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Amorphization and crystallization of Zr66.7−xCu33.3Nbx (x = 0, 2, 4) alloys during mechanical alloying
Journal of Alloys and Compounds 474 (2009) 152–157
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Amorphization and crystallization of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys during mechanical alloying Yan Wang a,b , Xiuxiu Chen b , Haoran Geng b,∗ , Zhongxi Yang a,b a b
The Key Laboratory of Liquid Structure and Heredity of Materials, Shandong University, 73 Jingshi Road, Jinan 250061, PR China School of Materials Science and Engineering, University of Jinan, 106 Jiwei Road, Jinan 250022, PR China
a r t i c l e
i n f o
Article history: Received 7 May 2008 Received in revised form 26 June 2008 Accepted 28 June 2008 Available online 3 August 2008 Keywords: Amorphous alloys Mechanical alloying X-ray diffraction Microstructure
a b s t r a c t In the present paper, the effect of Nb and different rotation speeds on the amorphization and crystallization of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys during mechanical alloying has been investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The results show that the minor addition of Nb can shorten the start time of the amorphization reaction, improve the glass forming ability of Zr–Cu alloys, but cannot promote the formation of a single amorphous phase at a lower rotation speed of 200 rpm. The glass forming ability of the Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys increases with increasing Nb additions. At a higher rotation speed of 350 rpm, a single amorphous phase of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) can be successfully fabricated. Moreover, the Nb addition into Zr–Cu alloys can accelerate the amorphization process and improve the stability of the amorphous phase against the mechanically induced crystallization. Furthermore, the amorphous Zr66.7 Cu33.3 phase gradually transforms into a metastable fcc-Zr2 Cu phase with increasing milling time. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In spite of the melt spinning technique, mechanical alloying (MA) does allow such opportunity for the cluster formation and homogenization of the glassy powders at the atomic scale through adequate milling periods. Hence, any significant concentration gradients or structural fluctuations in the glassy matrix can be avoided. Moreover, the introduction of lattice defects raises the free energy of the crystalline system and thus, destabilizes it with respect to the amorphous state. Minor alloying additions or microalloying technologies were key metallurgical practices and dominant concepts for developing new metallic crystalline materials in the later half of the 20th century. It was found that appropriate minor alloying additions were very effective in increasing glass forming ability (GFA), enhancing thermal stability and improving magnetic and mechanical properties for some bulk metallic glasses (BMGs). Moreover, the minor addition approach can also represent a feasible way to develop and design novel BMGs. Based on their atomic sizes [1], these elements can be categorized into three groups: (1) small metalloid elements like C, B, Si, (2) intermediate transition metals such as Fe, Ni, Co, Cu, Mo, Zn, Nb, and (3) large elements like Zr, Sn, Y, La and Ca.
Nb, as a kind of transition metals with intermediate atomic sizes, has been selected as minor alloying elements in various systems [2–8]. The Nb addition can make the atomic size more consecutive for some alloy systems, which effectively increases the atomic packing density [9]. Moreover, the addition of Nb atoms can also enhance the viscosity of undercooled liquid alloys and decrease the atomic diffusion coefficient [10]. In addition, the GFA and properties of Cu–Zr alloys are very sensitive to the minor additions [11–14]. Therefore, Cu–Zr is a model system for studying the mechanism of amorphization enhanced by the minor addition. The negative heat of mixing between Zr and Cu is −23 kJ/mol, but Cu–Nb and Zr–Nb have positive heats of mixing, +3 and +4 kJ/mol, respectively [15]. So the interactional force of atoms becomes more complicated and the formation of the amorphous phase may relate to a new reaction mechanism. In addition, El-Eskandarany and Inoue [16] have studied the effect of rotation speed on the solid-state phase transformtions of a Zr67 Cu33 alloy. They have found that the milling induced cyclic phase transformations are related to the rotation speed. The present paper aims to investigate the influence of Nb addition and different rotation speeds on the solid-state amorphization and phase evolution of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys during mechanical alloying. 2. Experimental
∗ Corresponding author. Tel.: +86 531 82765317; fax: +86 531 82765317. E-mail address: mse [email protected] (H. Geng). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.137
Powder mixtures of elemental Zr (<70 m, 99.9 wt.% purity), Cu (<70 m, 99.9 wt.% purity) and Nb (<70 m, 99.9 wt.% purity) with nominal compositions of
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Fig. 1. XRD patterns of the as-milled Zr66.7 Cu33.3 powders after (a) 3 h, (b) 16 h, (c) 30 h (d) 44.5 h, (e) 60 h and (f) 99 h of the MA time at a rotation speed of 200 rpm.
Zr66.7 Cu33.3 , Zr64.7 Cu33.3 Nb2 and Zr62.7 Cu33.3 Nb4 were mechanically alloyed in a high energy ball mill (Fritsch P6) at a rotation milling speed of 200 and 350 revolutions per minute (rpm). The chromium steel vial and balls were used in the present work. Mechanical alloying was carried out at room temperature in an Ar atmosphere with a ball-to-powder weight ratio of 17:1. In order to avoid an increase in the vial temperature, the milling process was periodically interrupted every 30 min and then halted for 10 min. In addition, the milling process was interrupted at various time intervals in order to remove small amounts of the milled products for analysis and characterization. Microstructural characterization of the as-milled powders was made using a Rigaku D/max-RB X-ray diffractometer (XRD) with Cu K␣ radiation, a Philips CM 20 transmission electron microscope (TEM), and a scanning electron microscope (SEM, LEO 1530 VP). Thermal analysis of the as-milled powders was carried out using a differential scanning calorimeter (DSC, NETZSCH STA 409) at a heating rate of 10 K min−1 in a purified argon atmosphere.
3. Results and discussion Fig. 1 shows the XRD patterns of the as-milled Zr66.7 Cu33.3 powders after selected MA times at a rotation speed of 200 rpm. The as-milled products at the initial stage (3 h) are just a polycrystalline mixture of starting reactant powders, indicated by sharp Bragg peaks corresponding to elemental Zr and Cu (Fig. 1(a)). With increasing milling time to 44.5 h, the as-milled powders are still composed of Zr and Cu, but a broadening of crystalline diffraction peaks is observed due to decreasing grain size and increasing atomic level strain (Fig. 1(b)–(d)). A broad diffuse hump appears on the XRD pattern after 60 h of MA (as highlighted by an arrow in Fig. 1(e)), indicating the formation of an amorphous phase. However, the broad diffraction peaks of Zr (1 0 0) and Cu (1 1 1) are still present on the XRD pattern at this stage. Further increasing the MA time to 99 h, the Zr (1 0 0) diffraction peak can still be observed on the XRD pattern, but all diffraction peaks of Cu disappear, suggesting that the resulting milling products are composed of unprocessed nanocrystalline Zr and the amorphous phase (Fig. 1(g)). Fig. 2 shows the XRD patterns of the as-milled Zr64.7 Cu33.3 Nb2 and Zr62.7 Cu33.3 Nb4 powders after different MA times at a rotation speed of 200 rpm. The starting materials comprise crystalline Zr, Cu and Nb, as shown in Fig. 2. With increasing milling time to 16 h, the as-milled Zr64.7 Cu33.3 Nb2 powders are crystalline Zr and Cu with decreasing grain sizes, and no amorphous phase can be detected at this stage (Fig. 2(a)). After 30 h of MA, the amorphous phase is dominant in the as-milled Zr64.7 Cu33.3 Nb2 powders together with a small amount of unprocessed nanocrystalline Zr. Prolonging the MA time to 89 h, the as-milled powders are still composed of the amorphous phase and nanocrystalline Zr (Fig. 2(a)). In comparison, the
Fig. 2. XRD patterns of the as-milled (a) Zr64.7 Cu33.3 Nb2 and (b) Zr62.7 Cu33.3 Nb4 powders after different MA times at a rotation speed of 200 rpm.
as-milled Zr62.7 Cu33.3 Nb4 alloy is dominantly amorphous after 16 h of MA, as shown in Fig. 2(b). With increasing milling time to 49 h, the unprocessed crystalline Zr phase can still be detected although the amorphous phase is dominant at this stage. Normally, a shift of diffraction peaks towards lower angles on the XRD patterns indicates the formation of solid solutions. In our work, however, the shift of Zr peaks on the XRD patterns is negligible with increasing milling time up to 99 h for Zr66.7 Cu33.3 and 89 h for Zr64.7 Cu33.3 Nb2 (Figs. 1 and 2). This suggests that the nanocrystalline Zr instead of Zr(Cu) or Zr(Cu, Nb) solid solutions exists in the as-milled products. To verify the XRD results, some samples were selected to be examined using TEM and selected-area electron diffraction (SAED). Fig. 3 shows the TEM microstructures and corresponding SAED patterns of the as-milled Zr66.7 Cu33.3 and Zr62.7 Cu33.3 Nb4 alloys at a rotation speed of 200 rpm. Fig. 3(a) shows several nano-sized particles from the as-milled Zr66.7 Cu33.3 alloy after 99 h of MA. The areas “A” and “B” in Fig. 3(a) are identified to be crystalline and amorphous phases by the SAED patterns, as shown in Fig. 3(b) and (c), respectively. Fig. 3(d) shows a typical microstructure of the asmilled Zr62.7 Cu33.3 Nb4 alloy after 49 h of MA. The corresponding SAED pattern consists of diffraction rings, indicating the nanocrystalline nature of the particle in Fig. 3(d), as shown in Fig. 3(e). The diffraction rings are indexed as (1 0 0), (1 0 1), (1 1 0) and (1 0 3) reflections of Zr from the inner ring to the outer one, respectively. The examination of other particles verifies the existence of the
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Fig. 3. TEM images showing the microstructure of the as-milled powders at a rotation speed of 200 rpm. (a) Zr66.7 Cu33.3 after 99 h of the MA time and (d) Zr62.7 Cu33.3 Nb4 after 49 h of the MA time. (b and c) SAED patterns corresponding to “A” and “B” areas in (a), respectively. (e) SAED pattern corresponding to (d). The white and black circles indicate the selected areas.
amorphous phase in the as-milled Zr62.7 Cu33.3 Nb4 alloy. The TEM and diffraction results are consistent with the XRD patterns shown in Figs. 1 and 2(b). Table 1 presents the start time of the amorphization reaction (ts ) at a lower rotation speed of 200 rpm for Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys. Although a single amorphous phase cannot be obtained through MA at the lower rotation speed, it is obvious that a minor addition of Nb into Zr–Cu can accelerate the amorphization process and effectively shorten the MA time for the amorphization of the Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys, as shown in Figs. 1 and 2 and Table 1. Furthermore, the more the Nb addition into Zr–Cu, the less ts is needed (Table 1). The reasons for this can be described as follows: The heat of mixing, atomic size mismatch and plasticity behavior, all of which can influence the start time of amorphization reactions during MA [17]. Nb has a Goldschmidt radius of 0.143 nm between the atomic radii of Zr (0.162 nm) and Cu (0.128 nm) and it is predicted that the marginal atomic size mismatches among all constituents. Microalloying with the transitional metal Nb makes the atomic size more consecutive for all constituents. It is difficult for Nb to diffuse in Zr–Cu alloys, and the atomic arrangement becomes out-of-order. In addition, the introduction of an additional element enhances the “confusion principle” [18]. In microstructure, the minor addition favors the formation of the unique atomic dense-packing configurations with small free volumes. Moreover, the addition of an element with a positive heat of mixing was shown to lead to phase separation, and increased the plasticity of BMGs [19]. In comparison with the addition of 2 at.% Nb, the addition of 4 at.% Nb can further increase the mixing disorder and improve the packing density. In order to clarify whether a single amorphous phase can be obtained at higher rotation speeds, the influence of the rotation speed on solid-state phase transformations of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) has also been considered. Fig. 4 presents the XRD patterns obtained from the Zr66.7 Cu33.3 samples subjected to MA for different milling times at a rotation speed of 350 rpm. After 4 h
of MA, the amorphous phase is dominant in the as-milled powders together with a small amount of unprocessed nanocrystalline Zr (Fig. 4(a)). With increasing milling time to 8 h, the as-milled Zr66.7 Cu33.3 powders are completely amorphous (Fig. 4(c)). Further increasing milling time to 30 h, the as-milled product is still a single amorphous phase, as shown in Fig. 4(d) and (e). In order to explore the stability of the amorphous Zr66.7 Cu33.3 phase against MA, the powders were continuously milled up to 110 h under the same ball milling conditions. With increasing milling time to 50 h, the broad diffuse hump disappears and four crystalline diffraction peaks appear at 2 = 31◦ –35◦ , 37◦ –42◦ , 54◦ –59◦ , 65◦ –69◦ (Fig. 4(f)). This suggests that the amorphous phase transforms into a new metastable phase which is characterized by the diffraction peaks from the reflections of (1 1 1), (2 0 0), (2 2 0) and (3 1 1). The analysis of the Bragg peaks for the new phase confirms the formation of a metastable fcc-Zr2 Cu phase. The formation of the metastable fcc-Zr2 Cu phase has also been verified by the TEM results. Fig. 5 shows the TEM microstructure and corresponding SAED pattern of the as-milled Zr66.7 Cu33.3 alloys after 70 h of MA at a rotation speed of 350 rpm. It is obvious that the particle in Fig. 5(a) consists of a nanocrystalline phase with an fcc structure, as shown in Fig. 5(b).
Table 1 The start time of the amorphization reaction (ts ) at a rotation speed of 200 rpm and the finish time of the amorphization reaction (tf ) at a rotation speed of 350 rpm for Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys. Alloys (at.%)
ts (h)
tf (h)
Zr66.7 Cu33.3 Zr64.7 Cu33.3 Nb2 Zr62.7 Cu33.3 Nb4
60 30 16
8 4 4
Fig. 4. XRD patterns of the as-milled Zr66.7 Cu33.3 powders after (a) 4 h, (b) 6 h, (c) 8 h (d) 16 h, (e) 30 h, (f) 50 h, (g) 70 h, (h) 90 h and (i) 110 h of the MA time at a rotation speed of 350 rpm.
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Fig. 5. (a) TEM image and (b) corresponding SAED pattern of the as-milled Zr66.7 Cu33.3 powders after 70 h of the MA time at a rotation speed of 350 rpm. The white circle indicates the selected area.
The SAED pattern are indexed as (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections from the inner ring to the outer one. Similar results have been reported for the Zr67 Cu33 alloy by El-Eskandarany and Inoue [16]. Further increasing the milling time to 110 h, no phase transformation occurs and the metastable fcc-Zr2 Cu phase is present in the as-milled products (Fig. 4(g)–(i)). This demonstrates that the fcc-Zr2 Cu phase produced under the present milling conditions is so stable and capable of withstanding the shear and impact stresses which are generated by the milling media. Fig. 6 shows the DSC traces of the as-milled Zr66.7 Cu33.3 powders after different milling times at a rotation speed of 350 rpm. After 8 h of the MA time, a broad exothermic peak can be observed on the DSC trace, indicating the crystallization of the amorphous phase in the as-milled powders (Fig. 6(a)). The glass transition temperature Tg and the crystallization temperature Tx are determined to be 710 and 766 K, respectively. So the supercooled liquid region (Tx ), which is defined as the difference between Tx and Tg (Tx = Tx − Tg ), is about 56 K. With increasing milling time to 16 h, the DSC trace of the as-milled alloy also shows a broad exothermic peak but the glass transition temperature cannot be identified (Fig. 6(b)). The crystallization temperature is about 771 K, slightly higher than that of the as-milled alloy after 8 h of MA. After 70 h of MA, no endothermic or exothermic peak can be observed on the DSC trace (Fig. 6(c)). This is in agreement with the XRD results shown in Fig. 4(g). At this stage, the as-milled powders are composed of the metastable fcc-Zr2 Cu phase and no amorphous phase exists. Fig. 7 shows the morphology of the as-milled Zr66.7 Cu33.3 powders after 70 h of the MA time at different rotation speeds. When the
Fig. 6. DSC traces of the as-milled Zr66.7 Cu33.3 powders after different MA times at a rotation speed of 350 rpm.
rotation speed is 200 rpm, the large particles agglomerated through cold welding are still not smashed completely (Fig. 7(a)). The size distribution of the particles is inhomogeneous and the particle size is about 2–50 m. With increasing rotation speed to 350 rpm, the morphology of the as-milled powders is still particle-like (Fig. 7(b)). The size distribution of the particles is uniform and the particle size is only several microns.
Fig. 7. SEM images showing the morphology of the as-milled Zr66.7 Cu33.3 powders after 70 h of the MA time at a rotation speed of (a) 200 and (b) 350 rpm, respectively.
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Fig. 8. XRD patterns of the as-milled (a) Zr64.7 Cu33.3 Nb2 and (b) Zr62.7 Cu33.3 Nb4 powders after different MA times at a rotation speed of 350 rpm.
Fig. 8 shows the XRD patterns of the as-milled Zr64.7 Cu33.3 Nb2 and Zr62.7 Cu33.3 Nb4 powders after different MA times at a rotation speed of 350 rpm. After 4 h of MA, the as-milled Zr64.7 Cu33.3 Nb2 powders are completely amorphous, as shown in Fig. 8(a). With increasing milling time to 30 h, the as-milled product is still a single amorphous phase. Further increasing milling time to 50 h, the mechanically induced crystallization occurs and one sharp diffraction peak of a crystalline phase is highlighted by an arrow (Fig. 8(a)). At this stage, however, the amorphous phase is dominant. The crystallization process continues with increasing milling time to 70 h, and the diffraction peak of the crystalline phase is also marked by an arrow. It should be noticed that the amorphous phase is still dominant even after 70 h of MA. As for Zr62.7 Cu33.3 Nb4 , the as-milled product is completely amorphous after 4 h of MA, as shown in Fig. 8 (b). Moreover, the as-milled product is still a single amorphous phase even after 70 h of MA. The present results show that a single amorphous Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) phase can be obtained through MA at a higher rotation speed of 350 rpm. The finish time of the amorphization reaction (tf ) at the higher rotation speed for the Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys is listed in Table 1. It is obvious that the Nb addition can accelerate the amorphization process and shorten the tf . It should be noticed that the effect of 2 at.% Nb on the amorphization of Zr–Cu is comparable to that of 4 at.%
Nb. According to the results reported in the literature, there are contrary standpoints on the effect of Nb additions on the GFA in some alloy systems. Recently, Sharma et al. [9] reported that the addition of Nb can increase the GFA of Fe–Ni–Zr–B alloys, and they noted that glass formation was accompanied by contraction of the crystalline lattice. On the contrary, the addition of 2 at.% Nb has deteriorated the effect on the GFA of Fe72−x Al5 Ga2 P11 C6 B4 Mx [2]. The single amorphous phase of Zr66.7 Cu33.3 formed by MA is metastable and can transform into a metastable fcc-Zr2 Cu phase with further increasing milling time, as shown in Fig. 4. Similar mechanically driven amorphization and crystallization during MA have also been found in Zr66.7−x Ni33.3 Cx (x = 0, 1, 3) alloys [20]. The factors associated with the milling induced solid-state devitrification can be summarized as: (1) heterogeneity in the mechanically alloyed powders; (2) temperature rises during the ball milling process; (3) introduction of contamination to the ball-milled powders; and (4) lattice imperfections [21]. It should be noticed that the longer milling time does not cause the cyclic phase transformations (amorphous–crystalline–amorphous phase) of mechanically alloyed Zr66.7 Cu33.3 alloy, but leads to the homogenization of the metastable fcc-Zr2 Cu phase in this work. However, the cyclic phase transformations of mechanically alloyed Co75 Ti25 [22], and Zr67 Cu33 powders have been reported by El-Eskandarany and Inoue [16]. The authors argued that one possible factor is the introduction of contamination in the ball-milled powders and another possible factor is the temperature rise during the milling. The formation enthalpy of the metastable fcc phase is comparable to the amorphous phase, and the energy barrier between these two phases is rather low to allow such a cyclic transformation [16]. Moreover, the cyclic amorphous–crystalline phase transformation can only be observed in the moderate milling process. The different milling conditions may be the reason for the difference between the present results and the literature. In addition, the Nb addition can improve the stability of the amorphous Zr–Cu–Nb phase against the mechanically induced crystallization during the prolonged milling, as shown in Fig. 8. The stability of the amorphous Zr62.7 Cu33.3 Nb4 alloy is higher than that of Zr64.7 Cu33.3 Nb2 . The Nb addition makes the atomic size more consecutive for all constituents. It is difficult for Nb to diffuse in the amorphous Zr–Cu–Nb alloys. Furthermore, the packing density of the Zr–Cu–Nb alloys becomes higher with increasing Nb concentration. In addition, the present results show that the rotation speed of ball milling has a significant influence on the formation of the amorphous Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) phase by MA. The higher rotation speed (350 rpm) can not only sharply shorten ts but also promote the formation of the single amorphous Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) phase. El-Eskandarany and Inoue [16] have reported that the milling induced cyclic phase transformations of a Zr67 Cu33 alloy are related to the rotation speed. The velocity and impact frequency of milling balls increase with increasing rotation speed. During the milling, the relationship of average velocity vb , average impact frequency f and rotation speed ˝ can be expressed as [23]: b = f =
a1 + 4a2 [a3 + mp (1 + RBp )] · a5 · ˝ a4 [a3 + mp (1 + RBp )]
a1 + 4a2 [a3 + mp (1 + RBp )] · (mp · RBP )1/3 · a5 · ˝ b1 [a3 + mp (1 + RBp )] · rb
(1)
(2)
where a1 –a5 and b1 are constants related to the geometrical size and quality of the ball mill and vial. mp is the powder weight. RBP is the ball-to-powder weight ratio and rb is the radius of milling balls. Eqs. (1) and (2) state that the b and f are proportional to the rotation speed. Firstly, increasing the rotation speed can improve the velocity of the milling balls and thus enhance the energy of
Y. Wang et al. / Journal of Alloys and Compounds 474 (2009) 152–157
the powders transferred by the impact behavior of the balls. Secondly, the increase of the rotation speed can improve the collision frequency of the milling balls and also increase the energy captured by the powders, which can enhance the diffusion driving force among atoms and accelerate the solid-state reaction. Therefore, the rotation speed plays an important role in the formation of the amorphous phase by MA. 4. Conclusions The minor addition of Nb into Zr–Cu alloys can obviously shorten the start time of the amorphization reaction, improve the glass forming ability of the alloys, but cannot promote the formation of the single amorphous phase at a lower rotation speed of 200 rpm. In addition, the glass forming ability of the Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys increases with increasing Nb additions. A single amorphous Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) phase can be obtained through mechanical alloying at a higher rotation speed of 350 rpm. Moreover, the Nb addition into Zr–Cu alloys can accelerate the amorphization process, shorten the finish time of amorphization, and improve the stability of the amorphous phase against the mechanically induced crystallization at the higher rotation speed. Prolonging the ball milling time can induce the mechanical crystallization of the amorphous Zr66.7 Cu33.3 phase, which gradually transforms into a metastable fcc-Zr2 Cu phase. Acknowledgements The authors acknowledge the equipment donation from the Alexander von Humboldt Foundation (Bonn, Germany). The SEM
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and TEM experiments from Ruhr University Bochum (Bochum, Germany) are acknowledged. The authors also thank the support from the Key Subject (Laboratory) Research Foundation of Shandong Province). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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Amorphization and crystallization of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys during mechanical alloying Yan Wang a,b , Xiuxiu Chen b , Haoran Geng b,∗ , Zhongxi Yang a,b a b
The Key Laboratory of Liquid Structure and Heredity of Materials, Shandong University, 73 Jingshi Road, Jinan 250061, PR China School of Materials Science and Engineering, University of Jinan, 106 Jiwei Road, Jinan 250022, PR China
a r t i c l e
i n f o
Article history: Received 7 May 2008 Received in revised form 26 June 2008 Accepted 28 June 2008 Available online 3 August 2008 Keywords: Amorphous alloys Mechanical alloying X-ray diffraction Microstructure
a b s t r a c t In the present paper, the effect of Nb and different rotation speeds on the amorphization and crystallization of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys during mechanical alloying has been investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The results show that the minor addition of Nb can shorten the start time of the amorphization reaction, improve the glass forming ability of Zr–Cu alloys, but cannot promote the formation of a single amorphous phase at a lower rotation speed of 200 rpm. The glass forming ability of the Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys increases with increasing Nb additions. At a higher rotation speed of 350 rpm, a single amorphous phase of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) can be successfully fabricated. Moreover, the Nb addition into Zr–Cu alloys can accelerate the amorphization process and improve the stability of the amorphous phase against the mechanically induced crystallization. Furthermore, the amorphous Zr66.7 Cu33.3 phase gradually transforms into a metastable fcc-Zr2 Cu phase with increasing milling time. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In spite of the melt spinning technique, mechanical alloying (MA) does allow such opportunity for the cluster formation and homogenization of the glassy powders at the atomic scale through adequate milling periods. Hence, any significant concentration gradients or structural fluctuations in the glassy matrix can be avoided. Moreover, the introduction of lattice defects raises the free energy of the crystalline system and thus, destabilizes it with respect to the amorphous state. Minor alloying additions or microalloying technologies were key metallurgical practices and dominant concepts for developing new metallic crystalline materials in the later half of the 20th century. It was found that appropriate minor alloying additions were very effective in increasing glass forming ability (GFA), enhancing thermal stability and improving magnetic and mechanical properties for some bulk metallic glasses (BMGs). Moreover, the minor addition approach can also represent a feasible way to develop and design novel BMGs. Based on their atomic sizes [1], these elements can be categorized into three groups: (1) small metalloid elements like C, B, Si, (2) intermediate transition metals such as Fe, Ni, Co, Cu, Mo, Zn, Nb, and (3) large elements like Zr, Sn, Y, La and Ca.
Nb, as a kind of transition metals with intermediate atomic sizes, has been selected as minor alloying elements in various systems [2–8]. The Nb addition can make the atomic size more consecutive for some alloy systems, which effectively increases the atomic packing density [9]. Moreover, the addition of Nb atoms can also enhance the viscosity of undercooled liquid alloys and decrease the atomic diffusion coefficient [10]. In addition, the GFA and properties of Cu–Zr alloys are very sensitive to the minor additions [11–14]. Therefore, Cu–Zr is a model system for studying the mechanism of amorphization enhanced by the minor addition. The negative heat of mixing between Zr and Cu is −23 kJ/mol, but Cu–Nb and Zr–Nb have positive heats of mixing, +3 and +4 kJ/mol, respectively [15]. So the interactional force of atoms becomes more complicated and the formation of the amorphous phase may relate to a new reaction mechanism. In addition, El-Eskandarany and Inoue [16] have studied the effect of rotation speed on the solid-state phase transformtions of a Zr67 Cu33 alloy. They have found that the milling induced cyclic phase transformations are related to the rotation speed. The present paper aims to investigate the influence of Nb addition and different rotation speeds on the solid-state amorphization and phase evolution of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys during mechanical alloying. 2. Experimental
∗ Corresponding author. Tel.: +86 531 82765317; fax: +86 531 82765317. E-mail address: mse [email protected] (H. Geng). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.06.137
Powder mixtures of elemental Zr (<70 m, 99.9 wt.% purity), Cu (<70 m, 99.9 wt.% purity) and Nb (<70 m, 99.9 wt.% purity) with nominal compositions of
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Fig. 1. XRD patterns of the as-milled Zr66.7 Cu33.3 powders after (a) 3 h, (b) 16 h, (c) 30 h (d) 44.5 h, (e) 60 h and (f) 99 h of the MA time at a rotation speed of 200 rpm.
Zr66.7 Cu33.3 , Zr64.7 Cu33.3 Nb2 and Zr62.7 Cu33.3 Nb4 were mechanically alloyed in a high energy ball mill (Fritsch P6) at a rotation milling speed of 200 and 350 revolutions per minute (rpm). The chromium steel vial and balls were used in the present work. Mechanical alloying was carried out at room temperature in an Ar atmosphere with a ball-to-powder weight ratio of 17:1. In order to avoid an increase in the vial temperature, the milling process was periodically interrupted every 30 min and then halted for 10 min. In addition, the milling process was interrupted at various time intervals in order to remove small amounts of the milled products for analysis and characterization. Microstructural characterization of the as-milled powders was made using a Rigaku D/max-RB X-ray diffractometer (XRD) with Cu K␣ radiation, a Philips CM 20 transmission electron microscope (TEM), and a scanning electron microscope (SEM, LEO 1530 VP). Thermal analysis of the as-milled powders was carried out using a differential scanning calorimeter (DSC, NETZSCH STA 409) at a heating rate of 10 K min−1 in a purified argon atmosphere.
3. Results and discussion Fig. 1 shows the XRD patterns of the as-milled Zr66.7 Cu33.3 powders after selected MA times at a rotation speed of 200 rpm. The as-milled products at the initial stage (3 h) are just a polycrystalline mixture of starting reactant powders, indicated by sharp Bragg peaks corresponding to elemental Zr and Cu (Fig. 1(a)). With increasing milling time to 44.5 h, the as-milled powders are still composed of Zr and Cu, but a broadening of crystalline diffraction peaks is observed due to decreasing grain size and increasing atomic level strain (Fig. 1(b)–(d)). A broad diffuse hump appears on the XRD pattern after 60 h of MA (as highlighted by an arrow in Fig. 1(e)), indicating the formation of an amorphous phase. However, the broad diffraction peaks of Zr (1 0 0) and Cu (1 1 1) are still present on the XRD pattern at this stage. Further increasing the MA time to 99 h, the Zr (1 0 0) diffraction peak can still be observed on the XRD pattern, but all diffraction peaks of Cu disappear, suggesting that the resulting milling products are composed of unprocessed nanocrystalline Zr and the amorphous phase (Fig. 1(g)). Fig. 2 shows the XRD patterns of the as-milled Zr64.7 Cu33.3 Nb2 and Zr62.7 Cu33.3 Nb4 powders after different MA times at a rotation speed of 200 rpm. The starting materials comprise crystalline Zr, Cu and Nb, as shown in Fig. 2. With increasing milling time to 16 h, the as-milled Zr64.7 Cu33.3 Nb2 powders are crystalline Zr and Cu with decreasing grain sizes, and no amorphous phase can be detected at this stage (Fig. 2(a)). After 30 h of MA, the amorphous phase is dominant in the as-milled Zr64.7 Cu33.3 Nb2 powders together with a small amount of unprocessed nanocrystalline Zr. Prolonging the MA time to 89 h, the as-milled powders are still composed of the amorphous phase and nanocrystalline Zr (Fig. 2(a)). In comparison, the
Fig. 2. XRD patterns of the as-milled (a) Zr64.7 Cu33.3 Nb2 and (b) Zr62.7 Cu33.3 Nb4 powders after different MA times at a rotation speed of 200 rpm.
as-milled Zr62.7 Cu33.3 Nb4 alloy is dominantly amorphous after 16 h of MA, as shown in Fig. 2(b). With increasing milling time to 49 h, the unprocessed crystalline Zr phase can still be detected although the amorphous phase is dominant at this stage. Normally, a shift of diffraction peaks towards lower angles on the XRD patterns indicates the formation of solid solutions. In our work, however, the shift of Zr peaks on the XRD patterns is negligible with increasing milling time up to 99 h for Zr66.7 Cu33.3 and 89 h for Zr64.7 Cu33.3 Nb2 (Figs. 1 and 2). This suggests that the nanocrystalline Zr instead of Zr(Cu) or Zr(Cu, Nb) solid solutions exists in the as-milled products. To verify the XRD results, some samples were selected to be examined using TEM and selected-area electron diffraction (SAED). Fig. 3 shows the TEM microstructures and corresponding SAED patterns of the as-milled Zr66.7 Cu33.3 and Zr62.7 Cu33.3 Nb4 alloys at a rotation speed of 200 rpm. Fig. 3(a) shows several nano-sized particles from the as-milled Zr66.7 Cu33.3 alloy after 99 h of MA. The areas “A” and “B” in Fig. 3(a) are identified to be crystalline and amorphous phases by the SAED patterns, as shown in Fig. 3(b) and (c), respectively. Fig. 3(d) shows a typical microstructure of the asmilled Zr62.7 Cu33.3 Nb4 alloy after 49 h of MA. The corresponding SAED pattern consists of diffraction rings, indicating the nanocrystalline nature of the particle in Fig. 3(d), as shown in Fig. 3(e). The diffraction rings are indexed as (1 0 0), (1 0 1), (1 1 0) and (1 0 3) reflections of Zr from the inner ring to the outer one, respectively. The examination of other particles verifies the existence of the
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Fig. 3. TEM images showing the microstructure of the as-milled powders at a rotation speed of 200 rpm. (a) Zr66.7 Cu33.3 after 99 h of the MA time and (d) Zr62.7 Cu33.3 Nb4 after 49 h of the MA time. (b and c) SAED patterns corresponding to “A” and “B” areas in (a), respectively. (e) SAED pattern corresponding to (d). The white and black circles indicate the selected areas.
amorphous phase in the as-milled Zr62.7 Cu33.3 Nb4 alloy. The TEM and diffraction results are consistent with the XRD patterns shown in Figs. 1 and 2(b). Table 1 presents the start time of the amorphization reaction (ts ) at a lower rotation speed of 200 rpm for Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys. Although a single amorphous phase cannot be obtained through MA at the lower rotation speed, it is obvious that a minor addition of Nb into Zr–Cu can accelerate the amorphization process and effectively shorten the MA time for the amorphization of the Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys, as shown in Figs. 1 and 2 and Table 1. Furthermore, the more the Nb addition into Zr–Cu, the less ts is needed (Table 1). The reasons for this can be described as follows: The heat of mixing, atomic size mismatch and plasticity behavior, all of which can influence the start time of amorphization reactions during MA [17]. Nb has a Goldschmidt radius of 0.143 nm between the atomic radii of Zr (0.162 nm) and Cu (0.128 nm) and it is predicted that the marginal atomic size mismatches among all constituents. Microalloying with the transitional metal Nb makes the atomic size more consecutive for all constituents. It is difficult for Nb to diffuse in Zr–Cu alloys, and the atomic arrangement becomes out-of-order. In addition, the introduction of an additional element enhances the “confusion principle” [18]. In microstructure, the minor addition favors the formation of the unique atomic dense-packing configurations with small free volumes. Moreover, the addition of an element with a positive heat of mixing was shown to lead to phase separation, and increased the plasticity of BMGs [19]. In comparison with the addition of 2 at.% Nb, the addition of 4 at.% Nb can further increase the mixing disorder and improve the packing density. In order to clarify whether a single amorphous phase can be obtained at higher rotation speeds, the influence of the rotation speed on solid-state phase transformations of Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) has also been considered. Fig. 4 presents the XRD patterns obtained from the Zr66.7 Cu33.3 samples subjected to MA for different milling times at a rotation speed of 350 rpm. After 4 h
of MA, the amorphous phase is dominant in the as-milled powders together with a small amount of unprocessed nanocrystalline Zr (Fig. 4(a)). With increasing milling time to 8 h, the as-milled Zr66.7 Cu33.3 powders are completely amorphous (Fig. 4(c)). Further increasing milling time to 30 h, the as-milled product is still a single amorphous phase, as shown in Fig. 4(d) and (e). In order to explore the stability of the amorphous Zr66.7 Cu33.3 phase against MA, the powders were continuously milled up to 110 h under the same ball milling conditions. With increasing milling time to 50 h, the broad diffuse hump disappears and four crystalline diffraction peaks appear at 2 = 31◦ –35◦ , 37◦ –42◦ , 54◦ –59◦ , 65◦ –69◦ (Fig. 4(f)). This suggests that the amorphous phase transforms into a new metastable phase which is characterized by the diffraction peaks from the reflections of (1 1 1), (2 0 0), (2 2 0) and (3 1 1). The analysis of the Bragg peaks for the new phase confirms the formation of a metastable fcc-Zr2 Cu phase. The formation of the metastable fcc-Zr2 Cu phase has also been verified by the TEM results. Fig. 5 shows the TEM microstructure and corresponding SAED pattern of the as-milled Zr66.7 Cu33.3 alloys after 70 h of MA at a rotation speed of 350 rpm. It is obvious that the particle in Fig. 5(a) consists of a nanocrystalline phase with an fcc structure, as shown in Fig. 5(b).
Table 1 The start time of the amorphization reaction (ts ) at a rotation speed of 200 rpm and the finish time of the amorphization reaction (tf ) at a rotation speed of 350 rpm for Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys. Alloys (at.%)
ts (h)
tf (h)
Zr66.7 Cu33.3 Zr64.7 Cu33.3 Nb2 Zr62.7 Cu33.3 Nb4
60 30 16
8 4 4
Fig. 4. XRD patterns of the as-milled Zr66.7 Cu33.3 powders after (a) 4 h, (b) 6 h, (c) 8 h (d) 16 h, (e) 30 h, (f) 50 h, (g) 70 h, (h) 90 h and (i) 110 h of the MA time at a rotation speed of 350 rpm.
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Fig. 5. (a) TEM image and (b) corresponding SAED pattern of the as-milled Zr66.7 Cu33.3 powders after 70 h of the MA time at a rotation speed of 350 rpm. The white circle indicates the selected area.
The SAED pattern are indexed as (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections from the inner ring to the outer one. Similar results have been reported for the Zr67 Cu33 alloy by El-Eskandarany and Inoue [16]. Further increasing the milling time to 110 h, no phase transformation occurs and the metastable fcc-Zr2 Cu phase is present in the as-milled products (Fig. 4(g)–(i)). This demonstrates that the fcc-Zr2 Cu phase produced under the present milling conditions is so stable and capable of withstanding the shear and impact stresses which are generated by the milling media. Fig. 6 shows the DSC traces of the as-milled Zr66.7 Cu33.3 powders after different milling times at a rotation speed of 350 rpm. After 8 h of the MA time, a broad exothermic peak can be observed on the DSC trace, indicating the crystallization of the amorphous phase in the as-milled powders (Fig. 6(a)). The glass transition temperature Tg and the crystallization temperature Tx are determined to be 710 and 766 K, respectively. So the supercooled liquid region (Tx ), which is defined as the difference between Tx and Tg (Tx = Tx − Tg ), is about 56 K. With increasing milling time to 16 h, the DSC trace of the as-milled alloy also shows a broad exothermic peak but the glass transition temperature cannot be identified (Fig. 6(b)). The crystallization temperature is about 771 K, slightly higher than that of the as-milled alloy after 8 h of MA. After 70 h of MA, no endothermic or exothermic peak can be observed on the DSC trace (Fig. 6(c)). This is in agreement with the XRD results shown in Fig. 4(g). At this stage, the as-milled powders are composed of the metastable fcc-Zr2 Cu phase and no amorphous phase exists. Fig. 7 shows the morphology of the as-milled Zr66.7 Cu33.3 powders after 70 h of the MA time at different rotation speeds. When the
Fig. 6. DSC traces of the as-milled Zr66.7 Cu33.3 powders after different MA times at a rotation speed of 350 rpm.
rotation speed is 200 rpm, the large particles agglomerated through cold welding are still not smashed completely (Fig. 7(a)). The size distribution of the particles is inhomogeneous and the particle size is about 2–50 m. With increasing rotation speed to 350 rpm, the morphology of the as-milled powders is still particle-like (Fig. 7(b)). The size distribution of the particles is uniform and the particle size is only several microns.
Fig. 7. SEM images showing the morphology of the as-milled Zr66.7 Cu33.3 powders after 70 h of the MA time at a rotation speed of (a) 200 and (b) 350 rpm, respectively.
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Fig. 8. XRD patterns of the as-milled (a) Zr64.7 Cu33.3 Nb2 and (b) Zr62.7 Cu33.3 Nb4 powders after different MA times at a rotation speed of 350 rpm.
Fig. 8 shows the XRD patterns of the as-milled Zr64.7 Cu33.3 Nb2 and Zr62.7 Cu33.3 Nb4 powders after different MA times at a rotation speed of 350 rpm. After 4 h of MA, the as-milled Zr64.7 Cu33.3 Nb2 powders are completely amorphous, as shown in Fig. 8(a). With increasing milling time to 30 h, the as-milled product is still a single amorphous phase. Further increasing milling time to 50 h, the mechanically induced crystallization occurs and one sharp diffraction peak of a crystalline phase is highlighted by an arrow (Fig. 8(a)). At this stage, however, the amorphous phase is dominant. The crystallization process continues with increasing milling time to 70 h, and the diffraction peak of the crystalline phase is also marked by an arrow. It should be noticed that the amorphous phase is still dominant even after 70 h of MA. As for Zr62.7 Cu33.3 Nb4 , the as-milled product is completely amorphous after 4 h of MA, as shown in Fig. 8 (b). Moreover, the as-milled product is still a single amorphous phase even after 70 h of MA. The present results show that a single amorphous Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) phase can be obtained through MA at a higher rotation speed of 350 rpm. The finish time of the amorphization reaction (tf ) at the higher rotation speed for the Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys is listed in Table 1. It is obvious that the Nb addition can accelerate the amorphization process and shorten the tf . It should be noticed that the effect of 2 at.% Nb on the amorphization of Zr–Cu is comparable to that of 4 at.%
Nb. According to the results reported in the literature, there are contrary standpoints on the effect of Nb additions on the GFA in some alloy systems. Recently, Sharma et al. [9] reported that the addition of Nb can increase the GFA of Fe–Ni–Zr–B alloys, and they noted that glass formation was accompanied by contraction of the crystalline lattice. On the contrary, the addition of 2 at.% Nb has deteriorated the effect on the GFA of Fe72−x Al5 Ga2 P11 C6 B4 Mx [2]. The single amorphous phase of Zr66.7 Cu33.3 formed by MA is metastable and can transform into a metastable fcc-Zr2 Cu phase with further increasing milling time, as shown in Fig. 4. Similar mechanically driven amorphization and crystallization during MA have also been found in Zr66.7−x Ni33.3 Cx (x = 0, 1, 3) alloys [20]. The factors associated with the milling induced solid-state devitrification can be summarized as: (1) heterogeneity in the mechanically alloyed powders; (2) temperature rises during the ball milling process; (3) introduction of contamination to the ball-milled powders; and (4) lattice imperfections [21]. It should be noticed that the longer milling time does not cause the cyclic phase transformations (amorphous–crystalline–amorphous phase) of mechanically alloyed Zr66.7 Cu33.3 alloy, but leads to the homogenization of the metastable fcc-Zr2 Cu phase in this work. However, the cyclic phase transformations of mechanically alloyed Co75 Ti25 [22], and Zr67 Cu33 powders have been reported by El-Eskandarany and Inoue [16]. The authors argued that one possible factor is the introduction of contamination in the ball-milled powders and another possible factor is the temperature rise during the milling. The formation enthalpy of the metastable fcc phase is comparable to the amorphous phase, and the energy barrier between these two phases is rather low to allow such a cyclic transformation [16]. Moreover, the cyclic amorphous–crystalline phase transformation can only be observed in the moderate milling process. The different milling conditions may be the reason for the difference between the present results and the literature. In addition, the Nb addition can improve the stability of the amorphous Zr–Cu–Nb phase against the mechanically induced crystallization during the prolonged milling, as shown in Fig. 8. The stability of the amorphous Zr62.7 Cu33.3 Nb4 alloy is higher than that of Zr64.7 Cu33.3 Nb2 . The Nb addition makes the atomic size more consecutive for all constituents. It is difficult for Nb to diffuse in the amorphous Zr–Cu–Nb alloys. Furthermore, the packing density of the Zr–Cu–Nb alloys becomes higher with increasing Nb concentration. In addition, the present results show that the rotation speed of ball milling has a significant influence on the formation of the amorphous Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) phase by MA. The higher rotation speed (350 rpm) can not only sharply shorten ts but also promote the formation of the single amorphous Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) phase. El-Eskandarany and Inoue [16] have reported that the milling induced cyclic phase transformations of a Zr67 Cu33 alloy are related to the rotation speed. The velocity and impact frequency of milling balls increase with increasing rotation speed. During the milling, the relationship of average velocity vb , average impact frequency f and rotation speed ˝ can be expressed as [23]: b = f =
a1 + 4a2 [a3 + mp (1 + RBp )] · a5 · ˝ a4 [a3 + mp (1 + RBp )]
a1 + 4a2 [a3 + mp (1 + RBp )] · (mp · RBP )1/3 · a5 · ˝ b1 [a3 + mp (1 + RBp )] · rb
(1)
(2)
where a1 –a5 and b1 are constants related to the geometrical size and quality of the ball mill and vial. mp is the powder weight. RBP is the ball-to-powder weight ratio and rb is the radius of milling balls. Eqs. (1) and (2) state that the b and f are proportional to the rotation speed. Firstly, increasing the rotation speed can improve the velocity of the milling balls and thus enhance the energy of
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the powders transferred by the impact behavior of the balls. Secondly, the increase of the rotation speed can improve the collision frequency of the milling balls and also increase the energy captured by the powders, which can enhance the diffusion driving force among atoms and accelerate the solid-state reaction. Therefore, the rotation speed plays an important role in the formation of the amorphous phase by MA. 4. Conclusions The minor addition of Nb into Zr–Cu alloys can obviously shorten the start time of the amorphization reaction, improve the glass forming ability of the alloys, but cannot promote the formation of the single amorphous phase at a lower rotation speed of 200 rpm. In addition, the glass forming ability of the Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) alloys increases with increasing Nb additions. A single amorphous Zr66.7−x Cu33.3 Nbx (x = 0, 2, 4) phase can be obtained through mechanical alloying at a higher rotation speed of 350 rpm. Moreover, the Nb addition into Zr–Cu alloys can accelerate the amorphization process, shorten the finish time of amorphization, and improve the stability of the amorphous phase against the mechanically induced crystallization at the higher rotation speed. Prolonging the ball milling time can induce the mechanical crystallization of the amorphous Zr66.7 Cu33.3 phase, which gradually transforms into a metastable fcc-Zr2 Cu phase. Acknowledgements The authors acknowledge the equipment donation from the Alexander von Humboldt Foundation (Bonn, Germany). The SEM
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and TEM experiments from Ruhr University Bochum (Bochum, Germany) are acknowledged. The authors also thank the support from the Key Subject (Laboratory) Research Foundation of Shandong Province). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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