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Phase separation in Zr56-xGdxCo28Al16 metallic glasses (0 ⩽ x ⩽ 20)
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ScienceDirect Acta Materialia 66 (2014) 262–272 www.elsevier.com/locate/actamat
Phase separation in Zr56-xGdxCo28Al16 metallic glasses (0 6 x 6 20) J.H. Han a,b,⇑, N. Mattern a, U. Vainio c, A. Shariq d, S.W. Sohn e, D.H. Kim e, J. Eckert a,b a
IFW Dresden, Institut fu¨r Komplexe Materialien, Helmholtzstr. 20, D-01069 Dresden, Germany b TU Dresden, Institut fu¨r Werkstoffwissenschaft, D-01062 Dresden, Germany c HASYLAB at DESY, Notkestr. 85, D-22603 Hamburg, Germany d FhG Center Nanoelectronic Technology, Koenigsbrueckerstr. 180, D-01099 Dresden, Germany e Center for Non-crystalline Materials, Department of Metallurgical Engineering, Yonsei University, 134 Shinchondong, Seodaemungu, Seoul 120-749, Republic of Korea Received 27 May 2013; received in revised form 30 September 2013; accepted 4 November 2013 Available online 25 November 2013
Abstract The influence of Gd addition on the microstructure of Zr56Co28Al16 metallic glasses was investigated for the exchange of Zr by up to 20 at.% Gd. Due to the large positive enthalpy of mixing between Zr and Gd, liquid–liquid phase separation occurs during rapid quenching of the melt. For a low concentration of Gd (x = 2 at.%), a homogeneous amorphous structure is obtained for the as-quenched state. Early stages of spinodal decomposition are observed in the as-quenched state of the glasses with x = 5 and 10 at.% Gd. Gd-enriched clusters 4–7 nm in size are formed, as shown by atom probe tomography (APT). Annealing below the crystallization temperature Tx leads to an increase in the amplitude of compositional fluctuations and the analysis of the spatial atomic distribution by APT provides direct evidence of the spinodal character of the decomposition by uphill diffusion of Gd into the clusters. For higher Gd content (x = 15 and 20 at.%), a coarsened microstructure of the phase-separated glass is obtained due to growth and coalescence while quenching the melt. The microstructure formation is essentially determined by the thermodynamic properties of the metastable undercooled liquid. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Metallic glasses; Spinodal decomposition; Small angle X-ray scattering; Three-dimensional atom probe (3DAP)
1. Introduction Metallic glasses (MGs) with a variety of promising properties, such as high mechanical strength, extended elastic limit and high hardness, have been developed in a number of different alloy systems by rapid quenching of the melt [1,2]. In contrast, the plastic deformability of MGs is rather poor in most cases, which significantly confines their applicability. Many efforts have been made to improve the plasticity of MGs by introducing heterogeneities or by creating glass–crystal composites [3–5]. One way to prepare heterogeneous glasses is by exploring phase
⇑ Corresponding author at: IFW Dresden, Institut fu¨r Komplexe Materialien, Helmholtzstr. 20, D-01069 Dresden, Germany. Tel.: +49 351 4659 246; fax: +49 351 4659 452. E-mail address: [email protected] (J.H. Han).
separation. For this, an alloy system with good glass-forming ability is modified by adding elements with a strong demixing tendency, as demonstrated for Zr–La–Cu–Ni–Al [6], Ni–Nb–Y [7] and Gd–Hf–Co–Al [8], for example. It was also reported by Park et al. [9] that the Cu–Zr–Al glasses with a small addition of Gd exhibit not only enhanced glass-forming ability (GFA) but also enhanced plasticity. This has been explained as being related to the phase-separation-induced local chemical heterogeneity in the glassy matrix. Recently, the composition dependence of phase separation for Cu–Zr–Gd MG has been investigated more systematically regarding the development of the early stage of phase separation [10]. In this work, Zr56-xGdxCo28Al16 (0 6 x 6 20 at.%) glassy alloys were synthesized by rapid quenching of the melt, and how the Gd content influences the development of phase separation in these alloys was investigated. The
1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.11.013
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quaternary alloy system is a suitable candidate for synthesizing phase separated MGs, because ternary Zr–Co–Al [11] and Gd–Co–Al [12] alloys are bulk glass forming systems and the enthalpy of mixing (DHmix) between Zr and Gd is +9 kJ mol 1 [13]. We will show how the Gd content influences the development of phase separation and corresponding microstructures under different conditions such as at elevated temperatures and isothermal annealing.
of 532 nm, with a pulse duration of 10 ps and a laser spot size of <10 lm. Pulse laser atom probe analyses were carried out at 27 K with an average detection rate of 0.005 ions pulse 1.
2. Experimental
3.1.1. Microstructure Fig. 1 shows the bright-field transmission electron microscope (BF-TEM) images and the corresponding selected area electron diffraction (SAED) patterns obtained from the cross-section of as-spun Zr56-xGdxCo28Al16 (x = 5, 10 and 20) glassy ribbons. For x = 5 (Fig. 1a), the absence of any significant contrast in BF-TEM image accompanied by the corresponding SAED pattern (inset) consisting of a single diffuse halo was revealed and the same holds true for x = 0 and 2. This suggests that the ribbons with low Gd additions may possess a monolithic glassy structure without any secondary glassy phase or crystallites. On the other hand, the BF-TEM image for x = 10 clearly shows the presence of heterogeneities of 10 nm in size with an inter-cluster distance of 20 nm (marked by arrows). Two diffuse halo rings in the corresponding SAED pattern (inset in Fig. 1b) and their radial intensity profile (shown in Fig. 2b) confirm that the regions with dark contrast are Gd-enriched clusters. For x = 20 at.% (Fig. 1c), the heterogeneous microstructure consisting of a Zr-rich glassy matrix and Gd-rich glassy phases are further coarsened by coalescence. As also shown in SAED patterns (insets of Fig. 1) and the radial intensity profiles (Fig. 2b), the diffraction intensity of the inner diffuse halo, which corresponds to the Gd-rich glassy phase, increases with increasing Gd content. These results suggest that, for higher Gd contents (x > 10 at.%) in the Zr–Gd–Co– Al system, liquid–liquid phase separation into Zr- and Gd-rich glassy phases takes place during quenching.
Zr56-xGdxCo28Al16 (x = 0, 2, 5, 10, 15 and 20 at.%) ingots were alloyed from high-purity elements (99.9%) using arc melting under argon atmosphere. To ensure efficient mixing, the ingots were re-melted at least three times. Rapidly quenched ribbon samples of the alloys were then fabricated by single-roller melt spinning using a quartz crucible and a wheel speed of 30 m s 1. The casting temperature was T = 1627–1677 K. The resulting ribbons had a width of 4 mm and a thickness of 40 lm. The structures of the melt-spun ribbons were characterized by X-ray diffraction (XRD: Panalytical X’pert Pro) with Co Ka radiation for a 2h range of 20–100°. Differential scanning calorimetry (DSC) measurements were performed employing a Diamond DSC (Perkin-Elmer) with a heating rate of 40 K min 1. The glass transition temperature Tg and the crystallization temperature Tx were determined as the onsets of the respective events, using the two-tangents method. Microstructural investigations were carried out with a scanning electron microscope (SEM; Zeiss Gemini 1530) and transmission electron microscopes (TEM; Philips CM 20 FEG and JEOL 2100F FEG) operated at 200 kV. A focused ion beam (FIB; Crossbeam 1540 XB, Zeiss) was used for TEM sample preparation. Small-angle X-ray scattering (SAXS) was measured at the B1 synchrotron beam line of HASYLAB/DESY using an energy of 16.025 keV. For these experiments, ribbon samples were placed on a heating stage which was used under vacuum for in situ measurement. SAXS patterns were registered by a PILATUS 300 k area detector. The SAXS data were obtained from q values of 0.1–2.9 nm 1 due to the longest possible sample-to-detector distance of 3.6 m. The temperature was increased in steps of 10 K. For each temperature, the background was measured, followed by a calibration standard (glassy carbon) and subsequent measurement of the sample. For isothermal analysis, the samples were heated at 100 K min 1 up to the corresponding temperature and the measurements were immediately started (10 min per pattern), then repeated for up to 6 h. Simultaneously, wide-angle X-ray scattering (WAXS) curves were recorded from q values of 10–40 nm 1 by a linear MYTHEN strip detector. A dual-focus ion beam system (FEI Strata 400) was used for the fabrication of the atom probe tomography (APT) tips. The APT specimens were analyzed using a local electrode atom probe (LEAP 3000 X Sie) equipped with a diode-pumped solid-state laser operated at the second harmonic frequency
3. Results 3.1. Characterization of the as-quenched states
3.1.2. X-ray and electron diffraction The amorphous structure of the as-quenched ribbons was also confirmed by XRD and radial intensity profiles obtained from the SAED patterns of TEM investigation. Fig. 2a shows the room temperature XRD patterns of asquenched Zr56-xGdxCo28Al16 (0 6 x 6 20 at.%) ribbons. The diffuse character of the XRD patterns for all as-quenched ribbons for x = 0, 2, 5 and 10 reveals their amorphous structure. In the XRD patterns for x = 15 and 20, a few weak additional peaks are also observed, indicating the presence of some unknown crystalline phases. This is due to partial crystallization at the surface region during quenching. As marked in Fig. 2a, decomposition into two amorphous phases is indicated by the broadened and asymmetric shape of the first diffuse maximum, which becomes more pronounced for higher Gd contents. The radial intensity profiles of electron
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Fig. 1. TEM bright-field images and corresponding SAED patterns of the (a) x = 5, (b) x = 10 and (c) x = 20 as-spun ribbons.
Fig. 2. (a) XRD patterns obtained for the Zr56-xGdxCo28Al16 (0 6 x 6 20) melt-spun ribbons. The XRD pattern for the Gd60Co30Al10 glass is also shown (open circles) for comparison [15]. (b) Radial intensity profiles obtained from the TEM SAED patterns for x = 2, 5, 10 and 20.
diffraction patterns shown in Fig. 2b clearly exhibit a splitting into two maxima for x P 10. The additional diffuse maxima at q 21.5 nm 1 are related to the formation of a Gd-enriched phase which increases in volume fraction for higher Gd contents. The SAED patterns correspond to selected regions of the samples and vary with the position on the sample. Obviously, in the XRD patterns the contributions are averaged over all local composition variations due to the much larger scattering volume. The comparison with the XRD pattern of a Gd60Co30Al10 monolithic glass (plotted as open circles) shows the position of the first maximum at a different position q 23.4 nm 1, which implies that the Gd-enriched phase in the phase separated glasses, x = 10–20 in this work for instance, is different in composition. 3.1.3. Thermal behavior The thermal behavior of the glasses upon continuous heating was characterized by DSC (Fig. 3). The occurrence of a glass transition between Tg = 710–760 K and the crystallization event with a strong exothermic peak at Tx = 775–820 K reveals the glassy nature of the Zr56-xGdx
Co28Al16 ribbons. Two-step crystallization accompanied by two exothermic events, which correspond to the formation of ZrCoAl (P63/mmc), ZrCo (Pm-3m) and Zr6Al2Co (P-62m) phases, for all the samples, is typical for the Zr–Co–Al glasses, as reported earlier [14]. With a small addition of Gd, i.e. x = 2 and 5, the glass transition temperature Tg and the crystallization temperature Tx decrease compared to that of x = 0 glass (indicated by arrows in Fig. 3). For the alloys with further increased Gd addition (x = 10–20), Tg and the supercooled liquid region become indistinguishable. Even though the TEM and XRD results show the character of a two-phase glass for x = 15 and 20, there are no indications for either a glass transition or a crystallization related to the Gd-rich glassy phase, which is expected to occur in the temperature range of T = 600–650 K (see the DSC curve for the Gd55Co25Al20 [8] glass indicated by the dashed line in Fig. 3). However, this might be due either to the very small fraction and size of the Gd-rich glassy phase or to the crystallization for the Gd-rich glassy phase presents in the vicinity of thermal events for the Zr-rich glassy phase. Similar results were reported for Zr–Gd–Cu–Al glasses with up to 15 at.%
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Fig. 3. DSC scans of Zr56-xGdxCo28Al16 (0 6 x 6 20) glasses at a heating rate of 40 K min 1. Glass transition (endothermic events after Tg) and crystallization (exothermic peaks) events confirm the amorphous state of the samples. Arrows indicate Tg. The DSC curve of Gd55Co25Al20 glass is also shown for comparison (dashed line) [8].
Gd addition, where also no distinct crystallization of a Gdrich phase was observed [9]. However in this case, an additional exothermic peak corresponding to the crystallization of a Gd-rich phase was clearly observed for Gd contents P20 at.%. 3.1.4. SAXS of as-quenched states The different degrees of the heterogeneities of the Zr56-xGdxCo28Al16 glassy ribbons (0 6 x 6 20) in the asquenched state can be clearly seen by comparison of the SAXS curves shown in Fig. 4. For x = 0 and 2, homoge-
Fig. 4. SAXS curves of as-quenched Zr56-xGdxCo28Al16 glassy ribbons (0 6 x 6 20).
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neous glassy structure without any indication for phase separation in the as-quenched state is ascertained by the very low SAXS intensity. The increase in SAXS intensity at very low scattering angle q < 0.4 nm 1 is not due to chemical heterogeneities or internal microstructure, but it is probably caused by surface roughness of the as-spun ribbon samples. This has also been observed for other homogeneous metallic glasses [16,17]. In the SAXS curve for x = 5, there was a slight increase in intensity over the entire scattering range, with a rather shallow maximum around qmax 1.0 nm 1. This indicates that only very small heterogeneities have been formed during rapid quenching. For x = 10, a distinct interference maximum is observed at qmax 0.4 nm 1, corresponding to the presence of compositional fluctuations with a dominant correlation length. The relationship f = 2p/qmax between the correlation length f and the position of the maximum qmax gives a distance of 16 nm. Comparing with TEM micrographs reveals that the value of f is in good agreement with the average inter-cluster distance for x = 10 (15–20 nm in Fig. 1b). For x = 15, the intensity of the interference maximum becomes higher and the position is shifted to a lower scattering angle (qmax 0.23 nm 1) corresponding to the correlation length of 27 nm. This implies that the compositional fluctuation is enlarged in amplitude and width for higher Gd content. On the other hand, a polydisperse character of the microstructure for x = 20 is revealed by a monotonic SAXS curve without a particular maximum. 3.1.5. The microstructure of as-quenched Zr51Gd5Co28Al16 and Zr46Gd10Co28Al16 as observed by APT The microstructures for x = 5 and 10, consisting of very fine heterogeneities as revealed from SAXS data, have been investigated in detail by APT. Chemical compositions of Zr51Gd5Co27Al17 for x = 5 and Zr46Gd10Co27Al17 for x = 10, respectively, were derived from the counted total number of atoms from the APT data. These compositions are in good agreement with the nominal composition of the samples. Fig. 5 shows reconstructed three-dimensional elemental maps for x = 5 and 10 glasses in the as-quenched state. The spatial distribution of Gd-enriched clusters is distinguished by dark-yellow iso-concentration surfaces for concentrations of Gd: 9 at.% for x = 5 and 30 at.% for x = 10, respectively. These values of Gd iso-concentration are chosen from the half maximum of the Gd concentration profiles. The used values are arbitrary to a certain extent due to the continuous variation of concentration between the clusters and matrix. The microstructure of the as-quenched samples in Fig. 5 clearly illustrates that the Gd-enriched clusters are randomly distributed throughout the sample volume, with size of 4 nm for x = 5 and 7 nm for x = 10, respectively. The corresponding concentration profiles along a cylinder of 2 nm in diameter and 8 nm in length through a path of matrix–cluster–matrix are shown in Fig. 6. The interface between the cluster and the surrounding matrix exhibits a continuous element distribution. The concentration profiles clearly demon-
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Fig. 5. Spatial distribution of constituent elements as analyzed by APT. (a) x = 5 and (b) x = 10 glasses in the as-quenched state.
Fig. 6. Concentration depth profiles of (a) x = 5 and (b) x = 10 glasses from selected cylinders of 2 nm in diameter.
strate the small compositional fluctuation in the Gd content, i.e. Gd enrichment at the middle of the profiles is accompanied by a simultaneous depletion of the other three elements Zr, Co and Al. The width of the profile of the Gd content confirms the size of the clusters. A comparison of the Gd concentrations of the x = 5 and 10 glasses in the as-quenched state (Fig. 6a and b) indicates that with higher Gd content, the clustering has been further developed via phase separation during rapid quenching. The alloy x = 5 was shown to be a completely homogeneous glass by BF-TEM observation and XRD, yet the existence of phase separation into Gd-enriched clusters and Zr-enriched glassy matrix was confirmed by the APT analysis. Since the heterogeneity in this glass is only a local compositional fluctuation with a small degree of Gd enrichment (less than 20 at.%), it is not easy to be detected by TEM investigation. The results of other experiments dealing with the early state of spinodal decomposition in metallic glass also have shown this effect, as reported in Refs. [18,19]. The proximity method that averages interfacial properties over a predefined surface irrespective of its convexity
[20] was used to characterize in detail the microstructure of the whole volume of the samples. Fig. 7a and b shows the proximity histograms of the as-quenched x = 5 and 10 glasses, respectively. The curves in the proximity histograms represent the average spatial concentration distribution for each constituent element of at least 50 clusters and their surroundings. A deficiency of Gd at the interface between the cluster and the surrounding matrix on the Gd concentration curve (marked by arrows) can be clearly observed from both sets of curves. An uphill diffusion of Gd atoms from the surrounding into the clusters accompanied by depletion of the other constituent elements can also be concluded. These factors provide direct evidence for the spinodal character of the decomposition. By taking the full width of half maximum (FWHM) value of the Gd concentration curve within a cluster regime, a cluster sizes of 3.8 nm for x = 5 and 6.9 nm for x = 10 are estimated. The averaged chemical compositions of the Gd-enriched clusters are measured to be Zr38Gd26Co21Al15 for x = 5 and Zr14Gd58Co10Al18 for x = 10, respectively. The more pronounced compositional fluctuation observed from x = 10 is indicative of further developed phase separation
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Fig. 7. Proximity histograms for (a) x = 5 and (b) x = 10 glasses in the as-quenched state. A statistical average of the element compositions within the Gdenriched clusters and their surrounding matrix is presented from the analysis of more then 50 clusters.
via spinodal decomposition. Table 1 summarizes the values of cluster size, number density and cluster composition as well as the composition of the surrounding matrix in detail. 3.2. In situ SAXS/WAXS investigation 3.2.1. Stepwise heating Fig. 8a shows the SAXS curves of monolithic Zr56Co28Al16 MG at different temperatures upon stepwise heating, together with simultaneously recorded WAXS patterns (inset in the upper right corner). The SAXS curve drawn by a dashed line for the as-quenched state (T = 300 K) exhibits a very low intensity, indicating a homogeneous amorphous structure without any type of heterogeneities. As also reflected by the diffuse maxima of the WAXS patterns shown in the inset, the glassy structure is preserved up to T = 743 K. At T = 753 K, we observe the additional SAXS intensity distributed over a broad q range. Simultaneously we observe a weak sharpening of the WAXS pattern, which means that the decomposition here is caused by nanocrystallization. The temperature dependence of the half width of WAXS peaks is shown in Fig. 8b. A drastic decrease in the width can be seen at the temperature range of T = 753–763 K, which is indicative of growths of the nanocrystallites. At T = 823 K the SAXS intensity of the Zr56Co28Al16 glass distinctly increases (distinguished by the dash-dotted line) due to crystallization and growth. Therefore, the homogeneous glassy structure of the asquenched state of the Zr56Co28Al16 glass is preserved up to T = 743 K and then nano-crystallization sets in during further heating as a precursor of the crystallization. These SAXS/WAXS results for x = 0 glass clearly show a typical thermal behavior of monolithic MG in accordance with the DSC results. Note that the SAXS curves for room temperature (T = 300 K) in Figs. 8 and 9 are multiplied by a factor of 0.5 for better visibility. In order to compare the structural evolution as a function of temperature for alloys with different Gd addition,
in situ SAXS (Fig. 9a–c) and corresponding WAXS (Fig. 9d–f) for Zr56-xGdxCo28Al16 (x = 2, 5 and 10) are displayed in Fig. 9. A very low SAXS intensity is observed for x = 2 in the as-quenched state (dashed line), which is also retained up to a temperature of T = 743 K. In the temperature range of T = 753–813 K, a similar behavior to the x = 0 glass was observed. At T = 823 K, a strong increase is evident in the SAXS intensity, which is related to crystal growth in accordance with the WAXS data. Further development of crystallization is clearly observed from SAXS and WAXS for T = 833–873 K. Therefore, the asquenched state of this ribbon exhibits a homogeneous glassy structure without any indications of phase separation during cooling, and then upon heating nano-crystallization occurs. The addition of 2 at.% Gd only affects the glass transition and crystallization temperature slightly. The distinct thermal behavior of nano-crystallization of a homogeneous glass as seen for x = 0 and 2 in comparison to phase separation into two glassy phases will be discussed later with the isothermal annealing experiments. For x = 5 glass (Fig. 9b and e), a low SAXS intensity is also observed for the as-quenched ribbon and is retained upon heating up to T = 673 K. However, a completely different behavior of the SAXS is observed for temperatures between T = 693 K and T = 793 K. A weak interference maximum representing the presence of compositional fluctuations appears around q 0.8 nm 1 at T = 693 K. A dominant correlation length f 8 nm is estimated. Upon heating to T = 733 K, the intensities of the maxima increase and the position of the maxima slightly shift to lower scattering angle q 0.7 nm 1. Such behavior is typical for the phase separation by the spinodal decomposition mechanism [21]. For further steps of heating up to T = 793 K, while the SAXS patterns seem to be varying only slightly in the intensity, corresponding WAXS patterns (shown in Fig. 9e) become sharper. This implies that the nano-crystallization discussed above for x = 0 and 2 takes place within the Zr-rich matrix phase. Again, the shape and the intensity of the SAXS curves change
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Table 1 Comparison of microstructural parameters calculated by proximity histograms (inter-cluster distances are also estimated using values of cluster number densities). Zr51Gd5Co28Al16 Gd-Cluster Matrix Cluster density Inter-cluster distance Zr46Gd10Co28Al16 Gd-cluster Matrix Cluster density Inter-cluster distance
As-quenched
10 min @ 723 K
Size (nm)
Zr
Gd
Co
Al
Size (nm)
Zr
Gd
Co
Al
3.8 ± 0.6
38.2 ± 1.4 50.4 ± 0.5
25.6 ± 1.3 5.4 ± 0.5
21.1 ± 0.9 26.7 ± 0.4
15.1 ± 1.0 17.5 ± 0.3
3.9 ± 0.6
36.5 ± 2.5 50.2 ± 0.4
35.0 ± 2.1 5.3 ± 0.3
14.6 ± 1.2 27.1 ± 0.4
13.9 ± 1.5 17.4 ± 0.3
15.3 ± 0.8 49.1 ± 0.5
58.6 ± 1.0 5.8 ± 0.5
10.5 ± 0.6 28.8 ± 0.2
15.6 ± 0.4 16.3 ± 0.3
0.0017 nm 3 8.4 ± 0.5 nm 6.9 ± 1 0.0002 nm 17 ± 1 nm
0.0018 nm 3 8.2 ± 0.5 nm 13.6 ± 0.9 48.5 ± 0.3
58.3 ± 1.4 6.2 ± 0.5
9.6 ± 0.5 28.5 ± 0.2
3
abruptly when crystallization completely sets in at T = 813 K. The evidence of phase separation by spinodal decomposition is more pronounced from the SAXS curves for x = 10, as shown in Fig. 9c. A strong interference maximum at q 0.4 nm 1 is observed in the SAXS curve of the as-quenched ribbon (dashed line). This indicates that phase separation by spinodal decomposition had already occurred with a dominant correlation length of f 16 nm during quenching. This value is in good agreement with the inter-cluster distance obtained from TEM micrographs and the APT results. Upon heating to T = 783 K the interference maximum increases in intensity whereas the position of the maxima qmax remains. The diffuse character of corresponding WAXS patterns in Fig. 9f reveals the preserved glassy structure of the ribbon up to T = 783 K. A distinct asymmetric shape is also observed in the WAXS patterns for T 6 783 K, which is in accordance with the results of XRD and the radial intensity profiles of the SAED patterns. An additional diffuse maximum around q = 22 nm 1 indicates the presence of an additional Gdrich amorphous phase. 3.2.2. Isothermal annealing Fig. 10a and b shows the in situ SAXS curves for x = 5 glass under isothermal annealing at two different temperatures, Tiso = 693 K and Tiso = 723 K, as a function of time t. For both, the interference maxima increase in the height or the intensity with annealing time. The positions of the maxima q@723K 0:85 nm 1 and q@723K 0:75 nm 1 max max remain nearly constant during the heat treatment, which is typical for the early stage of spinodal decomposition. However, as shown in Fig. 10b, a slight shift toward smaller q values (Dq 0.1 nm 1) has been observed at a very early stage of annealing (t 6 20 min). Within this very early time regime a small value of the correlation length change Df 1 nm is estimated. The corresponding WAXS patterns in the insets prove that the phase separation takes place within the glassy state well below crystallization. Fig. 10c shows the time evolution of the SAXS intensities at the position of the
18.5 ± 1.1 16.8 ± 0.4
7.1 ± 1 0.0003 nm 15 ± 1 nm
3
maxima for annealing temperatures Tiso = 693 K and Tiso = 723 K. The semi-logarithmic plot shows that the intensity exponentially increases in the earliest stage of annealing. Later on the increase in the intensity diminishes and reaches a stationary state at the later stage of annealing. Since both SAXS curves are obtained from the same thermally activated process of decomposition, they can be transformed into each other by a scaling factor. As expected, the decomposition process develops much faster at the higher temperature. As revealed by in situ SAXS/WAXS for Zr56-xGdxCo28 Al16 (x = 0, 2, 5 and 10) shown in Fig. 8 and 9, nanocrystallization is observed for x = 0 and 2 Fig. 11a shows the in situ SAXS curves and corresponding WAXS patterns (inset) of the Zr56Co28Al16 glass under isothermal annealing at temperature Tiso = 753 K as a function of time. In SAXS, a weak and very broad distributed “maximum” appears and gradually grows. During isothermal annealing especially the intensity at lower scattering angle becomes enhanced (note the log scale), shifting the intensity distribution to lower q values. The SAXS curve, then, reaches a stationary state at the later stage of annealing. Simultaneously the intensity of the WAXS peak increases (inset) together with a decrease in the half width. For comparison, in Fig. 11b the width of corresponding WAXS patterns is shown with that of Zr51Gd5Co28Al16 glass annealed at Tiso = 723 K as a function of annealing time. The width for x = 0 exhibits a continuous decrease while the width for x = 5 remains almost constant upon annealing. Such a difference proves that the common feature observed of the SAXS in this work – an evolution of the interference maximum upon heating – does in fact stem from different reasons, i.e. a nano-crystallization for the x = 0 and 2 and phase separation by a spinodal decomposition for x = 5 and 10, respectively. In order to characterize the microstructure of the annealed Zr51Gd5Co28Al16 and Zr46Gd10Co28Al10 glasses, APT was performed again. Fig. 12a and b shows the proximity diagrams for Zr51Gd5Co28Al16 and Zr46Gd10Co28Al16 glasses annealed at T = 723 K and T = 773 K for 10 min, respectively. The values of cluster composition, size and
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Fig. 8. (a) In situ SAXS and WAXS (inset) at different temperatures for Zr56Co28Al16 glass (x = 0). (b) Temperature dependence of FWHM for the corresponding WAXS patterns.
Fig. 9. In situ SAXS (a–c) curves and corresponding WAXS (d–f) curves of Zr56-xGdxCo28Al16 (x = 2, 5 and 10) glasses at room temperature (dashed curves) and at elevated temperatures upon stepwise heating (10 K or 20 K steps) from 573 K up to 873 K: (a, d) x = 2; (b, e) x = 5; (c, f) x = 10.
density as well as the composition of the surrounding matrix were also calculated and are summarized in Table 1 for comparison. In the proximity diagram for x = 5, it is clearly seen that the amplitude of the compositional fluctuation for the Gd significantly increases after the
annealing treatment, whereas the average cluster size and the number density remain almost constant (see Table 1). This supports the view that the decomposition develops by a spinodal mechanism clearly distinguished from the concentration profiles for a nucleation and growth
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mechanism [21]. On the other hand, it can be seen that the concentration profile of Gd for x = 10 exhibits no distinct change in the amplitude (see Fig. 12b) but only slight changes in cluster composition and size of the clusters. As stated before, this is also due to the fact that the glass x = 10 is characterized by the final stage of spinodal decomposition in the as-quenched state. Annealing the glass leads to only slight subsequent growth. 4. Discussion The phase separation and decomposition of the liquid in the binary Zr–Gd system extends to the quaternary Zr–Gd–Co–Al liquid. The structure formation of phaseseparated Zr56-xGdxCo28Al16 (0 6 x 6 20 at.%) glasses can be understood from the phase diagram. Based on the experimental results in this work, a schematic drawing of
the pseudo-binary section of the quaternary Zr–Gd–Co–Al phase diagram with miscibility gap (colored region) is shown in Fig. 13. The results suggest that the critical temperature of liquid–liquid phase separation Tc, i.e. the boundary of the miscibility gap (bimodal line in Fig. 13), strongly depends on the composition of the alloys. The metastable miscibility gap exists at least up to 20 at.% Gd. A homogeneous amorphous structure for x = 2 implies that Tc is far below the glass transition temperature Tg and the composition of the glass is out of the miscibility gap. APT characterized the earliest stage of spinodal decomposition in the as-quenched state for x = 5 glass due to the fact that Tc is just slightly above Tg. As proved by in situ SAXS/WAXS (Fig. 9b) and heat treatment (Figs. 10 and 12), further development of the decomposition process can be triggered by annealing the glass. For the alloys with higher Gd content (x P 10 at.%), further developed
Fig. 10. In situ isothermal SAXS/WAXS curves of x = 5 glass at (a) Tiso = 693 K for 350 min; (b) Tiso = 723 K for 120 min. (c) SAXS intensities at the position of first maxima as a function of time.
Fig. 11. (a) In situ isothermal SAXS and WAXS (inset) curves of Zr56Co28Al16 (x = 0) glass at Tiso = 753 K for 120 min. (b) The values of FWHM of WAXS patterns depending on the isothermal annealing time for x = 0 and 5.
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Fig. 12. Proximity histograms for annealed (a) x = 5 and (b) x = 10 glasses. Corresponding concentration profiles of Gd in as-quenched state are also shown as a dashed line for comparison.
Fig. 13. Schematic drawing of the binary section of the Zr56-xGdxCo28Al16 phase diagram.
stages of decomposition are obtained for the as-quenched state. Because Tc is much higher than Tg for these glasses, therefore, phase separation can be further developed during quenching the melt. The structure formation of phase separation by spinodal decomposition is usually described by the Cahn–Hilliard– Cook (CHC) theory [22]. Upon spinodal decomposition, two liquids of an immiscible system spontaneously separate and form domains with growth of coherent composition fluctuations in the initial stage. As shown by the SAXS curves under isothermal condition for x = 5 (Fig. 10), the intensities of the first maximum increase exponentially during the early stage of annealing, as it is predicted by the linear CHC theory. Afterwards, the growth of the amplitude shows a deceleration effect at the later stages of annealing in accordance with Tsakalakos’ approximation, which may explain the deviation from the exponential increase in the SAXS intensity [23]. Furthermore, one can expect that the stationary state where the growth of the amplitude
diminishes and reaches a saturated state will follow. On the other hand, the position of SAXS maximum for x = 5 initially shifts toward smaller q value for time t 6 20 min, then remains at a constant q value for t 6 120 min. This “initial shift” to smaller q values reflects the immediate coarsening of the compositional fluctuation at the very early stage of spinodal decomposition. However, the interface between the cluster and surrounding is only a concentration gradient and to some extent the cluster size is arbitrary as discussed with APT results. Therefore, the value of correlation length change Df 1 nm estimated from the SAXS data for x = 5 is still either negligible or too small to be interpreted as the increase in cluster size during annealing. All these observed features typify a spinodal decomposition process for early and intermediate stages in x = 5 [21]. Nano-crystallization is also a kind of phase separation, however fundamentally different from liquid–liquid phase separation. In general, it is difficult to distinguish which mechanism is present for early stage, if only a limited set of structural data is available. Especially the time dependence of a nucleation and growth mechanism is different from that of a spinodal mechanism [21]. Therefore SAXS measurement under isothermal annealing conditions is a suitable method for a correct interpretation in combination with APT as demonstrated here. 5. Conclusions The Zr–Gd–Co–Al system fulfills the prerequisites to form a phase separated metallic glass, i.e. Zr–Co–Al as well as Gd–Co–Al are well known glass-forming alloys and the Zr–Gd binary system exhibits a positive enthalpy of mixing inducing the demixing tendency. An almost homogeneous amorphous structure is obtained if the critical temperature of the miscibility gap is below the glass transition temperature (x = 2). Early stages of spinodal decomposition or
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further developed decomposition stages are observed for the alloys with higher Gd content (x P 5) in the asquenched state. In these cases, a critical temperature higher than glass transition temperature was concluded. Direct evidence of the early stage of spinodal decomposition was obtained by APT investigation, which revealed Gd-enriched clusters of 4–7 nm in size for x = 5 and 10. In situ SAXS/WAXS measurement at elevated temperature proved the progress of the spinodal decomposition by intensified amplitudes of the compositional fluctuations whereas the position of the maximum remains almost constant. Acknowledgements The authors thank B. Bartusch, S. Donath, M. Frey, D. Lohse and B. Opitz for technical assistance. Valuable discussions with B. Schwarz, M. Stoica and Y. Zhang are gratefully acknowledged. Financial support of the Deutsche Forschungsgemeinschaft DFG (Ma1531/10) is gratefully acknowledged. This work was also supported by the Global Research Laboratory (GRL) Program of the Korea Ministry of Education, Science and Technology. References [1] Johnson WL. MRS Bull 1999;24:42. [2] Inoue A. Acta Mater 2000;48:279–306. [3] Wang WH. Prog Mater Sci 2007;52:540–96.
[4] Pauly S, Liu G, Wang G, Ku¨hn U, Mattern N, Eckert J. Acta Mater 2009;57:5445–53. [5] Hays CC, Kim CP, Johnson WL. Phys Rev Lett 2000;84:2901–4. [6] Ku¨ndig AA, Ohnuma M, Ping DH, Ohkubo T, Hono K. Acta Mater 2004;52:2441–8. [7] Mattern N, Ku¨hn U, Gebert A, Gemming T, Zinkevich M, Wendrock H, et al. Scripta Mater 2005;53:271–4. [8] Han JH, Mattern N, Schwarz B, Gorantla S, Gemming T, Eckert J. Intermetallics 2012;20:115–22. [9] Park ES, Kyeong JS, Kim DH. Scripta Mater 2007;57:49–52. [10] Mattern N, Shariq A, Schwarz B, Vainio U, Eckert J. Acta Mater 2012;60:1946–56. [11] Wada T, Qin F, Wang X, Yoshimura M, Inoue A. J Mater Res 2009;24:2941–8. [12] Chen D, Takeuchi A, Inoue A. Mater Sci Eng A 2007;457:226–30. [13] Takeuchi A, Inoue A. Mater Trans 2005;46:2817–29. [14] Wada T, Zhang T, Inoue A. Mater Trans 2002;43:2843–6. [15] Schwarz B, Podmilsak B, Mattern N, Eckert J. J Mag Mag Mater 2010;322:2298–303. [16] Mattern N, Goerigk G, Vainio U, Miller MK, Gemming T, Eckert J. Acta Mater 2009;57:903–8. [17] Wang X, Cao QP, Chen YM, Hono K, Zhong C, Jiang QK, et al. Acta Mater 2011;59:1037–47. [18] Martin I, Ohkubo T, Ohnuma M, Deconihout B, Hono K. Acta Mater 2004;52:4427–35. [19] Oh JC, Ohkubo T, Kim YC, Fleury E, Hono K. Scripta Mater 2005;53:165–9. [20] Hellman OC, Vandenbroucke JA, Rusing J, Isheim D, Seidman DN. Mater Res Soc Symp Proc 1999;578:395–400. [21] Binder K, Fratzl P. In: Kostorz G, editor. Phase transformations in material. Weinheim: Wiley-VCH; 2001. [22] Cahn JW, Hilliard JE. J Chem Phys 1958;28:258–67. [23] Tsakalakos T, Dugan MP. J Mater Sci 1985;20:1301–9.
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Phase separation in Zr56-xGdxCo28Al16 metallic glasses (0 6 x 6 20) J.H. Han a,b,⇑, N. Mattern a, U. Vainio c, A. Shariq d, S.W. Sohn e, D.H. Kim e, J. Eckert a,b a
IFW Dresden, Institut fu¨r Komplexe Materialien, Helmholtzstr. 20, D-01069 Dresden, Germany b TU Dresden, Institut fu¨r Werkstoffwissenschaft, D-01062 Dresden, Germany c HASYLAB at DESY, Notkestr. 85, D-22603 Hamburg, Germany d FhG Center Nanoelectronic Technology, Koenigsbrueckerstr. 180, D-01099 Dresden, Germany e Center for Non-crystalline Materials, Department of Metallurgical Engineering, Yonsei University, 134 Shinchondong, Seodaemungu, Seoul 120-749, Republic of Korea Received 27 May 2013; received in revised form 30 September 2013; accepted 4 November 2013 Available online 25 November 2013
Abstract The influence of Gd addition on the microstructure of Zr56Co28Al16 metallic glasses was investigated for the exchange of Zr by up to 20 at.% Gd. Due to the large positive enthalpy of mixing between Zr and Gd, liquid–liquid phase separation occurs during rapid quenching of the melt. For a low concentration of Gd (x = 2 at.%), a homogeneous amorphous structure is obtained for the as-quenched state. Early stages of spinodal decomposition are observed in the as-quenched state of the glasses with x = 5 and 10 at.% Gd. Gd-enriched clusters 4–7 nm in size are formed, as shown by atom probe tomography (APT). Annealing below the crystallization temperature Tx leads to an increase in the amplitude of compositional fluctuations and the analysis of the spatial atomic distribution by APT provides direct evidence of the spinodal character of the decomposition by uphill diffusion of Gd into the clusters. For higher Gd content (x = 15 and 20 at.%), a coarsened microstructure of the phase-separated glass is obtained due to growth and coalescence while quenching the melt. The microstructure formation is essentially determined by the thermodynamic properties of the metastable undercooled liquid. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Metallic glasses; Spinodal decomposition; Small angle X-ray scattering; Three-dimensional atom probe (3DAP)
1. Introduction Metallic glasses (MGs) with a variety of promising properties, such as high mechanical strength, extended elastic limit and high hardness, have been developed in a number of different alloy systems by rapid quenching of the melt [1,2]. In contrast, the plastic deformability of MGs is rather poor in most cases, which significantly confines their applicability. Many efforts have been made to improve the plasticity of MGs by introducing heterogeneities or by creating glass–crystal composites [3–5]. One way to prepare heterogeneous glasses is by exploring phase
⇑ Corresponding author at: IFW Dresden, Institut fu¨r Komplexe Materialien, Helmholtzstr. 20, D-01069 Dresden, Germany. Tel.: +49 351 4659 246; fax: +49 351 4659 452. E-mail address: [email protected] (J.H. Han).
separation. For this, an alloy system with good glass-forming ability is modified by adding elements with a strong demixing tendency, as demonstrated for Zr–La–Cu–Ni–Al [6], Ni–Nb–Y [7] and Gd–Hf–Co–Al [8], for example. It was also reported by Park et al. [9] that the Cu–Zr–Al glasses with a small addition of Gd exhibit not only enhanced glass-forming ability (GFA) but also enhanced plasticity. This has been explained as being related to the phase-separation-induced local chemical heterogeneity in the glassy matrix. Recently, the composition dependence of phase separation for Cu–Zr–Gd MG has been investigated more systematically regarding the development of the early stage of phase separation [10]. In this work, Zr56-xGdxCo28Al16 (0 6 x 6 20 at.%) glassy alloys were synthesized by rapid quenching of the melt, and how the Gd content influences the development of phase separation in these alloys was investigated. The
1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.11.013
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quaternary alloy system is a suitable candidate for synthesizing phase separated MGs, because ternary Zr–Co–Al [11] and Gd–Co–Al [12] alloys are bulk glass forming systems and the enthalpy of mixing (DHmix) between Zr and Gd is +9 kJ mol 1 [13]. We will show how the Gd content influences the development of phase separation and corresponding microstructures under different conditions such as at elevated temperatures and isothermal annealing.
of 532 nm, with a pulse duration of 10 ps and a laser spot size of <10 lm. Pulse laser atom probe analyses were carried out at 27 K with an average detection rate of 0.005 ions pulse 1.
2. Experimental
3.1.1. Microstructure Fig. 1 shows the bright-field transmission electron microscope (BF-TEM) images and the corresponding selected area electron diffraction (SAED) patterns obtained from the cross-section of as-spun Zr56-xGdxCo28Al16 (x = 5, 10 and 20) glassy ribbons. For x = 5 (Fig. 1a), the absence of any significant contrast in BF-TEM image accompanied by the corresponding SAED pattern (inset) consisting of a single diffuse halo was revealed and the same holds true for x = 0 and 2. This suggests that the ribbons with low Gd additions may possess a monolithic glassy structure without any secondary glassy phase or crystallites. On the other hand, the BF-TEM image for x = 10 clearly shows the presence of heterogeneities of 10 nm in size with an inter-cluster distance of 20 nm (marked by arrows). Two diffuse halo rings in the corresponding SAED pattern (inset in Fig. 1b) and their radial intensity profile (shown in Fig. 2b) confirm that the regions with dark contrast are Gd-enriched clusters. For x = 20 at.% (Fig. 1c), the heterogeneous microstructure consisting of a Zr-rich glassy matrix and Gd-rich glassy phases are further coarsened by coalescence. As also shown in SAED patterns (insets of Fig. 1) and the radial intensity profiles (Fig. 2b), the diffraction intensity of the inner diffuse halo, which corresponds to the Gd-rich glassy phase, increases with increasing Gd content. These results suggest that, for higher Gd contents (x > 10 at.%) in the Zr–Gd–Co– Al system, liquid–liquid phase separation into Zr- and Gd-rich glassy phases takes place during quenching.
Zr56-xGdxCo28Al16 (x = 0, 2, 5, 10, 15 and 20 at.%) ingots were alloyed from high-purity elements (99.9%) using arc melting under argon atmosphere. To ensure efficient mixing, the ingots were re-melted at least three times. Rapidly quenched ribbon samples of the alloys were then fabricated by single-roller melt spinning using a quartz crucible and a wheel speed of 30 m s 1. The casting temperature was T = 1627–1677 K. The resulting ribbons had a width of 4 mm and a thickness of 40 lm. The structures of the melt-spun ribbons were characterized by X-ray diffraction (XRD: Panalytical X’pert Pro) with Co Ka radiation for a 2h range of 20–100°. Differential scanning calorimetry (DSC) measurements were performed employing a Diamond DSC (Perkin-Elmer) with a heating rate of 40 K min 1. The glass transition temperature Tg and the crystallization temperature Tx were determined as the onsets of the respective events, using the two-tangents method. Microstructural investigations were carried out with a scanning electron microscope (SEM; Zeiss Gemini 1530) and transmission electron microscopes (TEM; Philips CM 20 FEG and JEOL 2100F FEG) operated at 200 kV. A focused ion beam (FIB; Crossbeam 1540 XB, Zeiss) was used for TEM sample preparation. Small-angle X-ray scattering (SAXS) was measured at the B1 synchrotron beam line of HASYLAB/DESY using an energy of 16.025 keV. For these experiments, ribbon samples were placed on a heating stage which was used under vacuum for in situ measurement. SAXS patterns were registered by a PILATUS 300 k area detector. The SAXS data were obtained from q values of 0.1–2.9 nm 1 due to the longest possible sample-to-detector distance of 3.6 m. The temperature was increased in steps of 10 K. For each temperature, the background was measured, followed by a calibration standard (glassy carbon) and subsequent measurement of the sample. For isothermal analysis, the samples were heated at 100 K min 1 up to the corresponding temperature and the measurements were immediately started (10 min per pattern), then repeated for up to 6 h. Simultaneously, wide-angle X-ray scattering (WAXS) curves were recorded from q values of 10–40 nm 1 by a linear MYTHEN strip detector. A dual-focus ion beam system (FEI Strata 400) was used for the fabrication of the atom probe tomography (APT) tips. The APT specimens were analyzed using a local electrode atom probe (LEAP 3000 X Sie) equipped with a diode-pumped solid-state laser operated at the second harmonic frequency
3. Results 3.1. Characterization of the as-quenched states
3.1.2. X-ray and electron diffraction The amorphous structure of the as-quenched ribbons was also confirmed by XRD and radial intensity profiles obtained from the SAED patterns of TEM investigation. Fig. 2a shows the room temperature XRD patterns of asquenched Zr56-xGdxCo28Al16 (0 6 x 6 20 at.%) ribbons. The diffuse character of the XRD patterns for all as-quenched ribbons for x = 0, 2, 5 and 10 reveals their amorphous structure. In the XRD patterns for x = 15 and 20, a few weak additional peaks are also observed, indicating the presence of some unknown crystalline phases. This is due to partial crystallization at the surface region during quenching. As marked in Fig. 2a, decomposition into two amorphous phases is indicated by the broadened and asymmetric shape of the first diffuse maximum, which becomes more pronounced for higher Gd contents. The radial intensity profiles of electron
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Fig. 1. TEM bright-field images and corresponding SAED patterns of the (a) x = 5, (b) x = 10 and (c) x = 20 as-spun ribbons.
Fig. 2. (a) XRD patterns obtained for the Zr56-xGdxCo28Al16 (0 6 x 6 20) melt-spun ribbons. The XRD pattern for the Gd60Co30Al10 glass is also shown (open circles) for comparison [15]. (b) Radial intensity profiles obtained from the TEM SAED patterns for x = 2, 5, 10 and 20.
diffraction patterns shown in Fig. 2b clearly exhibit a splitting into two maxima for x P 10. The additional diffuse maxima at q 21.5 nm 1 are related to the formation of a Gd-enriched phase which increases in volume fraction for higher Gd contents. The SAED patterns correspond to selected regions of the samples and vary with the position on the sample. Obviously, in the XRD patterns the contributions are averaged over all local composition variations due to the much larger scattering volume. The comparison with the XRD pattern of a Gd60Co30Al10 monolithic glass (plotted as open circles) shows the position of the first maximum at a different position q 23.4 nm 1, which implies that the Gd-enriched phase in the phase separated glasses, x = 10–20 in this work for instance, is different in composition. 3.1.3. Thermal behavior The thermal behavior of the glasses upon continuous heating was characterized by DSC (Fig. 3). The occurrence of a glass transition between Tg = 710–760 K and the crystallization event with a strong exothermic peak at Tx = 775–820 K reveals the glassy nature of the Zr56-xGdx
Co28Al16 ribbons. Two-step crystallization accompanied by two exothermic events, which correspond to the formation of ZrCoAl (P63/mmc), ZrCo (Pm-3m) and Zr6Al2Co (P-62m) phases, for all the samples, is typical for the Zr–Co–Al glasses, as reported earlier [14]. With a small addition of Gd, i.e. x = 2 and 5, the glass transition temperature Tg and the crystallization temperature Tx decrease compared to that of x = 0 glass (indicated by arrows in Fig. 3). For the alloys with further increased Gd addition (x = 10–20), Tg and the supercooled liquid region become indistinguishable. Even though the TEM and XRD results show the character of a two-phase glass for x = 15 and 20, there are no indications for either a glass transition or a crystallization related to the Gd-rich glassy phase, which is expected to occur in the temperature range of T = 600–650 K (see the DSC curve for the Gd55Co25Al20 [8] glass indicated by the dashed line in Fig. 3). However, this might be due either to the very small fraction and size of the Gd-rich glassy phase or to the crystallization for the Gd-rich glassy phase presents in the vicinity of thermal events for the Zr-rich glassy phase. Similar results were reported for Zr–Gd–Cu–Al glasses with up to 15 at.%
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Fig. 3. DSC scans of Zr56-xGdxCo28Al16 (0 6 x 6 20) glasses at a heating rate of 40 K min 1. Glass transition (endothermic events after Tg) and crystallization (exothermic peaks) events confirm the amorphous state of the samples. Arrows indicate Tg. The DSC curve of Gd55Co25Al20 glass is also shown for comparison (dashed line) [8].
Gd addition, where also no distinct crystallization of a Gdrich phase was observed [9]. However in this case, an additional exothermic peak corresponding to the crystallization of a Gd-rich phase was clearly observed for Gd contents P20 at.%. 3.1.4. SAXS of as-quenched states The different degrees of the heterogeneities of the Zr56-xGdxCo28Al16 glassy ribbons (0 6 x 6 20) in the asquenched state can be clearly seen by comparison of the SAXS curves shown in Fig. 4. For x = 0 and 2, homoge-
Fig. 4. SAXS curves of as-quenched Zr56-xGdxCo28Al16 glassy ribbons (0 6 x 6 20).
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neous glassy structure without any indication for phase separation in the as-quenched state is ascertained by the very low SAXS intensity. The increase in SAXS intensity at very low scattering angle q < 0.4 nm 1 is not due to chemical heterogeneities or internal microstructure, but it is probably caused by surface roughness of the as-spun ribbon samples. This has also been observed for other homogeneous metallic glasses [16,17]. In the SAXS curve for x = 5, there was a slight increase in intensity over the entire scattering range, with a rather shallow maximum around qmax 1.0 nm 1. This indicates that only very small heterogeneities have been formed during rapid quenching. For x = 10, a distinct interference maximum is observed at qmax 0.4 nm 1, corresponding to the presence of compositional fluctuations with a dominant correlation length. The relationship f = 2p/qmax between the correlation length f and the position of the maximum qmax gives a distance of 16 nm. Comparing with TEM micrographs reveals that the value of f is in good agreement with the average inter-cluster distance for x = 10 (15–20 nm in Fig. 1b). For x = 15, the intensity of the interference maximum becomes higher and the position is shifted to a lower scattering angle (qmax 0.23 nm 1) corresponding to the correlation length of 27 nm. This implies that the compositional fluctuation is enlarged in amplitude and width for higher Gd content. On the other hand, a polydisperse character of the microstructure for x = 20 is revealed by a monotonic SAXS curve without a particular maximum. 3.1.5. The microstructure of as-quenched Zr51Gd5Co28Al16 and Zr46Gd10Co28Al16 as observed by APT The microstructures for x = 5 and 10, consisting of very fine heterogeneities as revealed from SAXS data, have been investigated in detail by APT. Chemical compositions of Zr51Gd5Co27Al17 for x = 5 and Zr46Gd10Co27Al17 for x = 10, respectively, were derived from the counted total number of atoms from the APT data. These compositions are in good agreement with the nominal composition of the samples. Fig. 5 shows reconstructed three-dimensional elemental maps for x = 5 and 10 glasses in the as-quenched state. The spatial distribution of Gd-enriched clusters is distinguished by dark-yellow iso-concentration surfaces for concentrations of Gd: 9 at.% for x = 5 and 30 at.% for x = 10, respectively. These values of Gd iso-concentration are chosen from the half maximum of the Gd concentration profiles. The used values are arbitrary to a certain extent due to the continuous variation of concentration between the clusters and matrix. The microstructure of the as-quenched samples in Fig. 5 clearly illustrates that the Gd-enriched clusters are randomly distributed throughout the sample volume, with size of 4 nm for x = 5 and 7 nm for x = 10, respectively. The corresponding concentration profiles along a cylinder of 2 nm in diameter and 8 nm in length through a path of matrix–cluster–matrix are shown in Fig. 6. The interface between the cluster and the surrounding matrix exhibits a continuous element distribution. The concentration profiles clearly demon-
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Fig. 5. Spatial distribution of constituent elements as analyzed by APT. (a) x = 5 and (b) x = 10 glasses in the as-quenched state.
Fig. 6. Concentration depth profiles of (a) x = 5 and (b) x = 10 glasses from selected cylinders of 2 nm in diameter.
strate the small compositional fluctuation in the Gd content, i.e. Gd enrichment at the middle of the profiles is accompanied by a simultaneous depletion of the other three elements Zr, Co and Al. The width of the profile of the Gd content confirms the size of the clusters. A comparison of the Gd concentrations of the x = 5 and 10 glasses in the as-quenched state (Fig. 6a and b) indicates that with higher Gd content, the clustering has been further developed via phase separation during rapid quenching. The alloy x = 5 was shown to be a completely homogeneous glass by BF-TEM observation and XRD, yet the existence of phase separation into Gd-enriched clusters and Zr-enriched glassy matrix was confirmed by the APT analysis. Since the heterogeneity in this glass is only a local compositional fluctuation with a small degree of Gd enrichment (less than 20 at.%), it is not easy to be detected by TEM investigation. The results of other experiments dealing with the early state of spinodal decomposition in metallic glass also have shown this effect, as reported in Refs. [18,19]. The proximity method that averages interfacial properties over a predefined surface irrespective of its convexity
[20] was used to characterize in detail the microstructure of the whole volume of the samples. Fig. 7a and b shows the proximity histograms of the as-quenched x = 5 and 10 glasses, respectively. The curves in the proximity histograms represent the average spatial concentration distribution for each constituent element of at least 50 clusters and their surroundings. A deficiency of Gd at the interface between the cluster and the surrounding matrix on the Gd concentration curve (marked by arrows) can be clearly observed from both sets of curves. An uphill diffusion of Gd atoms from the surrounding into the clusters accompanied by depletion of the other constituent elements can also be concluded. These factors provide direct evidence for the spinodal character of the decomposition. By taking the full width of half maximum (FWHM) value of the Gd concentration curve within a cluster regime, a cluster sizes of 3.8 nm for x = 5 and 6.9 nm for x = 10 are estimated. The averaged chemical compositions of the Gd-enriched clusters are measured to be Zr38Gd26Co21Al15 for x = 5 and Zr14Gd58Co10Al18 for x = 10, respectively. The more pronounced compositional fluctuation observed from x = 10 is indicative of further developed phase separation
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Fig. 7. Proximity histograms for (a) x = 5 and (b) x = 10 glasses in the as-quenched state. A statistical average of the element compositions within the Gdenriched clusters and their surrounding matrix is presented from the analysis of more then 50 clusters.
via spinodal decomposition. Table 1 summarizes the values of cluster size, number density and cluster composition as well as the composition of the surrounding matrix in detail. 3.2. In situ SAXS/WAXS investigation 3.2.1. Stepwise heating Fig. 8a shows the SAXS curves of monolithic Zr56Co28Al16 MG at different temperatures upon stepwise heating, together with simultaneously recorded WAXS patterns (inset in the upper right corner). The SAXS curve drawn by a dashed line for the as-quenched state (T = 300 K) exhibits a very low intensity, indicating a homogeneous amorphous structure without any type of heterogeneities. As also reflected by the diffuse maxima of the WAXS patterns shown in the inset, the glassy structure is preserved up to T = 743 K. At T = 753 K, we observe the additional SAXS intensity distributed over a broad q range. Simultaneously we observe a weak sharpening of the WAXS pattern, which means that the decomposition here is caused by nanocrystallization. The temperature dependence of the half width of WAXS peaks is shown in Fig. 8b. A drastic decrease in the width can be seen at the temperature range of T = 753–763 K, which is indicative of growths of the nanocrystallites. At T = 823 K the SAXS intensity of the Zr56Co28Al16 glass distinctly increases (distinguished by the dash-dotted line) due to crystallization and growth. Therefore, the homogeneous glassy structure of the asquenched state of the Zr56Co28Al16 glass is preserved up to T = 743 K and then nano-crystallization sets in during further heating as a precursor of the crystallization. These SAXS/WAXS results for x = 0 glass clearly show a typical thermal behavior of monolithic MG in accordance with the DSC results. Note that the SAXS curves for room temperature (T = 300 K) in Figs. 8 and 9 are multiplied by a factor of 0.5 for better visibility. In order to compare the structural evolution as a function of temperature for alloys with different Gd addition,
in situ SAXS (Fig. 9a–c) and corresponding WAXS (Fig. 9d–f) for Zr56-xGdxCo28Al16 (x = 2, 5 and 10) are displayed in Fig. 9. A very low SAXS intensity is observed for x = 2 in the as-quenched state (dashed line), which is also retained up to a temperature of T = 743 K. In the temperature range of T = 753–813 K, a similar behavior to the x = 0 glass was observed. At T = 823 K, a strong increase is evident in the SAXS intensity, which is related to crystal growth in accordance with the WAXS data. Further development of crystallization is clearly observed from SAXS and WAXS for T = 833–873 K. Therefore, the asquenched state of this ribbon exhibits a homogeneous glassy structure without any indications of phase separation during cooling, and then upon heating nano-crystallization occurs. The addition of 2 at.% Gd only affects the glass transition and crystallization temperature slightly. The distinct thermal behavior of nano-crystallization of a homogeneous glass as seen for x = 0 and 2 in comparison to phase separation into two glassy phases will be discussed later with the isothermal annealing experiments. For x = 5 glass (Fig. 9b and e), a low SAXS intensity is also observed for the as-quenched ribbon and is retained upon heating up to T = 673 K. However, a completely different behavior of the SAXS is observed for temperatures between T = 693 K and T = 793 K. A weak interference maximum representing the presence of compositional fluctuations appears around q 0.8 nm 1 at T = 693 K. A dominant correlation length f 8 nm is estimated. Upon heating to T = 733 K, the intensities of the maxima increase and the position of the maxima slightly shift to lower scattering angle q 0.7 nm 1. Such behavior is typical for the phase separation by the spinodal decomposition mechanism [21]. For further steps of heating up to T = 793 K, while the SAXS patterns seem to be varying only slightly in the intensity, corresponding WAXS patterns (shown in Fig. 9e) become sharper. This implies that the nano-crystallization discussed above for x = 0 and 2 takes place within the Zr-rich matrix phase. Again, the shape and the intensity of the SAXS curves change
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Table 1 Comparison of microstructural parameters calculated by proximity histograms (inter-cluster distances are also estimated using values of cluster number densities). Zr51Gd5Co28Al16 Gd-Cluster Matrix Cluster density Inter-cluster distance Zr46Gd10Co28Al16 Gd-cluster Matrix Cluster density Inter-cluster distance
As-quenched
10 min @ 723 K
Size (nm)
Zr
Gd
Co
Al
Size (nm)
Zr
Gd
Co
Al
3.8 ± 0.6
38.2 ± 1.4 50.4 ± 0.5
25.6 ± 1.3 5.4 ± 0.5
21.1 ± 0.9 26.7 ± 0.4
15.1 ± 1.0 17.5 ± 0.3
3.9 ± 0.6
36.5 ± 2.5 50.2 ± 0.4
35.0 ± 2.1 5.3 ± 0.3
14.6 ± 1.2 27.1 ± 0.4
13.9 ± 1.5 17.4 ± 0.3
15.3 ± 0.8 49.1 ± 0.5
58.6 ± 1.0 5.8 ± 0.5
10.5 ± 0.6 28.8 ± 0.2
15.6 ± 0.4 16.3 ± 0.3
0.0017 nm 3 8.4 ± 0.5 nm 6.9 ± 1 0.0002 nm 17 ± 1 nm
0.0018 nm 3 8.2 ± 0.5 nm 13.6 ± 0.9 48.5 ± 0.3
58.3 ± 1.4 6.2 ± 0.5
9.6 ± 0.5 28.5 ± 0.2
3
abruptly when crystallization completely sets in at T = 813 K. The evidence of phase separation by spinodal decomposition is more pronounced from the SAXS curves for x = 10, as shown in Fig. 9c. A strong interference maximum at q 0.4 nm 1 is observed in the SAXS curve of the as-quenched ribbon (dashed line). This indicates that phase separation by spinodal decomposition had already occurred with a dominant correlation length of f 16 nm during quenching. This value is in good agreement with the inter-cluster distance obtained from TEM micrographs and the APT results. Upon heating to T = 783 K the interference maximum increases in intensity whereas the position of the maxima qmax remains. The diffuse character of corresponding WAXS patterns in Fig. 9f reveals the preserved glassy structure of the ribbon up to T = 783 K. A distinct asymmetric shape is also observed in the WAXS patterns for T 6 783 K, which is in accordance with the results of XRD and the radial intensity profiles of the SAED patterns. An additional diffuse maximum around q = 22 nm 1 indicates the presence of an additional Gdrich amorphous phase. 3.2.2. Isothermal annealing Fig. 10a and b shows the in situ SAXS curves for x = 5 glass under isothermal annealing at two different temperatures, Tiso = 693 K and Tiso = 723 K, as a function of time t. For both, the interference maxima increase in the height or the intensity with annealing time. The positions of the maxima q@723K 0:85 nm 1 and q@723K 0:75 nm 1 max max remain nearly constant during the heat treatment, which is typical for the early stage of spinodal decomposition. However, as shown in Fig. 10b, a slight shift toward smaller q values (Dq 0.1 nm 1) has been observed at a very early stage of annealing (t 6 20 min). Within this very early time regime a small value of the correlation length change Df 1 nm is estimated. The corresponding WAXS patterns in the insets prove that the phase separation takes place within the glassy state well below crystallization. Fig. 10c shows the time evolution of the SAXS intensities at the position of the
18.5 ± 1.1 16.8 ± 0.4
7.1 ± 1 0.0003 nm 15 ± 1 nm
3
maxima for annealing temperatures Tiso = 693 K and Tiso = 723 K. The semi-logarithmic plot shows that the intensity exponentially increases in the earliest stage of annealing. Later on the increase in the intensity diminishes and reaches a stationary state at the later stage of annealing. Since both SAXS curves are obtained from the same thermally activated process of decomposition, they can be transformed into each other by a scaling factor. As expected, the decomposition process develops much faster at the higher temperature. As revealed by in situ SAXS/WAXS for Zr56-xGdxCo28 Al16 (x = 0, 2, 5 and 10) shown in Fig. 8 and 9, nanocrystallization is observed for x = 0 and 2 Fig. 11a shows the in situ SAXS curves and corresponding WAXS patterns (inset) of the Zr56Co28Al16 glass under isothermal annealing at temperature Tiso = 753 K as a function of time. In SAXS, a weak and very broad distributed “maximum” appears and gradually grows. During isothermal annealing especially the intensity at lower scattering angle becomes enhanced (note the log scale), shifting the intensity distribution to lower q values. The SAXS curve, then, reaches a stationary state at the later stage of annealing. Simultaneously the intensity of the WAXS peak increases (inset) together with a decrease in the half width. For comparison, in Fig. 11b the width of corresponding WAXS patterns is shown with that of Zr51Gd5Co28Al16 glass annealed at Tiso = 723 K as a function of annealing time. The width for x = 0 exhibits a continuous decrease while the width for x = 5 remains almost constant upon annealing. Such a difference proves that the common feature observed of the SAXS in this work – an evolution of the interference maximum upon heating – does in fact stem from different reasons, i.e. a nano-crystallization for the x = 0 and 2 and phase separation by a spinodal decomposition for x = 5 and 10, respectively. In order to characterize the microstructure of the annealed Zr51Gd5Co28Al16 and Zr46Gd10Co28Al10 glasses, APT was performed again. Fig. 12a and b shows the proximity diagrams for Zr51Gd5Co28Al16 and Zr46Gd10Co28Al16 glasses annealed at T = 723 K and T = 773 K for 10 min, respectively. The values of cluster composition, size and
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Fig. 8. (a) In situ SAXS and WAXS (inset) at different temperatures for Zr56Co28Al16 glass (x = 0). (b) Temperature dependence of FWHM for the corresponding WAXS patterns.
Fig. 9. In situ SAXS (a–c) curves and corresponding WAXS (d–f) curves of Zr56-xGdxCo28Al16 (x = 2, 5 and 10) glasses at room temperature (dashed curves) and at elevated temperatures upon stepwise heating (10 K or 20 K steps) from 573 K up to 873 K: (a, d) x = 2; (b, e) x = 5; (c, f) x = 10.
density as well as the composition of the surrounding matrix were also calculated and are summarized in Table 1 for comparison. In the proximity diagram for x = 5, it is clearly seen that the amplitude of the compositional fluctuation for the Gd significantly increases after the
annealing treatment, whereas the average cluster size and the number density remain almost constant (see Table 1). This supports the view that the decomposition develops by a spinodal mechanism clearly distinguished from the concentration profiles for a nucleation and growth
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mechanism [21]. On the other hand, it can be seen that the concentration profile of Gd for x = 10 exhibits no distinct change in the amplitude (see Fig. 12b) but only slight changes in cluster composition and size of the clusters. As stated before, this is also due to the fact that the glass x = 10 is characterized by the final stage of spinodal decomposition in the as-quenched state. Annealing the glass leads to only slight subsequent growth. 4. Discussion The phase separation and decomposition of the liquid in the binary Zr–Gd system extends to the quaternary Zr–Gd–Co–Al liquid. The structure formation of phaseseparated Zr56-xGdxCo28Al16 (0 6 x 6 20 at.%) glasses can be understood from the phase diagram. Based on the experimental results in this work, a schematic drawing of
the pseudo-binary section of the quaternary Zr–Gd–Co–Al phase diagram with miscibility gap (colored region) is shown in Fig. 13. The results suggest that the critical temperature of liquid–liquid phase separation Tc, i.e. the boundary of the miscibility gap (bimodal line in Fig. 13), strongly depends on the composition of the alloys. The metastable miscibility gap exists at least up to 20 at.% Gd. A homogeneous amorphous structure for x = 2 implies that Tc is far below the glass transition temperature Tg and the composition of the glass is out of the miscibility gap. APT characterized the earliest stage of spinodal decomposition in the as-quenched state for x = 5 glass due to the fact that Tc is just slightly above Tg. As proved by in situ SAXS/WAXS (Fig. 9b) and heat treatment (Figs. 10 and 12), further development of the decomposition process can be triggered by annealing the glass. For the alloys with higher Gd content (x P 10 at.%), further developed
Fig. 10. In situ isothermal SAXS/WAXS curves of x = 5 glass at (a) Tiso = 693 K for 350 min; (b) Tiso = 723 K for 120 min. (c) SAXS intensities at the position of first maxima as a function of time.
Fig. 11. (a) In situ isothermal SAXS and WAXS (inset) curves of Zr56Co28Al16 (x = 0) glass at Tiso = 753 K for 120 min. (b) The values of FWHM of WAXS patterns depending on the isothermal annealing time for x = 0 and 5.
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Fig. 12. Proximity histograms for annealed (a) x = 5 and (b) x = 10 glasses. Corresponding concentration profiles of Gd in as-quenched state are also shown as a dashed line for comparison.
Fig. 13. Schematic drawing of the binary section of the Zr56-xGdxCo28Al16 phase diagram.
stages of decomposition are obtained for the as-quenched state. Because Tc is much higher than Tg for these glasses, therefore, phase separation can be further developed during quenching the melt. The structure formation of phase separation by spinodal decomposition is usually described by the Cahn–Hilliard– Cook (CHC) theory [22]. Upon spinodal decomposition, two liquids of an immiscible system spontaneously separate and form domains with growth of coherent composition fluctuations in the initial stage. As shown by the SAXS curves under isothermal condition for x = 5 (Fig. 10), the intensities of the first maximum increase exponentially during the early stage of annealing, as it is predicted by the linear CHC theory. Afterwards, the growth of the amplitude shows a deceleration effect at the later stages of annealing in accordance with Tsakalakos’ approximation, which may explain the deviation from the exponential increase in the SAXS intensity [23]. Furthermore, one can expect that the stationary state where the growth of the amplitude
diminishes and reaches a saturated state will follow. On the other hand, the position of SAXS maximum for x = 5 initially shifts toward smaller q value for time t 6 20 min, then remains at a constant q value for t 6 120 min. This “initial shift” to smaller q values reflects the immediate coarsening of the compositional fluctuation at the very early stage of spinodal decomposition. However, the interface between the cluster and surrounding is only a concentration gradient and to some extent the cluster size is arbitrary as discussed with APT results. Therefore, the value of correlation length change Df 1 nm estimated from the SAXS data for x = 5 is still either negligible or too small to be interpreted as the increase in cluster size during annealing. All these observed features typify a spinodal decomposition process for early and intermediate stages in x = 5 [21]. Nano-crystallization is also a kind of phase separation, however fundamentally different from liquid–liquid phase separation. In general, it is difficult to distinguish which mechanism is present for early stage, if only a limited set of structural data is available. Especially the time dependence of a nucleation and growth mechanism is different from that of a spinodal mechanism [21]. Therefore SAXS measurement under isothermal annealing conditions is a suitable method for a correct interpretation in combination with APT as demonstrated here. 5. Conclusions The Zr–Gd–Co–Al system fulfills the prerequisites to form a phase separated metallic glass, i.e. Zr–Co–Al as well as Gd–Co–Al are well known glass-forming alloys and the Zr–Gd binary system exhibits a positive enthalpy of mixing inducing the demixing tendency. An almost homogeneous amorphous structure is obtained if the critical temperature of the miscibility gap is below the glass transition temperature (x = 2). Early stages of spinodal decomposition or
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further developed decomposition stages are observed for the alloys with higher Gd content (x P 5) in the asquenched state. In these cases, a critical temperature higher than glass transition temperature was concluded. Direct evidence of the early stage of spinodal decomposition was obtained by APT investigation, which revealed Gd-enriched clusters of 4–7 nm in size for x = 5 and 10. In situ SAXS/WAXS measurement at elevated temperature proved the progress of the spinodal decomposition by intensified amplitudes of the compositional fluctuations whereas the position of the maximum remains almost constant. Acknowledgements The authors thank B. Bartusch, S. Donath, M. Frey, D. Lohse and B. Opitz for technical assistance. Valuable discussions with B. Schwarz, M. Stoica and Y. Zhang are gratefully acknowledged. Financial support of the Deutsche Forschungsgemeinschaft DFG (Ma1531/10) is gratefully acknowledged. This work was also supported by the Global Research Laboratory (GRL) Program of the Korea Ministry of Education, Science and Technology. References [1] Johnson WL. MRS Bull 1999;24:42. [2] Inoue A. Acta Mater 2000;48:279–306. [3] Wang WH. Prog Mater Sci 2007;52:540–96.
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