Evolution of the microstructural and mechanical properties of BAM-11 bulk metallic glass during ion irradiation and annealing

Journal of Nuclear Materials 523 (2019) 299e309

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Evolution of the microstructural and mechanical properties of BAM-11 bulk metallic glass during ion irradiation and annealing J. Brechtl a, *, S. Agarwal b, M.L. Crespillo c, T. Yang c, H. Bei d, S.J. Zinkle a, b, c, d, ** a

The Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN, 37996, USA Department of Nuclear Engineering, University of Tennessee, Knoxville, TN, 37996, USA c Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN, 37996, USA d Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA b

h i g h l i g h t s
BAM-11 bulk metallic glass (BMG) was irradiated with 9 MeV Ni3þ ions to 10 dpa.
Specimens were irradiated at 25  C, 290  C, and 360  C.
No significant changes in properties observed in samples irradiated up to 290  C.
Significant hardening and crystallization occurred during irradiation at 360  C.
XRD indicates crystallization attributed to thermal, and not irradiation effects.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 December 2018 Received in revised form 30 May 2019 Accepted 5 June 2019 Available online 6 June 2019

Ion irradiation and annealing experiments were performed on Zr52.5Cu17.9Ni14.6Al10Ti5 “BAM-11” bulk metallic glass (BMG) specimens to evaluate their irradiation- and temperature-induced microstructural and mechanical property evolution. For the ion irradiations, samples were exposed to 9 MeV Ni3þ ions to a midrange (~1.5 nm depth) dose of 10 displacements per atom (dpa) at temperatures ranging from 25 to 360  C. A separate set of samples were annealed at 150  C, 200  C and 300  C with respective heating times of 96, 72, and 48 h. Bulk X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterization revealed that the alloy did not crystallize during irradiation up to 290  C, although partial crystallization occurred at 360  C throughout the unirradiated and irradiated regions of the sample. Transmission electron microscopy (TEM) characterization suggested that some of the irradiated region retained an amorphous structure, supporting the idea of partial crystallization. XRD analysis revealed that the crystallization which occurred in the sample irradiated at 360  C was caused by thermal effects, and not irradiation displacement damage, and may be due to slight impurity contamination in the ingot. Nanoindentation experiments showed that only a slight amount of hardening was observed in the specimen irradiated at room temperature and 290  C. However, significant hardening occurred in the sample irradiated at 360  C (~18% increase) as well as the unirradiated annealed specimens. For the sample irradiated at the highest temperature, the substantial increase in hardness was attributed to the partial crystallization of the alloy due to thermal, rather than irradiation effects. Overall, the results of the XRD and nanoindentation characterizations indicate good stability during irradiation at 25e290  C but suggest that the BAM-11 BMG is not suitable for irradiation environments where temperatures exceed 300  C for prolonged periods of time. Three different extrapolation models were employed to study how irradiation and annealing modify the indentation size effect (ISE) in the BAM-11 BMG. The poor linear fitting, as exhibited by all of the underlying equations, indicate that a new ISE model is needed to quantify nanoindentation mechanical properties in BMGs. © 2019 Elsevier B.V. All rights reserved.

Keywords: Bulk metallic glass Irradiation effects Nanoindentation Indentation size effect Crystallization

* Corresponding author. ** Corresponding author. The Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN 37996, USA. E-mail addresses: [email protected] (J. Brechtl), [email protected] (S.J. Zinkle). https://doi.org/10.1016/j.jnucmat.2019.06.010 0022-3115/© 2019 Elsevier B.V. All rights reserved.

1. Introduction In terms of material properties, metallic glasses can exhibit

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exceptional hardness [1,2], toughness [3], high strength [4e8], wear and corrosion resistance [9e13]. These alloys do not work harden like crystalline alloys, and exhibit deformation in the form of shear bands [14,15]. It is worth noting that although the percent strain within a shear band is quite large, it does little to contribute to the overall plastic strain [16]. In terms of inelastic deformation, “liquid-like” zones [17] or “fertile sites” [18] have been suggested as a major contributing factor to the plastic strain in BMGs [19]. An important stability criterion for amorphous materials is their resistance to radiation-induced recrystallization [20]. In some amorphous alloys, local recrystallization may occur in the thermal spike during multidisplacement cascades [21]. Due to their amorphous nature, BMGs do not contain conventional crystalline matter defects such as twin boundaries, grain boundaries, or dislocations [22]. This lack of crystalline structure in amorphous alloys may provide substantial advantages in terms of radiation tolerance due to the lack of lattice structure necessary for Frenkel pair defect formation [23,24]. Furthermore, the addition of metalloids such as silicon appear to distort the short-range order of the metallic glass, leading to the high stability of its bulk form. This combination of proven attractive unirradiated mechanical properties, good formability, and potential for radiation resistance make BMGs an appealing candidate for structural applications in fission and fusion reactors. Three mechanisms have been suggested to explain irradiation induced crystallization [25]. The first mechanism involves an increase in the free energy of the amorphous phase through the introduction of free volume and anti-free volume like defects [26]. The second mechanism is the postulated formation of a crystalline nanoscale domain in the vicinity of the above defect sites by chance through a decrease in the local strain that is caused by the associated loss of excess volume [25]. The final mechanism is associated with increased atomic diffusion from irradiation damage events (radiation enhanced diffusion). There have been a number of investigations that have examined the irradiation induced microstructural and mechanical property changes of Zr based BMGs. However, several of these studies have reported contradictory results. For instance, some studies have reported that these alloys crystallized at temperatures well below the glass transition temperature [27e30], while another study did not observe such an effect [31]. In addition, hardening was observed after irradiation in one study [30], whereas radiationinduced softening was reported in other investigations [32,33]. Importantly, other investigations have found a link between displacement damage and an increase in the free volume content of the glass [34,35]. A few prior studies that examined the room temperature radiation stability of a similar alloy to BAM-11 BMG, namely Zr55Ni5Al10Cu30 metallic glass, found conflicting results [27,28,31]. For instance, Nagata et al. [27] found that crystallization occurred upon irradiation by 500 keV Auþ ions at room temperature to a maximum fluence of 8  1016 ions/cm2 [600 displacements per atom (dpa)]. At half of the above dose, transmission electron microscopy (TEM) bright field (BF) imaging revealed the formation of nano-crystallites with a size of 50 nm, which consisted of face centered cubic (fcc) Zr2Ni and Zr2Cu phases. Work performed by Carter et al. [28], using X-ray diffraction (XRD) and TEM, found a similar enhanced crystallization reaction when the same alloy was irradiated by 1 MeV Cuþ ions (T < 50  C) to a fluence of 1  1016 ions/cm2 (20 dpa). They found that the crystalline intermetallic phases Cu10Zr7 and (Nix, Cu1-x)Zr2 formed during irradiation. In contrast, another investigation did not observe crystallization [31]. For the latter study, specimens which consisted of 50e100 mm foils were irradiated at room temperature with 150e500 keV Hþ, Cuþ, Agþ and Auþ ions to fluences up to 8  1016 ions/cm2. XRD results

indicated that the amorphous alloys did not crystallize under any irradiation condition. Please note that the dpa values for the above and subsequent investigations were cross-checked using the “quick” calculation mode (based on the Kinchin-Pease model [36]), via the Stopping and Range of Ions in Matter program (SRIM). Chen et al. examined the effects of 70 keV Ar irradiation on the microstructure of Zr50.7Al12.3Cu28Ni9 metallic glass thin foils [29]. Here, they irradiated the specimens at room temperature to peak fluences of 2.0  1015 (3.3 dpa) and 1.4  1016 ions/cm2 (24 dpa). For the lower dose irradiation, no structural changes were observed under TEM. On the other hand, the alloy was found to undergo crystallization when exposed to the higher fluence. It was found that the crystallized structures consisted of either fcc Cu phase or fcc AlNi3 phase, with diameters of 10e30 nm. Additionally, Cu- and Ni- enriched and depleted nanoprecipitates with diameters of 30e50 nm and 5e20 nm were also observed. They suggested that the nanocrystals probably act as the nuclei for the formation of the Cu- and Ni- enriched nanoprecipitates in the matrix. In another study, the effects of irradiation on the Zr61.5Cu21.5Fe5Al12 BMG were examined [30]. The specimens were irradiated at room temperature by 300 keV Arþ to peak fluences ranging from 3  1015 (3.7 dpa) to 3  1016 ions/cm2 (37 dpa). TEM showed that the samples remained amorphous at the lowest dose. However, at a dose of 1  1016 ions/cm2, fcc nanocrystals with an average size of 5 nm were observed in the matrix. Moreover, analysis of the selected area electron diffraction (SAED) patterns obtained using TEM indicated that irradiation at same dose led to a decrease in the interatomic spacings of the atoms. Nanoindentation experiments revealed that the hardness and Young's modulus were significantly higher for the samples irradiated to doses of 1  1016 and 3  1016 ions/cm2 as compared to the as-cast condition. It was surmised that the increase in the above values were due to the formation of the nanocrystals that can hinder the propagation of shear bands in the matrix. Recently, Perez-Bergquist et al. studied the radiation response of the same material investigated here, namely Zr52.5Cu17.9Ni14.6Al10Ti5 BAM-11 BMG [32,37,38]. The samples were irradiated with 3 MeV Niþ to fluences ranging from 4.2  1013e 4.2  1014 ions/cm2 (peak doses of 0.1 e 1 dpa) while exposed to temperatures ranging from ambient to 200  C. In addition, some pristine samples were pre-annealed to 300  C for 2 days before room temperature irradiation. TEM microscopy revealed that the samples did not crystallize under all conditions. Furthermore, it was found that irradiation led to a decrease in the nanoindentation hardness, while exposure to the higher temperatures, as discussed above, increased the hardness. Other studies have also found that irradiation led to the softening in BMGs [33,39]. For instance, Raghavan et al. [33] found similar irradiation induced softening and increased shear band formations in nickel irradiated Vitreloy 1 (Zr41.2Ti13.8Cu12.5Ni10Be22.5) BMG. Here, the alloy was irradiated at room temperature by Niþ ions to doses ranging from 0.1 to 100 dpa and energies varying between 2.5 and 15 MeV. It was found that irradiation decreased the nanoindentation hardness by ~17% (up to a dose of 10 dpa) as compared to the as-cast condition. They hypothesized that the reason for the softening in the alloy was caused by the creation of free-volume defects or nano-voids due to irradiation. This enhancement in the free volume content in Zr based BMGs via ion irradiation has been observed in other investigations [34,35,40]. Based on the aforementioned studies, irradiation can produce crystallization and mechanical property changes in BMGs. To advance our knowledge on the irradiation response of this material system, this investigation will examine whether irradiation produces a shift in the crystallization temperature of the BAM-11 BMG. Secondly, this study will gain insight on how irradiation and

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temperature modifies the nanoindentation properties of this material. To carry out this investigation, nanoindentation tests and microstructural characterization techniques were performed on specimens irradiated by 9 MeV Ni3þ to midrange doses of 10 dpa at temperatures ranging from 25 to 360  C. Nanoindentation tests were performed to measure the depth dependent hardness, while XRD and TEM methods were conducted to examine whether the BMG remained amorphous during irradiation. To gain insight into the effects of irradiation and temperature on the indentation size effect (ISE) of the BAM-11 BMG, two different extrapolation models were employed. The first model used is based on the Nix-Gao extrapolation method, while the other has been used to examine glassy polymers. The experimental details will be outlined in the next section. Furthermore, results will be presented in the following section, and the implications of the results will be detailed in the discussion section. 2. Experimental The Zr52.5Cu17.9Ni14.6Al10Ti5 (BAM-11) BMG alloy was fabricated at Oak Ridge National Laboratory (ORNL) by arc melting in an argon atmosphere using a mixture of base metals with the following purities: 99.5% Zr, 99.99% Cu, 99.99% Ni, 99.99% Al, and 99.99% Ti. The rod was then remelted and drop cast into a water-cooled 7 mm diameter cylindrical Cu mold in a Zr-gettered helium atmosphere. Sections of the drop cast rod were evaluated via XRD and differential scanning calorimetry, which both confirmed the material to be fully amorphous. Test specimens were prepared from the as-cast rods by electrical discharge machining. After fabrication, one set of BAM-11 BMG samples were annealed in a vacuum (106 torr) furnace at three different temperatures, namely 150, 200, and 300  C and respective times of 48, 72 and 96 h. To avoid oxidation, specimens were cooled in the furnace before removal. After annealing, ~200 mm were removed from the surface by grinding to minimize possible near-surface artifacts. This process was conducted via a Minimet 1000 polishing/grinding system. To ensure a high-quality surface, colloidal silica particles were used during the final polishing step. Ion irradiations were performed on another set of specimens at the Ion Beam Materials Laboratory at the University of Tennessee, Knoxville (UTK). After grinding and polishing steps similar to that used for the annealed samples, the specimens with nominal dimensions of 3 mm diameter by 0.4e0.5 mm thick were irradiated by 9 MeV Ni3þ ions at three different temperatures of 25, 290, and 360  C. The time for each irradiation was approximately 7 h. The ion flux in the range of ~9.5  1011 cm2s1 was used to achieve the final fluence of 2.4  1016 cm2. The steady state temperature of the sample surface was assessed via a K-type (chromel-alumel) thermocouple, which was spot welded to the surface of an adjacent dummy sample (composed of FeCrAl alloy). A second thermocouple was attached using a molybdenum clip to the surface of the top plate outside of the beam region. This thermocouple was used to estimate a baseline measurement of temperature that would not be affected by beam heating. More details on the experimental apparatus are provided elsewhere [41]. Adjustable beam slits were used to define the irradiation area of 4  9 mm2 on the sample surface. The ion beam was defocused in the horizontal and vertical directions over a wider area with the aim of defining a homogeneous irradiated region. Beam homogeneity was within 10% throughout the irradiated area, which was validated by checking the ion beam induced luminescence (IL) on Al2O3 (used as a scintillator) monitored with a CCD camera (more details can be found elsewhere [42,43]). Low beam current densities with 4.8  102 nA/cm2 were used to avoid beam heating and

301

charge accumulation on the samples [41]. Ion range and the depth-dependent dpa and Ni atomic percent (at. %) values were calculated using SRIM via the quick KinchinPease simulation option (version 2013) [36,44], see Fig. 1. Threshold displacement energies of 40 eV, and a density of 6.66 g cm3 were assumed. The irradiation fluence and corresponding dpa values were calculated taking into consideration all host elements present in the material. As can be seen in the figure, the 9 MeV Ni3þ ions had a projected range of approximately ~3 mm. The midrange and peak doses were calculated to be 10 and 25 dpa, respectively. For this work, to minimize inaccuracies associated with surface effects and diffusional broadening of the implanted ions [45e47], an intermediate depth (~1.5 mm) with a midrange dose of 10 dpa (see the dotted line in the figure), was selected to examine the effects of the above dose on the microstructure and mechanical properties. To observe whether any changes occurred in the hardness and microstructure of the region of the alloy exposed to the midrange dose, nanoindentation and XRD characterizations were performed at the corresponding depth. At the given fluence, the calculated peak Ni concentration is 0.42 at.% at a depth of ~3.4 mm and is ~0.01 at.% at a depth of 1.5 mm. The calculated ionization stopping power values at the surface and the midrange region were 5.0 keV/nm and 2.1 keV/nm, respectively. XRD measurements were carried out on the as-cast and irradiated specimens at the Joint Institute for Advanced Materials (JIAM) Diffraction Facility at the UTK. XRD was performed using a PANalytical Empyrean diffractometer equipped with a Xe proportional detector. The X-ray consisted of a Cu beam with a K-alpha wavelength of 1.54 Å, and the diffraction angle 2q ranged between 20 and 80 . Glancing angles ranging from 0.5 to 1 were used to examine whether any crystallization occurred in the irradiation region. The optimized glancing angles were calculated using the method described in Ref. [48]. In conjunction with the XRD experiment, TEM characterization was performed to check for irradiation induced crystallization in the alloy. Based on the XRD results, TEM was only performed on the specimen irradiated at the highest temperature of 360  C. The TEM characterization was conducted in the Low Activation Materials Development and Analysis (LAMDA) laboratory at ORNL. The electron transparent TEM foils were fabricated using an FEI Quanta Dual-beam focused ion beam (FIB)/SEM with a final thinning step of 2 kV Gaþ ions at a glancing angle of about 4 to minimize ion beam milling damage. After the thinning step, the sample was polished using a Fischione Nanomill 4 keV Arþ ion polisher at low incident angles. The samples were then examined and analyzed in a

Fig. 1. Left) Irradiation damage (dpa) profile (corresponding to a fluence of 2.4  1016 cm2) versus depth for the BAM-11 BMG. Right) at. % Ni versus depth. Both graphs were obtained from SRIM 2013 simulation using quick Kinchin-Pease calculations (see text for more details). Dashed line points out the depth and dpa that was chosen for this study.

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JEOL JEM-2100F TEM/STEM at 200 kV using BF imaging. Nano-indentation hardness tests were performed at room temperature using a KLA-Tencor G200 Nano-indenter with a Berkovich diamond (3-sided pyramidal tip) in continuous stiffness measurement mode with a constant loading rate of 400 mNs1. For statistical accuracy, ~25 indents were made where hardness was measured as a function of depth from the point of contact of the nanoindenter with the surface to a depth of ~2,500 nm. The hardness data from the surface to ~100 nm from the specimen surface was omitted due to large data scatter associated with surface roughness. Hardness was calculated using the Oliver and Pharr method [49,50]. The area function of the tip, in addition to the machine stiffness for the nanoindenter, was calibrated by indenting a standard fused silica sample [32]. For the first set of samples, which were only annealed, Vickershardness tests were also performed in conjunction with the nanoindentation tests. For the former test, a Buehler Micromet 5103 was used to indent the as-cast and annealed specimens. For the indentations, a load of 1,000 gf (gram force) and a dwell time of 15 s were used in which the indents were made in accordance with the American Society for Testing and Materials (ASTM) E384-16 standards [51]. The hardness measurements were then compared with the corresponding results of the nanoindentation ISE analysis. 2.1. Indentation size effect analysis To examine the influence of irradiation and annealing on the ISE [52e55] of the BMG, the hardness was plotted according to the following equations [52,53]:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffi h* H ¼ H0 1 þ 1 h 0 0

H ¼ H 0 @1 þ

sffiffiffiffiffiffi 1 h*2 A h

(1)

(2)

where H is the nanoindentation hardness, H0 and H’0 are the hardness associated with either the statistically stored dislocations or the statistically stored reactive (SSR) soft-zone defects, h is the indentation depth, and h*1, h*2 are length-scale terms that characterize the depth dependence of the hardness. It should be mentioned that Eq. (1) was first derived by Nix and Gao [52] to examine the ISE in crystalline alloys while the Eq. (2) was developed by Lam and Chong to examine the ISE in glassy polymers [54], and later applied by the same authors to quantify ISE behavior in metallic glasses [53]. The extrapolated (bulk) hardness values H0 and H’0, which are related to the statistically stored dislocations (fertile sites) in the absence of sites associated with geometrically necessary dislocations (strain-gradient hardness), are written as [55]:

pffiffiffi pffiffiffiffiffi H0 ¼ 3 3amb rs 0

H0 ¼

pffiffiffi 3 3 mð1 þ jlnnks Þ6=5 8

and soft-zones. 3. Results Fig. 2 shows the glancing X-ray diffraction patterns, with normalized intensity, for the as-cast and 9 MeV Ni3þ irradiated samples. Results show that there was no evidence for radiation induced crystallization at the investigated temperatures of 25 and 290  C, which is characterized by a broad hump at angles ranging from 30 to 46 . However, in the sample irradiated at 360  C, there were a few small peaks that were observed in the pattern. To help distinguish whether these anomalous peaks were a part of the background data, a fit was applied to the original data which was then deducted from the original XRD results. The fitted data was calculated using the Savitzky-Golay filtering method [56]. The deducted results, along with the original pattern and the corresponding fitted profile data, are displayed in Fig. 3(a). Furthermore, lines representing the mean (m) and three standard deviations (s) away from the mean of the deducted data were plotted. As can be seen, the peaks centered around 35 , 38 , 45 , and 54 are beyond three standard deviations from the mean, suggesting that they are not a part of the background pattern. These peaks also indicate that partial crystallization occurred in the specimen during the high temperature irradiation. Fig. 3(b), which displays the pattern for the background Si plate, shows no signs of spurious peaks that could have resulted in the anomalous peaks that were observed in Fig. 3(a). This suggestion of partial crystallization was further supported by the follow up TEM BF images (please see supplementary file), where it was observed that the sample retained some amorphous structure during irradiation at the highest temperature. To examine whether the crystallization in the sample was induced by thermal or irradiation effects, glancing XRD was also performed on the unirradiated side. The corresponding patterns are displayed in Fig. 4, and as can be observed, the patterns are quite similar. However, we can note some important differences from this figure. Firstly, the peak at ~45 is more pronounced in the pattern representing the unirradiated side. Secondly, there is an increase in the intensity values for the angles beyond 60 . Thirdly, the peak centered around 38  C for the as-cast sample is shifted slightly to the left for the irradiated specimens. However, the overall similarity in the patterns from the irradiated and unirradiated sides of the specimen indicates that partial crystallization was thermally induced. In contrast the XRD results, the atomic structure of the lift-out specimen that was examined by TEM, showed that it had remained amorphous during irradiation. This result indicates that the crystallization did not occur uniformly throughout the

(3)

(4)

here rs is the density of statistically stored dislocations, and nks is the total number of statistically stored polymer kinks [54]. It is important to note that since the Nix-Gao model [Eq. (1)] is based on dislocation strengthening, it may not be applicable to metallic glasses because they do not contain such defects. However, BMGs do contain other types of defects that can accommodate plastic deformation, such as liquid-like sites, shear transformation zones,

Fig. 2. Glancing X-ray diffraction patterns for BAM-11 BMG samples in the as-cast and Ni3þ irradiated condition (9 MeV, 10 dpa) at different temperatures.

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Fig. 5. Bulk X-ray diffraction patterns for the BAM-11 BMG sample irradiated by Ni3þ ions (9 MeV, 10 dpa) at 360  C (unirradiated and irradiated sides) and the specimen annealed at 500  C for 2.5 h.

Fig. 3. Glancing X-ray diffraction patterns of the (a) irradiated side, the corresponding fitted data, and the pattern for the deducted data for the BAM-11 BMG specimen irradiated by Ni3þ at 360  C (9 MeV, 10 dpa) and (b) the Si background plate. The deducted data was overlaid with statistical bounds to distinguish outlier peaks.

Fig. 4. Glancing X-ray diffraction patterns of the irradiated and unirradiated sides of the BAM-11 BMG specimen irradiated by Ni3þ at 360  C (9 MeV, 10 dpa).

material. Fig. 5 compares the bulk XRD measurements for the sample irradiated at 360  C (irradiated and unirradiated sides) for 7 h with the unirradiated specimen that was annealed at 500  C for 2.5 h. This comparison was intended to examine the XRD behavior at ~33  C below vs. ~100  C above the BAM-11 glass transition temperature [10] of 393  C. As can be seen, the patterns for the 360  C irradiated sample are quite similar to the one corresponding to the 500  C thermally annealed specimen. One must note that there are

more distinct peaks (angles between 30 and 50 ) in the pattern for the sample annealed at 500  C, which indicates that this specimen underwent more extensive crystallization as compared to the 360  C irradiated condition. Fig. 6(a) shows the nanoindentation hardness as a function of indenter depth for the as-cast and the ion irradiated samples. The hardness data below a depth of ~100 nm from the specimen surface was discarded due to large data scatter associated with surface roughness. As can be seen in the graph, the hardness decreased with respect to indentation depth for all conditions, exhibiting a clear indentation size effect even for the as-cast unirradiated condition. For this study, the nanoindentation hardness at a midrange dose of 10 dpa, and at the end of the ion range region, was analyzed. Since the nanoindenter tip is sensitive to material properties up to 5e10 times beyond its penetration depth [37,57], an indentation depth of approximately 200 nm and 450 nm corresponds to the midrange depth of 1.5 mm and projected range of 3 mm, respectively. Quantitative evaluation of the depth-dependent hardness [as presented in Fig. 6(a)] indicates that an indent depth of 450 nm approximately corresponds to the transition between the ion irradiated and unirradiated regions. To mark the approximate irradiation- and nonirradiation-dominant regimes in the alloy, a vertical line was inserted into Fig. 6(a). Fig. 6(b) shows a close-up of the irradiated region (100e450 nm indent depths). In the irradiated region, the hardness for the room temperature condition is within one standard deviation of the values for the as-cast condition, signifying a slight but not statistically significant increase in the hardness of the material. The irradiated hardness progressively increased with increasing irradiation temperature up to the maximum investigated temperature of 360  C. At indent depths beyond 450 nm, where the tip predominantly sensed the unirradiated regions, the hardness slightly increased between the as-cast condition and the room temperature and 290  C irradiation conditions [see unirradiated region in Fig. 6(a)]. On the other hand, the hardness values for the unirradiated region exhibited a large increase for the 360  C exposure condition, where the sample partially recrystallized. The corresponding hardness data for the indentation depths of 200 and 450 nm are presented in Table 1. The nanoindentation hardness curves for the as-cast and the unirradiated annealed specimens are displayed in Fig. 7. In all cases, a pronounced indentation size effect is observable where the measured hardness increases rapidly for small indent depths. As compared to the as-cast specimen, the ISE is most pronounced in the thermally annealed samples. Furthermore, the hardness was

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Fig. 7. Nanoindentation hardness vs. indentation depth for BAM-11 as-cast and annealed samples at different temperatures ranging from 150  C to 300  C and respective heating times of 96, 72, and 48 h for depths ranging from 100 to 2500 nm. Each data point is based on the mean values for ~25 indents.

Fig. 6. Nanoindentation hardness vs. indentation depth for BAM-11 as-cast and irradiated samples (9 MeV Ni3þ to 10 dpa) at temperatures ranging from 25 to 360  C. The data was for indent depths ranging from (a) 100e2500 nm (irradiated and unirradiated regions) and (b) the irradiated region (100e450 nm). Each data point is based on the mean values for ~25 indents.

higher at all indent depths for the annealed samples, as compared to the as-received specimen. Moreover, the hardness values for the sample heated at 150  C for 96 h and 200  C for 72 h were superimposed on each other. On the other hand, the hardness values exhibited by the specimens heated at 300  C for 48 h was significantly higher as compared to the other conditions. This significant increase in the hardness suggests that short-range atomic rearrangement occurred in the specimen annealed at 300  C. The ISE hardness analysis based on the Nix-Gao model [Eq. (1)] is plotted in Fig. 8(a)e(c) for the as-cast and the 9 MeV Ni3þ ion

irradiated specimens. The vertical line in the plot at 2.2 mm1 [Fig. 7(a)] corresponds to an indenter depth of 450 nm, which is near the expected transition between the hardness dominated by the unirradiated and ion-irradiated regions. As apparent in all the figures, the data did not behave in a linear fashion versus the independent variable (h1) at all indentation depths. The ISE hardness for the Nix-Gao model [Eq. (1)], with regards to the as-cast and unirradiated specimens that were annealed at temperatures ranging from 150  C to 300  C, is plotted in Fig. 9. Similar to the ion irradiated and as-cast results (Fig. 8), the data did not behavior in a linear fashion with respect to the independent variable (h1). Also, the extrapolated hardness did not change much for the unirradiated specimens annealed at 150  C for 96 h and 200  C for 72 h, as compared to the as-cast condition. However, annealing at 300  C led to an apparent increase in the extrapolated hardness. Table 2 compares the extrapolated nanoindentation bulk hardness obtained from applying Eq. (1) with the bulk Vickers hardness data for the as-cast and the unirradiated specimens annealed at temperatures ranging from 150 to 300  C. For the data obtained using Nix-Gao [Eq. (1)], the hardness was roughly constant for the as-cast and 150 and 200  C unirradiated annealed samples. The hardness was slightly higher for the unirradiated sample annealed at 300  C, as compared to the as-cast state. A similar trend was observed in the data obtained using the Vickers hardness approach. For every condition, the bulk hardness values from the Nix-Gao extrapolation model were 11e17% higher than the values obtained from the Vicker's hardness tests. It should be mentioned that in contrast to the nanoindentation results, where the hardness was higher for all the annealed samples (all indentation depths), the Vickers hardness was lower for the samples annealed at 150  C and 200  C, as compared to the as-cast condition. The ISE hardness for the Lam and Chong model [Eq. (2)], as applied to the as-cast material and the specimens irradiated by 9 MeV Ni3þ ions, is plotted in Fig. 10(a)e(c). Similar to Fig. 8(a), the vertical line in the plot at ~1.5 mm0.5 [Fig. 10(a)] corresponds to the expected transition between the hardness dominated by the unirradiated and ion-irradiated regions. Comparable to Fig. 8(a)e(c),

Table 1 Summary of nanoindentation results on as-cast and irradiated samples (9 MeV Ni3þ, dose of 10 dpa) at depths of 200 and 450 nm, corresponding to the midrange and end-ofrange regions for the ion irradiated samples.

Average Hardness (GPa)

Depth (nm)

As-cast

25  C irr.

290  C irr.

360  C irr.

200 450

7.4 ± 0.6 6.8 ± 0.2

8.0 ± 0.2 6.8 ± 0.7

8.1 ± 0.4 7.0 ± 0.1

8.8 ± 0.4 7.4 ± 0.1

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Fig. 8. H2 vs. 1/h (with extrapolated hardness Ho) for BAM-11 BMG in the as-cast and irradiated (9 MeV Ni3þ to 10 dpa at different temperatures ranging from 25 to 360  C) samples for depths ranging from (a) 150e2500 nm (irradiated and unirradiated region), (b) the unirradiated region (450 nme2500 nm), and (c) the ion irradiated region (100e450 nm).

Fig. 9. H vs. 1/h (with extrapolated hardness Ho) for BAM-11 BMG using Eq. (1) in the as-cast vs. annealed samples at temperatures of 150  C, 200  C, and 300  C (respective heating times of 96, 72, and 48 h). Here the data corresponded to depths ranging from 100 to 2500 nm.

Table 2 Comparison of the extrapolated nanoindentation hardness H0 [Eq. (1)] and Vickers hardness (1,000 gf) results for the as-cast and annealed samples. Condition

H0 (GPa)

Vickers (GPa)

Percent Diff. (%)

As-cast 150  C 96 h 200  C 72 h 300  C 48 h

5.7 5.8 5.6 6.2

5.1 ± 0.01 4.9 ± 0.03 5.0 ± 0.07 5.3 ± 0.05

11.1 16.8 11.3 15.7

the fitted values based on this second model for the as-cast and the irradiated specimens did not behave in a linear fashion versus 1/ h0.5 throughout the entire indentation region. However, the overall deviation from linear behavior was much less pronounced than observed for the Nix-Gao plots. The corresponding fitting

parameters are presented in Table 3. As can be seen in Table 3, for the near-surface irradiated region, the hardness was ~5.8e6.2 GPa for the as-cast and samples irradiated up to 290  C. Furthermore, the reduction was most pronounced for the specimen irradiated at 25  C (9%), while the sample bombarded at 290  C only softened by about 3%. Table 4 summarizes the derived hardness parameters, as assessed from the Lam and Chong model, for indentation depths representative of the unirradiated region (500e~2500 nm). For the unirradiated region, the extrapolated values of the bulk hardness for the as-cast and irradiated samples up to 290  C are ~4.2e4.6 GPa. Comparing the results for the unirradiated and irradiated regions for the specimen irradiated at 360  C, H’0 was 6.2 and 6.5 GPa, respectively. Apparently, there is no significant difference in H’0 for either region of the specimen irradiated at the highest temperature, where partial recrystallization occurred throughout the sample during the prolonged exposure. For all conditions, as listed in Table 4, the characteristic depth values, i.e. h*, were one to two orders of magnitude greater than those from the near-surface (ion irradiated) region, as listed in Table 3. However, for 360  C (partially crystallized), both the irradiated and unirradiated regions have similar h*2 values. Table 5 compares the extrapolated nanoindentation bulk and Vickers hardness data, as obtained from applying Eq. (2), for the ascast and unirradiated annealed specimens for temperatures ranging from 150  C to 300  C. Similar to Table 2, both the extrapolated nanoindentation and Vickers hardness values were comparable for the as-cast, 150, and 200  C annealing conditions. With regards to the Vickers data, the hardness was slightly higher for the sample annealed at 300  C as compared to the as-cast state. For every condition, the bulk hardness values evaluated using the Lam and Chong extrapolation model were lower by 2e17% as compared to the values obtained from the Vickers hardness tests. Fig. 11 displays the ISE hardness values obtained from Eq. (2), for the as-cast and samples annealed at 150e300  C. Unexpectedly, the extrapolated hardness values for the annealed samples were lower

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Fig. 10. H vs. 1/h0.5 (with extrapolated hardness Ho) for BAM-11 BMG using Eq. (2) in the as-cast vs. irradiated samples with 9 MeV Ni3þ to 10 dpa at different temperatures ranging from 25 to 360  C for the (a) depths ranging from 100 to 2500 nm, (b) the unirradiated region (450 nme2500 nm), and (c) the irradiated region (100e450 nm).

Table 3 Comparison of the results for the parameters h*2, H’0, and the percent change in H0 [Eq. (2)] for the as-cast and the 9 MeV Ni3þ irradiated BAM-11 BMG for the irradiated region (100e450 nm). Condition

h*2 (nm)

H’0 (GPa)

Change H0 (%)

As-cast 9 MeV Ni3þ 25  C 9 MeV Ni3þ 290  C 9 MeV Ni3þ 360  C

3.1 15.2 11.2 19.2

6.4 5.8 6.2 6.2

e 9.4 3.1 3.1

Table 4 Comparison of the results for the parameters h*2, H’0, and the percent change in H0 [Eq. (2)] for the as-cast and the 9 MeV Ni3þ irradiated BAM-11 BMG for the unirradiated region (500e~2500 nm). Condition

h*2 (nm)

H’0 (GPa)

Change H0 (%)

As-cast 9 MeV Ni3þ 25  C 9 MeV Ni3þ 290  C 9 MeV Ni3þ 360  C

210 130 130 18

4.2 4.5 4.6 6.5

e 7.1 9.5 55.8

Fig. 11. H vs. 1/h0.5 (with extrapolated hardness Ho) for BAM-11 BMG using Eq. (2) in the as-cast vs. annealed samples at temperatures of 150 , 200  C, and 300  C (respective heating times of 96, 72, and 48 h). Here the data corresponded to depths ranging from 100 to 2500 nm.

hardness values from fitting the Lam and Chong model [Eq. (2)] may not be valid. Table 5 Comparison of extrapolated nanoindentation hardness, H’0 [Eq. (2)], and Vickers hardness (1,000 gf) results for the as-cast and annealed samples. Condition

H’0 (GPa)

Vickers (GPa)

Percent Diff. (%)

As-cast 150  C 96 h 200  C 72 h 300  C 48 h

5.0 4.4 4.2 4.6

5.1 ± 0.01 4.9 ± 0.03 5.0 ± 0.07 5.3 ± 0.05

2.0 10.8 17.4 14.1

as compared to the as-cast specimen when fitting the full set of nanoindentation data for 100e2,500 nm depths. These values are not in agreement with the trend observed in Fig. 8 for the depth dependent hardness, which suggests that the extrapolated

4. Discussion For this investigation, the evolution of the microstructural and hardness properties of BAM-11 BMG during high temperature ion irradiation (midrange dose of 10 dpa) and annealing was examined. The hardness measurements were carried out using both nanoindentation and Vickers hardness experiments. Since currently there does not exist any theoretically valid models to analyze the ISE in BMGs, three existing extrapolation models on crystalline alloys, glassy polymers, and thin film metallic glass were applied to the data. The results of the glancing XRD characterization, as observed in Fig. 2, indicate that the BAM-11 BMG remained amorphous during

J. Brechtl et al. / Journal of Nuclear Materials 523 (2019) 299e309

the ion irradiation up to 290  C but partially crystallized when bombarded by ions at 360  C. This partial crystallization at 360  C appears to be thermally-induced rather than radiation-induced or -enhanced, since similar partial crystallization was detected by glancing XRD performed on the opposite side (see Fig. 4) of the specimen that was only exposed to the annealing temperature of 360  C. Subsequent TEM characterization of the irradiated region provide further evidence that the sample only partially crystallized during irradiation, and that this crystallization occurred in a heterogenous manner throughout the matrix. The crystallization at a temperature of ~30  C below the reported BAM-11 glass transition temperature of 393  C [3] may have been caused by oxygen impurities in the sample. For instance, He et al. reported that a concentration of approximately 706 atomic parts per million oxygen led to a reduction of 50e60  C in Tg of Zr52.5Cu17.9Ni14.6Al10Ti5 (BAM-11) BMG [58]. Analysis of the outlier peaks, as observed in Fig. 3(a), reveal the presence of a couple crystalline phases in the matrix. Here, the peaks at 35 and 45 are most likely associated with a tetragonal Zr2Ni intermetallic structure, while the peaks at 38 and 54 correspond to the Zr2(Ni0.67O0.33) oxide phase [58,59]. The oxide phase may be a result of oxygen impurities that are incorporated into the cubic Zr2Ni structure [60,61]. To further crosscheck the above results, bulk XRD measurements were also performed on a separate specimen that was annealed at 500  C for 2.5 h. The temperature of 500  C was chosen since it is above the crystallization temperature of the alloy [3,61]. Although, as can be seen in Fig. 5, the diffraction pattern obtained from the 500  C sample exhibited more defined peaks as compared to the 360  C sample, both curves superimpose on each other. This result further confirms that the 360  C specimen only partially crystallized due to heating during the irradiation experiment. As shown in Fig. 6(a)-(b), all BAM-11 BMG specimens exhibited a pronounced indentation size effect on the measured nanoindentation hardness, particularly for indent depths below ~200 nm. Moreover, only modest changes in hardness are observed following 9 MeV Ni3þ ion irradiation to midrange doses of ~10 dpa at 25e290  C (25 dpa at peak damage region). The slight increase in hardness may be partially due to surface modifications induced by the ion irradiation damage [62]. However, a cursory examination of the irradiated surfaces using an optical microscope (magnification of 10X) did not reveal any significant changes in the surface morphology following irradiation. Increased hardening was observed in the ion irradiated region and unirradiated substrate after irradiation at 360  C for ~7 h and is attributed to partial crystallization associated with thermal annealing effects. This hardening, when combined with the XRD results from Figs. 2, 4 and 5, provides very compelling evidence that the partial crystallization that occurred in the BAM-11 BMG was caused by thermal effects rather than irradiation displacement damage. Importantly, the above results also suggest that the material properties are not significantly affected by exposure during irradiation to temperatures at or below 290  C. The nanoindentation hardness vs. indentation depth data for the as-cast and unirradiated annealed samples, as displayed in Fig. 7, indicates annealing to temperatures ranging from 150 to 300  C led to a significant increase in the hardness of the material. A slight increase in hardness due to annealing of BAM-11 BMG has been reported in a previous study where the nanoindentation hardness (350 nm) increased by ~4% after being heated at 310  C for 30 min [63]. Similar to Fig. 6(a), all thermally annealed BAM-11 BMG specimens exhibited a pronounced indentation size effect on the measured nanoindentation hardness. Although the current investigation did not perform XRD on the unirradiated anneal samples, it was assumed that the specimens remained amorphous

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as a previous study found that the material did not crystallize during exposure to 300  C for 48 h, as determined from TEM characterization [37]. This assumption is further confirmed by work performed by Li et al., where X-ray diffraction patterns revealed that the BAM-11 BMG did not crystallize when heated at 300  C for a week [64]. From the results of both experiments on the irradiated and unirradiated annealed samples, it can therefore be concluded that the BAM-11 BMG is limited to temperatures of 300  C or lower due to thermally induced partial crystallization that occurs during long-term (7 h) exposure at 360  C, or possibly at 300  C for 48 h. Typically, the crystallized form of a metallic glass is harder than its amorphous counterpart. As discussed previously, the effects of 300 keV Arþ room temperature irradiation to a dose of 37 dpa also led to significant increase in the hardness of Zr61.5Cu21.5Fe5Al12 BMG, as compared to the as-cast condition [30]. As revealed by TEM characterization, these increases in hardness were accompanied by the formation of fcc nanocrystals during irradiation. However, unlike this investigation, where the increase in the hardness was a result of irradiation induced crystallization, the present study found that the enhanced hardness was caused by crystallization due to thermal effects. In terms of the ISE behavior for the as-cast, irradiated, and unirradiated annealed specimens, the poor linear fit between shallow (<450 nm indent depth) and deep indents as displayed in Figs. 8(a) and 9 implies that the model proposed by Nix and Gao [52], as described by Eq. (1), is not quantitatively valid for the indentation hardness of the BAM-11 BMG. With respect to the irradiated specimens, the hardness values decreased at a progressively faster rate for indent depths beyond the transition point, even though the indentation hardness at these relatively deep indents should be predominantly associated with the nonirradiated substrate (which should not exhibit any variation in hardness vs. depth). The discrepancy with the first equation is somewhat unexpected since the Nix-Gao model was successfully used to describe the ISE in BAM-11 BMG during a previous investigation [65]. However, based on physics considerations it is not surprising that the Nix-Gao model does not provide a good fit to the data, since it is based on proximity to geometrically necessary dislocations to induce plastic deformation (and glasses do not contain dislocations). Moreover, the reason for the deviation may be the result of surface defects formed during mechanical polishing. This effect of surface polishing on the model is inherent in the 1/h dependence in Eq. (1), where this term becomes more significant at shallower depths. Finally, the significantly larger H0 values for the specimen irradiated at 360  C (for the unirradiated region), probably corresponds to the partial crystallization of the specimen. As can be seen in Figs. 10(a) and 11, the model as derived by Lam and Chong [54], i.e. Eq. (2), also provided a poor, but slightly better fit to the nanoindentation data for the as-cast, irradiated, and unirradiated annealed samples. Thus, it can be inferred from the results that this model is also not valid to accurately quantify the nanoindentation hardness of the BAM-11 BMG. Similar to the first model, the significantly larger H0 values for the specimen irradiated at 360  C most likely corresponds to the partial crystallization of the specimen. In addition to the above, an extrapolation model that was recently applied to a metallic glass thin film [55] was used to analyze the ISE, but with no success. This failure occurred not only because the data could not be fit in a linear fashion, but also because the underlying equation produced negative values for the characteristic length, h*. This problem with h* is further compounded by its underlying relation h* f11c , where cL denotes the L fraction of fertile sites contained in the metallic glass. Here, a negative value of h* implies that the fertile sites comprise more than 100% of the matrix, which is not possible.

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In addition to the above, there were more conflicting results with regards to the extrapolated hardness values. For instance, Eq. (2) gave H’0 values that were lower for all irradiation temperatures as compared to the as-cast condition, while for Eq. (1), only the room temperature irradiation led to a decrease in H0. As for the unirradiated annealed specimens, there were discrepancies when comparing the extrapolated hardness and the Vickers hardness results. For Eq. (1), the extrapolated hardness values were higher than the Vickers hardness, while the opposite was true for Eq. (2). Furthermore, as compared to the as-cast state, the Vickers hardness was found to be less for the samples annealed at 150  C and 200  C, which appears contradictory to the results for the depth dependent hardness. Since the standard deviation ranged from 0.01 to 0.07, these changes appear to be statistically significant. The above discrepancy may be the result of different deformation responses between the macroindentation and the ISE from nanoindentation. The above discrepancies, when combined with the insufficiency of all three models to properly fit the data, suggests that a new model is needed to properly analyze the ISE in this BMG system. Perez-Bergquist et al. examined the response of BAM-11 BMG when exposed to 3 MeV Niþ to a range of irradiation dose and thermal conditions [32,37]. In Ref. [32], it was found that samples irradiated to a midrange dose of 0.1 dpa at 200  C did not experience significant changes in hardness. In another study [37], it was found that the hardness decreased after being annealed at 300  C for 48 h and subsequently irradiated to a midrange dose of 10 dpa (room temperature). For both investigations, TEM characterization revealed that the material did not crystallize under any irradiation or annealing condition. This resistance to phase change is vitally important since the metallic glass can become extremely brittle upon crystallization. Based on the results of the current and previous studies, it can be concluded that the effects of ion irradiation on the microstructure and mechanical properties of BAM-11 BMG appears to be relatively modest for doses up to ~10 dpa at 25e290  C. 5. Conclusions Results of the ~10 dpa ion irradiation and thermal annealing experiments revealed several interesting conclusions. Firstly, the maximum operating temperature for the BAM-11 BMG (Tg ¼ 393  C) appears to be limited to 300  C or lower due to thermally induced partial crystallization that occurs during long-term (7e24 h) exposures. Importantly, there does not appear to be any pronounced radiation-enhanced crystallization of the irradiated specimens, as revealed by XRD characterization. Secondly, only modest changes in hardness were observed following 9 MeV Ni ion irradiation to midrange doses of ~10 dpa at 25e290  C (25 dpa at peak damage region). Increased hardening was observed in the ion irradiated region and the unirradiated substrate after irradiation at 360  C for ~7 h, and is attributed to partial crystallization associated with thermal annealing effects. Furthermore, the results of TEM characterization provided evidence that the sample only partially crystallized during irradiation at the highest temperature. Finally, all of the BAM-11 BMG specimens exhibited a pronounced indentation size effect on the measured nanoindentation hardness and elastic modulus, particularly for indentation depths below ~200 nm. Three different extrapolation models were applied to examine the ISE in the BAM-11 BMG, although they were not successful. The Nix-Gao model [52] is a poor fit to the nanoindentation data for the BAM-11 BMG, which is not considered surprising since the physics for this model is only valid for crystalline specimens. An alternative ISE model for noncrystalline materials, as proposed by Lam and Chong [54], provides an incrementally better fit to the nanoindentation data, but overall it does not provide a good fit. The

final model, as used to study thin film metallic glass [55], was unsuccessful since like its predecessors, did not provide a linear fit to the data. Furthermore, it provided nonsensical values for the characteristic length, h* and the fraction of fertile sites in the matrix. Therefore, it is suggested here that a new model should be proposed to study the ISE in BMGs. Acknowledgements This research was sponsored in part by the Office of Fusion Energy Sciences, U.S. Department of Energy under contract DEAC05-00OR22725 with UT-Battelle, LLC and grant # DE-SC0006661 with the University of Tennessee. The XRD testing utilized UT's Joint Institute for Advanced Materials. The TEM characterization utilized ORNL's Low Activation Materials Development and Analysis User Facility. The authors would like to thank Drs. Michael Koehler and John Salasin for helpful discussions regarding the XRD method and analysis. The authors would also like to thank Dr. William A. Hanson for insight regarding irradiation damage effects. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jnucmat.2019.06.010. References [1] S.V. Madge, A. Caron, R. Gralla, G. Wilde, S.K. Mishra, Novel W-based metallic glass with high hardness and wear resistance, Intermetallics 47 (2014) 6e10. [2] M.N.M. Patnaik, R. Narasimhan, U. Ramamurty, Spherical indentation response of metallic glass, Acta Mater. 52 (11) (2004) 3335e3345. [3] C.T. Liu, L. Heatherly, D.S. Easton, C.A. Carmichael, J.H. Schneibel, C.H. Chen, J.L. Wright, M.H. Yoo, J.A. Horton, A. Inoue, Test environments and mechanical properties of Zr-base bulk amorphous alloys, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 29 (7) (1998) 1811e1820. [4] F.-F. Wu, K.C. Chan, S.-S. Jiang, S.-H. Chen, G. Wang, Bulk metallic glass composite with good tensile ductility, high strength and large elastic strain limit, Sci. Rep. 4 (2014) 5302. [5] C.J. Gilbert, R.O. Ritchie, W.L. Johnson, Fracture toughness and fatigue-crack propagation in a Zr-Ti-Ni-Cu-Be bulk metallic glass, Appl. Phys. Lett. 71 (1997) 476e478. [6] Q. Wang, Y. Yang, H. Jiang, C.T. Liu, H.H. Ruan, J. Lu, Superior tensile ductility in bulk metallic glass with gradient amorphous structure, Sci. Rep. 4 (2014). [7] X. Gu, G.J. Shiflet, F.Q. Guo, S.J. Poon, Mg-Ca-Zn bulk metallic glasses with high strength and significant ductility, J. Mater. Res. 20 (8) (2005) 1935e1938. [8] J.M. Park, Y.C. Kim, W.T. Kim, D.H. Kim, Ti-based bulk metallic glasses with high specific strength, Mater. Trans. 45 (2) (2004) 595e598. [9] A.L. Greer, K.L. Rutherford, I.M. Hutchings, Wear resistance of amorphous alloys and related materials, Int. Mater. Rev. 47 (2) (2002) 87e112. [10] T. Xu, S. Pang, H. Li, T. Zhang, Corrosion resistant Cr-based bulk metallic glasses with high strength and hardness, J. Non-Cryst. Solids 410 (2015) 20e25. [11] S.J. Poon, G.J. Shiflet, V. Ponnambalam, V.M. Keppens, R. Taylor, G. Petculescu, Synthesis and properties of high-manganese iron-based bulk amorphous metals as non-ferromagnetic amorphous steel alloys, Mater. Res. Soc. Symp. Proc., Boston, Ma (2003) 167e177. [12] W.H. Peter, R.A. Buchanan, C.T. Liu, P.K. Liaw, M.L. Morrison, J.C.A. Carmichael, J.L. Wright, Localized Corrosion behavior of a zirconium-based bulk metallic glass relative to its crystalline state, Intermetallics 10 (11e12) (2002) 1157e1162. [13] G. Li, Y.Q. Wang, L.M. Wang, Y.P. Gao, R.J. Zhang, Z.J. Zhan, L.L. Sun, J. Zhang, W.K. Wang, Wear behavior of bulk Zr41Ti14Cu12.5Ni10Be22.5 metallic glasses, J. Mater. Res. 17 (8) (2002) 1877e1880. [14] S.V. Madge, Toughness of bulk metallic glasses, Metals 5 (3) (2015) 1279e1305. [15] M. Miller, P.K. Liaw, Bulk Metallic Glasses: an Overview, Springer, New York, 2008. [16] C.A. Schuh, T.C. Hufnagel, U. Ramamurty, Overview No.144 - mechanical behavior of amorphous alloys, Acta Mater. 55 (12) (2007) 4067e4109. [17] T. Egami, Mechanical failure and glass transition in metallic glasses, J. Alloy. Comp. 509 (2011) S82eS86. [18] Y.Q. Cheng, A.J. Cao, H.W. Sheng, E. Ma, Local order influences initiation of plastic flow in metallic glass: effects of alloy composition and sample cooling history, Acta Mater. 56 (18) (2008) 5263e5275. [19] W. Song, X. Meng, Y. Wu, D. Cao, H. Wang, X. Liu, X. Wang, Z. Lu, Improving plasticity of the Zr46Cu46Al8 bulk metallic glass via thermal rejuvenation, Sci.

J. Brechtl et al. / Journal of Nuclear Materials 523 (2019) 299e309 Bull. 63 (13) (2018) 840e844. [20] T. Nagase, Y. Umakoshi, Thermal stability and electron irradiation effect on Zrbased amorphous alloys, J. Appl. Phys. 93 (2) (2003) 912e918. [21] H. Trinkaus, Local stress-relaxation in thermal spikes as a possible cause for creep and macroscopic stress-relaxation of amorphous solids under irradiation, J. Nucl. Mater. 223 (2) (1995) 196e201. [22] M. Telford, Bulk metallic glass, Mater. Today 7 (3) (2004) 36e43. [23] M. Nastasi, Q. Su, L. Price, J.A.C. Santana, T.Y. Chen, R. Balerio, L. Shao, Superior radiation tolerant materials: amorphous silicon oxycarbide, J. Nucl. Mater. 461 (2015) 200e205. [24] S.J. Zinkle, L.L. Snead, Designing radiation resistance in materials for fusion energy, in: D.R. Clarke (Ed.), Annual Review of Materials Research vol. 44, Annual Reviews, Palo Alto, 2014, pp. 241e267. [25] T. Nagase, T. Sanda, A. Nino, W. Qin, H. Yasuda, H. Mori, Y. Umakoshi, J.A. Szpunar, MeV electron irradiation induced crystallization in metallic glasses: atomic structure, crystallization mechanism and stability of an amorphous phase under the irradiation, J. Non-Cryst. Solids 358 (3) (2012) 502e518. [26] T. Egami, K. Maeda, D. Srolovitz, V. Vitek, Local structure of amorphous metals, J. Phys. Colloq. 41 (C8) (1980) C8-272eC8-275. [27] S. Nagata, M. Sasase, K. Takahiro, B. Tsuchiya, A. Inouye, S. Yamamoto, T. Shikama, Ion induced structural modification and nano-crystalline formation of Zr-Al-Ni-Cu metallic glasses, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 267 (8e9) (2009) 1514e1517. [28] J. Carter, E.G. Fu, M. Martin, G. Xie, X. Zhang, Y.Q. Wang, D. Wijesundera, X.M. Wang, W.-K. Chu, S.M. McDeavitt, L. Shao, Ion irradiation induced nanocrystal formation in amorphous Zr55Cu30Al10Ni5 alloy, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 267 (17) (2009) 2827e2831. [29] H.C. Chen, G.Q. Cao, R.D. Liu, G. Wang, L. Yan, X.T. Zhou, Ion irradiation induced element-enriched and depleted nanostructures in Zr-Al-Cu-Ni metallic glass, J. Appl. Phys. 118 (3) (2015) 4. [30] W.D. Luo, B. Yang, G.L. Chen, Effect of Arþ ion irradiation on the microstructure and properties of Zr-Cu-Fe-Al bulk metallic glass, Scripta Mater. 64 (7) (2011) 625e628. [31] S. Nagata, S. Higashi, B. Tsuchiya, K. Toh, T. Shikama, K. Takahiro, K. Ozaki, K. Kawatusra, S. Yamamoto, A. Inouye, Ion irradiation effects on amorphization and thermal crystallization in Zr-Al-Ni-Cu alloys, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 257 (1e2) (2007) 420e423. [32] A.G. Perez-Bergquist, H. Bei, K.J. Leonard, Y. Zhang, S.J. Zinkle, Effects of ion irradiation on Zr52.5Cu17.9Ni14.6Al10Ti5 (BAM-11) bulk metallic glass, Intermetallics 53 (2014) 62e66. €beli, R. Erni, U. Ramamurty, J. Michler, [33] R. Raghavan, B. Kombaiah, M. Do Nanoindentation response of an ion irradiated Zr-based bulk metallic glass, Mater. Sci. Eng. 532 (2012) 407e413. [34] X.L. Bian, G. Wang, H.C. Chen, L. Yan, J.G. Wang, Q. Wang, P.F. Hu, J.L. Ren, K.C. Chan, N. Zheng, A. Teresiak, Y.L. Gao, Q.J. Zhai, J. Eckert, J. Beadsworth, K.A. Dahmen, P.K. Liaw, Manipulation of free volumes in a metallic glass through Xe-ion irradiation, Acta Mater. 106 (2016) 66e77. [35] F. Hori, N. Onodera, Y. Fukumoto, A. Ishii, A. Iwase, A. Kawasuso, A. Yabuuchi, M. Maekawa, Y. Yokoyama, A study of defects in electron- and ion-irradiated ZrCuAl bulk glassy alloy using positron annihilation techniques, in: S.J. Buckman, J.P. Sullivan, C. Makochekanwa, R. White (Eds.), 12th International Workshop on Slow Positron Beam Techniques, Iop Publishing Ltd, Bristol, 2011, p. 1. [36] J.F. Ziegler, M.D. Ziegler, J.P. Biersack, SRIM - the stopping and range of ions in matter, Nucl. Instrum. Methods Phys. Res. B 268 (2010) 1818e1823. [37] A.G. Perez-Bergquist, J. Brechtl, H. Bei, Y. Zhang, S.J. Zinkle, Effects of ion irradiation on BAM-11 bulk metallic glass, in: Fusion Materials Semiannual Progress Report for Period Ending June 30, 2014 vol. 184, U.S. Department of Energy, 2014. DOE/ER- 0313/56. [38] A.G. Perez-Bergquist, H. Bei, Y. Zhang, S.J. Zinkle, Effects of ion irradiation on BAM-11 bulk metallic glass, in: Fusion Materials Semiannual Progress Report for Period Ending June 30, 2013 vol. 154, U.S. Department of Energy, 2013. DOE/ER- 0313/54. [39] Z. Hu, Z.Q. Zhao, Y.P. Hu, J.S. Xing, T. Lu, B.C. Wei, Effect of ion irradiation on mechanical behaviors of Ti40Zr25Be30Cr5 bulk metallic glass, Mater. Res. Ibero Am. J. Mater. 15 (5) (2012) 713e717. [40] N. Onodera, A. Ishii, K. Ishii, A. Iwase, Y. Yokoyama, Y. Saitoh, N. Ishikawa, A. Yabuuchi, F. Hori, Irradiation effect of swift heavy ion for Zr50Cu40Al10 bulk

[41] [42]

[43]

[44] [45]

[46]

[47]

[48] [49]

[50]

[51] [52] [53]

[54] [55]

[56] [57]

[58]

[59]

[60]

[61]

[62] [63]

[64]

[65]

309

glassy alloy, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 314 (2013) 122e124. M.L. Crespillo, J.T. Graham, Y. Zhang, W.J. Weber, Temperature measurements during high flux ion beam irradiations, Rev. Sci. Instrum. 87 (2) (2016) 7. M.L. Crespillo, J.T. Graham, Y. Zhang, W.J. Weber, Insitu luminescence monitoring of ion-induced damage evolution in SiO2 and Al2O3, J. Lumin. 127 (2016) 208e218. Y. Zhang, M.L. Crespillo, H. Xue, K. Jin, C.H. Chen, C.L. Fontana, J.T. Graham, W.J. Weber, New ion beam materials laboratory for materials modification and irradiation effects research, Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 338 (2014) 19e30. J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. 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. P.J. Doyle, K.M. Benensky, S.J. Zinkle, Modeling the impact of radiationenhanced diffusion on implanted ion profiles, J. Nucl. Mater. 509 (2018) 168e180. J.B. Whitley, P. Wilkes, G.L. Kulcinski, H.V. Smith Jr., Depth dependent damage profile in nickel irradiated with nickel or copper ions, J. Nucl. Mater. 79 (1) (1979) 159e169. M.F. Toney, T.C. Huang, S. Brennan, Z. Rek, X-ray depth profiling of iron oxide thin films, J. Mater. Res. 3 (2) (1988) 351e356. W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (6) (1992) 1564e1583. W.C. Oliver, G.M. Pharr, Measurement of hardness and elastic modulus by instrumented indentation: advances in understanding and refinements to methodology, J. Mater. Res. 19 (1) (2004) 3e20. ASTM E384-16, Standard Test Method for Microindentation Hardness of Materials, ASTM International, West Conshohocken, PA, 2016. www.astm.org. W.D. Nix, H. Gao, Indentation size effects in crystalline materials: a law for strain gradient plasticity, J. Mech. Phys. Solics 46 (3) (1998) 411e425. D.C.C. Lam, A.C.M. Chong, Model and experiments on strain gradient hardening in metallic glass, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 318 (1e2) (2001) 313e319. D.C.C. Lam, A.C.M. Chong, Indentation model and strain gradient plasticity law for glassy polymers, J. Mater. Res. 14 (9) (1999) 3784e3788. C. Wang, Q.P. Cao, T. Feng, X.D. Wang, D.X. Zhang, S.X. Qu, J.Z. Jiang, Indentation size effects of mechanical behavior and shear transformation zone in thin film metallic glasses, Thin Solid Films 646 (2018) 36e43. S.J. Orfanidis, Introduction to Signal Processing, Prentice Hall, Upper Saddle River, NJ, 1996. L.E. Samuels, T.O. Mulhearn, An experimental investigation of the deformed zone associated with indentation hardness impressions, J. Mech. Phys. Solids 5 (2) (1957) 125e134. G. He, Z. Bian, G.L. Chen, Structures and properties of a Zr-based bulk glass alloy after annealing, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 270 (2) (1999) 291e298. G. He, Z. Bian, G.L. Chen, Investigation of phases on a Zr-based bulk glass alloy, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process. 279 (1e2) (2000) 237e243. Z. Altounian, E. Batalla, J.O. Stromolsen, J.L. Walter, The influence of oxygen and other impurities on the crystallization of NiZr2 and related metallic glasses, J. Appl. Phys. 61 (1) (1987) 149e155. X. Gu, L.Q. Xing, T.C. Hufnagel, Glass-forming ability and crystallization of bulk metallic glass (HfxZr1-x)52.5Cu17.9Ni14.6Al10Ti5, J. Non-Cryst. Solids 311 (1) (2002) 77e82. K. Zhang, Z. Hu, F.J. Li, B.C. Wei, Viscous surface flow induced on Ti-based bulk metallic glass by heavy ion irradiation, Appl. Surf. Sci. 390 (2016) 941e945. W.H. Jiang, F.X. Liu, H. Choo, P.K. Liaw, Effect of structural relaxation on mechanical behavior of a Zr-based bulk-metallic glass, Mater. Trans. 48 (7) (2007) 1781e1784. W. Li, H. Bei, Y. Tong, W. Dmowski, Y.F. Gao, Structural heterogeneity induced plasticity in bulk metallic glasses: from well-relaxed fragile glass to metal-like behavior, Appl. Phys. Lett. 103 (17) (2013) 171910. J.I. Jang, B.G. Yoo, Y.J. Kim, J.H. Oh, I.C. Choi, H.B. Bei, Indentation size effect in bulk metallic glass, Scripta Mater. 64 (8) (2011) 753e756.