The Effect of SiC Whisker Addition on Bulk Amorphous Formation Abilities of La-Transition Metal-Al System

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1 The Effect of SiC Whisker Addition on Bulk Amorphous Formation Abilities of La-Transition Metal-Al System Takahito KOJIMA 1, Shuji AZUMO 2 and Katsuhisa NAGAYAMA 3 1 Graduate Student, Shibaura Institute of Technology, Tokyo, Japan, m207020@sic.shibaura-it.ac.jp 2 Graduate Student, Shibaura Institute of Technology, Tokyo, Japan, m705101@sic.shibaura-it.ac.jp 3 Department of Materials Science, Faculty of Engineering, Shibaura Institute of Technology, Tokyo, Japan, nagayama@sic.shibaura-it.ac.jp Abstract It is well known three empirical rules for the achievement of high glass-forming ability in bulk metallic glasses. However, a rule for amorphous phase with respect to undercooling behavior is not revealed. In this study, we elucidate effect of SiC addition on transition of undercooling and indicate amorphous formation depends on high undercooling. From this experiment result, it is suggested that an increase of the amorphous formation own effect of containerless solidification processes which are, gas jet flow type, splat quenching process and drop tube process. Additionally, amorphous formation ability increased by addition of SiC. Keywords: containerless process, undercooled melt, rapid solidification, SiC added La 55 Al 25 Cu 10 Ni 5 Co 5, droptube method, amorphous formation ability 1. Introduction It is reported that Rare Earth-Transition Metal-Al alloys have high supercooled melt formation and high bulk glass-forming ability by using rapid solidification process such as melt-spinning method, die-cast, suction method and various method 1, 2). La-transition metal-al alloys are one of the typical alloys with high glass-forming ability. It is well known three empirical rules for the achievement of high glass-forming ability in theses metallic glasses: (1) multicomponent alloy systems consisting of more than three elements; (2) significant difference in atomic size ratios above 12 % among the main three constituent elements; and (3) negative heats of mixing among their elements. The rules have showed a single role method as a guiding principle for selection of elements 3). However, a rule for amorphous phase with respect to undercooling behavior is not revealed. Solidification behavior with undercooling also can be confirmed by cooling curves and high speed camera images. Therefore, it is expected that bulk amorphous formation ability can be established by levitating process which enables easily observation of solidification behavior with undercooling in the form of cooling curves and high speed video camera images. Furthermore, we have reported about the effect of SiC addition in La-based bulk metallic glass against the enhancement of undercooling 4)5)6)7)8). In this study, we elucidate effect of SiC addition on transition of undercooling and indicated amorphous formation depends on high undercooling. 2. Experimental Ingots with composition of La 55 Al 25 Cu 10 Ni 5 Co 5 were prepared by arc melting the mixtures of pure La (99.9 mass%), Al (99.99 mass%), Cu (99.99 mass%), Ni (99.97 mass%) and Co (99.9 mass%) in argon gas atmosphere. La 55 Al 25 Cu 10 Ni 5 Co 5 samples with SiC whisker addition are levitated and solidified by using the electromagnetic levitation system and the gas jet flow type electromagnetic levitation system. Fig. 1 shows the schematic diagram of electromagnetic levitation system (a) and gas jet flow type electromagnetic levitation system (b). The electromagnetic levitation system enables the containerless solidification process whose mechanism is simple. However, the electromagnetic levitation gives heating at the cooling period due to levitating by electromagnetic power during solidification. (a) Inert gas (only at cooling) Two color pyrometer Sample Quartz holder Asymmetrical coil (b) Symmetric cylinder coil Inert gas Fig. 1 Schematic diagram of electromagnetic levitation system and gas jet flow type electromagnetic levitation system.

2 The gas jet flow type electromagnetic levitation process enables the containerless solidification process by using the inert gas jet flow. And this levitated melt was cooled by gas jet flow when the electromagnetic power was shut down. addition were produced by splat quenching process and drop tube process for comparison. Fig. 2 shows the schematic diagram of splat quenching system. Splat quenching process is one of the rapid solidification processes. As shown in Figure 2, the sample in the splat quenching process was heated and melted by electromagnetic power with asymmetric coil. When melt was shut off electro magnetic power, it was fallen by gravity. Falling melt was impacted with two pairs of copper plates. Moreover, the sample can be cooled by putting out the cooling gas from the ring nozzle above the copper plates. The temperature of the levitated sample was measured by two color pyrometer with Si and InGaAs as detecting component to examine solidification behavior with undercooling in these alloys. The levitated sample solidified by He gas jet. The structure of the solidified samples was examined by X-ray diffractometer (XRD) using CuKα radiation at room temperature. The differential thermal analysis (DTA) of samples was carried out to identify an amorphous phase and determine the crystallization temperature, T x, and the grass transition temperature, T g, with a heating rate of 0.33K/s. 3. Result and Discussion 3.1 Cooling curve Figs. 3, 4 shows the cooling curves of addition (0.5%) which were solidified by the and the gas jet flow type. Sample (a) and (c) are La 55 Al 25 Cu 10 Ni 5 Co 5. Sample (b) and (d) are La 55 Al 25 Cu 10 Ni 5 Co 5 samples with SiC whisker addition. In the electromagnetic levitation process, the highest heating temperature was heated 700K higher than the melting point to levitating the sample with stability. The samples were solidified by helium gas. After it began to have cooled, the stagnation of the temperature was observed in the vicinity of the melting point. In the gas jet flow type, samples were heated about 100K higher than melting points. The samples were solidified by helium gas when the high frequency power was shut down. Also, the levitated samples were kept from the levitating start to the termination of solidification in stable condition. After it began to have cooled, La 55 Al 25 Cu 10 Ni 5 Co 5 sample observed the crystal nucleation. However, the crystal nucleation was not observed to the detection limit of the temperature in La 55 Al 25 Cu 10 Ni 5 Co 5 with SiC whisker addition. 1372K (a) Cooling rate : 54K/s Tm = 795K 1443K (b) Cooling rate : 80K/s Tm =795K 755K 728K Time,t/s Fig. 3 Cooling curves of La 55 Al 25 Cu 10 Ni 5 Co 5 and samples with SiC whisker addition (0.5%) which are solidified by the. Two color pyrometer Sample Asymmetrical coil (a) 950K Cooling rate : 158K/s Tm=795K 752K 737K (b) 895K Tm = 795K Cooling rate : 112K/s Ring nozzle Cu plate Fig. 2 Schematic diagram of the splat quenching system Time,t/s Fig. 4 Cooling curves of La 55 Al 25 Cu 10 Ni 5 Co 5 and samples with SiC whisker addition (0.5%) which are solidified by the gas jet flow type.

3 3.2 XRD pattern Figs. 5, 6 shows the XRD pattern of addition (0.5%) which were solidified by the, the gas jet flow type and the splat quenching process. Sample (a) and (e) were solidified by the. Sample (b) and (f) were solidified by the gas jet flow type. Sample (c), (d), (g) and (h) were solidified by splat quenching process. (a) Electromagnetic levitation process (b) Gas jet flow type (c) Splat quenching process (d) Splat quenching process :αla θ Fig. 5 XRD patterns of La 55 Al 25 Cu 10 Ni 5 Co 5 samples are levitated and solidified by the electromagnetic levitation process, the gas jet flow type electromagnetic levitation process and the splat quenching process. process. As can be seen from the XRD pattern in Fig. 5 (a) and (b), the samples consist of crystal phases and amorphous phase. The crystal phases are predominant in the XRD pattern. However the XRD pattern for Fig. 6 (e) and (f) reveal the characteristic broad diffraction pattern typical of an amorphous structure and the sample (e) and (f) consists mainly of an amorphous phase. This is thought to be an effect of the SiC addition. Also, undercooling has increased by the SiC addition in the gas jet flow type. Therefore, it is thought that the increase of the undercooling by the SiC addition remarkably did amorphous formation. Sample (c) and (g) was made from melted. As can be seen from the XRD pattern for Fig. 5 (c) and Fig. 6 (g) reveal the characteristic broad diffraction pattern typical of an amorphous structure and the sample (e) and (f) consists mainly of an amorphous phase. Sample (d) and (h) was made from undercooled melte. Melt was shut off electro magnetic power when it was undercooled about 50K more than the melting point. As can be seen from the XRD pattern in Fig. 5 (d) and Fig. 5 (h), the samples consist of crystal phases and amorphous phase. 3.3 DTA curves Figs. 7 and 8 shows the DTA curves of the samples corresponding to Fig. 5 and 6 which obtained during continuous heating with 0.33 K/s. In sample (d), (e), (f), (g) and (h) DTA curves, the exothermic peak for the crystallization is detected. Heating rate : 0.33K/s (e) Electromagnetic levitation process :αla (a) Electromagnetic levitation process (b) Gas jet flow type (f) Gas jet flow type (g) Splat quenching process (h) Splat quenching process (c) Splat quenching process Tg = 463K Tx=509K (d) Splat quenching process Tg = 460K Tx = 529K θ Fig. 6 XRD patterns of La 55 Al 25 Cu 10 Ni 5 Co 5 samples with SiC whisker addition (0.5%) which are idified by the, the gas jet flow type and the splat quenching Temperature,T/K Fig. 7 DTA curves of La 55 Al 25 Cu 10 Ni 5 Co 5 samples which are solidified by the electromagnetic levitation process, the gas jet flow type electromagnetic levitation process and the splat quenching process.

4 Heating rate : 0.33K/s (e) Electromagnetic levitation process Tg = 476K Tg = 478K Tx = 563K (f) Gas jet flow type (g) Splat quenching process Tx = 550K Tx = 532K Tg = 473K rapidly from undercooled melt has increased glass -forming ability better than that of melt. As a result, it is suggested that the generation of the undecooled melt result greatly from an amorphous formation. 3.4 MICROSTRUCTURE Fig. 9 shows the microstructure of addition (0.5%) which are solidified by the, the gas jet flow type and the splat quenching process. (h) Splat quenching process Tg = 474K Tx = 563K Temperature,T/K Fig. 8 DTA curves of La 55 Al 25 Cu 10 Ni 5 Co 5 samples with SiC whisker addition (0.5%) which are solidified by the, the gas jet flow type and the splat quenching process. The crystallization temperature is 529K, 563K, 550k, 532K and 563K for sample (d), (e), (f), (g) and (h), respectively. However the exothermic peak for sample (c) is double peak. The glass transition temperature is 476K, 478K, 473K and 474K for sample (e), (f) (g) and (h) respectively. But the glass transition temperature is 463K and 460K for sample (c) and (d). This difference may be caused by the difference of the local structure in amorphous phase. Table 1 shows undercooled melt-forming ability(δ T x (=T x -T g ) )and glass-forming ability (T g /T L ) for amorphous La 55 Al 25 Cu 10 Ni 5 Co 5 and samples with SiC whisker addition (0.5%). As can be seen from Table 1, Samples which are solidified by the splat quenching process, it is suggested that the undercooled melt-forming ability (ΔT x ) has increased by the SiC addition. As a result, it is suggested that an amorphous formation is promoted by the SiC addition in the rapid solidification. Moreover, Samples which cooled Fig. 9 shows microstructure of La 55 Al 25 Cu 10 Ni 5 Co 5 and samples with SiC whisker addition which are solidified by the, the gas jet flow type and the splat quenching process. Table 1 ΔT x (=T x -T g ) and T g /T L for amorphous addition (0.5%). sample ΔTx(K) Tg/TL remarks (c) splat quenching (melt) (d) splat quenching (undercooled melt) (e) electromagnetic levitation (f) gas jet flow type electromagnetic levitation (g) splat quenching (melt) (h) splat quenching (undercooled melt)

5 As can be seen from the microstructure in Fig. 9 (a), (b), (e) and (f), the crystal phases were predominant in La 55 Al 25 Cu 10 Ni 5 Co 5 samples. But La 55 Al 25 Cu 10 Ni 5 Co 5 samples with SiC whisker addition (0.5%) consisted of crystal phases and amorphous phase. On the other side sample (c) and (g) became an amorphous single phase. But sample (d) and (h) precipitated crystal phase. The undercooled melt is very unstable, and makes the crystal by a little impact. Therefore, it is thought that because the precipitation rate of the crystal had pulled out the cooling rate with splat quenching, the crystal precipitated it. 3.5 DROP TUBE PROCESS Fig. 10 shows the schematic diagram of drop tube system. This method is able to do non-convection solidification. In drop tube process, the quartz nozzle was set in the vacuum chamber mounted at the top of the drop tube which has the 3 m fall length. The drop tube was evacuated and backfilled He gas. The sample was heated and melted by electromagnetic power with induction coil. The melt was ejected from the hole in the point of the quartz nozzle by putting out the He gas in the quartz nozzle. The ejected melt shaped into droplets and solidified during free fall. The spherical samples collected at bottom of the drop tube were classified into several groups according to their diameter. The sample used La 55 Al 25 Cu 10 Ni 5 Co 5 sample by this experiment. The diameter of the hole in the point of the quartz nozzle was 200μm in this experiment. 2 shows T g, T x, ΔT x (=T x -T g ) and T g /T L for amorphous La 55 Al 25 Cu 10 Ni 5 Co 5 and samples which was made by drop tube process. In sample (a), (b) and (c) DTA curves, the exothermic peak for the crystallization is detected. The crystallization temperature is 524K, 523K and 563K for sample (a), (b) and (c), respectively. The glass transition temperature is 466K, 462K and 462K for sample (a), (b) and (c) respectively. There was little change depending on the particle size in ΔT x and T g /T L. Moreover, T g and T x were similar to Figure 7 (b). Table 2 (f) is La 55 Al 25 Cu 10 Ni 5 Co 5 sample with SiC whisker addition which was made by drop tube process. Because the number of samples that was able to be gathered was little, it measured it mixing the sample from the diameter 212μm to 710μm. It obtained during continuous heating with 0.33 K/s. In sample (f) DTA curves, the exothermic peak for the crystallization is detected. The crystallization temperature is 585K. The glass transition temperature is 512K. The result in which ΔT x = 73K, and T g /T L = were obtained. Both the crystallization temperature and the glass transition temperature were shifted to the high temperature side by addition SiC whisker. Fig. 11 shows SEM backscattered electron micrographs and line analysis result by EDX of Table 2 (f). The shape of the SiC whisker used to experiment is φ1 10μm. As can be seen from Fig. 11 (a), the same size as SiC whisker crystal was composed in amorphous matrix. These crystals were analyzed line analysis by EDX. As a result, the ratio of Si was high on the line. Thus, SiC whisker exists without resolving in the amorphous matrix. (a) (b) b SiC whisker a amorphous matrix 10μm 3μm Fig. 10 Schematic diagram of the drop tube system. Fig. 11 (a) SEM backscattered electron micrographs of La 55 Al 25 Cu 10 Ni 5 Co 5 with SiC whisker addition and (b) line analysis result by EDX of La 55 Al 25 Cu 10 Ni 5 Co 5 with SiC whisker addition. Table 2 ΔT x (=T x -T g ) and T g /T L for amorphous La 55 Al 25 Cu 10 Ni 5 Co 5 samples which was made by drop tube process. sample (a) (b) Tg(K) Tx(K) ΔTx(K) Tg/TL diameter(μm) 212~ ~500 (c) ~710 Table (f) ~710

6 In this result, it is suggested that the sample with SiC addition keeps the state of the undercooled melt steady compared with the no addition sample. It is maybe that the diffusion speed slows by the SiC addition. As a result, the SiC addition originates greatly in an amorphous formation in drop tube process too. 4. Conclusion By the, the gas jet flow type, the splat quenching process amd the drop tube process, the bulk amorphous formation, effect of SiC adding and the local structure were examined and then we obtained the following results. (1) High undercooling and high bulk amorphous formation ability were obtained by adding SiC whisker which generally act as miclo heterogeneous nucleation site. (2) Because of undercooling, amorphous ability was increased. (3) In splat quenching process, the generation of the undecooled melt result greatly from an amorphous formation. (4) In splat quenching process and drop tube process, the SiC addition originates greatly in an amorphous formation References 1) A. Inoue, T. Zhang, and T. Masumoto., Mater. Trans. JIM, 31, , ) A. Inoue, T. Nakamura, T. Sugita, T. Zhang and T. Masumoto., Mater. Trans. JIM, 34, , ) Akira Takeuchi, Akihisa Inoue., Bulletin of the Japan Institute of Metals Materia Japan, 42, , ) S. Azumo and K. Nagayama., J.Japan Inst. Metals, 69, , ) K. Nagayama, S. Utuno and S. Azumo., J.Japan Inst. Metals, 70, , ) S. Azumo, S. Utuno and K. Nagayama., J Japan Soc. Microgravity Appl., 23, 8-14, ) S. Azumo, S. Utuno, and K. Nagayama., Materials Transaction Japan Institute of Metals, 47, , ) S. Azumo and K. Nagayama., Materials Transaction Japan Institute of Metals, 47, , 2006