Fillability and Imprintability of High-strength Ni-based Bulk Metallic Glass Prepared by the Precision Die-casting Technique

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1 Materials Transactions, Vol. 45, No. 4 (2004) pp to 1244 Special Issue on Bulk Amorphous, Nano-Crystalline and Nano-Quasicrystalline Alloys-V #2004 The Japan Institute of Metals Fillability and Imprintability of High-strength Ni-based Bulk Metallic Glass Prepared by the Precision Die-casting Technique Mamoru Ishida 1, Hideki Takeda 1, Daichi Watanabe 1, Kenji Amiya 1, Nobuyuki Nishiyama 1, Kazuhiko Kita 2, Yasunori Saotome 3 and Akihisa Inoue 4 1 RIMCOF-Tohoku University laboratory, Sendai , Japan 2 Sendai Institute of Material Science and Technology, YKK Corporation, Kurokawa , Japan 3 Department of Nano-Material Systems, Graduate School of Eng., Gunma University, Kiryu , Japan 4 Institute for Materials Research, Tohoku University, Sendai , Japan The fillability and imprintability of a Ni-based bulk metallic glass (BMG) prepared by the precision die-casting was investigated. A threedimensional microgear made of Ni-based BMG is successfully prepared by the precision die-casting technique. The cast Ni-based BMG has excellent fillability against a microindentation formed by Vickers indentation, and the filling area obtained using a confocal scanning microscope reaches 99%. In addition, the cast Ni-based BMG exhibits excellent imprintability of the die surface even on a nanometer scale. It is therefore concluded that the Ni-based BMG is suitable as a material for micromachines due to its superior fillability and imprintability. (Received November 7, 2003; Accepted January 16, 2004) Keywords: imprintability, surface roughness, precision die-casting, micromachine parts 1. Introduction It has been reported 1) that amorphization for most metallic alloys by rapid solidification is usually achieved in the case of high cooling rates exceeding 10 4 K/s. The necessity of the high cooling rates causes a limitation on the sample size resulting in limited industrial application. Recently, new amorphous alloys in Ln-, 2,3) Zr-, 4,5) Cu- 6,7) and Ni-based 8 10) systems which are called bulk metallic glasses (BMGs) with a large supercooled liquid region have been reported. The critical cooling rates for the glass formation of these glassy alloys are typically K/s. The low critical cooling rates enable us to obtain BMGs prepared by a conventional copper mold casting technique. For the Ni-based BMG, a maximum diameter of 3 mm, superior mechanical properties of 3000 MPa 8) and excellent corrosion resistance 9) have been reported. Thus, the development of BMG is expected for promoting its industrial applications applying its superior properties. 11) In particular, in the field of micromachines or micro electro-mechanical systems (MEMS), these bulk metallic glasses have been expected as promising materials because of their homogeneity on a nanoscopic scale. Generally, micro machine parts such as microgears with complicated three-dimensional shapes are prepared by lithography, micromachining and chemical or ion etching. 12) However, the materials prepared by such methods do not necessarily satisfy the property requirement. In order to promote the application to micromachines, it is required to develop both suitable materials and preparation methods. From such a viewpoint, the micro- or nanoformability of BMGs has been investigated. It has already been reported that BMGs have superior formability on micrometer or nanometer scales ) We assume that the net-shaped diecasting technique for the preparation of micromachine parts leads to an expansion of application fields of BMGs. The BMGs prepared by the precision die-casting (PDC) technique have two advantages, namely, a high precision level and superior imprintability. The present authors have reported that Zr-based BMG prepared by the PDC technique exhibits excellent dimensional accuracy on the micrometer scale. The application of BMG for an optical precision device has also been reported. 15,16) However, the fillability and imprintability of BMG prepared by the PDC technique are still unclear. In the present paper, we present the preparation of a Ni-based BMG microgear by the PDC technique. In addition, we discuss the fillability into microindentation and imprintability of the Ni-based BMG. 2. Experimental Procedures In order to clarify the surface imprintability of Ni-based BMG, two alloys, Ni 53 Nb 20 Zr 8 Ti 10 Co 6 Cu 3 and Al-Si-Cu commercial die-cast alloy (ADC12), were used in this study. The mother alloy was prepared by arc melting the mixture of the pure metals (up to 99.9% purity) in an argon atmosphere. The mother alloy was remelted and cast using the PDC equipment shown in Fig. 1. The mother alloy was completely remelted in the ceramic sleeve using high-frequency induc- Cavity Die Plunger Vacuum Chamber Die Molten Alloy Induction Coil Sleeve Fig. 1 Schematic illustration of the precision die-casting equipment used in the present study.

2 1240 M. Ishida et al. (a) Die A Cavity (b) φ 1.7mm φ 0.6mm 0.4mm Specimen Die B 0.7mm A s W d H s Evaluation Surface Fig. 2 Schematic illustrations of die (a) and specimen (b). A d H d (a) (b) (c) Vickers-indented Die 5.3 µmr y 0.1 µmr y 6.2 µm Fig. 3 Schematic illustrations of top planes of die B prepared by electricdischarge machining (a), polishing (b) and Vickers indentation (c). Fig. 4 Index of fillability into indentation W d : Width of Vickers indentation =50µm H d : Depth of Vickers indentation=6.2µm A d : Indentation area of die H s : Filling height of specimen A s : Filling area of specimen R f =A s / A d : Percentage of filling area Cross-sectional illustration of indented die and imprinted specimen. tion heating and injected into the mold by moving both the plunger and the sleeve. The temperature of the molten alloy was monitored using a radiation thermometer and fed back by a PID controller. The entire process was conducted in vacuum (under Pa) to avoid oxidation. The maximum plunger speed and injection pressure were 3.0 m/s and 30 MPa, respectively. The structures of the cast specimens were examined by microarea X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Figure 2 shows the schematic illustration of dies and a specimen. The cavity of the die-a (upper die) consists of two cylindrical holes with different diameters of 1.7 mm and 0.6 mm and heights of 0.3 mm and 0.4 mm, respectively. The cavity has the simplified geometry of a microgear. To investigate the fillability and imprintability, the top plane of the die-b (lower die) was prepared with a different finish as shown in Fig. 3. The surface roughness of the top plane was made to be 5.3 mmr y by electric-discharge machining (a), and 0.1 mmr y by polishing (b). The microindentation with 6.2 mm depth was prepared by Vickers indentation. The surface image and roughness were obtained using a confocal scanning microscope 17,18) (CSM; Lasertec HD-100D). Figure 4 shows the cross-sectional illustration of the indented die and imprinted specimen. We evaluated the depth of indentation (H d ), the indentation area (A d ), filling height (H s ) and filling area (A s ) by three-dimensional image processing. The evaluation index of fillability was obtained by calculating the filling ratio (R f ¼ A s =A d ). 3. Results and Discussion 3.1 Net-shaped forming of microgear by the PDC technique Figure 5 shows the external appearance of the microgear called a sun-carrier made of Ni-based BMG using the PDC technique. The sun-carrier consists of a microgear, carrier plate and three pins. The outer diameter of the gear is 300µm Fig. 5 External appearance of Ni-based BMG sun carrier prepared by the precision die-casting technique mm, teeth of 14 and the module of 0.04 is located on a carrier plate of 1.7 mm in diameter. At the bottom of the carrier plate, three set of pins with a diameter of 0.30 mm and a length of 0.45 mm are located for rotating planetary gears. A conventional sun-carrier was assembled of five individually machined parts. The PDC technique enables us to prepare the parts with a complicated three-dimensional shape through even just one process. Figure 6 shows the XRD patterns taken from the surface and cross-sectional region of the cast BMG sun-carrier. Each pattern consists only of a broad peak, and there is no significant peak corresponding to the crystalline phase. The structures of sun-carriers injected at temperatures ranging from 1400 to 1690 K show the same results. On the basis of XRD results, the sun-carrier was revealed to have a glassy structure. In addition, the thermal stability was evaluated by DSC, as shown in Fig. 7. The glass

3 Fillability and Imprintability of High-strength Ni-based Bulk Metallic Glass Prepared by the Precision Die-casting Technique 1241 N 53 Nb 20 Ti 10 Zr 8 Co 6 Cu 3 Cr Kα Ni 53 Nb 20 Ti 10 Zr 8 Co 6 Cu K/s Intensity (arb.unit) Cross Section Surface Exothermic 1.0W/g sun carrier θ Fig. 6 Microarea XRD patterns taken from the cross section and surface of Ni-based BMG sun carrier. transition temperature (T g ), crystallization temperature (T x ) and supercooled liquid region (T x ) are 842 K, 893 K and 51 K, respectively. The heat of crystallization for a Ni-based microgear is the same as that for melt-spun ribbon. The DSC results are consistent with the XRD results. Micromachine parts such as the sun-carrier require surface flatness, because the function of microparts is highly sensitive to the surface smoothness. Therefore, we investigated the fillability and imprintability of cast BMG using a simplified die shape, as shown in Fig. 3. melt spun ribbon Tx Temperature, T / K Fig. 7 DSC curve of Ni-based BMG sun carrier. The curve of the meltspun ribbon is also shown for comparison. 3.2 Fillability of BMG In order to clarify the fillability of Ni-based BMG into microgroove, we evaluated the filling height (H s ) and filling ratio (R f ) of cast BMG compared with crystalline aluminum die-cast alloy (ADC12). Figure 8 shows the three-dimensional image and cross-sectional profile of cast BMG (a) (b) and ADC12 (c) (d). The injection temperature is 1688 K for Tg (a) 10µm (c) 10µm (b) (d) 1µm 5µm 1µm 5µm H s = 6.2 µm R f = 99 % H s = 5.9 µm R f = 84 % Fig. 8 Three-dimensional image and cross-sectional profiles of cast BMG (a) (b) and ADC12 (c) (d).

4 1242 M. Ishida et al. Height, H s / µm Depth of indentation, H d BMG ADC Reduced Injection Temperature, T i / T m Fig. 9 Effect of reduced injection temperature T i =T m on the height H s of specimens as an evaluation index of fillability. T i is injection temperature during precision die-casting and T m is melting temperature of alloy. BMG and 1150 K for ADC12. The BMG specimen has a considerably flat surface and the indentation is very sharp as compared with the ADC12 specimen. H s and R f of BMG are 6.2 mm and 99%, although H s and R f of ADC12 are 5.9 mm and 84%, respectively. In addition, the tip of indentation of the ADC12 specimen shows a certain curvature and there is a crater. In order to investigate the effect of injection conditions on the fillability, the relationship between the reduced injection temperature T i =T m and H s was investigated. Figure 9 shows H s as a function of T i =T m. The melting temperature is 1320 K for Ni-based BMG, and 850 K for ADC12. In the case of BMG, H s increases with increasing T i =T m, and reaches the originally die indentation depth (H d ) at T i =T m ¼ 1:28. In contrast, H s of ADC12 is saturated at 5.9 mm with increasing T i =T m. It is considered that the difference in fillability between BMG and ADC12 is due to the difference in fluidity during their cooling process. In the case of a crystalline alloy such as ADC12, the viscosity of the alloy will increase drastically at the solidification temperature, and the fluidity of the alloy will also decrease. 19) This assumption suggests that the molten alloy filled into the indentation is rapidly cooled to below the solidification temperature, therefore the fluidity of the alloy is not sufficient to fill the indentation. The cooling rate for BMGs is assumed to be almost the same as that for ADC12. However, BMG might retain its fluidity even below T m. It is well known that a glass-forming alloy easily exhibits undercooling and an undercooled liquid still exists between T m to T g. Since the viscosity of an undercooled liquid gradually increases, it appears that the cast BMG takes a much longer time to fill the indentation, resulting in the difference in fillability between BMG and ADC Surface imprintability of BMG It is reported that BMG deformed by viscous flow transcribes the die surface correctly on a nanometer scale. It was concluded that the homogeneous atomic configuration of BMGs is the origin for the excellent imprintability. 12,13) In general, the imprintability of cast alloy obtained by a conventional die-casting technique is nearly on the micrometer scale. As mentioned above, the imprintability of cast microparts is one of the important factors from the viewpoint of their functioning and wear resistance. Therefore, the surface roughnesses of the die and specimen were evaluated. Figure 10 shows the surface morphologies of the die and cast BMG observed by CSM. A crater type machining mark can be seen in the image of the die surface, and the surface roughness of the die is evaluated to be 5.1 mmr y. As seen in Fig. 10(b), the cast BMG is well transcribed on the die surface, and it looks like a mirror image of the die surface. Figure 11 shows the CSM surface images of the polished die (a), cast BMG (b) and surface roughness profiles of the die and cast BMG (c), respectively. As seen in Fig. 11(b), the surface of the cast BMG specimen is quite flat and there is no shrinkage corresponding to solidification. In order to investigate the surface roughness of both, the matched surface profiles were taken from the line A B. Both profiles exhibit (a) Die (b) BMG 100 µm 100 µm Fig. 10 Surface morphologies of die prepared by electric discharge machining (a) and imprinted cast BMG (b).

5 Fillability and Imprintability of High-strength Ni-based Bulk Metallic Glass Prepared by the Precision Die-casting Technique 1243 (a) Die (c) A B Die 0.2µm 25µm (b) BMG B 25 µm A Height (arb.unit) BMG Superpose 25 µm A B Fig. 11 Surface morphologies of polished die (a), cast BMG (b), and surface profiles of the die and the cast BMG. Surface roughness were measured along the line A-B. the same maximum surface roughness of 100 nm and the profiles of both surfaces are similar. For comparison of the two profiles, the inverted profile of BMG is superposed upon the profile of the die. The maximum difference (d max ) between the profiles is evaluated to be less than 50 nm. It is concluded that the cast BMG prepared by the PDC technique exhibits excellent imprintability on the die surface on a nanometer scale. Figure 12 shows the relationship between the surface roughness of the die (R yd ) and cast BMG specimen (R ys ). A proportional relationship can be seen between R yd and R ys in the range from 100 nmr y to 5.5 mmr y. If the surface roughness of the die can be decreased, the surface of the imprinted cast BMG will become smoother. It is therefore concluded that the cast Ni-based BMG exhibits excellent imprintability on a nanometer scale, and is highly useful for preparing micromachine parts. 4. Summary We studied microscopic transcription characteristics of Ni 53 Nb 20 Zr 8 Ti 10 Co 6 Cu 3 bulk metallic glass. The results obtained are summarized as follows. (1) A three-dimensional net-shaped microgear was successfully prepared by the precision die-casting technique. The as-cast microgear made of BMG has a glassy state in the injection temperature range from 1300 to 1690 K. (2) Fillability of BMG into a microindentation formed by Fig. 12 Relationship between surface roughness of die R yd and surface roughness of cast BMG R ys. Vickers indentation increases with injection temperature, and the height H s reaches the original indentation depth at T i =T m ¼ 1:28. It is concluded that the fillability of BMG is superior to that of crystalline die-cast alloy. (3) The cast BMG transcribes the die surface correctly on a nanometer scale, and the surface roughness of the cast

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