Phase distribution and phase structure control through a high gradient magnetic field during the solidification process

Phase distribution and phase structure control through a high gradient magnetic field during the solidification process

Materials and Design 29 (2008) 1796–1801 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/ma...

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Materials and Design 29 (2008) 1796–1801

Contents lists available at ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

Phase distribution and phase structure control through a high gradient magnetic field during the solidification process Xi Li a,b,*, Zhongming Ren a, Yves Fautrelle b a b

Department of Material Science and Engineering, Shanghai University, Shanghai 200072, PR China EPM-Madylam, ENSHMG, BP 38402, St. Martin d’Heres Cedex, France

a r t i c l e

i n f o

Article history: Received 24 January 2007 Accepted 27 March 2008 Available online 3 April 2008 Keywords: Gradient magnetic field Magnetic force Phase separation Element separation

a b s t r a c t Influence of a high gradient magnetic field on the phase distribution and phase structure has been investigated experimentally. It was found that the application of a gradient magnetic field is capable of controlling the distribution of the primary phase in alloys. As a consequence, an axial gradient magnetic field causes the primary MnBi phase in Bi–6wt.%Mn alloy and the primary Si phase in Al–18wt.%Si alloy to segregate from the matrix; and the application of a radial gradient magnetic field produces a ringlike MnBi phase-rich layer. The application of an axial gradient magnetic field can produce the even refining structure during the electromagnetic vibration process of pure Al through restraining the formation of crystal rains. The gradient magnetic field also affects microstructure by aligning the phase and controlling the phase aggregation process. Moreover, the investigation on the influence of an axial gradient magnetic field on the distribution of the element indicates that the field caused the Mn solute in Bi–6wt.%Mn alloy to separate from the Bi matrix. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction

an application of a gradient magnetic field is capable of separating the phase and solute from the matrix. Moreover, it was also found that the application of a gradient magnetic field can change the microstructure and improve the refining structure.

Many of the mechanical properties of material depend on the structure and distribution of phase in the structure, so, controlling the structure and distribution of phase has been focused on by research workers. Since the structure is almost determined in the solidification process, it is essential to control the solidification structure. Many different methods have been used for this purpose and the imposition of external fields (including electric field, magnetic field and their complex fields) is often applied to improve the structure and the distribution of the phase and solute. It is well known that the electromagnetic field has a great influence on the segregation of elements and in alloys during electromagnetic stirring (EMS) solidification processes [1,2] and the application of an electromagnetic force is capable of separating the no-metallic inclusion from the melt-metal and refines the grains [3–6]. Recently, a high magnetic field is extensively used during the solidification process and many interesting and valuable phenomena have been found [7–11]. For example, it has been found that an application of a high magnetic field is capable of aligning and levitating the nonmagnetic substance and changing the phase transformation [10,11]. In this paper, the phase and solute separation under a high gradient magnetic field has been investigated and results indicate that

Solidification apparatus under a high magnetic field is schematically shown in Fig. 1. It consists of a superconducting magnet, heating furnace, graphite crucible and temperature controller. The superconducting magnet can produce a vertical magnetic field with a maximum value of 14 T. The fields have an axial symmetry and the field profile is approximately as shown in Fig. 2 [12]. The temperature in the heating furnace could reach around 1000 °C with the precision of 1 °C. Bi–6wt.%Mn and Al–18wt.%Si alloys were prepared with high pure substance (4 N) in a vacuum induction furnace and cast specimens with the diameter of 10 mm and length of 50 mm. Specimens sealed in the graphite tube with its axis along the magnetic field direction were heated in the furnace. The electromagnetic vibration device and the detail of the experiment under a high magnetic field can be found in Ref. [13]. Samples obtained from the experiment were polished and etched. Macrostructure photo was taken with a Leica and the microstructure was examined using Leica optical microscope.

* Corresponding author. Address: EPM-Madylam, ENSHMG, BP 38402, St. Martin d’Heres Cedex, France. Tel.: +33 476825262; fax: +33 476825249. E-mail address: [email protected] (X. Li).

Fig. 3 shows the structure of Bi–6wt.%Mn alloy solidified from 345 °C to 262 °C (eutectic point) at a cooling rate of 1 °C/min under

0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.03.012

2. Experimental

3. Result 3.1. Effect of a gradient magnetic field on the phase segregation in alloys

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3.2. Effect of a gradient magnetic field on the distribution of the refining grains of pure Al 1 2 3 +

4 5

0

Bmax

6

_

Fig. 1. Schematic diagram of the experimental device of metal solidification under the magnetic field: (1) Sample frame, (2) water-cool cover, (3) heat furnace, (4) superconductor magnet, (5) sample, (6) controlling temperature system.

Moreover, it is well known that owing to the difference in density between the solid and liquid phase of pure metal, crystal rains will form during the electromagnetic vibration process, which will often cause the uneven distribution of the refining structure. Therefore, it is valuable to restrain the formation of crystal rains and improve the refining structure. To restrain crystal rains, a magnetic force is applied during the electromagnetic vibration process. Fig. 6 shows the structure of pure Al under a negative gradient magnetic field during the electromagnetic vibration process with a certain electromagnetic force density of about 10 N. Fig. 6a shows the structure with a gradient magnetic field of B = 0.5 T, Bz dBz/ dz = 0.95 T2/m, it can be observed that the refined grains appear only at the bottom region of the sample. This should be attributed to the formation of crystal rains during the electromagnetic vibration and the magnetic force under gradient magnetic field of B = 0.5 T, BzdBz/dz = 0.95 T2/m is not enough to restrain crystal rains. However, imposed of a moderate magnetic force under the gradient magnetic field of B = 1.5 T, Bz dBz/dz = 8.5 T2/m, the refined grains evenly distribute on the whole sample. This should be attributed to the part restraint of crystal rains by the upward magnetic force under a higher gradient magnetic field. An application of a higher magnetic force under the magnetic field of B = 6 T, Bz dBz/dz = 150 T2/m, the refined grains only appear on the upper region of the sample; which is because the larger magnetic force restrains crystal rains completely. The above experimental result implies that an application of a moderate magnetic force is capable of improving the refining structure and gaining an even refining structure through restraining crystal rains during the electromagnetic vibration process. 3.3. Effect of a gradient magnetic field on the microstructure and distribution of solute

Fig. 2. Field profiles in the 8 T magnet in Ref. [8]. Gz and Gr are the vertical and radial components of grad (B2a =2).

a positive gradient magnetic field of B = 10 T, Bz dBz/dz = 400 T2/m. It can be observed that the phase segregation occurs. As a consequence, the primary MnBi phases segregate from the Bi matrix and are located at the bottom region of the sample (Fig. 3a). Moreover, it can be observed that elliptical MnBi phases are separated from the club-shaped MnBi and located at the upper region of the primary phase (Fig. 3b and c). Fig. 4 shows the structure of Bi–6wt.%Mn alloy solidified from 345 °C to 262 °C (eutectic point) at a cooling rate of 1 °C/min in the center of the magnetic field [i.e., under a radial gradient magnetic field]. It can be observed that the primary MnBi phases are separated from the Bi matrix and located at the periphery region of the sample. Moreover, it is also observed that the elliptical MnBi phases are separated from the club-shaped MnBi ones and located at the inner region of the ring-like structure. Fig. 5 shows the structure of Al–18wt.%Si solidified from 800 °C to 577 °C (eutectic point) at a cooling rate of 10 °C/min under a positive gradient magnetic field of B = 10 T, Bz dBz/dz = 400 T2/m. It can be observed that the primary Si separates from the Al matrix and is located at the upper region of the sample.

The effect of a gradient magnetic field on the microstructure and the solute segregation was investigated. Fig. 7 shows the microstructure of the Bi–6wt.%Mn alloy solidified from 380 °C at a cooling rate of 1 °C/min under the magnetic field. Fig. 7b–d shows microstructures located at different positions of a gradient magnetic field as shown in Fig. 7a, respectively. It can be observed that the microstructure in the upper region of the gradient magnetic field (i.e., subjected to a smaller magnetic force) is composed of the elliptical MnBi phase, and the microstructure on the middle of the sample is composed of the polymer of elliptical MnBi phase, however, at the lower region of the gradient magnetic field (i.e., subjected to a smaller magnetic force), the polymer of the elliptical MnBi phase amalgamates and forms into a rod-like structure. Fig. 7e and f shows microstructure solidified without and with a 10 T uniform magnetic field. It can be observed that the application of a 10 T magnetic field is capable of aligning and aggregating the elliptical MnBi phase. However, comparison of the microstructure under a uniform and a gradient magnetic fields show that an application of a gradient magnetic field has enhanced the aggregating course. Effect of a gradient magnetic field on the distribution of the solute was also investigated. To do this, the sample of Bi– 6wt.%Mn alloy was heated to 600 °C and held for 18 h under a positive gradient magnetic field of B = 10 T, BzdBz/dz = 400 T2/m, and then quenched. Fig. 8 shows the quenching structure and it can be found that Mn solute is separated from the Bi matrix and located at the bottom region of the sample. This implies that an application of a gradient magnetic field is capable of separating the solute from the matrix and controlling the distribution of the element.

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a

b

Elliptical MnBi Bi

400µm

c

Club-shaped MnBi

2mm

400µm

Fig. 3. Solidification structures of Bi–6wt.%Mn alloy solidified from 345 °C at a cooling rate of 0.1 °C/min under a positive gradient magnetic field: (a) Longitudinal structure, no MnBi phases on the top region of the sample; (b) microstructure at the site denoted by the rectangle in Fig. 3a; (c) microstructure at the site denoted by the rectangle in Fig. 3a.

a b

Bi 200µm

1mm

Elliptical MnBi

Club-shaped MnBi

c d Bi

200µm 1mm Fig. 4. Solidification structures of Bi–6wt.%Mn alloy solidified from 345 °C at a cooling rate of 0.1 °C/min under a radial gradient magnetic field of B = 10 T: (a) Macrostructure on the transverse section, no MnBi phases in the center region of the sample; (b) microstructure at the site denoted by the rectangle in Fig. 4a; (c) macrostructure on the longitudinal section; (d) microstructure at the site denoted by the rectangle in Fig. 4c.

4. Discussion The above experimental results show that an application of a gradient magnetic field can control the distribution of the primary phase in alloys. This should be attributed to the application of the magnetic force under a gradient magnetic field. Generally, a unit volume of material in a one-dimensional

magnetic field gradient experiences a magnetic force (~ F m ) defined as: 1 dBz ~ F m ¼ vV Bz l0 dz

ð1Þ

where, l0 is the permeability of the free space, vV volume magnetic susceptibility, B magnetic flux density, z a site-coordinate. Thus,

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a

1 dBz ~ F fm ¼ ðvPV  vM Bz VÞ l0 dz

b

Primary Si

200µm

c

Eutectic Si

ð2Þ

where, vPV and vM V are the volume susceptibilities of the primary phase and the matrix, respectively. The effective magnetic force works as a ‘‘driving force” to tend to separate the primary phase from the matrix. Consequently, under a radial gradient magnetic field, if ignoring viscous forces and convective flows, the force (~ F fm ) will cause the primary phase to separate from the matrix. Therefore, the formation of the ring-like MnBi phase layer should be attributed to the effective magnetic force under a radial gradient magnetic field. Since the primary MnBi phase and the matrix Bi are the ferromagnetic and diamagnetic substance, respectively; in a radial gradient magnetic field as shown in Fig. 2, the MnBi phase will be subjected to the outward force along the radial direction. As a consequence, the MnBi phase will transfer outward and form the ring-like structure at last. Under an axial gradient magnetic field, the effective gravitational force ~ GV ¼ ðqP  qM Þ~ g is applied at the same time. The total force F acting on the primary phase under a gradient magnetic field is g þ ðvPV  vM F ¼~ GV þ F fm ¼ ðqP  qM Þ~ VÞ

200µm

2mm

Fig. 5. Structures of Al–18wt.%Si alloy solidified from 800 °C at a cooling rate of 0.15 °C/min under a positive gradient magnetic field: (a) Macrostructure on the longitudinal section; (b) and (c) microstructure at the site denoted by the rectangle in Fig. 5a.

when an alloy is solidified under a gradient magnetic field, the primary phase and the matrix will experience a magnetic force at the same time. However, owing to the difference in the susceptibilities between the primary phase and the matrix, the magnetic forces are different. Thus, for the primary phase, an effective magnetic force ~ F fV will introduce as follow:

1 dBz Bz l0 dz

ð3Þ

Thus, if ignoring viscous forces and convective flows, the total force ~ F will decide if the primary phase will separate from the matrix. The above experimental results show that the primary MnBi and Si phases are segregated from the respective matrixes and this should be attributed to this total force. In the same mechanism, owing to the difference in the susceptibilities between solid and liquid of pure metal, an effective magnetic force is applied to the solid. Thus, it is possible to control the solidification behavior of pure metal by using this force. The above experimental result has shown that an application of an upward magnetic field can restrain crystal rains and improve the refining structure during the electromagnetic vibration. Owing to the difference in the susceptibilities between the elements, an application of the magnetic force is capable of separating elements as shown in Fig. 8.

Electric pole

a

b

1cm

c

1cm

1cm

Fig. 6. Effect of the magnetic force on the distribution of the refined grains in pure aluminum during the electromagnetic vibration process under a negative gradient magnetic field. (a) B = 0.5 T, I = 20A; (b) B = 1.5 T, I = 6.5A; (c) B = 6 T, I = 1.6A.

1800

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c

b

d

a

D C B

80µm

80µm

80µm

e

f

100µm

B

100µm

Fig. 7. Solidification structures of Bi–6wt.%Mn alloy solidified from 380 °C at a cooling rate of 0.15 °C/min under a 10 T magnetic field Under a positive gradient magnetic field: (a) lower gradient field, (b) middle gradient field, (c) higher gradient field; (d) under a uniform magnetic field: (e) without the magnetic field, (f) section parallel to the uniform magnetic field.

of the magnetization force on the alignment and aggregation of the primary phase. It is well known that the magnetic field can align the phase with the easy magnetic axis along the magnetic field direction; and at the same time, owing to the interaction between magnetic phases, the aggregation will occurs. Since the magnetic force is capable of affecting the transfer of the phase, an application of a magnetic force changes the aggregating process; as a consequence, the microstructure is affected.

a

b Mn

400µm

2mm Fig. 8. Microstructures of Bi–6wt.%Mn alloy quenched at 600 °C after holding at this temperature for 18 h under a positive gradient magnetic field of B = 10 T, BzdBz/ dz = 400 T2/m.

Moreover, the above experimental results indicate that the application of a gradient magnetic field is capable of changing the microstructure (Fig. 7). This should be attributed to the effect

5. Conclusions 1. An application of a gradient magnetic field can affect the distribution of primary phases in alloy; as a consequence, a positive axial gradient magnetic field causes the primary MnBi phase in Bi–6wt.% Mn alloy and the primary Si in Al–18wt.%Si alloy to move towards the top and bottom regions of the sample, respectively. A radial gradient magnetic field causes the MnBi phase to be located at the periphery region and to form a special ring-like structure. 2. A gradient magnetic field is capable of affecting the distribution of the refined grains of the pure Al during the electromagnetic vibration process and under a moderate gradient magnetic field can produce an even refining structure. 3. The gradient magnetic field can affect the microstructure of the MnBi phase in Bi–Mn alloy through aligning the phase and affecting the aggregation of the phase. The influence of the gradient magnetic field on the distribution of the solute is investigated and the result indicates that Mn solute is separated from Bi matrix under a high gradient magnetic field.

X. Li et al. / Materials and Design 29 (2008) 1796–1801

Acknowledgements This work is supported by the European Space Agency through the IMPRESS project and the Natural Science Foundation of China (Nos. 50234020, 50225416 and 59871026). One of the authors (LX) is also grateful for an Egide/Eiffel Doctorate Scholarship. The authors are indebted to Prof. R. Moreau and Prof. T. Duffar in CNRS, Grenoble, for helpful and fruitful discussions. References [1] Garnier M. In: Proceedings of the international symposium on electromagnetic processing of materials. Nagoya: ISIJ; 1994. p. 1.

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