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Electron irradiation induced crystallization behavior in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 amorphous alloys
Materials Science and Engineering A352 (2003) 251 /260 www.elsevier.com/locate/msea
Electron irradiation induced crystallization behavior in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 amorphous alloys T. Nagase a, Y. Umakoshi b,* a
Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan b Department of Materials Science and Engineering, Graduate School of Engineering and Handai Frontier Research Center, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan Received 27 September 2002
Abstract Electron irradiation effects on the phase stability and microstructure change of conventional Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass were examined. The amorphous phase was unstable under electron irradiation at 298 K and f.c.c.Zr2Cu precipitates were formed. Although b.c.t.-Zr2Cu with coarse grains precipitated from the amorphous phase by thermal annealing, a nanocrystalline structure composed of f.c.c.-Zr2Cu precipitates and amorphous matrix was obtained by electron irradiation. There was a threshold total dose density for electron irradiation induced crystallization. The density for Zr65.0Al7.5Cu27.5 was larger than that for Zr66.7Cu33.3. The b.c.t.-Zr2Cu precipitates in thermal equilibrium were unstable under electron irradiation and were transformed to f.c.c.-Zr2Cu through the amorphous phase. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron irradiation; Zirconium-based alloy; Phase stability; Thermal stability; Crystallization; Amorphization
1. Introduction New amorphous alloys called metallic glasses have been found in Mg- [1], Lanthanide- [2], Zr- [3,4], Fe- [5], Pd /Cu- [6], Ti- [7], Ni- [8] and Co- [9] based alloys. Metallic glasses show extremely high thermal stability against crystallization and a wide supercooled liquid region below the crystallization temperature. Largescale bulk amorphous alloys can be produced by consolidation of glassy powders or conventional casting at low cooling rates. Development of these alloys extends their application field. Inoue et al. proposed three empirical rules to obtain metallic glasses [10 /13] and a large number of the glasses have been discovered. More recently, amorphous alloys have been of great interest, not only as superior functional materials, but
* Corresponding author. Tel.: /81-6-6879-7494; fax: /81-6-68797495. E-mail address: [email protected] (Y. Umakoshi).
also as precursors to obtain nanocrystalline structure by crystallization. The nanocrystalline alloys prepared by crystallization of an amorphous phase show novel properties which cannot be obtained for amorphous single phase alloys or highly controlled crystalline structure alloys even at the same alloy compositions [14 /21]. Satisfaction of the three empirical rules for obtaining metallic glasses does not allow nanocrystalline structure to be obtained from an amorphous phase. The expansion of a supercooled liquid region induces high phase stability of the glassy phase against crystallization and results in an increase in crystallization temperature. Therefore, once nuclei form, they grow rapidly in the amorphous phase resulting in coarse grain size precipitates. Only some limited Zr-based metallic glasses [22,23] can form the nanocrystalline structure through crystallization of the amorphous phase by thermal annealing. We found that, not only thermal annealing, but also electron irradiation could stimulate the crystallization of an amorphous phase. Nanocrystalline structure was obtained by electron irradiation in metallic
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(02)00896-1
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glasses in which nanostructure cannot be realized by annealing [24,25]. This novel result implies new application of amorphous alloys to nano technology. However, no clear criterion to obtain nanocrystalline structure by electron irradiation induced crystallization has been identified. This paper reports the effect of electron irradiation on the amorphous phase in conventional Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass. The electron irradiation induced crystallization behavior of the amorphous phase is discussed focusing on different types of amorphous alloy and the difference in crystallization process between thermal annealing and electron irradiation.
2. Experimental procedure Master ingots of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 alloys were prepared by arc melting in a purified Ar atmosphere to examine two typical but different type Zr-based amorphous alloys of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. Zr66.7Cu33.3 does not satisfy three empirical rules for obtaining metallic glass, while Zr65.0Al7.5Cu27.5 does satisfy the rules and has the widest supercooled liquid region among ternary Zr /Al /Cu alloys. Rapidly quenched ribbons with a cross section of :/2.0 /0.02 mm were produced from the ingots at a rotation speed of 42 ms 1 by a single roller meltspinning method in an Ar atmosphere. Some specimens were annealed at 20 K lower temperature than the glass transition: at 593 K for Zr66.7Cu33.3 and 623 K for Zr65.0Al7.5Cu27.5. The thermal properties of the meltspun ribbons were examined by differential scanning calorimetry (DSC) at a heating rate of 0.67 Ks 1 in an Ar atmosphere. Structures of the melt-spun, annealed and electron irradiated ribbons were examined by X-ray diffractometry using Cu-Ka radiation, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HREM). TEM and HREM observations were carried out in a JEM-3010 operating at 300 kV and a JEM-2010 electron microscope operating at 200 kV, respectively. Thin foils for these observations were prepared from the ribbons by twin jet polishing in a solution of 30% nitric acid and 70% methanol at :/243 K. The foils were electron irradiated at 298 K by an ultra-high voltage electron microscope (UHVEM) H-3000 operating with the acceleration voltage of 2000 kV. The applied dose rate was in the range of 1.4 /1024 to 4.0 /1024 m 2 s 1 with the maximum dose density of 6.4 /1027 m 2. Change in the microstructure during electron irradiation was examined by bright field (BF) images and selected area diffraction (SAD) patterns in UHVEM.
3. Results 3.1. Thermal properties of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 In DSC measurements, two melt-spun ribbons show the anomalous endothermic reaction due to the glass transition, followed by a wide supercooled liquid region and then the sharp exothermic peak due to the crystallization. Fig. 1 shows the glass transition temperature (Tg), the onset of crystallization temperature (Tx ) and the temperature interval of the supercooled liquid region (DTx ), corresponding to the difference between Tg and Tx. The thermal stability of amorphous alloys is evaluated by DTx ; the stability increases with increasing value of DTx . The Tg and Tx increase with the addition of Al to the Zr/Cu binary amorphous alloy and as a result, the temperature interval of the supercooled liquid region extends :/90 K for the Zr65.0Al7.5Cu27.5 metallic glass. Increase of the DTx is mainly due to the increase of Tx , indicating that the Al addition is effective in improving the thermal stability by suppression of the crystallization. Fig. 2 shows TTT diagrams of Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass which were constructed by isothermal annealing ribbons in a DSC furnace. The Tg was measured by conventional
Fig. 1. Thermal properties of Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass as melt-spun.
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Fig. 2. TTT diagram of Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass as melt-spun.
DSC measurement at a heating rate of 0.67 Ks 1. It should be noticed that there is an extremely wide supercooled liquid region above Tg; Zr65.0Al7.5Cu27.5, which can maintain the supercooled liquid state for a prolonged interval period reaching :/104 s just above Tg. In the figure, the onset of crystallization shifts to a higher temperature and the time interval for maintaining the supercooled liquid region becomes longer because of the Al addition. These results are in good agreement with the thermal properties of Tg, Tx and DTx , as shown in Fig. 1. 3.2. Change in microstructure of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 by thermal annealing To examine the change in microstructure of Zr66.7Cu33.3 by thermal annealing, partially and fully crystallized specimens corresponding to (A) and (B) in Fig. 1, respectively, were prepared. Only featureless contrast is observed over the entire BF image for the melt-spun alloy in Fig. 3(a). The SAD pattern consists only of halo rings with no spotty diffraction patterns. The X-ray diffraction (XRD) pattern also consisted only of broad halo peaks. These results indicate that the meltspun alloy is composed of an amorphous single phase. In partially crystallized Zr66.6Cu33.3 (Fig. 3b), a typical two-phase structure consisting of crystalline precipitates and amorphous matrix is observed. The crystallization occurs in the nucleation and growth process. The precipitates which have ellipsoidal morphology and average grain size of 2 mm are identified as b.c.t.Zr2Cu phase from electron diffraction patterns. Smooth interface between the precipitates and the amorphous matrix suggests that growth of the precipitates occurs in the polymorphic mode with little redistribution of solute elements at the interface. Fig. 3(c) shows the TEM microstructure of a fully crystallized alloy, together with typical electron diffraction patterns of the crystalline
Fig. 3. TEM microstructures and the corresponding SAD patterns of Zr66.7Cu33.3 amorphous alloy as melt-spun and annealed under Tg. (a) As melt-spun; (b) annealed at 593 K for 7 /103 s indicated by (A) in Fig. 2; and (c) annealed at 593 K for 2 /104 s indicated by (B) in Fig. 2.
phase. The b.c.t.-Zr2Cu precipitates with a coarse grain size of /4 mm are seen. Fig. 4 shows the TEM microstructures and corresponding SAD patterns of Zr65.0Al7.5Cu27.5 as melt-spun (a), annealed at 623 K for 1.4 /105 s (b) and 3 /105 s (c), corresponding to annealing period (C) and (D) in Fig. 2, respectively. The BF image and the corresponding SAD pattern in Fig. 4(a) clearly show that the melt-
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in partially or fully crystallized specimen. Thus, it is very difficult to obtain nanocrystalline structure in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 by thermal annealing. Fig. 5 shows XRD patterns of Zr66.7Cu33.3 annealed at 593 K for 2 /104 s and Zr65.0Al7.5Cu27.5 annealed at 623 K for 3 /105 s, together with the data of Zr65.0Al7.5Cu27.5 annealed at 1143 K for 5.76 /104 s as an example of the equilibrium crystalline structure. Only the sharp diffraction peaks corresponding to b.c.t.Zr2Cu are observed in the two alloys just after crystallization. Although there is no significant difference in the position of the (103) peak for b.c.t.-Zr2Cu in Zr66.7Cu33.3 annealed at 593 K and Zr65.0Al7.5Cu27.5 annealed at 1143 K, the (103) peak marked by (a) for Zr65.0Al7.5Cu27.5 annealed at 623 K is shifted to slightly lower angle. This suggests that the alloy composition of b.c.t.-Zr2Cu precipitates in Zr66.7Cu33.3 at 593 K and in Zr65.0Al7.5Cu27.5 at 623 K differs. The EDX analysis indicates that Al dissolves in b.c.t.-Zr2Cu precipitates of Zr65.0Al7.5Cu27.5 annealed at 623 K but does not in the equilibrium phase annealed at 1143 K. The Al is rejected from Zr2Cu precipitates and forms Zr2Al precipitates during annealing at 1143 K.
3.3. Change in microstructure of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 by electron irradiation Electron irradiation on an amorphous phase can induce the crystallization of Zr- and Fe-based metallic glasses [23,24]. Phase stability of the amorphous phase against electron irradiation can be evaluated by the total
Fig. 4. TEM microstructures and the corresponding SAD patterns of Zr65.0Al7.5Cu27.5 metallic glass as melt-spun and annealed under Tg. (a) As melt-spun; (b) annealed at 623 K for 1.4 /105 s indicated by (C) in Fig. 2; and (c) annealed at 623 K for 3 /105 s indicated by (D) in Fig. 2.
spun alloy is composed of an amorphous single phase. Precipitates in Fig. 4(b) show ruggedness shapes and an average size of 300 nm. Fig. 4(c) shows a typical electron diffraction pattern of crystalline precipitate of b.c.t.Zr2Cu, similar to that in Zr66.7Cu33.3. The size of b.c.t.Zr2Cu precipitate is somewhat smaller than that in Zr66.7Cu33.3, although the annealing temperature is high. No nanocrystalline structures were obtained, however,
Fig. 5. X-ray diffraction patterns of Zr66.7Cu33.3 amorphous alloy annealed at 593 K for 2 /104 s, Zr65.0Al7.5Cu27.5 metallic glass annealed at 623 K for 3 /105 s and 1143 K for 5.76 /104 s.
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Fig. 6. The threshold total dose density for electron irradiation induced crystallization of Fe- and Zr-based amorphous alloys as a function of D/Tx .
dose density required for the irradiation induced crystallization of the amorphous phase. Fig. 6 shows the threshold total dose density of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 together with the data of Fe-based amorphous alloy and metallic glass as a function of DTx . Although the total dose density for Fe-based alloys is higher than that for Zr-based alloys, an increase in DTx raises the total dose and stabilizes amorphous phase against electron irradiation. Fig. 7 shows the TEM microstructures and the corresponding SAD patterns of Zr66.7Cu33.3 electron irradiated at the dose density of 3.2 /1027 m 2 (a) and Zr65.0Al7.5Cu27.5 at 2.5 /1027 m 2 (b). BF images show bright and dark nanoscale granular contrasts. Debye rings corresponding to nanocrystalline precipitates together with a halo ring corresponding to an amorphous phase are observed in the SAD patterns. Many crystalline precipitates of several 10 nm grain size are seen. No significant differences in TEM microstructure and the corresponding SAD pattern are observed between Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. A nanocrystalline structure composed of precipitates and amorphous phase is formed by electron irradiation in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. The intensity profile of the SAD pattern analyzed using a PICTRO STAT DIGITAL 400 image analyzer is shown in Fig. 8. The diffraction peaks can be indexed as f.c.c.-Zr2Cu reflections in Zr66.7Cu33.3. Precipitation of b.c.t.-Zr2Cu in equilibrium crystalline and quasicrystalline Zr2Cu phases is not confirmed from the SAD pattern. The crystal structure of precipitates formed from the amorphous phase in Zr65.0Al7.5Cu27.5
Fig. 7. TEM microstructures and the corresponding SAD patterns of Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass electron irradiated at 298 K at an acceleration voltage of 2000 kV. (a) Zr66.7Cu33.3 at the dose density of 3.2 /1027 m 2; and (b) Zr65.0Al7.5Cu27.5 at the dose density of 2.5 /1027 m 2.
under electron irradiation is the same as that in Zr66.7Cu33.3. More detailed microstructure after electron irradiation induced crystallization was examined by HREM observation. Fig. 9 shows HREM images and the corresponding SAD patterns of Zr66.7Cu33.3 electron irradiated at the dose density of 6.4 /1027 m 2 (a, c) and Zr65.0Al7.5Cu27.5 irradiated at 5.0 /1027 m 2 (b, d); where in each of these the dose density is about twice as large as that in Fig. 7. The Debye rings are identified as f.c.c.-Zr2Cu phase in the two specimens. No change in constituent phases with increase of dose density is observed. In HREM images, crystalline lattice images with nanoscale grain size of :/10 nm in an amorphous phase are seen; the lattice images are randomly oriented in each region. This indicates that the crystallization progresses in the nucleation and subsequent growth process and each nanocrystalline f.c.c.-Zr2Cu phase precipitates from an amorphous phase with random orientations. No significant coarsening of the precipitates occurs as dose density increases. Electron irradiation is very effective in synthesizing nanocrystalline structure not only in conventional Zr-based amorphous
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3.4. Change in microstructure of b.c.t.-Zr2Cu precipitates in thermal equilibrium under electron irradiation
Fig. 8. The intensity profile of electron diffraction rings in Zr66.7Cu33.3 amorphous alloy electron irradiated at 6.4 /1027 m 2.
alloys, but also in Zr-based metallic glasses, while nanocrystalline structure is rarely formed by thermal annealing.
The metastable f.c.c.-Zr2Cu phase precipitated from the amorphous phase under electron irradiation instead of b.c.t.-Zr2Cu phase, while equilibrium b.c.t.-Zr2Cu was formed by thermal annealing in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. To learn the phase transformation behavior under electron irradiation, the effect of the irradiation on the b.c.t.-Zr2Cu obtained by annealing was examined, as shown in Fig. 10. For annealed Zr66.7Cu33.3, BF image (a) shows the bend contour and the SAD pattern (b) corresponding to the b.c.t.-Zr2Cu is observed. The bend contour is weakened and dark and bright granular contrasts :/ 10 nm in size together with a featureless contrast appearing at the central part of the irradiated area at a dose density of 1.1 /1027 m 2. The halo rings corresponding to an amorphous phase and Debye rings are visible accompanied by a decrease in the intensity of the diffraction spot of b.c.t.-Zr2Cu. The Debye rings are due to the f.c.c.-Zr2Cu, but none for the b.c.t.-Zr2Cu and quasicrystalline phase are present. A duplex structure composed of nanoscale f.c.c.-Zr2Cu precipitates and amorphous phase is formed through the electron irradiation induced phase transformation of b.c.t.-
Fig. 9. HREM images and the corresponding SAD patterns of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 ribbons electron irradiated at 298 K at an acceleration voltage of 2000 kV. (a, c) Zr66.7Cu33.3 at the dose density of 6.4 /1027 m 2; and (b, d) Zr65.0Al7.5Cu27.5 at the dose density of 5.0 /1027 m 2.
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Fig. 10. Change in TEM microstructures and the corresponding SAD patterns of b.c.t.-Zr2Cu precipitates in Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass during electron irradiation. (a, e) Zr66.7Cu33.3 alloy before irradiation; (b, f) Zr66.7Cu33.3 alloy irradiated at the dose density of 1.1 /1027 m 2; (c, g) Zr65.0Al7.5Cu27.5 alloy before irradiation; and (d, h) Zr65.0Al7.5Cu27.5 alloy irradiated at the dose density of 1.2 /1027 m 2.
Zr2Cu. The same result was obtained in Zr65.0Al7.5Cu27.5. Annihilation of b.c.t.-Zr2Cu and amorphization in Zr65.0Al7.5Cu27.5 occurred within a
lower dose density than those in Zr66.7Cu33.3. The phase stability of b.c.t.-Zr2Cu differs under electron irradiation and during annealing is different.
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4. Discussion 4.1. Crystallization of the amorphous phase under electron irradiation The results in the present study clearly show that electron irradiation can introduce crystallization of an amorphous phase not only in conventional amorphous alloys, but also in metallic glasses. The promotion of atomic diffusion in an amorphous phase is necessary for crystallization and during thermal annealing, this diffusion is controlled by the thermal activation process. The atomic diffusion is also promoted by electron irradiation. The elastic collision between irradiation electrons and constituent atoms occurs in an amorphous phase under electron irradiation. The energy of the electrons is proportional to the acceleration voltage. If high energy is transferred to the collision atoms from irradiation electrons, dynamic displacement of these atoms is induced. This phenomenon is called the electron knock-on effect and it promotes the atomic diffusion. There is a threshold acceleration voltage for introducing atomic displacement by electron irradiation; in crystalline alloys this voltage is reported to be 400 kV for Cu [26 /28], 140 kV [26,29,30] and 170 kV [31] for Al atom. Since the acceleration voltage of 400 kV is believed to be sufficient for direct displacement of Zr atoms in crystalline alloys [32], 2000 kV may be enough to introduce dynamic atomic diffusion in Zr-based amorphous phase. The electron knock-on effect occurs not only in an amorphous phase, but also in crystalline phases. Crystalline phases cannot maintain their original structure under electron irradiation if the displacement of atoms by the electron knock-on effect occurs more frequently than the original positions of atomic sublattices recover by thermal diffusion. Only crystalline phases which can maintain their original structure under electron irradiation can precipitate from an amorphous phase. The f.c.c.-Zr2Cu crystalline phase precipitates from the amorphous phase in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 by electron irradiation, while the thermal equilibrium b.c.t.-Zr2Cu phase is unstable and is transformed to other phases under electron irradiation. This indicates that f.c.c.-Zr2Cu has a high phase stability to maintain its original structure against electron irradiation. We have suggested that there are two important factors for electron irradiation induced crystallization of an amorphous phase: (1) the atomic diffusion promoted by electron irradiation in an amorphous phase; and (2) the phase stability of crystalline phases against electron irradiation [24,25]. High phase stability of a crystalline phase to maintain the original crystal structure is necessary for electron irradiation induced crystallization from an amorphous phase together with atomic diffusion to change the glassy structure. The crystallization of the amorphous phase in Zr66.7Cu33.3
and Zr65.0Al7.5Cu27.5 is caused by the satisfaction of these factors at 2000 kV. 4.2. Difference in crystallization behavior of amorphous phase by thermal annealing and electron irradiation There are three different characteristics in common between crystallization by thermal annealing and electron irradiation induced crystallization: (1) the species of the crystalline precipitation phase; (2) the stability of the amorphous phase against crystallization in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5; and (3) the formation of nanocrystalline structure. Characteristic (2) and (3) are discussed in Section 4.3 and Section 4.4, respectively. The f.c.c.-Zr2Cu crystalline phase precipitates from the amorphous phase under electron irradiation. This is a metastable phase and cannot be observed during crystallization by thermal annealing. The electron irradiation can induce the atomic displacement of constituent atoms in crystalline phase, resulting in the introduction of displacement defects, such as vacancies and interstitial atoms. The amorphization of crystalline alloys often results from accumulation of numerous displacement defects. In Zr/Cu binary alloys, it was reported that ZrCu and Zr7Cu10 crystalline phases were not stable and amorphization occurred under electron irradiation [33]. The amorphization of b.c.t.-Zr2Cu phase in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 was confirmed in the present study; this phase did not precipitate from the amorphous phase under electron irradiation. In contrast, f.c.c.-Zr2Cu phase is stable against electron irradiation and the transformation to another phase does not occur. The f.c.c.-Zr2Cu crystalline phase in Zr67Cu33 was reported to form during high energy ball milling which can introduce a great number of lattice defects in crystals [34]. Therefore, the f.c.c.Zr2Cu may show high phase stability against the introduction of defects by electron irradiation. The stability of amorphous and crystalline phases against electron irradiation and annealing is responsible for a difference in phase transformation behavior. 4.3. Phase stability of the amorphous and crystalline phases in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 against electron irradiation Crystalline phase precipitated from the amorphous phase during thermal annealing and under electron irradiation in Zr66.7Cu33.3 is the same as that in Zr65.0Al7.5Cu27.5. The thermal stability of Zr65.0Al7.5Cu27.5 is much higher than that of Zr66.7Cu33.3. Electron irradiation-induced crystallization from the amorphous phase is disturbed by large DTx in Zr-based alloys. As the degree of DRP atomic structure increases, the atomic configuration becomes more closely packed and the atomic interaction nature among
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constituent atoms increases within a short range. An increase in the degree of DRP atomic structure suppresses not only the atomic diffusion during thermal annealing, but also the displacement rate of constituent atoms in an amorphous phase by the electron knock-on effect. The increase in the degree of DRP structure and DTx through the satisfaction of the three empirical rules may suppress the atomic displacement rate by electron irradiation and induce an increase in the threshold total dose density for crystallization in Zr65.0Al7.5Cu27.5. Since the threshold acceleration voltage for the electron knock-on effect depends on the atomic species, there is a difference in threshold total dose density between Feand Zr-based alloys. Thermal equilibrium b.c.t.-Zr2Cu precipitates annihilated during electron irradiation and amorphization occurred followed by the formation of f.c.c-Zr2Cu phase. This suggests that the phase stability of f.c.c.-Zr2Cu, b.c.t.-Zr2Cu and amorphous phase by thermal annealing and electron irradiation is different. Since the amorphous phase in Zr65.0Al7.5Cu27.5 is more stable than that in Zr66.7Cu33.3, amorphization of b.c.t.Zr2Cu by electron irradiation occurs easily in Zr65.0Al7.5Cu27.5. 4.4. Nanocrystallization in metallic glass It is known that the nanocrystalline structure is not easily formed by crystallization of an amorphous phase in metallic glasses. Formation of this structure requires the simultaneous satisfaction of the ease of homogeneous nucleation of the crystalline phase and difficulty of subsequent crystal growth. But the three empirical rules to obtain metallic glasses are not always favorable for obtaining nanocrystalline alloys through crystallization of the amorphous phase. The three empirical rules cause the formation of a unique glassy structure with an extremely high degree of DRP structure from topological and chemical points of view. Inoue et al. suggested that this structure is effective in suppressing the nucleation of a crystalline phase because of the high liquid/solid interface energy and low crystal growth rate due to the rearrangement of constituent elements on a long range scale [10 /13]. When redistribution of constituent elements at the phase boundary hardly occurs and crystallization is suppressed, crystallization progresses accompanied by the simultaneous precipitation of more than two kinds of crystalline phase. This crystallization mode is extremely unfavorable for achieving nanocrystallization of an amorphous phase. The activation process of atomic diffusion under electron irradiation is different from that during thermal annealing; this difference strongly affects the nucleation and growth rate of crystalline phases. Considering atomic species and direction in atomic movement, highly frequent homogeneous atomic displacement in an amorphous phase can be induced by electron
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irradiation at the extremely high acceleration voltage of 2000 kV. Highly frequent homogeneous nucleation may occur under electron irradiation. The crystal growth can be discussed from the adsorption and dispersion of atoms at the liquid /solid interface [35]. The growth rate is determined by the difference between the adsorption and dispersion rates. Under electron irradiation, the rates of adsorption and dispersion strongly depend not only on atomic interaction nature, but also on the interaction between high energy electron and constituent atoms. The difference between these rates under electron irradiation is believed to be smaller than that during thermal annealing. This is one reason for the subsequent low growth rate of f.c.c.-Zr2Cu under electron irradiation. In addition, the suppression of crystal growth based on this mechanism does not necessarily cause change in the crystallization mode in the simultaneous precipitation of more than two kinds of crystalline phase. The formation of nanocrystalline structure in Zr65.0Al7.5Cu27.5 can be explained by the unique nucleation and growth mechanism based on the electron knock-on effect. This novel result implies that there is a strong possibility that electron irradiation on amorphous phase can extend the application fields of nanocrystalline alloys.
5. Conclusions Effect of electron irradiation on crystallization behavior of an amorphous phase and the phase stability of thermal equilibrium precipitates in conventional Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass was examined, focusing on different types of amorphous alloy and the difference between thermal annealing and electron irradiation. The results were summarized and the following conclusions were reached. (1) The amorphous phase could not maintain a fully glassy structure under electron irradiation with the acceleration voltage of 2000 kV at 298 K in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. Crystallization from the amorphous phase occurred by electron irradiation. (2) B.c.t.-Zr2Cu precipitated from the amorphous phase during thermal annealing in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5, while the electron irradiation induced f.c.c.-Zr2Cu precipitates and the precipitation of the equilibrium b.c.t.-Zr2Cu could not be confirmed in either Zr-based alloy. The b.c.t.-Zr2Cu precipitates in thermal equilibrium annihilated by electron irradiation and amorphous phase appeared followed by formation of f.c.c.-Zr2Cu. This is due to the difference between thermodynamical phase stability and phase stability under the dynamic atomic displacement by electron irradiation. (3) Electron irradiation was very effective in synthesizing nanocrystalline structure from an amor-
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phous phase not only in Zr-based conventional amorphous alloy, but also in Zr-based metallic glass.
[9] [10] [11] [12]
Acknowledgements
[13]
The authors are grateful to Professor H. Mori and Dr T. Sakata of the Research Center for Ultra-High Voltage Electron Microscopy, Osaka University for operating the H-3000 UHVEM. T. Nagase would like to thank the Japan Society for the Promotion of Science (JSPS) for his research fellowship. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. This work was carried out at the Strategic Research Base ‘Handai Frontier Research Center’ supported by the Japanese Government’s Special Coordination Fund for Promoting Science and Technology.
[14]
References
[25] [26] [27]
[1] A. Inoue, K. Ohtera, K. Kita, T. Masumoto, Jpn. J. Appl. Phys. 27 (1988) L2248. [2] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 30 (1989) 965. [3] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 31 (1990) 177. [4] A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. [5] A. Inoue, Y. Shinohara, G.S. Gook, Mater. Trans. JIM 36 (1995) 1427. [6] A. Inoue, N. Nishiyama, T. Matsuda, Mater. Trans. JIM 37 (1996) 181. [7] K. Amiya, N. Nishiyama, A. Inoue, T. Masumoto, Mater. Sci. Eng. A179/A180 (1994) 692. [8] X.M. Wang, I. Yoshii, A. Inoue, Mater. Trans. JIM 40 (1999) 1130.
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[28] [29] [30] [31] [32]
[33] [34] [35]
T. Itoi, A. Inoue, Mater. Trans. JIM 41 (1999) 1256. A. Inoue, Mater. Trans. JIM 39 (1995) 866. A. Inoue, Mater. Sci. Eng. A226 /228 (1997) 357. A. Inoue, T. Zhang, A. Takeuchi, Mater. Sci. Forum 269 /272 (1998) 855. A. Inoue, A. Takeuchi, T. Zhang, Metall. Mater. Trans. 29A (1998) 1779. Y.H. Kim, A. Inoue, T. Masumoto, Mater. Trans. JIM 31 (1990) 747. A. Inoue, K. Nakazato, Y. Kawamura, T. Masumoto, Mater. Sci. Eng. A179/A180 (1994) 654. Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. K. Suzuki, A. Makino, N. Kataoka, T. Masumoto, Mater. Trans. JIM 32 (1991) 93. J.J. Croat, Appl. Phys. Lett. 37 (1980) 1096. E.F. Kneller, R. Hawing, IEEE Trans. Magn. 27 (1991) 3588. A. Inoue, Y. Tanaka, Y. Miyauchi, T. Masumoto, Sci. Rep. RITU A39 (1994) 147. A. Yokoyama, H. Komiyama, A. Inoue, T. Masumoto, H.M. Kimura, J. Catal. 68 (1981) 355. C. Fan, A. Inoue, Mater. Trans. JIM 38 (1997) 1040. C. Fan, A. Takeuchi, A. Inoue, Mater. Trans. JIM 40 (1999) 42. T. Nagase, Y. Umakoshi, N. Sumida, Sci. Tech. Adv. Mater. 3 (2002) 119. T. Nagase, Y. Umakoshi, Mater. Sci. Eng. A343 (2003) 13. M.J. Markin, Phil. Mag. 18 (1968) 637. P. Jung, R.L. Chaplin, H. Fenzl, K. Leichelt, P. Wombacher, Phys. Rev. B8 (1973) 553. M. Kiritani, N. Yoshida, H. Tanaka, Y. Maehara, J. Phys. Soc. Jap. 38 (1975) 1677. A. Wolfenden, Rad. Eff. 21 (1973) 197. M. Kiritani, N. Yoshida, H. Tanaka, J. Phys. Soc. Jpn. 36 (1974) 36. H.M. Simpson, R.L. Chaplin, Phys. Rev. 185 (1969) 958. L.M. Howe, D. Phillips, H. Zou, J. Forster, R. Siegele, J.A. Davis, A.T. Motta, J.A. Faldowski, P.R. Okamoto, Nucl. Inst. Meth. Phys. Res. B118 (1996) 663. H. Mori, H. Fujita, Suppl. Trans. JIM 29 (1988) 93. M. Sherif El-Eskandarany, A. Inoue, Metall. Mater. Trans. A33 (2002) 135. K.A. Jackson, J. Cryst. Growth 3 /4 (1968) 507.
Electron irradiation induced crystallization behavior in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 amorphous alloys T. Nagase a, Y. Umakoshi b,* a
Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan b Department of Materials Science and Engineering, Graduate School of Engineering and Handai Frontier Research Center, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan Received 27 September 2002
Abstract Electron irradiation effects on the phase stability and microstructure change of conventional Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass were examined. The amorphous phase was unstable under electron irradiation at 298 K and f.c.c.Zr2Cu precipitates were formed. Although b.c.t.-Zr2Cu with coarse grains precipitated from the amorphous phase by thermal annealing, a nanocrystalline structure composed of f.c.c.-Zr2Cu precipitates and amorphous matrix was obtained by electron irradiation. There was a threshold total dose density for electron irradiation induced crystallization. The density for Zr65.0Al7.5Cu27.5 was larger than that for Zr66.7Cu33.3. The b.c.t.-Zr2Cu precipitates in thermal equilibrium were unstable under electron irradiation and were transformed to f.c.c.-Zr2Cu through the amorphous phase. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron irradiation; Zirconium-based alloy; Phase stability; Thermal stability; Crystallization; Amorphization
1. Introduction New amorphous alloys called metallic glasses have been found in Mg- [1], Lanthanide- [2], Zr- [3,4], Fe- [5], Pd /Cu- [6], Ti- [7], Ni- [8] and Co- [9] based alloys. Metallic glasses show extremely high thermal stability against crystallization and a wide supercooled liquid region below the crystallization temperature. Largescale bulk amorphous alloys can be produced by consolidation of glassy powders or conventional casting at low cooling rates. Development of these alloys extends their application field. Inoue et al. proposed three empirical rules to obtain metallic glasses [10 /13] and a large number of the glasses have been discovered. More recently, amorphous alloys have been of great interest, not only as superior functional materials, but
* Corresponding author. Tel.: /81-6-6879-7494; fax: /81-6-68797495. E-mail address: [email protected] (Y. Umakoshi).
also as precursors to obtain nanocrystalline structure by crystallization. The nanocrystalline alloys prepared by crystallization of an amorphous phase show novel properties which cannot be obtained for amorphous single phase alloys or highly controlled crystalline structure alloys even at the same alloy compositions [14 /21]. Satisfaction of the three empirical rules for obtaining metallic glasses does not allow nanocrystalline structure to be obtained from an amorphous phase. The expansion of a supercooled liquid region induces high phase stability of the glassy phase against crystallization and results in an increase in crystallization temperature. Therefore, once nuclei form, they grow rapidly in the amorphous phase resulting in coarse grain size precipitates. Only some limited Zr-based metallic glasses [22,23] can form the nanocrystalline structure through crystallization of the amorphous phase by thermal annealing. We found that, not only thermal annealing, but also electron irradiation could stimulate the crystallization of an amorphous phase. Nanocrystalline structure was obtained by electron irradiation in metallic
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(02)00896-1
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glasses in which nanostructure cannot be realized by annealing [24,25]. This novel result implies new application of amorphous alloys to nano technology. However, no clear criterion to obtain nanocrystalline structure by electron irradiation induced crystallization has been identified. This paper reports the effect of electron irradiation on the amorphous phase in conventional Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass. The electron irradiation induced crystallization behavior of the amorphous phase is discussed focusing on different types of amorphous alloy and the difference in crystallization process between thermal annealing and electron irradiation.
2. Experimental procedure Master ingots of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 alloys were prepared by arc melting in a purified Ar atmosphere to examine two typical but different type Zr-based amorphous alloys of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. Zr66.7Cu33.3 does not satisfy three empirical rules for obtaining metallic glass, while Zr65.0Al7.5Cu27.5 does satisfy the rules and has the widest supercooled liquid region among ternary Zr /Al /Cu alloys. Rapidly quenched ribbons with a cross section of :/2.0 /0.02 mm were produced from the ingots at a rotation speed of 42 ms 1 by a single roller meltspinning method in an Ar atmosphere. Some specimens were annealed at 20 K lower temperature than the glass transition: at 593 K for Zr66.7Cu33.3 and 623 K for Zr65.0Al7.5Cu27.5. The thermal properties of the meltspun ribbons were examined by differential scanning calorimetry (DSC) at a heating rate of 0.67 Ks 1 in an Ar atmosphere. Structures of the melt-spun, annealed and electron irradiated ribbons were examined by X-ray diffractometry using Cu-Ka radiation, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HREM). TEM and HREM observations were carried out in a JEM-3010 operating at 300 kV and a JEM-2010 electron microscope operating at 200 kV, respectively. Thin foils for these observations were prepared from the ribbons by twin jet polishing in a solution of 30% nitric acid and 70% methanol at :/243 K. The foils were electron irradiated at 298 K by an ultra-high voltage electron microscope (UHVEM) H-3000 operating with the acceleration voltage of 2000 kV. The applied dose rate was in the range of 1.4 /1024 to 4.0 /1024 m 2 s 1 with the maximum dose density of 6.4 /1027 m 2. Change in the microstructure during electron irradiation was examined by bright field (BF) images and selected area diffraction (SAD) patterns in UHVEM.
3. Results 3.1. Thermal properties of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 In DSC measurements, two melt-spun ribbons show the anomalous endothermic reaction due to the glass transition, followed by a wide supercooled liquid region and then the sharp exothermic peak due to the crystallization. Fig. 1 shows the glass transition temperature (Tg), the onset of crystallization temperature (Tx ) and the temperature interval of the supercooled liquid region (DTx ), corresponding to the difference between Tg and Tx. The thermal stability of amorphous alloys is evaluated by DTx ; the stability increases with increasing value of DTx . The Tg and Tx increase with the addition of Al to the Zr/Cu binary amorphous alloy and as a result, the temperature interval of the supercooled liquid region extends :/90 K for the Zr65.0Al7.5Cu27.5 metallic glass. Increase of the DTx is mainly due to the increase of Tx , indicating that the Al addition is effective in improving the thermal stability by suppression of the crystallization. Fig. 2 shows TTT diagrams of Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass which were constructed by isothermal annealing ribbons in a DSC furnace. The Tg was measured by conventional
Fig. 1. Thermal properties of Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass as melt-spun.
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Fig. 2. TTT diagram of Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass as melt-spun.
DSC measurement at a heating rate of 0.67 Ks 1. It should be noticed that there is an extremely wide supercooled liquid region above Tg; Zr65.0Al7.5Cu27.5, which can maintain the supercooled liquid state for a prolonged interval period reaching :/104 s just above Tg. In the figure, the onset of crystallization shifts to a higher temperature and the time interval for maintaining the supercooled liquid region becomes longer because of the Al addition. These results are in good agreement with the thermal properties of Tg, Tx and DTx , as shown in Fig. 1. 3.2. Change in microstructure of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 by thermal annealing To examine the change in microstructure of Zr66.7Cu33.3 by thermal annealing, partially and fully crystallized specimens corresponding to (A) and (B) in Fig. 1, respectively, were prepared. Only featureless contrast is observed over the entire BF image for the melt-spun alloy in Fig. 3(a). The SAD pattern consists only of halo rings with no spotty diffraction patterns. The X-ray diffraction (XRD) pattern also consisted only of broad halo peaks. These results indicate that the meltspun alloy is composed of an amorphous single phase. In partially crystallized Zr66.6Cu33.3 (Fig. 3b), a typical two-phase structure consisting of crystalline precipitates and amorphous matrix is observed. The crystallization occurs in the nucleation and growth process. The precipitates which have ellipsoidal morphology and average grain size of 2 mm are identified as b.c.t.Zr2Cu phase from electron diffraction patterns. Smooth interface between the precipitates and the amorphous matrix suggests that growth of the precipitates occurs in the polymorphic mode with little redistribution of solute elements at the interface. Fig. 3(c) shows the TEM microstructure of a fully crystallized alloy, together with typical electron diffraction patterns of the crystalline
Fig. 3. TEM microstructures and the corresponding SAD patterns of Zr66.7Cu33.3 amorphous alloy as melt-spun and annealed under Tg. (a) As melt-spun; (b) annealed at 593 K for 7 /103 s indicated by (A) in Fig. 2; and (c) annealed at 593 K for 2 /104 s indicated by (B) in Fig. 2.
phase. The b.c.t.-Zr2Cu precipitates with a coarse grain size of /4 mm are seen. Fig. 4 shows the TEM microstructures and corresponding SAD patterns of Zr65.0Al7.5Cu27.5 as melt-spun (a), annealed at 623 K for 1.4 /105 s (b) and 3 /105 s (c), corresponding to annealing period (C) and (D) in Fig. 2, respectively. The BF image and the corresponding SAD pattern in Fig. 4(a) clearly show that the melt-
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in partially or fully crystallized specimen. Thus, it is very difficult to obtain nanocrystalline structure in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 by thermal annealing. Fig. 5 shows XRD patterns of Zr66.7Cu33.3 annealed at 593 K for 2 /104 s and Zr65.0Al7.5Cu27.5 annealed at 623 K for 3 /105 s, together with the data of Zr65.0Al7.5Cu27.5 annealed at 1143 K for 5.76 /104 s as an example of the equilibrium crystalline structure. Only the sharp diffraction peaks corresponding to b.c.t.Zr2Cu are observed in the two alloys just after crystallization. Although there is no significant difference in the position of the (103) peak for b.c.t.-Zr2Cu in Zr66.7Cu33.3 annealed at 593 K and Zr65.0Al7.5Cu27.5 annealed at 1143 K, the (103) peak marked by (a) for Zr65.0Al7.5Cu27.5 annealed at 623 K is shifted to slightly lower angle. This suggests that the alloy composition of b.c.t.-Zr2Cu precipitates in Zr66.7Cu33.3 at 593 K and in Zr65.0Al7.5Cu27.5 at 623 K differs. The EDX analysis indicates that Al dissolves in b.c.t.-Zr2Cu precipitates of Zr65.0Al7.5Cu27.5 annealed at 623 K but does not in the equilibrium phase annealed at 1143 K. The Al is rejected from Zr2Cu precipitates and forms Zr2Al precipitates during annealing at 1143 K.
3.3. Change in microstructure of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 by electron irradiation Electron irradiation on an amorphous phase can induce the crystallization of Zr- and Fe-based metallic glasses [23,24]. Phase stability of the amorphous phase against electron irradiation can be evaluated by the total
Fig. 4. TEM microstructures and the corresponding SAD patterns of Zr65.0Al7.5Cu27.5 metallic glass as melt-spun and annealed under Tg. (a) As melt-spun; (b) annealed at 623 K for 1.4 /105 s indicated by (C) in Fig. 2; and (c) annealed at 623 K for 3 /105 s indicated by (D) in Fig. 2.
spun alloy is composed of an amorphous single phase. Precipitates in Fig. 4(b) show ruggedness shapes and an average size of 300 nm. Fig. 4(c) shows a typical electron diffraction pattern of crystalline precipitate of b.c.t.Zr2Cu, similar to that in Zr66.7Cu33.3. The size of b.c.t.Zr2Cu precipitate is somewhat smaller than that in Zr66.7Cu33.3, although the annealing temperature is high. No nanocrystalline structures were obtained, however,
Fig. 5. X-ray diffraction patterns of Zr66.7Cu33.3 amorphous alloy annealed at 593 K for 2 /104 s, Zr65.0Al7.5Cu27.5 metallic glass annealed at 623 K for 3 /105 s and 1143 K for 5.76 /104 s.
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Fig. 6. The threshold total dose density for electron irradiation induced crystallization of Fe- and Zr-based amorphous alloys as a function of D/Tx .
dose density required for the irradiation induced crystallization of the amorphous phase. Fig. 6 shows the threshold total dose density of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 together with the data of Fe-based amorphous alloy and metallic glass as a function of DTx . Although the total dose density for Fe-based alloys is higher than that for Zr-based alloys, an increase in DTx raises the total dose and stabilizes amorphous phase against electron irradiation. Fig. 7 shows the TEM microstructures and the corresponding SAD patterns of Zr66.7Cu33.3 electron irradiated at the dose density of 3.2 /1027 m 2 (a) and Zr65.0Al7.5Cu27.5 at 2.5 /1027 m 2 (b). BF images show bright and dark nanoscale granular contrasts. Debye rings corresponding to nanocrystalline precipitates together with a halo ring corresponding to an amorphous phase are observed in the SAD patterns. Many crystalline precipitates of several 10 nm grain size are seen. No significant differences in TEM microstructure and the corresponding SAD pattern are observed between Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. A nanocrystalline structure composed of precipitates and amorphous phase is formed by electron irradiation in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. The intensity profile of the SAD pattern analyzed using a PICTRO STAT DIGITAL 400 image analyzer is shown in Fig. 8. The diffraction peaks can be indexed as f.c.c.-Zr2Cu reflections in Zr66.7Cu33.3. Precipitation of b.c.t.-Zr2Cu in equilibrium crystalline and quasicrystalline Zr2Cu phases is not confirmed from the SAD pattern. The crystal structure of precipitates formed from the amorphous phase in Zr65.0Al7.5Cu27.5
Fig. 7. TEM microstructures and the corresponding SAD patterns of Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass electron irradiated at 298 K at an acceleration voltage of 2000 kV. (a) Zr66.7Cu33.3 at the dose density of 3.2 /1027 m 2; and (b) Zr65.0Al7.5Cu27.5 at the dose density of 2.5 /1027 m 2.
under electron irradiation is the same as that in Zr66.7Cu33.3. More detailed microstructure after electron irradiation induced crystallization was examined by HREM observation. Fig. 9 shows HREM images and the corresponding SAD patterns of Zr66.7Cu33.3 electron irradiated at the dose density of 6.4 /1027 m 2 (a, c) and Zr65.0Al7.5Cu27.5 irradiated at 5.0 /1027 m 2 (b, d); where in each of these the dose density is about twice as large as that in Fig. 7. The Debye rings are identified as f.c.c.-Zr2Cu phase in the two specimens. No change in constituent phases with increase of dose density is observed. In HREM images, crystalline lattice images with nanoscale grain size of :/10 nm in an amorphous phase are seen; the lattice images are randomly oriented in each region. This indicates that the crystallization progresses in the nucleation and subsequent growth process and each nanocrystalline f.c.c.-Zr2Cu phase precipitates from an amorphous phase with random orientations. No significant coarsening of the precipitates occurs as dose density increases. Electron irradiation is very effective in synthesizing nanocrystalline structure not only in conventional Zr-based amorphous
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3.4. Change in microstructure of b.c.t.-Zr2Cu precipitates in thermal equilibrium under electron irradiation
Fig. 8. The intensity profile of electron diffraction rings in Zr66.7Cu33.3 amorphous alloy electron irradiated at 6.4 /1027 m 2.
alloys, but also in Zr-based metallic glasses, while nanocrystalline structure is rarely formed by thermal annealing.
The metastable f.c.c.-Zr2Cu phase precipitated from the amorphous phase under electron irradiation instead of b.c.t.-Zr2Cu phase, while equilibrium b.c.t.-Zr2Cu was formed by thermal annealing in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. To learn the phase transformation behavior under electron irradiation, the effect of the irradiation on the b.c.t.-Zr2Cu obtained by annealing was examined, as shown in Fig. 10. For annealed Zr66.7Cu33.3, BF image (a) shows the bend contour and the SAD pattern (b) corresponding to the b.c.t.-Zr2Cu is observed. The bend contour is weakened and dark and bright granular contrasts :/ 10 nm in size together with a featureless contrast appearing at the central part of the irradiated area at a dose density of 1.1 /1027 m 2. The halo rings corresponding to an amorphous phase and Debye rings are visible accompanied by a decrease in the intensity of the diffraction spot of b.c.t.-Zr2Cu. The Debye rings are due to the f.c.c.-Zr2Cu, but none for the b.c.t.-Zr2Cu and quasicrystalline phase are present. A duplex structure composed of nanoscale f.c.c.-Zr2Cu precipitates and amorphous phase is formed through the electron irradiation induced phase transformation of b.c.t.-
Fig. 9. HREM images and the corresponding SAD patterns of Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 ribbons electron irradiated at 298 K at an acceleration voltage of 2000 kV. (a, c) Zr66.7Cu33.3 at the dose density of 6.4 /1027 m 2; and (b, d) Zr65.0Al7.5Cu27.5 at the dose density of 5.0 /1027 m 2.
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Fig. 10. Change in TEM microstructures and the corresponding SAD patterns of b.c.t.-Zr2Cu precipitates in Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass during electron irradiation. (a, e) Zr66.7Cu33.3 alloy before irradiation; (b, f) Zr66.7Cu33.3 alloy irradiated at the dose density of 1.1 /1027 m 2; (c, g) Zr65.0Al7.5Cu27.5 alloy before irradiation; and (d, h) Zr65.0Al7.5Cu27.5 alloy irradiated at the dose density of 1.2 /1027 m 2.
Zr2Cu. The same result was obtained in Zr65.0Al7.5Cu27.5. Annihilation of b.c.t.-Zr2Cu and amorphization in Zr65.0Al7.5Cu27.5 occurred within a
lower dose density than those in Zr66.7Cu33.3. The phase stability of b.c.t.-Zr2Cu differs under electron irradiation and during annealing is different.
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4. Discussion 4.1. Crystallization of the amorphous phase under electron irradiation The results in the present study clearly show that electron irradiation can introduce crystallization of an amorphous phase not only in conventional amorphous alloys, but also in metallic glasses. The promotion of atomic diffusion in an amorphous phase is necessary for crystallization and during thermal annealing, this diffusion is controlled by the thermal activation process. The atomic diffusion is also promoted by electron irradiation. The elastic collision between irradiation electrons and constituent atoms occurs in an amorphous phase under electron irradiation. The energy of the electrons is proportional to the acceleration voltage. If high energy is transferred to the collision atoms from irradiation electrons, dynamic displacement of these atoms is induced. This phenomenon is called the electron knock-on effect and it promotes the atomic diffusion. There is a threshold acceleration voltage for introducing atomic displacement by electron irradiation; in crystalline alloys this voltage is reported to be 400 kV for Cu [26 /28], 140 kV [26,29,30] and 170 kV [31] for Al atom. Since the acceleration voltage of 400 kV is believed to be sufficient for direct displacement of Zr atoms in crystalline alloys [32], 2000 kV may be enough to introduce dynamic atomic diffusion in Zr-based amorphous phase. The electron knock-on effect occurs not only in an amorphous phase, but also in crystalline phases. Crystalline phases cannot maintain their original structure under electron irradiation if the displacement of atoms by the electron knock-on effect occurs more frequently than the original positions of atomic sublattices recover by thermal diffusion. Only crystalline phases which can maintain their original structure under electron irradiation can precipitate from an amorphous phase. The f.c.c.-Zr2Cu crystalline phase precipitates from the amorphous phase in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 by electron irradiation, while the thermal equilibrium b.c.t.-Zr2Cu phase is unstable and is transformed to other phases under electron irradiation. This indicates that f.c.c.-Zr2Cu has a high phase stability to maintain its original structure against electron irradiation. We have suggested that there are two important factors for electron irradiation induced crystallization of an amorphous phase: (1) the atomic diffusion promoted by electron irradiation in an amorphous phase; and (2) the phase stability of crystalline phases against electron irradiation [24,25]. High phase stability of a crystalline phase to maintain the original crystal structure is necessary for electron irradiation induced crystallization from an amorphous phase together with atomic diffusion to change the glassy structure. The crystallization of the amorphous phase in Zr66.7Cu33.3
and Zr65.0Al7.5Cu27.5 is caused by the satisfaction of these factors at 2000 kV. 4.2. Difference in crystallization behavior of amorphous phase by thermal annealing and electron irradiation There are three different characteristics in common between crystallization by thermal annealing and electron irradiation induced crystallization: (1) the species of the crystalline precipitation phase; (2) the stability of the amorphous phase against crystallization in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5; and (3) the formation of nanocrystalline structure. Characteristic (2) and (3) are discussed in Section 4.3 and Section 4.4, respectively. The f.c.c.-Zr2Cu crystalline phase precipitates from the amorphous phase under electron irradiation. This is a metastable phase and cannot be observed during crystallization by thermal annealing. The electron irradiation can induce the atomic displacement of constituent atoms in crystalline phase, resulting in the introduction of displacement defects, such as vacancies and interstitial atoms. The amorphization of crystalline alloys often results from accumulation of numerous displacement defects. In Zr/Cu binary alloys, it was reported that ZrCu and Zr7Cu10 crystalline phases were not stable and amorphization occurred under electron irradiation [33]. The amorphization of b.c.t.-Zr2Cu phase in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 was confirmed in the present study; this phase did not precipitate from the amorphous phase under electron irradiation. In contrast, f.c.c.-Zr2Cu phase is stable against electron irradiation and the transformation to another phase does not occur. The f.c.c.-Zr2Cu crystalline phase in Zr67Cu33 was reported to form during high energy ball milling which can introduce a great number of lattice defects in crystals [34]. Therefore, the f.c.c.Zr2Cu may show high phase stability against the introduction of defects by electron irradiation. The stability of amorphous and crystalline phases against electron irradiation and annealing is responsible for a difference in phase transformation behavior. 4.3. Phase stability of the amorphous and crystalline phases in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5 against electron irradiation Crystalline phase precipitated from the amorphous phase during thermal annealing and under electron irradiation in Zr66.7Cu33.3 is the same as that in Zr65.0Al7.5Cu27.5. The thermal stability of Zr65.0Al7.5Cu27.5 is much higher than that of Zr66.7Cu33.3. Electron irradiation-induced crystallization from the amorphous phase is disturbed by large DTx in Zr-based alloys. As the degree of DRP atomic structure increases, the atomic configuration becomes more closely packed and the atomic interaction nature among
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constituent atoms increases within a short range. An increase in the degree of DRP atomic structure suppresses not only the atomic diffusion during thermal annealing, but also the displacement rate of constituent atoms in an amorphous phase by the electron knock-on effect. The increase in the degree of DRP structure and DTx through the satisfaction of the three empirical rules may suppress the atomic displacement rate by electron irradiation and induce an increase in the threshold total dose density for crystallization in Zr65.0Al7.5Cu27.5. Since the threshold acceleration voltage for the electron knock-on effect depends on the atomic species, there is a difference in threshold total dose density between Feand Zr-based alloys. Thermal equilibrium b.c.t.-Zr2Cu precipitates annihilated during electron irradiation and amorphization occurred followed by the formation of f.c.c-Zr2Cu phase. This suggests that the phase stability of f.c.c.-Zr2Cu, b.c.t.-Zr2Cu and amorphous phase by thermal annealing and electron irradiation is different. Since the amorphous phase in Zr65.0Al7.5Cu27.5 is more stable than that in Zr66.7Cu33.3, amorphization of b.c.t.Zr2Cu by electron irradiation occurs easily in Zr65.0Al7.5Cu27.5. 4.4. Nanocrystallization in metallic glass It is known that the nanocrystalline structure is not easily formed by crystallization of an amorphous phase in metallic glasses. Formation of this structure requires the simultaneous satisfaction of the ease of homogeneous nucleation of the crystalline phase and difficulty of subsequent crystal growth. But the three empirical rules to obtain metallic glasses are not always favorable for obtaining nanocrystalline alloys through crystallization of the amorphous phase. The three empirical rules cause the formation of a unique glassy structure with an extremely high degree of DRP structure from topological and chemical points of view. Inoue et al. suggested that this structure is effective in suppressing the nucleation of a crystalline phase because of the high liquid/solid interface energy and low crystal growth rate due to the rearrangement of constituent elements on a long range scale [10 /13]. When redistribution of constituent elements at the phase boundary hardly occurs and crystallization is suppressed, crystallization progresses accompanied by the simultaneous precipitation of more than two kinds of crystalline phase. This crystallization mode is extremely unfavorable for achieving nanocrystallization of an amorphous phase. The activation process of atomic diffusion under electron irradiation is different from that during thermal annealing; this difference strongly affects the nucleation and growth rate of crystalline phases. Considering atomic species and direction in atomic movement, highly frequent homogeneous atomic displacement in an amorphous phase can be induced by electron
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irradiation at the extremely high acceleration voltage of 2000 kV. Highly frequent homogeneous nucleation may occur under electron irradiation. The crystal growth can be discussed from the adsorption and dispersion of atoms at the liquid /solid interface [35]. The growth rate is determined by the difference between the adsorption and dispersion rates. Under electron irradiation, the rates of adsorption and dispersion strongly depend not only on atomic interaction nature, but also on the interaction between high energy electron and constituent atoms. The difference between these rates under electron irradiation is believed to be smaller than that during thermal annealing. This is one reason for the subsequent low growth rate of f.c.c.-Zr2Cu under electron irradiation. In addition, the suppression of crystal growth based on this mechanism does not necessarily cause change in the crystallization mode in the simultaneous precipitation of more than two kinds of crystalline phase. The formation of nanocrystalline structure in Zr65.0Al7.5Cu27.5 can be explained by the unique nucleation and growth mechanism based on the electron knock-on effect. This novel result implies that there is a strong possibility that electron irradiation on amorphous phase can extend the application fields of nanocrystalline alloys.
5. Conclusions Effect of electron irradiation on crystallization behavior of an amorphous phase and the phase stability of thermal equilibrium precipitates in conventional Zr66.7Cu33.3 amorphous alloy and Zr65.0Al7.5Cu27.5 metallic glass was examined, focusing on different types of amorphous alloy and the difference between thermal annealing and electron irradiation. The results were summarized and the following conclusions were reached. (1) The amorphous phase could not maintain a fully glassy structure under electron irradiation with the acceleration voltage of 2000 kV at 298 K in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5. Crystallization from the amorphous phase occurred by electron irradiation. (2) B.c.t.-Zr2Cu precipitated from the amorphous phase during thermal annealing in Zr66.7Cu33.3 and Zr65.0Al7.5Cu27.5, while the electron irradiation induced f.c.c.-Zr2Cu precipitates and the precipitation of the equilibrium b.c.t.-Zr2Cu could not be confirmed in either Zr-based alloy. The b.c.t.-Zr2Cu precipitates in thermal equilibrium annihilated by electron irradiation and amorphous phase appeared followed by formation of f.c.c.-Zr2Cu. This is due to the difference between thermodynamical phase stability and phase stability under the dynamic atomic displacement by electron irradiation. (3) Electron irradiation was very effective in synthesizing nanocrystalline structure from an amor-
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phous phase not only in Zr-based conventional amorphous alloy, but also in Zr-based metallic glass.
[9] [10] [11] [12]
Acknowledgements
[13]
The authors are grateful to Professor H. Mori and Dr T. Sakata of the Research Center for Ultra-High Voltage Electron Microscopy, Osaka University for operating the H-3000 UHVEM. T. Nagase would like to thank the Japan Society for the Promotion of Science (JSPS) for his research fellowship. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan. This work was carried out at the Strategic Research Base ‘Handai Frontier Research Center’ supported by the Japanese Government’s Special Coordination Fund for Promoting Science and Technology.
[14]
References
[25] [26] [27]
[1] A. Inoue, K. Ohtera, K. Kita, T. Masumoto, Jpn. J. Appl. Phys. 27 (1988) L2248. [2] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 30 (1989) 965. [3] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 31 (1990) 177. [4] A. Peker, W.L. Johnson, Appl. Phys. Lett. 63 (1993) 2342. [5] A. Inoue, Y. Shinohara, G.S. Gook, Mater. Trans. JIM 36 (1995) 1427. [6] A. Inoue, N. Nishiyama, T. Matsuda, Mater. Trans. JIM 37 (1996) 181. [7] K. Amiya, N. Nishiyama, A. Inoue, T. Masumoto, Mater. Sci. Eng. A179/A180 (1994) 692. [8] X.M. Wang, I. Yoshii, A. Inoue, Mater. Trans. JIM 40 (1999) 1130.
[15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
[28] [29] [30] [31] [32]
[33] [34] [35]
T. Itoi, A. Inoue, Mater. Trans. JIM 41 (1999) 1256. A. Inoue, Mater. Trans. JIM 39 (1995) 866. A. Inoue, Mater. Sci. Eng. A226 /228 (1997) 357. A. Inoue, T. Zhang, A. Takeuchi, Mater. Sci. Forum 269 /272 (1998) 855. A. Inoue, A. Takeuchi, T. Zhang, Metall. Mater. Trans. 29A (1998) 1779. Y.H. Kim, A. Inoue, T. Masumoto, Mater. Trans. JIM 31 (1990) 747. A. Inoue, K. Nakazato, Y. Kawamura, T. Masumoto, Mater. Sci. Eng. A179/A180 (1994) 654. Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. K. Suzuki, A. Makino, N. Kataoka, T. Masumoto, Mater. Trans. JIM 32 (1991) 93. J.J. Croat, Appl. Phys. Lett. 37 (1980) 1096. E.F. Kneller, R. Hawing, IEEE Trans. Magn. 27 (1991) 3588. A. Inoue, Y. Tanaka, Y. Miyauchi, T. Masumoto, Sci. Rep. RITU A39 (1994) 147. A. Yokoyama, H. Komiyama, A. Inoue, T. Masumoto, H.M. Kimura, J. Catal. 68 (1981) 355. C. Fan, A. Inoue, Mater. Trans. JIM 38 (1997) 1040. C. Fan, A. Takeuchi, A. Inoue, Mater. Trans. JIM 40 (1999) 42. T. Nagase, Y. Umakoshi, N. Sumida, Sci. Tech. Adv. Mater. 3 (2002) 119. T. Nagase, Y. Umakoshi, Mater. Sci. Eng. A343 (2003) 13. M.J. Markin, Phil. Mag. 18 (1968) 637. P. Jung, R.L. Chaplin, H. Fenzl, K. Leichelt, P. Wombacher, Phys. Rev. B8 (1973) 553. M. Kiritani, N. Yoshida, H. Tanaka, Y. Maehara, J. Phys. Soc. Jap. 38 (1975) 1677. A. Wolfenden, Rad. Eff. 21 (1973) 197. M. Kiritani, N. Yoshida, H. Tanaka, J. Phys. Soc. Jpn. 36 (1974) 36. H.M. Simpson, R.L. Chaplin, Phys. Rev. 185 (1969) 958. L.M. Howe, D. Phillips, H. Zou, J. Forster, R. Siegele, J.A. Davis, A.T. Motta, J.A. Faldowski, P.R. Okamoto, Nucl. Inst. Meth. Phys. Res. B118 (1996) 663. H. Mori, H. Fujita, Suppl. Trans. JIM 29 (1988) 93. M. Sherif El-Eskandarany, A. Inoue, Metall. Mater. Trans. A33 (2002) 135. K.A. Jackson, J. Cryst. Growth 3 /4 (1968) 507.