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Crystallization behaviour and thermal stability of two aluminium-based metallic glass powder materials
Materials Science and Engineering A 530 (2011) 432–439
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Crystallization behaviour and thermal stability of two aluminium-based metallic glass powder materials X.P. Li a , M. Yan a , B.J. Yang b , J.Q. Wang b,∗ , G.B. Schaffer a , M. Qian a,∗∗ a b
The University of Queensland, School of Mechanical and Mining Engineering, ARC Centre of Excellence for Design in Light Metals, Brisbane, QLD 4072, Australia Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 2 October 2010 Received in revised form 6 July 2011 Accepted 26 September 2011 Available online 7 October 2011 Keywords: Aluminium alloys Metallic glass Crystallization Powder
a b s t r a c t The crystallization behaviour and thermal stability of two Al-based metallic glass powder materials, Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 , have been investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electron microscopy. Both alloy powders show a distinct three-stage crystallization process with a similar gap of ∼75 K between the onset crystallization temperature (Tx ) and the second crystallization temperature. Crystallization occurs by the precipitation and growth of fccAl, without intermetallic formation. The apparent activation energy for each stage of crystallization was determined from DSC analyses and the phases resulting from each crystallization stage were identified by XRD and electron microscopy. The critical cooling rate for each alloy powder was calculated from the DSC data. These results are necessary to inform the consolidation of amorphous powder particles of Al85 Ni5 Y6 Co2 Fe2 or Al86 Ni6 Y4.5 Co2 La1.5 into thick (>1 mm) metallic glass components. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Aluminium based bulk metallic glass (BMG) alloys are difficult to fabricate because of their high critical cooling rates (∼103 –104 K/s) [1,2] compared to Fe-, Zr-, Cu-, Pd-, Mg-, La-, Nd-based metallic glass materials [3–8]. As a result, it has taken over two decades to design and make a few bulk (∼1 mm thick) aluminium based metallic glass alloys [9] since Inoue et al. reported high strength aluminium based metallic glasses in 1988 [10]. Al86 Ni6 Y4.5 Co2 La1.5 (BMG) and Al85 Ni5 Y6 Co2 Fe2 (nearly BMG) are two such recent developments [9,11] which possess outstanding glass forming ability among all aluminium alloy compositions known to date. Given the challenges encountered in developing Al-based BMG and the limited thickness (∼1 mm at present) that can be achieved, alternative approaches to the fabrication of Al-based BMG alloys are desired. One possibility is to make amorphous powder particles and then consolidate them through a powder metallurgy route such as spark plasma sintering or hot pressing. Recent experimental work has indicated that it is practical to gas atomize both Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 into amorphous powders [9]. To fabricate Al-based BMG much thicker than 1 mm from powder, it is necessary to understand the thermal stability and crystallization behaviour of these alloys in their powder form.
∗ Corresponding author. ∗∗ Corresponding author. Fax: +61 7 3346 7015. E-mail addresses: [email protected] (J.Q. Wang), [email protected] (M. Qian). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.09.107
This study investigates the thermal stability and the crystallization behaviour of amorphous Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 powders with respect to a range of heating rates. It is also important to understand the crystallization behaviour because recent work has shown that controlled crystallization can lead to much improved mechanical properties of metallic glasses [12–17]. 2. Experimental Amorphous aluminium alloys Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 were gas-atomized in high purity nitrogen. The as-atomized powders were sieved to <25 m. They were examined and studied using XRD and scanning electron microscopy (SEM) using a JEOL JSM 7001F, operated at 15 kV with a spot size in the order of 10 nm. Their thermal stability and crystallization kinetics were investigated using non-isothermal DSC with a Netzsch A409C DSC under a continuous flow of pure argon. To determine the critical cooling rates, a range of non-isothermal DSC studies were conducted, where the powder samples (sample weight is fixed at 15 mg) were heated to 873 K at a constant heating rate of 20 K/min and then cooled at rates of 5, 10, 20, 30 and 40 K/min. To determine the activation energy for crystallization, powder samples were heated at rates of 5, 10, 20, 30 and 40 K/min to 873 K and subsequently cooled to room temperature at a constant cooling rate of 20 K/min without isothermal holding. Based on these studies and the peak temperatures determined, powder samples were heated to the identified peak temperature in the DSC furnace and then quenched to room temperature with a forced
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Fig. 1. XRD patterns of the as-gas-atomized alloy powders.
nitrogen gas flow to retain the microstructural features at each peak temperature. The treated powder samples were analysed in detail using XRD, SEM and transmission electron microscopy (TEM) in a JEOL JEM 2100. The TEM samples were prepared from powder particles using an xT Nova NanoLab 200 focused ion beam (FIB).
Fig. 3. Non-isothermal DSC scans of the metallic glass powder at different heating rates: (a) Al86 Ni6 Y4.5 Co2 La1.5 powder and (b) Al85 Ni5 Y6 Co2 Fe2 powder.
3. Results and discussion 3.1. Microstructure of the as-gas-atomized powder Fig. 1 shows the XRD patterns obtained from the as-gasatomized powders of Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 . Each spectrum displayed a broad diffuse maximum at angles corresponding to ∼40◦ , typical of amorphous materials. Fig. 2 shows a typical SEM back-scattered view of the microstructure of an asatomized powder particle from each composition. A featureless microstructure was observed for the Al86 Ni6 Y4.5 Co2 La1.5 powder (Fig. 2a), indicative of its essentially amorphous state. However, it is noted that the XRD patterns show a few tiny Bragg peaks (fccAl) for both compositions, with those for the Al85 Ni5 Y6 Co2 Fe2 alloy powder being more noticeable. It is likely that the Al85 Ni5 Y6 Co2 Fe2 alloy powder examined has contained a small amount of fcc-Al. 3.2. Characteristic crystallization events
Fig. 2. SEM back-scattered images of as-gas-atomized metallic glass powder particles: (a) Al86 Ni6 Y4.5 Co2 La1.5 and (b) Al85 Ni5 Y6 Co2 Fe2 .
Fig. 3 shows the non-isothermal DSC curves of the two powder materials with respect to different heating rates. The curves show transformation from amorphous to crystalline in three steps
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Fig. 5. (a) Kissinger plots, (b) Augis and Bennett plots, and (c) Ozawa plots for the Al85 Ni5 Y6 Co2 Fe2 powder. Fig. 4. (a) Kissinger plots, (b) Augis and Bennett plots, and (c) Ozawa plots for the Al86 Ni6 Y4.5 Co2 La1.5 powder.
of the non-isothermal DSC curve with respect to different heating rates for both compositions. The onset crystallization temperature Tx and crystallization peak temperatures Tp1 , Tp2 , Tp3 are indicated in Fig. 3 for each powder and their values are listed in Table 1. The apparent activation energy for each characteristic transformation can be derived from the Kissinger equation [18] according to the shift
ln
B T2
=−
E +C RT
(1)
where B is the heating rate, E is the apparent activation energy, C is a constant, R is the gas constant and T is the absolute characteristic temperature such as Tx , Tp1 , Tp2 , Tp3 indicated on the DSC curves.
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Table 1 Characteristic temperatures of Al86 Ni6 Y4.5 Co2 La1.5 and Al85 Ni5 Y6 Co2 Fe2 glass powders: onset crystallization temperature (Tx ) and three crystallization peak temperatures (Tp1 , Tp2 , Tp3 ). Powder
Heating rate (◦ C/min)
Tx (◦ C)
Tp1 (◦ C)
Tp2 (◦ C)
Tp3 (◦ C)
Al86 Ni6 Y4.5 Co2 La1.5
5 10 20 30 40 5 10 20 30 40
269.7 273.5 277.8 279.8 281.8 258.7 263.3 268.8 271.1 274.6
273.3 277.1 282 285.2 289.4 272.1 277.5 281.1 285.6 290.2
325.5 329.9 335.8 338.8 341.8 331.6 337.1 343.1 347 351.4
366.6 374.8 385 389.9 395 388.8 397.8 407.8 413.6 419.5
Al85 Ni5 Y6 Co2 Fe2
Fig. 4a shows the Kissinger plots for Al86 Ni6 Y4.5 Co2 La1.5 and Fig. 5a for Al85 Ni5 Y6 Co2 Fe2 . To assess the accuracy of the apparent activation energy terms determined by the Kissinger approach, two other widely used methods, the Augis–Bennett method [19] and the Ozawa method [20], were also used to estimate the apparent activation energy for each characteristic transformation. The three methods are all fundamentally similar. Although the Kissinger approach is widely applied, the use of the Augis–Bennett [19] and Ozawa [20] methods will permit an indirect assessment of the accuracy of the activation energy data obtained from the Kissinger approach. The Augis–Bennett method [19] is described by
ln
B T − T0
=−
E +C RT
(2)
where T0 is the initial temperature of the DSC curves, and the Ozawa method [20] by ln (B) = −
E +C RT
(3)
The Augis–Bennett plots for each alloy are shown in Figs. 4b and 5b while the Ozawa plots are shown in Figs. 4c and 5c. Excellent linear relationships were obtained for each characteristic transformation with each of the three approaches, demonstrating a high level of consistency between them. Table 2 summarizes the apparent activation energy values extracted from Figs. 4 and 5. The activation energy values calculated using the three methods are consistent with each other, confirming the reliability of the data. The values of Ex , Ep1 and Ep2 are higher for the Al86 Ni6 Y4.5 Co2 La1.5 metallic glass powder than for the Al85 Ni5 Y6 Co2 Fe2 metallic glass powder while the apparent activation energy Ep3 for Tp3 is similar for Al86 Ni6 Y4.5 Co2 La1.5 and Al85 Ni5 Y6 Co2 Fe2 . The activation energy Ep3 is much less important than each of the other three energy terms Ex , Ep1 and Ep2 from a crystallization point of view. This is because crystallization has already occurred to a readily detectable extent at Tp2 (see Fig. 10 for peak 2) and intermetallic phases form and grow to a great extent after Tp2 . Hence, in most cases, the processing temperature should not exceed Tp2 in order to keep the amorphous nature and prevent intermetallic phases from forming and growing. The apparent activation energy consists of two parts: one is related to nucleation and the other is related to crystal growth [21]. It is a measure of the thermodynamic barrier to crystallization. Higher crystallization activation energies generally favour thermal stability. The higher onset crystallization temperature (Tx ) and activation energies (Ex , Ep1 and Ep2 ) suggest that the Al86 Ni6 Y4.5 Co2 La1.5 metallic glass powder has higher resistance or better thermal stability against crystallization than the Al85 Ni5 Y6 Co2 Fe2 metallic glass powder. The limited data available in the literature shows that the crystallization activation energies for Al84 Gd6 Ni7 Co3 glass powder are Ep1 = 283 ± 2 kJ/mol and Ep2 = 210 ± 4 kJ/mol [22], and Ep = 270 ± 5 kJ/mol for Al70 Y16 Ni10 Co4
glass powder [23]. The Al86 Ni6 Y4.5 Co2 La1.5 glass powder thus shows higher activation energies of Ep1 and Ep2 (see Table 2) compared with these two Al-based glass powder materials, suggesting higher resistance to crystallization. It should be noted that the activation energies (particularly the Ex value) determined for Al86 Ni6 Y4.5 Co2 La1.5 are higher than those for Al85 Ni5 Y6 Co2 Fe2 and other Al-based MGs reported previously [22,23]. This is consistent with several recent studies [24–27], which showed that metallic glasses with higher GFA tend to show better thermal stability due to their more efficient and stable atomic packing in local structures. Another point to note is that the activation energy for the onset crystallization (Ex ) is actually higher than the activation energies (Ep ) for each peak crystallization event, especially for the Al86 Ni6 Y4.5 Co2 La1.5 MG. This agrees with Surreddi et al. [22] and McKay et al. [28] who reported similar observations for the crystallization of the Al84 Gd6 Ni7 Co3 and Al70 Ni13 Si17 , respectively. However, different observations have been reported; Zhang et al. [29–31] found that the Ex is lower than Ep for several Al-based MGs, which have different compositions from the above-mentioned. A recent study of the atomic structures in three Al-based MGs including Al86 Ni6 Y4.5 Co2 La1.5 has revealed that atomic pare factors such as the atomic pair species, pair fractions and their corresponding bond distances play an essential role in determining the actual GFA [32] and therefore the thermal stability because of its connections with the GFA [24–27]. All these atomic pair factors are closely related to the glass composition or the solute species. This suggests that the activation energy value for each characteristic event is alloy composition dependent in addition to its dependency on the annealing history. As a result, both types of observations could be possible. 3.3. Critical cooling rates The critical cooling rate (Rc ) is an important parameter that reflects the glass forming ability of a metallic alloy [33]. In the context of amorphous alloy powder, the critical cooling rate offers an important indication of the practicality of the amorphous alloy being gas-atomized into amorphous alloy powder. The Rc value can be estimated from the non-isothermal DSC analyses of the powder samples using the relationship described by Eq. (4) [34] ln R = ln Rc −
B (Tl − Txc )2
(4)
where R is the actual cooling rate, Tl is the liquidus temperature of the alloy (also referred to as offset temperature), and Txc is the onset temperature of solidification or the temperature at which the first major nucleation events occur (designated as Tn in solidification). The Tl value can be obtained from the DSC heating curve while the Txc value is obtainable from the DSC cooling curve. Fig. 6 shows the DSC cooling curves obtained from heating the two compositions of powder at a fixed heating rate of 20 K/min to
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Table 2 Apparent activation energy values for characteristic events of Al86 Ni6 Y4.5 Co2 La1.5 and Al85 Ni5 Y6 Co2 Fe2 powders (kJ/mol), calculated using the Kissinger, Augis–Bennet and Ozawa methods. Powder
Method
Ex (kJ/mol)
Al86 Ni6 Y4.5 Co2Lai.5
Kissinger method Augis and Bennett method Ozawa method Kissinger method Augis and Bennett method Ozawa method
418.18 416.93 427.22 312.01 310.53 320.94
Al85 Ni5 Y6 Co2 Fe2
± ± ± ± ± ±
7.20 7.20 7.19 13.61 13.59 13.58
Ep i (kJ/mol) 324.45 323.34 333.71 294.10 292.92 303.33
± ± ± ± ± ±
30.03 29.99 30.03 31.55 31.52 31.54
Ep 2 (kJ/mol) 380.24 380.08 390.36 327.02 326.99 337.28
± ± ± ± ± ±
16.97 16.95 17.04 19.29 19.26 19.28
Ep 3 (kJ/mol) 250.23 250.82 261.12 250.29 251.21 261.55
± ± ± ± ± ±
6.51 6.48 6.48 6.83 6.79 6.74
1273 K, followed by cooling at five different cooling rates. Fig. 7 displays the DSC heating curves for both compositions of powder at 20 K/min to 1273 K. Since the heating rate is fixed, the DSC heating curves are almost identical from each run for each powder. Only one DSC heating curve is therefore presented in Fig. 7 for each composition. By plotting ln R versus 1/(Tl − Txc )2 , the Rc value can be obtained from the ln R-intercept if the resultant relationship is linear. A good linear relationship was found to exist between ln R and 1/(Tl − Txc )2 for each alloy (Fig. 8). This provides an estimate of Rc = 1000 K/s for Al86 Ni6 Y4.5 Co2 La1.5 and Rc = 1500 K/s for Al85 Ni5 Y6 Co2 Fe2 . This indicates that both alloys can readily be gas-atomized into amorphous powders,
Fig. 7. DSC heating curves for each composition of amorphous powder. Heating rate: 20 K/min; samples size: 15 mg.
consistent with our observations. In addition, the results suggest that Al86 Ni6 Y4.5 Co2 La1.5 alloy powder has better glass forming ability than the Al85 Ni5 Y6 Co2 Fe2 alloy powder. 3.4. Powder crystallization behaviour Powder samples of both alloy compositions were heated to their respective peak temperatures and cooled rapidly to room temperature with a forced nitrogen gas flow without isothermal holding. Fig. 9 shows the SEM back-scattered images obtained from the powder samples of Al86 Ni6 Y4.5 Co2 La1.5 subject to different treatments
Fig. 6. Non-isothermal DSC scans of the two alloy powders at different cooling rates: (a) Al86 Ni6 Y4.5 Co2 La1.5 powder and (b) Al85 Ni5 Y6 Co2 Fe2 powder.
Fig. 8. The critical cooling rates for the two metallic glass alloy powders.
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Fig. 9. SEM back-scattered images of the Al86 Ni6 Y4.5 Co2 La1.5 powder treated at different peak temperatures with inset TEM images showing the fcc-Al crystals: (a) peak 1, (b) peak 2, (c) peak 3. The right-hand side images are views of the left side images at higher magnification. The insets show the difference in the size of the nanocrystals (with a dark contrast) in each microstructure.
and Fig. 10 shows the corresponding XRD patterns. Heating powder samples of Al86 Ni6 Y4.5 Co2 La1.5 to the first peak temperature (Tp1 ) at 20 K/min led to the formation of ∼5 nm diameter primary crystals in the amorphous matrix (Fig. 9a). These nanocrystals are fcc-Al crystallites according to the XRD spectrum (Fig. 10). At Tp1 , the microstructure is composed of the amorphous matrix and Al nanocrystals. Continuing to heat the powder to the second peak temperature (Tp2 ) results in three noticeable changes, (i) growth of the Al nanocrystals to ∼20 nm (Fig. 9b); (ii) increased crystallization (Fig. 10), where the diffuse peak for the amorphous matrix has shrunk obviously, and (iii) the formation of two Co-containing crystalline phases, (Ni,Co)3 Al4 and Al2 CoY, detectable by XRD (Fig. 10). The two changes (i) and (ii) are the main processes occurring at this stage according to the SEM back-scattered images and XRD patterns. Full crystallization occurred when the powder was further heated to the third peak temperature (Tp3 ); the amorphous matrix disappeared (Fig. 9c) and the XRD spectrum is typical of that of a crystalline material; no diffuse peak is detected (Fig. 10). In addition, a fourth crystalline phase, Al2.12 La0.88 , is detected (Fig. 10). The Fe-containing alloy was found to crystallize faster and more easily than the La containing alloy. Fig. 11 shows the SEM
Fig. 10. XRD patterns of the Al86 Ni6 Y4.5 Co2 La1.5 metallic glass powder in the as-gasatomized state and after being heated to different peak crystallization temperatures (peak 1, peak 2, peak 3) at 20 K/min.
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Fig. 11. SEM back-scattered images of the Al85 Ni5 Y6 Co2 Fe2 powder heated to different peak temperatures at 20 K/min: (a) peak 1, (b) peak 2, (c) peak 3. The right-hand side images are taken from the left-hand side samples at a higher magnification.
back-scattered images obtained from the powder samples of Al85 Ni5 Y6 Co2 Fe2 after similar treatments and Fig. 12 shows the corresponding XRD diffraction patterns. Crystallized phases are readily noticeable at the first peak temperature from the SEM back-scattered image (Fig. 11a) while TEM was necessary to detect the crystalline phases in the powder of Al86 Ni6 Y4.5 Co2 La1.5 at the first peak temperature (Fig. 9a). This agrees with the differences in the three apparent activation energy terms Ex , Ep1 and Ep2 discussed earlier between the two compositions of powder (see Table 2). The subsequent crystallization is similar to that of the Al86 Ni6 Y4.5 Co2 La1.5 powder except for the formation of different crystalline phases (Fig. 12). It is noted that the second stage of crystallization of the Al85 Ni5 Y6 Co2 Fe2 powder resulted in the formation of an intermediate AlNiY phase, which disappeared in the third stage of crystallization. Full crystallization at the third peak temperature led to the formation of Al9 Co2 , Al3 Y, and Al3 (Ni,Fe) phases in the crystalline Al solid solution matrix (Figs. 11 and 12). The Tp2 temperature can be treated as the maximum nanocrystallization temperature for controlled or partial crystallization. It is encouraging to note that both alloy powders showed a fairly wide gap between Tx and Tp2 , being about 75 K for each according to
Fig. 12. XRD patterns of the Al85 Ni5 Y6 Co2 Fe2 metallic glass powder in the as-gasatomized state and after being heated to different peak crystallization temperatures (peak 1, peak 2, peak 3) at 20 K/min.
X.P. Li et al. / Materials Science and Engineering A 530 (2011) 432–439
Fig. 3. This may serve as a useful processing window for controlled crystallization for each alloy powder. 4. Summary The crystallization process and thermal stability of two advanced Al-based metallic glass powder materials, Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 , have been investigated. The following findings are made from the study. (1) Both Al86 Ni6 Y4.5 Co2 La1.5 powder and Al85 Ni5 Y6 Co2 Fe2 powder show a three-stage crystallization process with a gap of ∼75 K between the Tx and the second crystallization temperature. The main crystallization process is the precipitation and growth of fcc-Al. (2) The critical cooling rates were determined to be ∼1000 K/s for the Al86 Ni6 Y4.5 Co2 La1.5 powder and ∼1500 K/s for the Al85 Ni5 Y6 Co2 Fe2 powder. (3) The Al86 Ni6 Y4.5 Co2 La1.5 powder has better thermal stability than the Al85 Ni5 Y6 Co2 Fe2 powder according to the apparent activation energy data obtained for the onset crystallization and the subsequent first and second crystallization stages for each alloy powder and the results of a detailed microstructural study of their crystallization processes. (4) The crystallization of both alloy powders starts with the formation of fcc-Al nanocrystals in the amorphous matrix. The second stage of crystallization of the Al86 Ni6 Y4.5 Co2 La1.5 powder leads to the growth of fcc-Al nanocrystals while two Co-containing crystalline phases (Ni,Co)3 Al4 and Al2 CoY form. The second stage of crystallization of the Al85 Ni5 Y6 Co2 Fe2 powder results in the formation of an intermediate AlNiY phase, which disappears in the third stage of crystallization. At the third peak temperature, the fully crystallized Al86 Ni6 Y4.5 Co2 La1.5 powder is composed of fcc-Al, Al2 CoY, Al2.12 La0.88 and (Ni,Co)3 Al4 while the fully crystallized Al85 Ni5 Y6 Co2 Fe2 powder consists of fcc-Al, Al9 Co2 , Al3 Y and Al3 (Ni,Fe), respectively. Acknowledgements This work is supported by the Australian Research Council (ARC) and the National Key Basic Research Program of China (Grant No. 2007CB613906). We also acknowledge assistance from the Australian Microscopy & Microanalysis Research Facility (AMMRF).
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References [1] A. Inoue, Prog. Mater. Sci. 43 (1998) 365–520. [2] P. Dong, W.L. Hou, X.C. Chang, M.X. Quan, J.Q. Wang, J. Alloys Compd. 436 (2007) 118–123. [3] W.H. Wang, C. Dong, C.H. Shek, Mater. Sci. Eng. R 44 (2004) 45–89. [4] Z.P. Lu, C.T. Liu, J.R. Thompson, W.D. Porter, Phys. Rev. Lett. 92 (2004). [5] M. Yan, J. Zou, J. Shen, Acta Mater. 54 (2006) 3627–3635. [6] N. Nishiyama, A. Inoue, Mater. Trans. JIM 38 (1997) 464. [7] Z.Y. Chang, X.M. Huang, L.Y. Chen, M.Y. Ge, Q.K. Jiang, X.P. Nie, J.Z. Jiang, Mater. Sci. Eng. A 517 (2009) 246–248. [8] Q.F. Li, K.Q. Qiu, X. Yang, Y.L. Ren, X.G. Yuan, T. Zhang, Mater. Sci. Eng. A 491 (2008) 420–424. [9] B.J. Yang, J.H. Yao, J. Zhang, H.W. Yang, J.Q. Wang, E. Ma, Scripta Mater. 61 (2009) 423–426. [10] A. Inoue, K. Ohtera, A.P. Tsai, T. Masumoto, Jpn. J. Appl. Phys. 2 Lett. 27 (1988) L479–L482. [11] H.W. Yang, J.Q. Wang, Scripta Mater. 55 (2006) 359–362. [12] H.S. Kim, J.K. Lee, S.Y. Shin, T.S. Kim, Crystallization behavior and mechanical property of Cu based metallic glass powders, in: S.J.L. Kang, M.Y. Huh, N.M. Hwang, H. Homma, K. Ushioda, Y. Ikuhara (Eds.), Recrystallization and Grain Growth III, Pts 1 and 2, Trans Tech Publications Ltd, Stafa-Zurich, 2007, pp. 1317–1322. [13] J. Das, M.B. Tang, K.B. Kim, R. Theissmann, F. Baier, W.H. Wang, J. Eckert, Phys. Rev. Lett. 94 (2005). [14] K. Mondal, T. Ohkubo, T. Toyama, Y. Nagai, M. Hasegawa, K. Hono, Acta Mater. 56 (2008) 5329–5339. [15] F.X. Qin, X.M. Wang, A. Inoue, Intermetallics 15 (2007) 1337–1342. [16] N.S. Barekar, S. Pauly, R.B. Kumar, U. Kuhn, B.K. Dhindaw, J. Eckert, Mater. Sci. Eng. A 527 (2010) 5867–5872. [17] S. Mula, K. Mondal, S. Ghosh, S.K. Pabi, Mater. Sci. Eng. A 527 (2010) 3757–3763. [18] V.A. Khonik, K. Kitagawa, H. Morii, J. Appl. Phys. 87 (2000) 8440–8443. [19] J.A. Augis, J.E. Bennett, J. Therm. Anal. 13 (1978) 283–292. [20] H.R. Wang, Y.L. Gao, Y.F. Ye, G.H. Min, Y. Chen, X.Y. Teng, J. Alloys Compd. 353 (2003) 200–206. [21] Z.Z. Yuan, X.D. Chen, B.X. Wang, Y.J. Wang, J. Alloys Compd. 407 (2006) 163–169. [22] K.B. Surreddi, S. Scudino, M. Sakaliyska, K.G. Prashanth, D.J. Sordelet, J. Eckert, J. Alloys Compd. 491 (2010) 137–142. [23] K.G. Prashanth, S. Scudino, B.S. Murty, J. Eckert, J. Alloys Compd. 477 (2009) 171–177. [24] O.A. Lafi, M.M.A. Imran, J. Alloys Compd. 509 (2011) 5090–5094. [25] P. Gargarella, C.S. Kiminami, M.F. de Oliveira, C. Bolfarini, W.J. Botta, J. Alloys Compd. 495 (2010) 334–337. [26] Y. Wang, J.F. Wang, C.C. Li, Mater. Sci. Eng. A 528 (2011) 1623–1627. [27] D.B. Miracle, D.V. Louzguine-Luzgin, L.V. Louzguina-Luzgina, A. Inoue, Int. Mater. Rev. 55 (2010) 218–256. [28] B.J. McKay, P. Cizek, P. Schumacher, K.A.Q. O’Reilly, Mater. Sci. Eng. A 304 (2001) 240–244. [29] Z.H. Zhang, Y.Z. Zhou, E.J. Lavernia, J. Alloys Compd. 466 (2008) 189–200. [30] Z.H. Zhang, D. Witkin, E.J. Lavernia, J. Non-Cryst. Solids 351 (2005) 1646–1652. [31] Z.H. Zhang, N. Yang, Y.Z. Zhou, E.J. Lavernia, Philos. Mag. 88 (2008) 737–753. [32] M. Yan, S. Kohara, J.Q. Wang, K. Nogita, G.B. Schaffer, M. Qian, Scripta Mater. 65 (2011) 755–758. [33] S. Guo, Z.P. Lu, C.T. Liu, Intermetallics 18 (2010) 883–888. [34] A. Inoue, T. Zhang, T. Masumoto, J. Non-Cryst. Solids 156 (1993) 473–480.
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Crystallization behaviour and thermal stability of two aluminium-based metallic glass powder materials X.P. Li a , M. Yan a , B.J. Yang b , J.Q. Wang b,∗ , G.B. Schaffer a , M. Qian a,∗∗ a b
The University of Queensland, School of Mechanical and Mining Engineering, ARC Centre of Excellence for Design in Light Metals, Brisbane, QLD 4072, Australia Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 2 October 2010 Received in revised form 6 July 2011 Accepted 26 September 2011 Available online 7 October 2011 Keywords: Aluminium alloys Metallic glass Crystallization Powder
a b s t r a c t The crystallization behaviour and thermal stability of two Al-based metallic glass powder materials, Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 , have been investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electron microscopy. Both alloy powders show a distinct three-stage crystallization process with a similar gap of ∼75 K between the onset crystallization temperature (Tx ) and the second crystallization temperature. Crystallization occurs by the precipitation and growth of fccAl, without intermetallic formation. The apparent activation energy for each stage of crystallization was determined from DSC analyses and the phases resulting from each crystallization stage were identified by XRD and electron microscopy. The critical cooling rate for each alloy powder was calculated from the DSC data. These results are necessary to inform the consolidation of amorphous powder particles of Al85 Ni5 Y6 Co2 Fe2 or Al86 Ni6 Y4.5 Co2 La1.5 into thick (>1 mm) metallic glass components. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Aluminium based bulk metallic glass (BMG) alloys are difficult to fabricate because of their high critical cooling rates (∼103 –104 K/s) [1,2] compared to Fe-, Zr-, Cu-, Pd-, Mg-, La-, Nd-based metallic glass materials [3–8]. As a result, it has taken over two decades to design and make a few bulk (∼1 mm thick) aluminium based metallic glass alloys [9] since Inoue et al. reported high strength aluminium based metallic glasses in 1988 [10]. Al86 Ni6 Y4.5 Co2 La1.5 (BMG) and Al85 Ni5 Y6 Co2 Fe2 (nearly BMG) are two such recent developments [9,11] which possess outstanding glass forming ability among all aluminium alloy compositions known to date. Given the challenges encountered in developing Al-based BMG and the limited thickness (∼1 mm at present) that can be achieved, alternative approaches to the fabrication of Al-based BMG alloys are desired. One possibility is to make amorphous powder particles and then consolidate them through a powder metallurgy route such as spark plasma sintering or hot pressing. Recent experimental work has indicated that it is practical to gas atomize both Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 into amorphous powders [9]. To fabricate Al-based BMG much thicker than 1 mm from powder, it is necessary to understand the thermal stability and crystallization behaviour of these alloys in their powder form.
∗ Corresponding author. ∗∗ Corresponding author. Fax: +61 7 3346 7015. E-mail addresses: [email protected] (J.Q. Wang), [email protected] (M. Qian). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.09.107
This study investigates the thermal stability and the crystallization behaviour of amorphous Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 powders with respect to a range of heating rates. It is also important to understand the crystallization behaviour because recent work has shown that controlled crystallization can lead to much improved mechanical properties of metallic glasses [12–17]. 2. Experimental Amorphous aluminium alloys Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 were gas-atomized in high purity nitrogen. The as-atomized powders were sieved to <25 m. They were examined and studied using XRD and scanning electron microscopy (SEM) using a JEOL JSM 7001F, operated at 15 kV with a spot size in the order of 10 nm. Their thermal stability and crystallization kinetics were investigated using non-isothermal DSC with a Netzsch A409C DSC under a continuous flow of pure argon. To determine the critical cooling rates, a range of non-isothermal DSC studies were conducted, where the powder samples (sample weight is fixed at 15 mg) were heated to 873 K at a constant heating rate of 20 K/min and then cooled at rates of 5, 10, 20, 30 and 40 K/min. To determine the activation energy for crystallization, powder samples were heated at rates of 5, 10, 20, 30 and 40 K/min to 873 K and subsequently cooled to room temperature at a constant cooling rate of 20 K/min without isothermal holding. Based on these studies and the peak temperatures determined, powder samples were heated to the identified peak temperature in the DSC furnace and then quenched to room temperature with a forced
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Fig. 1. XRD patterns of the as-gas-atomized alloy powders.
nitrogen gas flow to retain the microstructural features at each peak temperature. The treated powder samples were analysed in detail using XRD, SEM and transmission electron microscopy (TEM) in a JEOL JEM 2100. The TEM samples were prepared from powder particles using an xT Nova NanoLab 200 focused ion beam (FIB).
Fig. 3. Non-isothermal DSC scans of the metallic glass powder at different heating rates: (a) Al86 Ni6 Y4.5 Co2 La1.5 powder and (b) Al85 Ni5 Y6 Co2 Fe2 powder.
3. Results and discussion 3.1. Microstructure of the as-gas-atomized powder Fig. 1 shows the XRD patterns obtained from the as-gasatomized powders of Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 . Each spectrum displayed a broad diffuse maximum at angles corresponding to ∼40◦ , typical of amorphous materials. Fig. 2 shows a typical SEM back-scattered view of the microstructure of an asatomized powder particle from each composition. A featureless microstructure was observed for the Al86 Ni6 Y4.5 Co2 La1.5 powder (Fig. 2a), indicative of its essentially amorphous state. However, it is noted that the XRD patterns show a few tiny Bragg peaks (fccAl) for both compositions, with those for the Al85 Ni5 Y6 Co2 Fe2 alloy powder being more noticeable. It is likely that the Al85 Ni5 Y6 Co2 Fe2 alloy powder examined has contained a small amount of fcc-Al. 3.2. Characteristic crystallization events
Fig. 2. SEM back-scattered images of as-gas-atomized metallic glass powder particles: (a) Al86 Ni6 Y4.5 Co2 La1.5 and (b) Al85 Ni5 Y6 Co2 Fe2 .
Fig. 3 shows the non-isothermal DSC curves of the two powder materials with respect to different heating rates. The curves show transformation from amorphous to crystalline in three steps
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Fig. 5. (a) Kissinger plots, (b) Augis and Bennett plots, and (c) Ozawa plots for the Al85 Ni5 Y6 Co2 Fe2 powder. Fig. 4. (a) Kissinger plots, (b) Augis and Bennett plots, and (c) Ozawa plots for the Al86 Ni6 Y4.5 Co2 La1.5 powder.
of the non-isothermal DSC curve with respect to different heating rates for both compositions. The onset crystallization temperature Tx and crystallization peak temperatures Tp1 , Tp2 , Tp3 are indicated in Fig. 3 for each powder and their values are listed in Table 1. The apparent activation energy for each characteristic transformation can be derived from the Kissinger equation [18] according to the shift
ln
B T2
=−
E +C RT
(1)
where B is the heating rate, E is the apparent activation energy, C is a constant, R is the gas constant and T is the absolute characteristic temperature such as Tx , Tp1 , Tp2 , Tp3 indicated on the DSC curves.
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Table 1 Characteristic temperatures of Al86 Ni6 Y4.5 Co2 La1.5 and Al85 Ni5 Y6 Co2 Fe2 glass powders: onset crystallization temperature (Tx ) and three crystallization peak temperatures (Tp1 , Tp2 , Tp3 ). Powder
Heating rate (◦ C/min)
Tx (◦ C)
Tp1 (◦ C)
Tp2 (◦ C)
Tp3 (◦ C)
Al86 Ni6 Y4.5 Co2 La1.5
5 10 20 30 40 5 10 20 30 40
269.7 273.5 277.8 279.8 281.8 258.7 263.3 268.8 271.1 274.6
273.3 277.1 282 285.2 289.4 272.1 277.5 281.1 285.6 290.2
325.5 329.9 335.8 338.8 341.8 331.6 337.1 343.1 347 351.4
366.6 374.8 385 389.9 395 388.8 397.8 407.8 413.6 419.5
Al85 Ni5 Y6 Co2 Fe2
Fig. 4a shows the Kissinger plots for Al86 Ni6 Y4.5 Co2 La1.5 and Fig. 5a for Al85 Ni5 Y6 Co2 Fe2 . To assess the accuracy of the apparent activation energy terms determined by the Kissinger approach, two other widely used methods, the Augis–Bennett method [19] and the Ozawa method [20], were also used to estimate the apparent activation energy for each characteristic transformation. The three methods are all fundamentally similar. Although the Kissinger approach is widely applied, the use of the Augis–Bennett [19] and Ozawa [20] methods will permit an indirect assessment of the accuracy of the activation energy data obtained from the Kissinger approach. The Augis–Bennett method [19] is described by
ln
B T − T0
=−
E +C RT
(2)
where T0 is the initial temperature of the DSC curves, and the Ozawa method [20] by ln (B) = −
E +C RT
(3)
The Augis–Bennett plots for each alloy are shown in Figs. 4b and 5b while the Ozawa plots are shown in Figs. 4c and 5c. Excellent linear relationships were obtained for each characteristic transformation with each of the three approaches, demonstrating a high level of consistency between them. Table 2 summarizes the apparent activation energy values extracted from Figs. 4 and 5. The activation energy values calculated using the three methods are consistent with each other, confirming the reliability of the data. The values of Ex , Ep1 and Ep2 are higher for the Al86 Ni6 Y4.5 Co2 La1.5 metallic glass powder than for the Al85 Ni5 Y6 Co2 Fe2 metallic glass powder while the apparent activation energy Ep3 for Tp3 is similar for Al86 Ni6 Y4.5 Co2 La1.5 and Al85 Ni5 Y6 Co2 Fe2 . The activation energy Ep3 is much less important than each of the other three energy terms Ex , Ep1 and Ep2 from a crystallization point of view. This is because crystallization has already occurred to a readily detectable extent at Tp2 (see Fig. 10 for peak 2) and intermetallic phases form and grow to a great extent after Tp2 . Hence, in most cases, the processing temperature should not exceed Tp2 in order to keep the amorphous nature and prevent intermetallic phases from forming and growing. The apparent activation energy consists of two parts: one is related to nucleation and the other is related to crystal growth [21]. It is a measure of the thermodynamic barrier to crystallization. Higher crystallization activation energies generally favour thermal stability. The higher onset crystallization temperature (Tx ) and activation energies (Ex , Ep1 and Ep2 ) suggest that the Al86 Ni6 Y4.5 Co2 La1.5 metallic glass powder has higher resistance or better thermal stability against crystallization than the Al85 Ni5 Y6 Co2 Fe2 metallic glass powder. The limited data available in the literature shows that the crystallization activation energies for Al84 Gd6 Ni7 Co3 glass powder are Ep1 = 283 ± 2 kJ/mol and Ep2 = 210 ± 4 kJ/mol [22], and Ep = 270 ± 5 kJ/mol for Al70 Y16 Ni10 Co4
glass powder [23]. The Al86 Ni6 Y4.5 Co2 La1.5 glass powder thus shows higher activation energies of Ep1 and Ep2 (see Table 2) compared with these two Al-based glass powder materials, suggesting higher resistance to crystallization. It should be noted that the activation energies (particularly the Ex value) determined for Al86 Ni6 Y4.5 Co2 La1.5 are higher than those for Al85 Ni5 Y6 Co2 Fe2 and other Al-based MGs reported previously [22,23]. This is consistent with several recent studies [24–27], which showed that metallic glasses with higher GFA tend to show better thermal stability due to their more efficient and stable atomic packing in local structures. Another point to note is that the activation energy for the onset crystallization (Ex ) is actually higher than the activation energies (Ep ) for each peak crystallization event, especially for the Al86 Ni6 Y4.5 Co2 La1.5 MG. This agrees with Surreddi et al. [22] and McKay et al. [28] who reported similar observations for the crystallization of the Al84 Gd6 Ni7 Co3 and Al70 Ni13 Si17 , respectively. However, different observations have been reported; Zhang et al. [29–31] found that the Ex is lower than Ep for several Al-based MGs, which have different compositions from the above-mentioned. A recent study of the atomic structures in three Al-based MGs including Al86 Ni6 Y4.5 Co2 La1.5 has revealed that atomic pare factors such as the atomic pair species, pair fractions and their corresponding bond distances play an essential role in determining the actual GFA [32] and therefore the thermal stability because of its connections with the GFA [24–27]. All these atomic pair factors are closely related to the glass composition or the solute species. This suggests that the activation energy value for each characteristic event is alloy composition dependent in addition to its dependency on the annealing history. As a result, both types of observations could be possible. 3.3. Critical cooling rates The critical cooling rate (Rc ) is an important parameter that reflects the glass forming ability of a metallic alloy [33]. In the context of amorphous alloy powder, the critical cooling rate offers an important indication of the practicality of the amorphous alloy being gas-atomized into amorphous alloy powder. The Rc value can be estimated from the non-isothermal DSC analyses of the powder samples using the relationship described by Eq. (4) [34] ln R = ln Rc −
B (Tl − Txc )2
(4)
where R is the actual cooling rate, Tl is the liquidus temperature of the alloy (also referred to as offset temperature), and Txc is the onset temperature of solidification or the temperature at which the first major nucleation events occur (designated as Tn in solidification). The Tl value can be obtained from the DSC heating curve while the Txc value is obtainable from the DSC cooling curve. Fig. 6 shows the DSC cooling curves obtained from heating the two compositions of powder at a fixed heating rate of 20 K/min to
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Table 2 Apparent activation energy values for characteristic events of Al86 Ni6 Y4.5 Co2 La1.5 and Al85 Ni5 Y6 Co2 Fe2 powders (kJ/mol), calculated using the Kissinger, Augis–Bennet and Ozawa methods. Powder
Method
Ex (kJ/mol)
Al86 Ni6 Y4.5 Co2Lai.5
Kissinger method Augis and Bennett method Ozawa method Kissinger method Augis and Bennett method Ozawa method
418.18 416.93 427.22 312.01 310.53 320.94
Al85 Ni5 Y6 Co2 Fe2
± ± ± ± ± ±
7.20 7.20 7.19 13.61 13.59 13.58
Ep i (kJ/mol) 324.45 323.34 333.71 294.10 292.92 303.33
± ± ± ± ± ±
30.03 29.99 30.03 31.55 31.52 31.54
Ep 2 (kJ/mol) 380.24 380.08 390.36 327.02 326.99 337.28
± ± ± ± ± ±
16.97 16.95 17.04 19.29 19.26 19.28
Ep 3 (kJ/mol) 250.23 250.82 261.12 250.29 251.21 261.55
± ± ± ± ± ±
6.51 6.48 6.48 6.83 6.79 6.74
1273 K, followed by cooling at five different cooling rates. Fig. 7 displays the DSC heating curves for both compositions of powder at 20 K/min to 1273 K. Since the heating rate is fixed, the DSC heating curves are almost identical from each run for each powder. Only one DSC heating curve is therefore presented in Fig. 7 for each composition. By plotting ln R versus 1/(Tl − Txc )2 , the Rc value can be obtained from the ln R-intercept if the resultant relationship is linear. A good linear relationship was found to exist between ln R and 1/(Tl − Txc )2 for each alloy (Fig. 8). This provides an estimate of Rc = 1000 K/s for Al86 Ni6 Y4.5 Co2 La1.5 and Rc = 1500 K/s for Al85 Ni5 Y6 Co2 Fe2 . This indicates that both alloys can readily be gas-atomized into amorphous powders,
Fig. 7. DSC heating curves for each composition of amorphous powder. Heating rate: 20 K/min; samples size: 15 mg.
consistent with our observations. In addition, the results suggest that Al86 Ni6 Y4.5 Co2 La1.5 alloy powder has better glass forming ability than the Al85 Ni5 Y6 Co2 Fe2 alloy powder. 3.4. Powder crystallization behaviour Powder samples of both alloy compositions were heated to their respective peak temperatures and cooled rapidly to room temperature with a forced nitrogen gas flow without isothermal holding. Fig. 9 shows the SEM back-scattered images obtained from the powder samples of Al86 Ni6 Y4.5 Co2 La1.5 subject to different treatments
Fig. 6. Non-isothermal DSC scans of the two alloy powders at different cooling rates: (a) Al86 Ni6 Y4.5 Co2 La1.5 powder and (b) Al85 Ni5 Y6 Co2 Fe2 powder.
Fig. 8. The critical cooling rates for the two metallic glass alloy powders.
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Fig. 9. SEM back-scattered images of the Al86 Ni6 Y4.5 Co2 La1.5 powder treated at different peak temperatures with inset TEM images showing the fcc-Al crystals: (a) peak 1, (b) peak 2, (c) peak 3. The right-hand side images are views of the left side images at higher magnification. The insets show the difference in the size of the nanocrystals (with a dark contrast) in each microstructure.
and Fig. 10 shows the corresponding XRD patterns. Heating powder samples of Al86 Ni6 Y4.5 Co2 La1.5 to the first peak temperature (Tp1 ) at 20 K/min led to the formation of ∼5 nm diameter primary crystals in the amorphous matrix (Fig. 9a). These nanocrystals are fcc-Al crystallites according to the XRD spectrum (Fig. 10). At Tp1 , the microstructure is composed of the amorphous matrix and Al nanocrystals. Continuing to heat the powder to the second peak temperature (Tp2 ) results in three noticeable changes, (i) growth of the Al nanocrystals to ∼20 nm (Fig. 9b); (ii) increased crystallization (Fig. 10), where the diffuse peak for the amorphous matrix has shrunk obviously, and (iii) the formation of two Co-containing crystalline phases, (Ni,Co)3 Al4 and Al2 CoY, detectable by XRD (Fig. 10). The two changes (i) and (ii) are the main processes occurring at this stage according to the SEM back-scattered images and XRD patterns. Full crystallization occurred when the powder was further heated to the third peak temperature (Tp3 ); the amorphous matrix disappeared (Fig. 9c) and the XRD spectrum is typical of that of a crystalline material; no diffuse peak is detected (Fig. 10). In addition, a fourth crystalline phase, Al2.12 La0.88 , is detected (Fig. 10). The Fe-containing alloy was found to crystallize faster and more easily than the La containing alloy. Fig. 11 shows the SEM
Fig. 10. XRD patterns of the Al86 Ni6 Y4.5 Co2 La1.5 metallic glass powder in the as-gasatomized state and after being heated to different peak crystallization temperatures (peak 1, peak 2, peak 3) at 20 K/min.
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Fig. 11. SEM back-scattered images of the Al85 Ni5 Y6 Co2 Fe2 powder heated to different peak temperatures at 20 K/min: (a) peak 1, (b) peak 2, (c) peak 3. The right-hand side images are taken from the left-hand side samples at a higher magnification.
back-scattered images obtained from the powder samples of Al85 Ni5 Y6 Co2 Fe2 after similar treatments and Fig. 12 shows the corresponding XRD diffraction patterns. Crystallized phases are readily noticeable at the first peak temperature from the SEM back-scattered image (Fig. 11a) while TEM was necessary to detect the crystalline phases in the powder of Al86 Ni6 Y4.5 Co2 La1.5 at the first peak temperature (Fig. 9a). This agrees with the differences in the three apparent activation energy terms Ex , Ep1 and Ep2 discussed earlier between the two compositions of powder (see Table 2). The subsequent crystallization is similar to that of the Al86 Ni6 Y4.5 Co2 La1.5 powder except for the formation of different crystalline phases (Fig. 12). It is noted that the second stage of crystallization of the Al85 Ni5 Y6 Co2 Fe2 powder resulted in the formation of an intermediate AlNiY phase, which disappeared in the third stage of crystallization. Full crystallization at the third peak temperature led to the formation of Al9 Co2 , Al3 Y, and Al3 (Ni,Fe) phases in the crystalline Al solid solution matrix (Figs. 11 and 12). The Tp2 temperature can be treated as the maximum nanocrystallization temperature for controlled or partial crystallization. It is encouraging to note that both alloy powders showed a fairly wide gap between Tx and Tp2 , being about 75 K for each according to
Fig. 12. XRD patterns of the Al85 Ni5 Y6 Co2 Fe2 metallic glass powder in the as-gasatomized state and after being heated to different peak crystallization temperatures (peak 1, peak 2, peak 3) at 20 K/min.
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Fig. 3. This may serve as a useful processing window for controlled crystallization for each alloy powder. 4. Summary The crystallization process and thermal stability of two advanced Al-based metallic glass powder materials, Al85 Ni5 Y6 Co2 Fe2 and Al86 Ni6 Y4.5 Co2 La1.5 , have been investigated. The following findings are made from the study. (1) Both Al86 Ni6 Y4.5 Co2 La1.5 powder and Al85 Ni5 Y6 Co2 Fe2 powder show a three-stage crystallization process with a gap of ∼75 K between the Tx and the second crystallization temperature. The main crystallization process is the precipitation and growth of fcc-Al. (2) The critical cooling rates were determined to be ∼1000 K/s for the Al86 Ni6 Y4.5 Co2 La1.5 powder and ∼1500 K/s for the Al85 Ni5 Y6 Co2 Fe2 powder. (3) The Al86 Ni6 Y4.5 Co2 La1.5 powder has better thermal stability than the Al85 Ni5 Y6 Co2 Fe2 powder according to the apparent activation energy data obtained for the onset crystallization and the subsequent first and second crystallization stages for each alloy powder and the results of a detailed microstructural study of their crystallization processes. (4) The crystallization of both alloy powders starts with the formation of fcc-Al nanocrystals in the amorphous matrix. The second stage of crystallization of the Al86 Ni6 Y4.5 Co2 La1.5 powder leads to the growth of fcc-Al nanocrystals while two Co-containing crystalline phases (Ni,Co)3 Al4 and Al2 CoY form. The second stage of crystallization of the Al85 Ni5 Y6 Co2 Fe2 powder results in the formation of an intermediate AlNiY phase, which disappears in the third stage of crystallization. At the third peak temperature, the fully crystallized Al86 Ni6 Y4.5 Co2 La1.5 powder is composed of fcc-Al, Al2 CoY, Al2.12 La0.88 and (Ni,Co)3 Al4 while the fully crystallized Al85 Ni5 Y6 Co2 Fe2 powder consists of fcc-Al, Al9 Co2 , Al3 Y and Al3 (Ni,Fe), respectively. Acknowledgements This work is supported by the Australian Research Council (ARC) and the National Key Basic Research Program of China (Grant No. 2007CB613906). We also acknowledge assistance from the Australian Microscopy & Microanalysis Research Facility (AMMRF).
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References [1] A. Inoue, Prog. Mater. Sci. 43 (1998) 365–520. [2] P. Dong, W.L. Hou, X.C. Chang, M.X. Quan, J.Q. Wang, J. Alloys Compd. 436 (2007) 118–123. [3] W.H. Wang, C. Dong, C.H. Shek, Mater. Sci. Eng. R 44 (2004) 45–89. [4] Z.P. Lu, C.T. Liu, J.R. Thompson, W.D. Porter, Phys. Rev. Lett. 92 (2004). [5] M. Yan, J. Zou, J. Shen, Acta Mater. 54 (2006) 3627–3635. [6] N. Nishiyama, A. Inoue, Mater. Trans. JIM 38 (1997) 464. [7] Z.Y. Chang, X.M. Huang, L.Y. Chen, M.Y. Ge, Q.K. Jiang, X.P. Nie, J.Z. Jiang, Mater. Sci. Eng. A 517 (2009) 246–248. [8] Q.F. Li, K.Q. Qiu, X. Yang, Y.L. Ren, X.G. Yuan, T. Zhang, Mater. Sci. Eng. A 491 (2008) 420–424. [9] B.J. Yang, J.H. Yao, J. Zhang, H.W. Yang, J.Q. Wang, E. Ma, Scripta Mater. 61 (2009) 423–426. [10] A. Inoue, K. Ohtera, A.P. Tsai, T. Masumoto, Jpn. J. Appl. Phys. 2 Lett. 27 (1988) L479–L482. [11] H.W. Yang, J.Q. Wang, Scripta Mater. 55 (2006) 359–362. [12] H.S. Kim, J.K. Lee, S.Y. Shin, T.S. Kim, Crystallization behavior and mechanical property of Cu based metallic glass powders, in: S.J.L. Kang, M.Y. Huh, N.M. Hwang, H. Homma, K. Ushioda, Y. Ikuhara (Eds.), Recrystallization and Grain Growth III, Pts 1 and 2, Trans Tech Publications Ltd, Stafa-Zurich, 2007, pp. 1317–1322. [13] J. Das, M.B. Tang, K.B. Kim, R. Theissmann, F. Baier, W.H. Wang, J. Eckert, Phys. Rev. Lett. 94 (2005). [14] K. Mondal, T. Ohkubo, T. Toyama, Y. Nagai, M. Hasegawa, K. Hono, Acta Mater. 56 (2008) 5329–5339. [15] F.X. Qin, X.M. Wang, A. Inoue, Intermetallics 15 (2007) 1337–1342. [16] N.S. Barekar, S. Pauly, R.B. Kumar, U. Kuhn, B.K. Dhindaw, J. Eckert, Mater. Sci. Eng. A 527 (2010) 5867–5872. [17] S. Mula, K. Mondal, S. Ghosh, S.K. Pabi, Mater. Sci. Eng. A 527 (2010) 3757–3763. [18] V.A. Khonik, K. Kitagawa, H. Morii, J. Appl. Phys. 87 (2000) 8440–8443. [19] J.A. Augis, J.E. Bennett, J. Therm. Anal. 13 (1978) 283–292. [20] H.R. Wang, Y.L. Gao, Y.F. Ye, G.H. Min, Y. Chen, X.Y. Teng, J. Alloys Compd. 353 (2003) 200–206. [21] Z.Z. Yuan, X.D. Chen, B.X. Wang, Y.J. Wang, J. Alloys Compd. 407 (2006) 163–169. [22] K.B. Surreddi, S. Scudino, M. Sakaliyska, K.G. Prashanth, D.J. Sordelet, J. Eckert, J. Alloys Compd. 491 (2010) 137–142. [23] K.G. Prashanth, S. Scudino, B.S. Murty, J. Eckert, J. Alloys Compd. 477 (2009) 171–177. [24] O.A. Lafi, M.M.A. Imran, J. Alloys Compd. 509 (2011) 5090–5094. [25] P. Gargarella, C.S. Kiminami, M.F. de Oliveira, C. Bolfarini, W.J. Botta, J. Alloys Compd. 495 (2010) 334–337. [26] Y. Wang, J.F. Wang, C.C. Li, Mater. Sci. Eng. A 528 (2011) 1623–1627. [27] D.B. Miracle, D.V. Louzguine-Luzgin, L.V. Louzguina-Luzgina, A. Inoue, Int. Mater. Rev. 55 (2010) 218–256. [28] B.J. McKay, P. Cizek, P. Schumacher, K.A.Q. O’Reilly, Mater. Sci. Eng. A 304 (2001) 240–244. [29] Z.H. Zhang, Y.Z. Zhou, E.J. Lavernia, J. Alloys Compd. 466 (2008) 189–200. [30] Z.H. Zhang, D. Witkin, E.J. Lavernia, J. Non-Cryst. Solids 351 (2005) 1646–1652. [31] Z.H. Zhang, N. Yang, Y.Z. Zhou, E.J. Lavernia, Philos. Mag. 88 (2008) 737–753. [32] M. Yan, S. Kohara, J.Q. Wang, K. Nogita, G.B. Schaffer, M. Qian, Scripta Mater. 65 (2011) 755–758. [33] S. Guo, Z.P. Lu, C.T. Liu, Intermetallics 18 (2010) 883–888. [34] A. Inoue, T. Zhang, T. Masumoto, J. Non-Cryst. Solids 156 (1993) 473–480.