j m a t e r r e s t e c h n o l . 2 0 1 9;8(4):3603–3611 Available online at www.sciencedirect.com www.jmrt.com.br Original Article Bridging the local configurations and crystalline counterparts of bulk metallic glass by nanocalorimetry Bingge Zhao a,b , Bin Yang c , Javier Rodríguez-Viejo d , Mannan Wu a , Christoph Schick c,e , Qijie Zhai a , Yulai Gao a,b,∗ a Center for Advanced Solidification Technology (CAST), School of Materials Science and Engineering, Shanghai University, 99 Shangda Road, 200444 Shanghai, PR China b Laboratory for Microstructures, Institute of Materials, Shanghai University, 99 Shangda Road, 200444 Shanghai, PR China c Institute of Physics and Competence Centre CALOR, University of Rostock, Albert-Einstein-Street 23-24, Rostock 18051, Germany d Physics Department, Universitat Autoǹoma de Barcelona, Bellaterra 08193, Spain e Kazan Federal University, 18 Kremlyovskaya Street, Kazan 420008, Russian Federation a r t i c l e i n f o a b s t r a c t Article history: The structural understanding of crystallization in bulk metallic glasses (BMGs) has attracted Received 19 December 2018 much attention while rapid crystallization occurring under controllable conditions is less Accepted 30 May 2019 involved. In this study, a Ce68 Al10 Cu20 Co2 (at.%) BMG was thermally devitrified by differential Available online 10 June 2019 scanning calorimetry (DSC) and nanocalorimetry. At a heating rate of 10 K/min by DSC, AlCe3 and Ce are the major crystalline phases after devitrification while Al13 Co4 quasicrystals and Keywords: Ce are the dominant phases in the crystallization products at a heating rate of 5000 K/s by Nanocalorimetry nanocalorimetry. Attributing to the covalent-like bond in Al–Co atom pairs, Al13 Co4 qua- Nucleation sicrystals precipitate in the primary crystallization and work as the precursors associating Crystal growth local atomic configurations in the glassy state with crystalline phases after crystallization. Metallic glass Attributing to the enhanced mobility of Cu atoms, compositional redistribution occurs in Quasicrystal the as-cast sample. On nanocalorimetry heating, an unambiguous discrepancy in the nucle- ation and growth of the nano-sized Al13 Co4 quasicrystals is thus triggered, contributing to an obvious difference in the crystal size. This research unveils the distinct crystallization behaviors of Ce-based BMG on rapid heating. The formation of quasicrystals demonstrates the multi-stage crystallization on rapid heating and bridges the structural gap between local atomic configurations of metallic glasses and crystalline phases. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction Since the first preparation of bulk metallic glasses (BMGs) by Duwez [1], tremendous attention from both the fun- ∗ Corresponding author. damental and practical disciplines has been paid to this E-mail: ylgao@shu.edu.cn (Y. Gao). area attributing to their intriguing structural and mechanical https://doi.org/10.1016/j.jmrt.2019.05.023 2238-7854/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). 3604 j m a t e r r e s t e c h n o l . 2 0 1 9;8(4):3603–3611 properties [2–5]. According to the classical nucleation the- Differential fast scanning calorimetry (DFSC), a nanocalori- ory (CNT), atom motions slow down and enthalpy decreases metric instrument designed by Schick [32], achieves in situ rapidly upon quenching the metallic melt below its melting heating and quenching at rates up to 105 K/s, and it has temperature. With different undercooling, various nucleation been widely employed to reveal the rapid phase transitions and growth behaviors are expected, changing the final struc- of metals [20,26,27,33–36]. Although high-temperature DFSC ture and mechanical properties [6–11]. If crystallization on sensor became available recently [25], alloys with low crys- cooling is totally suppressed, the undercooled liquid falls out tallization temperature are still preferred to guarantee the of equilibrium, and therefore an amorphous state is pre- full crystallization in studying the rapid crystallization of bulk served. Compared with crystalline phases, an excess Gibbs metallic glasses. Ce-based BMG is deemed as an ideal can- free energy is trapped in the metallic glass. Driven by such didate because of its high glass forming ability (GFA), low excess energy, BMG undergoes glass transition and crystal- crystallization temperature, and good stability [37–39]. In this lization at elevated temperatures. Up to now, the kinetics paper, Ce68 Al10 Cu20 Co2 (at.%) BMG samples were crystallized and structure evolution on crystallization in various BMGs by DSC and DFSC respectively, revealing that the crystal- have been synthetically demonstrated by thermal analy- lization products are evidently affected by heating rate. On sis and structural characterization [12,13]. However, most of DSC heating at 10 K/min, Ce and AlCe3 with a small frac- these studies were performed under near-equilibrium con- tion of Al5 Co2 were detected after crystallization. Whereas at ditions. For example, the heating rates of the conventional the heating rate of 5000 K/s by DFSC, Al13 Co4 with the qua- differential scanning calorimetry (DSC) are usually centered sicrystalline structure, which precipitates in the early stage at tens of K/min [14]. In view of the pronounced rela- of crystallization, was captured. Based on the diffusion of Co tionship between solidification behavior and cooling rate under these two conditions, the whole crystallization trajec- [15], it is extremely attractive to comparatively figure out tory was depicted. According to the atom pairs in this BMG what is able to happen on rapid heating. However, due and Gibbs free energy in Al–Co binary system, the role of to the deficiency of conventional DSC, a novel calorimetry Co element in the formation and evolution of Al–Co inter- that can provide enhanced heating capability is vigorously metallic compounds was determined. As a result, primary needed. With the development of microelectromechanical and secondary crystallization were proposed during the rapid systems (MEMS) technology, nanocalorimetry, which was ini- devitrification of Ce68 Al10 Cu20 Co2 BMG. Cu atoms, on the other tially designed by Allen and Hellman et al., provides a hand, have higher mobility and can diffuse even at room tem- solution to this problem attributing to its ultrafast heating perature, leading to a composition fluctuation in the as-cast rate as high as 106 K/s [16,17]. Furthermore, the sensitiv- sample. Both the nucleation and growth of Al13 Co4 quasicrys- ity of nanocalorimetry is substantially improved, making it tals on following heating are influenced by the diffusion of Cu, capable of capturing weak thermal signals and revealing causing the discrepancy in quasicrystal sizes. phase transitions in nanoscale systems [18–20]. On rapid heating, it has been demonstrated that both crystallization kinetics and structure evolutions are possible to change. 2. Methods For example, a curved Kissinger plot depicting the crystal- lization kinetics of Ge2 Sb2 Te5 (at.%) was obtained by Greer, Ce68 Al10 Cu20 Co2 (at.%) master alloy was prepared by arc melt- confirming the growth-controlled crystallization on heating ing pure Ce, Al, Cu and Co in high-purity argon atmosphere. and a decoupling between crystal growth and viscous flow The purity of Ce is about 99.5 wt.% while other metals have in the deeply undercooled liquid [21]. This phenomenon is purity at least 99.9 wt.%. Then the ingot was melted and deemed to occur in many other fragile phase-change materials quenched by suction casting to get a cylindrical rod with (PCMs) [22,23]. In terms of metallic glasses, Vlassak introduced a diameter of 3 mm. The amorphous structure was exam- the nanocalorimetry to the rapid crystallization of Cu50 Zr50 ined by X-ray diffraction (XRD, D/Max-2200, Rigaku) with Cu (at.%) thin films, where the decoupling and crystallization K˛ radiation ( = 0.154056 nm) at a scanning rate of 4◦ /min. asymmetry on heating and cooling were demonstrated as High-resolution transmission electron microscopy (HRTEM, well [24]. Yang further identified the decoupling temperature JEM-2010F, JEOL) was used to further certify the amorphous in an Al-based metallic glass by heating at rates span- structure. Using plastic abrasive paper, micro-sized samples ning over six orders of magnitude [25]. With the help of were ground from the as-cast rod and then loaded onto nanocalorimetry, our previous studies realized in situ prepa- a copper mesh. The wedge-shaped edge of these samples ration of metallic glass and made it possible to tune the is sufficiently thin for HRTEM observation. Glass transition microstructure of metallic glass, and the structural effect temperature (Tg ), crystallization temperature (Tx ) and melt- in determining the crystallization behaviors were revealed ing temperature (Tm ) were determined by DSC (Diamond, [26,27]. Another remarkable issue about the rapid crystal- PerkinElmer Instruments) at heating rates of 5, 10, 20, 40, and lization by nanocalorimetry is the structure evolution. Some 80 K/min. equilibrium phases can be suppressed while some other new To identify crystallization products, one BMG sample was metastable phases can form under extreme non-equilibrium fully crystallized by heating it from 323 K to 573 K (before Tm , condition [28–30]. If the heating rate is further increased, crys- without melting) at 10 K/min using DSC, which was immedi- tallization may be totally suppressed [9,31]. Based on those ately followed by free cooling. Crystallization products were scientific outcomes, it is expected that nanocalorimetry opens then examined by XRD at a scanning rate of 4◦ /min. new avenues to rapid crystallization under extreme condi- As for DFSC measurement, XI 39395 sensor (Xensor Inte- tions. gration, Netherlands) with a measuring area of 60 ␮m × 70 ␮m j m a t e r r e s t e c h n o l . 2 0 1 9;8(4):3603–3611 3605 Fig. 1 – Characterization on the as-cast sample. (a) XRD pattern of the as-cast Ce68 Al10 Cu20 Co2 rod. The broad peak indicates the glassy nature of the sample. Insets are HRTEM image and corresponding SAED pattern, further indicating the amorphous state. (b) DSC curves of the as-cast samples. A dominating endothermic peak rather than the glass transition step is detected because of the enthalpy rejuvenation. was used. A small amount of silicon oil was spread on the For Ce68 Al10 Cu20 Co2 BMG, its Tg is close to room temperature, sensor to improve the thermal contact between sample and and enthalpy relaxation is possible to occur even just after suc- membrane. Micro-sized samples were cut from the as-cast tion casting. When the as-cast sample is heated, the relaxed rod. A sample having an estimated size of 20 ␮m was picked enthalpy rejuvenates, and an endothermic peak rather than a up using a copper wire and then positioned in the center of step change is detected at the glass transition, as indicated in the measuring area with the help of optical microscopy (Stemi Fig. 1b. 2000, Carl Zeiss). Then it was heated from 320 K to 621 K to By DSC heating from 320 K to 621 K at 10 K/min, the as-cast get full crystallization without melting, which was followed sample was fully crystallized. Both the DSC curve and cor- by quenching at 5000 K/s down to room temperature. After responding XRD pattern are shown in Fig. 2. The dominant quenching, this crystallized sample was thinned by focused phases are Ce and AlCe3 , which agrees well with the results in ion beam (FIB, Helios NanoLab 600i, FEI). Its morphology, Ref. [41]. In addition, Al5 Co2 is identified here after the crystal- structure and composition were then characterized by HRTEM lization by DSC. According to the Scherrer equation [42], the equipped with energy dispersive spectroscopy (EDS, Oxford). mean size of the crystals is roughly estimated to be 19 nm. For comparison, another as-cast sample was machined by FIB The lower heating rate by DSC guarantees the full crys- and characterized by HRTEM to show the structure after cast- tallization while DFSC can expose the initial crystallization ing. In addition to the TEM bright-field images, high angle stage by the ultrafast heating rate. Fig. 3a shows the DFSC annular dark field-scanning transmission electron microscopy heating curve at 5000 K/s. In contrast to the DSC traces in (HAADF-STEM) images were captured to distinguish the com- Fig. 1b, two endothermic peaks near Tg are detected in the position fluctuation both in the as-cast and as-crystallized sample. 3. Results and discussion The absence of sharp diffraction peaks in the XRD pat- tern (Fig. 1a) confirms the amorphous nature of the as-cast Ce68 Al10 Cu20 Co2 sample. HRTEM image and selected area electron diffraction (SAED) pattern further verify the amor- phous state. Fig. 1b represents DSC heating curves at different rates. Both glass transition and sharp crystallization peak are observed, confirming the XRD and HRTEM results in Fig. 1a. Tg , Tx and Tm are 358 K, 430 K and 641 K respectively at the heating rate of 5 K/min. They are considered as the reference temperatures for the following calorimetric measurements. Using Kissinger model, the activation energy of crystallization (Ea,c ) is estimated as 239.4 kJ/mol. This large value suggests Fig. 2 – XRD pattern of Ce68 Al10 Cu20 Co2 sample after that Ce68 Al10 Cu20 Co2 has a higher thermal stability against heating at 10 K/min. AlCe3 and Ce are the principal crystals. crystallization in Ce-based metallic glasses, agreeing with its The inset shows the DSC heating curve for full higher glass forming ability than those CeAlCu BMGs [38,40]. crystallization. 3606 j m a t e r r e s t e c h n o l . 2 0 1 9;8(4):3603–3611 Fig. 3 – Nanocalorimetry curves indicating the glass transition and crystallization of the as-cast sample. (a) DFSC heating curve at 5000 K/s. The glass transition and crystallization are patterned in green and blue respectively. (b) Temperature profile for step scanning. The temperature step between two cycles is 20 K with heating and cooling rates of 2000 K/s and 4000 K/s, respectively. (c) Heating curves of the step scanning on one sample. When the sample is scanned below Tg (Curves 1 and 2), there are no visible phase transitions. One reversible endothermic step corresponding to glass transition is detected when the sample is scanned between amorphous solid and undercooled liquid. Since the thermal history can be erased at temperature higher than Tg , the double glass transition in the as-cast sample is associated with room-temperature aging and corresponding structural heterogeneity caused by atom rearrangement. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) DFSC heating curve. To exclude the effect of silicone oil, sim- noteworthy that the thermal history including structure relax- ilar amount of silicone oil was spread on the sensor and ation can be erased when the metallic glass is heated to enter was scanned using the same nanocalorimetric profile. No the undercooled liquid region. If the phase separation occurs peaks corresponding to phase transitions of silicone oil are in the as-cast sample, the double glass transition should detected (Fig. S1), suggesting that the distinct behavior of always appear in the cycles without crystallization, which glass transition and crystallization in DFSC measurement disagrees with the nanocalorimetry curves shown in Fig. 3b. comes from Ce68 Al10 Cu20 Co2 . In addition, analogous results This contradictory case implies that the dual peaks in glass are observed in the nanocalorimetric measurement at 1000 K/s transition are related to the structure heterogeneity formed and 10,000 K/s (Fig. S2). A step scanning was performed on during the room-temperature aging after the suction cast- another sample to determine the dual endothermic peaks. ing of the alloy, and its mechanism will be discussed later in The temperature profile is displayed in Fig. 3b: the sample this paper. The absence of the dual endothermic peaks in the is heated at 2000 K/s from room temperature to 393 K. After DSC curves may be due to the limited sensitivity of this con- holding for 0.06 s, it is quenched down to room temperature ventional calorimetric method. As the temperature increases at 4000 K/s. Then the cycle with temperature step of 20 K is further (Curve 8), an exothermic event associated with the repeated until the final temperature reaches 553 K. The results crystallization of the undercooled liquid is detected. The next are shown in Fig. 3c. When the sample is cycled below Tg , there reheating curve (Curve 9) shows no traces of a glass transition, are no obvious peaks (Curves 1 and 2). But as the maximum indicating the crystalline nature of the sample before the tem- temperature on heating is above Tg (Curves 3–7), an endother- perature scan. The overall crystallization process consists of mic step rather than dual peaks is repeatedly observed, which two exothermic peaks, which will be demonstrated based on is the reversible characteristic of glass transition [43]. It is the structure characterization. j m a t e r r e s t e c h n o l . 2 0 1 9;8(4):3603–3611 3607 Fig. 4 – Structure characterization on Ce68 Al10 Cu20 Co2 after rapid crystallization by DFSC. (a) The bright-field TEM image showing the bright (A) and dark (B) contrasts in the sample. The contrast suggests the different crystallographic orientations. (b) HRTEM image showing the interface between area A and B. Apparent size variations are observed. (c) The residual amorphous phase after rapid crystallization. (d) HRTEM image showing Ce and Al13 Co4 . They are the main ordered phases after rapid crystallization. (e and f) The HRTEM images corresponding to region A and B respectively. Al13 Co4 are confirmed in both regions with an obvious size variation. Insets are the FFT patterns corresponding to the HRTEM image. TEM bright-field images of the sample that was crystal- conditions, and that is why a comparison between nucle- lized at 5000 K/s are displayed in Fig. 4. Regions with different ation rate and growth velocity is important [47]. To determine contrasts, marked as A and B respectively, are clearly dis- the relationship between crystal size and heating rate exper- tinguished (Fig. 4a), indicating variations in crystallographic imentally, further explorations are still needed. Apart from orientations after rapid crystallization. In the high-resolution the ordered phases, a few amorphous phases were still images, both regions contain a large number of nanocrystals retained after rapid crystallization (Fig. 4c). Theoretically, of different sizes, as displayed in Fig. 4b. In area A, most of crystallization can be suppressed by ultrahigh heating rate, the crystals are less than 5 nm while in area B, the crystals where amorphous structure can survive up to the melt- are several tens of nanometers. These nanocrystals suggest ing point [48,49]. Experimentally, this has been verified an extremely high nucleation rate but a slow growth velocity in Zr41 Ti14 Cu12 Ni10 Be23 (at.%) and Au49 Ag5.5 Pd2.3 Cu26.9 Si16.3 during crystallization, agreeing well with the growth-limited (at.%) metallic glasses [6,9]. This scenario of the mixed crystallization on heating [9,24]. As mentioned above, the crystalline-glassy structure highlights the crucial role of crys- mean size of the crystals after DSC heating is about 19 nm, tal growth in the formation of amorphous structure, as stated not far from those measured after fast heating. In contrast, by Greer [5]. In current case, ordered clusters nucleate on Tkatch found a reduction of crystal size from 320 nm to 48 nm previous cooling (suction casting). Then they grow on fast as the heating rate increased from 10 K/min to 104 K/s in FeB heating, where the growth rate is too sluggish to crystal- metallic glass [44]. While in the CuZrAl bulk metallic glass, the lize the whole sample, leaving the residual amorphous phase crystalline phase produced at 250 K/s is about 1.5 ␮m which is between different grains. This can be further demonstrated by two times smaller than that obtained at 750 K/s [45]. These two the nano-sized crystals in Fig. 4. studies contradict current results, implying the crystallization By the fast Fourier transform (FFT) patterns, the ordered of Ce68 Al10 Cu20 Co2 is insensitive to the heating rate. Based on phases observed in Fig. 4d are consistent with Ce (ICDD-JCPDS Farjas’s research [46], crystal size on crystallization is essen- card No. 65-3368) and Al13 Co4 (ICDD-JCPDS card No. 65-1165) tially dependent on the ratio between activation energies respectively. This result differs from the crystallization prod- of nucleation and growth rather than external experimental ucts at 10 K/min (Fig. 2). Furthermore, it is interesting to note 3608 j m a t e r r e s t e c h n o l . 2 0 1 9;8(4):3603–3611 that Al13 Co4 possesses an orthorhombic structure belonging should precipitate from the amorphous matrix because of its to Pmn21 space group, and this phase has been confirmed as a lower Gibbs free energy. However, in contrast to Al5 Co2 crys- quasicrystal [50,51]. Since Al13 Co4 phase exhibits 10-fold sym- tals with hexagonal structure, Al13 Co4 is a quasicrystalline metry in (1 0 0) plane [50], the typical diffraction pattern of a phase that is structurally similar with the local atomic con- quasicrystal is not recognized in the FFT pattern (Fig. 4e and figurations units of the metallic glass. As mentioned above, it f). The formation of Al13 Co4 quasicrystals can be illustrated as is the decreased nucleation barrier resulting from the struc- follows. In CeAlCuCo BMGs, Al-centered icosahedrons, which tural similarity that contributes to the precipitation of Al13 Co4 presumably form on quenching, are the predominant struc- rather than Al5 Co2 . Following the nucleation, quasicrystal tural building blocks which constitute the essential units [52]. growth occurs on heating, in which the diffusion of Co is con- These local units are far below the critical size of a nucleus sidered as the essential factor. As indicated by the free energy and cannot work as the heterogeneous sites like macroscopic change in Fig. 5, metastable Al13 Co4 quasicrystals evolve to impurities. As recognized, nucleation barrier arises from the stable crystalline Al5 Co2 phases with a sufficient diffusion of difference between local structure of the undercooled liquid Co at elevated temperatures. In summary, Al13 Co4 quasicrys- and the ordered crystals. Compared with a crystalline phase, tals can overcome the energy barrier and precipitate as the there is a significant structural similarity between local icosa- intermediate phase between local icosahedron structure and hedron configurations and quasicrystalline phases. Although equilibrium crystals, which corresponds to the statement on these local icosahedrons are very small, a decreased surface evolution of quasicrystals. energy and nucleation barrier are generated [53], facilitating Based on the step scanning measurements, relaxation is the formation of quasicrystals. This argument agrees with confirmed in this metallic glass, which has an impact on the consideration that the topographical short-range order the crystallization. As evidenced by a similar Ce68 Al10 Cu20 Fe2 dominates the nucleation of quasicrystals [54]. It has been metallic glass [39], crystallization mechanism can be changed demonstrated that the sp electrons of Al atoms can hybridize with relaxation, where multiple crystallization peaks are with the 3d electrons of late transition metals (such as Co, detected consequently. However, the crystallization products Ni, Fe) [38], which shortens the bond length and produces are the same in that study, implying the negligible effect of the covalent-like bond. According to Holland-Moritz’s study, relaxation on final phase formation. In the present experi- the bond length of Al–Co is only 2.535 Å [54] which is smaller ment, however, the crystallization products are significantly than Al–Cu (2.74 Å) and Al–Ce bond (3.356 Å) [55]. This situation distinguished, suggesting that the split of the crystallization suggests a high priority to form Al–Co compounds during the peak is caused by both the relaxation and the ultrahigh heating crystallization of Ce68 Al10 Cu20 Co2 . According to Al–Co binary rate. Since the composition of quasicrystals is totally differ- phase diagram [56], there are several kinds of Al–Co inter- ent from the amorphous matrix, a primary crystallization metallic compounds. Based on Sudavtsova’s study [57], Fig. 5 is deemed to occur, which is evidenced by the first crystal- schematically plots the dependence of Gibbs free energy on lization peak on DFSC heating curve. Furthermore, several Co content at the onset temperature of crystallization (500 K). kinds of ordered phases including some Ce-based crystals are In the light of the thermodynamics, Al5 Co2 , a stable phase, detected after crystallization, implying a complicated crys- tallization behavior in this multicomponent metallic glass. A secondary crystallization is deemed to occur in this alloy, cor- responding to the second crystallization peak in DFSC heating curve. On DSC heating (inset of Fig. 2), several minor exother- mal peaks after the main crystallization are detected while they are absent on DFSC heating (Fig. 3a). Consistent with this argument, Al5 Co2 that is identified on DSC heating is not observed in DFSC measurements. Since the quasicrys- talline phase, A113 Co4 , is thermodynamically metastable, it can evolve into stable Al5 Co2 by reaction with undercooled liquid at elevated temperatures. On the ultrafast heating by DFSC, this transition may shift to temperature higher than 621 K, which is beyond the crystallization temperature range. In other words, the evolution from Al13 Co4 to Al5 Co2 is limited, leaving some undercooled liquid, corresponding to the mixed glassy-crystalline structure shown in Fig. 4. In addition to the phase variations on DSC and DFSC Fig. 5 – Schematic depicting the evolution of Gibbs free crystallization, Al13 Co4 quasicrystals exhibit apparent size energy versus reaction coordinate on crystallization. discrepancy after rapid crystallization, as demonstrated in Al13 Co4 quasicrystals preferentially precipitate attributing Fig. 4b. To shed light on the mechanism behind this phe- to their structural similarity with Al-centered icosahedron nomenon, HAADF-STEM mode in addition to bright-field in BMG although Al5 Co2 is more thermodynamically stable. image was employed to determine the Z-contrast regions, as At elevated temperatures, metastable quasicrystals react pictured in Fig. 6a. Using EDS, composition fluctuations of with undercooled liquid to form stable crystals. The free Cu and Al are detected while Ce and Co are uniformly dis- energy of Al13 Co4 and Al5 Co2 is calculated based on Ref. tributed after rapid crystallization [58]. Associated with the [57]. TEM image shown in Fig. 4a, area A with refined nanocrystals j m a t e r r e s t e c h n o l . 2 0 1 9;8(4):3603–3611 3609 Fig. 6 – HAADF-STEM images of the (a) as-crystallized and (b) as-cast sample. Both Cu and Al segregate after crystallization, resulting in the contrasts in (a). In (b), only the segregation of Cu is detected while other elements are uniformly distributed. Insets in (b) are the high-resolution images for various areas, showing the amorphous structure. is rich in Al element while Cu element is abundant in area B with coarse grains. As a comparison, Fig. 6b demonstrates the 4. Conclusions HAADF-STEM image of the as-cast BMG sample in which dif- Rapid crystallization of Ce68 Al10 Cu20 Co2 BMG was realized by ferent contrasts are observed as well (more TEM images can nanocalorimetry. When heated at 5000 K/s, the sample shows be found in Fig. S3). In contrast to the as-crystallized sam- an altered crystallization behavior as compared with that crys- ple, only a fluctuation of Cu element is observed in the fully tallized at 10 K/min by conventional DSC, which digs out the amorphous regions. Ce68 Al10 Cu20 Co2 BMG has a Tg close to initial stage of crystallization and depicts the trajectory of room temperature, and it is therefore convincing that atoms devitrification. begin to rearrange and diffuse just after casting [37], presum- Multi-stage crystallization starting with the precipitation ably resulting in the composition discrepancy. Consequently, of Al13 Co4 quasicrystals proceeds in Ce68 Al10 Cu20 Co2 BMG. both the nucleation and growth, which are central to the crys- Working as the precursor on devitrification, this metastable tallization, are manifestly influenced on the following heating, phase bridges the structural gap between local atomic con- and various crystallization behaviors are expected in different figurations in metallic glass and crystallization products. Co regions. For Ce68 Al10 Cu20 Co2 system, Ce works as the matrix element plays the essential role in the nucleation of quasicrys- while Al and Cu are the main solute elements. Compared with tals attributing to its covalent-like bond with Al. Al, Cu atoms have a smaller radius, and a particular mobility Attributing to the particular mobility of Cu atoms, a compo- is therefore achieved [59–61]. After casting and being stored at sition redistribution is detected in the as-cast Ce68 Al10 Cu20 Co2 room temperature, Cu atoms can diffuse in the sample, induc- BMG, which causes distinct crystallization behaviors in the ing the composition heterogeneity in the amorphous matrix. different regions on the following heating. The enrichment For the atom pairs in Ce68 Al10 Cu20 Co2 BMG, only Cu and Co of Cu destabilizes the undercooled melt and promotes the displays a positive enthalpy of mixing (6 kJ/mol [61]). Accord- nucleation and growth of Al13 Co4 quasicrystals on crystalliza- ing to Inoue’s criteria [62,63], this positive mixing enthalpy can tion, resulting in the coarse crystals compared with Cu-poor reduce the glass forming ability and deteriorate the stability regions. of the undercooled melt. It is therefore easier for the nucle- The application of DFSC provides new insights into the ation to occur in the Cu-rich region. Although some external devitrification of bulk metallic glasses, making it feasible to factors such as heating rate [9] and pressure [64] can affect the control and quantify phase transformations under extremely crystallization behavior, the nucleation and growth rates are non-equilibrium conditions. the intrinsic factors controlling the crystallization of metal- lic glass. For the particular case of quasicrystals, chemical short-rang order is especially important in their growth [65]. Conflicts of interest In Cu-rich region, the repulsive force between Co and Cu is larger, and an additional energy for Al–Co pair is thus pro- The authors declare no conflicts of interest. duced as compared with Cu-poor zones. In other words, the growth for Al–Co is accelerated by Cu element in the Cu-rich Acknowledgements region. It is therefore proposed that both the facilitated nucle- ation and enhanced growth contribute to the coarse crystals This work is supported by the National Natural Science Foun- in the Cu-rich area. dation of China (Grant Nos. 51671123, 51171105 and 50971086), 3610 j m a t e r r e s t e c h n o l . 2 0 1 9;8(4):3603–3611 the Program for Professor of Special Appointment (Eastern [16] Minakov AA, Schick C. Ultrafast thermal processing and Scholar) at Shanghai Institutions of Higher Learning (Grant nanocalorimetry at heating and cooling rates up to 1 MK/s. No. TP2014042). BGZ thanks the support by China Postdoctoral Rev Sci Instrum 2007;78:073902. [17] Gao Y, Zhao B, Vlassak JJ, Schick C. Nanocalorimetry: door Science Foundation (Grant No. 2018M640376). CS acknowledge opened for in situ material characterization under extreme financial support from the Ministry of Education and Science non-equilibrium conditions. Prog Mater Sci 2019;104:53–137. of the Russian Federation (Grant No. 14.Y26.31.0019). [18] Lai SL, Ramanath G, Allen LH, Infante P. Heat capacity measurements of Sn nanostructures using a thin-film differential scanning calorimeter with 0.2 nJ sensitivity. Appl Appendix A. Supplementary data Phys Lett 1997;70:43–5. [19] Lee D, Vlassak JJ. Diffusion kinetics in binary CuZr and NiZr alloys in the super-cooled liquid and glass states studied by Supplementary data associated with this article can be found, nanocalorimetry. Scr Mater 2019;165:73–7. in the online version, at doi:10.1016/j.jmrt.2019.05.023. [20] Li L, Yang B, Zhao B, Abyzov AS, Schmelzer JWP, Schick C, et al. Rapid solidification behavior of nano-sized Sn droplets embedded in the Al matrix by nanocalorimetry. Mater Res references Express 2014;1:045012. [21] Orava J, Greer AL, Gholipour B, Hewak DW, Smith CE. Characterization of supercooled liquid Ge2 Sb2 Te5 and its [1] Jun WK, Willens RH, Duwez P. Non-crystalline structure in crystallization by ultrafast-heating calorimetry. Nat Mater solidified gold–silicon alloys. Nature 1960;187:869–70. 2012;11:279–83. [2] Kumar G, Desai A, Schroers J. Bulk metallic glass: the smaller [22] Chen B, Momand J, Vermeulen PA, Kooi BJ. Crystallization the better. Adv Mater 2011;23:461–76. kinetics of supercooled liquid Ge–Sb based on ultrafast [3] Matias TB, Asato GH, Ramasco BT, Botta WJ, Kiminami CS, calorimetry. Cryst Growth Des 2016;16:242–8. Bolfarini C. Processing and characterization of amorphous [23] Weber H, Orava J, Kaban I, Pries J, Greer AL. Correlating magnesium based alloy for application in biomedical ultrafast calorimetry, viscosity, and structural measurements implants. J Mater Res Technol 2014;3:203–9. in liquid GeTe and Ge15 Te85 . Phys Rev Mater 2018;2:093405. [4] Tantavisut S, Lohwongwatana B, Khamkongkaeo A, [24] Lee D, Zhao B, Perim E, Zhang H, Gong P, Gao Y, et al. Tanavalee A, Tangpornprasert P, Ittiravivong P. The novel Crystallization behavior upon heating and cooling in toxic free titanium-based amorphous alloy for biomedical Cu50 Zr50 metallic glass thin films. Acta Mater 2016;121:68–77. application. J Mater Res Technol 2018;7:248–53. [25] Yang B, Schmelzer JWP, Zhao B, Gao Y, Schick C. Glass [5] Greer AL. New horizons for glass formation and stability. Nat transition and primary crystallization of Al86 Ni6 Y4.5 Co2 La1.5 Mater 2015;14:542–6. metallic glass at heating rates spanning over six orders of [6] Schroers J, Masuhr A, Johnson WL, Busch R. Pronounced magnitude. Scr Mater 2019;162:146–50. asymmetry in the crystallization behavior during constant [26] Zhao B, Yang B, Abyzov AS, Schmelzer JWP, Rodríguez-Viejo J, heating and cooling of a bulk metallic glass-forming liquid. Zhai Q, et al. Beating homogeneous nucleation and tuning Phys Rev B 1999;60:11855–8. atomic ordering in glass-forming metals by nanocalorimetry. [7] Chen Z, Zhang Y, Wang S, Zhang JY, Tao Q, Zhang P. Nano Lett 2017;17:7751–60. Microstructure and mechanical properties of undercooled [27] Zhao B, Li L, Zhai Q, Gao Y. Formation of amorphous Fe80 C5 Si10 B5 eutectic alloy. J Alloys Compd 2018;747:846–53. structure in Sn3.5Ag droplet by in situ fast scanning [8] Schroers J, Wu Y, Busch R, Johnson WL. Transition from calorimetry controllable quenching. Appl Phys Lett nucleation controlled to growth controlled crystallization in 2013;103:131913. Pd43 Ni10 Cu27 P20 melts. Acta Mater 2001;49:2773–81. [28] Swaminathan P, Grapes MD, Woll K, Barron SC, LaVan DA, [9] Pogatscher S, Uggowitzer PJ, Löffler JF. In-situ probing of Weihs TP. Studying exothermic reactions in the Ni-Al system metallic glass formation and crystallization upon heating at rapid heating rates using a nanocalorimeter. J Appl Phys and cooling via fast differential scanning calorimetry. Appl 2013;113:143509. Phys Lett 2014;104:251908. [29] Molina-Ruiz M, Lopeandía AF, González-Silveira M, Anahory [10] Wang S, Chen Z, Feng LC, Liu YY, Zhang P, He YZ, et al. Y, Guihard M, Garcia G, et al. Formation of Pd2 Si on Nano-phase formation accompanying phase separation in single-crystalline Si (1 0 0) at ultrafast heating rates: an in-situ undercooled CoCrCuFeNi-3 at.% Sn high entropy alloy. Mater analysis by nanocalorimetry. Appl Phys Lett 2013;102:143111. Charact 2018;144:516–21. [30] Pogatscher S, Leutenegger D, Schawe JEK, Uggowitzer PJ, [11] Simon C, Gao J, Mao Y, Wilde G. Fast scanning calorimetric Löffler JF. Solid–solid phase transitions via melting in metals. study of nucleation rates and nucleation transitions of Nat Commun 2016;7:11113. Au–Sn alloys. Scr Mater 2017;139:13–6. [31] Johnson WL, Kaltenboeck G, Demetriou MD, Schramm JP, Liu [12] Kündig AA, Ohnuma M, Ohkubo T, Hono K. Early X, Samwer K, et al. Beating crystallization in glass-forming crystallization stages in a Zr–Cu–Ni–Al–Ti metallic glass. metals by millisecond heating and processing. Science Acta Mater 2005;53:2091–9. 2011;332:828–33. [13] Pauly S, Gorantla S, Wang G, Kühn U, Eckert J. [32] Zhuravlev E, Schick C. Fast scanning power compensated Transformation-mediated ductility in CuZr-based bulk differential scanning nano-calorimeter: 1. The device. metallic glasses. Nat Mater 2010;9:473–7. Thermochim Acta 2010;505:1–13. [14] Schick C, Lexa D, Leibowitz L. Differential scanning [33] Zhao B, Li L, Lu F, Zhai Q, Yang B, Schick C, et al. Phase calorimetry and differential thermal analysis. In: Kaufmann transitions and nucleation mechanisms in metals studied by EN, editor. Characterization of materials. Hoboken, NJ, USA: nanocalorimetry: a review. Thermochim Acta 2015;603:2–23. John Wiley & Sons, Inc.; 2012. p. 483–94. [34] Gao Y, Zhuravlev E, Zou C, Yang B, Zhai Q, Schick C. [15] Kurz W, Giovanola B, Trivedi R. Theory of microstructural Calorimetric measurements of undercooling in single development during rapid solidification. Acta Metall micron sized SnAgCu particles in a wide range of cooling 1986;34:823–30. rates. Thermochim Acta 2009;482:1–7. j m a t e r r e s t e c h n o l . 2 0 1 9;8(4):3603–3611 3611 [35] Wu W, Zhang Q, Zhao B, Zhang L, Zhai Q, Gao Y. [51] Kandaskalov D, Fournée V, Ledieu J, Gaudry É. Adsorption Solidification behavior of indium droplets embedded in properties of the o-Al13 Co4 (1 0 0) surface toward molecules aluminum by differential fast scanning calorimetry. J Therm involved in the semihydrogenation of acetylene. J Phys Anal Calorim 2018;135:2995–3003. Chem C 2014;118:23032–41. [36] Yang B, Perepezko JH, Schmelzer JWP, Gao Y, Schick C. [52] Xi X, Li LL, Zhang B, Wang W, Wu Y. Correlation of atomic Dependence of crystal nucleation on prior liquid overheating cluster symmetry and glass-forming ability of metallic glass. by differential fast scanning calorimeter. J Chem Phys Phys Rev Lett 2007;99:095501. 2014;140:104513. [53] Cheng YQ, Ma E. Atomic-level structure and [37] Zhang B, Zhao DQ, Pan MX, Wang WH, Greer AL. Amorphous structure-property relationship in metallic glasses. Prog metallic plastic. Phys Rev Lett 2005;94:205502. Mater Sci 2011;56:379–473. [38] Zhang B, Zhao DQ, Pan MX, Wang RJ, Wang WH. Formation [54] Holland-Moritz D. Short-range order and solid–liquid of cerium-based bulk metallic glasses. Acta Mater interfaces in undercooled metallic melts. Mater Sci Eng A 2006;54:3025–32. 2001;304-306:108–13. [39] Luo Q, Zhang B, Zhao DQ, Wang RJ, Pan MX, Wang WH. Aging [55] Han SW, Booth CH, Bauer ED, Huang PH, Chen YY, Lawrence and stability of cerium-based bulk metallic glass. Appl Phys JM. Lattice disorder and size-induced Kondo behavior in Lett 2006;88:151915. CeAl2 and CePt2+x . Phys Rev Lett 2006;97:097204. [40] Bian Z, Inoue A. Ultra-low glass transition temperatures in [56] Gille P, Bauer B. Single crystal growth of Al13 Co4 and Al13 Fe4 Ce-based bulk metallic glasses. Mater Trans 2005;46:1857–60. from Al-rich solutions by the Czochralski method. Cryst Res [41] Zhang B, Wang RJ, Zhao DQ, Pan MX, Wang WH. Superior Technol 2008;43:1161–7. glass-forming ability through microalloying in cerium-based [57] Shevchenko MA, Berezutskii VV, Ivanov MI, Kudin VG, alloys. Phys Rev B 2006;73:092201. Sudavtsova VS. Thermodynamic properties of alloys of the [42] Scherrer P. Bestimmung der Grösse und der inneren Struktur Al–Co and Al–Co–Sc systems. Russ J Phys Chem von Kolloidteilchen mittels Röntgenstrahlen. Nachr Ges Wiss 2014;88:729–34. 1918;26:98–100. [58] Zhao B, Li L, Zhai Q, Gao Y. Structure characterization of [43] Johari GP, Hallbrucker A, Mayer E. The glass–liquid transition Sn-based and Ce-based alloys treated by ultrafast scanning. of hyperquenched water. Nature 1987;330:552–3. In: Carpenter JS, Bai C, Hwang J-Y, Ikhmayies S, Li B, Monteiro [44] Tkatch VI, Limanovskii AI, Kameneva VY. Studies of SN, et al., editors. Characterization of minerals, metals, and crystallization kinetics of Fe40 Ni40P14 B6 and Fe80 B20 metallic materials. John Wiley & Sons, Inc.; 2014. p. 227–34. glasses under non-isothermal conditions. J Mater Sci [59] Angell CA, Ngai KL, McKenna GB, McMillan PF, Martin SW. 1997;32:5669–77. Relaxation in glass forming liquids and amorphous solids. J [45] Okulov IV, Soldatov IV, Sarmanova MF, Kaban I, Gemming T, Appl Phys 2000;88:3113–57. Edström K, et al. Flash Joule heating for ductilization of [60] Evenson Z, Naleway SE, Wei S, Gross O, Kruzic JJ, Gallino I, metallic glasses. Nat Commun 2015;6:7932. et al. ˇ relaxation and low-temperature aging in a Au-based [46] Farjas J, Roura P. Solid-phase crystallization under bulk metallic glass: from elastic properties to atomic-scale continuous heating: kinetic and microstructure scaling laws. structure. Phys Rev B 2014;89:174204. J Mater Res 2008;23:418–26. [61] Takeuchi A, Inoue A. Classification of bulk metallic glasses [47] Gerges J, Affouard F. Predictive calculation of the by atomic size difference, heat of mixing and period of crystallization tendency of model pharmaceuticals in the constituent elements and its application to characterization supercooled state from molecular dynamics simulations. J of the main alloying element. Mater Trans 2005;46:2817–29. Phys Chem B 2015;119:10768–83. [62] Inoue A. Stabilization of supercooled liquid and opening-up [48] Shneidman VA, Weinberg MC. Crystallization of rapidly of bulk glassy alloys. Proc Jpn Acad Ser B 1997;73:19–24. heated amorphous solids. J Non-Cryst Solids [63] Inoue A. Stabilization of metallic supercooled liquid and 1996;194:145–54. bulk amorphous alloys. Acta Mater 2000;48:279–306. [49] Orava J, Greer AL. Kissinger method applied to the [64] Zhang J, Wei YH, Qiu KQ, Zhang HF, Quan MX, Hua ZQ. crystallization of glass-forming liquids: regimes revealed by Crystallization kinetics and pressure effect on crystallization ultra-fast-heating calorimetry. Thermochim Acta of Zr55 Al10 Ni5 Cu30 bulk metallic glass. Mater Sci Eng A 2015;603:63–8. 2003;357:386–91. [50] Addou R, Gaudry E, Deniozou Th, Heggen M, Feuerbacher M, [65] Schroers J, Holland-Moritz D, Herlach DM, Urban K. Growth Gille P, et al. Structure investigation of the (1 0 0) surface of kinetics of quasicrystalline and polytetrahedral phases of the orthorhombic Al13 Co4 crystal. Phys Rev B 2009;80: Al–Pd–Mn, Al–Co, and Al–Fe from the undercooled melt. Phys 014203. Rev B 2000;61:14500–6.