Journal of Alloys and Compounds Effect of high magnetic ... - CiteSeerX

Journal of Alloys and Compounds 487 (2009) 612–617

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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom

Effect of high magnetic field on the primary dendrite arm spacing and segregation of directionally solidified superalloy DZ417G Tao Zhang a , Weili Ren a , Jianwen Dong a , Xu Li a , Zhongming Ren a,∗ , Guanghui Cao a , Yunbo Zhong a , Kang Deng a , Zuosheng Lei a , Jianting Guo b a b

Shanghai Key Laboratory of Modern Metallurgy & Material Processing, Shanghai University, Shanghai, 200072, China Institute of Metal Research, The Chinese Academy of Sciences, Shenyang, 110016, China

a r t i c l e

i n f o

Article history: Received 24 February 2009 Received in revised form 4 August 2009 Accepted 8 August 2009 Available online 15 August 2009 Keywords: High temperature alloys Directional solidification Microstructure High magnetic fields

a b s t r a c t High magnetic field has been applied to the directional solidification of superalloy DZ417G. The results show that high magnetic field can significantly influence the primary dendrite arm spacing and microsegregation. A 6 T magnetic field can decrease the primary dendrite arm spacing by 22% at a drawing speed of 40 ␮m/s. Nevertheless, when the magnetic field is greater than 6 T the primary dendrite arm spacing begins to increase. The microsegregation of Ti and Mo can be also decreased by about 28% and 40% respectively with a 6 T magnetic field at a drawing speed of 40 ␮m/s. These phenomena are attributed to the competitive relationship of the electromagnetic damping and thermoelectromagnetic convection caused by the high magnetic field. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The development of new and better superalloys has been an important topic in materials science for more than 50 years. One major driving force has been the demanding performance needs of higher efficiency aero-engines and industrial gas turbines. Directional solidification has been widely used in manufacturing the advanced gas turbine vanes and blades to achieve high operation performance [1]. Dendrite structures are frequently observed during the directional solidification of alloys. The dendritic spacings especially the primary dendrite arm spacings are important microstructural parameters resulting from the solidification process. They affect the microsegregation profiles and govern the formation of a second phase in the interdendritic regions, and consequently influence the properties of the materials [2]. Fine dendritic structures in casting are recognized to yield superior mechanical properties to coarser ones, notably with respect to tensile strength and ductility [3]. And the finer the primary dendrite arm spacing, the better mechanical properties it performs. So far, many processes have been developed to decrease the primary dendrite arm spacing, for example liquid metal cooling [1], zone melting liquid metal cooling [4], and directional rapid solidification by highly cooled bulk melt [5]. Nevertheless, these methods

∗ Corresponding author. Tel.: +86 21 56331102; fax: +86 21 56332939. E-mail addresses: [email protected] (W. Ren), [email protected] (Z. Ren). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.08.025

are all restricted in improving the thermal gradient and cooling rate, and they cannot be improved endlessly because of the practical process conditions. Another method which can dramatically optimize the dendritic structures is thirstily needed. And the high static magnetic field maybe act as the very role. With the advance of the superconducting magnets, strong magnetic fields have become readily available and are being applied to materials science. In this trend a lot of researches about the effect of strong magnetic field on the processing of materials have been done. The influence of high static magnetic field on dendrite growth behavior of directionally solidified Al–4.5%Cu is recently reported by Ren et al. [6–8]. The results show that a static magnetic field can influence the dendrite size and number, critical transformation speeds from cellular to dendrite crystal, growth direction, and so on. The effects of axial magnetic fields on the segregation during vertical Bridgman growth of Hg0.8 Cd0.2 Te were investigated by Watring [9]. The percent radial segregation was reduced from a value of 47% to 10% with the application of the magnetic field, which implies that the dendrite morphology and segregation of directional solidified alloys could be controlled by the static magnetic field. Till now, the effect of high static magnetic field on directionally solidified superalloys is rare in literature. A preliminary work has revealed that a high static magnetic field can dramatically increase per area dendrite number for directionally solidified superalloy DZ417G [10]. In the present work the influence of magnetic field on superalloy DZ417G is further investigated, it indicates that with different intensities of the field the influences are different and they are of dependence on the drawing speed. It may provide a new way to pro-

T. Zhang et al. / Journal of Alloys and Compounds 487 (2009) 612–617

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Fig. 2. The schematic illustration of the area counting method of the primary dendrite arm spacing.

3. Results and discussion 3.1. Effect of high magnetic field on the primary dendrite arm spacing

Fig. 1. The schematic illustration of the directional solidification apparatus in a high magnetic field. 1 – water-cooler, 2 – heating elements, 3 – melt, 4 – superconductor magnet, 5 – solid, 6 – LMC cylinder and 7 – withdraw rod.

duce the gas turbine vanes and blades with better microstructures and mechanical properties. 2. Experimental The superalloy DZ417G (C 0.18, Cr 8.96, Mo3.08, Co9.72, V 0.86, B 0.015, Al 5.41, Ti 4.50, Fe 0.23, P0.002, S0.002, Si0.04, Mn0.05, and Ni as balance, wt%) was used in the present work. The alloy ingot with a diameter of 100 mm was prepared in an induction furnace. The samples for directional solidification with a diameter of 2.8 mm and length of 90 mm were cut by electro-discharge machining from the cast ingots and enveloped in a high purity corundum tube with the inner diameter of 3 mm and length of 100 mm. A schematic illustration of the directional solidification apparatus in a high magnetic field is shown in Fig. 1. The experimental apparatus consists of a static superconductor magnet, Bridgman–Stockbarge type furnace, drawing speed and temperature controller. The magnet could produce a vertical static magnetic field with the adjustable intensity of 14 T. The temperature was controlled by a PID controller with a precision of ±1 ◦ C. A water-cooled cylinder containing liquid Ga–In–Sn metal (LMC) was used to cool down the specimen. The growth velocity was controlled by a withdrawing device and could be continuously adjusted between 0.5 and 104 ␮m/s. The apparatus was adjusted to make sure the liquid-solid interface was in the centre plane of the magnet where the magnetic field intensity is homogenous. The samples were made under various magnetic field intensities. The temperature gradient in the present study was 180 K/cm, and the growth velocities were 40 ␮m/s, 80 ␮m/s and 120 ␮m/s. The transverse microstructure was examined in the etched condition by optical microscope. The etchant is composed of CuSO4 (4 g), HCl (20 ml), H2 SO4 (1 ml) and H2 O (16 ml). The primary dendrite arm spacings 1 were measured by the area counting method on the transverse sections, which is shown in Fig. 2 with equation 1 = (A/N)0.5 , where A is the actual area of the region selected and it is chosen a certain value in this paper. N is the average number of primary dendrites in the area A, and more than 30 regions of 3 specimens prepared under the same conditions were measured for the data N in this paper. The microsegregation of the alloying element was determined by EDS (energy disperse spectroscopy). The segregation ratio (SR) is defined as SR = Ci /Cc , where Ci and Cc denote the average concentration of each alloying elements in the interdendrite and dendrite core, respectively. As shown in Fig. 3, three regions in the interdendrite were selected for each primary dendrite, more than three primary dendrites were analyzed for each sample, and the average SR was calculated.

Fig. 4 shows the transverse microstructure of directionally solidified superalloy DZ417G at 40 ␮m/s in various magnetic field intensities with a temperature gradient in the liquid of GL = 180 K/cm. It can be found that the per area dendrite number is dramatically increased with enhancing the intensity of applied magnetic field and reaches a maximum under 6 T intensity. Further increase of magnetic field results in a sharply decrease of the per area dendrite number. Compared with the case of no magnetic field, the application of a 6 T magnetic field increases the per area dendrite number by 62%. The dendrite number with 10 T magnetic field is similar to that without magnetic field. The changing trend of the per area dendrite number with the intensity of the magnetic field is similar with the one of the microsegregation of the thermoelectric material SiGe reported by Dold et al. [11]. However, the results above are different from the one in Ref. [6], in which the per area dendrite number of directionally solidified Al–4.5 wt.%Cu alloy is decreased with enhancing the intensity of magnetic field.

Fig. 3. Scanning areas of EDS in the dendrite structure.

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Fig. 4. Typical transverse microstructure of directionally solidified superalloy DZ417G with various intensities of magnetic field at the drawing speed of 40 ␮m/s (GL = 180 K/cm). (a) 0 T, (b) 2 T, (c) 6 T, (d) 8 T, (e) 10 T.

Fig. 5 shows the transverse microstructure of directionally solidified superalloy DZ417G in various intensities of magnetic field at a drawing speed of 80 ␮m/s with temperature gradient of 180 K/cm. It presents that the intensities of the strong magnetic field at the drawing speed have the same effect on the primary dendrite number as that of 40 ␮m/s (Fig. 4). The primary dendrite arm spacing (PDAS) of directionally solidified superalloy DZ417G with various magnetic field intensities and drawing speeds is shown in Fig. 6. The PDAS goes through a minimum value when the magnetic field is at 6 T. For the magnetic field less than 6 T, the PDAS decreases with increasing the magnetic field intensity. In comparison with that of no magnetic field, the PDAS is decreased by 22% with a 6 T magnetic field at a drawing speed of 40 ␮m/s, at 80 ␮m/s, the decrease is 16% and at 120 ␮m/s, the decrease is only 2% which maybe in the measured error. When the magnetic field is higher than 6 T, the PDAS increases dramatically. Compared to that of the 6 T magnetic field, a 10 T magnetic field can results in an increase of 23% and 22% for the drawing speed of 40 ␮m/s and 80 ␮m/s, separately. And for 120 ␮m/s the increase

is 10%. From the results above it can be also concluded that the effect of magnetic field on the PDAS is of dependence of the drawing speed. And the effect of magnetic field is weakened with increasing the drawing speeds, which is consistent with the results in Ref. [6]. The results above indicate that high magnetic field can significantly affect the primary dendrite arm spacing for directionally solidified superalloy DZ417G. The effect of magnetic field on electrical conductive melt has been studied for decades, and two main mechanisms have been recognized. One is the classical electromag− → − → → → netic braking (EMB) effect, FEMB = (− u × B ) × B , where , − u , and − → B denote the electrical conductivity, the liquid velocity field and the vertical applied magnetic field, respectively. The EMB effect can eliminate the turbulence and instability of the interface, which is not conducive to the development of the primary dendrite, so the EMB effect is considered to increase the PDAS. The other effect of magnetic field on the melt is thermoelectromagnetic convection (TEMC), which has been discussed by Ren et al. [6–8]. It results from the coupling between a thermoelectric current and magnetic field, − → FTEMC = JTE × B . Since the liquid and solid phases have the different


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Fig. 5. Typical transverse microstructure of directionally solidified superalloy DZ417G with various intensities of magnetic field at the drawing speed of 80 ␮m/s (GL = 180 K/cm) (a) 0 T, (b) 2 T, (c) 6 T, (d) 8 T, (e) 10 T.

thermoelectric power, this elementary part of a solid–liquid dendrite interface in a given temperature gradient can be looked upon as a thermocouple, as shown in Fig. 7. The current will flow in the dendrite. Assuming uniform currents (JTE ) in infinitely along dendrites and the temperature gradient (T) [12], the thermoelectric current JTE is expressed as: JTE =

s l2 (s − l )2

fs (˛s − ˛l )∇ T

(1)

where fs is the solid fractions, s , l , ˛s , ˛l , are the electrical conductivity and Seebeck coefficient for solid and liquid, respectively. The thermoelectric current will act the magnetic field to produce the Lorentz force, which results in a new flow. And the newly produced flow promotes the turbulence and instability of the solidification interface, which is helpful to the formation of the dendrite. When the magnetic field is applied to the directional solidification, EMB and TEMC influence the melt at the same time, but

they turn to a contest relationship and the solidified microstructure depends on the winner. In the present work the changes of the PDAS with the intensities of the magnetic field can be explained by the interaction of the EMB and TEMC. Fig. 8 is the schematic illustration of the relationship of FEMB and FTEMC as a function of the external field. As shown in Fig. 8, FEMB can be considered as a function of B2 and FTEMC is directly proportional to B. The different increasing velocities between FTEMC and FEMB result in a difference F because of their opposite directions when the magnetic field is applied. The changing trend of difference F with magnetic field intensity is shown as F in Fig. 8. And there is a B0 where F is the largest. For B < B0 , FTEMC is stronger than FEMB and F increases with increasing the magnetic field intensity, which results in the fast increase of the turbulence and instability of interface and the rapid decrease of PDAS (Fig. 4(b)). When B = B0 , F as well as the turbulence and instability of interface reach their maximum, and the PDAS reaches the minimum (Fig. 4(c)) at the same time. As the intensity increases further (B > B0 ), F begins to decrease

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Fig. 6. Effect of the magnetic field intensity on the primary dendrite arm spacing of directionally solidified superalloy DZ417G with various drawing speed (GL = 180 K/cm).

Fig. 9. The average SR of each alloying elements for directionally solidified superalloy DZ417G with different drawing speed (GL = 180 K/cm, B = 0 T).

and the EMB effect starts to play as a main role in the solidification gradually, which results in the increase of PDAS (Fig. 4(d) and (e)). 3.2. Effect of high magnetic field on the segregation ratio (SR)

Fig. 7. Sketch of thermoelectromagnetic convection in the interdendritic area.

Fig. 8. Schematic illustration of the relationship of FEMB and FTEMC as a function of the external field.

Fig. 9 is the average SR of each alloying elements for directionally solidified superalloy DZ417G with different drawing speeds without magnetic filed. It indicates that Ti, Mo are positive segregation elements (SR > 1), and the SR was reduced rapidly with the drawing speed increases from 40 ␮m/s to 120 ␮m/s. Fig. 10 is the average SR of directionally solidified superalloy DZ417G with various intensities of magnetic field at the drawing speed of 40 ␮m/s. It can be clearly observed that a 6 T magnetic field can dramatically decrease the segregation ratio of Ti, Mo by 28% and 40%, respectively. But when the magnetic field is 10 T, the microsegregation begins to increase, especially for the alloying element Mo. The correlation between SR and magnetic field intensity can be also explained by the interaction of FEMB and FTEMC . When the TEMC effect plays a main role in the solidification, the strengthened liquid flow is helpful to the transfer of the alloying elements. At the same time, the fined dendrite arm spacing which shortens the distance of diffusion and reduces the accumulation of the alloying elements is also of help to the decrease of the microsegregation.

Fig. 10. . The average SR of each alloying elements for directionally solidified superalloy DZ417G with various intensities of magnetic field at the drawing speed of 40 ␮m/s (GL = 180 K/cm).


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4. Conclusion

Acknowledgements

The high static magnetic field was found to significantly influence the primary dendrite arm spacing and microsegregation of the directionally solidified superalloy DZ417G. The primary dendrite arm spacing goes through a minimum at a 6 T magnetic field with increasing the intensities from 0 T to 10 T at various drawing speeds. The influence of magnetic field is dependent upon the drawing speed. With the increasing of the drawing speed the influence is weakened. Compared with that of no magnetic field, the segregation ratio of Ti and Mo were found to be decreased by 28% and 40%, respectively, with a 6 T magnetic field at the drawing speed of 40 ␮m/s. These phenomena are attributed to the correlation between the electromagnetic damping and thermoelectromagnetic convection caused by the high static magnetic field. This provides a new way to control the dendrite number and mechanical properties for directionally solidified superalloys. And the high static magnetic field may serve as a third method in improving the microstructures and mechanical properties of materials besides the cooling speed and the temperature gradient of the solid–liquid interface.

This research was supported by “973” projector for Ministry of Science and Technology of China (No. 2007CB616904), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0739), Shanghai Science and Technology Committee (071005103), Shanghai Educational Committee and National Natural Science Foundation of China (No. 50701031) References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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