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Spherulitic crystallization behavior of a metallic glass at high heating rates
Intermetallics 19 (2011) 1538e1545
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Spherulitic crystallization behavior of a metallic glass at high heating rates Hongqing Sun, Katharine M. Flores* Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2011 Accepted 28 May 2011 Available online 13 July 2011
Understanding crystallization in metallic glasses is an important consideration in the development of processing and manufacturing techniques which will utilize these unique alloys to full advantage. In the present work, the crystalline morphology and crystallization mechanisms of a Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 metallic glass are examined at heating rates spanning 4 orders of magnitude. In contrast to the multiphase nanocrystalline structure formed at low heating rates, high heating rates (>2.5 K/s) result in the formation of a non-equilibrium crystalline phase with spherulitic microstructure and uniform composition. Heating rates as fast as 103 K/s are found to be insufficient to avoid the formation of spherulites. The microstructure suggests that rapid heating suppresses phase separation and nucleation processes at the initial stage of crystallization, resulting in a growth-dominated crystallization behavior. The activation energy for spherulite formation is found to be less than that for nanocrystallization, which is attributed to the lack of phase separation and nucleation steps. Calculation of the Avrami exponent also indicates that spherulites form via the growth of quenched-in nuclei. The number density of quenched-in nuclei is estimated to be 1014 m3. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Multiphase intermetallics B. Metallic glasses B. Phase transformation C. Crystal growth C. Laser processing
1. Introduction Crystallization of metallic glass alloys is strongly history dependent due to a significant difference in the temperatures at which maximum nucleation and growth rates are achieved [1,2]. In most metallic systems, the maximum nucleation rate occurs at a much lower temperature than the maximum growth rate [1,3]. This phenomenon leads to the pronounced asymmetry of critical cooling and heating rates required to prevent crystallization [4,5]. For example, Hays et al. found that crystallization of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 occurred upon heating at rates of up to 500 K/s, which is at least two orders of magnitude larger than the critical cooling rate required to avoid crystallization from the melt [5]. The asymmetry has been attributed to phase separation or pre-existing nuclei that accelerate the crystallization process upon heating [1,6]. However, the crystallization temperatures of metallic glasses increase with the heating rate during non-isothermal tests [7,8], suggesting that some critical steps in the crystallization process may be bypassed at high heating rates. It has been reported that at low heating rates, polymorphic crystallization usually occurs in bulk metallic glasses (BMGs) since their compositions are close to deep eutectics [3,9,10]. Atomic
* Corresponding author. Tel.: þ614 292 9548; fax: þ614 292 1537. E-mail address: fl[email protected] (K.M. Flores). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.05.022
redistribution occurs during crystallization in order to reach the equilibrium compositions of the crystalline phases, resulting in the formation of multiple nano-scale crystalline phases. For instance, Zr2Cu, Zr2Ni and Zr3Al2 intermetallics are among the most stable equilibrium crystallization products in several Zr-based BMG alloys [9,11,12]. On the other hand, micro-scale spherical crystals or spherulites have also been observed as the crystalline products in both binary [13] and multicomponent metallic glasses [14e16]. Usually the spherulites are found distributed homogeneously within the amorphous matrix, with a characteristic appearance indicating radial growth from the center nucleation sites. Recent work on laser processing of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 BMG indicates that the formation of the spherulites is observed under high heating rate conditions. Spherulitic crystallization takes place in the heat-affected zones, where the heating rate is on the order of 103 K/s, and occurs within 0.2 s [17]. However, the detailed crystallization mechanisms upon heating, particularly at high heating rates, remain unclear. The present study examines the crystallization mechanisms and microstructural evolution for Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 metallic glass at heating rates spanning 4 orders of magnitude. Crystallization at low heating rates results in the formation of multi-phase nanocrystals, which are commonly observed in Zr-based BMGs when annealed in deep undercooling conditions [18e20]. In contrast, micro-scale spherulites form at high heating rates. We find that the activation energy required for spherulite formation at
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high heating rates is markedly lower than the activation energy for nanocrystallization at low heating rates, contributing to the observed asymmetry in critical heating and cooling rates. 2. Material and methods Ingots of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 (nominal at. %) were prepared by arc melting the high purity elemental constituents and casting into a Cu mold in a high purity argon atmosphere to produce 50 mm 50 mm 3 mm plates. The stated alloy composition is based on the fractions of the pure constituents prior to melting. Prior spot checks of the chemical composition of homogenized ingots and as-cast plates indicate that the nominal composition is typically correct to within less than 10% of each atomic fraction. As-cast specimens were crystallized at heating rates ranging from 0.2 K/s to 5.0 K/s using a differential scanning calorimeter (DSC). Specimens with dimensions 5 mm 5 mm 0.5 mm (average mass w 80 mg) were heated to 900 K at a constant heating rate in flowing argon gas. Higher heating rates of up to 50 K/s were achieved using a joule heating process on a GleebleÔ thermomechanical test frame. These higher heating rate experiments were conducted in vacuum (w105 mbar). Spherulitic crystallization behavior was also observed in the heat-affected zone of laser processed Zr58.5Cu15.6Ni12.8Al10.3Nb2.8. Laser processing was performed using an OptomecÔ 750 Laser Engineered Net Shaping (LENSÔ) system housed in a glovebox maintained at less than 15 ppm oxygen. The laser power was set to 150 W with a beam diameter of 1 mm and the travel speed of the laser spot relative to the substrate surface was set to 12.7 mm/s. The maximum heating rate obtained during laser processing is estimated to be on the order of 5000 K/s, based on finite element method simulations [21]. X-ray diffraction (XRD) using Cu-Ka radiation was performed on the amorphous and crystalline specimens. Microstructural characterization was performed via optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). TEM foils were prepared from sites of interest using a focused ion beam (FIB) operating at 30 kV. A low ion beam current of 93 pA was used for the final milling step to reduce damage to the structure during FIB processing. The milled foils were then extracted by ex situ lift-out and placed on carbon-coated copper TEM grids. The dimensions of the TEM foils were on the order of 15 mm by 8 mm. Detailed microstructural and compositional investigations were carried out using a FEI CM200 TEM under bright field (BF) conditions and a FEI Tecnai TEM in scanning mode (STEM) with a high angle annular dark field (HAADF) detector and energy dispersive spectrometry (EDS). It should be noted that the Cu fractions given by TEM EDS measurements may be high due to the effect of the Cu grids used to support the FIB-milled TEM foils. 3. Results Fig. 1 shows the XRD patterns of the various crystalline products of the Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 BMG heated at constant rates of 0.5 K/s, 5 K/s and 50 K/s, respectively, in comparison with the spherulites observed in the heat-affected zone of laser processed
Fig. 1. XRD profiles of crystalline Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 products obtained at heating rates of 0.5 K/s, 5 K/s, 50 K/s and w5000 K/s, compared with the profile for the initially amorphous alloy.
material as well as the as-cast glass. A broad peak was observed for the as-cast specimen, indicating that the material was amorphous before heating. After heating, the specimens are all crystalline, as illustrated by the multiple sharp Bragg diffraction peaks. Table 1 summarizes the three strongest crystalline peak positions in the XRD patterns for the different heating rates. Although phase identification is difficult due to the complexity of the five component alloy system, it is evident that the crystals produced at 5 K/s, 50 K/s, and w5000 K/s have similar crystalline structures, as shown by the similarity of most peak positions, in contrast with the structure formed at 0.5 K/s. Microstructural investigations of the crystallization products heated at different rates further demonstrate that the morphology depends on the heating rate. As shown in Fig. 2(a), a heating rate of 0.5 K/s results in a nanocrystalline structure with crystal sizes on the order of 100e200 nm. At a heating rate of 5 K/s (Fig. 2 (b)), spherulites with an average size of w2 mm are observed, corresponding to a number density of nuclei, N, of 1017 m3. Crystallization occurs by radial growth from central nuclei until the crystals impinge upon each other. At a higher heating rate of 50 K/s, the optical image shown in Fig. 2(c) demonstrates a similar formation of spherulites, but with a much larger average crystal size. Several of the nucleation sites are marked by arrows. Again, the spherulites grew until they impinged. The size and dense packing of the spherulites formed at 50 K/s is quite similar to that formed by laser processing at an estimated heating rate of w5000 K/s (Fig. 2(d)). The average crystal sizes for the spherulites shown in Fig. 2(c) and (d) are on the order of 15e20 mm, corresponding to N ¼ 1014 m3.
Table 1 Comparison of the positions of the three strongest XRD peaks (2q values) for the crystallized products of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 obtained at different heating rates. Heating Method
Heating rate
1st
2nd
3rd
Microstructure
Nuclei density
DSC DSC Gleeble Laser
0.5 K/s 5 K/s 50 K/s w5000 K/s
38.87 37.70 37.58 37.70
36.95 35.64 35.51 35.62
34.49 33.96 33.74 33.78
nanocrystals spherulites spherulites spherulites
w w w w
1021 1017 1014 1014
m3 m3 m3 m3
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Fig. 2. Microstructural morphologies of the crystallized Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 obtained at various heating rates. (a) BF TEM image of nanocrystalline structure formed at a heating rate of 0.5 K/s; (b) BF TEM image of micro-scale spherulites formed at a heating rate of 5 K/s; (c) optical image of spherulites formed at a heating rate of 50 K/s; (d) backscattered SEM image of spherulites in the heat-affected zone formed by laser processing at an estimated heating rate of w5000 K/s. The arrows in (c) identify several nucleation sites.
Fig. 3 compares the nano-scale crystals produced at a low heating rate and the micro-scale spherulites formed at high heating rates. The BF TEM images of the crystallized products are shown on the left-hand side, with the Z-contrast images on the right-hand side obtained using an STEM mode with an HAADF detector [22]. Several nanocrystalline phases with different local compositions are distinguishable in Fig. 3(a) and (b), which shows the specimen crystallized at 0.5 K/s. The nanocrystalline phases are observed throughout the specimen, and the boundaries between the nanocrystals are sharp and distinct. In contrast, the spherulites formed at higher heating rates exhibit a more or less uniform compositional distribution. The spherulites are polycrystalline, composed of closely packed columnar aggregates. The contrast differences of the adjacent aggregates under TEM BF imaging conditions results from the differences in crystallographic orientation (Fig. 3(c) and (e)). Notice that the aggregates spacing formed at 50 K/s is on the order of 200 nm (Fig. 3(e)), much larger than that produced at 5 K/s, which is on the order of 10 nm (Fig. 3(c)). Although phase contrast is distinct in the BF TEM images, the STEM images in Fig. 3(d) and (f) show little compositional contrast between the neighboring spherulitic aggregates. Indeed, no significant elemental partitioning occurred during the formation of the spherulites at 5 K/s as evidenced by the lack of contrast in Fig. 3(d). In comparison, a secondary phase is observed along the boundaries of the spherulitic aggregates formed at a heating rate of 50 K/s, as shown in Fig. 3(f). Compositional differences of the nanocrystals and the spherulites were investigated in greater detail via high resolution EDS line scans using STEM mode with an HAADF detector. Fig. 4(a) shows the high magnification STEM image of the nanocrystalline specimen
produced at 0.5 K/s. At least four different nanocrystalline phases, labeled A, B, C, and D in Fig. 4(a), are distinguishable with distinct boundaries between each other. Fig. 4(b) and (c) illustrate the EDS line scans performed across the boundaries between the neighboring crystals. Table 2 summarizes the EDS measurements of these four phases. The compositional evidence strongly suggests phase separation. Note that Nb is found primarily in region A and is virtually non-existent in phases B and D, while the Ni content of region A and the Al content of region D are much lower than the nominal values. In comparison, the compositional analysis of the spherulites formed at a heating rate of 50 K/s is illustrated in Fig. 5. Fig. 5(b) shows the EDS line scans across the spherulitic aggregate boundary highlighted in Fig. 5(a). The columnar aggregates are closely packed in the spherulites and are homogeneous in composition. A secondary phase with a distinct composition rich in Nb and a thickness of w20 nm is observed along the aggregate boundaries. As shown in Table 2, EDS analysis indicates that the chemical compositions of the spherulitic aggregates are very close to that of the parent amorphous material and completely different from any of the nanocrystals produced at the lower heating rate. The kinetics of the nanocrystallization and spherulitic crystallization processes were examined using DSC. Continuous heating DSC curves obtained at heating rates of 0.2e5.0 K/s are presented in Fig. 6. Each of the DSC traces exhibits an obvious endothermic event characteristic of the glass transition, followed by a significant exothermic event characteristic of the crystallization process. The glass transition temperatures (Tg), the crystallization onset (Tx) and peak temperatures (Tp), as well as the width of the supercooled liquid region (DTx ¼ TxTg) are summarized in Table 3. Notice that Tg, Tx and Tp are all shifted to higher temperatures as the heating
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Fig. 3. Detailed morphologies of (a)-(b) nanocrystals formed at 0.5 K/s, (c)-(d) micro-spherulites formed at 5 K/s, and (e)-(f) spherulitic aggregates formed at 50 K/s (a), (c) and (e) are BF TEM images; (b), (d) and (f) are HAADF STEM images.
rate increases. The single crystallization peaks observed in the DSC scans suggest that all of the nanocrystalline phases form within a similar temperature regime so that the exothermic heat flows detected by DSC overlap. 4. Discussion The Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 alloy investigated in the present study is an excellent glass former, as illustrated by its extended supercooled liquid region. The critical cooling rate required to achieve a fully amorphous structure has been shown to be w1.75 K/s for this alloy [5]. However, in the present study heating rates of up to w5000 K/s were not sufficient to bypass
crystallization, which motivates this study of crystallization mechanisms. Similar asymmetry in the crystallization behaviors during continuous heating of amorphous specimens and cooling from the stable melts has been observed in several bulk metallic glass-forming systems [4,23]. Schroers et al. indicated that quenched-in clusters or nuclei during cooling and their subsequent growth may contribute to the strong asymmetry between the heating from the amorphous state and cooling from the liquid state, and consequently result in a much higher critical heating rate than the critical cooling rate to avoid crystallization [4]. However, there are no published reports of completely different crystallization products resulting from different heating rates. Therefore, we now examine the mechanisms of crystallization upon heating.
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Fig. 4. (a) STEM image of the nanocrystalline structure produced at a heating rate of 0.5 K; (b) and (c) describe the corresponding EDS line scan results across the interfaces between nanocrystals A-B and C-D marked in (a), respectively.
In the present study, crystallization was observed during heating of the glass to a maximum temperature between the glass transition and the liquidus temperature of this alloy (w1100 K). The microstructural and XRD observations reveal that different crystallization products were obtained upon continuous heating at different rates. To investigate differences in the phase transformation mechanisms, the variation of three characteristic temperatures (Tg, Tx, and Tp) during continuous heating processes were fitted to the Kissinger equation [24]:
Table 2 Composition of the crystalline products of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 obtained at several heating rates. Heating rate
Composition (at. %)
Zr
Cu
Ni
Al
Nb
N/A
Nominal
58.5
15.6
12.8
10.3
2.8
0.5 K/s (Fig. 4)
A B C D Spherulitic aggregates Boundary phase Spherulitic aggregates Boundary phase
45.6 44.2 54.7 59.7 53.7 51.2 55.8 63.5
18.5 26.0 19.4 19.2 22.2 19.9 27.7 19.7
1.2 21.1 12.7 17.9 10.7 7.6 9.6 1.7
26.5 8.6 10.9 2.7 12.3 10.2 6.2 3.3
8.2 0.1 2.3 0.5 1.1 11.1 0.7 11.8
50 K/s (Fig. 5) w5000 K/s
T2 ln c B
! ¼
Ec AR ln Ec RTc
(1)
where B is the constant heating rate (dT/dt); Tc is the characteristic temperature of interest; Ec is the activation energy for the transition event occurring at the characteristic temperature of interest (e.g. the onset of crystallization); R is the gas constant; and A is a material constant. Using the characteristic temperatures identified as functions of heating rate by the DSC study, Kissinger plots are presented in Fig. 7 as lnðTc2 =BÞ versus 1000/Tc. The data for the glass transition events result in a straight line and an activation Eg ¼ 380.3 kJ/mol. This single valued activation energy indicates that the mechanism for the second-order transition from glass to supercooled liquid is not a function of the heating rate. In contrast, the onset and peak crystallization temperature data exhibit a deviation from linearity at heating rates greater than 2.5 K/s. Below this heating rate, the activation energy for the onset of the crystallization process is Ex ¼ 257.5 kJ/mol, and that for the peak is Ep ¼ 213.6 kJ/mol. At higher heating rates, where the transition from nanocrystallization to spherulite formation is observed, the slopes of the Kissinger plots decrease. This indicates a reduction in the activation energies, although there is insufficient DSC data to obtain a reliable value. A separate study of spherulite formation in the heat-affected zone formed during laser processing indicates
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Fig. 5. (a) STEM image of spherulitic aggregates formed at a heating rate of 50 K/s. (b) The corresponding EDS line scan across the indicated aggregate boundary.
that the activation energy for spherulitic crystallization at heating rates on the order of 103 K/s is Es ¼ 123.5 kJ/mol [21]. The variation in the activation energy indicates that while nanocrystallization occurs at lower temperatures than the formation and growth of the spherulites, the activation energy for nanocrystallization is higher than that for the spherulite formation. According to the EDS analysis presented in Table 2, the composition distribution of the nanocrystalline structure formed at low heating rates is inhomogeneous, indicating that phase separation occurred at low heating rates. It has been suggested that the combined free energy of the mixture of multiple amorphous phases could be less than that of the original homogeneous amorphous phase in a certain composition range [18]. As a result, phase separation into several amorphous phases with different compositions is commonly observed to precede nanocrystallization in the Zr-Cu-Ni-Al-Nb BMG systems [6,29,30]. Furthermore, it has been
demonstrated that in the Zr-Cu-Ni-Al-Nb alloy system, Cu (r ¼ 0.157 nm), Ni (r ¼ 0.162 nm), and to some degree Al (r ¼ 0.182 nm) can substitute for each other [9,31]. Therefore, the compositions of the nanocrystals are consistent with several known intermetallics: Zr5Al3Nb2 with some Cu substitutions in Region A, Zr(Cu, Ni) with some Al substitutions in Region B, and Zr2(Cu, Ni) in Region D, although caution is advised as the measured Cu fractions may be high due to the contribution of the Cu grids used to support the FIB-milled TEM foils. Notice that the composition in Region C is close to the nominal glass composition of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8, possibly corresponding to a quasicrystalline phase formed in the multicomponent system [32,33]. In contrast, there is no evidence of extensive phase separation as a precursor to crystallization during the rapid heating process that resulted in spherulite formation. The composition of the spherulitic aggregates is close to that of the amorphous alloy. Thus, the higher activation energy for crystallization measured at low heating rates where nanocrystallization was observed reflects the added energetic cost of a phase separation step. To compare the kinetics of the nanocrystallization and spherulitic crystallization mechanisms, the relative degree of crystallinity, X, at time t after the onset of crystallization was calculated assuming a linear relationship between the integral area under the crystallization peak and the percent crystallinity. The resulting behaviors at different heating rates are shown in Fig. 8(a). While the crystallization rate generally increases with increasing heating rate, note that the transformation rates for the three highest heating
Table 3 Comparison of the glass transition (Tg), crystallization onset (Tx), and crystallization peak (Tp) temperatures, and the width of the supercooled liquid regions (DTx) obtained via DSC at several heating rates.
Fig. 6. DSC traces of amorphous Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 specimens obtained at heating rates varying from 0.2 K/s to 5 K/s.
Heating rates (K/s)
Tg (K)
Tx (K)
Tp (K)
DTx (K)
0.2 0.3 0.5 0.7 0.8 1.3 1.7 2.5 3.3 4.2 5.0
663.4 671.3 675.3 678.2 680.4 684.1 687.3 690.3 692.6 695.6 698.3
757.4 770.9 778.1 784.9 787.9 796.0 800.4 809.6 820.2 831.7 850.7
763.1 778.6 785.9 794.0 796.9 810.2 816.0 831.7 845.9 862.0 887.6
94.0 99.6 102.8 106.7 107.5 111.9 113.1 119.3 127.6 136.1 152.4
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H. Sun, K.M. Flores / Intermetallics 19 (2011) 1538e1545 Table 4 Avrami exponents, n, obtained at several heating rates. Heating rates (K/s) n
Fig. 7. Kissinger plots for the glass transition, crystallization onset and crystallization peak temperatures obtained at various heating rates.
rates, 2.5 K/s, 3.3 K/s, and 4.2 K/s, are virtually indistinguishable. The kinetics of crystallization upon heating at a constant rate can be described by the Avrami equation [27]:
X ¼ 1 expðZc t n Þ
(2)
where the Avrami exponent, n, depends on the crystal nucleation and growth mechanisms, and Zc is a constant dependant on temperature. Because n is determined by both the nucleation and growth characteristics, it has been used to identify different types of phase transformation mechanisms. The values of n are determined for each heating rate from the slopes of the linearized data presented in Fig. 8(b) and are listed in Table 4. At the lower heating rates of 0.5 and 0.8 K/s, n is approximately 4.5; at heating rates of 2.5 K/s and greater the value of n decreases to the range of 2.1e2.6. Prior theoretical studies have shown that an n value of 4 indicates a constant nucleation rate throughout the transformation process
0.5 4.58
0.8 4.56
1.7 3.69
2.5 2.59
3.3 2.13
4.2 2.23
with three-dimensional diffusion-controlled growth [8,28], while a value of n ¼ 2.5 indicates that crystallization occurs by the growth of a fixed number of nuclei [28]. This indicates that the spherulites primarily form from pre-existing or quenched-in nuclei in the glass, while new crystals nucleate throughout the nanocrystallization process. In addition to the phase separation step at low heating rates, this continuous nucleation of new crystals may also contribute to the increased activation energy for crystallization under these conditions. Although the Avrami exponent suggests that the number of spherulite nuclei is fixed, a change in the number and spacing of the spherulites over a wider range of heating rates indicates that preexisting clusters or nuclei may be supplemented by nucleation during the continuous heating process. For example, the number density of spherulites increases by three orders of magnitude from N ¼ 1014 m3 at a heating rate of 50 K/s to N ¼ 1017 m3 at a heating rate of 5 K/s due to the longer time spent at the peak nucleation temperature at the lower heating rate. However, the number density of the spherulites becomes constant for heating rates greater than 50 K/s, as illustrated by the similar spherulite sizes shown in Fig. 2(c) and (d). Therefore, the quenched-in nuclei density in the as-cast material can be estimated as N ¼ 1014 m3. The spherulitic growth process begins with columnar aggregates growing radially from the quenched-in clusters or nuclei; this radial growth dominates the formation of the spherulites. Due to the relatively low diffusivity in the glass, the diffusion necessary for epitaxial growth cannot keep up with the crystallization process, and consequently different orientations are obtained within the spherulitic aggregates [25,26]. The variation in the crystallographic orientations results in the observed contrast difference in the BF TEM images shown in Fig. 3(c) and (e). The aggregates grow from the isolated nucleation sites until they interfere, ultimately consuming the glass. Although the formation of the spherulites involves the redistribution of the elements at the growth front, no compositional segregation was observed during the spherulitic crystallization at a heating rate of 5 K/s (Fig. 3(c) and (d)). However, EDS analysis indicates that there is an
Fig. 8. Crystallization rate as a function of heating rates. (a) Crystallization fraction X versus time and (b) fits to the linearized Avrami equation.
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elemental segregation along the spherulitic aggregate boundaries at higher heating rates (e.g. at 50 K/s in Fig. 5 or in the heat-affected zone produced by laser processing shown in Fig. 2(d)), wherein the material reaches a higher temperature with consequently high diffusivity. The crystalline phase appears to have rejected the Nb to the boundary phase. It has been reported that Zr preferentially reacts with Cu, Ni and Al to produce intermetallic compounds because of the negative mix enthalpies of mixing (DHmix ZrCu ¼ 23kJ=mol, DHZrNi ¼ 49kJ=mol mix and DHZrAl ¼ 44kJ=mol) [11,34]. On the contrary, Nb has a positive enthalpy of mixing with Zr (DHmix ZrNb ¼ 4kJ=mol) [34]. The ZreNb binary phase diagram [35] suggests that Nb is likely to form a bodycentered cubic (bcc) solid solution with Zr at a temperature above 960 K, providing an explanation of the partition of the Nb to form the boundary phase which is also enriched in Zr but depleted in Cu, Ni and Al, as shown in Fig. 5. On the other hand, the Nb remains supersaturated in the spherulites at a low heating rate, e.g. 5 K/s, with the spherulitic crystallization taking place below the critical temperature for the formation of bcc ZreNb phase. The resulting structure is uniform in composition. 5. Conclusions Although the Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 BMG is considered to have excellent stability and glass forming ability due to its low critical cooling rate, recent work on laser processing of the glass has shown that spherultic crystals form in the glass even at heating rates on the order of 103 K/s. In this work, the crystallization mechanisms of this glass were investigated at heating rates spanning 4 orders of magnitude in an effort to explain this heating/cooling asymmetry and the significant microstructural differences between low and high heating rate crystallization products. While micro-scale spherulites were observed at heating rates greater than 2.5 K/s, heating rates less than 2.5 K/s produced several nanocrystalline phases with distinct compositions. Calculations of the activation energy for crystallization and the Avrami exponent indicate that this nanocrystallization process occurs via phase separation, transient nucleation, and threedimensional diffusion-controlled growth. In contrast, the lower values for the activation energy and Avrami exponent during rapid heating indicate that nucleation is suppressed, so growth from quenched-in nuclei dominates the crystallization process. The rapid transformation from glass to spherulites was accomplished without evidence of prior phase separation. Based on spherulite size at heating rates of 50 K/s and 5000 K/s, the quenched-in nuclei density was estimated to be 1014 m3. Acknowledgements This work was funded by the Office of Naval Research under Award #N0014-08-1-1023. References [1] Schroers J, Johnson WL. History dependent crystallization of Zr41Ti14Cu12Ni10Be23 melts. Journal of Applied Physics 2000;88:44e8. [2] Schroers J. Processing of bulk metallic glass. Advanced Materials 2010;22: 1566e97. [3] Schroers J, Johnson WL. Crystallization of Zr41Ti14Cu12Ni10Be23. Materials Transactions, JIM 2000;41:1530e7. [4] Schroers J, Masuhr A, Johnson WL. Pronounced asymmetry in the crystallization behavior during constant heating and cooling of a bulk metallic glassforming liquid. Physical Review B 1999;60:11855e8. [5] Hays CC, Schroers J, Johnson WL. Vitrification and determination of the crystallization time scales of the bulk-metallic-glass-forming liquid Zr58.5Nb2.8Cu15.6Ni12.8Al10.3. Applied Physics Letters 2001;79:1605e7.
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[6] Gallino I, Shah MB, Busch R. Enthalpy relaxation and its relation to the thermodynamics and crystallization of the Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 bulk metallic glass-forming alloy. Acta Materialia 2007;55:1367e76. [7] Saotome Y, Roppongi K, Zhang T, Inoue A. Characteristic behavior of La55Al25Ni20 amorphous alloy under rapid heating. Materials Science and Engineering A 2001;304-306:743e6. [8] Qiao D, Fan C, Liao PK, Choo H. Crystallization kinetics of the Zr58Ni13.6Cu18Al10.4Nb1 bulk metallic glass. Advanced Engineering Materials 2006;8: 714e9. [9] Shadowspeaker L, Shah M, Busch R. On the crystalline equilibrium phases of the Zr57Cu15.4Ni12.6Al10Nb5 bulk metallic glass forming alloy. Scripta Materialia 2004;50:1035e8. [10] Aboki TAM, Brisset F, Souron JP, Dezellus A, Plaindoux P. Microstructure studies of Zr65Cu17.5Al7.5Ni10 and Zr65Cu15Al10Ni10 glass forming alloys: phase morphologies and undercooled melt solidification. Intermetallics 2008;16: 615e24. [11] He G, Loser W, Eckert J, Schultz L. Phase transformation and mechanical properties of Zr-base bulk glass-forming alloys. Materials Science and Engineering A 2003;352:179e85. [12] Sun YJ, Qu DD, Huang YJ, Liss K-D, Wei XS, Xing DW, et al. Zr-Cu-Ni-Al bulk metallic glasses with superhigh glass-forming ability. Acta Materialia 2009; 57:1290e9. [13] Ziewiec K. Kinetics of phase transformations in Ni87P13 alloy upon heating. Journal of Alloys and Compounds 2005;397:207e10. [14] Yano T, Yorikado Y, Akeno Y, Hori F, Yokoyama Y, Iwase A, et al. Relaxation and crystallization behavior of the Zr50Cu40Al10 metallic glass. Materials Transactions JIM 2005;46:2886e92. [15] Xia C, Xing L, Long W, Li Z, Li Y. Calculation of crystallization start line for Zr48Cu45Al7 bulk metallic glass at a high heating and cooling rate. Journal of Alloys and Compounds 2009;484:698e701. [16] Liu J, Zhang H, Fu H, Hu Z, Yuan X. In situ spherical B2 CuZr phase reinforced ZrCuNiAlNb bulk metallic glass matrix composite. Journal of Materials Research 2010;25:1159e63. [17] Sun H, Flores KM. Microstructural analysis of a laser-processed Zr-based bulk metallic glass. Metallurgical and Materials Transactions A 2010;41A:1752e7. [18] Pekarskaya E, Loffler JF, Johnson WL. Microstructural studies of crystallization of a Zr-based bulk metallic glass. Acta Materialia 2003;51:4045e57. [19] Liu XJ, Chen GL, Hou HY, Hui X, Yao KF, Lu ZP, et al. Atomistic mechanism for nanocrystallization of metallic glasses. Acta Materialia 2008;56:2760e9. [20] Kulik T. Nanocrystallization of metallic glasses. Journal of Non-Crystalline Solids 2001;287:145e61. [21] H. Sun, Microstructure evolution of bulk metallic glasses via laser processing, PhD dissertation in Materials Science and Engineering. 2010, The Ohio State University: Columbus, OH. [22] Williams DB, Carter CB. Transmission electron microscopy: a Textbook for materials science. New York: Springer; 1996. [23] Kui HW, Turnbull D. Melting of Ni40Pd40P20 glass. Applied Physics Letters 1985;47:796e7. [24] Kissinger HE. Variation of peak temperature with heating rate in differential thermal analysis. Journal of Research of the National Bureau of Standards 1956;57:217e21. [25] Granasy L, Pusztai T, Tegze G, Warren JA, Douglas JF. Growth and form of spherulites. Physical Review E 2005;72. 011605 1e15. [26] Granasy L, Pusztai T, Borzsonyi T, Toth G, Tegze G, Warren JA, et al. Phase filed theory of crystal nucleation and polycrystalline growth: a review. Journal of Materials Research 2006;21:309e19. [27] Jeziorny A. Parameters characterizing the kinetics of the non-isothermal crystallization of poly(ethylene terephthalate) determined by DSC. Polymer 1978;19:1142e4. [28] Burke J. The kinetics of phase transformations in metals. New York: Pergamon Press Inc; 1965. [29] Johnson WL. Bulk glass-forming metallic alloys: science and technology. MRS Bulletin 1999;24:42e56. [30] Loffler JF, Bossuyt S, Glade SC, Johnson WL, Wagner W, Thiyagarajan P. Crystallization of bulk amorphous Zr-Ti(Nb)-Cu-Ni-Al. Applied Physics Letters 2000;77:525e7. [31] Miracle DB. The efficient cluster packing model - an atomic structural model for metallic glasses. Acta Materialia 2006;54:4317e36. [32] Kuhn U, Eckert J, Mattern N, Schultz L. Formation of micrometer sized quasicrystals in slowly cooled Zr-Ti-Nb-Cu-Ni-Al alloys. Physica Status Solidi 2005;202:2436e41. [33] Kuhn U, Eckert J, Mattern N, Schultz L. As-cast quasicrystalline phase in a Zr-based multicomponent bulk alloy. Applied Physics Letters 2000;77: 3176e8. [34] Takeuchi A, Inoue A. Classification of bulk metallic glasses by atomic size difference, heat of mixing and period of constituent elements and its application to characterization of the main alloying element. Materials Transactions, JIM 2005;46:2817e29. [35] Okamoto H. Phase diagrams for binary alloys. Materials Park, OH: ASM International; 2000.
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Spherulitic crystallization behavior of a metallic glass at high heating rates Hongqing Sun, Katharine M. Flores* Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2011 Accepted 28 May 2011 Available online 13 July 2011
Understanding crystallization in metallic glasses is an important consideration in the development of processing and manufacturing techniques which will utilize these unique alloys to full advantage. In the present work, the crystalline morphology and crystallization mechanisms of a Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 metallic glass are examined at heating rates spanning 4 orders of magnitude. In contrast to the multiphase nanocrystalline structure formed at low heating rates, high heating rates (>2.5 K/s) result in the formation of a non-equilibrium crystalline phase with spherulitic microstructure and uniform composition. Heating rates as fast as 103 K/s are found to be insufficient to avoid the formation of spherulites. The microstructure suggests that rapid heating suppresses phase separation and nucleation processes at the initial stage of crystallization, resulting in a growth-dominated crystallization behavior. The activation energy for spherulite formation is found to be less than that for nanocrystallization, which is attributed to the lack of phase separation and nucleation steps. Calculation of the Avrami exponent also indicates that spherulites form via the growth of quenched-in nuclei. The number density of quenched-in nuclei is estimated to be 1014 m3. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Multiphase intermetallics B. Metallic glasses B. Phase transformation C. Crystal growth C. Laser processing
1. Introduction Crystallization of metallic glass alloys is strongly history dependent due to a significant difference in the temperatures at which maximum nucleation and growth rates are achieved [1,2]. In most metallic systems, the maximum nucleation rate occurs at a much lower temperature than the maximum growth rate [1,3]. This phenomenon leads to the pronounced asymmetry of critical cooling and heating rates required to prevent crystallization [4,5]. For example, Hays et al. found that crystallization of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 occurred upon heating at rates of up to 500 K/s, which is at least two orders of magnitude larger than the critical cooling rate required to avoid crystallization from the melt [5]. The asymmetry has been attributed to phase separation or pre-existing nuclei that accelerate the crystallization process upon heating [1,6]. However, the crystallization temperatures of metallic glasses increase with the heating rate during non-isothermal tests [7,8], suggesting that some critical steps in the crystallization process may be bypassed at high heating rates. It has been reported that at low heating rates, polymorphic crystallization usually occurs in bulk metallic glasses (BMGs) since their compositions are close to deep eutectics [3,9,10]. Atomic
* Corresponding author. Tel.: þ614 292 9548; fax: þ614 292 1537. E-mail address: fl[email protected] (K.M. Flores). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.05.022
redistribution occurs during crystallization in order to reach the equilibrium compositions of the crystalline phases, resulting in the formation of multiple nano-scale crystalline phases. For instance, Zr2Cu, Zr2Ni and Zr3Al2 intermetallics are among the most stable equilibrium crystallization products in several Zr-based BMG alloys [9,11,12]. On the other hand, micro-scale spherical crystals or spherulites have also been observed as the crystalline products in both binary [13] and multicomponent metallic glasses [14e16]. Usually the spherulites are found distributed homogeneously within the amorphous matrix, with a characteristic appearance indicating radial growth from the center nucleation sites. Recent work on laser processing of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 BMG indicates that the formation of the spherulites is observed under high heating rate conditions. Spherulitic crystallization takes place in the heat-affected zones, where the heating rate is on the order of 103 K/s, and occurs within 0.2 s [17]. However, the detailed crystallization mechanisms upon heating, particularly at high heating rates, remain unclear. The present study examines the crystallization mechanisms and microstructural evolution for Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 metallic glass at heating rates spanning 4 orders of magnitude. Crystallization at low heating rates results in the formation of multi-phase nanocrystals, which are commonly observed in Zr-based BMGs when annealed in deep undercooling conditions [18e20]. In contrast, micro-scale spherulites form at high heating rates. We find that the activation energy required for spherulite formation at
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high heating rates is markedly lower than the activation energy for nanocrystallization at low heating rates, contributing to the observed asymmetry in critical heating and cooling rates. 2. Material and methods Ingots of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 (nominal at. %) were prepared by arc melting the high purity elemental constituents and casting into a Cu mold in a high purity argon atmosphere to produce 50 mm 50 mm 3 mm plates. The stated alloy composition is based on the fractions of the pure constituents prior to melting. Prior spot checks of the chemical composition of homogenized ingots and as-cast plates indicate that the nominal composition is typically correct to within less than 10% of each atomic fraction. As-cast specimens were crystallized at heating rates ranging from 0.2 K/s to 5.0 K/s using a differential scanning calorimeter (DSC). Specimens with dimensions 5 mm 5 mm 0.5 mm (average mass w 80 mg) were heated to 900 K at a constant heating rate in flowing argon gas. Higher heating rates of up to 50 K/s were achieved using a joule heating process on a GleebleÔ thermomechanical test frame. These higher heating rate experiments were conducted in vacuum (w105 mbar). Spherulitic crystallization behavior was also observed in the heat-affected zone of laser processed Zr58.5Cu15.6Ni12.8Al10.3Nb2.8. Laser processing was performed using an OptomecÔ 750 Laser Engineered Net Shaping (LENSÔ) system housed in a glovebox maintained at less than 15 ppm oxygen. The laser power was set to 150 W with a beam diameter of 1 mm and the travel speed of the laser spot relative to the substrate surface was set to 12.7 mm/s. The maximum heating rate obtained during laser processing is estimated to be on the order of 5000 K/s, based on finite element method simulations [21]. X-ray diffraction (XRD) using Cu-Ka radiation was performed on the amorphous and crystalline specimens. Microstructural characterization was performed via optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). TEM foils were prepared from sites of interest using a focused ion beam (FIB) operating at 30 kV. A low ion beam current of 93 pA was used for the final milling step to reduce damage to the structure during FIB processing. The milled foils were then extracted by ex situ lift-out and placed on carbon-coated copper TEM grids. The dimensions of the TEM foils were on the order of 15 mm by 8 mm. Detailed microstructural and compositional investigations were carried out using a FEI CM200 TEM under bright field (BF) conditions and a FEI Tecnai TEM in scanning mode (STEM) with a high angle annular dark field (HAADF) detector and energy dispersive spectrometry (EDS). It should be noted that the Cu fractions given by TEM EDS measurements may be high due to the effect of the Cu grids used to support the FIB-milled TEM foils. 3. Results Fig. 1 shows the XRD patterns of the various crystalline products of the Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 BMG heated at constant rates of 0.5 K/s, 5 K/s and 50 K/s, respectively, in comparison with the spherulites observed in the heat-affected zone of laser processed
Fig. 1. XRD profiles of crystalline Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 products obtained at heating rates of 0.5 K/s, 5 K/s, 50 K/s and w5000 K/s, compared with the profile for the initially amorphous alloy.
material as well as the as-cast glass. A broad peak was observed for the as-cast specimen, indicating that the material was amorphous before heating. After heating, the specimens are all crystalline, as illustrated by the multiple sharp Bragg diffraction peaks. Table 1 summarizes the three strongest crystalline peak positions in the XRD patterns for the different heating rates. Although phase identification is difficult due to the complexity of the five component alloy system, it is evident that the crystals produced at 5 K/s, 50 K/s, and w5000 K/s have similar crystalline structures, as shown by the similarity of most peak positions, in contrast with the structure formed at 0.5 K/s. Microstructural investigations of the crystallization products heated at different rates further demonstrate that the morphology depends on the heating rate. As shown in Fig. 2(a), a heating rate of 0.5 K/s results in a nanocrystalline structure with crystal sizes on the order of 100e200 nm. At a heating rate of 5 K/s (Fig. 2 (b)), spherulites with an average size of w2 mm are observed, corresponding to a number density of nuclei, N, of 1017 m3. Crystallization occurs by radial growth from central nuclei until the crystals impinge upon each other. At a higher heating rate of 50 K/s, the optical image shown in Fig. 2(c) demonstrates a similar formation of spherulites, but with a much larger average crystal size. Several of the nucleation sites are marked by arrows. Again, the spherulites grew until they impinged. The size and dense packing of the spherulites formed at 50 K/s is quite similar to that formed by laser processing at an estimated heating rate of w5000 K/s (Fig. 2(d)). The average crystal sizes for the spherulites shown in Fig. 2(c) and (d) are on the order of 15e20 mm, corresponding to N ¼ 1014 m3.
Table 1 Comparison of the positions of the three strongest XRD peaks (2q values) for the crystallized products of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 obtained at different heating rates. Heating Method
Heating rate
1st
2nd
3rd
Microstructure
Nuclei density
DSC DSC Gleeble Laser
0.5 K/s 5 K/s 50 K/s w5000 K/s
38.87 37.70 37.58 37.70
36.95 35.64 35.51 35.62
34.49 33.96 33.74 33.78
nanocrystals spherulites spherulites spherulites
w w w w
1021 1017 1014 1014
m3 m3 m3 m3
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Fig. 2. Microstructural morphologies of the crystallized Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 obtained at various heating rates. (a) BF TEM image of nanocrystalline structure formed at a heating rate of 0.5 K/s; (b) BF TEM image of micro-scale spherulites formed at a heating rate of 5 K/s; (c) optical image of spherulites formed at a heating rate of 50 K/s; (d) backscattered SEM image of spherulites in the heat-affected zone formed by laser processing at an estimated heating rate of w5000 K/s. The arrows in (c) identify several nucleation sites.
Fig. 3 compares the nano-scale crystals produced at a low heating rate and the micro-scale spherulites formed at high heating rates. The BF TEM images of the crystallized products are shown on the left-hand side, with the Z-contrast images on the right-hand side obtained using an STEM mode with an HAADF detector [22]. Several nanocrystalline phases with different local compositions are distinguishable in Fig. 3(a) and (b), which shows the specimen crystallized at 0.5 K/s. The nanocrystalline phases are observed throughout the specimen, and the boundaries between the nanocrystals are sharp and distinct. In contrast, the spherulites formed at higher heating rates exhibit a more or less uniform compositional distribution. The spherulites are polycrystalline, composed of closely packed columnar aggregates. The contrast differences of the adjacent aggregates under TEM BF imaging conditions results from the differences in crystallographic orientation (Fig. 3(c) and (e)). Notice that the aggregates spacing formed at 50 K/s is on the order of 200 nm (Fig. 3(e)), much larger than that produced at 5 K/s, which is on the order of 10 nm (Fig. 3(c)). Although phase contrast is distinct in the BF TEM images, the STEM images in Fig. 3(d) and (f) show little compositional contrast between the neighboring spherulitic aggregates. Indeed, no significant elemental partitioning occurred during the formation of the spherulites at 5 K/s as evidenced by the lack of contrast in Fig. 3(d). In comparison, a secondary phase is observed along the boundaries of the spherulitic aggregates formed at a heating rate of 50 K/s, as shown in Fig. 3(f). Compositional differences of the nanocrystals and the spherulites were investigated in greater detail via high resolution EDS line scans using STEM mode with an HAADF detector. Fig. 4(a) shows the high magnification STEM image of the nanocrystalline specimen
produced at 0.5 K/s. At least four different nanocrystalline phases, labeled A, B, C, and D in Fig. 4(a), are distinguishable with distinct boundaries between each other. Fig. 4(b) and (c) illustrate the EDS line scans performed across the boundaries between the neighboring crystals. Table 2 summarizes the EDS measurements of these four phases. The compositional evidence strongly suggests phase separation. Note that Nb is found primarily in region A and is virtually non-existent in phases B and D, while the Ni content of region A and the Al content of region D are much lower than the nominal values. In comparison, the compositional analysis of the spherulites formed at a heating rate of 50 K/s is illustrated in Fig. 5. Fig. 5(b) shows the EDS line scans across the spherulitic aggregate boundary highlighted in Fig. 5(a). The columnar aggregates are closely packed in the spherulites and are homogeneous in composition. A secondary phase with a distinct composition rich in Nb and a thickness of w20 nm is observed along the aggregate boundaries. As shown in Table 2, EDS analysis indicates that the chemical compositions of the spherulitic aggregates are very close to that of the parent amorphous material and completely different from any of the nanocrystals produced at the lower heating rate. The kinetics of the nanocrystallization and spherulitic crystallization processes were examined using DSC. Continuous heating DSC curves obtained at heating rates of 0.2e5.0 K/s are presented in Fig. 6. Each of the DSC traces exhibits an obvious endothermic event characteristic of the glass transition, followed by a significant exothermic event characteristic of the crystallization process. The glass transition temperatures (Tg), the crystallization onset (Tx) and peak temperatures (Tp), as well as the width of the supercooled liquid region (DTx ¼ TxTg) are summarized in Table 3. Notice that Tg, Tx and Tp are all shifted to higher temperatures as the heating
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Fig. 3. Detailed morphologies of (a)-(b) nanocrystals formed at 0.5 K/s, (c)-(d) micro-spherulites formed at 5 K/s, and (e)-(f) spherulitic aggregates formed at 50 K/s (a), (c) and (e) are BF TEM images; (b), (d) and (f) are HAADF STEM images.
rate increases. The single crystallization peaks observed in the DSC scans suggest that all of the nanocrystalline phases form within a similar temperature regime so that the exothermic heat flows detected by DSC overlap. 4. Discussion The Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 alloy investigated in the present study is an excellent glass former, as illustrated by its extended supercooled liquid region. The critical cooling rate required to achieve a fully amorphous structure has been shown to be w1.75 K/s for this alloy [5]. However, in the present study heating rates of up to w5000 K/s were not sufficient to bypass
crystallization, which motivates this study of crystallization mechanisms. Similar asymmetry in the crystallization behaviors during continuous heating of amorphous specimens and cooling from the stable melts has been observed in several bulk metallic glass-forming systems [4,23]. Schroers et al. indicated that quenched-in clusters or nuclei during cooling and their subsequent growth may contribute to the strong asymmetry between the heating from the amorphous state and cooling from the liquid state, and consequently result in a much higher critical heating rate than the critical cooling rate to avoid crystallization [4]. However, there are no published reports of completely different crystallization products resulting from different heating rates. Therefore, we now examine the mechanisms of crystallization upon heating.
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Fig. 4. (a) STEM image of the nanocrystalline structure produced at a heating rate of 0.5 K; (b) and (c) describe the corresponding EDS line scan results across the interfaces between nanocrystals A-B and C-D marked in (a), respectively.
In the present study, crystallization was observed during heating of the glass to a maximum temperature between the glass transition and the liquidus temperature of this alloy (w1100 K). The microstructural and XRD observations reveal that different crystallization products were obtained upon continuous heating at different rates. To investigate differences in the phase transformation mechanisms, the variation of three characteristic temperatures (Tg, Tx, and Tp) during continuous heating processes were fitted to the Kissinger equation [24]:
Table 2 Composition of the crystalline products of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 obtained at several heating rates. Heating rate
Composition (at. %)
Zr
Cu
Ni
Al
Nb
N/A
Nominal
58.5
15.6
12.8
10.3
2.8
0.5 K/s (Fig. 4)
A B C D Spherulitic aggregates Boundary phase Spherulitic aggregates Boundary phase
45.6 44.2 54.7 59.7 53.7 51.2 55.8 63.5
18.5 26.0 19.4 19.2 22.2 19.9 27.7 19.7
1.2 21.1 12.7 17.9 10.7 7.6 9.6 1.7
26.5 8.6 10.9 2.7 12.3 10.2 6.2 3.3
8.2 0.1 2.3 0.5 1.1 11.1 0.7 11.8
50 K/s (Fig. 5) w5000 K/s
T2 ln c B
! ¼
Ec AR ln Ec RTc
(1)
where B is the constant heating rate (dT/dt); Tc is the characteristic temperature of interest; Ec is the activation energy for the transition event occurring at the characteristic temperature of interest (e.g. the onset of crystallization); R is the gas constant; and A is a material constant. Using the characteristic temperatures identified as functions of heating rate by the DSC study, Kissinger plots are presented in Fig. 7 as lnðTc2 =BÞ versus 1000/Tc. The data for the glass transition events result in a straight line and an activation Eg ¼ 380.3 kJ/mol. This single valued activation energy indicates that the mechanism for the second-order transition from glass to supercooled liquid is not a function of the heating rate. In contrast, the onset and peak crystallization temperature data exhibit a deviation from linearity at heating rates greater than 2.5 K/s. Below this heating rate, the activation energy for the onset of the crystallization process is Ex ¼ 257.5 kJ/mol, and that for the peak is Ep ¼ 213.6 kJ/mol. At higher heating rates, where the transition from nanocrystallization to spherulite formation is observed, the slopes of the Kissinger plots decrease. This indicates a reduction in the activation energies, although there is insufficient DSC data to obtain a reliable value. A separate study of spherulite formation in the heat-affected zone formed during laser processing indicates
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Fig. 5. (a) STEM image of spherulitic aggregates formed at a heating rate of 50 K/s. (b) The corresponding EDS line scan across the indicated aggregate boundary.
that the activation energy for spherulitic crystallization at heating rates on the order of 103 K/s is Es ¼ 123.5 kJ/mol [21]. The variation in the activation energy indicates that while nanocrystallization occurs at lower temperatures than the formation and growth of the spherulites, the activation energy for nanocrystallization is higher than that for the spherulite formation. According to the EDS analysis presented in Table 2, the composition distribution of the nanocrystalline structure formed at low heating rates is inhomogeneous, indicating that phase separation occurred at low heating rates. It has been suggested that the combined free energy of the mixture of multiple amorphous phases could be less than that of the original homogeneous amorphous phase in a certain composition range [18]. As a result, phase separation into several amorphous phases with different compositions is commonly observed to precede nanocrystallization in the Zr-Cu-Ni-Al-Nb BMG systems [6,29,30]. Furthermore, it has been
demonstrated that in the Zr-Cu-Ni-Al-Nb alloy system, Cu (r ¼ 0.157 nm), Ni (r ¼ 0.162 nm), and to some degree Al (r ¼ 0.182 nm) can substitute for each other [9,31]. Therefore, the compositions of the nanocrystals are consistent with several known intermetallics: Zr5Al3Nb2 with some Cu substitutions in Region A, Zr(Cu, Ni) with some Al substitutions in Region B, and Zr2(Cu, Ni) in Region D, although caution is advised as the measured Cu fractions may be high due to the contribution of the Cu grids used to support the FIB-milled TEM foils. Notice that the composition in Region C is close to the nominal glass composition of Zr58.5Cu15.6Ni12.8Al10.3Nb2.8, possibly corresponding to a quasicrystalline phase formed in the multicomponent system [32,33]. In contrast, there is no evidence of extensive phase separation as a precursor to crystallization during the rapid heating process that resulted in spherulite formation. The composition of the spherulitic aggregates is close to that of the amorphous alloy. Thus, the higher activation energy for crystallization measured at low heating rates where nanocrystallization was observed reflects the added energetic cost of a phase separation step. To compare the kinetics of the nanocrystallization and spherulitic crystallization mechanisms, the relative degree of crystallinity, X, at time t after the onset of crystallization was calculated assuming a linear relationship between the integral area under the crystallization peak and the percent crystallinity. The resulting behaviors at different heating rates are shown in Fig. 8(a). While the crystallization rate generally increases with increasing heating rate, note that the transformation rates for the three highest heating
Table 3 Comparison of the glass transition (Tg), crystallization onset (Tx), and crystallization peak (Tp) temperatures, and the width of the supercooled liquid regions (DTx) obtained via DSC at several heating rates.
Fig. 6. DSC traces of amorphous Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 specimens obtained at heating rates varying from 0.2 K/s to 5 K/s.
Heating rates (K/s)
Tg (K)
Tx (K)
Tp (K)
DTx (K)
0.2 0.3 0.5 0.7 0.8 1.3 1.7 2.5 3.3 4.2 5.0
663.4 671.3 675.3 678.2 680.4 684.1 687.3 690.3 692.6 695.6 698.3
757.4 770.9 778.1 784.9 787.9 796.0 800.4 809.6 820.2 831.7 850.7
763.1 778.6 785.9 794.0 796.9 810.2 816.0 831.7 845.9 862.0 887.6
94.0 99.6 102.8 106.7 107.5 111.9 113.1 119.3 127.6 136.1 152.4
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H. Sun, K.M. Flores / Intermetallics 19 (2011) 1538e1545 Table 4 Avrami exponents, n, obtained at several heating rates. Heating rates (K/s) n
Fig. 7. Kissinger plots for the glass transition, crystallization onset and crystallization peak temperatures obtained at various heating rates.
rates, 2.5 K/s, 3.3 K/s, and 4.2 K/s, are virtually indistinguishable. The kinetics of crystallization upon heating at a constant rate can be described by the Avrami equation [27]:
X ¼ 1 expðZc t n Þ
(2)
where the Avrami exponent, n, depends on the crystal nucleation and growth mechanisms, and Zc is a constant dependant on temperature. Because n is determined by both the nucleation and growth characteristics, it has been used to identify different types of phase transformation mechanisms. The values of n are determined for each heating rate from the slopes of the linearized data presented in Fig. 8(b) and are listed in Table 4. At the lower heating rates of 0.5 and 0.8 K/s, n is approximately 4.5; at heating rates of 2.5 K/s and greater the value of n decreases to the range of 2.1e2.6. Prior theoretical studies have shown that an n value of 4 indicates a constant nucleation rate throughout the transformation process
0.5 4.58
0.8 4.56
1.7 3.69
2.5 2.59
3.3 2.13
4.2 2.23
with three-dimensional diffusion-controlled growth [8,28], while a value of n ¼ 2.5 indicates that crystallization occurs by the growth of a fixed number of nuclei [28]. This indicates that the spherulites primarily form from pre-existing or quenched-in nuclei in the glass, while new crystals nucleate throughout the nanocrystallization process. In addition to the phase separation step at low heating rates, this continuous nucleation of new crystals may also contribute to the increased activation energy for crystallization under these conditions. Although the Avrami exponent suggests that the number of spherulite nuclei is fixed, a change in the number and spacing of the spherulites over a wider range of heating rates indicates that preexisting clusters or nuclei may be supplemented by nucleation during the continuous heating process. For example, the number density of spherulites increases by three orders of magnitude from N ¼ 1014 m3 at a heating rate of 50 K/s to N ¼ 1017 m3 at a heating rate of 5 K/s due to the longer time spent at the peak nucleation temperature at the lower heating rate. However, the number density of the spherulites becomes constant for heating rates greater than 50 K/s, as illustrated by the similar spherulite sizes shown in Fig. 2(c) and (d). Therefore, the quenched-in nuclei density in the as-cast material can be estimated as N ¼ 1014 m3. The spherulitic growth process begins with columnar aggregates growing radially from the quenched-in clusters or nuclei; this radial growth dominates the formation of the spherulites. Due to the relatively low diffusivity in the glass, the diffusion necessary for epitaxial growth cannot keep up with the crystallization process, and consequently different orientations are obtained within the spherulitic aggregates [25,26]. The variation in the crystallographic orientations results in the observed contrast difference in the BF TEM images shown in Fig. 3(c) and (e). The aggregates grow from the isolated nucleation sites until they interfere, ultimately consuming the glass. Although the formation of the spherulites involves the redistribution of the elements at the growth front, no compositional segregation was observed during the spherulitic crystallization at a heating rate of 5 K/s (Fig. 3(c) and (d)). However, EDS analysis indicates that there is an
Fig. 8. Crystallization rate as a function of heating rates. (a) Crystallization fraction X versus time and (b) fits to the linearized Avrami equation.
H. Sun, K.M. Flores / Intermetallics 19 (2011) 1538e1545
elemental segregation along the spherulitic aggregate boundaries at higher heating rates (e.g. at 50 K/s in Fig. 5 or in the heat-affected zone produced by laser processing shown in Fig. 2(d)), wherein the material reaches a higher temperature with consequently high diffusivity. The crystalline phase appears to have rejected the Nb to the boundary phase. It has been reported that Zr preferentially reacts with Cu, Ni and Al to produce intermetallic compounds because of the negative mix enthalpies of mixing (DHmix ZrCu ¼ 23kJ=mol, DHZrNi ¼ 49kJ=mol mix and DHZrAl ¼ 44kJ=mol) [11,34]. On the contrary, Nb has a positive enthalpy of mixing with Zr (DHmix ZrNb ¼ 4kJ=mol) [34]. The ZreNb binary phase diagram [35] suggests that Nb is likely to form a bodycentered cubic (bcc) solid solution with Zr at a temperature above 960 K, providing an explanation of the partition of the Nb to form the boundary phase which is also enriched in Zr but depleted in Cu, Ni and Al, as shown in Fig. 5. On the other hand, the Nb remains supersaturated in the spherulites at a low heating rate, e.g. 5 K/s, with the spherulitic crystallization taking place below the critical temperature for the formation of bcc ZreNb phase. The resulting structure is uniform in composition. 5. Conclusions Although the Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 BMG is considered to have excellent stability and glass forming ability due to its low critical cooling rate, recent work on laser processing of the glass has shown that spherultic crystals form in the glass even at heating rates on the order of 103 K/s. In this work, the crystallization mechanisms of this glass were investigated at heating rates spanning 4 orders of magnitude in an effort to explain this heating/cooling asymmetry and the significant microstructural differences between low and high heating rate crystallization products. While micro-scale spherulites were observed at heating rates greater than 2.5 K/s, heating rates less than 2.5 K/s produced several nanocrystalline phases with distinct compositions. Calculations of the activation energy for crystallization and the Avrami exponent indicate that this nanocrystallization process occurs via phase separation, transient nucleation, and threedimensional diffusion-controlled growth. In contrast, the lower values for the activation energy and Avrami exponent during rapid heating indicate that nucleation is suppressed, so growth from quenched-in nuclei dominates the crystallization process. The rapid transformation from glass to spherulites was accomplished without evidence of prior phase separation. Based on spherulite size at heating rates of 50 K/s and 5000 K/s, the quenched-in nuclei density was estimated to be 1014 m3. Acknowledgements This work was funded by the Office of Naval Research under Award #N0014-08-1-1023. References [1] Schroers J, Johnson WL. History dependent crystallization of Zr41Ti14Cu12Ni10Be23 melts. Journal of Applied Physics 2000;88:44e8. [2] Schroers J. Processing of bulk metallic glass. Advanced Materials 2010;22: 1566e97. [3] Schroers J, Johnson WL. Crystallization of Zr41Ti14Cu12Ni10Be23. Materials Transactions, JIM 2000;41:1530e7. [4] Schroers J, Masuhr A, Johnson WL. Pronounced asymmetry in the crystallization behavior during constant heating and cooling of a bulk metallic glassforming liquid. Physical Review B 1999;60:11855e8. [5] Hays CC, Schroers J, Johnson WL. Vitrification and determination of the crystallization time scales of the bulk-metallic-glass-forming liquid Zr58.5Nb2.8Cu15.6Ni12.8Al10.3. Applied Physics Letters 2001;79:1605e7.
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