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Mechanical alloyed Ti–Cu–Ni–Si–B amorphous alloys with significant supercooled liquid region
Intermetallics 10 (2002) 1271–1276 www.elsevier.com/locate/intermet
Mechanical alloyed Ti–Cu–Ni–Si–B amorphous alloys with significant supercooled liquid region I-Kuan Jenga, Pee-Yew Leea,*, Jium-Shyong Chenb, Rong-Ruey Jengb, Chien-Hung Yehb, Chung-Kwei Linc a Institute of Materials Engineering, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, Taiwan Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Lung-Tan, Taiwan c Department of Materials Science, Feng Chia University, Taichung, Taiwan
b
Received in revised form 16 July 2002; accepted 16 July 2002
Abstract This study examined the glass formation range of Ti94 x yCuxNiySi4B2 alloy powders synthesized by mechanical alloying technique. According to the results, after 5–7 h of milling, the mechanically alloyed powders were amorphous at compositions with (x+y) equal to 20–40%. For the compositions with (x+y) larger than 45% or smaller than 10%, the structure of ball-milled powders is a partial amorphous single phase or coexistent partial amorphous and crystalline phases, respectively. The thermal stability of the amorphous powders was also investigated by differential thermal analysis. As the results demonstrated, several amorphous powders were found to exhibit a wide supercooled liquid region before crystallization. The temperature interval of the supercooled liquid region defined by the difference between Tg and Tx, i.e. T(=Tx–Tg), are 52 K for Ti74Ni20Si4B2, 74 K for Ti64Ni30Si4B2, 58 K for Ti64Cu20Ni10Si4B2, and 61 K for Ti74Cu10Ni10Si4B2. # 2002 Published by Elsevier Science Ltd. Keywords: C. Mechanical alloying and milling
1. Introduction Recently, new metallic amorphous alloys with a wide supercooled liquid region exceeding 20K have been prepared in a number of Ti-based alloy systems, such as Ti–Ni–Cu, Ti–Ni–Cu–Al, Ti–Zr–Ni–Cu–Al, Ti–Ni–Cu– Sn, Ti–Ni–Cu–Si–B, Ti–Ni–Cu–Sn–Zr, and Ti–Ni–Cu– Be [1–5]. The supercooled liquid region is defined by the temperature range, T=Tx Tg, between the glass transition temperature (Tg) and crystallization temperature (Tx). The increase of T means that the stability of the supercooled liquid state against crystallization increases and, therefore, enables the formation of bulk amorphous alloys by conventional casting techniques at a low cooling rates ranging from 1.5 to 100 K/s. Indeed, the bulk amorphous Ti50Ni15Cu25Sn5Zr5 alloys with diameter up to 5 mm have been produced by Inoue [5]. These new alloys are expected to expand the application * Corresponding author. Tel.: +886-22462-2192; fax: +886-224625324.
fields of bulk amorphous alloys due to the unique properties, such as high tensile strength and relatively high corrosion resistance at room temperature. It is well-known that both high reduced glass transition temperature Tg/Tm and large supercooled liquid region T are essential for the formation of bulk amorphous alloys by rapid solidification [6]. However, these also restrict bulk glass formation to near-eutectic compositions where supercooling can be realized without nucleation of crystalline phases. An alternative way to prepare amorphous alloys is via solid-state amorphization reaction (SSAR processes) [7]. SSAR is a low temperature process; therefore, it circumvents the limitations of conventional alloying and allows forming amorphous samples for compositions which cannot be amorphized by casting techniques. The techniques to synthesize amorphous alloys via SSAR include hydrogenation, multilayer interdiffusion, and mechanical alloying (MA). As previous investigations demonstrated, amorphization by mechanical alloying has been observed for a variety of binary and ternary alloy systems [8–13].
0966-9795/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S0966-9795(02)00161-9
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The product material of mechanical alloying is in powdered form and is suitable for compaction and densification into various shapes. The objective of this paper is, therefore, to investigate the feasibility of preparing Ti-based amorphous alloys by mechanical alloying. The formation and thermal stability of mechanically alloyed Ti94 x yCuxNiySi4B2 powders was discussed.
2. Experimental procedure Elemental powders of Ti (99.9%, < 100 mesh), Cu (99.98%, < 100 mesh), Ni (99.9%, < 300 mesh), Si (99.9%, < 325 mesh), B (99.9%, < 100 mesh) were
weighed to yield the desired compositions: Ti94 x yCuxNiySi4B2 (x=0–40, y=0–45), and then canned into an SKH 9 high speed steel vial together with Cr steel balls under an argon-filled glove box, where a SPEX 8016 shaker ball mill was employed for MA. The overall mechanical alloying process was persisted for 5–7 h which were interrupted every 15 min for the first hour and every 30 min after that. Each interruption was followed by an equal length of time (30 min) to cool down the vials, and then a suitable quantity of the mechanically alloyed powders was extracted to examine the progress of amorphization reaction. Techniques used to examine the status of amorphization include X-ray diffraction, scanning electron microscopy (SEM),
Fig. 1. Cross sections of mechanically alloyed Ti64Ni30Si4B2 powders after different milling times. (a) 0.25 h, (b) 0.50 h, (c) 1.00 h, (d) 1.50 h, (e) 2.00 h, and (f) 5.00 h.
I.-K. Jeng et al. / Intermetallics 10 (2002) 1271–1276
Fig. 2. X-ray diffraction patterns of mechanically alloyed Ti64Ni30Si4B2 powders for different milling times.
and differential scanning calorimeter (DSC). The X-ray analysis was performed using a SIEMENS D-5000 diffractometer with a monochromatic Cu Ka radiation. The microstructure and the morphology of the mechanically alloyed powders were examined with a Hitachi S-4100 scanning electron microscope. The thermal stability of the as-milled powders was determined using a Dupont 2000 differential scanning calorimeter, where the sample was heated from room temperature to 700 C in a purified argon atmosphere at a rate of 40 K/min.
3. Results and discussion Fig. 1 displays the SEM micrographs of particle cross sections for mechanically alloyed Ti64Ni30Si4B2 powders at different milling stages. At the very beginning of milling (i.e. 0.25, 0.5 and 1 h of milling), a typical lamellar microstructure forms and the individual layer thickness as measured from Fig. 1(a)–(c) ranges from 1 to 10 mm. With the extension of milling, the particles are stressed continuously and this results in a decrease of the lamellar thickness, as shown in Fig. 1(d)–(e). After 5 hours of milling the refined lamellar microstructure is no longer detected by SEM [Fig. 1(f)]. X-ray diffraction analysis is another conventional technique for monitoring the progress of amorphization. Fig. 2 displays the X-ray diffraction patterns of
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the starting and as-milled Ti64Ni30Si4B2 powders as a function of milling time. The top curve shows the X-ray diffraction pattern of a mixture of pure crystalline Ti, Ni, Si and B peaks. After 1 h of ball milling, the peak intensities of Ti and Ni decreased rapidly, indicating a preferential alloying of these two elements. With further ball milling up to 6 h, all the crystalline peaks have disappeared; only a broad diffraction peak is left, indicating that amorphization is complete within the resolution of X-ray diffraction. The gradual decrease of the elemental X-ray peaks during the early milling stages is similar to what is observed for amorphous phase formation by mechanical alloying in many binary alloy systems [14–15]. Weeber and Bakker reported three different types of amorphization reactions by MA of binary elemental powder mixture (AxBy) [16]. Type I denotes that the ‘‘effective crystalline size’’ continuously decreases and the peaks’ positions shift, ultimately resulting in an amorphous alloy; i.e. the solid solution is an intermediate stage. The second type (type II) directly forms an amorphous alloy from the starting elemental powders; i.e. the diffraction peak intensities of the elemental decrease and a broad peak’s intensity of the amorphous alloy increase. For the final type, intermetallics form as intermediate products and further milling results in an amorphous alloy. In this study, as Fig. 2 illustrates, a broad diffraction peak, indicating the formation of amorphous alloy, was superimposed on the Ni(111) and Ti(011) diffraction peaks at around 2y=41 for 2 h ball-milled powder. This observation suggests that type II amorphization reaction is responsible for the formation of amorphous phase. Fig. 3 shows the corresponding X-ray diffraction patterns of Ti94 x yCuxNiySi4B2 alloy powders after 5–7 h MA treatment. The powders were amorphous for the Ti64Cu30Si4B2, Ti84 xCuxNi10Si4B2 (x=10–30), Ti74 x CuxNi20Si4B2 (x=0–20), Ti64 xCuxNi10Si4B2 (x=0–15), and Ti54Ni40Si4B2. The compositional dependence of amorphous phase for the Ti94 x yCuxNiySi4B2 mechanically alloyed powders was shown in Fig. 4. Further examination of Fig. 4 indicates that two types of phase field were observed for the ball-milled Ti94 x yCuxNiySi4B2 powders. For the compositions with (x+y) larger than 45% or smaller than 10%, the structure of ball-milled powders is a partial amorphous single phase or coexistent partial amorphous and crystalline phases, respectively. The fully amorphous phase only formed at compositions with (x+y) equal to 20–40%. As reported earlier, it is easier to prepare binary amorphous alloys by MA at the compositions with a large negative heat of mixing (Hm). The values of Hm, as calculated by the Miedema model [17], indicate Hm to be negative for Ti–Cu and Ti–Ni alloy systems and positive for Cu–Ni alloy system. Previous study on the mechanically alloyed Ti–Cu and Ti–Ni powders had shown that fully amorphous phase was formed in the
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Fig. 3. X-ray diffraction patterns of Ti–Cu–Ni–Si–B powders after 5–7 h mechanical alloying treatment.
composition ranges of Ti13Cu87–Ti90Cu10 and Ti30Ni70– Ti70Ni30, respectively [18,19]. For Cu–Ni alloy system, no amorphous phase was observed in the ball-milled powders with compositions ranging from Cu10Ni90– Cu70Ni30 [20]. In the present study, since Cu and Ni are miscible and total amount of Si and B are only 6%, therefore, the multicomponent compositions of Ti94 x yCuxNiySi4B2 alloys can be modified into a Ti100 x y (CuNi)x+y quasi-binary alloy system. For the ball-milled Ti94 x yCuxNiySi4B2 powders with (x+y) equal to 20–40%, the values of Hm is expected to be similar to that of Ti–Cu or Ti–Ni binary systems, and is presumably responsible for the formation of single amorphous phase as observed. On the other hand, for the compositions of ball-milled Ti94 x yCuxNiySi4B2 powders with (x+y) larger than 45% or smaller than 10%, the Ti94 x yCuxNiySi4B system can be treated as (CuNi)-based alloys (x+y> 45%) or Ti solid solution (x+y< 10%), and this implies existence of positive values of Hm is possibly accounting for the inability for the preparation of amorphous phase by mechanical alloying. The thermal stability of the Ti94 x yCuxNiySi4B2 amorphous powders was investigated by differential
scanning calorimetry. Fig. 5 shows the DSC curves of the several ball-milled Ti94 x yCuxNiySi4B2 alloys consisting of the amorphous single phase. It can be seen that all the alloys exhibit an endothermic heat event due to the glass transition at low temperatures followed
Fig. 4. Compositional dependence of amorphous phase for the Ti94 x yCuxNiySi4B2 mechanically alloyed powders.
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Fig. 5. DSC scans for the amorphous Ti74Ni20Si4B2, Ti64Ni30Si4B2, Ti64Cu20Ni10Si4B2, and Ti74Cu10Ni10Si4B2 alloys.
by a sharp exothermic heat release events indicating the successive stepwise transformations from supercooled liquid state to crystalline phases. The glass transition temperature, Tg, and the crystallization temperature, Tx, were defined as the onset temperatures of the endothermic and exothermic DSC events, respectively. T=Tx Tg is referred to as the supercooled liquid region. As shown in Fig. 5, a wide supercooled liquid region is found for every amorphous alloy. The T are 52 K for Ti74Ni20Si4B2, 74 K for Ti64Ni30Si4B2, 58 K for Ti64Cu20Ni10Si4B2, and 61 K for Ti74Cu10Ni10Si4B2. It is generally known that amorphous Ti–Cu and Ti– Ni alloys have been successfully prepared in a rather wide composition range by both rapid quenching and mechanical alloying process [18,19]. The Ti–Cu amorphous alloys exhibit only rather small supercooled liquid regions in a limited composition range and no supercooled liquid region is observed for Ti–Ni amorphous alloys. However, the simultaneous presence of several elements was found to cause a significant extension of the supercooled liquid region before crystallization. Choi-Yim et al. [21] have reported that the addition of Si can enhances the glass formability of Cu–Ti–Zr–Ni alloy system. Recently, Inoue et al. [22] also found the addition of B could induce the occurrence of amorphization in melt-spun Fe–Si–C crystalline alloys. Inoue et al. [23] have suggested that large atomic size ratios and attractive bonding nature between the constituent elements together with the difficulty of the redistribution of these elements for crystallization are dominant factors for the increase in glass-forming ability and the appearance of a wide supercooled liquid region. It can be seen that the difference in the atomic
sizes of the five constituent elements exceeds 11% (the atomic radii of Ti, Cu, Ni, Si, and B are 0.147, 0.128, 0.124, 1.32, and 0.98 nm respectively). It is believed that these atomic size differences lead to the highly dense random packed structure in the amorphous phase, which enables the achievement of a large liquid/solid interfacial energy and makes the redistribution of atoms on a large range scale difficult. In addition, the Ti–Cu and Ti–Ni atomic pairs also have a highly attractive bonding nature as is evidenced by the large negative heat of mixing Hm for their atomic pairs [17]. The above-mentioned characteristics of the alloy suppress nucleation and growth of the crystalline phase in the supercooled liquid phase, and thus lead to large glass forming ability and high thermal stability in the Ti94 x yCuxNiySi4B2 alloy.
4. Conclusions We have studied the amorphization behavior of Ti94 x yCuxNiySi4B2 alloy powders synthesized by mechanical alloying technique. Complete amorphization is feasible at compositions with (x+y) equal to 20–40%. Coexistence of partial amorphous and crystalline phases is obtained for the compositions with (x+y) larger than 45% or smaller than 10%. The amorphous phase in Ti74Ni20Si4B2, Ti64Ni30Si4B2, Ti64Cu10Ni10Si4B2, and Ti74Cu10Ni10Si4B2 exhibits a wide supercooled liquid region before crystallization. This demonstrates the potential of mechanical alloying as a versatile method for the formation of amorphous Ti-based alloys with wide supercooled liquid regions.
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Acknowledgements The authors are grateful for the financial support of this work by the National Science Council of Republic of China under Grant No. NSC 88–2216-E-019–006.
References [1] Zhang T, Inoue A, Masumoto T. Mater Sci Eng 1994;A181– 182:1423–6. [2] Inoue A, Nishiyama N, Amiya K, Zhang T, Masumoto T. Mater Lett 1994;19:131–5. [3] Zhang T, Inoue A. Mater Trans JIM 1998;39:1001–6. [4] Amiya K, Nishiyama N, Inoue A, Masumoto T. Mater Sci Eng 1994;A179–180:692–6. [5] Inoue A. Mater Sci Forum 312- 1999;314:307–14. [6] Inoue A. Proc Japan Acad 1997;73B:19–24. [7] Johnson WL. Prog Mater Sci 1986;30:81–134.
[8] Lee PY, Yang JL, Lin CK, Lin HM. Metall Mater Trans A 1997; 28A:1429–35. [9] El-Eskandarany MS, Aoki K, Suzuki K. J Appl Phys 1992; 72:2665–72. [10] Lee PY, Koch CC. J Mater Sci 1988;23:2837–45. [11] Hu CJ, Lee PY. Mater Chem and Phys 2002;74:13–18. [12] Eckert J. Mater Sci Forum 1999;312–314:3–12. [13] Eckert J, Seidel M, Schultz L. Mater Sci Forum 1996;225– 227:113–8. [14] Lee PY, Koch CC. J Non Cryst Solids 1987;94:88–100. [15] El-Eskandarany MS, Aoki K, Suzuki K. Metall Mater Trans A 1992;23A:2131–40. [16] Weeber AW, Bakker H. Phys B 1988;153:93–135. [17] Miedema AR, Dechatel PF, de Boer FR. Phys B 1980;100:1–28. [18] Politis C, Johnson WL. J Appl Phys 1986;60:1147–51. [19] Schwarz RB, Petrich RR, Saw CK. J Non-Cryst Solids 1987; 76:281–302. [20] Arce-Estrada EM, Torres-Gonzalez NE, Lopez-Hirata VM. Mater Sci Forum 1999;312–314:507–12. [21] Choi-Yim H, Busch HR, Johnson WL. J Appl Phys 1998; 83:7993–7. [22] Wang XM, Inoue A. Mater Trans JIM 1999;40:634–42. [23] Inoue A. Mater Trans JIM 1995;36:866–75.
Mechanical alloyed Ti–Cu–Ni–Si–B amorphous alloys with significant supercooled liquid region I-Kuan Jenga, Pee-Yew Leea,*, Jium-Shyong Chenb, Rong-Ruey Jengb, Chien-Hung Yehb, Chung-Kwei Linc a Institute of Materials Engineering, National Taiwan Ocean University, 2 Pei-Ning Road, Keelung, Taiwan Materials and Electro-Optics Research Division, Chung-Shan Institute of Science and Technology, Lung-Tan, Taiwan c Department of Materials Science, Feng Chia University, Taichung, Taiwan
b
Received in revised form 16 July 2002; accepted 16 July 2002
Abstract This study examined the glass formation range of Ti94 x yCuxNiySi4B2 alloy powders synthesized by mechanical alloying technique. According to the results, after 5–7 h of milling, the mechanically alloyed powders were amorphous at compositions with (x+y) equal to 20–40%. For the compositions with (x+y) larger than 45% or smaller than 10%, the structure of ball-milled powders is a partial amorphous single phase or coexistent partial amorphous and crystalline phases, respectively. The thermal stability of the amorphous powders was also investigated by differential thermal analysis. As the results demonstrated, several amorphous powders were found to exhibit a wide supercooled liquid region before crystallization. The temperature interval of the supercooled liquid region defined by the difference between Tg and Tx, i.e. T(=Tx–Tg), are 52 K for Ti74Ni20Si4B2, 74 K for Ti64Ni30Si4B2, 58 K for Ti64Cu20Ni10Si4B2, and 61 K for Ti74Cu10Ni10Si4B2. # 2002 Published by Elsevier Science Ltd. Keywords: C. Mechanical alloying and milling
1. Introduction Recently, new metallic amorphous alloys with a wide supercooled liquid region exceeding 20K have been prepared in a number of Ti-based alloy systems, such as Ti–Ni–Cu, Ti–Ni–Cu–Al, Ti–Zr–Ni–Cu–Al, Ti–Ni–Cu– Sn, Ti–Ni–Cu–Si–B, Ti–Ni–Cu–Sn–Zr, and Ti–Ni–Cu– Be [1–5]. The supercooled liquid region is defined by the temperature range, T=Tx Tg, between the glass transition temperature (Tg) and crystallization temperature (Tx). The increase of T means that the stability of the supercooled liquid state against crystallization increases and, therefore, enables the formation of bulk amorphous alloys by conventional casting techniques at a low cooling rates ranging from 1.5 to 100 K/s. Indeed, the bulk amorphous Ti50Ni15Cu25Sn5Zr5 alloys with diameter up to 5 mm have been produced by Inoue [5]. These new alloys are expected to expand the application * Corresponding author. Tel.: +886-22462-2192; fax: +886-224625324.
fields of bulk amorphous alloys due to the unique properties, such as high tensile strength and relatively high corrosion resistance at room temperature. It is well-known that both high reduced glass transition temperature Tg/Tm and large supercooled liquid region T are essential for the formation of bulk amorphous alloys by rapid solidification [6]. However, these also restrict bulk glass formation to near-eutectic compositions where supercooling can be realized without nucleation of crystalline phases. An alternative way to prepare amorphous alloys is via solid-state amorphization reaction (SSAR processes) [7]. SSAR is a low temperature process; therefore, it circumvents the limitations of conventional alloying and allows forming amorphous samples for compositions which cannot be amorphized by casting techniques. The techniques to synthesize amorphous alloys via SSAR include hydrogenation, multilayer interdiffusion, and mechanical alloying (MA). As previous investigations demonstrated, amorphization by mechanical alloying has been observed for a variety of binary and ternary alloy systems [8–13].
0966-9795/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S0966-9795(02)00161-9
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I.-K. Jeng et al. / Intermetallics 10 (2002) 1271–1276
The product material of mechanical alloying is in powdered form and is suitable for compaction and densification into various shapes. The objective of this paper is, therefore, to investigate the feasibility of preparing Ti-based amorphous alloys by mechanical alloying. The formation and thermal stability of mechanically alloyed Ti94 x yCuxNiySi4B2 powders was discussed.
2. Experimental procedure Elemental powders of Ti (99.9%, < 100 mesh), Cu (99.98%, < 100 mesh), Ni (99.9%, < 300 mesh), Si (99.9%, < 325 mesh), B (99.9%, < 100 mesh) were
weighed to yield the desired compositions: Ti94 x yCuxNiySi4B2 (x=0–40, y=0–45), and then canned into an SKH 9 high speed steel vial together with Cr steel balls under an argon-filled glove box, where a SPEX 8016 shaker ball mill was employed for MA. The overall mechanical alloying process was persisted for 5–7 h which were interrupted every 15 min for the first hour and every 30 min after that. Each interruption was followed by an equal length of time (30 min) to cool down the vials, and then a suitable quantity of the mechanically alloyed powders was extracted to examine the progress of amorphization reaction. Techniques used to examine the status of amorphization include X-ray diffraction, scanning electron microscopy (SEM),
Fig. 1. Cross sections of mechanically alloyed Ti64Ni30Si4B2 powders after different milling times. (a) 0.25 h, (b) 0.50 h, (c) 1.00 h, (d) 1.50 h, (e) 2.00 h, and (f) 5.00 h.
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Fig. 2. X-ray diffraction patterns of mechanically alloyed Ti64Ni30Si4B2 powders for different milling times.
and differential scanning calorimeter (DSC). The X-ray analysis was performed using a SIEMENS D-5000 diffractometer with a monochromatic Cu Ka radiation. The microstructure and the morphology of the mechanically alloyed powders were examined with a Hitachi S-4100 scanning electron microscope. The thermal stability of the as-milled powders was determined using a Dupont 2000 differential scanning calorimeter, where the sample was heated from room temperature to 700 C in a purified argon atmosphere at a rate of 40 K/min.
3. Results and discussion Fig. 1 displays the SEM micrographs of particle cross sections for mechanically alloyed Ti64Ni30Si4B2 powders at different milling stages. At the very beginning of milling (i.e. 0.25, 0.5 and 1 h of milling), a typical lamellar microstructure forms and the individual layer thickness as measured from Fig. 1(a)–(c) ranges from 1 to 10 mm. With the extension of milling, the particles are stressed continuously and this results in a decrease of the lamellar thickness, as shown in Fig. 1(d)–(e). After 5 hours of milling the refined lamellar microstructure is no longer detected by SEM [Fig. 1(f)]. X-ray diffraction analysis is another conventional technique for monitoring the progress of amorphization. Fig. 2 displays the X-ray diffraction patterns of
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the starting and as-milled Ti64Ni30Si4B2 powders as a function of milling time. The top curve shows the X-ray diffraction pattern of a mixture of pure crystalline Ti, Ni, Si and B peaks. After 1 h of ball milling, the peak intensities of Ti and Ni decreased rapidly, indicating a preferential alloying of these two elements. With further ball milling up to 6 h, all the crystalline peaks have disappeared; only a broad diffraction peak is left, indicating that amorphization is complete within the resolution of X-ray diffraction. The gradual decrease of the elemental X-ray peaks during the early milling stages is similar to what is observed for amorphous phase formation by mechanical alloying in many binary alloy systems [14–15]. Weeber and Bakker reported three different types of amorphization reactions by MA of binary elemental powder mixture (AxBy) [16]. Type I denotes that the ‘‘effective crystalline size’’ continuously decreases and the peaks’ positions shift, ultimately resulting in an amorphous alloy; i.e. the solid solution is an intermediate stage. The second type (type II) directly forms an amorphous alloy from the starting elemental powders; i.e. the diffraction peak intensities of the elemental decrease and a broad peak’s intensity of the amorphous alloy increase. For the final type, intermetallics form as intermediate products and further milling results in an amorphous alloy. In this study, as Fig. 2 illustrates, a broad diffraction peak, indicating the formation of amorphous alloy, was superimposed on the Ni(111) and Ti(011) diffraction peaks at around 2y=41 for 2 h ball-milled powder. This observation suggests that type II amorphization reaction is responsible for the formation of amorphous phase. Fig. 3 shows the corresponding X-ray diffraction patterns of Ti94 x yCuxNiySi4B2 alloy powders after 5–7 h MA treatment. The powders were amorphous for the Ti64Cu30Si4B2, Ti84 xCuxNi10Si4B2 (x=10–30), Ti74 x CuxNi20Si4B2 (x=0–20), Ti64 xCuxNi10Si4B2 (x=0–15), and Ti54Ni40Si4B2. The compositional dependence of amorphous phase for the Ti94 x yCuxNiySi4B2 mechanically alloyed powders was shown in Fig. 4. Further examination of Fig. 4 indicates that two types of phase field were observed for the ball-milled Ti94 x yCuxNiySi4B2 powders. For the compositions with (x+y) larger than 45% or smaller than 10%, the structure of ball-milled powders is a partial amorphous single phase or coexistent partial amorphous and crystalline phases, respectively. The fully amorphous phase only formed at compositions with (x+y) equal to 20–40%. As reported earlier, it is easier to prepare binary amorphous alloys by MA at the compositions with a large negative heat of mixing (Hm). The values of Hm, as calculated by the Miedema model [17], indicate Hm to be negative for Ti–Cu and Ti–Ni alloy systems and positive for Cu–Ni alloy system. Previous study on the mechanically alloyed Ti–Cu and Ti–Ni powders had shown that fully amorphous phase was formed in the
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Fig. 3. X-ray diffraction patterns of Ti–Cu–Ni–Si–B powders after 5–7 h mechanical alloying treatment.
composition ranges of Ti13Cu87–Ti90Cu10 and Ti30Ni70– Ti70Ni30, respectively [18,19]. For Cu–Ni alloy system, no amorphous phase was observed in the ball-milled powders with compositions ranging from Cu10Ni90– Cu70Ni30 [20]. In the present study, since Cu and Ni are miscible and total amount of Si and B are only 6%, therefore, the multicomponent compositions of Ti94 x yCuxNiySi4B2 alloys can be modified into a Ti100 x y (CuNi)x+y quasi-binary alloy system. For the ball-milled Ti94 x yCuxNiySi4B2 powders with (x+y) equal to 20–40%, the values of Hm is expected to be similar to that of Ti–Cu or Ti–Ni binary systems, and is presumably responsible for the formation of single amorphous phase as observed. On the other hand, for the compositions of ball-milled Ti94 x yCuxNiySi4B2 powders with (x+y) larger than 45% or smaller than 10%, the Ti94 x yCuxNiySi4B system can be treated as (CuNi)-based alloys (x+y> 45%) or Ti solid solution (x+y< 10%), and this implies existence of positive values of Hm is possibly accounting for the inability for the preparation of amorphous phase by mechanical alloying. The thermal stability of the Ti94 x yCuxNiySi4B2 amorphous powders was investigated by differential
scanning calorimetry. Fig. 5 shows the DSC curves of the several ball-milled Ti94 x yCuxNiySi4B2 alloys consisting of the amorphous single phase. It can be seen that all the alloys exhibit an endothermic heat event due to the glass transition at low temperatures followed
Fig. 4. Compositional dependence of amorphous phase for the Ti94 x yCuxNiySi4B2 mechanically alloyed powders.
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Fig. 5. DSC scans for the amorphous Ti74Ni20Si4B2, Ti64Ni30Si4B2, Ti64Cu20Ni10Si4B2, and Ti74Cu10Ni10Si4B2 alloys.
by a sharp exothermic heat release events indicating the successive stepwise transformations from supercooled liquid state to crystalline phases. The glass transition temperature, Tg, and the crystallization temperature, Tx, were defined as the onset temperatures of the endothermic and exothermic DSC events, respectively. T=Tx Tg is referred to as the supercooled liquid region. As shown in Fig. 5, a wide supercooled liquid region is found for every amorphous alloy. The T are 52 K for Ti74Ni20Si4B2, 74 K for Ti64Ni30Si4B2, 58 K for Ti64Cu20Ni10Si4B2, and 61 K for Ti74Cu10Ni10Si4B2. It is generally known that amorphous Ti–Cu and Ti– Ni alloys have been successfully prepared in a rather wide composition range by both rapid quenching and mechanical alloying process [18,19]. The Ti–Cu amorphous alloys exhibit only rather small supercooled liquid regions in a limited composition range and no supercooled liquid region is observed for Ti–Ni amorphous alloys. However, the simultaneous presence of several elements was found to cause a significant extension of the supercooled liquid region before crystallization. Choi-Yim et al. [21] have reported that the addition of Si can enhances the glass formability of Cu–Ti–Zr–Ni alloy system. Recently, Inoue et al. [22] also found the addition of B could induce the occurrence of amorphization in melt-spun Fe–Si–C crystalline alloys. Inoue et al. [23] have suggested that large atomic size ratios and attractive bonding nature between the constituent elements together with the difficulty of the redistribution of these elements for crystallization are dominant factors for the increase in glass-forming ability and the appearance of a wide supercooled liquid region. It can be seen that the difference in the atomic
sizes of the five constituent elements exceeds 11% (the atomic radii of Ti, Cu, Ni, Si, and B are 0.147, 0.128, 0.124, 1.32, and 0.98 nm respectively). It is believed that these atomic size differences lead to the highly dense random packed structure in the amorphous phase, which enables the achievement of a large liquid/solid interfacial energy and makes the redistribution of atoms on a large range scale difficult. In addition, the Ti–Cu and Ti–Ni atomic pairs also have a highly attractive bonding nature as is evidenced by the large negative heat of mixing Hm for their atomic pairs [17]. The above-mentioned characteristics of the alloy suppress nucleation and growth of the crystalline phase in the supercooled liquid phase, and thus lead to large glass forming ability and high thermal stability in the Ti94 x yCuxNiySi4B2 alloy.
4. Conclusions We have studied the amorphization behavior of Ti94 x yCuxNiySi4B2 alloy powders synthesized by mechanical alloying technique. Complete amorphization is feasible at compositions with (x+y) equal to 20–40%. Coexistence of partial amorphous and crystalline phases is obtained for the compositions with (x+y) larger than 45% or smaller than 10%. The amorphous phase in Ti74Ni20Si4B2, Ti64Ni30Si4B2, Ti64Cu10Ni10Si4B2, and Ti74Cu10Ni10Si4B2 exhibits a wide supercooled liquid region before crystallization. This demonstrates the potential of mechanical alloying as a versatile method for the formation of amorphous Ti-based alloys with wide supercooled liquid regions.
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Acknowledgements The authors are grateful for the financial support of this work by the National Science Council of Republic of China under Grant No. NSC 88–2216-E-019–006.
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