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On crystallization behavior and thermal stability of Cu64Zr36 metallic glass by controlling the melt temperature
NOC-18013; No of Pages 6 Journal of Non-Crystalline Solids xxx (2016) xxx–xxx
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On crystallization behavior and thermal stability of Cu64Zr36 metallic glass by controlling the melt temperature Xiao Cui a,b,⁎, Qi Dong Zhang a, Xiao Yun Li a, Fang Qiu Zu a,⁎⁎ a b
Institute of Liquid/Solid Metal Processing, School of Materials Science & Engineering, Hefei University of Technology, Hefei 230009, People's Republic of China College of Machinery and Engineering, Taishan University, Taian 271000, People's Republic of China
a r t i c l e
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Article history: Received 28 June 2016 Received in revised form 7 September 2016 Accepted 18 September 2016 Available online xxxx Keywords: Melt temperature Metallic glass Glass forming ability Thermal stability Crystallization behavior
a b s t r a c t Change in melt structure of Cu64Zr36 binary metallic glass (MG) was studied using resistivity and differential thermal analysis in continuous heating process, abnormal structural changes were observed with temperature elevating. Cu64Zr36 MG ribbons with different melt temperatures were prepared using single roller spinning method, experimental results based on resistivity, DSC, and XRD indicate that the thermal stability and glass forming ability (GFA) of Cu64Zr36 MGs are greatly affected by the melt temperature. With the increasing of the melt temperature, the crystallization temperature (Tx) and supercooled liquid region (ΔTx) shift from 780.7 K and 41.9 K of the MG with the lowest melt temperature of 1353 K towards to 800.3 K and 48.1 K of the MG with the highest melt temperature of 1653 K. The relationship of the melt temperature and GFA was further studied by injecting the melt into a cone-shape copper mould. The critical diameter can reach Ф2.5 mm of the Cu64Zr36 MG with the highest melt temperature of 1653 K. However, the Ф2.5 cross section of Cu64Zr36 MGs prepared with the melt temperature lower than 1653 K shows apparent Bragg peaks. © 2016 Published by Elsevier B.V.
1. Introduction Glass forming ability (GFA) and thermal stability are key properties for development and application of metallic glasses (MGs) [1–3]. There have been many efforts on improving the thermal stability and glass forming ability (GFA) of metallic glasses, such as adding a small quantity of high melting temperature elements [4–5], controlling the casting parameter [6] and overheating treatments [7–10]. As the cooling rate of preparation process of metallic glass is usually higher than 104 K/ s, they can be considered as the frozen-in-liquid. It is presumed that the melt state would affect the formation and configuration of metallic glasses, and furthermore affect their thermal stability. Fan and Inoue [10] reported that the melt temperature only has slight influence for the MGs which has typical polymorphous crystallization behaviors, but has significant influences for the MGs that has nano-crystallization tendency. Zhu et al. [7] studied the influence of casting temperature on the thermal-stability of a series Cu- and Zr-based MGs. Their results indicated that increasing the casting temperature could enhance the thermal stability of MGs, and it was considered to be attributed to the decrease in the amount of the local ordering clusters induced by elevating the casting temperature. And similar works have been reported by ⁎ Correspondence to: X. Cui, College of Machinery and Engineering, Taishan University, Taian 271000, People's Republic of China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (X. Cui), [email protected] (F.Q. Zu).
many groups in various MGs systems [7–11]. However, the former work that had been reported lacks direct guidance about the relationship between melt state and glass formation, the melt behaviors with respect to overheating temperature of MGs are rarely reported so far. In recent years, discontinuous structure changes of alloy melts upon heating have been observed using direct ways such as X-Ray and neutron diffraction methods [12–13], indirect ways such as internal friction technique [14–15] and electrical resistivity measurement [16–17] etc. In the present work, melt behaviors of Cu64Zr36 binary MGs were studied using electrical resistivity and differential thermal analysis (DTA) measurements in the heating process, discontinuous structure changes of Cu64Zr36 melt were found in a wide temperature range. Experimental results using resistivity, DSC, and XRD measurements show that the GFA, thermal stability, and crystallization behaviors of Cu64Zr36 MGs are greatly affected by the melt temperature. 2. Experiments The alloy ingot of nominal composition Cu64Zr36 was melted by arc melting mixtures of ultrasonically cleaned Zr and Cu pieces with high purity (99.9 at.% of Zr and 99.999 at.% of Cu). The arc melting was performed in a Ti-gettered high purity Argon atmosphere. The ingot was re-melted five times in the arc melter accompanied with electromagnetic stirring in order to ensure chemical homogeneity, and then crushed into pieces. The metallic glass ribbons with a thickness of about 50 μm were prepared by single roller melt-spinning method
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under high purity argon atmosphere. The circumferential velocity was about 28.8 m/s for all the samples, which would confirm the samples be cooled with the same rate. Cone-shape Cu64Zr36 BMGs were prepared using cooper mould injection casting method, the internal cavity of the copper mould is cone-shape with the taper angle of 14°. The quenching temperature in melt spinning process and injection casting process was monitored by the infrared thermo-scope, and four melt temperatures were chosen (1353 K, 1453 K, 1553 K, and 1653 K, respectively). The fragments used for melt-spinning and injection casting were crushed from the same master alloy ingot with the mass of 130 g. Resistivity measurement in this work was measured using direct current four probe method, and the voltage in resistivity measurement was measured by Keithley-2182 nano-voltmeter and the constant current was provided by the PF66M sourcemeter (with a constant current of 100 mA). The dimension of the ribbon specimen used for resistivity measurement is 45 mm × 4 mm. The resistivity results in this work had been normalized as ρN = ρ/ρ0, where ρN, ρ, ρ0, represent the normalized resistivity, the measured resistivity, and the initial resistivity before heating, respectively. The differential thermal analysis (DTA) was performed on a HCT-2 high temperature thermal analyzer. GFA associated with crystallization and thermal-stability behaviors was measured using Perkin-Elmer DSC-8000 differential scanning calorimeter (DSC). The DTA and DSC data were normalized with the same method as the resistivity data. The structures of as quenched ribbons and crystalline phases were checked using X-ray diffraction (XRD) (D/ MAX2500V, CuKα radiation).
The melt resistivity and DTA with respect to temperature which are shown in Fig. 1 can provide detailed information about the change in the melt structure with the temperature elevating. The resistivity changes discontinuously with the temperature increasing. The slight decrease in the temperature range 1353 K–1453 K, and a minimum of resistivity appears at 1440 K, which implies the absence of liquid-liquid structure transition (LLST) [16–17]. After that then, the resistivity begins to increase but has no linear relation, which suggests further LLST might appeared upon continuous heating. However, there are several factors like electron-phonon scattering that affect the resistivity of the metals and alloys. The relationship between the abnormal change of melt resistivity and LLST need to be verified by another experimental methods. From Fig. 1, The discontinuous change can also be seen on the DTA curve, an obvious endothermic phenomenon appears at the temperature range 1366 K–1427 K, and with the elevating of the melt
temperature, another obvious endothermic peak appears at 1518 K. The mutual verification of resistivity experiment and DTA measurement reveals that LLST exist in Cu64Zr36 alloy melt upon overheating. Similar melt behavior with respect to temperature on binary Cu50Zr50 metallic glass melt was reported by Zhou et al. [18], they used viscosity measurement upon cooling from 1653 K, and discontinuous change in viscosity during cooling is observed, which is attributed to an underlying liquid-liquid phase transition in the melts, and the viscosity result was verified by thermodynamic response. Liquid-liquid phase transition of CuZr binary and CuZr based metallic glasses were summarized based on the concept of fluid cluster in metallic melts, and the reversible liquid-liquid phase transition is considered to be contributed to the structural transition from the strongly ordered high-density liquids to the weak-local low-density liquids upon cooling [18]. The above results and discussions show that the melt structure is closely related to the temperature. As the quenching rate of BMGs is usually higher than 103 K/s, BMGs can be regard as the frozen-in liquid, i.e., the structure of BMGs is inherited from the melt. In order to study the influence of melt state on glass formation of Cu64Zr36 MGs, meltspun Cu64Zr36 MGs were prepared with four different melt temperatures (1353 K, 1453 K, 1553 K, and 1653 K). The structures of the ribbon samples prepared under different melt temperatures were characterized by X-Ray diffraction. Fig. 2 shows the XRD patterns of melt-spun Cu64Zr36 MGs, and it displays only broad diffraction maxima without appearance of sharp Bragg peaks, representing the feature of the samples are all amorphous. The amorphous nature of all the samples can also be confirmed by DSC measurements which can be seen in Fig. 3. The characteristic temperatures (Tg, Tx, Trg [19], ΔTx [20] and γ [21]) are determined from Fig. 3 and shown in Table 1. As shown in Fig. 3 and Table 1, the Tx of Cu64Zr36 metallic glass increases obviously with the elevating of the melt temperature. The 1653 K sample has the highest Tx of 800.3 K, nearly 20 K higher than that of the 1353 K sample. It is quite well-known that the melt homogeneity increases with temperature. More the homogeneity, less will be the competition from crystallization during glass formation. Further, the homogeneity of the melt structure would cause the distribution of the alloy elements more uniform, during to the melt heredity, element distribution in as prepared MGs will be homogeneous, thus, crystallization process will be much difficult than the one that with local order clusters. By comparing the GFA criteria in Table 1, the 1653 K sample has the best GFA among the MGs prepared with different melt temperatures. The results indicate that GFA and thermal stability of Cu64Zr36 MGs can be improved by controlling the melt temperature. Similar results were also reported in Cu-, Zr- and Al-based MGs [7–10].
Fig. 1. The electrical resistivity and DTA curves for Cu64Zr36 alloy melts with a heating rate of 10 K/min.
Fig. 2. XRD patterns of melt spin Cu64Zr36 metallic glasses with different melt temperatures.
3. Results and discussion
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Fig. 3. DSC scanning curves of the melt-spun Cu64Zr36 metallic glasses with different melt temperatures.
Due to the temperature limitation of the DSC measurement, the calorimetry behaviors cannot be observed above 823 K. So, auxiliary experiments using resistivity measurement are performed. The sharp decrease in the normalized resistivity curves shown in Fig. 4 reveals the onset of primary crystallization TER x [22]. As shown in Fig. 4, the resistivity of Cu64Zr36 MGs implies abnormal structural changes above 823 K, showing multi-stage crystallization behaviors. It should be mentioned that the variation tendency of the resistivity is various for MGs prepared with different melt temperatures. For 1353 K, 1453 K, and 1553 K sample, the process from stage A to stage B seems experience a second order-disorder transformation, because the resistivity increased from stage A to stage B. However, the resistivity of glassy alloy is mainly governed by scattering electrons resulting from the random structure which is analogous to that in liquid, whereas the resistivity of crystalline material is mainly caused by scattering electrons which induced by grain boundaries [23]. For the un-completely crystallized MG, the resistivity is caused by the localization of the residual amorphous structure and the precipitated crystal phases. Therefore, difference in the precipitate phases or micro-structure analysis of each crystalline stage could explain the difference in the variation tendency of the resistivity. In this work, we choose to examine the precipitate phase. In order to identify the crystalline phases that precipitate at various stage, each sample is heated to three specific stages (which are marked as A, B, and C in Fig. 4) with a heating rate of 20 K/min and rapidly quenched into water. The phases are identified using powder diffraction data by X-Ray diffraction, and the results are shown in Fig. 5(a–d). The precipitated phases of 1353 K sample and 1453 K sample at stage A and B are identical, which are Cu10Zr7 and Cu51Zr14, as shown in Fig. 5(a–b). However, for the 1553 K sample and 1653 K sample, no Cu51Zr14 phase precipitates at stage A and B, as shown in Fig. 5(c–d). Stage C is selected after the final decrease of the resistivity on heating process, and it is believed that the phases precipitate at this stage are final crystallization products of Cu64Zr36 MGs. As it can be seen from Fig. 5(a–d), the final crystallization phases are identical for all the samples, which are Cu10Zr7 and Cu8Zr3.
Table 1 Tg, Tx, ΔTx, Trg, γ and TER x parameters of the melt spin Cu64Zr36 metallic glasses with different melt temperatures.
1353 K 1453 K 1553 K 1653 K
Tg (K)
Tx (K)
ΔTx (K)
Trg
γ
TER x (K)
738.8 748.6 749.7 752.3
780.7 791.1 794.2 800.3
41.9 42.5 44.8 48.1
0.6051 0.6131 0.6140 0.6161
0.3985 0.4016 0.4030 0.4056
781.7 787.6 789.7 799.4
3
Fig. 4. Normalized electrical resistivity of the melt spin Cu64Zr36 MGs with different melt temperatures.
In the work of Wang et al. [23], Cu51Zr14 precipitated firstly when heating the Cu50Zr50 bulk metallic glasses, and disappeared with the temperature increasing. They conjectured that the precipitation of Cu51Zr14 was mainly caused by the quenched-in nuclei, and Cu51Zr14 phase features in many invariant points, especially for copper-rich compositions [24]. Due to the super-high cooling rate, the atomic configuration of the amorphous structure should be similar to the one of the liquid from which it was frozen-in. If the melt temperature is not high enough, some local order clusters or high melting temperature phases will survive in the melt. When the melt is quenched, these clusters or phases are frozen in an amorphous phase, as quenched in nuclei, and will provide the nucleation sites for precipitation of the primary crystals on heating [25]. In this work, the precipitation of Cu51Zr14 is considered to be caused by the local order Cu-rich clusters in liquid which will be frozen in an amorphous phase, as quenched in nuclei, and they may provide the nucleation sites for precipitation of Cu51Zr14. Liquid alloys are complex systems, and the effects of melt temperature on the local order clusters have been investigated by many research groups [17,25–28]. MGs quenched with higher melt temperature should have smaller quenched-in nuclei and more uniformed structures. These clusters will disappear gradually until the melt reaches a critical temperature. The critical temperature is generally about 1.3– 1.4Tm (Tm is the melt temperature) [11]. As shown in Fig. 1, an endothermic peak appears at 1518 K, it is indicated that the clusters in Cu64Zr36 melt will be dissolved in a large proportion when the temperature higher than 1540 K. As a result, there are no nucleation sites for Cu51Zr14 phases in the 1553 K and 1653 K samples. And the crystalline phases will precipitate in accordance with the Cu10Zr7-Cu8Zr3 eutectic system [29]. The above results show that increasing the melt temperature will be beneficial to the glass formation of Cu64Zr36 BMGs. In our previous research [30], only 1 mm rod of Cu64Zr36 BMG can be cast with fully amorphous structure using copper mould suction casting method. In order to get detailed information about the influence of melt temperature on GFA of Cu64Zr36 BMGs, Copper mould injection casting method was performed. The pieces of master alloy were induction heated to the above mentioned four different melt temperatures and injected to a copper mould with the internal cavity of cone-shape. As it is stated in the above text that Cu64Zr36 MG ribbon prepared with the melt temperature of 1673 K possesses the highest GFA and thermal-stability. The cross section of the cone-shape Cu64Zr36 BMG with the melt temperature of 1673 K was cut, and the structure of various cross section from Ф1.5 mm to Ф3 mm with a constant interval of 0.5 mm were examined using XRD. The results shown in Fig. 5(a) demonstrate that the cross section of Ф1.5, Ф2, and Ф2.5 display only broad
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Fig. 5. Phase identification results of the crystallized Cu64Zr36 metallic glasses at different stages.
diffraction maxima without appearance of sharp Bragg peaks, while the cross section of Ф3 shows sharp Bragg peaks. The results indicate that the GFA of Cu64Zr36 BMG is improved from 1 mm to 2.5 mm by controlling the melt temperature. For comparison, the structure of the Ф2.5 cross section of other MGs was also examined. As it can be seen in Fig. 6(b) the Ф2.5 cross sections of the cone-shape BMGs that prepared with the melt temperature of 1323 K, 1423 K, and 1573 K display sharp Bragg peaks. The results shown in Fig. 6 also show us another interesting phenomenon, the competing crystal phases on glass formation of Cu64Zr36 alloy is different with the difference of melt state. The competing crystal phase on glass formation of the cone shape sample with the Tq of 1653 K is Cu10Zr7, which is similar as that of the cone shape sample with the Tq of 1553 K. However, the competing crystal phases of the cone shape sample with the Tq of 1453 K and 1353 K are Cu10Zr7 and Cu51Zr14. As it has been discussed in the above text that Cu64Zr36 melt experienced structure change with the increasing of the melt temperature, before the structure change, there are many local order structures exist in the melt, and these clusters could form quenched in nuclei upon quenching. On crystallization process of Cu64Zr36 ribbon samples prepared with the Tq of 1323 K and 1423 K, Cu51Zr14 phase precipitated on primary crystallization stage. The precipitation of Cu51Zr14 is believed to be caused by the quenched in local order clusters. Correspondingly, the competing crystal phase on glass formation of Cu64Zr36 cone-shape sample prepared with the Tq of 1323 K and 1423 K is Cu10Zr7 and Cu51Zr14, the
precipitation of Cu51Zr14 is also believed to be caused by the residual local order clusters in the alloy melt. Elevating the melt temperature will make the local order clusters which can provide the nucleation site of Cu51Zr14 disappear gradually, as a result, the competing precipitation of Cu51Zr14 is restrained, i.e., the GFA of Cu64Zr36 BMG is improved by controlling the melt temperature via suppressing the competing precipitation of Cu51Zr14. As disorder systems, the atomic configuration of MGs has certain similarity with their original melts in terms of the inter-atomic distances and atomic coordination [31]. With the increasing of the melt temperature, the local order structures and high melting temperature phases will dissolve gradually. And as well, the fluctuation of the atomic concentration will varies to shorter waves with the melt temperature increasing (which is caused by high-frequency fluctuation), and the distribution of the elements becomes more and more uniform in the liquid [11,14]. The uniform distributed atoms accompanies with the drastic fluctuation of the atomic concentration will increase the degree of disorder. Then, the solid-liquid interface energy of MG with respect to its original melt will decrease, while the solid-liquid interface energy of the corresponding crystal with respect to its original melt will increase. In summary, with the increasing of the melt temperature, the glass formation tendency will be improved. In this work, 1653 K sample has the most uniformed distribution of the elements and highest degree of disorder. Cu64Zr36 MG prepared with the melt temperature of 1653 K has the highest GFA and thermal-stability.
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BMGs prepared with the Tq lower than 1653 K show apparent sharp Bragg peaks. It is believed that elevating the melt temperature can improve the GFA and thermal stability of Cu64Zr36 BMGs. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (Grant No. 50971053, 11304073) and the National Basic Research Program of China (Grant No. 2012CB825702). One of the authors (X. Cui) is grateful for the support of The Introduced Talents Project of Taishan University (Grant No. Y01-2014011) and Science and Technology Development Plan of Taian (Grant No. 2015GX2054). References
Fig. 6. Glass forming ability and competing crystal phase identification results of as prepared cone-shape Cu64Zr36 BMGs prepared with different Tq.
4. Conclusion Anomalous changes in melt behavior of Cu64Zr36 MG are studied using resistivity measurement and DTA measurement. Discontinuous structure changes are observed from both the resistivity and DTA curves with the elevating of the melt temperature. The observed structural changes are related with the pre-existed local order clusters in the low temperature melt, and these clusters will be dissolved with the temperature increasing. As the cooling rate on preparing metallic glasses is very high, the as quenched amorphous structure can be regarded as the frozen-in liquid structure. Experimental results using resistivity, DSC, and XRD show that the GFA, thermal stability and crystallization behaviors are greatly affected by the relevant melt temperature. With the increasing of the Tq, the Tx, ΔTx, Trg, and γ shift from 780.7 K, 41.9 K, 0.6051, and 0.3985 of the MG with the lowest Tq of 1353 K to 800.3 K, 48.1 K, 0.6161, and 0.4056 of the MG with the highest Tq of 1653 K. The intermetallic phase, Cu51Zr14, precipitates at the primary crystallization stage of the MGs prepared with the Tq of 1353 K and 1453 K. No Cu51Zr14 phase is found precipitate among the whole crystallization stage of the MGs prepared with the Tq of 1553 K and 1653 K. By injecting Cu64Zr36 melts with different temperatures into a copper mould with an internal cavity of cone-shape, the relationship of GFA and the relevant Tq is characterized by the cross section of the cone-shape samples. The Cu64Zr36 BMG with the highest Tq of 1653 K has the GFA of Ф2.5. However, the Ф2.5 cross section of Cu64Zr36
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On crystallization behavior and thermal stability of Cu64Zr36 metallic glass by controlling the melt temperature Xiao Cui a,b,⁎, Qi Dong Zhang a, Xiao Yun Li a, Fang Qiu Zu a,⁎⁎ a b
Institute of Liquid/Solid Metal Processing, School of Materials Science & Engineering, Hefei University of Technology, Hefei 230009, People's Republic of China College of Machinery and Engineering, Taishan University, Taian 271000, People's Republic of China
a r t i c l e
i n f o
Article history: Received 28 June 2016 Received in revised form 7 September 2016 Accepted 18 September 2016 Available online xxxx Keywords: Melt temperature Metallic glass Glass forming ability Thermal stability Crystallization behavior
a b s t r a c t Change in melt structure of Cu64Zr36 binary metallic glass (MG) was studied using resistivity and differential thermal analysis in continuous heating process, abnormal structural changes were observed with temperature elevating. Cu64Zr36 MG ribbons with different melt temperatures were prepared using single roller spinning method, experimental results based on resistivity, DSC, and XRD indicate that the thermal stability and glass forming ability (GFA) of Cu64Zr36 MGs are greatly affected by the melt temperature. With the increasing of the melt temperature, the crystallization temperature (Tx) and supercooled liquid region (ΔTx) shift from 780.7 K and 41.9 K of the MG with the lowest melt temperature of 1353 K towards to 800.3 K and 48.1 K of the MG with the highest melt temperature of 1653 K. The relationship of the melt temperature and GFA was further studied by injecting the melt into a cone-shape copper mould. The critical diameter can reach Ф2.5 mm of the Cu64Zr36 MG with the highest melt temperature of 1653 K. However, the Ф2.5 cross section of Cu64Zr36 MGs prepared with the melt temperature lower than 1653 K shows apparent Bragg peaks. © 2016 Published by Elsevier B.V.
1. Introduction Glass forming ability (GFA) and thermal stability are key properties for development and application of metallic glasses (MGs) [1–3]. There have been many efforts on improving the thermal stability and glass forming ability (GFA) of metallic glasses, such as adding a small quantity of high melting temperature elements [4–5], controlling the casting parameter [6] and overheating treatments [7–10]. As the cooling rate of preparation process of metallic glass is usually higher than 104 K/ s, they can be considered as the frozen-in-liquid. It is presumed that the melt state would affect the formation and configuration of metallic glasses, and furthermore affect their thermal stability. Fan and Inoue [10] reported that the melt temperature only has slight influence for the MGs which has typical polymorphous crystallization behaviors, but has significant influences for the MGs that has nano-crystallization tendency. Zhu et al. [7] studied the influence of casting temperature on the thermal-stability of a series Cu- and Zr-based MGs. Their results indicated that increasing the casting temperature could enhance the thermal stability of MGs, and it was considered to be attributed to the decrease in the amount of the local ordering clusters induced by elevating the casting temperature. And similar works have been reported by ⁎ Correspondence to: X. Cui, College of Machinery and Engineering, Taishan University, Taian 271000, People's Republic of China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (X. Cui), [email protected] (F.Q. Zu).
many groups in various MGs systems [7–11]. However, the former work that had been reported lacks direct guidance about the relationship between melt state and glass formation, the melt behaviors with respect to overheating temperature of MGs are rarely reported so far. In recent years, discontinuous structure changes of alloy melts upon heating have been observed using direct ways such as X-Ray and neutron diffraction methods [12–13], indirect ways such as internal friction technique [14–15] and electrical resistivity measurement [16–17] etc. In the present work, melt behaviors of Cu64Zr36 binary MGs were studied using electrical resistivity and differential thermal analysis (DTA) measurements in the heating process, discontinuous structure changes of Cu64Zr36 melt were found in a wide temperature range. Experimental results using resistivity, DSC, and XRD measurements show that the GFA, thermal stability, and crystallization behaviors of Cu64Zr36 MGs are greatly affected by the melt temperature. 2. Experiments The alloy ingot of nominal composition Cu64Zr36 was melted by arc melting mixtures of ultrasonically cleaned Zr and Cu pieces with high purity (99.9 at.% of Zr and 99.999 at.% of Cu). The arc melting was performed in a Ti-gettered high purity Argon atmosphere. The ingot was re-melted five times in the arc melter accompanied with electromagnetic stirring in order to ensure chemical homogeneity, and then crushed into pieces. The metallic glass ribbons with a thickness of about 50 μm were prepared by single roller melt-spinning method
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under high purity argon atmosphere. The circumferential velocity was about 28.8 m/s for all the samples, which would confirm the samples be cooled with the same rate. Cone-shape Cu64Zr36 BMGs were prepared using cooper mould injection casting method, the internal cavity of the copper mould is cone-shape with the taper angle of 14°. The quenching temperature in melt spinning process and injection casting process was monitored by the infrared thermo-scope, and four melt temperatures were chosen (1353 K, 1453 K, 1553 K, and 1653 K, respectively). The fragments used for melt-spinning and injection casting were crushed from the same master alloy ingot with the mass of 130 g. Resistivity measurement in this work was measured using direct current four probe method, and the voltage in resistivity measurement was measured by Keithley-2182 nano-voltmeter and the constant current was provided by the PF66M sourcemeter (with a constant current of 100 mA). The dimension of the ribbon specimen used for resistivity measurement is 45 mm × 4 mm. The resistivity results in this work had been normalized as ρN = ρ/ρ0, where ρN, ρ, ρ0, represent the normalized resistivity, the measured resistivity, and the initial resistivity before heating, respectively. The differential thermal analysis (DTA) was performed on a HCT-2 high temperature thermal analyzer. GFA associated with crystallization and thermal-stability behaviors was measured using Perkin-Elmer DSC-8000 differential scanning calorimeter (DSC). The DTA and DSC data were normalized with the same method as the resistivity data. The structures of as quenched ribbons and crystalline phases were checked using X-ray diffraction (XRD) (D/ MAX2500V, CuKα radiation).
The melt resistivity and DTA with respect to temperature which are shown in Fig. 1 can provide detailed information about the change in the melt structure with the temperature elevating. The resistivity changes discontinuously with the temperature increasing. The slight decrease in the temperature range 1353 K–1453 K, and a minimum of resistivity appears at 1440 K, which implies the absence of liquid-liquid structure transition (LLST) [16–17]. After that then, the resistivity begins to increase but has no linear relation, which suggests further LLST might appeared upon continuous heating. However, there are several factors like electron-phonon scattering that affect the resistivity of the metals and alloys. The relationship between the abnormal change of melt resistivity and LLST need to be verified by another experimental methods. From Fig. 1, The discontinuous change can also be seen on the DTA curve, an obvious endothermic phenomenon appears at the temperature range 1366 K–1427 K, and with the elevating of the melt
temperature, another obvious endothermic peak appears at 1518 K. The mutual verification of resistivity experiment and DTA measurement reveals that LLST exist in Cu64Zr36 alloy melt upon overheating. Similar melt behavior with respect to temperature on binary Cu50Zr50 metallic glass melt was reported by Zhou et al. [18], they used viscosity measurement upon cooling from 1653 K, and discontinuous change in viscosity during cooling is observed, which is attributed to an underlying liquid-liquid phase transition in the melts, and the viscosity result was verified by thermodynamic response. Liquid-liquid phase transition of CuZr binary and CuZr based metallic glasses were summarized based on the concept of fluid cluster in metallic melts, and the reversible liquid-liquid phase transition is considered to be contributed to the structural transition from the strongly ordered high-density liquids to the weak-local low-density liquids upon cooling [18]. The above results and discussions show that the melt structure is closely related to the temperature. As the quenching rate of BMGs is usually higher than 103 K/s, BMGs can be regard as the frozen-in liquid, i.e., the structure of BMGs is inherited from the melt. In order to study the influence of melt state on glass formation of Cu64Zr36 MGs, meltspun Cu64Zr36 MGs were prepared with four different melt temperatures (1353 K, 1453 K, 1553 K, and 1653 K). The structures of the ribbon samples prepared under different melt temperatures were characterized by X-Ray diffraction. Fig. 2 shows the XRD patterns of melt-spun Cu64Zr36 MGs, and it displays only broad diffraction maxima without appearance of sharp Bragg peaks, representing the feature of the samples are all amorphous. The amorphous nature of all the samples can also be confirmed by DSC measurements which can be seen in Fig. 3. The characteristic temperatures (Tg, Tx, Trg [19], ΔTx [20] and γ [21]) are determined from Fig. 3 and shown in Table 1. As shown in Fig. 3 and Table 1, the Tx of Cu64Zr36 metallic glass increases obviously with the elevating of the melt temperature. The 1653 K sample has the highest Tx of 800.3 K, nearly 20 K higher than that of the 1353 K sample. It is quite well-known that the melt homogeneity increases with temperature. More the homogeneity, less will be the competition from crystallization during glass formation. Further, the homogeneity of the melt structure would cause the distribution of the alloy elements more uniform, during to the melt heredity, element distribution in as prepared MGs will be homogeneous, thus, crystallization process will be much difficult than the one that with local order clusters. By comparing the GFA criteria in Table 1, the 1653 K sample has the best GFA among the MGs prepared with different melt temperatures. The results indicate that GFA and thermal stability of Cu64Zr36 MGs can be improved by controlling the melt temperature. Similar results were also reported in Cu-, Zr- and Al-based MGs [7–10].
Fig. 1. The electrical resistivity and DTA curves for Cu64Zr36 alloy melts with a heating rate of 10 K/min.
Fig. 2. XRD patterns of melt spin Cu64Zr36 metallic glasses with different melt temperatures.
3. Results and discussion
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Fig. 3. DSC scanning curves of the melt-spun Cu64Zr36 metallic glasses with different melt temperatures.
Due to the temperature limitation of the DSC measurement, the calorimetry behaviors cannot be observed above 823 K. So, auxiliary experiments using resistivity measurement are performed. The sharp decrease in the normalized resistivity curves shown in Fig. 4 reveals the onset of primary crystallization TER x [22]. As shown in Fig. 4, the resistivity of Cu64Zr36 MGs implies abnormal structural changes above 823 K, showing multi-stage crystallization behaviors. It should be mentioned that the variation tendency of the resistivity is various for MGs prepared with different melt temperatures. For 1353 K, 1453 K, and 1553 K sample, the process from stage A to stage B seems experience a second order-disorder transformation, because the resistivity increased from stage A to stage B. However, the resistivity of glassy alloy is mainly governed by scattering electrons resulting from the random structure which is analogous to that in liquid, whereas the resistivity of crystalline material is mainly caused by scattering electrons which induced by grain boundaries [23]. For the un-completely crystallized MG, the resistivity is caused by the localization of the residual amorphous structure and the precipitated crystal phases. Therefore, difference in the precipitate phases or micro-structure analysis of each crystalline stage could explain the difference in the variation tendency of the resistivity. In this work, we choose to examine the precipitate phase. In order to identify the crystalline phases that precipitate at various stage, each sample is heated to three specific stages (which are marked as A, B, and C in Fig. 4) with a heating rate of 20 K/min and rapidly quenched into water. The phases are identified using powder diffraction data by X-Ray diffraction, and the results are shown in Fig. 5(a–d). The precipitated phases of 1353 K sample and 1453 K sample at stage A and B are identical, which are Cu10Zr7 and Cu51Zr14, as shown in Fig. 5(a–b). However, for the 1553 K sample and 1653 K sample, no Cu51Zr14 phase precipitates at stage A and B, as shown in Fig. 5(c–d). Stage C is selected after the final decrease of the resistivity on heating process, and it is believed that the phases precipitate at this stage are final crystallization products of Cu64Zr36 MGs. As it can be seen from Fig. 5(a–d), the final crystallization phases are identical for all the samples, which are Cu10Zr7 and Cu8Zr3.
Table 1 Tg, Tx, ΔTx, Trg, γ and TER x parameters of the melt spin Cu64Zr36 metallic glasses with different melt temperatures.
1353 K 1453 K 1553 K 1653 K
Tg (K)
Tx (K)
ΔTx (K)
Trg
γ
TER x (K)
738.8 748.6 749.7 752.3
780.7 791.1 794.2 800.3
41.9 42.5 44.8 48.1
0.6051 0.6131 0.6140 0.6161
0.3985 0.4016 0.4030 0.4056
781.7 787.6 789.7 799.4
3
Fig. 4. Normalized electrical resistivity of the melt spin Cu64Zr36 MGs with different melt temperatures.
In the work of Wang et al. [23], Cu51Zr14 precipitated firstly when heating the Cu50Zr50 bulk metallic glasses, and disappeared with the temperature increasing. They conjectured that the precipitation of Cu51Zr14 was mainly caused by the quenched-in nuclei, and Cu51Zr14 phase features in many invariant points, especially for copper-rich compositions [24]. Due to the super-high cooling rate, the atomic configuration of the amorphous structure should be similar to the one of the liquid from which it was frozen-in. If the melt temperature is not high enough, some local order clusters or high melting temperature phases will survive in the melt. When the melt is quenched, these clusters or phases are frozen in an amorphous phase, as quenched in nuclei, and will provide the nucleation sites for precipitation of the primary crystals on heating [25]. In this work, the precipitation of Cu51Zr14 is considered to be caused by the local order Cu-rich clusters in liquid which will be frozen in an amorphous phase, as quenched in nuclei, and they may provide the nucleation sites for precipitation of Cu51Zr14. Liquid alloys are complex systems, and the effects of melt temperature on the local order clusters have been investigated by many research groups [17,25–28]. MGs quenched with higher melt temperature should have smaller quenched-in nuclei and more uniformed structures. These clusters will disappear gradually until the melt reaches a critical temperature. The critical temperature is generally about 1.3– 1.4Tm (Tm is the melt temperature) [11]. As shown in Fig. 1, an endothermic peak appears at 1518 K, it is indicated that the clusters in Cu64Zr36 melt will be dissolved in a large proportion when the temperature higher than 1540 K. As a result, there are no nucleation sites for Cu51Zr14 phases in the 1553 K and 1653 K samples. And the crystalline phases will precipitate in accordance with the Cu10Zr7-Cu8Zr3 eutectic system [29]. The above results show that increasing the melt temperature will be beneficial to the glass formation of Cu64Zr36 BMGs. In our previous research [30], only 1 mm rod of Cu64Zr36 BMG can be cast with fully amorphous structure using copper mould suction casting method. In order to get detailed information about the influence of melt temperature on GFA of Cu64Zr36 BMGs, Copper mould injection casting method was performed. The pieces of master alloy were induction heated to the above mentioned four different melt temperatures and injected to a copper mould with the internal cavity of cone-shape. As it is stated in the above text that Cu64Zr36 MG ribbon prepared with the melt temperature of 1673 K possesses the highest GFA and thermal-stability. The cross section of the cone-shape Cu64Zr36 BMG with the melt temperature of 1673 K was cut, and the structure of various cross section from Ф1.5 mm to Ф3 mm with a constant interval of 0.5 mm were examined using XRD. The results shown in Fig. 5(a) demonstrate that the cross section of Ф1.5, Ф2, and Ф2.5 display only broad
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Fig. 5. Phase identification results of the crystallized Cu64Zr36 metallic glasses at different stages.
diffraction maxima without appearance of sharp Bragg peaks, while the cross section of Ф3 shows sharp Bragg peaks. The results indicate that the GFA of Cu64Zr36 BMG is improved from 1 mm to 2.5 mm by controlling the melt temperature. For comparison, the structure of the Ф2.5 cross section of other MGs was also examined. As it can be seen in Fig. 6(b) the Ф2.5 cross sections of the cone-shape BMGs that prepared with the melt temperature of 1323 K, 1423 K, and 1573 K display sharp Bragg peaks. The results shown in Fig. 6 also show us another interesting phenomenon, the competing crystal phases on glass formation of Cu64Zr36 alloy is different with the difference of melt state. The competing crystal phase on glass formation of the cone shape sample with the Tq of 1653 K is Cu10Zr7, which is similar as that of the cone shape sample with the Tq of 1553 K. However, the competing crystal phases of the cone shape sample with the Tq of 1453 K and 1353 K are Cu10Zr7 and Cu51Zr14. As it has been discussed in the above text that Cu64Zr36 melt experienced structure change with the increasing of the melt temperature, before the structure change, there are many local order structures exist in the melt, and these clusters could form quenched in nuclei upon quenching. On crystallization process of Cu64Zr36 ribbon samples prepared with the Tq of 1323 K and 1423 K, Cu51Zr14 phase precipitated on primary crystallization stage. The precipitation of Cu51Zr14 is believed to be caused by the quenched in local order clusters. Correspondingly, the competing crystal phase on glass formation of Cu64Zr36 cone-shape sample prepared with the Tq of 1323 K and 1423 K is Cu10Zr7 and Cu51Zr14, the
precipitation of Cu51Zr14 is also believed to be caused by the residual local order clusters in the alloy melt. Elevating the melt temperature will make the local order clusters which can provide the nucleation site of Cu51Zr14 disappear gradually, as a result, the competing precipitation of Cu51Zr14 is restrained, i.e., the GFA of Cu64Zr36 BMG is improved by controlling the melt temperature via suppressing the competing precipitation of Cu51Zr14. As disorder systems, the atomic configuration of MGs has certain similarity with their original melts in terms of the inter-atomic distances and atomic coordination [31]. With the increasing of the melt temperature, the local order structures and high melting temperature phases will dissolve gradually. And as well, the fluctuation of the atomic concentration will varies to shorter waves with the melt temperature increasing (which is caused by high-frequency fluctuation), and the distribution of the elements becomes more and more uniform in the liquid [11,14]. The uniform distributed atoms accompanies with the drastic fluctuation of the atomic concentration will increase the degree of disorder. Then, the solid-liquid interface energy of MG with respect to its original melt will decrease, while the solid-liquid interface energy of the corresponding crystal with respect to its original melt will increase. In summary, with the increasing of the melt temperature, the glass formation tendency will be improved. In this work, 1653 K sample has the most uniformed distribution of the elements and highest degree of disorder. Cu64Zr36 MG prepared with the melt temperature of 1653 K has the highest GFA and thermal-stability.
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BMGs prepared with the Tq lower than 1653 K show apparent sharp Bragg peaks. It is believed that elevating the melt temperature can improve the GFA and thermal stability of Cu64Zr36 BMGs. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China (Grant No. 50971053, 11304073) and the National Basic Research Program of China (Grant No. 2012CB825702). One of the authors (X. Cui) is grateful for the support of The Introduced Talents Project of Taishan University (Grant No. Y01-2014011) and Science and Technology Development Plan of Taian (Grant No. 2015GX2054). References
Fig. 6. Glass forming ability and competing crystal phase identification results of as prepared cone-shape Cu64Zr36 BMGs prepared with different Tq.
4. Conclusion Anomalous changes in melt behavior of Cu64Zr36 MG are studied using resistivity measurement and DTA measurement. Discontinuous structure changes are observed from both the resistivity and DTA curves with the elevating of the melt temperature. The observed structural changes are related with the pre-existed local order clusters in the low temperature melt, and these clusters will be dissolved with the temperature increasing. As the cooling rate on preparing metallic glasses is very high, the as quenched amorphous structure can be regarded as the frozen-in liquid structure. Experimental results using resistivity, DSC, and XRD show that the GFA, thermal stability and crystallization behaviors are greatly affected by the relevant melt temperature. With the increasing of the Tq, the Tx, ΔTx, Trg, and γ shift from 780.7 K, 41.9 K, 0.6051, and 0.3985 of the MG with the lowest Tq of 1353 K to 800.3 K, 48.1 K, 0.6161, and 0.4056 of the MG with the highest Tq of 1653 K. The intermetallic phase, Cu51Zr14, precipitates at the primary crystallization stage of the MGs prepared with the Tq of 1353 K and 1453 K. No Cu51Zr14 phase is found precipitate among the whole crystallization stage of the MGs prepared with the Tq of 1553 K and 1653 K. By injecting Cu64Zr36 melts with different temperatures into a copper mould with an internal cavity of cone-shape, the relationship of GFA and the relevant Tq is characterized by the cross section of the cone-shape samples. The Cu64Zr36 BMG with the highest Tq of 1653 K has the GFA of Ф2.5. However, the Ф2.5 cross section of Cu64Zr36
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