provide finer resolution of the onset of defect clustering as measured by thermal diffusivity recovery, both on niobium and on other materials [21]. The authors thank Dr. Charles Settens for his assistance with XRD analysis. This work was supported in part by the MIT-SUTD International Design Center (IDC). C.A.D. acknowledges support from the DOE NNSA Stewardship Science Graduate Fellowship under cooperative Agreement No. DE-NA-0003864. M.P.S. acknowledges funding from the U.S. Nuclear Regulatory Commission's MIT Nuclear Education Faculty Development Program under Grant No. NRC-HQ-84-15-G-0045. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-14-19807. Data Availability Statement: The TGS signal processing code, along with all raw and processed data used in the creation of this manuscript, can be found in the GitHub repository for this study [28]. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jnucmat.2019.06.015. References [1] N. Soneda, Irradiation Embrittlement of Reactor Pressure Vessels (RPVs) in Nuclear Power Plants, Elsevier, 2014. [2] Z.B. Li, W.-Y. Lo, Y.R. Chen, J. Pakarinen, Y.Q. Wu, T. Allen, Y. Yang, Irradiation response of delta ferrite in as-cast and thermally aged cast stainless steel, J. Nucl. Mater. 466 (2015) 201e207. [3] F.A. Garner, L. Shao, M.B. Toloczko, S.A. Maloy, V.N. Voyevodin, Use of self-ion bombardment to study void swelling in advanced radiation-resistant alloys, in: 17th Int. Symp. Conf. On Environmental Degradation of Materials in Nuclear Power Systems, Ottawa, Canada, 2015. [4] S. Zinkle, G. Was, Materials challenges in nuclear energy, Acta Mater. 61 (3) (2013) 735e758. [5] B.D. Wirth, M.J. Caturla, T.D. de la Rubia, T. Khraishi, H. Zbib, Mechanical property degradation in irradiated materials: a multiscale modeling approach, Nucl. Instrum. Methods Phys. Res. B 180 (1e4) (2001) 23e31. [6] G.S. Was, Z. Jiao, E. Getto, K. Sun, A.M. Monterrosa, S.A. Maloy, O. Anderoglu, B.H. Sencer, M. Hackett, Emulation of reactor irradiation damage using ion beams, Scripta Mater. 88 (2014) 33e36. [7] S.J. Zinkle, L.L. Snead, Opportunities and limitations for ion beams in radiation effects studies: bridging critical gaps between charged particle and neutron irradiations, Scripta Mater. 143 (2018) 154e160. [8] J. Etoh, M. Sagisaka, T. Matsunaga, Y. Isobe, F.A. Garner, P.D. Freyer, Y. Huang, J.M.K. Wiezorek, T. Okita, Development of a nondestructive inspection method for irradiation-induced microstructural evolution of thick 304 stainless steel blocks, J. Nucl. Mater. 440 (1e3) (2013) 500e507. [9] J. Pakarinen, M. Khafizov, L. He, C. Wetteland, J. Gan, A.T. Nelson, D.H. Hurley, A. El-Azab, T.R. Allen, Microstructure changes and thermal conductivity reduction in uo2 following 3.9 MeV He 2þ ion irradiation, J. Nucl. Mater. 454 (1) (2014) 283e289. [10] M. Khafizov, C. Yablinsky, T. Allen, D. Hurley, Measurement of thermal conductivity in proton irradiated silicon, Nucl. Instrum. Meth. Phys. Res. B 325 (2014) 11e14. [11] R. Cheaito, C. Gorham, A. Misra, K. Hattar, P. Hopkins, Thermal conductivity measurements via time-domain thermoreflectance for the characterization of radiation induced damage, J. Mater. Res. 30 (9) (2015) 1403e1412. [12] K.H. Matlack, J.J. Wall, J.-Y. Kim, Jianmin Qu, L.J. Jacobs, H.-W. Viehrig, Evaluation of radiation damage using nonlinear ultrasound, J. Appl. Phys. 111 (5)
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