Y Zhang

D Räbiger
email: d.raebiger@hzdr.de

V Galindo

S Franke

B Willers

Grain refinement in Al-Si alloys induced by applying electric currents during solidification

Keywords: Grain refinement, Al-Si alloys, electric current, electrovortex flows

The solidification of Al -7 wt. % Si alloys under the influence of electric current pulses (ECP) through two parallel electrodes at the melt surface is investigated. An effective grain refinement was found if the ECP is applied during the initial solidification period (nucleation and recalescence). The grain size can be gradually reduced, which is likely due to the remelting process of high-order dendrite arms in the mushy zone driven by solute fluctuation and promoted by thermal fluctuation. This fragmentation process is mainly driven by electromagnetically forced convection. The grain refinement does not require the formation of nuclei from a solidified shell near the electrodes, which would result in a grain rain inside the sample.

Introduction

High-performance structural alloys play an important role in the daily life of human beings, particularly in the automobile and aircraft industries. The application of lightweight materials such as aluminium or magnesium alloys can reduce the weight of vehicles as well as the consumption of energy, which in turn has a positive effect on the energy efficiency and CO 2 -emissions of vehicles. In addition, the safety and stability of vehicles hast to be guaranteed or even promoted. Hence, investigation and further development of high-performance structural alloys became an important research topic in academic and industry world, especially for the development of high-performance iron, aluminum, magnesium and titanium alloys [1][2][3][4]. Various techniques for enhancing the tensile strength can be applied to obtain high-performance structural alloys, such as the solid-solution strengthening, precipitation strengthening, secondary phase strengthening, strain hardening, grain refinement, etc. [5][6][7]. However, most of these treatments result in an undesired influence on the plasticity of alloys. This can be avoided by achieving grain refinement during the solidification process. Different approaches to achieve microstructures with finer grains have been developed, such as the technique of adding grain refiner [8], ultrasonic vibration [9], electromagnetic vibration [10], electromagnetic stirring (EMS) [11], and the so-called electric current pulse (ECP) technique [12][13][14][15]. The ECP technique considers a procedure where pulses of high electrical current are ducted through a solidifying metal alloy. However, the mechanism of grain refinement by ECP is not fully understood yet and remains a subject for discussion. Different reasons for grain refinement have been suggested, such as the fragmentation of dendrites induced due to Joule heating by the electric current, the reduction of the nucleation activity energy, or the break out and the transport of little grains from the boundary by the periodic Lorentz force. In particular, Liao et al. [12] conducted solidification experiments in liquid aluminum where current pulses were supplied through facing electrodes located at the bottom and the top of the sample. The authors assume that magnetic pressure pulses occur in the sample and separate a large number of dendrite fragments from the wall and the mushy zone, respectively, forming a so-called grain rain. From observations of significantly disturbed liquid metal surface in experiments with parallel electrodes installed through the free surface Li et al. [13] concluded that the periodic Lorentz force may create shock waves within the liquid. An essential effect of electric pulses on the nucleation rate is also suggested by Barnak et al. [14]. The authors make speculations about a reduction in the free energy difference between solid and liquid state and an increase in the liquid-solid interfacial energy, respectively. Another recent study revealed the significant role of melt convection for grain refinement [15]. It was demonstrated that the interaction of the applied current with its own induced magnetic field causes a significant electro-vortex flow in the solidifying liquid. This paper presents an experimental study which should contribute to a better understanding of the grain refining effect by ECP. Systematic experiments are conducted to investigate the influence of several process parameters as the thermal boundary conditions and the duration of ECP application during the solidification process.

Experimental set-up and procedure Fig. 1(a) shows the experimental setup which comprises a water cooling system, a double-walled cylindrical mould (diameter 50 mm), a temperature measurement system and an electric current power supply. The cooling system consists of a copper chill which was drained by water and kept at 20 °C. NiCr -Ni thermocouples were located at the different heights along the vertical axis (figure 1(a)) of the samples. Each thermocouple was embedded in an INCONEL (trademark of INCO Alloys International, Huntington, WV) sheath with outer diameter of 1.5 mm and covered by Al 2 O 3 tubes. Temperature data were acquired at a frequency of 5 Hz with a resolution of 0.01°C and a measuring error of ±1 °C. Two cylindrical stainless steel bars having a diameter 8 mm were used as electrodes. The parallel electrodes were dipped into the melt through the free surface with a distance of 36 mm and a penetration depth of 10 mm. The lateral surface of the electrodes is coated by an electrically insulating material (boron-nitride, BN) so that the electric current enters the liquid metal only through the front surface of the electrodes. Fig. 1(b) shows the waveform of the rectangular electric current pulses applied. In this paper we present results obtained at a current intensity of 480 A (I 0 ), a frequency of 200 Hz (f) and a pulse length of 0.5 ms (t p ). An Al -7 wt. % Si hypoeutectic alloy was prepared using pure Al (99.99 wt. %) and Si (99.999 wt. %). The alloy was modified by Sr of 200 ppm in a clay-graphite crucible. For sample preparation the molten alloy was poured into a stainless mould of 50mm diameter and solidified. Each sample was cut at the same weight of 310 g. The preparatory sample was inserted into the double walled mould and heated up to 750 °C in a resistance furnace. After a holding time of 45mins the temperature was reduced from 750 °C to 720 °C which has been kept constant for another 30mins. Then, the mould with the melt was taken from the furnace and covered with a stainless steel lid. Both thermocouples and electrodes are fixed at the lid. The electrodes were preheated to 700 °C in order to avoid an immediate solidification at the electrodes. The electric current was applied during different stages of the cooling process. The sample was cut along the longitudinal section for metallographic analysis. One section was ground on SiC paper, etched in a solution of 60 mL HCl, 30 mL HNO 3 , 5 mL HF, and 5 mL H 2 O. A digital camera (Konica Minolta) was employed to obtain a photograph of the macrostructure of the longitudinal section. The other section was ground and polished from 6μm to 1μm, electro-etched of 50 mm × 30 mm in Barker etching reagent at 25 V, 15 Hz for 180 s, then examined by an optical microscope MeF4 (Leica Microsystems, Wetzlar). Polarized light was used for the quantitative analysis of the grain size. Image processing was carried out using the software Leica Application Suite V3.6 (Leica Microsystems). The grain size was measured by the linear intercept method in the software package aquinto a4i (Fa. Aquinto AG).

Results

The macrostructures of the solidified samples are presented in figure 2. The reference sample without applied ECP shows the vertical columnar structures aligned parallel with the temperature gradient. A columnar to equiaxed transition (CET) occurs at the middle of the sample and several coarse equiaxed grains become visible (figure 2(a)). A significant grain refinement of macrostructure is generated by the application of ECP, as shown in figure 2(b) and (c). The columnar growth is almost completely suppressed and finer equiaxed grains appear. While ECP was active during the entire solidification period in figure 2(b), the ECP treatment was switched off after the initial solidification period in figure 2(c), when a mushy zone with a significant solid fraction occurs. There are no remarkable differences between both samples with respect to the grain morphology. Li et. al [13] suggested a grain rain as the main mechanism for grain refinement by ECP, in particular, the electromagnetic force causes the detachment of a large number of nuclei from a thin solidified shell formed at the free surface. We conducted a special experiment to verify this assumption. The temperature of the free surface was controlled by a stainless steel block which was installed just above the melt surface at a distance of 7 mm. At first, the solidification was carried with a hot, preheated block in order to keep a high surface temperature and to avoid the formation of a solidified shell. Figure 3(a) shows cooling curves and the solidified macrostructure obtained from this experiment. The temperature of the sample with hot block measured at a height of 60mm (sample height is 65 mm) is about 618 °C at a point of time, when the greater part of the sample is already solidified. This temperature is higher as the liquidus temperature of about 615 °C. Thus, it is clear that the alloy is still liquid above 60 mm at the surface. Nevertheless, the macrostructure reveals a distinct grain refinement. The same experiment was repeated with a cold block at room temperature. The cooling curves in figure 3(b) shows a temperature of about 604 °C at a position of 60 mm. The temperature at higher positions is significantly lower which indicates the existence of a solidified layer at the surface. However, the comparison of the macrostructures dominated by equiaxed grains in both cases does not reveal a significant difference. The measurements of the grain size yield a mean value of 0.8 mm in both cases.

Obviously, the nuclei for equiaxed solidification do not necessarily originate from a solidified shell at the surface.

Conclusions

This paper presents some contributions to a better understanding of the grain refinement effect in Al -7 wt. % Si alloys with applied electric current pulse (ECP) technique.

(1) The application of ECP during the initial solidification period (nucleation and recalescence) is very efficient to achieve grain refinement in Al -7 wt. % Si alloys.

(2) The grain refinement does not require the formation of nuclei from a solidified shell near the electrodes, which would result in a grain rain inside the sample. Our results support the findings of Räbiger et al. [15] who suggested that the grain refinement can be ascribed to the forced melt flow driven by the Lorentz force. This convection covers the entire liquid zone and provokes increased fragmentation rates in the mushy zone growing from the bottom of the mould.

Fig. 1 :

 1 Fig.1: Schematic view of (a) the experimental setup, (b) typical waveform of an electric current pulse (ECP).

Fig. 2 :

 2 Fig. 2: Macrostructure in the longitudinal section of the solidified Al -7 wt. % Si alloy: (a) without ECP, (b) with ECP treatment, (c) ECP treatment during the initial solidified period.

Fig. 3 :

 3 Fig. 3: Cooling curves and macrostructures of Al -7 wt. % Si samples with ECP of I 0 = 480 A, f = 200 Hz, t P = 0.5 ms under different conditions: (a) hot block, (b) cold block (room temperature).

Acknowledgment

The research is supported by the German Helmholtz Association in form of the Helmholtz-Alliance "LIMTECH". Yunhu Zhang is very grateful to the support of China Scholarship Council.


Zhi-Long Zhao

Gao Jian-Jun

Liu Lin

The temperature effects of pulsed magnetic field on the directional solidification

Keywords: pulsed magnetic field, aluminum-silicon alloys, directional solidification, Joule heat

The pulsed magnetic fields (PMF) with magnetic intensities of 0.129~0.35 Tesla were respectively applied to two kinds of alloys of hypoeutectic Al-7% Si and eutectic Al-11.5% Si during directional solidification at the drawing rate of 130 μm/s. The influence of PMF on temperature gradients and the relative position of Solid/Liquid interface were investigated. The experimental results show that the temperature gradient ahead of solidifying interface was increased, and the width of mushy region in Al-7% Si alloy was decreased, and that the relative position of S/L interface in the Al-11.5% Si was dropped when the PMF was applied. The analysis suggested that the PMF would create more Joule heat in the liquid phase than that in solid phase, which effect will decrease the region of constitutional undercooling.

Introduction

With the aid of electromagnetic force produced by interaction between the electric current and magnetic field, electromagnetic techniques change the metal forming qualities and its internal microstructures by influence the melt flow, mass transfer and so on [1][2][3]; furthermore, the heat transfer and exchange which induced by the magnetic treatment can not be neglected during solidification. Pirmohammadi et al [4] approved that the heat transfer mechanisms within the enclosure depend strongly upon both the strength of the magnetic field as well as the Rayleigh number, while Lo [5] disclosed that the heat transfer rate is decreased with increasing the strength of the external magnetic field in the lower region of the Prandtl number. The PMF with a high strength can be produced by using discharging current of capacitors passing through solenoid coils, and then the magnetic strength will attenuate with a ultrahigh oscillating frequency within a certain lasting time. It has many advantages such as short relaxation time, strong magnetic penetration, and simple control process, etc., enriches the theory research of electromagnetic metallurgy, and indicates the application prospect in aspects of large ingot or casting solidification control. However, nowadays the mechanism of the PMF on the metal solidification process and the heat transfer are still not clear, and still lack systemic experimental works, therefore, based on the good controllable directional solidification technology this research work uses two compositions of aluminum silicon alloys, comparatively studies the effects of applying PMF on the temperature gradients, the change of solidifying interface position.

The experimental methods

The industrial Al-11.5%Si alloy and commercially pure aluminum (99.7%) were selected as raw materials. The alloy of Al-7wt%Si was blended by Al-11.5%Si and pure aluminum according to the composition proportion and melted in the crucible resistance furnace. The self-made experimental devices of magnetic pulse directional solidification with liquid quenching are illustrated in figure 1. The directional solidification of metallic test bars was realized by moving the heating furnace and cooling water jacket in the device. The pulling rate of 130m/s was determined by comparison of several experimental results to ensure the solid/liquid interface within the pulsed magnetic field generated between the magnetic dipoles during the directional solidification of molten metal. By regulating the discharge voltage to adjust the peak discharging current, thus the magnetic strength of PMF was changed. The experimental parameters are listed in table 1. No. 1#-5# samples are Al-7%Si alloys and No. 6#-10# samples are Al-11.5%Si alloys in the table. The alloy bar inserted into the corundum crucible of 7115 will be placed on the supporting rod. The furnace temperature was 900℃, after preserved the temperature for 20 minutes, the experiments were carried out according to the setting drawing velocities and peak strengths of PMF as listed in table 1. Two thermocouples 10 mm apart from each other were distributed into the samples. The temperature changes were real-time measured during solidification with the application of PMF. When the directional solidification entered the stable growth stage, the liquid-quenching was carried out. After being wiped and cleaned, the directional solidification test bar was cut for 10 mm length around the center line of liquid-solid interface to prepare the metallographic specimen. The mushy zone width of hypoeutectic samples was measured, and the position of S/L interface of eutectic samples was determined.

The experimental results and discussions

The effects of pulsed magnetic field on the temperature gradients ahead of the solidifying interface. Under the different magnetic strength of 0 T and 0.3 T, the cooling curves of two measuring temperature points with the interval of 10 mm apart were in real time obtained during the directional solidification, which shown in figure 2. It can be seen from the figure 2 that the PMF obviously influences the temperature distributions in the two kinds of Al-Si alloys during the solidification and intensifies the temperature fluctuation in the melt. The time difference, t, between the lower thermocouple which has been in the solidus temperature and upper thermocouple just entered the liquidus, can be obtained in the partial enlarged drawing of figure 2 (a), and (b). Therefore, the width of mushy region x can be calculated using the relation of x=10mm-t, in which  is the pulling velocity of furnace. The calculated results are listed in table 2. The width of mushy zone of Al-7%Si alloy can be actually measured on the surface of metallographic samples, which also are listed in the table 2. The results of mushy region width in table 2, whether calculated or actually measured, indicate a fact that the application of pulsed magnetic field can reduce the width of mushy region, and the width of mushy region decreases gradually with the increase of pulsed magnetic strength. Because the two thermocouples have a relatively fixed position, the temperature variation is a function of time during solidification, compare the two partial enlarge illustration in figure 2 (c), and (d), thus the temperature gradients ahead of solidifying interface can be calculated. The values of temperature gradients ahead of solid-liquid interface of eutectic sample are also listed in table 2. The temperature gradients in both alloys increase with the increasing of pulsed magnetic strength, which can be seen from table 2. The relative position of S/L interface of two kinds of Al-Si alloys under the effect of pulsed magnetic field Figure 3 shows a coordinate system is set up in the solidifying cell with a PMF. The origin of coordinates O lied in the intersection of upper surface of silicon-steel lamination core and axis of alundum tube. The direction of the X axes agrees with the directional solidification direction. The distances between S/L interface of Al-Si alloys and origin of coordinates O is x 0 , which indicates the relative position of S/L interface shown in figure 3. The values of x 0 are determined according to the position of S/L interface on the macroscopic surface of metallographic specimen after liquid-quenching and pulling duration to calculate. The variation of x 0 is shown in figure 4. It can be seen from figure 4 that x 0 increased with the increase of pulsed magnetic strength in the both Al-Si alloys during the solidification, which provide an evidence that the S/L interface move down when a PMF is applied, and the fall extent of S/L interface is

increased with the increasing of pulsed magnetic strength.

Analysis and discussion

Quoting the classical composition undercooling theory, the solute enrichment in front of solidifying interface causes liquidus temperature distribution from low to high, combining with the actual temperature gradient ahead of solid-liquid interface, so the constitutional supercooling zone is formed. Under the effect of PMF, the temperature gradient increases in both Al-Si alloys, the constitutional supercooling zone become narrow, which bring the reduce of the mushy region in the solidification of hypoeutectic Al-Si alloy and the stabilization of the solidifying growth interface in the eutectic Al-Si alloy. Moreover, the Joule heating effect of low frequency PMF on the metallic melts can not be overlooked. Because of the electrical resistivity of liquid phase superiors to solid phase during the solidification, the heating effects will mainly concentrate on the liquid phase, which promotes the temperature rise in melt, result in remelt of solid and move down the S/L interface. The electrical resistivity accords with the relation of

     T
, where  and  is temperature coefficients. L  and S  respectively refer to the electrical resistivities of liquid and solid. The L  is several times bigger than S  [6]. The temperature rise which caused with Joule heat can be calculated by

2 2 / cs t j T d    
, where j is induced current density, t is the time of applied PMF, d  is the material density, c is the specific heat capacity, s is sectional area. Thus, the temperature rise in liquid phase is several times bigger than that in solid phase, which increases temperature gradients ahead of S/L interface, decreases the width of mushy region. And the temperature rise effect of PMF brings about the move down of S/L interface.

Conclusion

The measuring works of the temperature fields and the relative position of S/L interface during directional solidification with a application of PMF were carried out in this work. The results show that temperature gradients ahead of solid-liquid interface are raised in two kinds of Al-Si alloys when a PMF is exerted during directional solidification, and that the temperature gradients are increased with enhancing the pulsed magnetic strength. The Joule heat induced by low frequency PMF increases the temperature difference in the liquid phase and solid phase. That is the reason of temperature gradients ahead of S/L interface increase, and brings about decline of the mushy region width of hypoeutectic Al-Si alloy. The relative position of S/L interface move down in the eutectic Al-Si alloys during directional solidification with exerting a PMF, is the another evidence of Joule heating effect of PMF.

Table 1 :Fig. 1 :

 11 Fig.1: Schematic diagram of directional solidification experimental device under the pulsed magnetic field.

Fig. 2 :

 2 Fig.2: The cooling curves of two kinds of Al-Si alloys.

Fig. 3 :Fig. 4 :

 34 Fig.3: The schematic diagram of coordinate system with the application of pulsed magnetic field.Fig.4: The relative position curves of S/L interface of two kinds of Al-Si alloys with different pulsed magnetic strength.

Table 2 :

 2 The relevant solidification parameters of two kinds of Al-Si alloys under different pulsed magnetic strength

	Number of samples	1#	2#	3#	4#	5#	6#
	Pulsed magnetic strength, B/T	0	0.247	0.3	0	0.247	0.3
	Calculated mushy region width,	6.88	5.13	4.54	-	-	-
	x/mm
	Real measured mushy region width,	7.0	5.9	4.9	-	-	-
	x/mm
	Temperature gradient ahead of S/L	4.63	6.19	6.41	5.13	5.22	8.25
	interface, G/℃mm -1

Acknowledgment

Financial support from the National Nature Science Funds (51374173, 51054007, and 50874093) is gratefully acknowledged.


J Pal
email: j.pal@hzdr.de

I Grants

S Eckert

G 1 Gerbeth

Physical model of Czochralski crystal growth in a horizontal magnetic field

Keywords: Czochralski Crystal Growth, Horizontal Magnetic Field, Flow Measurements

A low temperature liquid metal model of the Czochralski crystal growth process is considered experimentally under conditions of high aspect ratio. In this paper we focus on the influence of a horizontal magnetic field (HMF) on the radial flow field and present first results from related model experiments. The flow is measured by means of the ultrasound Doppler velocimetry (UDV).

Introduction

About 95% of the mono-crystalline silicon is produced by the Czochralski (CZ) method. In the process, a single seed crystal is attached to the free surface of molten high purity silicon in a cylindrical quartz crucible. Under rotation and simultaneous pulling of the seed the melt crystallizes and a mono-crystalline rod develops as the final product. This method is now routinely applied for the production of 300 mm diameter single crystals. Further size increase is certainly possible, though there are technical challenges to be solved. To grow a longer 450 mm crystal with the present equipment, a higher initial filling level would be needed. However, the melt instability is increasingly difficult to suppress by crucible/crystal rotation as the height of the melt is increased. The fluid flow inside the crucible serves as a carrier for mass and temperature and affects crucially the quality of the growing crystal. The horizontal magnetic field (HMF) may improve conditions in the melt during large silicon single crystal growth by the CZ technique. This observation might be counter-intuitive as the HMF evidently breaks the rotational symmetry. A previous study [1] has shown, that the HMF is not able to significantly delay the Rayleigh-Bénard (RB) instability in a rotating cylinder. It has been observed in a large aspect ratio rectangular box that an oscillating flow sets in soon after the linear onset [2]. We have started a longer-term research program to investigate the effect of an HMF on the thermally driven flow within a setup which has some striking similarities with the Czochralski (Cz) silicon crystal growth process. The research focuses on the development of the flow and temperature field under the influence of the asymmetrically applied magnetic field. The results should help for answering various questions, for instance, whether a stabilizing effect of the HMF on the flow can be expected in the Cz process or which consequences arises with respect to the mean heat flux expressed by the Nusselt number Nu(Ra,Ha) or regarding the local temperature field near the crystal edge. Besides, the ongoing experimental study is also considered as a benchmark for comparison with numerical codes. Therefore, to serve the latter purpose, the boundary conditions should be well defined. In the present paper we present first preliminary flow measurements from the ongoing measurement campaign.

Basic principles in modelling the Czochralski technique

The simplest model of the CZ facility might be a RB configuration, in particular a cylindrical cell heated from below and cooled at the top which is characterized by a height H and diameter 2R and adiabatic insulated side walls. Applying a temperature gradient T = T b -T t between the bottom and top sides, buoyant convection occurs when T exceeds some critical value. The dynamics of the mere thermally induced convection may be described by three control parameters. First of all the dimensionless Rayleigh number Ra is the crucial parameter in modelling buoyancy and describes its strength: Ra = gTH 3 /, where  is the thermal expansion coefficient ,  the kinematic viscosity,  the thermal diffusivity of the fluid, and g the gravitational acceleration. The second control parameter, the Prandtl number Pr, takes into account the heat transport within the fluid and is given by the ratio of the thickness of viscous and thermal boundary layers: Pr = . In general, molten metals and semiconductor melts are low Prandtl number fluids with Pr in the order of 10 -2 , which means that the heat diffuses quickly in comparison with the convective transport. The third control parameter, the aspect ratio a = H/(2R) concerns the geometry of the setup and affects crucially the developed convective pattern inside the melt. Owing to a, the variation of the convective patterns was studied numerically and experimentally in [3] for Pr  29; a single roll convective motion corresponding to the azimuthal wave number of m = 1 was found for a ≥ 0.63. This flow is often termed wind or large scale circulation (LSC) and is frequently observed in RB systems. More details on such wind can be found in [4] and references therein. For smaller a down to about 0.3 the authors of [3] observed m = 0, which is a torus and thus axisymmetric. The initial filling level in a real industrial CZ facility does not reach a = 1, it is even lower than a = 0.5 and decreases continuously during the process. One of the reasons not using high a up to 1 in practice might be the generation of non-axisymmetric modes implied by the RB configuration. The Hartmann number Ha=BL(/) 1/2 occurs as another control parameter if external magnetic fields are exposed to the system. Thereby is B the magnetic induction, L a typical length scale in the system, and  the electrical conductivity of the melt. The Hartmann number represents a measure for the ratio between the electromagnetic body force and the viscous force.

The experimental setup

The object of the investigation is a modified Rayleigh-Bénard (RB) configuration, a cylindrical melt column of variable aspect ratio homogeneously heated from below. The photo in Fig. 1 illustrates the experimental setup mounted between the HMF producing coils. As working fluid the ternary alloy GaInSn [5] was used because it remains liquid at room temperature and as distinguished from mercury it is non-poisonous. Moreover, its low Prandtl number is similar to that of molten silicon. The heating was realized by an electrical heating plate embedded in a massive copper disc to achieve isothermal boundary condition. Several thermocouples were installed inside the copper disc for monitoring its temperature. The upper thermal boundary condition in a Czochralski system is accounted for a partially cooled surface. The partial cooling in our experiment covers approximately the same fraction area as the crystal does in an industrial facility. It is realized with a circular heat exchanger (cold finger) mounted concentrically at the top of the experimental cell. The cold finger is optionally rotable. A precise control of the temperature of the cold finger is realized by supplying it with coolant fluid at high flow rate from a thermostat having a large reservoir. The latter is regulated by a PID circuit. The temperature of the cold finger is monitored at various positions. For the purpose to achieve adiabatic boundary conditions at the side walls a borosilicate glass pipe was chosen as experimental cell owing to its poor heat conductivity. Furthermore, the glass pipe allows the transmission of ultrasound for flow measurements. The drawing in Fig. 2 illustrates the cell and the adjustable holder for the ultrasound sensors. During the measurements, the apparatus was embedded in mineral wool to minimize the lateral heat loss. In the real Czochralski facility thermal radiation from the free hot silicon melt is a considerable heat sink. A distinctive feature of the present setup is the possibility to model the heat loss from the melt surface. The surface is therefore covered by an electrolyte layer which is cooled by a copper spiral completely immersed into this layer (cf. Fig. 1). Instead of pure water a weak aqueous solution of KOH was used as electrolyte to protect the GaInSn surface from oxidation. Flow velocities were measured by the UDV technique, the principle of operation is described in the pioneering work of [6]. Mainly two features render UDV predestinated for the present work. Firstly, it works for opaque media including liquid metals. Secondly, it allows the quasi-simultaneous measurement of an entire profile of the local velocity components in direction of the sound propagation along the ultrasonic beam. The readings of the 8 MHz transducers (type TR0805LS, Signal-Processing, Lausanne, Switzerland) were taken by a DOP2000 velocimeter (model 2032 from the same manufacturer).

Results

All measurements performed for the present paper were done under the same thermal boundary conditions. The bottom heater was always operated with a constant power of 1035 W. After a certain thermalization phase of the experimental cell a constant temperature gradient of T = 41 K was obtained between the heater and the cold finger, which corresponds to the Rayleigh number of Ra = 3.710 7 . The cell filling height was kept constant at H = 150 mm, which results in an aspect ratio of a=0.84. Hence, a single roll convection cell (wind) is expected. Six UDV sensors were positioned slightly below the liquid metal surface at different azimuthal positions to measure the radial flow velocity across the diameter. The sketch in Fig. 3 indicates these measurement positions. The connecting diameter 0°/180° is aligned perpendicular and the -90°/ +90° one parallel to the direction of the HMF. Fig. 4 shows velocity profiles measured without applied magnetic field. The findings suggest the occurrence of the wind in the meridional plane spanned by the positions -90°/+90°. Hence, the wind comes upward along the side wall to the top at +90°, crosses the cell along the diameter, descends at -90°, and closes along the diameter at the bottom. Concerning the flow structure a flow reversal behavior was observed. The large scale recirculation roll (wind) remains thereby as the main hallmark of the flow but it changes completely the flow direction. Such flow reversals for high Ra number under high aspect ratio conditions are well known in the literature, [7,8] to quote a few, and still under investigation. In the measurements done so far flow reversals were observed not only in the mere buoyant case but also for the case when the HMF was switched on. Fig. 7 summarizes measured velocity profiles in this cases.

Fig. 6: Velocity profiles at the azimuthal angle 0° where the flow is perpendicular to the HMF, and -45°, which is between the parallel and perpendicular one.

Conclusions and outlook

First flow measurements were performed in a laboratory CZ model using a low Prandtl number melt contained in a volume of high aspect ratio. The thermally driven fluid flow was exposed to a horizontal DC magnetic field. A strong damping of the flow was observed for the flow velocity parallel to the orientation of the HMF. Also flow reversals were observed during the measurements. Belonging to ongoing investigations, additional temperature measurements will be integrated in the vicinity to the rim of the crystal mockup (cold finger). Measurements as function of Ra, Nu, Ha, aspect ratio a, and crystal mockup rotation rate are on the way to investigate the questions pointed out in the Introduction and to compare the measurements with numerical simulations.

Fig. 1 :

 1 Fig. 1: Photo of the experimental setup mounted inside of the HMF coil system.

Fig. 2 :

 2 Fig. 2: The cell covering the liquid metal with its dimensions and the UDV sensor holder.

Fig. 3 :

 3 Fig. 3: Azimuthal positions of the UDV sensors where measurements were performed.

Fig. 4 :

 4 Fig. 4: Radial velocity profiles across the diameter measured at different azimuthal positions. A positive sign of the velocity indicates a flow direction moving away from the sensor.

Fig. 5 :

 5 Fig. 5: Velocity profiles at the azimuthal angle -90° for different feeding currents of the HMF coils.

Fig. 5

 5 Fig. 5 shows velocity profiles measured at -90° for the cases without and two different magnetic fields. The indicated coil feeding currents of I = 50/100 A correspond to the magnetic induction B = 29.3/59.5 mT, or Ha = 102/206. Even though the flow is aligned parallel to the HMF a strong damping in the flow velocity occurs, which is at the center (r = 89 mm) about 54% for Ha = 102 and 78% for Ha = 206. Fig. 6 displays the velocity profiles recorded at two other positions for Ha = 206. Firstly at = 0°, where the flow is perpendicular to the HMF, and, secondly at = -45°. A comparable velocity damping of about 83% and 73% occurs for the same Ha number at = -45° and = 0°, respectively. Even a partial change in the flow direction is observed at = 0° by applying a HMF with Ha = 206.

Fig. 7 :

 7 Fig. 7: Velocity profiles measured without (R#1) and with (R#2) occurred flow reversal.

Acknowledgment

Financial support of this research by the German Helmholtz Association in the frame of the Helmholtz-Alliance LIM-TECH is gratefully acknowledged.


J C Jie

H Chen

Y B Zhang

T M Wang

T J Li

Grain refinement mechanism of pure aluminum by application of pulsed magnetic field during solidification

Keywords: Pulsed magnetic field, Commercial purity Al, Grain refinement, Forced convection

In the present study, the effect of pulsed magnetic field (PMF) on the solidification structure of commercial purity Al was investigated. The results clearly show that the developed columnar grain is formed opposite to the heat release direction without PMF. However, a novel solidification structure, i.e. a refined equiaxed grain layer with a smooth interface inside the large columnar dendrites, is formed due to the application of PMF. The formation mechanism is ascribed to the agitation Lorentz force acting on the sample, the forced liquid flow and the crystal growth.

Introduction

Fine crystalline grain sizes enhance most mechanical properties in engineering alloys and consequently much effort is expended in trying to understand and control alloy grain size. Many methods can be used to refine the grain size of alloy, including adding inoculants [1][2], application of electromagnetic field including rotating or travelling magnetic field [3], supersonic field [4][5] or other methods etc. Of the intrinsic methods, electromagnetic stirring is the one of the most mature, have been applied in the continuous casting of steel and aluminum. The effect of a pulsed magnetic field (PMF) on the solidification of alloys or pure metal, especially on the grain refinement, has been confirmed by many investigations [6][7][8][9][10][11][12][13][14]. Many experiments and numerical simulations of this process have been carried out in recent years. For the case of alloys, the effect of PMF on the solidification structure was deeply investigated. Recently, it was found that the application of a PMF to an array of Al-15wt.%Cu dendrites led to an increase in dendrite fragmentation rate observed by synchrotron X-ray imaging. This phenomenon can be ascribed to instabilities in the local temperatureconcentration-curvature equilibrium and the rapid remelting of the most vulnerable microregions [12]. For pure metal, however, the grain refinement mechanism by PMF is still not clear. Not enough solute atoms exist to introduce the solute undercooling, which is helpful to dendrite fragmentation and then grain multiplication. Hence, some other mechanisms should be responsible for the grain refinement. For example, Zhao el at. ascribed the grain refinement to the forced convection induced by PMF which may encourage a thermal homogenization of the liquid zone ahead of the solidification front [15]. Understanding of the refining mechanism is still limited to phenomena where solid proof from experiments is absent. Although synchrotron X-ray imaging is a powerful method to in situ observe the solidification process which may uncover the refinement mechanism, it cannot be used for pure Al due to the similar densities between liquid and solid Al. In addition, the thickness of sample for synchrotron X-ray imaging analysis is only about 120-200 μm. The bulk convection during solidification is restrained which is very different from the actual situation. Hence, in the present study, we investigated the effect of self-prepared PMF on the grain size of commercial purity Al, and the corresponding refining and controlling mechanisms were also discussed.

Experimental

The commercial purity Al used in the present study was 99.76 wt.% and 0.18 wt.%Fe. The PMF was induced by a coil of 80 circles connected with a self-prepared power based on the principle of the discharge control of capacity into coils. The current I used was 500 A and the frequency was 10 Hz, and was measured to be sine wave which can be written as

5 0 0 s in 5 0 0 π ( 0 2 m s )    I t t
. Pure Al was heated to 720 o C in a resistance furnace. Then the liquid melt was poured into a steel pipe with an inner diameter of 45 mm and height of 90 mm which was placed on a water-cooled copper mold, and then placed into the PMF for different solidification times. The steel pipe was surrounded by three layers of asbestos to prevent the Al melt from solidification at the sidewall of mold, and was temperature hold in a heat treatment furnace at 700 o C. It should be noted that the cooling rates were different at various positions under different conditions due to the introduction of PMF. Specimens collected in this study were all cut across longitudinal section for macrostructural characterization in order to avoid experiment error. Then the specimens were ground and polished using standard metallographic procedures for macrostructure examination. The etched reagent for macrostructure was a mixed acid of 75 ml HCl, 25 ml HNO 3 and 5 ml HF. The grain size measurements were carried out by mean linear interception method using image analyzer.

Fig. 1 Schematic illustration of experimental setup

Results and discussion

The macrostructures of pure Al solidified under PMF with 500 A and 10 Hz for different times are shown in Fig. 2. In the case of solidification without PMF, the developed columnar grains start to grow vertically from the bottom of the mold in the opposite direction of the heat flow (see Fig. 2a). However, when the pure Al solidified on a water-cooling copper mold for 30s and then solidified under PMF with 500 A and 10 Hz for 20 s (see Fig. 2b), a novel solidification structure is formed that a refined equiaxed grain layer with grain size of 0.4 mm locates between the two layers which is composed of columnar dendrites. When the PMF time is extended to 40 s, the thickness of refined grain layer becomes larger and also a columnar grain layer is formed at the top of the sample. convection in the solidifying alloy melt [16][17][18]. Currently, the preferred scenario is the remelting of the necks of individual dendrite arms, which is based on the convective transport of solute and/or heat into the mushy zone that has been observed in transparent model substances [19] and in metal alloys [20]. However, for the case of commercial purity Al with little impurities solidified under electromagnetic field, the situation may be different because a little solute enrichment will segregate in front of the solid-liquid interface, and the mechanism is needed to be clarified and investigated. When the power is switched on, a pulsed current goes through the coils due to the discharge of the capacity, resulting in the introduction of PMF. Hence, an induced current is formed in the Al melt due to the Lenz law, and an induced Lorentz force in the melt can be given as [START_REF] Asai | Electromagnetic Processing of Materials[END_REF]:

 F J B (1)

   JB

(2) Then the time-averaged electromagnetic force per period is expressed as:

2 ( / 2 ) e x p ( 2 / )   F B R



(3) where J is the current density, B is the magnetic flux intensity, μ is magnetic permeability, δ is the electromagnetic skin depth which can be written as

1 / 2 ()       .
In the present study, the electromagnetic skin depth is estimated to be 2.4 mm. The above equation suggests that the time-averaged electromagnetic force reduces as an exponential trend. Furthermore, the induced Lorentz force is an agitation force acdcording to the calculated results (which will be published elsewhere), and can introduce a pressure gradient into the melt which results in a forced melt flow. During the crystal grow process, the temperature always plays an important role. As shown in Fig. 2a, the columnar grains are formed due to a vertical temperature gradient thanks to the use of a cooling copper mold. However, under PMF, the forced convection induced by the PMF encourages the thermal homogenization of the liquid zone, and the growth condition of the columnar grains is restricted. Thus, the refined equiaxed grain is formed rather than the columnar grain structure, as shown in the interlayer of Fig. 2b-c. When the power of PMF is switched off, the vertical temperature gradient is re-built. The equiaxed grains with a good growth condition or crystal orientation will grow to be the columnar again, final resulting in the formation of three-layer structure. The present study is beneficial to the investigation of columnar-equiaxed-transition (CET).

Conclusions

In the present study, the effect of pulsed magnetic field (PMF) on the solidification structure of commercial purity Al was investigated through a series of experiments. The results clearly shows that the developed columnar grain formed opposite to the heat release direction can change to be the refined equiaxed grains. However, a novel solidification structure, which a refined equiaxed grain layer with a smooth interface inside the large columnar dendrites, is formed due to the application of PMF. The corresponding formation mechanism is ascribed to the agitation Lorentz force acting on the surface of sample and the forced liquid flow.

Fig. 2

 2 Fig. 2 Pure Al solidified on a water-cooling copper mold for 30s and then solidified under PMF with 500 A and 10 Hz for different times (a) 0 s, (b) 20 s, (c) 40 sIn the present study, the formation of refined grain structure is achieved by the introduction of PMF with its intrinsic character, such as the introduction of forced convection, vibration, heat transfer, and Joule heat. Various investigations have demonstrated that a pronounced decrease in grain size can be achieved with increasing intensity of forced

Acknowledgement

The authors gratefully acknowledge the support of the Natural Science Foundation of China (No. 51471042, 51271042), and the Fundamental Research Funds for the Central Universities of China (DUT14RC(4)13).


Q C Zou

J C Jie

Y Dong

T J Li

Effect of Sn addition on the electromagnetic separation of primary Si from Al-30Si alloy during solidification

Keywords: Separation, Rotating magnetic field, Tin, Al-Si alloy

In the present study, the effect of Sn addition on separation of the primary Si from Al-30 at.% Si melt under rotating magnetic field (RMF) was investigated. The results clearly show that the application of RMF during solidification of alloy can separate the primary Si from the Al-Si-Sn melt and accumulate on the periphery of the ingots to form a Si-rich layer. Meanwhile, the content of primary Si in Si-rich layer increases with the increasing of Sn content. The separation mechanism is ascribed to a complex coupling process of the cooling condition, the flow field and the crystal growth. In addition, the Sn addition tends to enlarge the crystallization temperature range and enhance the separation efficiency.

Introduction

The massive and low-cost solar grade silicon (SOG-Si) feedstock is essential for the widespread use of solar cells. Normally, the photovoltaic (PV) industry is dependent on the high-cost semiconductor-grade silicon (SEG-Si) that is produced by the Siemens technology. To reduce the cost of raw materials, some low-cost technologies for SOG-Si production should be proposed. Metallurgical technologies [1], [2], and [3] are considered as a candidate to produce low-cost SOG-Si by removing the impurities from the metallurgical grade silicon (MG-Si). However, those methods still require long solidification processing time and high temperature. Recently, a novel metallurgical process, namely, the alloy solidification refining at low temperature has been proposed in Japan [4]. During alloy solidification process, the primary Si initially precipitates from supersaturated melt with decreasing temperature, during which more impurities, especially B and P, are enriched in the melt because of the strong segregation behavior [5] and [6], thereby gaining high-purity primary Si. Al [4][5][6], Cu [7], Fe [8], Sn [9,10], Na [11], etc. alloyed with silicon, can be used as solvent metals. Among, the hypereutectic Al-Si alloy has the advantages of low liquidus temperature, high solubility of Si in Al, and single eutectics without complex compound. Morita et al. measured the segregation coefficients of impurity elements decrease with the dropping temperature, especially segregation coefficient of B between primary Si and Al-Si melt is determined to be 0.49 (1473 K), 0.32 (1373 K), and 0.22 (1273 K) [12]. This result indicates that the Al-Si alloy is more useful for B removal at low temperature. Similarly, the Sn-Si alloy also has a simple binary eutectic phase diagram with limited solubility of tin in silicon, and contains no intermetallic compound. Besides, the moderate Sn-Si liquidus slope can produce a large amount of silicon precipitate with decreasing temperature, and the growth rate of silicon from Si-Sn alloy melt increased compared with it from Al-Si alloy melt [9,10]. Therefore, Sn based alloy is beneficial for silicon growth, and Al based alloy favors the removal of B. Preliminary studies in our group have found that application of rotating magnetic field (RMF) during solidification of Al-30Si alloy can separate the primary Si phase to the inner wall of crucible with 65~69 wt.% Si content, which supplies a solution to the collect problem of high-purity primary Si from Al-Si alloy [13]. However, the effect of Sn addition on the electromagnetic separation of primary Si from Al-Si melt has not been reported, which is useful to the future possible industrial application of this method. Hence, in the present work, the Al-Si alloys with different Sn contents solidified under RMF were studied to investigate the effect of Sn content on the separation effect. The crystallization behaviors and microstructure of the Al-Sn-Si alloys were investigated.

Experimental

For this experiment, the materials underwent induction heating in argon atmosphere to obtain the Al-30 at.% Si-x at.% Sn alloy, where x = 0, 2.5, 5, 7.5, 10, 15 (all in at.% unless otherwise stated). The alloys were melted in high purity clean graphite crucible. Afterward, the obtained alloy was maintained at 1373 K for 20 min in a SiC electric furnace, and then poured into a preheated cylindrical graphite crucible with an inner diameter of 60 mm at the temperature 50 K higher than liquidus temperature. The crucible was placed in an RMF with a frequency of 50 Hz and magnetic flux density of 25 mT which was induced by a three-phase-three-pole magnetic generator. The schematic diagram of the separation apparatus can be seen in our previous work [13]. The magnetic field was switched on when melt was cooled to 1103 K. TR-V1000 temperature measurement system was embedded in the centre of the mould to record temperature-time data related to solidification features. It is worth stressing that the thermal analysis only measure the cooling curves of alloy solidified without RMF because the induced strong flow by RMF can shock the thermocouple and cause errors. Before and after use, the K-type thermocouples were calibrated using high-purity aluminium, and the errors in the relative temperature measurements were estimated to be less than 0.3 K. Samples for metallography were taken at the tip of the thermocouple and prepared according to standard grinding and polishing procedures. After polishing, the specimens were etched with 2% HF acid. Microstructures were analysed using an optical microscope. The Si content of Si-rich layer was measured by X-ray fluorescence spectrometry (XRFS).

Results and discussion

In order to reveal the influence of Sn content on the separated effect of primary Si phase, the samples were sectioned 20 mm from the bottom surface along their cross-section planes (the height of sample is 75 mm). A quarter of the cross section of samples with different Sn contents is shown in Fig. 1. For the Al-Si-Sn alloy solidified without RMF, similar to the previous work [5], the primary Si phase tends to uniformly distribute in the alloy on the macroscopic view, as shown in Fig. 2a, c, e, andg. However, when solidified under RMF, a large amount of the primary Si phase is separated from the Al-30Si-Sn alloy and distributes in the periphery of the sample, as shown in Fig. 2b, d, f, andh. In addition, the thicknesses and distribution of Si-rich layer remain the same with increasing Sn content. In order to further exhibit the influence of Sn addition on the segregation of primary Si phase, the Si content of Si-rich layer was measured by XRFS. The Si contents in Si-rich layer of sample with different Sn contents solidified under RMF, as shown in Fig. 3. It can be seen that the Si contents in Si-rich layer increases with increasing Sn content. The Si content of Si-rich layer of Al-30% Si-10% Sn alloy is extended to be about 80 at.%, while it is only 63 at.% for the Al-30Si alloy.

Based on the review of past research [13], the formation of a Si-rich layer in Al-30Si alloy solidified under RMF is a complex coupling process of cooling conditions, flow field and crystal growth. The primary Si phase is first nucleated on the crucible wall or formed close to the wall where the temperature is lower during the solidification process. When an alternating magnetic field is imposed from outside of graphite mold, the primary flow induced by Lorentz force generates shearing force in melt, which is a rotation of the fluid around the cylinder axis. In the steady state, a secondary recirculating flow in the radial-meridional plane, which consists of a double-vortex structure, cause a strong outward flow from the axis of the ingot towards the solidification front where symmetrical segregation channels emerge [14]. Such the periodical flow structure played a substantial role for the separation of the primary Si, because it carried the bulk liquid metal with higher Si content which continuously flow towards the solidification front to promote the growth of the primary Si, resulting in the formation of the Si-rich layer with 63 at.% Si content on the periphery of the ingot, as shown Fig. 2b. To further prove the potency of the Sn added to the Al-30% Si, thermal analysis was carried out during solidification of the Al-30%Si-Sn alloy with different Sn contents. Fig. 4 shows that addition of 10 at.% Sn to Al-30 at.% Si caused a increase in nucleation temperature of primary Si from 1049 K to 1133 K and a drop in growth temperature of primary Si from 850 K to 834 K, resulting in a significant increase in the crystallization temperature range of the primary Si. Specifically, an extended crystallization temperature range will provide a longer separation time, which facilitates the growth and enhances the separation efficiency, as in shown Fig. 3.

Conclusions

In the present study, the effect of Sn addition on separation of the primary Si from Al-30 at.% Si melt under RMF was investigated. The results clearly show that the application of RMF during solidification of alloy can separate the primary Si from the Al-Si-Sn melt and accumulate on the periphery of the ingots to form a Si-rich layer. Meanwhile, the mass fraction of primary Si in Si-rich layer increases with the increasing of Sn content. The separation mechanism is ascribed to a complex coupling process of the cooling condition, the flow field and the crystal growth. In addition, the Sn addition tends to enlarge the crystallization temperature range and enhance the separation efficiency.

Fig. 1 :Fig. 2 :

 12 Fig. 1: A quarter of the cross section of Al-30at%Si alloys with different Sn contents: without RMF (a) 0 Sn, (c) 2.5% Sn, (e) 5% Sn, (g) 10% Sn; with RMF (b) 0 Sn, (d) 2.5 Sn, (f) 5 Sn, (h) 10 Sn.The microstructures at different positions in Fig.1were observed, and the corresponding results are shown in Fig.2. In the case of sample without imposition of RMF, it can be seen from Fig.2a(position 1) that the coarse plate-like primary Si phase distributes in the Al-Si alloy and is surrounded by the Al-Si eutectic structure. Fig.2b(position 2) shows the microstructure of Al-30%Si-10%Sn alloy, and no intermediate compound was found. During the solidification process of this ternary alloy, the primary Si first separated from the melt, and then binary eutectic αAl + Si formed, followed by the solidification of the ternary eutectic αAl +βSn + Si. In the case of sample solidified under RMF, the formed Si-rich layer on the periphery of the Al-30%Si alloy contains the large coarse primary Si plate and a small amount of needlelike eutectic structure, as shown in Fig.2c(position 3). Fig.2d(position 4) shows several small Si particles may combine together to form much larger particles in Al-30% Si alloy with 10% Sn addition. It is clear from the microstructures observed that addition of Sn can promote the precipitation of primary Si.

Fig. 3 :

 3 Fig. 3: The Si content in Si-rich layer of Al-30Si-Sn alloy with different Sn contents.

Fig. 4 :

 4 Fig. 4: The solidification curve of the Al-30Si-Sn alloy with different Sn contents.

Acknowledgment

The authors wish to express their appreciation to the support of the National Natural Science Foundation of China (No. 51471042).


Zhe Shen

Yunbo Zhong

Licheng Dong

Lijun Fan

Huai Wang

Chuanjun Li

Weili Ren

Zuosheng Lei

Zhongming Ren

Effect of axial magnetic field on the shape of copper ribbon crystal grown by Czochralski method

Keywords: axial magnetic field, ribbon crystal, crystal morphology, Czochralski

During the process of growing ribbon crystal by Czochralski method, Turbulent convection in copper melt was effectively suppressed by applying an axial magnetic field (magnetic induction B≤57mT). The changes of thermal fluctuation and flow field were measured and modeled. With the magnetic field increased gradually (from 0 to 57mT), the shape of ribbon crystal became regularly wider. We concluded that the axial magnetic field could promote to form a suitable temperature field for ribbon crystal growth by restraining thermal fluctuation and convection.

Introduction

Many silicon sheet growth techniques for solar cell substrates have been proposed in the past few decades [1][2][START_REF] Ruby | 25th IEEE Photovoltaic Specialists Conference Record[END_REF][START_REF] Barnett | 18th IEEE Photovoltaic Specialists Conference Record[END_REF][START_REF] Lange | [END_REF]. The advantages of such techniques which are the efficient use of silicon feed stock with no kerf loss and the elimination of mechanical sawing steps including etching to remove surface damage all focus on reducing costs and translating directly into improved economics. But these techniques also exist many disadvantages, such as high defect density, short minority carrier lifetime, high stress, especially mechanical integrity including flatness, uniform thickness, and surface roughness [START_REF] Bell | Growth of silicon sheets for photovoltaic applications[END_REF].

The process of growing crystal by Czochralski method under static magnetic fields has attracted much attention from researchers [START_REF] Suzuki | Semiconductor Silicon[END_REF][START_REF] Hoshikawa | Czochralski silicon crystal growth in the vertical magnetic field[END_REF][START_REF] Hoshi | Czochralski silicon crystals grown in a transverse magnetic field[END_REF]. Static magnetic fields have been used as a powerful tool to suppress convection and reduce fluctuation in electrically conducting liquids, thus changing temperature distribution in the melt and the crystal morphology during growth [START_REF] Robertson | Magnetic field effects on float-zone Si crystal growth: II. Strong transverse fields[END_REF]. The mechanical integrity of crystal (flatness, surface roughness) may be improved by applying static magnetic fields.

In this work, we study on the changes in the shape of copper ribbon crystal with and without an axial magnetic field (magnetic induction B≤57mT). The results of temperature measurement and numerical simulation both show that the turbulent convection in copper melt was suppressed by applying an axial magnetic field which can provide a suitable temperature field for ribbon crystal growth.

Experimental

Fig. 1 shows a schematic of the experimental apparatus used in this investigation of the growth of copper ribbon crystal in an axial magnetic field. A cylindrical water induction coil (200 mm ID, 300 mm OD and 150 mm in height) which could generate 57 mT in magnetic field strength in the square graphite crucible (100 mm in length, 40mm in width, 115 mm in height) was placed outside of the graphite heater (110 mm in length, 50 mm in width, 125 mm in height). The apparatus is different from the conventional CZ furnace, not only the graphite heater is designed to be square to provide a square hot zone for the growth of ribbon crystal, but also crucible and seed crystal will not rotate. The copper ribbon crystal was grown from the copper melt in argon gas kept at 1 atm. The copper seed crystal is 40 mm in width, 300 µm in thickness, 50 mm in length. The pulling rate is 240 µm/s. The magnetic field strength ranges from 0 to 57 mT.

With the magnetic field increased gradually, the shape of ribbon crystal became regularly wider. These changes shown in Fig. 2 were recorded by video camera.

In order to determine the accurate temperature distribution, three Pt-Rh thermocouples with a diameter of 0.3 mm, protected by quartz tubes of 4 mm OD, were placed at 10 mm below the melt surface at points A, B, and C, as was shown in Fig. 3.

Results and discussion

Crystal morphology

The most obvious effect produced by the presence of an axial magnetic field is the increase of the width of copper ribbon crystal shown in Fig. 2. With the increase of magnetic flux density, the width has increased. These results show that the axial magnetic field may change the temperature distribution in the melt and promote to form a suitable temperature field for ribbon crystal growth.

Temperature measurement

The results of temperature measurement are shown in Table . 1. The temperature difference between point A and B was 10.7℃ at 0 mT, and 12.6℃ at 57 mT. The temperature difference between point B and C was 3℃ at 0 mT, and 4℃ at 57 mT. These suggest that the axial magnetic field increases temperature gradients in the melt by suppressing horizontal thermal convection. Compare with the difference of temperature of point A, B, and C with and without the axial magnetic field, we conclude that the axial magnetic field also plays a very important role in restraining vertical thermal convection. . 1: Temperature distribution in the copper melt with and without an axial magnetic field

Numerical simulation

Fig. 4 and Fig. 5 show the numerical simulation results of the temperature distribution and flow field with and without an axial magnetic field. Compare with the results of temperature measurement, the changes of temperature distribution are much smaller. Because the error exists in the experiment of temperature measurement, and also numerical simulation has certain error with its idealized model.

Conclusions

Copper ribbon crystals were grown under the presence of an axially symmetric vertical magnetic field. With the magnetic field increased gradually (from 0 to 57mT), the shape of ribbon crystals became regularly wider. All the results of crystal morphology, experiment measurement and numerical simulation show that turbulent convection within a copper melt is effectively suppressed and temperature fluctuation in the melt is reduced by applying an axial magnetic field.

Fig. 1 :Fig. 2 :Fig. 3 :

 123 Fig. 1: Schematic diagram of the CZ copper ribbon crystal growth apparatus used in the present investigation in an axial magnetic field application

Fig. 4 :Fig. 5 :

 45 Fig. 4: The numerical simulation results of the temperature distribution with and without an axial magnetic field (magnetic induction B=57mT)

Acknowledgment

The authors gratefully acknowledged the financial support of Nature Science Foundation of China (Key Project No. 51034010), Science and Technology Commission of Shanghai Municipality (Key Project No. 13JC1402500), Natural Science Foundation of Jiangsu Province (BK2011501) and Innovation Fund for Technology Based Firms (12C26213202452)


Long Hou

Yanchao Dai

Zongbin Li

Zhongming Ren

Xi Li

The Effect of magnetic field on the microstructure and morphology of directionally solidified Ni-Mn-Ga alloys

Keywords: Shape memory alloys, Magnetic field, Directional solidification, Ni-Mn-Ga alloy

The effect of magnetic field on the microstructure and morphology of austenitic Ni-Mn-Ga alloy during directional solidification has been investigated experimentally. Results indicate that magnetic field changed the dendrite spacing and morphology of liquid-solid interface. The magnetic field also caused the channel segregation near the liquid-solid interface. These influences should be contributed to the thermoelectric magnetic convection (TEMC) on the specimen scales and the dendritic scales induced by the transverse magnetic field during the directional solidification.

Introduction

Ni-Mn-Ga alloys, as the best-known group of ferromagnetic shape memory alloys (FSMA), has recently attracted a great deal of interest due to its huge magnetostrain of up to 10%, which is of potential for application in actuators [1][2][3][4][5].

Indeed, the performance is strongly dependent on the microstructural configuration and crystallographic correlation of constituent martensitic variants. It is noteworthy that the microstructural configuration of martensitic variants and their crystallographic correlation deduced from the martensitic transformation that follows certain specific orientation relationships (OR) between the parent phase and the product phase. Thus, the microstructure and crystallographic orientation of parent austenitic phase are important for the performance of product martensite.

Recently, the application of magnetic field during directional solidification is an interesting research topic because abundant microstructural and morphologic changes occur in various alloys [6][7][8]. In this study, room-temperature austenitic Ni 48 Mn 30 Ga 22 alloy was designed to explore the effect of magnetic field on the microstructure and morphology during directional solidification. Indeed, insight into these aspects is of both theoretical significance and practical interest.

Experimental Procedures

Ni-Mn-Ga alloys with nominal composition of Ni 48 Mn 30 Ga 22 used in this study were prepared with high-purity Ni (99.99 wt.%), Mn (99.9 wt.%) and Ga (99.99 wt.%) in an Ar atmosphere. Stoichiometic amount of Ni, Mn and Ga were melted in an arc-melting furnace four times and suctioned into a quartz tube to cast specimens with a diameter of 3 mm and length of 150 mm. The cast specimen was enveloped in a tube of high-purity corundum with an inner diameter of 3 mm and length of 200 mm.

The experimental apparatus in a transverse magnetic field consists of an electromagnet and a Bridgman-Stockbarge-type furnace equipped with a pulling system and temperature controllers, as illustrated in our previous report [9]. The samples were prepared under various magnetic field intensities and growth speeds in order to examine the effect of the magnetic field on the microstructures during the directional solidification. The samples obtained in the experiments were sectioned and mechanically polished. The microstructures were examined using optical microscopy and scanning electron microscopy (SEM). The element distribution was measured by energy dispersed spectrum (EDS) by line scanning and map scanning. All samples were characterized using Hitachi SU70 scanning electron microscope (SEM), equipped with Octane plus EDS detector.

Results and discussion

Fig. 1 shows the longitudinal microstructure near the liquid/solid interface in directionally solidified Ni-Mn-Ga alloys at various growth speeds under different magnetic field intensities. It can be observed that the microstructure near the liquid/sololid interface transforms from planar to dendritic with the increase of growth speeds. At a low growth speed (1 μm/s), the interface of the sample prepared without magnetic field is almost planar with two caves caused by natural convection. When the magnetic field intensity is 0.1 T, the interface shows a cave only on one side. If the magnetic field intensity increases up to 0.8T, the interface becomes extremely planar. At higher growth speeds (i.e. 5 and 20 μm/s), the field has caused the planar to become cellular-dendrite or regular dendrite. When the magnetic field is applied, the interface shows the similar transformation as shown in the samples at lower growth speed. Moreover, it is worthy to note that channel segregations exist in the samples prepared under a magnetic field at 5 μm/s growth speed.

Furthermore, the magnetic field has caused that the dendrites become much shorter and thinner.

The solution distribution has a great effect on the microstructure of directional solidification. The distributions of solutes Ni, Mn and Ga were measured in the line scanning mode at the similar location in the interface front of the samples at 1 μm/s with and without magnetic field. As shown in Fig. 2a, the solute Mn is higher in the two sides and inter-dendrite zone. When a 0.1 T transverse magnetic field is applied, the solute Mn is higher in the one side and inter-dendrites, which is corresponding to the microstructure shown in Fig. 2. It also can be shown that the dendrite spacing become smaller while the curve of solute Mn shows more peaks which exist in the inter-dendrite. In order to further reveal the element distribution under magnetic field, EDS mapping scanning were measured in the sample in a 0.1 T magnetic field at 5 μm/s growth speed (Fig. 3). Fig. 3a shows the SEM image near the liquid/solid interface in the sample. Fig. 3b shows the EDS phase mapping containing two phases, blue area indicating Mn-poor zone and red indicating Mn-rich zone. It can be found that the solute Mn is rich in the inter-dendrite and channel segregation zone.

From the above experimental results, it can be learned that the application of magnetic field during the directional Fig. 2. The distributions of solutes Ni, Mn and Ga measured in the interface front of the samples with a growth speed of 1 μm/s with and without magnetic field.

Figure 3. EDS mapping of solid/liquid interfaces of Ni-Mn-Ga alloys solidified at a growth speed of 5μm/s under magnetic field with intensity of 0.1 T.

Figure 4. Schematic of the TEMC and its effects on the solidification structure during the transverse magnetic field: (a) TEMC near the liquid/solid interface and its effect on the solidification structure, and (b) formation of the channel caused by the flows induced by the TEMC. solidification influenced the liquid/solid interface morphology and led to the formation of channel segregation. This should contribute to the formation of the effect of solute migration in the interface and in the mushy zone. As is mentioned in Ref. [9], a Lorentz force will be generated near the solid/liquid interface during directional solidification under a magnetic field due to seebeck effect, which will induce a liquid convection called thermoelectric magnetic convection (TEMC), as is shown in Figures 4(a) and (b). The TEMC and the corresponding recirculation loops will cause the solute to move along the direction perpendicular to the magnetic field (i.e., the direction of the TEMC). As a result, the sloping interface will form. However, the TEMC at the crucible scale increases with the increase of the magnetic field and reaches a maximum value when B is of the order 0.1 T, the sloping amplitude reaches a maximum under a 0.1 T magnetic field and decreases with the increase of the magnetic field intensity (Fig. 1). On the other hand, the TEMC may result in the channel formation, as shown in Figure 4(b). Moreover, it is known that dendrite spacing readily adjusts in response to interdendritic constitutional supercooling. Therefore, the adjustment of the cellular/dendritic spacing can be contributed to the influence of the TEMC on solute transport, and thus interdendritic constitutional supercooling.

Conclusions

The effect of magnetic field on the microstructure and morphology of austenitic Ni-Mn-Ga alloy during directional solidification has been investigated experimentally. The main results are listed as follows:

1. The application of a transverse magnetic field refined the dendrites of directionally solidified Ni 48 Mn 30 Ga 22 at a higher growth speed (e. g. 5 μm and 20 μm).

2. The transverse magnetic caused tunnel segregation near liquid-solid interface.

3. The above results might be attributed to the solute migration caused by the thermoelectric magnetic convection (TEMC) induced by the transverse magnetic field during the directional solidification.

Figure 1 .

 1 Figure 1. The metallography images of austenitic Ni 48 Mn 30 Ga 22 alloys under different magnetic field (0, 0.1, 0.5 and 0.8 T) with different growth velocities (1, 5 and 20 μm/s).

Acknowledgement

This work is supported partly by the National Natural Science Foundation of China (Nos. 51271109 and 51171106) , and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.


Chunmei Liu

Yunbo Zhong

Tianxiang Zheng

Weili Ren

Zuosheng Lei

Zhongming Ren

The microstructure evolution of Fe-Si alloys solidified in a high static magnetic field

Keywords: Thermoelectric magnetic convection, Silicon steel, Solidification, Ferrite

The effect of magnetic field on the microstructure during the bulk solidification process of Fe-3wt.%Si alloys has been investigated experimentally. The results show that the ferrite turned into granular and bulk morphology as the increase of magnetic flux density (MFD). The microstructure in the region which is adjacent to the wall of corundum crucible was same in the samples with and without high static magnetic field (HSMF). When the MFD exceed 1.0 T, the ferrite was coarse. These phenomena were attributed to thermoelectric magnetic convection in solidification.

Introduction

During solidification,most models of the formation of microstructure neglect the influence of convection and only consider diffusional transport of heat or mass away from the growing crystal. However, at low growth rates, ever bulk solidification process, convection and its influence on microstructure formation and development can become dominant [1.2] and should not be neglected. Indeed, Charborty et al. [3][4][5][6][7] have investigated numerically the effect of convection on the solidification structure during electromagnetohydrodynamic stirring processing, and have shown that convection will affect the solidification structure significantly. The effect of a permanent magnetic field on convection in solidification has been discussed extensively [8]. It has been determined that the application of a magnetic field during the directional solidification of materials can significantly reduce the thermosolutal buoyant flow [9][10][11]. Albousiere et al. [12] suggested that this new flow was induced by the interaction between the magnetic field and thermoelectric effects, and Lehmann [8] subsequently offered some experimental evidence for thermoelectromagnetic convection(TEMC).

It has been found that a high magnetic field causes the breakdown of needle-like ferrite into granular and bulk shape. Here, we propose a theory for the magnetization and TEMC of a binary alloy under a high magnetic field, and explain the breakdown of the needle-like α ferrite phase through this theory.

Experiment

The Fe-Si alloy used in this study were prepared with high-purity Fe (99.99wt.%), Si(99.99wt.%) in an induction furnace. The alloy were placed in a high-purity corundum crucible(4 cm diameter) and heated to a certain temperature (1520℃ for the Fe-3wt.%Si alloy,1540℃ for the Fe-1wt.%Si),then heat insulation for 10 min and cast specimens by suction casting. Φ8mm ×10mm rods were cut from the cast specimen and were enveloped in a tube of high-purity square corundum for the bulk solidification experiments. The experimental setup we used for the solidification is the same as that shown in Ref [13] and consists of a superconductor magnet, water-cooling cover, heating furnace and controlling temperature system. The superconductor magnet can generate a high magnetic field up to 8T. The furnace was set in the room bore of the magnet and the temperature in it could reach 1600℃.

Results

Fig. 2 shows the microstructures of the Fe-1wt.%Si alloy solidified from 1540℃(melting state)at a cooling rate of 10K/min without and with a magnetic field, respectively. It can be observed that for the α ferrite phase in the Fe-1wt.%Si alloy, the effect of a high magnetic field on the growth of the α ferrite phase is weak. However, for the Fe-3wt.%Si alloy, an application of a high magnetic field has affected the dendrite growth significantly. Fig. 3 shows the microstructures of the Fe-3wt.%Si alloy solidified from 1520℃(melting state)at a cooling rate of 10K/min without and with a magnetic field, respectively. In the case of no field, it can be observed that the structure of the alloy has a usual disordered nature; however, after the application of a 0.4T and 0.8T magnetic field, the needle-like α ferrite phases away from the wall of corundum crucible were break down to granular ferrite, needle-like α ferrite phase in the case of no magnetic field becomes the mixture of needle-like and granular-like α ferrite phase in the application of a 0.4T and 0.8T. As the magnetic field exceed 1T, the granular-like α ferrite phase disappear totally and needle-like α ferrite phase reappear and even coarsening. This means that the Si content in the alloy has played an important role during the morphology change of the α ferrrite phase.

Discussion

From the above experimental results, it can be learned that the magnetic field can affect the morphology of α ferrite phase. The effect of a high magnetic field on the growth of the needle-like α ferrite phase in the bulk solidified Fe-3wt.%Si has been investigated experimentally and it has been found that an application of a high magnetic field has caused the break and the coarsening of the α ferrite phase. Moreover, it is found that the Si content in the alloy has played an important role during the morphology change. This is attributed to the forces produced on the dendrite during the bulk solidification under a high magnetic field. Normally, there exist two force:the magnetic force(MF) and thermoelectric magnetic force(TEMF) during solidification under a high static magnetic field. The TEMF in liquid will drive some motions [i.e.so called thermoelectric magnetic convection (TEMC)].

The driving force varies as B and the braking force varies as B2, therefore, as shown by Shercliff [14],there is an optimum strength of the magnetic field. When the MFD is below the optimum strength, TEMF will break the needle-like α ferrite phase and result in the increase of the granular-like α ferrite phase ahead of the needle-like α ferrite phase. When the MFD exceed the optimum strength, Lorentz forces impede the flow of the melt, resulting the coarsening of needle-like α ferrite phase. As is well-known , the electromagnetic properties of Si-steel are improved by increasing Si content. So for different Si content alloys, the breaking behaviors of the α ferrite phase are different, owing to the high magnetic field has caused the occurrence of the breakdown of needle-like α ferrite phase not in Fe-1wt.%Si but in Fe-3wt.%Si. This may be attributed that the difference of magnetic force of Fe-Si alloy. Thus, under a certain MFD, the dendrites in the Fe-3wt.%Si alloy is easier to break down than the one in the Fe-1wt.%Si alloy. It should be emphasized that the above experimental results may act as an experimental proof that the force imposed on the dendrite during the bulk solidification will induce the occurrence of the strip morphology to the granular morphology.

Conclusions

Effects of a high magnetic field on the ferrite phase morphology transition in bulk solidified Fe-based alloys has been investigated experimentally. Results show that for different Si content in Fe-Si alloy, the effect of a high magnetic field on the growth of the α ferrite phase is different. Indeed, it has been found that the growth of the α ferrite phase in the Fe-1wt.%Si alloy becomes coarsening as the MFD increase. However, for the Fe-3wt.%Si alloy, a certain magnetic field has caused the occurrence of the needle-like to granular-like transitions of the α ferrite phase. This may be attributed to the thermoelectric magnetic force (TEMF) on the dendrite caused by the interaction between the thermoelectric current and the magnetic field. Above results may act as an experimental proof that the force imposed on the dendrite during the bulk solidification will induce the occurrence of the strip morphology to the granular morphology. Further, above results are discussed from the TEMF imposed on the dendrite and the different of the magnetic force of α ferrite phase in the different Si content.

Fig. 1 Fig. 1 :

 11 Fig.1shows the phase diagram of the Fe-Si alloy near Fe and the temperature profiles for the solidification procedure. The samples obtained from the experiment were cut along the direction parallel and perpendicular to the magnetic field. After machining off the surface, the longitudinal ( parallel to the magnetic field direction) were examined in the etched condition by optical microscope.

Fig. 2 :Fig. 3 :

 23 Fig.2:Microstructure of the Fe-1wt.%Si alloy solidified from melting state at a cooling rate of 10K/min:(a)0T;(b)0.4T;(c)0.8T;(d)1T;(e)2T;(f)4T;(g)6T.longitudinal microstructure

Acknowledgements

The authors gratefully acknowledged the financial support of Nature Science Foundation of China (Key Project No. 51034010), Science and Technology Commission of Shanghai Municipality (Key Project No. 13JC1402500), Natural Science Foundation of Jiangsu Province (BK2011501) and Innovation Fund for Technology Based Firms (12C26213202452)