Numerical and experimental study of electron beam floating zone melting of Iridium single crystal

Accepted Manuscript Title: Numerical and experimental study of electron beam floating zone melting of Iridium single crystals Authors: Jieren Yang, Hu Wang, Binqiang Wang, Rui Hu, Yi Liu, Ximing Luo PII: DOI: Reference:

S0924-0136(17)30288-1 http://dx.doi.org/doi:10.1016/j.jmatprotec.2017.07.016 PROTEC 15313

To appear in:

Journal of Materials Processing Technology

Received date: Revised date: Accepted date:

5-5-2017 8-7-2017 10-7-2017

Please cite this article as: Yang, Jieren, Wang, Hu, Wang, Binqiang, Hu, Rui, Liu, Yi, Luo, Ximing, Numerical and experimental study of electron beam floating zone melting of Iridium single crystals.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2017.07.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Numerical and experimental study of electron beam floating zone melting of Iridium single crystals

Jieren Yang1)*, Hu Wang1), Binqiang Wang1), Rui Hu1), Yi Liu2), Ximing Luo2)

1) State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China 2) Kuming Institute of Precious Metals, Kuming 650106, China

*Corresponding author: Jieren Yang State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, Shaanxi Province, P. R. China Tel: +86 29 8846 0361, Fax: 86 29 8846 0294 E-mail address: [email protected]

Graphical Abstract

1

Rotating

(c)

Chuck Melting zone

Height of Iridium rod (mm)

Iridium rod

(d)

Iridium rod

(e) 6 mm 1 mm

EB gun

(f)

2 mm

(b)

(a) Rod diameter (mm)

In the current work, numerical model was first established to investigate the electron beam floating zone melting (EBFZM) of Iridium (a). The temperature distribution in Iridium rod, especially the melting zone, was calculated and analyzed with different processing parameters (b). Based on the simulated results, an approach for parameter optimization was proposed and applied successfully on the EBFZM of Iridium rod (c and d) with a diameter of 20 mm. Equiaxed grains (e) rapidly evolved into coarsen grains (f) via a 40-mm growth, which indicated that a single-crystal Iridium could be obtained in this way.

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Abstract Both numerical and experimental approaches were conducted to investigate the electron beam floating zone melting (EBFZM) of iridium crystal. A finite element model was established and the temperature fields under different processing parameters were calculated and discussed. The heating power, the rod diameter and the movement of heating source significantly influence the temperature distribution in the iridium rod. Once the heating starts, the temperature quickly increases and gradually reaches the steady state. The melting zone enlarges with the increase of heating power, and the critical power for obtaining a complete melting zone is 5.5 kW～6.0 kW in 20 mm-diameter Iridium rod. With the upward movement of heating source, the melting zone obviously enlarges and the lower solid/liquid (S/L) interface becomes more planar. Then how to get a complete melting zone with a suitable volume and the control of a planar S/L interface were discussed. An optimized processing window was proposed and applied to the EBFZM of iridium crystal. An iridium rod 20 mm in diameter was successfully produced and the microstructural morphologies indicated that the initial polycrystalline microstructure will evolve into single crystal.

Keywords: Single-crystal Iridium; EBFZM; numerical calculation; temperature field; solid/liquid interface.

1.

Introduction Among the platinum group metals (PGMs), iridium is considered as a superhigh-temperature

material that can be applied beyond 2200 °C in oxidizing atmosphere due to its high melting point, attractive mechanical properties and good chemical stability, even though its melting and fabrication difficulties were indicated by Hunt (1987). In the fields of aerospace and high energy physics, (Franco-Ferreira and Gerg, 1997) reported that PuO2 fuel can be sealed in an iridium container and provides the nuclear energy for deep space probes. Czochralslri (1974) successfully applied a 3

single-crystal iridium crucible (purity ≥99.995%) to produce single-crystal yttrium aluminium garnet, which is the core component in a high-power laser. Besides, study in Schreck et al. (1999) revealed that single-crystal iridium is the best substrate material for the epitaxial growth of semiconductor materials. Iridium is a face-centred cubic (FCC) metal, however, Panfilov et al. (2009) found that its room-temperature plasticity is considerably low and cleavage fracture easily occurs after a small deformation. The intrinsic brittleness of iridium has not been fundamentally understood so far and is probably due to the high stacking fault energy in the single-crystal structure, as revealed in Cawkwell et al. (2005). In particular, the ductility of single-crystal iridium is highly dependent on the crystal orientation. Research has demonstrated that single-crystal iridium is a strongly anisotropic material and exhibits the optimal ductility with the [110] orientation. Therefore, Yermakov et al. (1990) pointed out that the deformation and shaping of iridium crystal should be based on the solidification control of [110] oriented single crystal. However, public reporting on the iridium single crystal is rare, which is due to the difficulty of experiments and the military sensitivity. Anyway, there are main two problems that limit the production of iridium single crystal. Firstly, in term of that the superhigh melting point of iridium (2447 °C), Zee et al. (2001) emphasized that no suitable crucible can be chosen as the melting container. Encouragingly, some works conducted by (Glebovsky and Semenov, 1995) and Council (2001) confirmed that refractory single-crystal materials can be prepared by electron beam floating zone melting (EBFZM), such as tungsten (W) and molybdenum (Mo) with a diameter of 30 mm and length of 150 mm. Also, (Boehlerta and Bingert, 2001) reported that a 125 mm-length rod of Ti-Al-Nb alloy was directionally solidified via a float-zone melting. Note that the density of iridium is 22.56 g/cm3, which is 17 % higher than that of W, so the stabilizing of iridium melt during floating melting is a tough process. Secondly, the well growth of single crystals and the close control of the crystalline orientation are difficult. Szeliga et al. (2016) proposed that a slightly 4

convex solid/liquid (S/L) interface is beneficial to the single-crystal growth. Therefore, a melting–solidifying parameter window should be investigated and then adopted for the preparation of single-crystal iridium. In this work, EBFZM was attempted to prepare iridium single crystals. Based on this, a finite element (FE) model was established to investigate the temperature field in the EBFZM of iridium. Then the temperature evolution involving the steady state, the morphologies of the melting zone and the solidification interface were numerically investigated under different processing parameters. Further, the possibility of producing iridium single crystals by EBFZM was experimentally tried and evaluated.

2.

Experimental method

2.1 Procedure of EBFZM The EBFZM equipment applied to prepare iridium single crystals is presented in Fig. 1. It mainly consists of the furnace body, electron beam (EB) gun system, water-cooled system and vacuum system. The size of the furnace chamber is about 700 mm in diameter and 1000 mm in height. The main technical parameters are as follows: ——Maximum power of EB gun: 25 kW ——Limit of rod diameter: ≤ 40 mm ——Movable distance of EB gun in height: ≤ 500 mm ——Adjustable velocity of EB gun: 0.1～200 mm/min ——Ultimate vacuum: ≤ 10-4 Pa The EBFZM of iridium single crystals can be performed as the following sequence, which is basically illustrated in Fig. 2(a): ——Rod installation. Both ends (top and bottom) of the iridium rod are connected with the chuck that is a rotatable component in the furnace. The EB gun, similar to the induction coil, surrounds the 5

iridium rod and is adjusted at a specific initial height. ——Establishment of vacuum environment. Checking the sealing parts, starting up the water-cooled system, closing the furnace door and pumping the vacuum. The vacuum in the furnace chamber can be evacuated to ~10-4 Pa after 2 hours using a molecular pump. ——EBFZM of iridium rod. Raising the power of the EB gun to a preset value, which is processed in slow steps. When the melting zone reaches a steady state, moving the EB gun upward at a controlled velocity. Importantly, in the whole EBFZM process, a stable melting zone is required, that is, the melt must not collapse, and this can be realized by optimizing the power and the moving velocity during EBFZM. ——Experiment end. Turning off the power of the EB gun. Meanwhile, keeping the water-cooled system and vacuum system working, for the experimental security. After about 2 hours cooling, recovering the atmosphere in the chamber, opening the door and removing the directionally solidified iridium rod.

2.2 Sample characterization The samples were wire-cut from the iridium rod and polished on the cross-section. Then the iridium samples were processed for metallographic observation after electrolytic corrosion. The etching solution is composed of 100 ml saturated NaCl solution and 3~5 g HF. The iridium samples were treated as the anode and graphite rod was the cathode. The electrolytic corrosion was performed under a voltage of 15~25 v and a current of 2~3 A. Further, an optical microscopy (GX71-OLYMPUS) was employed to analyze the microstructure of the iridium crystals.

3.

Modelling of EBFZM

3.1 Physical FE model Based on the physical process of EBFZM of iridium, the FE model was established and conducted 6

by FORTRAN programming. As seen in Fig. 2(a), the iridium rod is 200 mm in height and 20 mm in diameter. Note that the actual diameter in simulation would be changed according to the parameter adjustment. The grid size in the FE model is 1 mm × 1 mm. The EB gun, which is a movable heating source, is set to 20 mm as the initial position. The heating range of the EB gun in height is 6.0 mm according to the actual condition. Fig. 2(b) shows the calculation process of EBFZM. It can be seen in the flow chart that there are several aspects dominating the calculation path. Firstly, the flow field will be calculated once the local temperature exceeds the melting point. Then the governing equations involving convection heat transfer are applied. Secondly, it is considered that the rod temperature reaches a steady state if the temperature change is less than 0.1 K between two adjacent steps. Subsequently, the heating source moves upward, that is, the boundary conditions related to the heating source are modified. Finally, the programming will terminate when reaching the preset time.

3.2 Physical parameters The melting point (Tm) of pure Iridium is 2720 K in programming. The density of solid Iridium at room temperature is 22.56 g/cm3, while the liquid density is a temperature-dependent variable, which was expressed by the equation  (T )  19.5 103  0.85(T  Tm )(kg  m-3 ) based on the results from Ishikawa et al. (2005). Further, the thermal conductivity and the molar heat capacity of pure iridium are presented in Fig. 3 (a) and (b), as reported in (Savit︠s︡kiĭ and Savin, 1978) and (Cagran and Pottlacher, 2006). Due to lack of thermophysical parameters of Iridium at high temperature, the values beyond the temperature range in Fig.3 were obtained by the fitting and interpolating calculation based on the existing data. For simplifying the FE model and saving time, the emissivity and the melt viscosity were considered as constant that are 0.35 and 6.0×10-3 Pa·s respectively, which referring to (Cagran 7

and Pottlacher, 2006) and Ishikawa et al. (2012). In addition, the thermal expansion and contraction in heating process were neglected.

3.3 Governing equations and boundary conditions The initial temperature of the iridium rod was set as 20 °C before heating. The temperature of the iridium rod gradually increases under the action of EB heating. The circular EB gun with the height range of 6.0 mm is the only heating source in the numerical model. In the axisymmetric two-dimensional model, the heating power is transferred to heat flux on the surface (the height range of the EB gun). So the relationship between the heating power and the heat flux can be expressed as: q  P /d

(1)

where q, P and d in equation (1) represent the heat flux, heating power and rod diameter respectively. The high-temperature surface originating from the EB heating source will cause the temperature gradient in the iridium and heat transfer occurs from the high temperature region to the ends of the iridium rod. Kermanpur et al. (2011) established a FE model to calculate physical fields in the levitation melting of metals. Based on that, the transition heat transfer in iridium rod is governed by:

 c T / t     2T / x 2   2T / y 2 

(2)

where ρ, c, T, t and λ in equation (2) are the iridium density, the molar heat capacity, the temperature, the time and the thermal conductivity respectively. When the melting zone forms, the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) method was applied to solve the Navier–Stokes equation, so the temperature field coupled with the flow field can be calculated. The surrounding temperature is set as a constant (20 °C), and thermal radiation on the rod surface will occur once the rod temperature exceeds 20 °C. Yang et al. (2013) applied a governing equation to express the heat exchange caused by thermal radiation during directional solidification:

 T / n     b T 4  Ts4   0

(3) 8

where n, ξ, σb and Ts in equation (3) are the normal vector, the emissivity, the Stefan-Boltzmann constant (5.67 ×108 W/m2·k4) and the surrounding temperature. Among these, ξ represents the radiation from the iridium surface to the surrounding furnace.

4.

Results and discussion

4.1 Temperature change in heating process When the rod diameter is 20 mm and the heating power is 6.0 kW, the temperature rise at points 1 to 6 (as marked in Fig. 2) is illustrated in Fig. 4 (a). At the initial stage, the temperature at those positions near the heating source rapidly increases, as revealed at points 1 and 2. When the heating time is 40 s, the temperature reaches about 2000 °C and 1500 °C respectively. With the action of thermal conduction, the heat gradually transfers from the heating source to the far side, resulting in the slow rise of temperature at the ends of the iridium rod. It can be found that the temperature at point 4 only rises to 130 °C when heating to 130 s, which is considerably lower than at points 1 and 2. Then the temperature increase gradually slows down, especially for the region around the heating source. Study in Yang et al. (2016) showed that it was caused by the intensive heat radiation on the directionally solidified rod. As the EB heating is prolonged, at the time of 120～125 s, point 1 first reaches the melting point and liquid iridium forms. It is worth noting that the rate of increase is considerably slow during the period 200～500 s and the temperature at each point basically reaches the steady value after 1000 s, which means that the temperature distribution in the whole iridium rod is in a dynamic equilibrium state. The absorbing power is equal to the radiation power, which is proportional to the fourth-order temperature in the iridium rod, as expressed mathematically in Yang et al. (2013). Under the output power of 6.0 kW, the steady temperatures of points 1 to 6 are 2913 °C, 2411 °C, 917 °C, 1625 °C, 1030 °C and 490 °C respectively. The temperature at point 3 is considerably low, even though the position is close to the heating source, which indicates that a massive amount of heat is taken away 9

by the boundary radiation. Further, the height (x=10 mm)–temperature–time relationship is presented in Fig. 4(b). During the initial tens of seconds, the temperature along the central axis rapidly rises, especially within the height range of the heating source (temperature rise appears clearly at the height of about 25 mm). Due to the temperature gradient from the heating source to the rod ends, the heating temperature dramatically decreases towards both ends of the iridium rod. The temperature at the far end (point 6) is higher than that at the near end (point 3) at each moment. With the enhancing of heat radiation, the centre temperature tends to be stable under a given power. Actually, the highest temperature along the centre axis only rises by 5.9 %, from 400 s (2400 °C) to 1500 s (2543 °C). Therefore, it can be predicted that the temperature change at each point in the iridium rod exhibits a similar rule, that is, a rapid rise at the early stage and gradually reaching steady state.

4.2 Temperature distribution and solidification interface 4.2.1

Effect of heating power

The steady temperature distribution in the iridium rod under different heating powers is displayed in Fig. 5. The black dotted line in Fig. 5 represents the S/L interface, as well as in Figs. 7, 9 and 10. The highest temperature appears on the surface at the height of the EB gun. When the power is 5.0 kW, the iridium rod begins to melt and only a few portions of material exceed the melting point, as seen in Fig. 5(a). A small melting zone forms under the power of 5.5 kW, and in this case, the power is not high enough to melt the centre material and the deepest melting zone is about 5 mm. The zone melting of iridium cannot be conducted with an impenetrable melting zone. A full melting at the section area is the basis for the crystal growth and the orientation control in EBFZM, as well as in other directional solidification processes. When the power further rises to 6.0 kW, as shown in Fig. 5(c), a melting zone with the height of 20 mm emerges and the S/L interface is convex. It can be acknowledged that a slight convex S/L 10

interface can accelerate the competition growth and eliminate these grains with non-preferred orientations, which would lead to the formation of single crystals. Fig. 5(d) demonstrates that the height of the melting zone rises to 90 mm under the power of 7.0 kW. The morphology of the S/L interface is still convex and smoother than that in the case of 6.0 kW. It can be speculated that the iridium melt will collapse under an overlarge melting zone, because the hydrostatic pressure of the iridium melt cannot be balanced with its surface tension and the EB pressure. The results in Zee et al. (2001) indicated that float melting with a focused EB is an effective method to produce refractory metal single-crystals, and the melting and solidification should be in a controlled manner which is highly power-sensitive. The relationship between the heating power and the steady temperature along the specific directions is revealed in Fig. 6. First, the steady temperature at each point significantly increases with the increase of the heating power. When the power increases from 4.0 to 7.0 kW, the highest steady temperature along the axial and the radial direction clearly increases from 1830 °C to 2790 °C and 2065 °C to 3093 °C respectively. By contrast, the steady temperature change at the end of the iridium rod presents just a small rise. As shown in Fig. 6 (a), the temperature increase is only 90 °C (from 835 °C to 925 °C) and 112 °C (from 452 °C to 564 °C) for points 3 (y=0 mm) and 6 (y=200 mm) when the power increases from 4.0 kW to 7.0 kW. Secondly, there is a critical power affecting the melting range in the iridium rod. For the temperature distribution in the centre axial direction, no melting zone forms under the power of 5.5 kW, which has been verified in Fig. 5. The centre material begins to melt when the power is over 5.5 kW and further enlarges with increase of the power. Similarly, the rule can be also identified in Fig. 6 (b) that no melt exists in the case of 4.0 kW, while each position is in the melt state in the radial direction when the power is 7.0 kW. At the same height, the centre temperature is 15～18 % lower than that on the heated surface.

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4.2.2

Effect of rod diameter

The diameter of the iridium rod is also an important factor influencing the temperature distribution in EBFZM. The temperature field at the steady state of the iridium rod with different diameters is displayed in Fig. 7. As can be found in Fig. 7(a), only 2.8 kW is required to melt the iridium rod; however, in this case the melting zone is incomplete, that is, the central material is still solid. With a slight rise of heating power, the complete melting pool forms and the melt height is 20 mm, as plotted in Fig. 7 (b). Li et al. (2015) studied the effect of growth angle on the floating zone stability of high-temperature pure metals, founding that the melt volume should be carefully controlled by balancing with the surface tension and EB pressure. So the processing window in EBFZM is very narrow for those iridium rods with a small diameter. By comparing Fig. 7 (c) and Fig. 7 (a), the required power for melting the iridium rod is almost tripled when the diameter enlarges from 10 mm to 30 mm. Again, Göbel et al. (1986) calculated the temperature field in a floating melting of molybdenum (30 mm in diameter), indicating that both the lower and upper interface morphology is moderately convex in the steady state. A complete melting zone with a diameter of 30 mm and height of about 25 mm appears under the power of 9.0 kW, as indicated in Fig. 7 (d). However, in this case the S/L interface is more convex than in Fig. 7(b), which means that the elimination process of stray crystals would be promoted. However, control of the crystal orientation is difficult under a curved S/L interface. The steady temperature at point 1 (surface) and point 2 (centre) under different parameters is presented in Fig. 8. For the same position, increased power should be applied to obtain the same steady temperature for the enlarged rod diameter. With a diameter of 10 mm, the temperature at point 1 just exceeds the melting point under the power of 2.8 kW. By contrast, it is about 6 kW for reaching the same state when the rod diameter is 25 mm. Also, the dotted lines in Fig. 8 demonstrated that under the same power, the steady temperature in a thin rod is higher than that in a thick rod, which is attributed to the increasing heat accumulation in the centre region. In addition, the 12

temperature gap between the surface and centre will increase with the enhanced power, and this can cause non-uniformity of the temperature in the radial direction.

4.3 Temperature evolution in EBFZM The formation of a complete melting zone with a suitable volume provides the initial temperature conditions for zone-melting of the iridium rod. The steady temperature contour of iridium under the power of 5.8 kW is plotted in Fig. 9 (a). Based on the analysis in Section 4.2.1, the required power for forming a complete melting pool should be higher than 5.5 kW, and the melt volume will be oversized if the power is increased beyond 6.0 kW. Therefore, the initial power of 5.8 kW is selected as the preheating parameter.

It is believed that the temperature in the iridium rod reaches the steady state at 1100 s and then the the EB gun starts to move upward. When the moving distance is 50 mm, the transition temperature fields under 1.0 mm/min, 10 mm/min and 60 mm/min are presented in Fig. 9 (b), Fig. 9 (c) and Fig. 9 (d) respectively. With increase of the moving rate, the temperature field in the melting zone changes significantly, which is considerably different from the initial state. First, the melting zone clearly enlarges and meanwhile the maximum temperature in the iridium melt increases. Under a low moving rate such as 1.0 mm/min, there is enough time for heat to accumulate in the iridium rod, and the heat cannot be taken away in time from the surface, so the temperature quickly rises with the upward movement of the EB gun. It can be predicted that under the same power, the farther the EB gun is from the rod ends (bottom or top), the higher the steady temperature is. Therefore, in order to heighten the stability of the melting zone, decreased power should be considered when the gun is moving. Secondly, the convex S/L interface under the initial state becomes a plane via an upward movement of 50 mm. This is because the distance from the S/L interface to the heating source 13

gradually increases, which indicates that the morphology of the S/L interface is barely influenced by the non-uniform temperature field around the heating source. The isothermals in Fig. 9 (b) and (c) indicate that the temperature distribution is more uniform in the radial direction near the ends of the iridium rod. When a quick moving rate is applied, such as 60 mm/min, the time for the temperature decrease is too short for solidification, therefore the position of the S/L interface below is relatively low compared with the cases of 1.0 mm/min and 10 mm/min. Moreover, the faster the moving rate is, the closer the heating source is to the upper S/L interface. The material beyond the height of 40 mm has not melted in the initial state and is gradually heated following the movement of the EB gun. As a result, massive melting appears at a low moving rate, as seen in Fig. 9 (b). By contrast, when the movement of the EB gun is fast, the upward expansion of the S/L interface will be lagging due to the inadequate heat transfer from the heating source. Further, to explore the temperature evolution in the Iridium rod in EBFZM, the temperature fields at different stages under the power of 5.8 kW and the moving rate of 60 mm/min were calculated, and the results are plotted in Fig. 10. By comparing Fig. 9 (a) and Fig. 10 (a), it can be found that the lower S/L interface becomes more planar after moving 20 mm and the whole melting zone slightly enlarges. With the upward movement of the heating source (EB gun), the volume of the molten zone continuously expands and the S/L interface is gradually farther away from the heating source (EB gun), which is beneficial to form a plane interface. As stated above, the rod temperature in the solidified Iridium rod decreases slowly (the heat absorption is generally faster than the thermal radiation during the moving process of EB gun), which accounts for the lagged upward movement of the solidification interface. On the other hand, once movement of the EB gun is fast, the heating time for the rod within the heating region is limited and the centre can not be fully melted, as demonstrated by Fig. 10 (c) and (d). In this case, studies in Fu et al. (2008) and Yang et al. (2013) indicated that the upper S/L interface in directional solidification becomes more and more concave, which would disturb the competitive growth among oriented grains and hinder the formation of 14

single crystal. Therefore, an over-quick movement of the EB gun in the zone melting of iridium rod should be avoided. 4.4 Optimized EBFZM of iridium crystal Based on the temperature calculation of EBFZM of iridium crystal, several processing parameters were under close control in the experiment, which was summarized as follows: (i) a preheated process that takes at least about 1000 seconds was required before EBFZM, which was to obtain a steady temperature field in the iridium rod; (ii) the applied heating power should exceed a critical value for melting and must not cause the collapse of the iridium melt, which indicated that an over-high power was not desirable. Also, the appropriate heating power would be heightened with the increase of the rod diameter; (iii) the temperature in the melting zone and the melt volume clearly increased once the heating source moves upward, which was not beneficial for stabilizing the iridium melt. So a relatively low power should be adopted, as discussed in Li et al. (2015); (iv) a massive amount of heat would gradually accumulate in the iridium rod during EBFZM, especially under a slow moving rate. The calculated results indicated that a moderately fast moving rate was suitable for the EBFZM. Some PGMs (platinum group metals) and refractory metal single crystals up to ～50 mm in diameter can be produced using EBFZM. As reported in Glebovsky et al. (1995), single-crystal Mo rod with the diameter beyond 30 mm was successfully prepared. However, for these heavy platinum group metals (Os, Ir, Pt), the density is too large (～22 g/cm3) to successful production. The maximum size is mainly limited by the heating power, the configuration of EB gun, the physical attributions of material, as well as the processing parameters. For the iridium rod with the diameter of 20 mm, we notice that the feasible processing window for melting iridium is considerably narrow due to the strict experimental conditions. The microstructure of as-cast iridium before EBFZM is presented in Fig. 11 (a). It was a typical block morphology with an average grain size of 800 μm. The grain boundary was irregular, which could be attributed to the 15

solidification and cooling process. The main applied processing parameters are listed below: (i) the preheated heating power was 5.5～6.0 kW; (ii) the processing power in EBFZM gradually decreased and was within the range of 4.0～5.5 kW; (iii) the moving rate of the electron gun was 10～30 mm/min, depending on the performing power and moving distance. Fig. 11 (b) shows the surface pattern of the iridium rod after solidification from EBFZM. The surface wrinkle layer with a thickness of 0.6～0.8 mm was caused by the upward gradual melting and solidification in EBFZM. Actually, in many levitating directional solidification, the regular wrinkle was an inevitable surface morphology and they were observed in Wang et al. (2015) and Cho et al. (1998). For the floating zone melting of Iridium rod, the hydrostatic pressure of the Iridium melt was balanced with its self-tension and the constraining pressure provided by electron beam. With the upward moving of EB gun and the continuous melting of Iridium rod, the local surface tension and constraining pressure can not provide the enough force to support the hydrostatic pressure, especially for the lower position of melting zone. Therefore, a small layer of Iridium melt collapsed downward and then rapidly solidified, as seen as the typical oscillation mark in Fig.11(b). In the current study the movement distance of the EB gun was 40 mm, and the cross-section microstructure of the iridium rod after EBFZM is presented in Fig. 10 (c). It can be clearly seen that these initial fine irregular grains evolved into few coarse grains with a size of 5～10 mm, which proved that massive grains were eliminated in the melting and competition growth. Only [100] orientation was detected in the preliminary XRD result, which indicated and that [100] may be the predominant orientation in the EBFZM of the Iridium rod. The detailed data and discussion involving competitive growth and orientation control will be presented in a separate work. The experimental results indicated that an iridium single crystal 20 mm in diameter will be successfully produced following the continuous movement of heating source. The preparation of iridium single crystals and the control of crystalline orientation will be studied in further work.

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5.

Conclusions (1) A FE model based on EBFZM was established and can be used to investigate the temperature

field in Iridium rod; (2) The temperature quickly increases and gradually reaches the steady state under the action of EM heating and thermal radiation; (3) The melting zone enlarges with the increase of heating power. The critical power for obtaining a complete melting zone in 20 mm-diameter Iridium rod is within 5.5 kW～6.0 kW; (4) With the upward movement of the heating source, the lower S/L interface becomes more planar and the melting zone significantly enlarges, especially under a slow moving rate; (5) An iridium rod 20 mm in diameter was successfully produced by EBFZM. The initial polycrystalline microstructure evolved into coarse grains, which indicated that the Iridium single crystal would form.

Acknowledgement The current study was financially supported by the Joint Funds of the National Natural Science Foundation of China (Grant NO. U1202273)

17

References Boehlert, C.J., Bingert, J.F., 2001. Microstructure, tensile, and creep behavior of O + BCC Ti2AlNb alloys processed using induction-float-zone melting. J. Mater. Process. Technol. 117, 400-408. Cagran, C., Pottlacher. G., 2006. Physical properties and normal spectral emissivity of iridium up to 3500K, 16th Symposium on Thermophysical Properties 1-12. Cawkwell, M.J., Nguyen-Manh, D., Woodward, C., Pettifor, D.G., Vitek, V., 2005. Origin of brittle cleavage in iridium. Science. 309(5737),1059-1062. Cho, Y.W., OH, Y.J., Chung, S.H., Shim, J.D., 1998. Mechanism of surface slab with rectangular quality improvement cold crucible mold in continuous cast, ISIJ International, 38(7): 723-729. Council N., 2001. Thermionics quo vadis?: An assessment of the DTRA's advanced thermionics research and development program, first ed., The National Academic Press, Washington. Czochralslri, B.C., 1974. Growth of oxide single crystals:iridium crucibles and their use. Platin. Met. Rev. 18(3), 86-91. Franco-Ferreira, E.A., Gerg, T.G., 1997. Long life radioisotopic power sources encapsulated in platinum metal alloys. Platin. Met. Rev. 41(4), 154-163. Fu, H.Z., Guo, J.J., Liu, L., Li,J.S., 2008. Directional solidification and processing of advanced materials. In: Fu, H.Z., (Ed), Directional solidification of single phase alloy and interface stability. Science Press, Beijing, pp. 223-275. Glebovsky, V.G., Semenov, V.N., 1995. Growing single crystals of high-purity refractory metals by electron-beam zone melting. High Temp. Mater. Processes. 14(2), 121-130. Göbel, R., Jurisch, M., Löser, W., Martuzāne, E., Martuzāns, B., 1986. Temperature distribution of a floating zone in electron beam zone melting of molybdenum. Cryst. Res. Technol. 21(8), 1015-1022. Hunt, L.B., 1987.A history of Iridium: Overcoming the difficulties of melting and fabrication. Platin. 18

Met. Rev. 31(1), 32-41. Ishikawa, T., Paradis, P. F., Fujii, R., Saita, Y., Yoda, S., 2005. Thermophysical Property Measurements of Liquid and Supercooled Iridium by Containerless Methods. Int. J. Thermophys. 26(3), 893-904. Ishikawa, T., Paradis, P. F., Okada, J. T., Watanabe, Y., 2012. Viscosity measurements of molten refractory metals using an electrostatic levitator. Meas. Sci. Technol. 23(23), 91-95. Kermanpur, A., Jafari, M., Vaghayenegar, M., 2011. Electromagnetic-thermal coupled simulation of levitation melting of metals. J. Mater. Process. Technol. 211, 222-229. Li, S., Geng, Z., Hu, R., Liu, Y., Luo, X., 2015. Effect of growth angle and solidification rate on the floating zone stability for processing of high-temperature pure metals. Acta metall. Sin. 51(1), 114-120. Li, S., Geng, Z., Hu, R., Liu, Y., Luo, X., 2015. Effects of growth angle and solidification rate on crystal growth of precious metal prepared by electron beam floating zone method. Acta Metall. Sin. 64, 292-299. Panfilov, P., Yermakov, A., Antonova, O.V., Pilyugin, V.P., 2009. Plastic deformation of polycrystalline iridium at room temperature. Platin. Met. Rev. 53(3), 138-146. Savit︠s︡kiĭ, E. M., Savin, I. V., 1978. Physical metallurgy of platinum metals, first ed., Pergamon Press, Moscow, 33(2). Schreck, M., Roll, H., Stritzker, B., 1999. Diamond/Ir/SrTiO3: A material combination for improved heteroepitaxial diamond films. Appl. Phys. Lett. 74(5), 650-652. Szeliga, D., Kubiak K., Sieniawski, J., 2016. Control of liquidus isotherm shape during solidification of Ni-based superalloy of single crystal platforms. J. Mater. Process. Technol. 234, 18-26. Wang, Y.L.,

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Yang, J.R., Chen, R.R., Ding, H.S., Guo, J.J., Han, J.C., Fu, H.Z., 2013. Thermal characteristics of induction heating in cold crucible used for directional solidification. Appl. Therm. Eng. 59(1–2), 69-76. Yang, J.R., Chen, R.R., Ding, H.S., Guo, J.J., Su, Y.Q., Fu, H.Z., 2013. Flow field and its effect on microstructure in cold crucible directional solidification of Nb containing TiAl alloy. J. Mater. Process. Technol. 213, 1355-1363. Yang, J.R., Chen, R.R., Guo, J.J., Fu, H.Z., 2016. Temperature distribution in bottomless electromagnetic cold crucible applied to directional solidification. Int. J. Heat Mass Transfer 100, 131-138. Yermakov, A., Panfilov, P., Adamesku, R., 1990. The main features of plastic deformation of Iridium single crystals. J. Mater. Sci. Lett. 9(6), 696-697. Zee, R.H., Xiao, Z., Chin, B.A., Liu, J., 2001. Processing of single crystals for high temperature applications. J. Mater. Process. Technol. 113(1), 75-80.

20

Furnace chamber

Melting zone

Control panel

EB gun Vacuum system

Fig. 1. High vacuum EBFZM equipment for preparing iridium single crystals Physical modeling

Rotating

Height of Iridium rod (mm)

6

5

FE mesh and material parameters

Chuck Iridium rod

Governing equations and boundary conditions One step

One step

Temperature calculation

4

No

Meshed element Steady judgment Yes

Moving

6 mm

Melting judgment Yes

Gun moving

Flow calculation

1 2 3

No

Electron gun

No

Time judgment Yes

(a)

Rod diameter (mm)

(b)

END

Fig. 2. FE modelling of EBFZM of iridium: (a) meshed FE model and schematic diagram; (b) calculation process.

21

-1

140

120

100

80

(a) 60

40

-1

-1

Heat Capacity (Cp / J•mol •K )

50

-1

Thermal conductivity (W•m •K )

160

0

500

1000

1500

2000

2500

3000

3500

30 20 10

(b)

0 0

Temperature (K)

500

1000

1500

2000

2500

Temperature (K)

Fig. 3. Temperature-dependent physical properties of Iridium: (a) Thermal conductivity; (b) Molar heat capacity.

3000

2000

Liquid Point 1 (1, 23)

Temperature ( C)

2500

o

o

Temperature of feature points ( C)

2600 2400

Point 2 (10, 23) 2000 Point 4 (10, 100) 1500 Point 5 (10, 150) 1000 Point 3 (10, 1)

(a)

0 0

200

400

Point 6 (10, 200)

600

Heating time (s)

800

1000

1200 800 400

50 s

1500 s 800 s 400 s 200 s 100 s

200

500

Solid

1600

1200

150 r h 100 eig 50 ht (m m)

1000 100 (s) me i t g tin

10 s

Ba

(b)

2s

0

2

10

a He

Fig. 4. Temperature rising and reaching steady state at feature points in iridium rod: (a) temperature rising under 6.0 kW; (b) transition temperature distribution at different heating times.

22

220 (a)

200

(c)

(b)

(d)

Height of Iridium rod (mm)

180 160 140 120 100 80 60

Melting zone

40

Melt

20 0 0

20 40 60 80 100 120 140 Diameter of Iridium rod (mm)

Fig. 5. Effect of heating power on the temperature distribution of iridium rod (rod diameter 20 mm): (a) 5.0 kW; (b) 5.5 kW; (c) 6.0 kW; (d) 7.0 kW 3000

3200

Liquid

3000

Steady temperature ( C)

2400

o

o

Steady temperature ( C)

2700

2100

6 kW

Solid 7 kW

1800

4 kW

1500 1200 900

5.5 kW 5 kW

600 0

25

50

75

100

125

7 kW

2600 2400

6 kW

2200

5.5 kW 5 kW

2000 1800

300

(a)

Liquid 2800

150

175

200

Height of Iridium bar (mm) (x=10 mm)

4 kW

Solid

1600

(b)

0

2

4

6

8

10

12

14

16

18

20

Diameter of Iridium bar (mm) (y=23 mm)

Fig. 6. Temperature distribution along different directions in iridium rod (rod diameter 20 mm): (a) centre axis; (b) lateral direction (height 23 mm)

23

220 200

(a)

(c)

(b)

(d)

Height of Iridium rod (mm)

180 160 140 120 100 80 60

Melting zone

40 20 0 0

30 60 90 120 150 Diameter of Iridium rod (mm)

Fig. 7. Effect of rod diameter on the temperature distribution in iridium rod: (a) 2.8 kW; (b) 3.0 kW; (c) 8.0 kW; (d) 9.0 kW 3600

mm, P1 mm, P2 mm, P1 mm, P2

o

Steady temperature ( C)

3400 3200

mm, P1 mm, P2 mm, P1 mm, P2

mm, P1 mm, P2

3000

Liquid

2800 2600 2400 2200 2000

Solid

1800 1600

2

3

4

5

6

7

8

9

10

11

Heating power (kW)

Fig. 8. Diameter and power-dependent steady temperature at feature points

24

220 (a)

200

(b)

(d)

(c)

Height of Iridium rod (mm)

180 160 140 120 100 80 60 40

Moving

Melting zone

20 0 0

20 40 60 80 100 120 140 Diameter of Iridium rod (mm)

Fig. 9. Effect of moving rate on the temperature distribution in iridium rod under the heating power of 5.8 kW: (a) initial status; (b) 1.0 mm/min; (c) 10 mm/min; (d) 60 mm/min

25

220 (a)

(b)

(d)

(c)

200

Height of Iridium rod (mm)

180 160 140 120 100 80 Melting zone

60 40

Moving

20 0 0

20 40 60 80 100 120 140 Diameter of Iridium rod (mm)

Fig. 10. Evolution of temperature distribution during zone-melting of iridium crystal. The upward movement distance of the EB gun is: (a) 20 mm; (b) 40 mm; (c) 80 mm; (d) 100 mm.

26

Wrinkle layer

1 mm

(a)

(b)

(c)

2 mm

Fig. 11. EBFZM of iridium crystal: (a) initial microstructure on cross-section; (b) surface morphology of iridium rod after EBFZM; (c) cross-section microstructure after EBFZM.

27