Letter pubs.acs.org/NanoLett
Effects of Helium Implantation on the Tensile Properties and Microstructure of Ni73P27 Metallic Glass Nanostructures Rachel Liontas,† X. Wendy Gu,† Engang Fu,‡ Yongqiang Wang,‡ Nan Li,‡ Nathan Mara,‡ and Julia R. Greer*,§,∥ †
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States Los Alamos National Laboratory, Los Alamos, New Mexico 87544, United States § Division of Engineering and Applied Sciences and ∥The Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United States ‡
S Supporting Information *
ABSTRACT: We report fabrication and nanomechanical tension experiments on as-fabricated and helium-implanted ∼130 nm diameter Ni73P27 metallic glass nanocylinders. The nanocylinders were fabricated by a templated electroplating process and implanted with He+ at energies of 50, 100, 150, and 200 keV to create a uniform helium concentration of ∼3 atom % throughout the nanocylinders. Transmission electron microscopy imaging and through-focus analysis reveal that the specimens contained ∼2 nm helium bubbles distributed uniformly throughout the nanocylinder volume. In situ tensile experiments indicate that helium-implanted specimens exhibit enhanced ductility as evidenced by a 2-fold increase in plastic strain over as-fabricated specimens with no sacrifice in yield and ultimate tensile strengths. This improvement in mechanical properties suggests that metallic glasses may actually exhibit a favorable response to high levels of helium implantation. KEYWORDS: Amorphous, Ni−P, irradiation, ductility, mechanical properties
M
hardening, embrittlement, and a degradation of mechanical properties,20−27 irradiating metallic glasses may actually improve mechanical properties. Recent work on Ga+ ion irradiation of metallic glasses via a focused ion beam (FIB) found that such irradiation increased tensile ductility by a factor of 3 in Ni80P20 nanopillars17 and caused originally brittle Pt57.5Cu14.7Ni5.3P22.5 nanowires to exhibit postyielding strains of up to 2%.16 In another study, irradiation with Ni atoms led to reduced yield strength and smaller stress drops in a Zr41.2Ti13.8Cu12.5Ni10Be22.5 metallic glass under compression, which implies enhanced ductility upon irradiation.19 Molecular dynamics simulations on Zr50Cu40Al10 metallic glass nanopillars revealed that increasing irradiation caused the plastic deformation mode to switch from localized shear banding to homogeneous shear flow.18 These studies suggest that irradiation is capable of enhancing the mechanical performance of otherwise brittle metallic glasses, thus rendering metallic glass components and thin coatings suitable candidates for irradiation-intensive applications, such as space and nuclear energy.
etallic glasses offer a variety of desirable properties including corrosion resistance, high strength, a large elastic limit, and excellent soft magnetic properties.1−3 However, the use of metallic glasses in structural applications is limited by their brittle failure: tensile loading of metallic glasses at room temperature typically leads to catastrophic failure by localization of strain into narrow regions, known as shear bands.4 Much research has been focused on understanding the ability of metallic glasses to deform without sudden failure and on developing strategies to alleviate their inherently brittle behavior. Three such strategies are (1) creating a more liquid-like metallic glass structure,5,6 (2) fabricating composites that contain metallic glasses,7−9 and (3) reducing the external dimensions of metallic glasses to the nanoscale.10−15 The first two strategies require changing the material composition. The third approach involves reducing the size of a metallic glass feature to below some critical length scale on the order of ∼100 nm, which has been found to induce a brittle-to-ductile transition in compression10−12 and tension.13,14 Recently, ion irradiation has emerged as another promising strategy to induce ductility in metallic glasses.16−19 While irradiating crystalline materials with high-energy ions causes the formation of self-interstitials, vacancies, and voids, which lead to © 2014 American Chemical Society
Received: June 4, 2014 Revised: July 21, 2014 Published: August 1, 2014 5176
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Figure 1. (a) Schematic representation of the fabrication of Ni73P27 nanoscale mechanical testing specimens, starting with (1) an electroplating template consisting of a 500 μm thick Si substrate coated with 20 nm of Ti, 100 nm of Au, and 500 nm of PMMA patterned with e-beam lithography to contain an array of vertical pores, (2) Ni73P27 is electroplated into the vertical pores with inset depicting the electrodeposition growth process, (3) electroplating is continued until the Ni73P27 deposits above the PMMA layer to create tensile “mushroom caps” on the nanocylinders, (4) the PMMA is dissolved in acetone, resulting in a free-standing array of Ni73P27 tensile nanocylinders. (b) Electroplating condition. (c) SEM image of asfabricated Ni73P27 tensile specimens.
metallic glass nanomechanical testing specimens has limited dedicated nanomechanical studies of irradiated metallic glasses. One viable technique reported by Kumar et al. utilizes hot embossing to create metallic glass molds, which can subsequently be crystallized and used to form additional metallic glass replicas with resultant feature sizes as small as 13 nm.38 Another technique developed by Burek et al.39 and utilized in the current study, involves lithographically patterning vertical pores in resist then electroplating into those pores to form arrays of vertically oriented nanocylinders.39 We report the fabrication of Ni73P27 metallic glass nanotensile cylinders with ∼130 nm diameters and ∼500 nm heights via templated electroplating. The specimens are implanted with He+ ions and strained by an in situ nanomechanical instrument, which allows correlation of tensile stress−strain data with morphological evolution of the specimens. We discuss the effects of helium implantation on the subsequent microstructure and mechanical properties of the specimens. Fabrication of specimens was performed using the FIB-less templated electroplating process developed by Burek et al.39 In this process, 130 nm diameter nanotensile cylinders were formed by first utilizing electron-beam lithography to create a poly(methyl methacrylate) (PMMA) template containing vertical pores, and subsequently electroplating Ni73P27 metallic glass into those vertical pores. A nickel−phosphorus metallic glass was chosen because it is amenable to electrodeposition.40−44 Electroplating was performed until the Ni73P27 material deposited above the PMMA layer and formed “mushroom caps” on top of the nanocylinders. These “mushroom caps” serve as grips for the tensile tester to grab during mechanical testing of individual nanocylinders. The fabrication process is illustrated in Figure 1 and described in
Certain irradiation conditions can produce helium in solid materials, such as transmutation reactions in nuclear reactor environments. The low solubility of helium in metals leads to its precipitation into bubbles and voids, which cause embrittlement of crystalline metals and alloys.20,28−30 However, the effect of such helium irradiation on the tensile properties of metallic glasses is not well understood because most experimental studies on irradiated metallic glass have focused on FIB-induced Ga+ ion irradiation,16,17 due to the ease of accessing such irradiation with SEM-FIB dual beam systems. Studying the tensile properties of metallic glasses upon helium irradiation, specifically implantation where the incoming helium ends up in the metallic glass material, would be particularly relevant for radiation applications where helium byproducts are produced in materials, such as neutron irradiations, high energy proton irradiations, and nuclear reactors. Ion beam irradiation with particle accelerators allows irradiation of materials with small dimensions by a variety of ions, including He+, and has been shown to be a time- and cost-effective method for studying radiation damage in these small-scale specimens.31−34 Small-scale nanotensile specimens fabricated without the use of ion irradiation, that is, without the FIB, are well-suited for such studies because the small size of the specimens allows full penetration by ion accelerators, the ensuing nanotension tests enable quantitative assessment of mechanical properties, including ductility, under known stress states, and FIB-less fabrication avoids the complicating effects of additional irradiation. The effects of FIB irradiation have been shown to include ductility enhancement in metallic glass,16,17 modification of local atom arrangements,35,36 and even surface crystallization.37 The scarcity of “FIB-less” fabrication techniques for individual 5177
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Figure 2. (a) Schematic of helium implantation into Ni73P27 nanocylinders. (b) Helium implantation conditions. (c) Helium concentration and (d) displacements per atom profiles as calculated by SRIM for each implantation energy (dashed lines) and the total sum (solid line) from all energies, overlaid on a typical specimen geometry.
Because SRIM assumes the He+ ions are entering a material that is infinite in lateral dimensions, it likely overestimates the helium concentration and dpa for our ∼130 nm nanocylinder specimens as the limited lateral dimensions of such specimens allow some of the incoming He+ ions to escape from the external nanocylinder surfaces. Figure 3 displays characterization of the as-fabricated and helium-implanted nanocylinders. SEM images of representative individual tensile nanocylinders are shown in Figure 3a for the as-fabricated sample and Figure 3f for the helium-implanted sample. These images reveal that the helium-implanted specimens have a smoother exterior than the as-fabricated specimens with no noticeable swelling, blistering, or flaking occurring as a result of the implantation. An increase in the smoothness of specimen surfaces upon helium implantation agrees with earlier work reporting a decrease in surface roughness upon ion bombardments.46−48 The lack of any noticeable swelling, blistering, or flaking in the heliumimplanted specimens is consistent with studies on helium implantation in bulk metallic glasses in which these morphological changes were observed only at radiation levels above an extremely high critical fluence of 1017−1018 ions cm−2.49−54 On the basis of these studies, it is likely that the ∼1016 ions cm−2 fluence used in the current work is below the critical fluence necessary for blistering in Ni73P27. The chemical composition of the samples was analyzed using energy-dispersive X-ray spectroscopy (EDX). EDX was conducted in an SEM (Nova 200 Nanolab, FEI Co.) at 4 keV on five as-fabricated and five helium-implanted nanocylinders randomly chosen from different regions on the samples. EDX analysis shown in Figure 3b,g reveals that both the as-fabricated and helium-implanted samples contain ratios of Ni to P corresponding to a chemical composition of ∼73 atom % Ni and ∼27 atom % P. As measured by EDX, the only significant difference in chemical composition between the two samples was that the helium-implanted sample had nearly twice
more detail in the Supporting Information. After specimens were fabricated, the 1 cm2 substrate on which the specimens were grown contained thousands of nanocylinders and was cleaved into halves with one of the halves subsequently implanted with He+ and the other half left as-fabricated. Nanomechanical experiments conducted on the individual nanocylinders from each half allowed a direct comparison between helium-implanted and as-fabricated specimens that had been fabricated under completely identical conditions and thus contained identical original microstructure and chemical composition. The implantations were performed at room temperature, with the helium implanted vertically into the nanocylinders in the form of He+, as shown schematically in Figure 2a, using several different energies and fluences, Figure 2b, selected to create a relatively uniform helium concentration profile throughout the nanocylinders. The helium concentration profiles and displacements per atoms (dpa) were calculated using SRIM software45 with the Kinchin-Pease model, the bulk density for Ni73P27 (6.985 g cm−3), and the default threshold displacement energies for Ni and P (25 eV). The profiles for each implantation energy are shown in Figure 2c,d. The resultant radiation damage within the specimens, measured in dpa, was nonuniform with more damage occurring near the top of the specimens, as expected for the vertical top-down nature of the implantation. Implanting He+ vertically from above the sample results in more He+ ions traveling through the top of the specimens than the bottom, consequently there are more collisions and thus more damage in the top of the specimens. SRIM calculations estimated the damage to be 3.00 ± 0.78 dpa and the helium concentration to be 3.18 ± 0.49 atom % at penetration depths of 150 − 650 nm, which are dimensions corresponding to the length of a typical nanocylinder. Helium penetration depths of less than ∼150 nm are located above the nanocylinder in the “mushroom cap” and penetration depths greater than ∼650 nm are in the underlying Au substrate. 5178
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Figure 3. Sample characterization of (a−e) as-fabricated and (f−h) helium-implanted samples. SEM images of a typical (a) as-fabricated and (f) Heimplanted Ni73P27 specimen. EDX spectrum from a typical (b) as-fabricated and (g) He-implanted Ni73P27 specimen. Inset tables in top right corners shows chemical composition obtained from five representative (b) as-fabricated and (g) He-implanted Ni73P27 nanocylinders. (c) Diffraction pattern, (d) bright-field, and (e) dark-field TEM images on an as-fabricated Ni73P27 nanocylinders. (h) A through-focal TEM series on a representative Heimplanted Ni73P27 nanocylinder with defocus values spanning −2 μm, −1 μm, 0, +1 μm, and +2 μm, from left to right, with example bubbles surrounded by red squares to guide the eye.
atoms, and the amount of carbon and oxygen is highest at the sample surfaces. The microstructure of the nanocylinders was examined using TEMs operating at 200 and 300 keV (TF20 and TF30, FEI Co.), and nanocylinders were prepared for TEM analysis by two methods, both of which avoided the use of FIB so as to prevent extraneous Ga+ ion irradiation. The first TEM sample preparation method involved moving individual nanocylinders from the Au substrate to a Cu TEM grid (Ted Pella) by attaching a nanomanipulator needle (OmniProbe 200, Oxford Instruments) inside an SEM (Nova 600 Nanolab, FEI Co.) to the “mushroom cap” of a specimen using electrostatic forces and then transferring that specimen to the TEM grid also with only electrostatic forces. Once on the grid, a small amount of W was deposited onto the nanocylinder’s “mushroom cap” with
the amount of carbon and oxygen as the as-fabricated sample; any difference in helium concentration is indiscernible by EDX because helium is too light for detection. The presence of carbon and oxygen in both samples may be caused by exposure to air, residual solvent used to strip the PMMA, or small amounts of unremoved PMMA. The helium-implanted sample likely contains more carbon and oxygen due to carbon and oxygen exposure in the ion accelerator and increased exposure to air during transport to ion beam facilities (the as-fabricated sample was stored in a N2 environment). These carbon and oxygen impurities do not form a continuous layer on the specimen surfaces and thus do not bear load, which renders their effect on mechanical properties negligible. Further, the EDX results over-represent the amount of carbon and oxygen present because this technique is more sensitive to surface 5179
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Figure 4. Mechanical behavior of as-fabricated and He-implanted samples under tension. Top right: video stills from in situ SEM for (a−c) asfabricated and (a′−c′) He-implanted samples during (a, a′) initial contact between grip and sample, (b, b′) just before failure, and (c, c′) after failure. Top left: engineering stress−strain for as-fabricated and helium-implanted samples. Bottom: elastic modulus (E), yield strength (σyield), ultimate tensile strength (σUTS), and plastic strain (εp) for both sample types.
helium implantation. Multiple helium bubbles are visible in the through-focal TEM images shown in Figure 3h with the bubbles appearing lighter in the under-focused images and darker in the overfocused images. From the through-focal images, the average helium bubble diameter was measured to be 2.25 ± 0.85 nm. Several through-focal series of images were taken both at different locations within the same nanocylinder and on multiple nanocylinders with no significant changes in the apparent bubble size and distribution from location to location or from cylinder to cylinder. This consistency of the through-focal images suggests that the helium implantation did indeed result in a uniform distribution and concentration of helium, as expected based on the SRIM-calculated uniform helium concentration. In situ uniaxial tensile experiments on individual metallic glass nanocylinders were conducted at a constant nominal strain rate of 0.01 s−1 to the point of failure in the InSEM, a combined SEM (Quanta SEM, FEI Co.) and nanoindenter (Nanomechanics, Inc.). The nanoindenter arm was fitted with a diamond tensile grip. After accounting for response from the load frame, support spring, and substrate compliances, the raw load and displacement due to deformation of the specimen were recorded. Using load−displacement data, engineering stresses and strains were calculated using the initial nanocylinder diameter and gauge length and by assuming homogeneous deformation. Elastic modulus was calculated from the slope of the loading curve, using stress−strain data from the initial loading until yielding occurred. The yield point was determined by visually inspecting the data for the first deviation from linear elastic loading and the yield stress was defined as the stress at this yield point. More details on the mechanical testing setup and methods are included in Supporting Information. Figure 4 contains tensile engineering stress versus strain data for five as-fabricated and five helium-implanted samples.
the electron-beam in an SEM (Nova 200 Nanolab, FEI Co.) to secure the specimen to the TEM grid. The second TEM sample preparation method involved physically rubbing a TEM grid containing a holey carbon film (Ted Pella) on the surface of the substrates to cause some specimens to attach to the carbon film. After rubbing, the TEM grid was examined by SEM to find appropriately placed nanocylinders located over holes in the carbon film. Nanocylinders positioned in this way could be imaged without interference from the carbon film. These two TEM sample preparation methods were complementary as the first method allowed selectively in choosing nanocylinders for TEM analysis and the second method allowed picking up many nanocylinders simultaneously to conduct “brute-force” TEM and ensure that the microstructure observed in specimens prepared by the first method was representative across many specimens. To visualize the helium bubbles, a common technique for studying radiation damage with TEM is through focus imaging,55 which was used to view the helium bubbles by taking a through-focal series of images on the helium-implanted nanocylinders at defocus values of −2 μm, −1 μm, 0, +1 μm, and +2 μm. TEM analysis is shown in Figure 3c−e for the as-fabricated specimens and Figure 3h and Supporting Information Figure S.1 for the helium-implanted specimens. Both specimen types have diffuse ring electron diffraction patterns, which confirm the amorphous microstructure of the nanocylinders. The amorphous microstructure was further corroborated by the featureless and uniform contrast bright-field and dark-field TEM images shown for the as-fabricated nanocylinders in Figure 3c,d. Similar featureless and uniform contrast bright-field and dark-field images were obtained for the helium-implanted sample as well and are included in the Supporting Information, Figure S1. These images verify that unlike the crystallization observed in other irradiated metallic glasses,37,56,57 the current specimens maintained an amorphous microstructure during 5180
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able to accommodate applied strain without failure before the STZs coalesce and form a catastrophic shear band. During helium implantation, the incoming He+ ions knock the metallic glass atoms from their native positions. For the implantation conditions used in this work, corresponding to a SRIMcalculated damage of 3.00 ± 0.78 dpa, the metallic glass atoms underwent a significant number of knock-on events. These knock-on events likely disrupted the local icosahedral symmetries between metallic glass atoms, which generated additional free volume within the metallic glass, and facilitated a more uniform distribution of STZs. As discussed by others,17,18 irradiation shifts native metallic glass atoms away from equilibrium and propels them to higher potential energy states, thereby lowering the activation energy for atomic rearrangements and lending greater probability to noncatastrophic plastic deformation. This line of reasoning is consistent with the observed ∼2-fold increase in plastic strain of the heliumimplanted specimens compared with the as-fabricated specimens. On the basis of this reasoning, the potential energy landscape in the as-fabricated metallic glass samples is closer to equilibrium, where diffuse local atomic activity and plastic deformation is inhibited. We speculate that compared to the helium-implanted specimens, the as-fabricated specimens contain a more localized free volume and heterogeneous distribution of STZs that more readily coalesce into shear bands. These arguments are consistent with previous studies that have reported a deformability enhancement in metallic glass nanostructures as a result of FIB-induced Ga+ ion bombardments. Magagnosc et al. found that Ga+ ion irradiation caused originally brittle Pt57.5Cu14.7Ni5.3P22.5 nanowires to exhibit tensile ductility with up to 2% post yielding strains.16 Chen et al. found that Ga+ ion bombardment into the surface of Ni80P20 nanopillars led to a 3-fold increase in plastic strain17 The difference in the extent of irradiation-induced increase in plastic strain between Chen et al. (3-fold increase)17 and the current work (2-fold increase) may arise from slight differences in the composition of the constituent Ni−P metallic glass and the dissimilar nature of the irradiating ions. The experiments of Chen et al. involved bombardment with Ga+ ions, which have limited penetration due to the large size of the Ga+ ions and relatively low FIB energies used. In contrast, samples of the current work were implanted with He+, which resulted in a distribution of high-pressure helium bubbles throughout the volume of the nanocylinders. At equilibrium, the pressure of helium gas inside a bubble is balanced by surface tension, thus to estimate the pressure difference between the inside and outside of a spherical bubble, we use the well-known Laplace pressure relationship:
Helium implantation appears to lower the elastic modulus, which was measured to be 47.9 ± 5.20 GPa for the as-fabricated specimens and 38.2 ± 5.25 GPa for the helium-implanted specimens. This decrease in modulus upon helium implantation may be because the helium-implanted specimens contain ∼3.2 atom % helium and thus have a higher porosity. A relation between elastic modulus, (E), porosity (ϕ), and elastic modulus of the nonporous monolithic material (Eo) was developed by Lu et al.58 E = E0(1 − 2ϕ)(1 + 4ϕ2)
(1)
Using eq 1 with the elastic moduli obtained from the slope of linear elastic loading in our experiments, E0 = 47.9 GPa and E = 38.2 GPa, results in a porosity of ϕ = 0.125, which is higher than anticipated based on the lack of any measurable volume change in the nanocylinders upon helium implantation. This seeming discrepancy in modulus is not unreasonable because uniaxial tension experiments at the nanoscale often have significant variability in measured moduli data especially when the moduli are measured from the loading portions of the nanomechanical data.59−61 The relation by Lu et al. in eq 1 is qualitatively useful for illustrating the general trend of elastic modulus decreasing with porosity but should not be used for precise quantification of porosity. All specimens broke within the nanocylinder at a ∼45° angle with respect to the substrate (images of fracture surfaces are included in Figure S2 of the Supporting Information), which indicates that both as-fabricated and helium-implanted specimens failed by a similar mechanism of shear banding without significant necking. Prior to failure, the specimens underwent some plastic deformation, which was quantified by measuring plastic strain (εp), defined as the total strain at fracture (εf) less the strain at the yield point (εy), that is εp = εf − εy. Heliumimplanted specimens were on average capable of withstanding around two times greater plastic strain before failure (εp = 1.215 ± 0.296%) as compared with the as fabricated specimens (εp = 0.598 ± 0.032%). The yield strength (σyield) and ultimate tensile strength (σUTS) for both specimen types were similar at values of 1.89 ± 0.169 and 2.04 ± 0.185 GPa for the asfabricated specimens and 1.98 ± 0.307 and 2.21 ± 0.324 GPa for the helium-implanted specimens. The similarity in the strengths between helium-implanted and as-fabricated samples suggests that helium implantation did not alter the failure mechanism. In summary, these tensile tests revealed that implanting He+ into Ni73P27 metallic glass nanocylinders increases ductility by a factor of 2 without a sacrifice in strength. The mechanical results imply that instead of being a detriment, irradiation may actually enhance ductility in metallic glasses. This is the opposite trend seen in irradiation on crystalline materials, where the large body of literature indicates irradiation leads to embrittlement.20,21,27,28,30 The substantial difference in mechanical response upon irradiation between crystalline materials and metallic glasses likely arises from the disordered microstructure of metallic glasses, which undergo deformation by atomic rearrangements within ∼100 atom-sized regions, often called shear transformation zones (STZs),62,63 instead of the dislocation-motion inherent to plasticity of crystalline materials. Deformation in metallic glasses occurs by collective rearrangements and coalescence of multiple STZs. At yield stress, some STZs coalesce to form large planar bands, know as shear bands.4 The formation and propagation of shear bands are localized events that depend upon the distribution of STZs. Hence, a more uniform distribution of STZs is better
ΔP = Pinside − Poutside =
2γ r
(2)
where γ is the surface energy of the surrounding material and r is the bubble radius. For Ni73P27 metallic glass, γ was measured to be 3 J/m2 using contact angle goniometry and the bubble radius from the TEM images was ∼1 nm, which results in a pressure difference of ΔP ∼ 6 GPa between the inside of the bubble and the surrounding Ni73P27. The presence of such high-pressure helium bubbles likely reduces the free volume available for plastic deformation and thus the extent of ductility brought about by displacements from He+ ion bombardments. This suggests that helium implantation leads to two competing effects on metallic glass ductility: (1) bombardments from 5181
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incoming He+ ions enhances ductility by increasing free volume in the metallic glass, and (2) the presence of helium segregated into high-pressure bubbles reduces ductility by decreasing free volume in the metallic glass. The dominant effect depends on how the displacements per atom (a measure of bombardments) compare with the implanted helium concentration. The increased ductility observed in the current work suggests that the effect of bombardments dominates for the implantation conditions used, which correspond to SRIM-calculated dpa of 3.00 ± 0.78 and helium concentration of 3.18 ± 0.49 atom %. This work demonstrates a promising new pathway for developing materials that can maintain and even improve their mechanical properties in irradiated environments. By utilizing an electroplating-based nanofabrication technique we created isolated tensile Ni−P metallic glass nanostructures without the use of focused-ion beam irradiation. We then utilized an ion accelerator to uniformly implant helium throughout the samples at a concentration of 3.2 atom % with resultant damage of ∼3 dpa. In situ uniaxial tensile tests on the asfabricated ∼130 nm diameter Ni73P27 metallic glass nanostructures and on identical helium-implanted ones indicate that helium implantation enhanced postelastic deformability by a factor of 2 without any sacrifice in strength. The absence of dramatic changes in mechanical properties upon helium implantation suggests that the small ∼2 nm sized helium bubbles produced as a result of the helium implantation in this study were not large enough to serve as sufficient stress concentrators, thus the mechanical response of the heliumimplanted nanocylinders did not transition to the regime where stress-concentrating helium bubbles would act as flaws that induce failure. Our work suggests that metallic glasses may respond to helium irradiation in a beneficial way, a finding diametrically opposite to the behavior of crystalline metals and alloys, for which irradiation has been widely reported to induce hardening, embrittlement, catastrophic failure, and dramatic changes in mechanical properties. For example, in a study where 3 atom % He was implanted into pure Ni at room temperature under conditions similar to those used in this work, the hardness increased from 1.1 to 5.8 GPa.64 The results of the current work are even more promising because the amount of helium implanted, 3.2 atom %, is over an order of magnitude larger than what a nuclear reactor would typically experience (typically 0.2 atom % for fusion reactors).30 Consequently, the improvement in mechanical properties of the studied metallic glass system upon very high levels of helium implantation suggests that metallic glasses may be particularly well suited for irradiation-intensive applications, involving high levels of implanted ions. Caution should be taken because a significant limitation of metallic glasses may be their glass transition temperature (Tg), which restricts their use to applications below Tg in order to prevent crystallization.
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Author Contributions
R.L. fabricated samples, characterized samples (by SEM, EDX, and TEM), and analyzed the mechanical data. R.L and X.W.G. carried out the in situ tensile experiments and X.W.G. trained R.L. on various techniques. R.L and X.W.G. performed SRIM calculations, mounted and removed samples for the ion beam implantation, and monitored the ion beam during implantation. E.F. and Y.W. helped perform the ion beam implantations. R.L. wrote the manuscript with input from X.W.G., J.R.G., N.M., Y.W., N.L., and E.F. The experiments were designed by R.L., X.W.G., and J.R.G. and J.R.G. supervised the project. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the U.S. Department of Energy through J.R.G.’s Early Career Research Program under Grant DE-SC0006599. Additional financial support was provided by X.W.G.’s National Defense Science and Engineering Graduate Fellowship and R.L.’s National Science Foundation Graduate Research Fellowship. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant DGE-1144469. Any opinions, findings, and conclusions or recommendations expressed in the material are those of the authors and do not necessarily reflect the views of the National Science Foundation. The authors thank David Chen and Kelly Guan for developing the Ni−P electroplating conditions and Dongchan Jang and Carol Garland for TEM assistance. The authors also thank the Kavli Nanoscience Institute (KNI) at Caltech for support and availability of cleanroom facilities, and the Center for Integrated Nanotechnologies (CINT) user program for use of ion beam facilities at Los Alamos National Laboratory. This work was performed in part at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DEAC52-06NA25396.
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ASSOCIATED CONTENT
S Supporting Information *
Supporting figure and methods. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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dx.doi.org/10.1021/nl502074d | Nano Lett. 2014, 14, 5176−5183
