Assessing continuous casting of precious bulk metallic glasses

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Assessing continuous casting of precious bulk metallic glasses Fabian Haaga, Romuald Saugetb, Güven Kurtuldua, Silke Prades-Rödelb, Jürgen E.K. Schawec, ⁎ Andreas Blatterb, Jörg F. Löfflera, a b c

Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland PX Services SA, 2304 La Chaux-de-Fonds, Switzerland Mettler-Toledo GmbH, Analytical, 8603 Schwerzenbach, Switzerland

ARTICLE INFO

ABSTRACT

Keywords: Bulk metallic glasses Continuous casting Casting simulation Heat transfer X-ray diffraction Differential scanning calorimetry

This study describes investigations of continuous casting for the bulk metallic glass-forming alloys Pd43Ni10Cu27P20 and Pt57.3Cu14.6Ni5.3P22.8, respectively. Continuous casting of the Pd-based alloy readily produced fully amorphous rods of 10 mm diameter and > 500 mm length. In contrast, the Pt-based alloy always underwent crystallization during the casting process, even at smaller rod diameters down to 2 mm. In search of the reason for this difference, we simulated the process using ProCAST. Via adapting the heat transfer coefficients for all the relevant interfaces of the continuous casting setup we obtained good agreement between the experimental temperature data and the simulations. The calculations revealed that the maximum cooling rates achievable with this industrial setup ranged from 15 K/s to 17 K/s. This is well above the critical cooling rate for glass formation of the Pd-based alloy, but below that of the Pt-based alloy, as inferred from literature data and corroborated by fast differential scanning calorimetry. These findings illustrate how detailed casting simulations can predict the feasibility of semi-industrial bulk metallic glass production techniques.

1. Introduction Since the first metallic glass was discovered in the Au–Si system [1], many further alloy families have been investigated and potential fields of application assessed [2,3]. Precious metal alloys were among the first glass formers discovered [4,5]. They possess good glass-forming ability and some bulk metallic glasses (BMGs) can reach critical diameters of up to 80 mm [6]. They also feature excellent superplastic forming properties owing to their moderate glass transition temperature Tg (100 °C for Au- [7], 240 °C for Pt- [8], and 300 °C for Pd-based BMGs [9]), combined with a large supercooled liquid region (SCLR) of up to 100 °C. Their formability, luster and aesthetic appeal make precious metal BMGs a prime material for potential applications in the jewelry and watch industry [10]. Moreover, these BMGs can be hallmarked, which is an important aspect in the luxury domain as their precious metal content satisfies a fineness standard (weight parts per thousand of 750, 850 and 500 for the Au, Pt and Pd-based BMGs, respectively). However, economical production of large quantities of amorphous feedstock material is essential for such applications. Continuous casting is a processing route which may fulfill this requirement. Continuously cast rods with a diameter of about 10 mm might be a suitable and economical starting material for the production of jewelry or watch

⁎

parts by superplastic forming, rolling or machining. Earlier studies showed the potential of various continuous casting techniques for nonprecious-metal BMGs, such as that of Bridgman [11,12], twin-roller casting [13,14], or horizontal continuous casting [15]. The latter was also computationally studied [16], revealing the importance of pulling speed, but not much detail was given on the background of the simulations. The interfacial heat transfer coefficient h, mostly unknown, is a decisive parameter for the cooling rates achieved and therefore for vitrification. Lee et al. [17] calculated the cooling behavior during twinroller casting using different h values between 1000 Wm−2 K−1 and 5000 Wm−2 K−1, and accordingly performed experiments firstly to determine h and secondly to find the optimal rolling speed for this casting method. In this paper we present vertical continuous casting results for precious metal BMGs (Pd43Ni10Cu27P20 and Pt57.3Cu14.6Ni5.3P22.8) gained using semi-industrial casting equipment for precious metal alloys and commercially available casting simulation software. By measuring a large set of temperature data during the casting experiments, we were able to find reasonable values for the heat transfer coefficients h – a parameter which is widely unexplored in BMG casting – for all the important interfaces, and to model the process with great accuracy. We then applied it to vary the main experimental parameters – rod

Corresponding author. E-mail address: [email protected] (J.F. Löffler).

https://doi.org/10.1016/j.jnoncrysol.2018.09.035 Received 24 July 2018; Accepted 24 September 2018 0022-3093/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Fabian Haag, et al., Journal of Non-Crystalline Solids, https://doi.org/10.1016/j.jnoncrysol.2018.09.035

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

F. Haag, et al.

diameter and pulling speed – to assess the process for both alloys investigated.

Fig. 2a depicts the temperature evolution in the die as recorded with the thermocouples during the casting process of Pd43Ni10Cu27P20. Fig. 2b shows the temperature evolution measured 35 mm below the exit from the die with an infrared thermometer (Optris LS LT, spot size: 1 mm, see inset to Fig. 2b). The measured values were corrected with respect to the emissivity of the alloy by reinserting a cast rod into the empty die and reheating the system until black body temperatures similar to those during actual casting were measured using the IR thermometer. Simultaneously, two type-K thermocouples were clamped onto the rod to determine its actual surface temperature. The compositions of the cast rods were verified using atomic absorption spectroscopy as well as X-ray fluorescence equipped with a wavelength-dispersive detector. Microstructural analyses were performed by means of X-ray diffraction (XRD, PANalytical X'Pert, Cu Kα radiation) and differential scanning calorimetry (DSC) at heating and cooling rates of 20 K/min (Mettler-Toledo DSC1). Complementary fast differential scanning calorimetry (FDSC) measurements were carried out using a Mettler-Toledo Flash DSC 2+ with an UFH 1 sensor to obtain detailed information about the onset of crystallization during the continuous undercooling of the Pt-based alloy. This system enables measurements in a temperature range between −95 °C and 1000 °C with typical cooling rates of up to 40,000 K/s. The measurements were performed on tiny samples from the cast rod with a typical thickness of 20 μm and a cross-section of 40 μm × 40 μm. The sample mass was varied between 0.6 and 1.4 μg, and no size effect was observed [18]. Each measurement was repeated at least three times.

2. Continuous casting of bulk metallic glasses 2.1. Experimental procedure A feedstock of Pd43Ni10Cu27P20 and Pt57.3Cu14.6Ni5.3P22.8 was prealloyed in an argon-purged (99.996%), closed container of a vacuuminduction unit. Feedstock batches of approximately 1 kg were then recast by continuous casting under 99.8% pure nitrogen atmosphere. The machine used is commercially available (Indutherm CC 400, equipped with a 15 kW induction furnace; see Fig. 1a) and widely applied for the alloying and casting of precious metal semi-finished products on industrial scales. Dies with variable cross-sections can be inserted below the graphite crucible (see Fig. 1b) before pulling the melt under controlled flow conditions. In this study, cylindrical dies made of graphite with an inner diameter of 10 mm and a length of 100 mm were mounted between the graphite crucible and the cooling unit (copper, water-cooled). Three thermocouples (type K) were inserted into the die at the positions marked in Fig. 1b to record the temperature distribution in the die during casting. A graphite starter rod (diameter: 10 mm, length: 400 mm) was inserted through the die up to the bottom orifice of the graphite crucible. The pre-alloy was melted in the crucible and kept at 1000 °C for several minutes until an equilibrated temperature distribution was reached. The casting process was then started by pulling the graphite starter from the crucible through the die in an intermittent way at a constant speed (averaged over pulling steps and short intermittent pauses), which is a central processing parameter for the continuously cast BMG rods with a length of > 500 mm (Fig. 1c).

2.2. Computational casting simulations The commercial casting simulation software ProCAST was used to model the process described above. In the following we present the Fig. 1. Continuous casting of BMGs. (a) The rods were produced in an Indutherm CC 400 continuous casting machine. (b) Casting simulations using ProCAST were performed using a computational replica of the casting machine. (c) Rods with a length of > 500 mm could be reproducibly processed, left. The Pd-based rods show the high metallic luster typical of the glassy state; the Pt-based rods lack this luster, right.

2

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

F. Haag, et al.

Fig. 2. Process temperature from experiment and simulation. (a) Temperature data from the thermocouples positioned in the die (see Fig. 1b) compared to various simulation results depending on the heat transfer coefficient, h. (b) Exit temperatures from calibrated IR thermometer (see inset) and simulation output. (c) Flow chart explaining the parameter optimization process.

details of the material database, meshing and process parameters, and of the iterative approach deployed to derive reasonable heat transfer coefficients by reconciling computed temperatures with measured data. The most important alloy properties required for casting computations (onset (end) of melting Tm,onset (Tm,end), glass transition temperature Tg, dynamic viscosity η, mass density ρ, specific heat capacity cp, and thermal conductivity λth) were gathered from literature and are

summarized in Table 1. The λth of the Pd-alloy was also assigned to the Pt-alloy due to the lack of more specific data and given the constitutional similarity between the two alloys. The effect of the error introduced by this approximation remains marginal because the dominant heat flow occurs through the die, ruled by the heat transfer coefficients discussed below, rather than along the alloy rod.

Table 1 Overview of relevant material properties and corresponding references. Pd-base

Pt-base

Tm,onset

533 °C [this work]

491 °C [this work]

Tm,end

589 °C [this work]

558 °C [this work]

Tg

303 °C–308 °C [this work]

220 °C [24,26]

η

Free volume model [27]

Calculated from Vogel-Fulcher-Tamann parameters [24]; alternatives available [10,26]

ρ

9.38 g/cm3 at room temperature; ρ(T) calculated from specific volume [19]

15.3 g/cm3 at room temperature [28]; ρ(T) calculated from the thermal expansion αth [20]

cp

[29]

[24]

λth

Reasonably assumed equivalent to Pd40Ni10Cu30P20 [30]

Same reference [30]; lack of data for Pt-based alloys

3

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

F. Haag, et al.

Triangular meshing was carried out for all major parts of the casting machine using the algorithm implemented in ProCAST with the grid sizes given in Table 2. The materials and starting temperatures T0 are also included. In line with these conditions, the time step for the calculations was set to a maximum of 2.5 ms. Crucible and cooler were set under heat-bath conditions at 1000 °C and 60 °C, respectively. The flowchart in Fig. 2c sketches the procedure applied to determine the heat transfer coefficients h. During an initial temperature equilibration phase of 240 s, a steady-state heat flow from the crucible to the cooler via the die was established (Fig. 2a). This phase was used to determine hcrucible-die, hdie-cooler, and hdie-starter to match the set of measured temperature data TABC,i (shaded area in Fig. 2a). Subsequently, the starter was moved downwards at a velocity of 1.5 mm/s, allowing the melt to flow into the cylindrical die cavity, where the interfacial heat transfer coefficient hrod-die plays an essential role. This value (together with hrodstarter) was determined to fit well the experimental data TABC,ii in Fig. 2a. The final reference point for adjusting the set of h coefficients was the exit temperature measured with the IR thermometer (Fig. 2b).

The reliability of the simulation output improved in this way is seen in particular in the area of the kink between TABC,i and TABC,ii in Fig. 2a and in the general order of magnitude of Texit in Fig. 2b. Table 3 Optimized values for h at respective interfaces. Interface

h

Crucible-die

No interface, modeled as one part instead

Die-cooler

6000

W m2K

Die-starter

h (T ) =

Rod-starter

10,000

Rod-die

Table 2 Simulation parameters for the parts of the casting machine. Mesh grid size; no. of elements

T0

Crucible

Graphite

6 mm; 11,986

1000 °C

Melt pool

Pd43Ni10Cu27P20, Pt57.3Cu14.6Ni5.3P22.8

3 mm; 12,606

1000 °C

Die

Graphite

2 mm; 35,583

470 °C

Starter

Graphite

2 mm; 8780

97 °C

Cooling unit Cast BMG rod

Copper Pd43Ni10Cu27P20, Pt57.3Cu14.6Ni5.3P22.8

3 mm; 31,362 1 mm; 106,000

60 °C 1000 °C

, if T < 473 K W 2,047.5 2 mK , if T

, if 473 K

T < 673 K

673 K

W m2K

W , if T < 473 K m2K W W 2.5 2 2 T + 2,932.5 2 , if 473 K mK mK W 1,250 2 , if T 673 K mK 1,750

h (T ) =

Material

W m2K W 7.5 2 2 T mK W 3,000 2 mK 1,500

T < 673 K

Although a linear approximation of h with temperature may still appear rather rudimentary, it mimics the casting conditions far better than using constants. The reasons for this improvement are expected to stem from the mismatch of the thermal expansion coefficients between the alloys (αth,BMG > 30 × 10−6 K−1 [19,20]) and the die material (αth,graphite ≈ 7 × 10−6 K−1 [21]), which may affect the heat transfer conditions along the varying die temperature. Nevertheless, the h values found should be considered rather a robust average for this process than a property of the rod-die interface. In this study it was, for example, neglected that the die cavity is slightly conical with an angle of 1°, which avoids clogging, to simplify the meshing procedure. This might also have an influence on the radial heat transfer along the die axis. Fig. 3a shows the XRD spectra of continuously cast rods. As supported by the broad diffraction halo, amorphous Pd43Ni10Cu27P20 rods of diameter 10 mm can easily be produced by vertical continuous casting. BMG rods longer than 500 mm (Fig. 1c) were obtained at 1.5 mm/s, the slowest pulling speed possible. In contrast, all attempts to produce fully amorphous Pt57.3Cu14.6Ni5.3P22.8 rods using the same vertical continuous casting process failed. Instead, the Pt-based alloy underwent crystallization during solidification, as evidenced by the distinct diffraction peaks in Fig. 3a. In line with the XRD results, the DSC traces of the rods (Fig. 3b) show a glass transition followed by crystallization events for the Pd-alloy but not for the Pt-alloy. These observations may in fact suggest that continuous casting using the present equipment does not lead to vitrification of Pt57.3Cu14.6Ni5.3P22.8.

3. Results and discussion By following the route described above, a set of h values was collected from the continuous-casting experiments and simulations using Pd43Ni10Cu27P20. The best set of h values is summarized in Table 3. Following this procedure, it became evident that the assumption of constant hdie-starter and hrod-die within the range determined in similar studies [17] cannot match the experimental data satisfactorily in all comparisons. The calculated curves for hstarter-die = hrod−2 −1 K , 2000 Wm−2 K−1, 3000 Wm−2 K−1} included die = {1000 Wm in Fig. 2a and b show less agreement with experimental data than when temperature-dependent h values were used. We achieved an increased conformity by using linear h-functions between 200 °C and 400 °C, which is the range of die temperatures recorded by the thermocouples. The minimum and maximum of 1250 Wm−2 K−1 and 3000 Wm−2 K−1, respectively, lie within a realistic range of heat transfer coefficients.

4

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

F. Haag, et al.

Fig. 3. Microstructure of 10 mm diameter rods. (a) Discrete peaks and a diffuse halo in the XRD spectra reveal crystalline and amorphous microstructures for the Ptand Pd-based alloys, respectively. (b) Tg and Tx can be determined from the DSC scans of the Pd-based alloy, whereas only a melting event between 490 °C and 560 °C can be observed for the Pt-based alloy.

In terms of applications in the luxury domain, however, Pt-based alloys are potentially more interesting than Pd-based alloys. In the market, Pt is perceived more valuable and nobler than Pd, and it exhibits higher resistance to oxidation. In addition, the fineness of the Ptbased alloy is higher; it can be hallmarked 850 Pt, compared to 500 Pd (the next higher standard fineness for Pd is 950, probably inaccessible as a BMG). For this reason, we also tried to assess the presented casting technique for the Pt-based glass-forming alloy, reasonably assuming the heat transfer coefficients to be similar to those of the Pd-based alloy (Table 3) in equivalent experiments. Fig. 4 presents the steady-state temperature profiles (i.e. when TABC,ii ≈ const.) extracted from the centers of the rods. It reveals that the cooling behavior is similar for both alloys, if only the material database is changed while maintaining all other simulation parameters. The cooling rates, computed from the first derivatives of temperature with time, are comparable at approximately 15 K/s to 17 K/s. According to literature this is clearly higher than the critical cooling rate reported for Pd43Ni10Cu27P20 (0.2 K/s [22], after fluxing), but slightly lower than that documented for Pt57.3Cu14.6Ni5.3P22.8 (≈20 K/s [23,24], after fluxing).

rod center was cooled to below the respective values for the onset of melting. It is evident that Pd43Ni10Cu27P20 can be vitrified with ease using the current technique, in line with experiment and simulation, even without a flux treatment, because the cooling curve passes the crystallization nose at a great distance. For the Pt-based alloy, however, the situation is different: the various cooling paths in Fig. 5a and b are close to or intersect the nose of the crystallization curve. The latter was reproduced from an earlier study [24], which (according to the authors) suffered from high scatter in the onset time of crystallization upon cooling. These TTT-data are also incomplete, because the maximum cooling rate Ṫmax was too slow to access crystallization in the vicinity of the transformation nose in conventional DSC (Ṫmax = 2 K/s). Using the classical crystallization theory based on the Turnbull-Fisher (TF) approach for nucleation and the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model, the authors therefore fitted the crystallization curve only to the values measured below the nose temperature, i.e. upon heating. For these reasons and because the cooling paths obtained from the simulations are very close to the extrapolated nose of the crystallization curve, we performed continuous cooling experiments using fast differential scanning calorimetry (FDSC) [25]. The samples were quenched from 625 °C (i.e. from above their melting temperature) to room temperature at varying cooling rates Ṫ between 0.1 and 60 K/s. The onset time of crystallization tx,onset was calculated from the onset temperature of crystallization Tx,onset via tx,onset = (Tm,onset – Tx,onset)/Ṫ. The continuous cooling transformation (CCT) data obtained for the non-fluxed Pt-based alloy are included with error bars in Fig. 5 together with the results of the simulations. Using FDSC and a sample mass of < 1 μg, it was possible to reproduce the postulated critical cooling rate of 20 K/s for the fluxed Pt-based glass-former, but crystallization clearly sets in earlier than predicted by the extrapolated DSC literature data. The FDSC scatter of the onset times is 14% on average, which is much less than the standard deviation of 43% (78 s) for the average of 180 s to form a crystalline fraction of 5% at 703 K, as reported in [24]. The critical cooling rate derived from the FDSC data lies above 20 K/s and ranges up to 90 K/s (which might be due to the missing fluxing effect), and all cooling paths intersect the CCT data. The question then arises as to whether the cooling rate can be substantially increased to above the critical rate by changing the process parameters. The simulation results in Fig. 5a show that decreasing the rod diameter from 10 mm to 2 mm at constant pulling speed indeed lowers the exit temperature, but only marginally affects the cooling path. On the other hand, the pulling speed shows a stronger, but still limited effect on cooling rate in the critical interval between Tm,onset and the temperature at which the rod exits the die (Fig. 5b). Ramping up the speed to 2.0 mm/s (much higher speeds would not be accessible

Fig. 4. Simulation of temperature and cooling rate. No significant difference in cooling behavior can be deduced between the simulation data (diameter: 10 mm, pulling speed: 1.5 mm/s) for the Pd-based and Pt-based alloys. The zero position corresponds to the entrance into the die (see Fig. 1).

Fig. 5a shows the cooling profiles calculated at a casting speed of 1.5 mm/s for a 10 mm Pd-based rod and for Pt-based rods of various diameters ranging from 2 to 10 mm. The time was set to zero when the 5

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

F. Haag, et al.

experimentally for safety reasons) increases the average cooling rate from 16.1 K/s to 17.1 K/s. Because these values are still lower than the critical cooling rate for fluxed Pt57.3Cu14.6Ni5.3P22.8 under laboratory conditions, we conjecture that even virtually improving the glassforming ability by measures such as using a higher-purity N2 atmosphere or adding a fluxing agent to the melt would not allow for a continuous casting production of amorphous Pt57.3Cu14.6Ni5.3P22.8 feedstock material. To verify the outcome of the simulations, we produced a Pt-based rod of 2 mm diameter via continuous casting and determined its microstructure by XRD and DSC. In fact, Fig. 6 confirms that the cooling conditions are not sufficient to produce amorphous Ptbased rods of 2 mm diameter via a semi-industrial process, which agrees well with the predictions of the casting simulations.

4. Summary and conclusions In summary, we investigated the possibility of industrial continuous casting of the BMG-forming alloys Pd43Ni10Cu27P20 and Pt57.3Cu14.6Ni5.3P22.8, respectively. Fully amorphous rods of 10 mm diameter and > 500 mm length were readily obtained with the Pdbased alloy, while the Pt-based alloy always crystallized upon solidification. Inferring the refined interfacial heat transfer coefficients involved in the process from the vast temperature data set recorded during casting, we were able to establish a robust simulation model which closely matches the real continuous casting process. The computationally intensive simulation helps us to better understand this process in the context of precious metal BMG casting. In line with

Fig. 5. Assessing the glass-forming ability during vertical continuous casting. (a) Cooling curves at 1.5 mm/s pulling speed for rod diameters of 10 mm (Pd) and 2–10 mm (Pt) together with TTT (Pd, Pt) and CCT (Pt) diagrams. The gap between the cooling curve and the crystallization “nose” is large for Pd43Ni10Cu27P20, whereas the cooling curves for Pt57.3Cu14.6Ni5.3P22.8 pass the “nose” of the TTT diagram in its proximity (data and fit [24]). However, the curves for the Pt-based alloy clearly intersect the CCT data measured in the present study by FDSC, which indicates the onset of crystallization. Reducing the diameter of the die does not significantly increase the cooling rate. (b) Simulated higher pulling speeds of 2 mm/s have a more pronounced, though insufficient effect to vitrify the Pt-based alloy.

Fig. 6. XRD and DSC results from Pt-based rods of 2 mm diameter. (a) The XRD spectrum reveals discrete Bragg peaks similar to the one shown in Fig. 3a, i.e. the 2 mm rod is still fully crystalline. (b) Only melting can be observed in DSC, which illustrates that the cooling conditions are not sufficient to produce amorphous Ptbased rods via semi-industrial continuous casting, as predicted by the casting simulations.

6

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

F. Haag, et al.

experimental experience, the simulation results suggest that Pd43Ni10Cu27P20 can be vitrified with ease under all processing parameters investigated, while Pt57.3Cu14.6Ni5.3P22.8 always crystallizes, even for diameters down to 2 mm. In fact, the cooling rates of the Ptbased alloy, calculated for various rod diameters and pulling speeds, always stayed below the critical cooling rate necessary for glass formation as derived from TTT and CCT data. In conclusion, we explained via detailed casting simulations the feasibility of up-scaled production of BMG rods for Pd43Ni10Cu27P20 and its non-feasibility for Pt57.3Cu14.6Ni5.3P22.8 using an industrial continuous casting process. The set of simulation parameters we used can be deployed in an attempt to optimize the vertical continuous casting of BMGs in general and of the Pt-based alloy in particular, by modifying machine parameters such as die geometry and cooling design. Overall, this study nicely illustrates how detailed simulation of a semi-industrial casting process can aid the understanding of the processes required to produce BMGs rods of 50 cm in length or more. In general, the better understanding of thermal history gained by such simulations may also help to reduce the number of scrap parts and/or help us to obtain amorphous alloys with a tailored microstructure, extending from monolithic glasses to BMG composites with a controlled fraction of crystalline phase.

https://doi.org/10.1016/j.msea.2006.02.301. [11] Y. Li, H.Y. Liu, H.A. Davies, H. Jones, Easy glass formation in Mg64Ni21Nd15 by Bridgman solidification, Mater. Sci. Eng. A A179–180 (1994) 628–631. [12] J.W. Qiao, S. Wang, Y. Zhang, P.K. Liaw, G.L. Chen, Large plasticity and tensile necking of Zr-based bulk-metallic-glass-matrix composites synthesized by the Bridgman solidification, Appl. Phys. Lett. 94 (2009) 2007–2010, https://doi.org/ 10.1063/1.3118587. [13] J.G. Lee, H. Lee, Y.S. Oh, S. Lee, N.J. Kim, Continuous fabrication of bulk amorphous alloy sheets by twin-roll strip casting, Intermetallics 14 (2006) 987–993, https://doi.org/10.1016/j.intermet.2006.01.039. [14] A. Urata, N. Nishiyama, K. Amiya, A. Inoue, Continuous casting of thick Fe-base glassy plates by twin-roller melt-spinning, Mater. Sci. Eng. A 448 (451) (2007) 269–272, https://doi.org/10.1016/j.msea.2006.02.341. [15] T. Zhang, X. Zhang, W. Zhang, F. Jia, A. Inoue, H. Hao, Y. Ma, Study on continuous casting of bulk metallic glass, Mater. Lett. 65 (2011) 2257–2260, https://doi.org/ 10.1016/j.matlet.2011.04.033. [16] B.W. Zhou, S.J. Yin, R.H. Tang, H.S. Yang, B. Ya, B.Y. Jiang, Y. Fang, X.G. Zhang, Study on fabrication of bulk metallic glassy composites by horizontal continuous casting method, J. Alloys Compd. 660 (2016) 39–43, https://doi.org/10.1016/j. jallcom.2015.11.108. [17] J.G. Lee, S.S. Park, S.B. Lee, H.T. Chung, N.J. Kim, Sheet fabrication of bulk amorphous alloys by twin-roll strip casting, Scr. Mater. 53 (2005) 693–697, https:// doi.org/10.1016/j.scriptamat.2005.05.018. [18] S. Pogatscher, D. Leutenegger, A. Hagmann, P.J. Uggowitzer, J.F. Löffler, Characterization of bulk metallic glasses via fast differential scanning calorimetry, Thermochim. Acta 590 (2014) 84–90, https://doi.org/10.1016/j.tca.2014.06.007. [19] I.R. Lu, G.P. Görler, H.J. Fecht, R. Willnecker, Investigation of specific volume of glass-forming Pd-Cu-Ni-P alloy in the liquid, vitreous and crystalline state, J. NonCryst. Solids 312–314 (2002) 547–551, https://doi.org/10.1016/S0022-3093(02) 01767-2. [20] S. Mukherjee, J. Schroers, Z. Zhou, W.L. Johnson, W.K. Rhim, Viscosity and specific volume of bulk metallic glass-forming alloys and their correlation with glass forming ability, Acta Mater. 52 (2004) 3689–3695, https://doi.org/10.1016/j. actamat.2004.04.023. [21] P.S. Gaal, Graphite thermal expansion reference for high temperature, Therm. Expans. SOLIDS — 1973 Int. Symp. ASCE, 1974, pp. 102–116, , https://doi.org/10. 1063/1.2945915. [22] I.R. Lu, G. Wilde, G.P. Görler, R. Willnecker, Thermodynamic properties of Pd-based glass-forming alloys, J. Non-Cryst. Solids 250–252 ( (1999) 577–581, https://doi. org/10.1016/S0022-3093(99)00135-0. [23] J. Schroers, W.L. Johnson, R. Busch, Crystallization kinetics of the bulk-glassforming Pd43Ni10Cu27P20 melt, Appl. Phys. Lett. 77 (2000) 1158–1160, https://doi. org/10.1063/1.1289033. [24] B.A. Legg, J. Schroers, R. Busch, Thermodynamics, kinetics, and crystallization of Pt57.3Cu14.6Ni5.3P22.8 bulk metallic glass, Acta Mater. 55 (2007) 1109–1116, https://doi.org/10.1016/j.actamat.2006.09.024. [25] F. Haag, S. Geisel, G. Kurtuldu, J.F. Löffler, Bulk metallic glass casting investigated using high-speed infrared monitoring and complementary fast scanning calorimetry, Acta Mater. 151 (2018) 416–423, https://doi.org/10.1016/j.actamat.2018. 02.029. [26] I. Gallino, O. Gross, G. Dalla Fontana, Z. Evenson, R. Busch, On the kinetic and thermodynamic fragility of the Pt60Cu16Co2P22 and Pt57.3Cu14.6Ni5.3P22.8 bulk metallic glasses, J. Alloys Compd. 615 (2015) S35–S39, https://doi.org/10.1016/j. jallcom.2013.12.006. [27] G.J. Fan, H.-J. Fecht, E.J. Lavernia, Viscous flow of the Pd43Ni10Cu27P20 bulk metallic glass-forming liquid, Appl. Phys. Lett. 84 (2004) 487–489, https://doi.org/10. 1063/1.1644052. [28] J. Schroers, Processing of Bulk Metallic Glass, Adv. Mater. 22 (2010) 1566–1597, https://doi.org/10.1002/adma.200902776. [29] G.J. Fan, J.F. Löffler, R.K. Wunderlich, H.J. Fecht, Thermodynamics, enthalpy relaxation and fragility of the bulk metallic glass-forming liquid Pd43Ni10Cu27P20, Acta Mater. 52 (2004) 667–674, https://doi.org/10.1016/j.actamat.2003.10.003. [30] U. Harms, T. Shen, R. Schwarz, Thermal conductivity of Pd40Ni40−xCuxP20 metallic glasses, Scr. Mater. 47 (2002) 411–414, https://doi.org/10.1016/S1359-6462(02) 00160-4.

Acknowledgements This research was supported within the framework of the VitriMetTech ITN network FP7-PEOPLE-2013-ITN-607080. The authors would like to thank Denis Huelin for experimental assistance during casting. References [1] W. Klement, R.H. Willens, P. Duwez, Non-crystalline structure in solidified GoldSilicon alloys, Nature 187 (1960) 869–870, https://doi.org/10.1038/187869b0. [2] A.L. Greer, Metallic glasses…on the threshold, Mater. Today 12 (2009) 14–22, https://doi.org/10.1016/S1369-7021(09)70037-9. [3] A. Inoue, A. Takeuchi, Recent development and application products of bulk glassy alloys, Acta Mater. 59 (2011) 2243–2267, https://doi.org/10.1016/j.actamat.2010. 11.027. [4] H.S. Chen, Thermodynamic considerations on the formation and stability of metallic glasses, Acta Metall. 22 (1974) 1505–1511, https://doi.org/10.1016/00016160(74)90112-6. [5] A.J. Drehman, A.L. Greer, D. Turnbull, Bulk formation of a metallic glass: Pd40Ni40P20, Appl. Phys. Lett. 41 (1982) 716, https://doi.org/10.1063/1.93645. [6] N. Nishiyama, K. Takenaka, H. Miura, N. Saidoh, Y. Zeng, A. Inoue, The world's biggest glassy alloy ever made, Intermetallics 30 (2012) 19–24, https://doi.org/10. 1016/j.intermet.2012.03.020. [7] J. Schroers, B. Lohwongwatana, W.L. Johnson, A. Peker, Gold based bulk metallic glass, Appl. Phys. Lett. 87 (2005) 404–406, https://doi.org/10.1063/1.2008374. [8] J. Schroers, W.L. Johnson, Highly processable bulk metallic glass-forming alloys in the Pt-Co-Ni-Cu-P system, Appl. Phys. Lett. 84 (2004) 3666–3668, https://doi.org/ 10.1063/1.1738945. [9] A. Inoue, N. Nishiyama, Extremely low critical cooling rates of new Pd-Cu-P base amorphous alloys, Mater. Sci. Eng. A 226–228 (1997) 401–405, https://doi.org/10. 1016/S0921-5093(97)80051-2. [10] J. Schroers, B. Lohwongwatana, W.L. Johnson, A. Peker, Precious bulk metallic glasses for jewelry applications, Mater. Sci. Eng. A 449–451 (2007) 235–238,

7