Nanoscale Structural Evolution and Anomalous Mechanical Response

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Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX

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Nanoscale Structural Evolution and Anomalous Mechanical Response of Nanoglasses by Cryogenic Thermal Cycling Wei-Hong Liu,† B. A. Sun,‡ Herbert Gleiter,§,⊥ Si Lan,∥,# Yang Tong,†,○ Xun-Li Wang,∥ Horst Hahn,⊥ Yong Yang,† Ji-Jung Kai,† and C. T. Liu*,† †

Centre for Advanced Structural Materials, Department of Mechanical and Biomechanical Engineering, City University of Hong Kong, Hong Kong, PR China ‡ Institute of Physics, Chinese Academy of Sciences, 100190 Beijing, PR China § Senior member of the Institute for Advanced Study, City University of Hong Kong, Hong Kong, PR China ⊥ Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), 76021 Karlsruhe, Germany ∥ Department of Physics and Material Science, City University of Hong Kong, Hong Kong, PR China # Herbert Gleiter Institute of Nanoscience, School of Materials Science and Engineering, Nanjing University of Science and Technology, 200 Xiaolingwei Avenue, Nanjing, PR China ○ Division of Materials Science and Technology, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA S Supporting Information *

ABSTRACT: One of the central themes in the amorphous materials research is to understand the nanoscale structural responses to mechanical and thermal agitations, the decoding of which is expected to provide new insights into the complex amorphous structural-property relationship. For common metallic glasses, their inherent atomic structural inhomogeneities can be rejuvenated and amplified by cryogenic thermal cycling, thus can be decoded from their responses to mechanical and thermal agitations. Here, we reported an anomalous mechanical response of a new kind of metallic glass (nanoglass) with nanoscale interface structures to cryogenic thermal cycling. As compared to those metallic glasses by liquid quenching, the Sc75Fe25 (at. %) nanoglass exhibits a decrease in the Young’s modulus but a significant increase in the yield strength after cryogenic cycling treatments. The abnormal mechanical property change can be attributed to the complex atomic rearrangements at the short- and medium- range orders due to the intrinsic nonuniformity of the nanoglass architecture. The present work gives a new route for designing high-performance metallic glassy materials by manipulating their atomic structures and helps for understanding the complex atomic structure−property relationship in amorphous materials. KEYWORDS: nanoglasses, metallic glasses, structural evolution, cryogenic thermal treatment, atomic structures

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defects.2 The lack of a long-range atomic structure leads to the nanoscale heterogeneity in the distribution of the inherent defects, containing densely packed atomic clusters and loosely packed defective domains.2 Extensive studies20−25 reveal that these loosely bonded sites,23,26 essentially act as the preferred regions that initiate the glassy structural destabilization caused by either high temperatures or applied shear stresses. Ketov et. al show that the inherent atomic structural inhomogeneities of MGs can be rejuvenated and amplified by cryogenic thermal cycling, and this leads to changes of a decreased yield strength (YS) but an enhanced plastic strain before a final fracture. These changes on mechanical properties are believed to associate with a nonaffine thermal strain produced during

etallic glasses (MGs) constitute a new class of disordered materials, whose microstructures are essentially featureless under a conventional microscopy in contrast to crystalline materials where grain boundaries, dislocations and other structural features can be readily identified and manipulated.1,2 Tremendous research efforts have been dedicated to the fundamental studies of their various properties and related applications;3−8 however, a lack of the understanding on microstructural and property modifications has greatly obstructed their structural applications.9−14 The nanoglass (NG) architecture, which introduces defects (glass−glass interfaces) into common MGs by consolidating nanoscale amorphous particles, permits the controlled properties modification by varying their chemical and/or defect microstructures through methods used for crystalline materials.1,15−20 MGs not only inherit the disordered atomic configuration from the liquid state but also a large number of quench-in © XXXX American Chemical Society

Received: March 13, 2018 Revised: May 17, 2018

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DOI: 10.1021/acs.nanolett.8b01007 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. (a) Typical load−displacement curves obtained at different loading times (tL) with a spherical indenter (noted that the peak load of 600 μN is way below the yield point of the Sc75Fe25 NG) and (b) the comparison of volume fractions of loosely bonded regions in the Sc75Fe25 NG with common Zr-, Cu- and Ni-based MGs.

before testing on our samples and it is less than 0.1 nm/s. The nanoindentation system possesses an achievable resolution of ∼1 nm in displacement and ∼1 μN in load after calibration and an ultrafast data acquisition capacity (with a maximum of ∼30 000 points per second). This makes unusually high loading rates possibly be achieved. To reveal the anelasticity, the loading time (tL) was systematically varied from 0.005 to 0.4 s, and the holding time and unloading time was set at 0.1 s. This was performed under load control up to a maximum load (Pmax) of 600 μN, which is way below the yield load of the Sc75Fe25 NG. To confirm the unique mechanical response on the Sc75Fe25 was not the consequence of machine response but of materials, we carry out a similar experiment on the standard material fused quartz (Figure S1, Supporting Information). The Young’s modulus and hardness were determined by the standard Oliver and Pharr method from the load−displacement (P−h) curve (Figure S2, Supporting Information) obtained by a Berkovich indentation,29 the area function of which was calibrated using fused quartz. In addition, the atomic force microscope (AFM) image of the indentation was presented in Figure S3 in the Supporting Information. The cumulative distribution curves of Young’s modulus and hardness of the Sc75Fe25 NG include 30−80 data points for each state. We conducted in situ scanning electron microscope (SEM) microcompression tests on nanopillars30 with top diameters of 330−430 nm and aspect ratios (height/diameter) of 1.8−3.0, at room temperature using a PI 85 Picoindenter (Hysitron Inc.) with a flat 3-μm punch diamond tip inside a FEI Quanta 450 FEG SEM, under displacement-control mode and at a strain rate of around 5 × 10−3 s−1. High-energy and high-resolution X-ray scattering experiment was carried out using the 11-ID-C high-energy beamline at the Advanced Photon Source, Argonne National Laboratory. The energy of the synchrotron radiation was set to 105.09 keV, which corresponds to a wavelength of λ = 0.0117418 nm. A Perkin-Elmer amorphous silicon detector was used to record the scattering data and the distance between the 2D detector and sample was adjusted to about 30.8 cm in order to cover high-Q range up to 30 Å−1, where Q is the magnitude of

thermal cycling due to the existence of nanoscale structural inhomogeneities. The NG has more significant structural inhomogeneities at nanoscales. Here, we aim to treat a Sc75Fe25 NG containing nanoscale interfaces with cryogenic thermal cycling, since previous studies on the characterization of the static microstructure of glassy grains and interfacial regions of NGs applying various advanced technologies (such as transmission electron microscopy (TEM),27 Mössbauer spectroscopy,16 and small-angle X-ray scattering17) revealed that the NG has an intrinsic structural/chemical inhomogeneity which is much larger scale than that in common MGs. Thus, it is intriguing to see how these nanoscale inhomogeneities response to thermal agitation of cryogenic thermal cycling and mechanical stress. We utilized a thermal agitation as cycling between room-temperature (293 K) and cryogenic temperature (77 K) to modulate its microstructure and mechanical property. An unusual mechanical response was found for the Sc75Fe25 NG and its atomic structural origin was decoded by the subsequent high-resolution synchrotron radiation. The Sc75Fe25 NG specimens were produced by inert-gas condensation at KIT, Karlsruhe, Germany, which consist of glassy grains of diameter of ∼10 nm and glassy−glassy interfaces of ∼1 nm thick with a reduced atomic density.21,28 The composition of about Sc85Fe15 was found in the interfacial region and the composition ranges from Sc70Fe20N10 to Sc50Fe30N20 inside of the glassy grains.27 The as-prepared Sc75Fe25 sample was inserted into liquid nitrogen for 1 min, then dried at room-temperature for 1 min; samples were treated with 10 such cycles. Noted that before the 10 roomtemperature−77 K cycles, the samples were first inserted into liquid nitrogen for a 10-min hold. In order to address whether these structural and property changes are due to cycling or the time spent at 77 K, the sample was also inserted into liquid nitrogen and held for 1 day to compared with the cycled sample. Nanoindentation tests were performed by a TI 950 Tribo Indenter system (Hysitron Inc., Minneapolis, MN) with a 2.4 μm spherical indenter and a Berkovich indenter at room temperature, respectively. We conducted the drift correction B

DOI: 10.1021/acs.nanolett.8b01007 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters momentum transfer.31,32 The in-house software PDFgetx2 was used to correct the background scattering, multiple scattering, absorption, Oblique incidence, Compton scattering, Florescence correction and Laue diffuse scattering, etc. The local structure of the Sc75Fe25 NG was investigated for two different samples: (a) as-prepared and (b) 10 cycles between roomtemperature and 77 K. The scattering data, as a function of the momentum transfer Q (=4π(sin θ)/λ, where λ is the wavelength and 2θ is the scattering angle), was collected at room temperature for 3 s × 5 frames with step size 67 μm. The reduced atomic pair distribution function (G(r), where r is the atomic pair distance) was obtained by Fourier transforming Q(S(Q)−1).33 It is noted that the result presented in the main text is an analysis of the data collected from an area of 500 μm × 500 μm. Although a number of experiments2,23,34,35 and numerical simulations36 on the atomic-scale glassy structure have revealed the existence of loosely packed regions and densely packed regions in MGs, direct observations of the nanoscale heterogeneity are still missing, and quantitative analysis of the heterogeneity in different MGs remains poorly known due to the limited resolution of current measurement methods. According to Tanaka et al.37 and Huo et al.,38 under mechanical agitations, the distinction between the loosely packed zones and densely packed defective sites with different viscosities is their relaxation time to mechanical loading. Therefore, if the experimental time is set to be shorter than the relaxation time of the loosely packed regions but longer than that of the densely packed regions, the two regions can be distinguished by the quasi-static mechanical loading. As presented in Figure 1a, an interesting result was obtained from the fast-spherical nanoindentations. The load−displacement (P−h) curves were acquired with a 2.4 μm spherical indenter at a peak load PH = 600 μN. At a relatively low loading rate (600 μN/0.4 s = 1500 μN/s), it exhibits an apparent elastic behavior; but as the loading rate increases to 60000 μN/s (600 μN/0.01 s), a mechanical hysteresis loop starts to emerge, and expands strikingly with the further increase of the stress rate (600 μN/ 0.005 s = 120000 μN/s). Huo et al.38 proposed at a simple constitutive relation that is able to correlate this nanoscale relaxation dynamics of the loosely bonded regions and densely bonded regions with the microscale “quasi-static” shear modulus. The P−h relation for the loading process of spherical nanoindentations for anelastic materials can be depicted by the standard linear solid model in rheology, which is composed of two springs and one dashpot, and is known as a threeparameter viscoelastic model:38 h(t )3/2 =

̇ − ν)G IItc 3P(1 3p(t )(1 − ν) − 8 R GI 8 R G I(G I + G II) ⎡ ⎛ t ⎞⎤ ⎢1 − exp⎜ − ⎟⎥ ⎢⎣ ⎝ tc ⎠⎥⎦

loading curve in line with the three-parameter viscoelastic model. According to Hertzian theory, the GI of the Sc75Fe25 NG can be first obtained via fitting the loading curve from the slow indentation (tL = 0.4 s) where the load and unloading curves overlap with each other. With the known GI, then GII and η from fast indentations (tL= 0.005 and 0.01 s) were extracted by fitting the loading curves that deviate significantly from the unloading curves with the three-parameter viscoelastic model.38,39 A comparison of the experimental and theoretical loading curves at tL = 0.01 s and PH = 600 μN was shown in Figure S4 in the Supporting Information. At least 10 loading curves with tL = 0.005 and 0.01s were fitted, and the GI and GII extracted from different loading curves appear to be very stable at ∼26.8 and ∼11 GPa, respectively, and the apparent relaxation time tc is 0.0035 s. Thus, the volume fraction of the loosely bonded regions of the Sc75Fe25 NG can be roughly estimated to be ∼29%, as shown in Figure 1b. Figure 1b and Table 1 compare the volume fraction of the loose-bonded Table 1. Volume Fractions of Loosely Bonded Regions in the As-Prepared Sc75Fe25 NG and Several Representative Zr-, Cu-, and Ni-Based MGsa

MGs

μ

GI

GII

GII/μ (%)

Zr52.5Cu17.9Ni14.6Al10Ti5 Zr41.2Ti13.8Ni10Cu12.5Be22.5 Zr46.75Ti8.25Cu7.5Ni10Be27.5 Zr57.5Nb5Cu15.4Ni12Al10 Zr48Nb8Ni12Cu14Be18 Zr57Nb5Cu15.4Ni12.6Al10 Zr55Ti5Cu20Ni10Al10 Zr65Al10Ni10Cu15 Zr54Cu46 Ni50Nb50 Ni45Ti20Zr25Al10 Cu57.5Hf27.5Ti15 Cu50Zr50 Cu64Zr36 Cu46Zr42Al7Y5 Sc75Fe25 NG

39.6 47.1 47.6 36.5 44.4 36.6 36.9 35.7 38.54 50.7 45.7 40.7 39.1 41.25 37.14 37.8

34.1 37.4 37.2 30.8 34.3 32 31 30.3 30 48.2 40.2 37.3 32 34 31 26.8

5.5 9.7 10.4 5.7 10.1 4.6 5.9 5.4 8.54 2.5 5.5 3.4 7.1 7.25 6.14 11

13.8 20.6 21.8 15.6 22.7 12.6 16 15.1 22.2 4.9 12 8.4 18.2 17.6 16.5 29.1

average volume fraction (%) ∼14 ∼18

∼8 ∼15

∼29

Note: μ = GI + GII, GPa; GII/μ is the volume fraction of looselybonded regions, %. a

regions of the Sc75Fe25 NG and several selected representative MGs measured by fast-spherical indentation tests. Compared with common MGs, the Sc75Fe25 NG contains a significantly higher volume of loosely bonded defective regions (∼29%). This suggests more plasticity channels for deformation under mechanical loading. This result provides a more compelling evidence than the interfaces regions, which can account for the good ductility of the Sc75Fe25 NG as compared with its chemically identical MG counterpart reported in previous studies.21,28 The nanoscale chemical and structural heterogeneities in the Sc75Fe25 NG make it to be a good candidate for microstructure and property modifications by thermal agitations via cycling between room-temperature (293 K) and cryogenic temperature of 77 K. In order to investigate the effect of cryogenic thermal cycling on the mechanical response of the Sc75Fe25 NG, we carried out in situ SEM compression tests on nanopillars

(1)

Here R denotes the indenter tip radius, ν the Poisson’s ratio, Ṗ the loading rate, the entropic modulus GII deriving from the possible configuration of the loosely bonded regions, GI the atomic bonding strength modulus of the densely bonded regions, tc the apparent relaxation time, tc = η(GI + GII)/ (GIGII), and η the viscosity of the dashpot. Therefore, the three unknown viscoelastic properties, i.e., GII GI and the apparent relaxation time tc, can be extracted by deriving the experimental C

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Nano Letters fabricated from the as-prepared and cryogenic thermal cycled Sc75Fe25 NG. Each test was performed on five pillars with top diameters of 330−430 nm and aspect ratios (height/diameter) of 1.8−3.0 to avoid data deviation. Figure 2 shows two

Table 2. YS (GPa) of All Ten Nanopillar Compression Tests on the As-Prepared and Cryogenic Thermal Cycled Sc75Fe25 NG YS

A

B

C

D

E

error bar

as-prepared 10-min hold +10 cycles

1.34 1.72

1.23 1.51

1.18 1.58

1.38 1.5

1.31 1.53

0.08 0.09

It should be noted that the measured pillar’s Young’s modulus from the microcompressions has a strong geometry dependence,43,44 and thus cannot precisely describe the accurate Young’s modulus of the Sc75Fe25 NG. As a result, we carried out numerous nanoindentation tests with a Berkovich tip to measure the material’s basic mechanical properties (see the detailed methods in the Supporting Information). The Young’s modulus and hardness were determined on the unloading curve at a full load by the standard Oliver and Pharr method. 29 The cumulative distributions of the Young’s modulus and hardness for the Sc75Fe25 NG were presented in Figure 3, corresponding to Figure 2. Representative engineering stress−strain curves for the nanopillar tests of the as-prepared and the cryogenic thermal cycled Sc75Fe25 NG.

representative engineering stress−strain curves for the nanopillar compression tests of the as-prepared and the cryogenic thermal cycled Sc75Fe25 NG. The middle area of the crosssection was used to calculate the stress, and the inset of the Figure 2 shows how the dimension of nanopillars was measured. The YS is indicated by the first sharp “burst” on the stress−strain curve, corresponding to a shear-banding onset. As shown in Figure 2, except the difference in the YS, the two curves show the same signature: elastic loading followed by a plastic yield, and as the plastic deformation proceeded, the stress continued to increase until the test was stopped at a selected displacement. Different from previous studies on conventional MGs as reported by Ketov et al,40 both the asprepared and treated Sc75Fe25 NG showed excellent ductility, i.e. both samples could be steadily deformed with a large height reduction exceeding 50%. Since once the compressive strain exceeds 50%, the sample will not be fractured along the primary shear plane, so we stopped the test at the selected displacement. In addition, we could also see obvious “pop-in” events on the two stress−strain curves. It is known that the strain “pop” or “burst” in the stress−strain curves of MGs corresponds to the process of shear-banding.41 A conventional MG forms only one major shear plane upon yielding,42 however, the as-prepared and thermal-cycled NG display a much more complex serrated flow behavior with many small serrations and few large ones, indicating a more stable shear stability due to the interaction of multiple shear bands. This behavior is not surprising since it had been confirmed previously by Wang et al.21,28 But, surprisingly, we also see that the thermal cycling of the Sc75Fe25 NG down to 77 K induced an apparent YS enhancement, as shown in Figure 2 and Table 2, which is different from that of conventional MGs.40 The other stress−strain curves were included in Figure S5 in the Supporting Information, and an analysis of all the data was presented in Table 2.

Figure 3. Cumulative distributions of (a) Young’s modulus and (b) the hardness of the Sc75Fe25 NG at the as-prepared state, quenching directly into liquid nitrogen (77 K) and holding for 1 day state, and cryogenically thermal cycled state.

three states: (1) as-prepared state without any further treatments, (2) directly quenching into liquid nitrogen (77 K) and holding for 1 day, and (3) thermal cycling between room-temperature and 77 K for 10 cycles, with a 10 min holding at 77 K first. It shows the median hardness value of 4.59 GPa for the as-prepared sample increases to 4.89 GPa (by 6.5% increase) after 10 room temperature−77 K cycles, but the increased hardness was accompanied by a decreased Young’s modulus by 8.2%, i.e., from 70.0 to 65.5 GPa. This result D

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Nano Letters furtherly confirmed our previous finding, that is, the cryogenic thermal cycling induces an increased YS but a decreased Young’s modulus in the Sc75Fe25 NG. The property changes of the Sc75Fe25 NG indicate that the cryogenically thermal cycling treatment should induce some changes in the glassy structure, but these changes are too subtle to be detectable with the conventional X-ray methods. So, the high-resolution synchrotron diffraction, which is able to extract the local atomic structure information from the scattering data,31,32 has been employed to study the subtle atomic rearrangement induced by the thermal agitation treatment. As shown in Figure 4a, generally, the synchrotron diffraction data

blue arrow) appeared, indicating the formation of more ordered atomic clusters. Furthermore, a Fourier transform of Q(S(Q)−1)33 was performed to obtain a detailed analysis of the short-range order (SRO) structure through the reduced atomic pair distribution function (PDF), which describes the atomic density distribution as a function of the interatomic distance. Figure 4b shows the first atomic shell abnormally expands after 10 thermal cycles, suggestion a more disordered local structure at SRO. While three of the four small peaks on the second atomic shell become much sharper, indicating a more ordered atomic structure at MRO, this result is in consistence with the structure factors analysis in Figure 4a. Furthermore, an analysis of the detailed first atomic shell in the inset of Figure 4b suggests an atomic reordering and a change of the interatomic bonding distance. The peak profile for the Sc−Sc atomic pair becomes sharper and shifts toward a larger interatomic distance, suggesting an increased atomic ordering and an increased interatomic distance of the Sc−Sc bond. Also, the Sc−Fe atomic pairs peak becomes relatively weaker after the cycling. In other words, the thermal cycling clearly induces the formation of more Sc−Sc pairs and less Sc−Fe pairs in the first nearest-neighbor shell. This change in the chemical SRO, which is newly detected, could result in the abnormal expansion of the first atomic shell, and we believe it is responsible for the decrease in the Young’s modulus of the Sc75Fe25 NG after the thermal cycling. All the evidence suggest that the thermal cycling of the asprepared Sc75Fe25 NG leads to some structural and property changes, and our study indicates that these changes are induced essentially by the cycling rather than the time spent at 77 K. This is due to the evidence that the sample held at 77 K for as long as 48 h did not give more obvious property changes as compared with the 10 thermal cycles (see Figure 3). To the best of our knowledge, the structural and property changes in the Sc75Fe25 NG are related to the atomic rearrangements induced by the nonaffine thermal expansion-contraction produced during the process of cryogenically thermal cycling due to the inherited nanoscale inhomogeneities of the Sc75Fe25 NG. The detail mechanism for the strength enhancement of NGs may involve the complex mechanical interaction due to the nonaffine plastic deformation at multiple scales. The first reason is the nanoscale structural heterogeneity in the as-prepared Sc75Fe25 NG. According to previous reports, NGs consist of two kinds of noncrystalline regions: glassy grains that have the atomic structure of MGs produced by fast quenching and the interface regions that interconnected the glassy grains; the interfaces were well characterized for their atomic and electronic structures.2,23,46,47 Refer to this proposed structure of nanoglasses,46,47 the atomic structure schematic of the Sc75Fe25 NG can be illustrated in Figure 5, where the structural backbones are the defective interfacial regions (light blue balls) which connect the glassy grains (deep yellow) together. Furthermore, it is generally accepted that the microstructural feature of MGs contains two parts: loosely packed regions embedded on the densely packed regions. Thus, according to the structure model, the glassy grain region in NGs should contain discrete sites with loosely bonded atoms (light cyan) embedding in the tightly bonded atomic matrix (deep yellow). On the basis of the above structural modeling, we constructed the schematic diagram in Figure 5. The positron annihilation spectroscopy measurements by Fang et al.

Figure 4. Comparison of (a) structure factors S(Q) and (b) PDF of the Sc75Fe25 NG at the as-prepared and 10 cycles between room temperature and 77 K with 10 min hold states. The inset of Figure 4a is an enlarged portion of S(Q) 2.2 < Q (Å−1) < 3.4 and the inset of Figure 4b is the enlarged of the first atomic shell.

of the Sc75Fe25 samples with and without thermal cycling show a characteristic of metallic liquids and glasses, with a first sharp diffraction peak, followed by a second peak and a shoulder (Figure 4a).45 However, a small shoulder was also visible on the detailed first peak (marked by a black arrow, see the inset of Figure 4a), which reflects the information on the medium range order (MRO) structure, suggesting the possible formation of a small number of atomic clusters in the as-prepared state. After the 10 cryogenic thermal cycles, this shoulder becomes stronger and sharper, and a new shoulder around 2.7 Å−1 (marked by a E

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Furthermore, it is well-known that the modulus is strongly dependent on the atomic bonding force, while the strength/ hardness is essentially controlled by the density and mobility of defect structures in materials under stresses (such as dislocation densities, grain sizes and defect configurations in crystalline materials). A well-known effect is indicated by the Hall−Petch equation, which indicates a shape increase of the YS without any changes of the Young’s modulus. As previous reports,40,51 the rejuvenation of the Zr- and Labased MGs induced by the cryogenic thermal cycling treatment can be attributed to the nonaffine thermal strain due to the intrinsic atomic-scale inhomogeneity in MGs. While viewing at a much larger nanoscale, these MGs could be considered as a relatively “more homogeneous” solid. Here, the samples we used for the thermal cycling are a new kind of MGs which contain both nanoscale glassy grains and glassy grain interfaces, as indicated in Figure 5. The cryogenic cycling treatment induces more complex atomic rearrangements in NGs than in conventional MGs, due to complex interactions between glassy grains and grain−grain interfaces. A carefully characterization of the nanostructural changes by the highly sensitive synchrotron irradiation on the Sc75Fe25 NG samples before and after the cryogenic treatments revealed the formation of more atomic clusters at the MRO (Figure 4a) and a more disordered local structure at the first atomic shell (Figure 4b). It is due to these clusters which increase the YS and hardness of the cryogenically treated Sc75Fe25 NG samples. Furthermore, a recent study, reporting that it was the MRO structure that dominates the plastic deformation of MGs at room temperature rather than the atomic pairs in the SRO.52,53 This provides a strong support to our conclusion. So, it makes sense that the increased interatomic distance in the first nearest neighbor caused a decreased Young’s modulus, meanwhile the formation of more atomic cluster in the MRO induced an increased YS and hardness. On the other hand, the larger scale heterogeneities between glassy grains and glassy interfaces would induce residual internal stresses during the cycling process in the Sc75Fe25 NG and the residual stresses so generated also would result in an increase of YS. All the analyses above suggest that the nondestructive cryogenically thermal cycling treatment is a simple but an effective and useful method to manipulate the structural and mechanical properties of metallic glassy materials with the heterogeneous nature, especially for NGs with more complex structural, compositional and electronic heterogeneities as compared with MGs. However, the effect of thermal agitations on the structural and property changes of metallic glassy materials show a great dependence on the starting material and its heterogeneous feature, it is worthy further investigation in the future. In summary, the thermal agitation is a useful way to manipulate the structure and property of the Sc75Fe25 NG with nanoscale structural and chemical heterogeneities. A thermal cycling of the Sc75Fe25 NG between 77 K and roomtemperature induced an abnormal property change, involving an increased YS and a decreased Young’s modulus. The property change suggests the occurrence of complex atomic rearrangements which were revealed by the high energy synchrotron diffraction. The structural analysis indicates that the cryogenic thermal agitation not only leads to the formation of atomic clusters at the MRO but also induces the abnormal expansion of the first atomic shell at the SRO. It was these new structure features that lead to the opposite change trend of the

Figure 5. Microstructural model of the Sc75Fe25 NGs

showed the volume fraction of the interfacial regions occupy as much as 35% throughout the volume.27 The second reason is the chemically inhomogeneous microstructure of the as-consolidated Sc75Fe25 NG as reported previously.27 The composition of about Sc85Fe15 was found in the interfacial region and the compositions ranging from Sc70Fe20N10 to Sc50Fe30N20 inside of the glassy clusters.27 Furthermore, the enhanced Sc concentration at the glass−glass interfaces furtherly changes the electronic structure of the Sc75Fe25 NG at the interfaces and hence the interatomic potentials at the interfaces.46,48 All these evidence together indicate a severe local variation of elastic modulus and thermal expansion coefficient in the Sc75Fe25 NG at the atomic scale. This atomic level fluctuation suggests that when the temperature imposed on the sample was changed, nonaffine thermal strain develops and the significant internal stresses so generated can cause irreversible local atomic rearrangements and thus property changes of the Sc75Fe25 NG. As a matter of fact, previously, Ketov and Greer et al.40 had exploited the nanoscale structural heterogeneity of MGs to modulate their atomic structure and mechanical property. It shows that the thermal cycling of a Zr- (Zr62Cu24Fe5Al9) based MG down to a cryogenic temperature repeatedly can rejuvenate its glassy structure to less relaxed states with a higher energy, leading to an improvement of plasticity and a simultaneous decrease of Young’s modulus and hardness. As compared to these MGs produced by liquid quenching, the Sc75Fe25 NG with nanoscale interface structures shows an anomalous mechanical response after the cryogenic thermal cycling, i.e., an evident decreased Young’s modulus but an apparent increased YS. Now let us discuss the possible physical origin of the opposite trend of the Young’s modulus and YS in the Sc75Fe25 NG. It should be noted that although a general impression is that the trend of the elastic modulus would go along with the magnitude of hardness,49 but this rule neither has an analytical support nor is generally obeyed by materials.50 For example, it is generally accepted that cold rolled metallic crystalline alloys with closed packed atoms generally result in an enhanced strength/hardness and an unchanged Young’s modulus. F

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Letter

Nano Letters

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magnitudes of Young’s modulus and YS. It is our belief that the nanoscale structural and chemical heterogeneities in the Sc75Fe25 NG lead to a locally dependent on the thermal expansion coefficient, which must induce a nonaffine thermal strain upon the thermal agitation thus atomic rearrangements. The results reported here clearly evidence that the cryogenic thermal cycling is a simple and useful way to modify the microstructure and property of metallic glassy materials for engineering applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b01007. Load−displacement curves, AFM images, loading curves, and engineering stress−strain curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(C.T. Liu) E-mail: [email protected]. ORCID

C. T. Liu: 0000-0001-7888-9725 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial supports by the General Research Fund of Hong Kong. Grants CityU 7004686 and CityU 8730038 are acknowledged. B. A. Sun expresses thanks for the support from the National Science Foundation of China (Grant Nos.51671121).

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REFERENCES

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DOI: 10.1021/acs.nanolett.8b01007 Nano Lett. XXXX, XXX, XXX−XXX