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Electro growth of AlCu eutectic alloy
Materials Characterization 161 (2020) 110157
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Electro growth of AleCu eutectic alloy b
b
Sercan Basit , Semih Birinci , Necmettin Maraşlı a b
T
a,⁎
Department of Metallurgical and Materials Engineering, Faculty of Chemistry and Metallurgical Engineering, Yıldız Technical University, 34210 İstanbul, Turkey Department of Metallurgical and Materials Engineering, Graduate School of Natural and Applied Sciences, Yıldız Technical University, 34210 İstanbul, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Solidification under electric field Aluminium‑copper alloy Microstructure Grain size Hardness
The effects of directions and magnitudes of static electrical field on solidification of Ale33 wt.%Cu eutectic alloy were investigated. For this purpose, a new directional solidification apparatus was specially designed to solidify the AleCu eutectic alloy under static high electrical field (E). For the first time, the AleCu molten eutectic alloy was solidified under different directions; parallel and antiparallel of the solid liquid interface growth direction (E+ and E−, respectively) and magnitudes (7–10 kV/cm) of static high electrical fields. The lamellar spacing (λ), eutectic grain size (EGS) and hardness (HB) of the AleCu eutectic alloy solidified with different values of E+ and E− were measured with standards methods. It was observed that the static electrical field is an effective control parameter on the directional solidification and the values of λ, EGS and HB are increased and decreased with increasing the values of E+ and E−, respectively in the AleCu eutectic alloy. Finally, the dependency of λ, EGS and HB values on E+ and E− values were obtained with linear regression analysis in the AleCu eutectic alloy.
1. Introduction The directional solidified binary or ternary eutectics may produce well aligned regular structures consisting of fibrous (rod-like) or laminated components [1]. Such structures can provide significant increases in creep resistance with respect to high temperature resistance, breakage properties or conventional casting alloys [2]. The eutectic compositions are the basic alloy of many engineering materials. Because the eutectic alloys have a low melting point and excellent casting capability [3]. It is generally preferred to use eutectic or near eutectic compounds in cast alloys [4]. By controlling the alloys solidification parameters; composition of alloy (X), temperature gradient (G) and growth rate (V) with directional solidification, it can be determined how the microstructures and thus basic mechanical and physical properties of the eutectic alloys such as lamellar spacings (λ), microhardness (HV), electrical resistivity (ρ) etc. are changed according to these parameters. So far, numerous theoretical and experimental studies have been carried out on the directional solidification of binary and ternary metallic alloys to reveal the relationship between the solidification parameters and the material properties [5–17]. Most effective controlling parameter for directional solidification is found to be growth rate and it is also limited. Recently, new external control parameters such as alternate current (AC) field, direct current (DC) field, high current density electropulsing, magnetic field and DC electrical field on directional solidification were
⁎
wildly investigated [18–23]. Applying AC and DC electric fields, the magnetic field and DC electro pulsing at high current density to metallic materials have shown to influence the microstructure and mechanical properties of materials [18–23]. The effect of pulse electric field on alloy element distribution and migration in Fe-based binary alloys was investigated and found to be the applied electric field alters both the migration and the distribution of the alloying elements [19]. In the crystal growth process, it was shown that the microstructure formation can be regulated by electrical and magnetic field generated by externally applied alternating current in the BieMn eutectic system [20]. The effect of electro pulsing plus static magnetic field, and a pulse electromagnetic field on the microstructures of the material were examined by Manuwong [21]. The researcher in this study claimed that Lorentz force and magnetic flux were the predominant parameters in grain refinement and enhancing the solute diffusion [21]. The effect of high strength static magnetic field combined with alternating current on the solidification process of immiscible alloys was investigated [22]. In this study, it was concluded that solidified alloys in correctly selected electric-magnetic fields can show a microstructure without macrophase segregation. They have achieved homogeneous solid structure under the conditions of 85 N/cm3 and frequency 50 Hz of electric magnetic body forces. In addition, researchers agree that the mechanism of the electric field effect remains unclear [18,19]. The applied electric field was found to be a major effect on the solidification of AleCu eutectic alloys
Corresponding author. E-mail address: [email protected] (N. Maraşlı).
https://doi.org/10.1016/j.matchar.2020.110157 Received 28 August 2019; Received in revised form 17 January 2020; Accepted 21 January 2020 Available online 22 January 2020 1044-5803/ © 2020 Elsevier Inc. All rights reserved.
Materials Characterization 161 (2020) 110157
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consists of heating furnace, cylindrical cooling tank and static high electrical field system. The heating furnace was constructed from an alumina tube (90 mm ID, 110 mm OD and 150 mm long). Kanthal-A resistance wire insulated with ceramic bids was wound on the tube to give a hot zone; 100 mm long from 20 mm bottom of alumina tube and the heating element was covered with fire clay cement. The alumina tube was placed into Yutong stone placed into metal carcass. The furnace was set up with the tube axis vertical. The heating furnace temperature was controlled by with an on/off temperature controller using a K-type 0.5 mm thick a thermocouple insulated but fixed to the heater windings. The cylindrical cooling tank is made of stainless steel. It had a 38 mm ID, 42 mm OD, and 150 mm in length and both ends of stainless steel bore was closed. All joints on the cooling tank were made by welding and tested for air leaks by placing under water using pressurized air. A 50 mm length of the cooling tank was vertically inserted into heating furnace alumina tube at the bottom of heating furnace. The temperature of water in the cooling tank was kept at 291 K using a heating/refrigerating circulating bath in the present work. Static high electrical field line was set by connecting a FUG HCP 1400–35,000 type DC high voltage power supply with the graphite specimen crucible, directly placed on stainless steel cooling tank and placing a stainless steel disc connected to earth at a few mm far from the graphite specimen crucible in series or vice versa as shown in Fig. 2. FUG HCP 1400–35,000 type DC high voltage power supply gives 35 kV with maximum 40 mA outputs. As shown in Fig. 1, a space maker is used to sensitively determine the electrical field on the specimen. The space maker was made by welding vertically a stainless steel screw (200 mm in length and 4 mm in diameter) at the centre of the stainless steel disc (4 mm thick and 49 mm in diameter) and hanging it at the ceramic brick disc centre with nuts as shown in Fig. 1. The spaces of 6–12 mm between the specimen and stainless steel disc were set using screw and nuts and measured from photographs of space maker. The space between the specimen and stainless steel disc were measured from the photographs of the space maker with an accuracy of ± 0.5 mm. The fixed distances between the specimen and stainless steel disc were measured as about 18 mm from the photographs of the space maker. The fractional uncertainty in the measurements of the fixed distances between the specimen and stainless steel disc is about 2.7%.
[23]. In the experiment, liquid metal was used directly as one of the electrodes and a static electric field which is normal to the growth direction of liquid-solid interface was applied [23]. They observed that both positive and negative electrical fields increased the lamellar spacing and grain size as it reduced the number of grains. Despite all these study and investigation, there is a not sufficient study on solidification of metallic alloys under static high electric field. So far, the static high electrical field was not applied to be parallel or antiparallel of the solid-liquid interface growth direction to see real effects on growth rate in the literature. Besides previous studies do not have a suitable prototype design for producing an industrial cast under uniform electrical fields. Thus, the effects of directions and magnitudes of static high DC electric field on the microstructure and hardness in the Ale33 wt.%Cu eutectic alloy were aimed to investigated in present work. For this purpose, a new directional solidification apparatus under static high electrical field might be a prototype for industrial production is designed. Directions of the static high electrical fields are chosen to be parallel and antiparallel of the solid liquid interface growth direction to see clearly its effects on the microstructure. The AleCu eutectic alloy at near the eutectic composition is solidified under the different directions and magnitudes of static high electrical field and the microstructure parameters (the lamellar spacing and eutectic grain size) and hardness of the AleCu eutectic alloy are measured. Finally, the dependency of microstructure parameters and hardness on direction and magnitude of static high electrical field are obtained with regression analysis. 2. Experimental set up and procedure 2.1. Experimental apparatus for directional solidification under high static electric field An experimental apparatus for directional solidification under static high electric field is specially designed and its schematic illustration is shown in Fig. 1. As can be seen from Fig. 1, the experimental apparatus DC power source or earth connection cable Screw Stainless steel nut Ceramic brick
2.2. Sample preparation
Stainless steel washer Outer alumina tube
The specimen produced with none electrical field and static high electrical fields for measuring the lamellar spacing, eutectic grain size and hardness must be solidified under certain and same solidification or growth conditions otherwise an experimental error comes from different microstructural length scale due to different solidification conditions or growth rates. To prevent length scale problems, the directional solidification conditions with none electrical field such as composition of alloy, the preheating temperature of heating furnace, liquid alloy temperature and cooling water temperature and flow rate were identified and kept constant and same size graphite crucibles were used for directional solidifications under static high electric fields. In present study, Ale33 wt% Cu alloy was prepared with the 4 N purity of Al and Cu metallic elements supplied by Alfa Aesar company. A volume of approximately 45 cm3 metals were melted in a graphite crucible (10 mm ID × 40 mm OD, 80 mm in depth and 100 mm in length) to form the molten alloy using the vacuum melting furnace. When the sufficient metals of Al and Cu were melted, the molten alloy was mixed with one end closed an alumina tube (1.2 mm ID × 2 mm OD and 200 mm in length) within 20 min intervals at least 3 times to get homogenisation. The solidification furnace under static electrical field was preheated up to 150 K above the eutectic melting point of AleCu alloy. During the preheating of solidification furnace, there was no water into the cooling tank. After stirring, the molten alloy into
Inner alumina tube Stainless steel disc
Ceramic insulation
Molten alloy
Metal carcass
TC alumina tube
Heating wires
Graphite Crucible
Ceramic plate Stainless steel cooling tank Water outlet Water inlet
DC power source or earth connection cable
Fig. 1. Experimental set up for directional solidification under static electrical field. 2
Materials Characterization 161 (2020) 110157
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Al+3e
Stainless steel disc E+
Cu+e
F+
Eutectic liquid
HIGH DC POWER SUPPLY
S- L interface growth direction
Solid- liquid interface Eutectic solid
Current (mA) (a)
Al+3e E F
Current (mA)
Cu+e Eutectic liquid
HIGH DC POWER SUPPLY
S- L interface growth direction
Solid- liquid interface Eutectic solid
(b) Fig. 2. Schematic illustrations of liquid atom charging and atomic mass transfers from liquid to solid with static electrical field; (a) positive electrical field and (b) negative electrical field.
the cooling water was turn on and the input power for heating furnace was turn off to directionally solidify the specimen from bottom to top under static high electrical field. When the heating furnace temperature reached to 70 K below the eutectic melting temperature of alloy, the high DC voltages generator power was turned off. The time for applying high voltage was about 25 min during the solidification of molten alloy. Finally the samples were quickly taken from the graphite crucible and rapidly quenched by throwing it into a water tank at a temperature of 25 degrees. The solidification experiments are carried out under the different directions (positive and negative) and magnitudes of high electric fields (7–10 kV/cm). In addition, a specimen was solidified under the same solidification conditions with none the electric field for a reference specimen. Electric field can be expressed as 1 V/m, as well as 1 N/C. Output voltages of the high DC voltages generator power were directly read from front panel of generator with an accuracy of 0.3%. The estimated error in the measurements of electrical field, E can be expressed as
graphite crucible was taken out from the vacuum melting furnace and placed on the cooling tank inserted into the heating alumina tube at the bottom of heating furnace. Static high electrical field directions were chosen as positive, E+ (parallel to the direction of solid-liquid interface growth direction) and negative, E− (antiparallel to the direction of solid-liquid interface growth direction). In present work, the solid liquid interface growth direction was vertically from bottom of specimen to top of specimen i.e. +y direction. Thus, the output of high DC power supply was connected to the cooling tank and the stainless steel disc was connected to earth to get positive static electrical field (E+) into molten alloy as shown in Fig. 2(a). The output of high DC power supply was connected to the stainless steel disc and the cooling tank was connected to earth to get negative static electrical field (E−) into molten alloy as shown in Fig. 2(b). Thus, the negative electrical field direction is opposite the direction of solid-liquid interface growth i.e. –y direction. To see clearly the effects of electrical field on the microstructures, the applied electrical field force on the charged liquid atoms must be equal or bigger than the sum of surface tension and fluidity force of liquid atoms at least. Therefore, the critical electrical field value dependence on kind of alloys and it must be identified. Thus, the required critical electrical value for the Al Cu eutectic alloy was found to be 6 kV/cm by doing a few experimental works. The space between the graphite crucible and the stainless steel disc was set as 6–12 mm. The required electric field was applied avoiding of possible electric arc, electropulsing or electric current using high DC voltages generator. After applying the required voltage to molten alloy,
∣
ΔE ΔV ΔX∗ ∣=∣ ∣+∣ ∣ E V ΔX
(1)
where ΔV is the uncertainty in the voltage reading, V is the applied voltage, ΔX⁎ is the uncertainty in the distance measurements, and ΔX is the distance between fixed distances between the specimen and stainless steel disc. The total estimated error in the measurements of electrical field, E is sum of the estimated error in the measurements of voltage reading and fixed distances between the specimen and stainless steel disc. Thus, the 3
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Fig. 3. Typical optical images of AleCu eutectic alloy solidified under different values of positive static electric fields; (a) none electrical field (b) 8.1 kV/cm (c) 8.8 kV/cm and (d) 10.0 kV/cm.
estimated error in the measurements of electrical field, E is about 3.0%.
Tables 1–6 [5]. The statistical error in the measurement of the grain size is twice of the statistical error in the measurement of the eutectic spacing and thus it is about 20%.
2.3. Microstructure observation and measurements of lamellar spacing and eutectic grain size
2.4. Hardness measurement The solidified AleCu specimen dimension was about 10 mm in diameter and 80 mm in length. For metallographic progress, a part of 3–5 mm length from the top of sample was removed to eliminate oxide layer possibility and the rest of specimen was then cut into 10 mm lengths. The cross sections of 3 different pieces cut at 5, 15 and 25 mm below from top of rest specimen were mounted using a transparent thermoplastic resin. Grinding and polishing processes were carried out by MiniTech 263 polishing machine. After polishing, the samples were etched with Keller agent (1.5 ml HCl, 1 ml HF, and 2.5 ml HNO3, 95 ml H2O) to expose the microstructures of the samples for 35–40 s. Microstructure of AleCu eutectic alloy is well known as lamellar structure of Alα with CuAl2 (θ). The microstructure images were taken from cross sections of samples using a Nikon ECLIPSE MA100 model optical reverse metal microscope. Typical optical images of AleCu eutectic alloy solidified under none electrical field and different directions and magnitudes of static high electric field are shown in Fig. 3 and Fig. 4. Lamellar spacing and grain size were measured from optical images taken from cross sections of specimen using the NIS-Elements Version 4.30 software supplied by Nikon Company. The lamellar spacing is considered utilising linear intercept method [16]. The lamellar spacing was measured according to Eq. (2)
λ=
Δx n−1
Hardness is one of the most important parameters that give an idea about the mechanical properties of the materials. The Brinell hardness, HB can be expressed as
HB =
2F πD[D −
D2 − d2 ]
=
2mg πD[D −
D2 − d2 ]
(3)
where m is the used load mass standardized (10 g to 50), g is the gravitational acceleration (9.81 m/s2), d is the track diameter and D is the ball diameter. The measurements of hardness for directionally solidified samples under different directions and magnitudes of static electrical field were made with a standardized Brinell device, has 2.5 mm diameter steel ball using a 62.5 kg load. For hardness measurements, Bulut rbov-200 accredited, calibrated hardness tester was used. A hardness value for each transverse section is the average of at least 10 measured values. In order to check the accuracy of the applied load value, the d/D ratio (where d is the track diameter and D is the ball diameter) was found in the range of 0.4–0.5. The measured ratio values of 0.4–0.5 were found to be in the range of the required ratio values of 0.20–0.70 for sensitive hardness measurements with hardness tester. In the hardness measurements, the values of sample radius (r), the ball diameter (D) and track diameter (d) are measured. Thus, the estimated experimental error in the measurement of HB is the sum of fractional uncertainty of the sample radius and diagonal length (d) and it can be expressed as
(2)
where λ is the lamellar spacing, Δx is total distance of n lamellar and n is the number of lamella. For the statistical reliability at least 10 measurements for each cross section were made. So, the statistical error in the measurement of the eutectic spacing is about 10% and given in
∣
ΔHB ΔD Δd ∣ = 2∣ ∣ + 2∣ ∣ HB D d
(4)
The values of r, D and d were measured from photographs sample 4
Materials Characterization 161 (2020) 110157
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Fig. 4. Typical optical images of AleCu eutectic alloy solidified under different values of negative static electric fields; (a) none electrical field (b) 7.8 kV/cm (c) 8.9 kV/cm and (d) 10.2 kV/cm.
Table 1 Lamella spacing of AleCu eutectic alloy solidified under different values of static positive electrical field. Positive static electrical field, E+ (kV/cm)
0
8.1
8.8
10.0
Distance from top of sample (mm)
Lamellar spacing, λ+ (μm)
0.837 0.780 0.798 0.805 ± 0.07
1.16 0.81 0.825 0.932 ± 0.09
1.493 1.031 1.006 1.177 ± 0.07
1.938 1.396 1.68 1.671 ± 0.11
5 15 25
Average lamellar spacing, λ+,
Average,
(μm)
Table 2 Lamella spacing of AleCu eutectic alloy solidified under different values of static negative electrical field. Negative static electrical field, E− (kV/cm)
0
7.8
8.9
10.2
Distance from top of sample (mm)
Lamellar spacing, λ− (μm)
0.837 0.780 0.798 0.805 ± 0,07
0.817 0.7711 0.7955 0.795 ± 0.09
0.6126 0.6444 0.5610 0.606 ± 0.06
0.4680 0.4472 0.4778 0.464 ± 0.03
5 15 25
Average lamellar spacing, λ−,
Average
(μm)
Table 3 Eutectic grain size of AleCu eutectic alloy solidified under different values of static positive electrical field. Positive static electrical field, E+ (kV/cm)
0
8.1
8.8
10.0
Distance from top of sample (mm)
Eutectic grain size, EGS+ (μm2)
25,814.08 26,729.27 23,209.07 25,250.81 ± 5050.16
26,998.84 31,007.45 24,847.46 27,617.92 ± 5523.58
31,202.41 33,555.78 25,652.84 30,137.01 ± 6027.40
44,209.35 34,248.14 38,894.1 39,117.2 ± 7823.44
5 15 25
Average eutectic grain size, EGS+, (μm2)
Average
5
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Table 4 Eutectic grain size of AleCu eutectic alloy solidified under different values of static negative electrical field. Negative static electrical field, E− (kV/ cm)
0
7.8
8.9
10.2
Distance from top of sample (mm)
Eutectic grain size, EGS− (μm2)
25,814.08 26,729.27 23,209.07 25,250.81 ± 5050.16
26,713.78 23,957.21 23,820.54 24,830.51 ± 4966.10
20,993.85 22,589.55 22,545.8 22,042.87 ± 4408.57
14,333.82 13,855.05 12,208.20 13,465.49 ± 2693.09
5 15 25
Average eutectic grain size, EGS−, (μm2)
Average
Table 5 Brinell hardness of AleCu eutectic alloy solidified under different values of static positive electrical field. Positive static electrical field, E+ (kV/cm) −2
Brinell hardness, ΗΒ+ (kg mm
Average Brinell hardness, ΗΒ+,
)
Average
(kg mm−2)
0
8.1
8.8
10.0
Distance from top of sample (mm)
160.46 159.48 163.97 161.30 ± 6.45
158.99 158.43 160.91 159.44 ± 6.38
154.846 157.922 159.23 157.33 ± 6.29
116.792 137.32 139.778 131.30 ± 5.25
5 15 25
Table 6 Brinell hardness of AleCu eutectic alloy solidified under different values of static negative electrical field. Negative static electrical field, E− (kV/cm) −2
Brinell hardness, ΗΒ− (kg mm
)
Average Brinell hardness, ΗΒ− (kg mm−2)
0
7.8
8.9
10.2
Distance from top of sample (mm)
160.46 159.48 163.97 161.30 ± 6.45
162.45 163.43 164.14 163.34 ± 6.53
169.424 167.886 175.764 171.02 ± 6.84
182.47 184.638 194.322 187.14 ± 7.49
5 15 25
positively charged liquid atoms reduces the number of liquid atom transfer from liquid phase to solid phase i.e. reduce the mass transfer rate from liquid phase to solid phase as shown in Fig. 2(a). This means the solid liquid interface growth rate decreases and the lamellar spacing increases with increasing the value of F+ effects on positively charged liquid atoms under the positive static electrical field as shown in Fig. 2(a). The value of λ−,Average decreases with increasing the value of E− as can be seen from Fig. 4 and Table 2. In the case of negative electrical field, first the molten AleCu eutectic liquid alloy is connected to earth and thus, the liquid Al and Cu atoms lose the electron, or electrons, in their highest energy level and become positively charged ions (Al+3 and Cu+). Lateral, the required d.c. voltage was applied to the stainless steel disc and the direction of electrical field force (F−) effects on positively charged liquid Al+3 and Cu+ ions is antiparallel to the solidliquid growth direction. In this situation, the negative electrical field force (F−) increases the rate of liquid Al+3 and Cu+ ions transfer from liquid to solid i.e. increases the mass transfer rate from liquid phase to solid phase as shown in Fig. 2(b). Thus, the solid liquid interface growth rate (V) is increased and the lamellar spacing (λ) is decreased by increasing the value of negative static electrical force (F−) effects on positively charged liquid atoms as shown in Fig. 2(b). The variations of λAverage values with E+ and E− values are plotted and it was given in Fig. 5. As shown in Table 1, Table 2 and Fig. 5, the value of λ+, Average increases with increasing the value of E+ while the value of λ−, Average decreases with increasing the value of E−. The λ+, Average value of 1.67 μm with 10.0 kV/cm electrical field is the twice of the λ+, Average value of 0.805 μm with none electric field while the λ−, Average value of 0.464 μm with 10.2 kV/cm electrical field is about half of the λ+, Average value of 0.805 μm with none electric field. As can be seen from Fig. 5, the data form straight lines, the linear regression analysis gives the dependences of λ on E as,
and the indentation traces. The fractional uncertainty for measurements of D and d are about 1%, respectively. Thus, total estimated experimental error in the measurement of microhardness is about 4%. 3. Results and discussions 3.1. Influence of the static electrical field on lamellar spacing and eutectic grain size As mentioned above, the lamellar spacing of AleCu eutectic alloy solidified under none electrical field and different directions and magnitudes of static electrical field were measured from optical images taken from cross sections of samples by linear intercept method [16]. Typical optical images of the AleCu eutectic alloy solidified with different directions and magnitudes of static electrical field are shown in Fig. 3 and Fig. 4. For each step of solidification (under none electrical field and different directions and magnitudes of electrical field), the lamellar spacing measurements were made from three different cross sections of each sample to see whether there is any difference or not on the lamellar spacing along to length of samples? Measurements show there is no apparent difference in the microstructure along the length of rest specimen whether the electric field in negative or positive electric field because of the potential voltages is identic along the sample lengths. The measured values of λ from different the pieces of rest sample and also average value of λAverage for the AleCu eutectic alloy solidified under none electrical field and different directions and magnitudes of static high electrical field are given in Table 1 and Table 2. It is well known that the application of an electric field to conducting liquid solutions can produce substantial changes in composition due to differences in ionic mobility. The value of λ+,Average increases with increasing the value of E+ for the AleCu eutectic alloy as can be seen from Fig. 3 and Table 1. The reason for this can be explained as fallows. The liquid atoms are positively charged with the applied positive high voltages. Thus, the direction of electrical field force effects on the positive charged liquid atom (F+) is parallel to the solid-liquid growth direction and the static electrical field force (F+) effects on the
λ + = k1 × E+n
(5)
m λ − = k2 × E− −
(6)
where k1 and k2 are constants and n and m are exponent values of E+ 6
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Lamellar spacing, λ (μm)
2.0
parameter for directional solidification. The exponent value of E+ and E− are equal or bigger than 2.00 for AleCu eutectic alloy and also the exponent value of E+ is about 40% bigger than the exponent value of E−. Typical optical images of the eutectic grains for the AleCu eutectic alloy solidified with different directions and magnitudes of static electrical fields are shown in Fig. 6 and Fig. 7. The eutectic grain size measurements were also made from the cross sections of three pieces of each sample for each step of solidifications. Eutectic grain sizes measured from optical images for E+ and E− are given in Table 3 and Table 4. The variations of average eutectic grain size (EGSAverage) with the values of E+ and E− are plotted and they are given in Fig. 8. The dependence of the eutectic grain size values (EGSAverage) on the values of E+ and E− were obtained by linear regression analysis and found as
1 0.9 8.8 0.7 0.6 0.5 0.4
λ+ (μm) =1.672 E+2.77
λ− (μm) = 0.482 E−−2.00
0.3 0.9
0.8
1
Electrical Field (V μm−1)
EGS(+,Average) (μm2) = 38618.85 × E1.69 +
(9)
EGS(−,Average) (μm2) = 13978.73 × E−2.66
(10)
and E−, respectively. The relationships between the λAverage and E+ and E− were determined by using linear regression analysis and found as
As shown in Fig. 8, the value of EGS+, Average increases with increasing the value of E+ while the value of EGS−, Average decreases with increasing the value of E−. The exponent value of 1.69 related with E+ is about half of the exponent value of 2.66 related with E−. This means the value of EGS+, Average dependence on the value of E− is stronger than the value of EGS−, Average dependence on the value of E+ in the AleCu eutectic system.
λ (+,Average) (μm) = 1.672 × E+2.77
(7)
3.2. Hardness dependence on directions and magnitudes of electrical fields
2.00 λ (−,average) (μm) = 0.482 × E− −
(8)
The measured values of Brinell hardness (HB) for directionally solidified samples under different directions and magnitudes of static electrical field are given in Table 5 and Table 6. The variations of HB with E+ and E− are plotted and they were shown in Fig. 9. The relationships between them were obtained by linear regression analysis as
Fig. 5. Lamellar spacing variations with the directions and magnitudes of static electrical field in the AleCu eutectic alloy.
The exponent values of E+ and E− related with the values of λ(+,average) and λ(−,average) were found to be 2.77 and 2.00, respectively. Thus, the static electrical field becomes more effective a control
Fig. 6. Typical optical images of AleCu eutectic grains solidified under different values of positive static electric fields; (a) none electrical field (b) 8.1 kV/cm (c) 8.8 kV/cm and (d) 10.0 kV/cm. 7
Materials Characterization 161 (2020) 110157
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Fig. 7. Typical optical images of AleCu eutectic grains solidified under different values of negative static electric fields; (a) none electrical field (b) 7.8 kV/cm (c) 8.9 kV/cm and (d) 10.2 kV/cm. 50000 45000
HB+= 103.83 E+−0.96
Brinell hardness, HB (kg mm−2)
2
Eutectic grain size, EGS (μm )
40000 35000 30000 25000
20000
15000
EGS+= 38618.85 E+1.69 EGS−= 14955.32 E−−2.88
200
HB−= 5.51 E−0.51
150
10000 0.8
0.9
1
800
Electrical field (V μm−1)
900
1000
Electrical field (V mm−1)
Fig. 8. Eutectic grain sizes variations with the directions and magnitudes of static high electrical field in the AleCu eutectic alloy.
HB(+,Average) (kg mm−2) = 103.83 × E+−0.96
(11)
HB(−,Average) (kg mm−2) = 5.51 × E−0.51
(12)
Fig. 9. Brinell hardness variations with the directions and magnitudes of static electrical field in the AleCu eutectic.
4. Conclusions From present results of microstructure and hardness analysis in the AleCu eutectic alloy solidified under different directions and magnitudes of static high electrical field, the conclusions can be made as follows,
As shown in Fig. 9, the value of HB+, Average decreases with increasing the value of E+ while the value of HB−, Average increases with increasing the value of E−. In the hardness case, the exponent value of E+ is about twice of the exponent value of E− i.e. the dependence of HB value on the value of E+ is stronger than the dependence of HB value on the value of E− in the AleCu eutectic alloy.
1. The applied electric field constitutes an electrical force on liquid atoms or molecules and the electrical field force leads an increment or decrement in the number of atoms or molecules transferring from liquid phase to solid phase when its direction is the parallel or 8
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antiparallel to the solid liquid interface growth direction. The negative and positive electric fields decrease and increase the microstructure parameters such as lamellar spacing and grain size and mechanical properties such as hardness of the eutectic AleCu alloy, respectively. 2. The static electric field can be considered an effective control parameter for the directional solidification of the AleCu alloy to affect the growth rate when its direction is the parallel or antiparallel to the solid liquid interface growth direction. 3. The critical value of static high electrical filed in both positive and negative electrical field directions for the AleCu eutectic alloy was found to be 6 kV/cm. 4. There is no apparent difference in the microstructure along the specimen whether the electric field in negative or positive electric field.
[4] Ü. Bayram, N. Maraşlı, Influence of growth rate on eutectic spacings, microhardness and ultimate tensile strength in the directionally solidified Al-Cu-Ni eutectic alloy, Metall. and Materi. Trans. B 49B (2018) 3293–3305. [5] E. Çadirli, M. Gündüz, E. Çadirli, M. Gündüz, The dependence of lamellar spacing on growth rate and temperature gradient in the lead–tin eutectic alloy, J. Mater. Process Tech. 97 (2000) 74–81. [6] K.A. Jackson, J.D. Hunt, Lamellar and eutectic growth, Trans. Met. Soc. AIME 236 (1966) 1129–1142. [7] R.M. Jordan, J.D. Hunt, The growth of lamellar eutectic structures in the Pb-Sn and Al-CuAl2 systems, Metall. Trans. 2 (1971) 3401–3410. [8] K.B. Kim, J. Lui, N. Maraşlı, J.D. Hunt, The effect of different atomic volumes in the three phases during lamellar eutectic growth. A comparison of experiment and theory in the Al-CuAl2 system, Acta Metall. Mater. 43 (6) (1995) 2143–2147. [9] R. Trivedi, J.T. Mason, J.D. Verhoeven, W. Kurz, Eutectic spacing selection in leadbased alloy systems, Metall. Trans. 22A (1991) 2523–2533. [10] P. Magnin, R. Trivedi, Eutectic growth: a modification of the Jackson–Hunt theory, Acta Metall. Mater. 39 (1991) 453–467. [11] C.A. Norlund, R. Trivedi, Eutectic interface configurations during melting, Metall. Mater. Trans. A 31 (4) (2000) 1261–1269. [12] Y.J. Chen, S.H. Davis, Dynamics and instability of triple junctions of solidifying eutectics: flow-modified morphologies, Acta Mater. 50 (9) (2002) 2269–2284. [13] M. Gündüz, H. Kaya, E. Çadirli, A. Özmen, Interflake spacings and undercoolings in Al–Si irregular eutectic alloy, Mater. Sci. Eng. A 369 (2004) 215–229. [14] E. Çadırlı, A. Ülgen, M. Gündüz, Directional solidification of the aluminium copper eutectic alloy, Mater. Trans. JIM 40 (1999) 989–996. [15] H. Kaya, E. Çadırlı, M. Gündüz, Eutectic growth of unidirectionally solidified bismuth-cadmium alloy, J. Mater. Process. Technol. 183 (2007) 310–320. [16] S.C. Gill, W. Kurz, Rapid solidification Al-Cu alloys—I. Experiment determination of the microstructure selection MAP, Acta Metall. 41 (12) (1993) 3563–3573. [17] H.P. Wang, P. Lü, X. Cai, B. Zhai, J.F. Zhao, B. Wei, Rapid solidification kinetics and mechanical property characteristics of Ni–Zr eutectic alloys processed under electromagnetic levitation state, Mater. Sci. Eng. A 772 (14) (2020) 138660. [18] H. Conrad, Influence of an electric or magnetic field on the liquid–solid transformation in materials and on the microstructure of the solid, Mater. Sci. Eng. A 287 (2005) 205–212. [19] Z.Y. Liao, H.C. Wang, P. Hong, X. Li, J. Lie, S.J. Wang, et al., Effect of pulsed electric field on the distribution and migration of P, S, and Si elements of Fe based alloys, Adv. Mater. Res. 194–196 (2011) 371–374. [20] Y. Ma, L.L. Zheng, D.J. Larson, Microstructure formation during BiMn/Bi eutectic growth with applied alternating electric fields, J. Cryst. Growth 62 (2004) 620–630. [21] T. Manuwong, Solıdıfıcatıon of Metal Alloys in Pulse Electromagnetıc Fıelds, Chulalongkorn University, Thailand, 2015. [22] Y. Zhong, J. Wang, T. Zheng, Y. Fautrelle, Z. Ren, Homogeneous hypermonotectic alloy fabricated by electric-magnetic-compound field assisting solidification, Mater. Today Proc. 2 (2015) S364–S372. [23] B. Liu, Z. Zhao, Y. Wang, Z. Chen, The solidification of Al-Cu binary eutectic alloy with electric fields, J. Cryst. Growth 271 (2004) 294–301.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was supported financially by the Scientific and Technical Research Council of Turkey (TUBİTAK) under contract no. 118M695. The authors are grateful to the Scientific and Technical Research Council of Turkey (TUBİTAK) for their financial supports. References [1] Ü. Bayram, N. Maraşlı, Thermal conductivity and electrical resistivity dependences on growth rate in the directionally solidified Al–Cu–Ni eutectic alloy, J. Alloys Compd. 753 (2018) 695–702. [2] R. Caram, S. Milenkovic, Microstructure of Ni-Ni3Si eutectic alloy produced by directional solidification, J. Cryst. Growth 198–199 (1999) 844–849. [3] W. Kurz, D.J. Fisher, Fundamentals of Solidification, Third edition, Trans Tech, Publications, Aedermannsdorf, Switzerland, 1989, pp. 94–95.
9
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Electro growth of AleCu eutectic alloy b
b
Sercan Basit , Semih Birinci , Necmettin Maraşlı a b
T
a,⁎
Department of Metallurgical and Materials Engineering, Faculty of Chemistry and Metallurgical Engineering, Yıldız Technical University, 34210 İstanbul, Turkey Department of Metallurgical and Materials Engineering, Graduate School of Natural and Applied Sciences, Yıldız Technical University, 34210 İstanbul, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Solidification under electric field Aluminium‑copper alloy Microstructure Grain size Hardness
The effects of directions and magnitudes of static electrical field on solidification of Ale33 wt.%Cu eutectic alloy were investigated. For this purpose, a new directional solidification apparatus was specially designed to solidify the AleCu eutectic alloy under static high electrical field (E). For the first time, the AleCu molten eutectic alloy was solidified under different directions; parallel and antiparallel of the solid liquid interface growth direction (E+ and E−, respectively) and magnitudes (7–10 kV/cm) of static high electrical fields. The lamellar spacing (λ), eutectic grain size (EGS) and hardness (HB) of the AleCu eutectic alloy solidified with different values of E+ and E− were measured with standards methods. It was observed that the static electrical field is an effective control parameter on the directional solidification and the values of λ, EGS and HB are increased and decreased with increasing the values of E+ and E−, respectively in the AleCu eutectic alloy. Finally, the dependency of λ, EGS and HB values on E+ and E− values were obtained with linear regression analysis in the AleCu eutectic alloy.
1. Introduction The directional solidified binary or ternary eutectics may produce well aligned regular structures consisting of fibrous (rod-like) or laminated components [1]. Such structures can provide significant increases in creep resistance with respect to high temperature resistance, breakage properties or conventional casting alloys [2]. The eutectic compositions are the basic alloy of many engineering materials. Because the eutectic alloys have a low melting point and excellent casting capability [3]. It is generally preferred to use eutectic or near eutectic compounds in cast alloys [4]. By controlling the alloys solidification parameters; composition of alloy (X), temperature gradient (G) and growth rate (V) with directional solidification, it can be determined how the microstructures and thus basic mechanical and physical properties of the eutectic alloys such as lamellar spacings (λ), microhardness (HV), electrical resistivity (ρ) etc. are changed according to these parameters. So far, numerous theoretical and experimental studies have been carried out on the directional solidification of binary and ternary metallic alloys to reveal the relationship between the solidification parameters and the material properties [5–17]. Most effective controlling parameter for directional solidification is found to be growth rate and it is also limited. Recently, new external control parameters such as alternate current (AC) field, direct current (DC) field, high current density electropulsing, magnetic field and DC electrical field on directional solidification were
⁎
wildly investigated [18–23]. Applying AC and DC electric fields, the magnetic field and DC electro pulsing at high current density to metallic materials have shown to influence the microstructure and mechanical properties of materials [18–23]. The effect of pulse electric field on alloy element distribution and migration in Fe-based binary alloys was investigated and found to be the applied electric field alters both the migration and the distribution of the alloying elements [19]. In the crystal growth process, it was shown that the microstructure formation can be regulated by electrical and magnetic field generated by externally applied alternating current in the BieMn eutectic system [20]. The effect of electro pulsing plus static magnetic field, and a pulse electromagnetic field on the microstructures of the material were examined by Manuwong [21]. The researcher in this study claimed that Lorentz force and magnetic flux were the predominant parameters in grain refinement and enhancing the solute diffusion [21]. The effect of high strength static magnetic field combined with alternating current on the solidification process of immiscible alloys was investigated [22]. In this study, it was concluded that solidified alloys in correctly selected electric-magnetic fields can show a microstructure without macrophase segregation. They have achieved homogeneous solid structure under the conditions of 85 N/cm3 and frequency 50 Hz of electric magnetic body forces. In addition, researchers agree that the mechanism of the electric field effect remains unclear [18,19]. The applied electric field was found to be a major effect on the solidification of AleCu eutectic alloys
Corresponding author. E-mail address: [email protected] (N. Maraşlı).
https://doi.org/10.1016/j.matchar.2020.110157 Received 28 August 2019; Received in revised form 17 January 2020; Accepted 21 January 2020 Available online 22 January 2020 1044-5803/ © 2020 Elsevier Inc. All rights reserved.
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consists of heating furnace, cylindrical cooling tank and static high electrical field system. The heating furnace was constructed from an alumina tube (90 mm ID, 110 mm OD and 150 mm long). Kanthal-A resistance wire insulated with ceramic bids was wound on the tube to give a hot zone; 100 mm long from 20 mm bottom of alumina tube and the heating element was covered with fire clay cement. The alumina tube was placed into Yutong stone placed into metal carcass. The furnace was set up with the tube axis vertical. The heating furnace temperature was controlled by with an on/off temperature controller using a K-type 0.5 mm thick a thermocouple insulated but fixed to the heater windings. The cylindrical cooling tank is made of stainless steel. It had a 38 mm ID, 42 mm OD, and 150 mm in length and both ends of stainless steel bore was closed. All joints on the cooling tank were made by welding and tested for air leaks by placing under water using pressurized air. A 50 mm length of the cooling tank was vertically inserted into heating furnace alumina tube at the bottom of heating furnace. The temperature of water in the cooling tank was kept at 291 K using a heating/refrigerating circulating bath in the present work. Static high electrical field line was set by connecting a FUG HCP 1400–35,000 type DC high voltage power supply with the graphite specimen crucible, directly placed on stainless steel cooling tank and placing a stainless steel disc connected to earth at a few mm far from the graphite specimen crucible in series or vice versa as shown in Fig. 2. FUG HCP 1400–35,000 type DC high voltage power supply gives 35 kV with maximum 40 mA outputs. As shown in Fig. 1, a space maker is used to sensitively determine the electrical field on the specimen. The space maker was made by welding vertically a stainless steel screw (200 mm in length and 4 mm in diameter) at the centre of the stainless steel disc (4 mm thick and 49 mm in diameter) and hanging it at the ceramic brick disc centre with nuts as shown in Fig. 1. The spaces of 6–12 mm between the specimen and stainless steel disc were set using screw and nuts and measured from photographs of space maker. The space between the specimen and stainless steel disc were measured from the photographs of the space maker with an accuracy of ± 0.5 mm. The fixed distances between the specimen and stainless steel disc were measured as about 18 mm from the photographs of the space maker. The fractional uncertainty in the measurements of the fixed distances between the specimen and stainless steel disc is about 2.7%.
[23]. In the experiment, liquid metal was used directly as one of the electrodes and a static electric field which is normal to the growth direction of liquid-solid interface was applied [23]. They observed that both positive and negative electrical fields increased the lamellar spacing and grain size as it reduced the number of grains. Despite all these study and investigation, there is a not sufficient study on solidification of metallic alloys under static high electric field. So far, the static high electrical field was not applied to be parallel or antiparallel of the solid-liquid interface growth direction to see real effects on growth rate in the literature. Besides previous studies do not have a suitable prototype design for producing an industrial cast under uniform electrical fields. Thus, the effects of directions and magnitudes of static high DC electric field on the microstructure and hardness in the Ale33 wt.%Cu eutectic alloy were aimed to investigated in present work. For this purpose, a new directional solidification apparatus under static high electrical field might be a prototype for industrial production is designed. Directions of the static high electrical fields are chosen to be parallel and antiparallel of the solid liquid interface growth direction to see clearly its effects on the microstructure. The AleCu eutectic alloy at near the eutectic composition is solidified under the different directions and magnitudes of static high electrical field and the microstructure parameters (the lamellar spacing and eutectic grain size) and hardness of the AleCu eutectic alloy are measured. Finally, the dependency of microstructure parameters and hardness on direction and magnitude of static high electrical field are obtained with regression analysis. 2. Experimental set up and procedure 2.1. Experimental apparatus for directional solidification under high static electric field An experimental apparatus for directional solidification under static high electric field is specially designed and its schematic illustration is shown in Fig. 1. As can be seen from Fig. 1, the experimental apparatus DC power source or earth connection cable Screw Stainless steel nut Ceramic brick
2.2. Sample preparation
Stainless steel washer Outer alumina tube
The specimen produced with none electrical field and static high electrical fields for measuring the lamellar spacing, eutectic grain size and hardness must be solidified under certain and same solidification or growth conditions otherwise an experimental error comes from different microstructural length scale due to different solidification conditions or growth rates. To prevent length scale problems, the directional solidification conditions with none electrical field such as composition of alloy, the preheating temperature of heating furnace, liquid alloy temperature and cooling water temperature and flow rate were identified and kept constant and same size graphite crucibles were used for directional solidifications under static high electric fields. In present study, Ale33 wt% Cu alloy was prepared with the 4 N purity of Al and Cu metallic elements supplied by Alfa Aesar company. A volume of approximately 45 cm3 metals were melted in a graphite crucible (10 mm ID × 40 mm OD, 80 mm in depth and 100 mm in length) to form the molten alloy using the vacuum melting furnace. When the sufficient metals of Al and Cu were melted, the molten alloy was mixed with one end closed an alumina tube (1.2 mm ID × 2 mm OD and 200 mm in length) within 20 min intervals at least 3 times to get homogenisation. The solidification furnace under static electrical field was preheated up to 150 K above the eutectic melting point of AleCu alloy. During the preheating of solidification furnace, there was no water into the cooling tank. After stirring, the molten alloy into
Inner alumina tube Stainless steel disc
Ceramic insulation
Molten alloy
Metal carcass
TC alumina tube
Heating wires
Graphite Crucible
Ceramic plate Stainless steel cooling tank Water outlet Water inlet
DC power source or earth connection cable
Fig. 1. Experimental set up for directional solidification under static electrical field. 2
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Al+3e
Stainless steel disc E+
Cu+e
F+
Eutectic liquid
HIGH DC POWER SUPPLY
S- L interface growth direction
Solid- liquid interface Eutectic solid
Current (mA) (a)
Al+3e E F
Current (mA)
Cu+e Eutectic liquid
HIGH DC POWER SUPPLY
S- L interface growth direction
Solid- liquid interface Eutectic solid
(b) Fig. 2. Schematic illustrations of liquid atom charging and atomic mass transfers from liquid to solid with static electrical field; (a) positive electrical field and (b) negative electrical field.
the cooling water was turn on and the input power for heating furnace was turn off to directionally solidify the specimen from bottom to top under static high electrical field. When the heating furnace temperature reached to 70 K below the eutectic melting temperature of alloy, the high DC voltages generator power was turned off. The time for applying high voltage was about 25 min during the solidification of molten alloy. Finally the samples were quickly taken from the graphite crucible and rapidly quenched by throwing it into a water tank at a temperature of 25 degrees. The solidification experiments are carried out under the different directions (positive and negative) and magnitudes of high electric fields (7–10 kV/cm). In addition, a specimen was solidified under the same solidification conditions with none the electric field for a reference specimen. Electric field can be expressed as 1 V/m, as well as 1 N/C. Output voltages of the high DC voltages generator power were directly read from front panel of generator with an accuracy of 0.3%. The estimated error in the measurements of electrical field, E can be expressed as
graphite crucible was taken out from the vacuum melting furnace and placed on the cooling tank inserted into the heating alumina tube at the bottom of heating furnace. Static high electrical field directions were chosen as positive, E+ (parallel to the direction of solid-liquid interface growth direction) and negative, E− (antiparallel to the direction of solid-liquid interface growth direction). In present work, the solid liquid interface growth direction was vertically from bottom of specimen to top of specimen i.e. +y direction. Thus, the output of high DC power supply was connected to the cooling tank and the stainless steel disc was connected to earth to get positive static electrical field (E+) into molten alloy as shown in Fig. 2(a). The output of high DC power supply was connected to the stainless steel disc and the cooling tank was connected to earth to get negative static electrical field (E−) into molten alloy as shown in Fig. 2(b). Thus, the negative electrical field direction is opposite the direction of solid-liquid interface growth i.e. –y direction. To see clearly the effects of electrical field on the microstructures, the applied electrical field force on the charged liquid atoms must be equal or bigger than the sum of surface tension and fluidity force of liquid atoms at least. Therefore, the critical electrical field value dependence on kind of alloys and it must be identified. Thus, the required critical electrical value for the Al Cu eutectic alloy was found to be 6 kV/cm by doing a few experimental works. The space between the graphite crucible and the stainless steel disc was set as 6–12 mm. The required electric field was applied avoiding of possible electric arc, electropulsing or electric current using high DC voltages generator. After applying the required voltage to molten alloy,
∣
ΔE ΔV ΔX∗ ∣=∣ ∣+∣ ∣ E V ΔX
(1)
where ΔV is the uncertainty in the voltage reading, V is the applied voltage, ΔX⁎ is the uncertainty in the distance measurements, and ΔX is the distance between fixed distances between the specimen and stainless steel disc. The total estimated error in the measurements of electrical field, E is sum of the estimated error in the measurements of voltage reading and fixed distances between the specimen and stainless steel disc. Thus, the 3
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Fig. 3. Typical optical images of AleCu eutectic alloy solidified under different values of positive static electric fields; (a) none electrical field (b) 8.1 kV/cm (c) 8.8 kV/cm and (d) 10.0 kV/cm.
estimated error in the measurements of electrical field, E is about 3.0%.
Tables 1–6 [5]. The statistical error in the measurement of the grain size is twice of the statistical error in the measurement of the eutectic spacing and thus it is about 20%.
2.3. Microstructure observation and measurements of lamellar spacing and eutectic grain size
2.4. Hardness measurement The solidified AleCu specimen dimension was about 10 mm in diameter and 80 mm in length. For metallographic progress, a part of 3–5 mm length from the top of sample was removed to eliminate oxide layer possibility and the rest of specimen was then cut into 10 mm lengths. The cross sections of 3 different pieces cut at 5, 15 and 25 mm below from top of rest specimen were mounted using a transparent thermoplastic resin. Grinding and polishing processes were carried out by MiniTech 263 polishing machine. After polishing, the samples were etched with Keller agent (1.5 ml HCl, 1 ml HF, and 2.5 ml HNO3, 95 ml H2O) to expose the microstructures of the samples for 35–40 s. Microstructure of AleCu eutectic alloy is well known as lamellar structure of Alα with CuAl2 (θ). The microstructure images were taken from cross sections of samples using a Nikon ECLIPSE MA100 model optical reverse metal microscope. Typical optical images of AleCu eutectic alloy solidified under none electrical field and different directions and magnitudes of static high electric field are shown in Fig. 3 and Fig. 4. Lamellar spacing and grain size were measured from optical images taken from cross sections of specimen using the NIS-Elements Version 4.30 software supplied by Nikon Company. The lamellar spacing is considered utilising linear intercept method [16]. The lamellar spacing was measured according to Eq. (2)
λ=
Δx n−1
Hardness is one of the most important parameters that give an idea about the mechanical properties of the materials. The Brinell hardness, HB can be expressed as
HB =
2F πD[D −
D2 − d2 ]
=
2mg πD[D −
D2 − d2 ]
(3)
where m is the used load mass standardized (10 g to 50), g is the gravitational acceleration (9.81 m/s2), d is the track diameter and D is the ball diameter. The measurements of hardness for directionally solidified samples under different directions and magnitudes of static electrical field were made with a standardized Brinell device, has 2.5 mm diameter steel ball using a 62.5 kg load. For hardness measurements, Bulut rbov-200 accredited, calibrated hardness tester was used. A hardness value for each transverse section is the average of at least 10 measured values. In order to check the accuracy of the applied load value, the d/D ratio (where d is the track diameter and D is the ball diameter) was found in the range of 0.4–0.5. The measured ratio values of 0.4–0.5 were found to be in the range of the required ratio values of 0.20–0.70 for sensitive hardness measurements with hardness tester. In the hardness measurements, the values of sample radius (r), the ball diameter (D) and track diameter (d) are measured. Thus, the estimated experimental error in the measurement of HB is the sum of fractional uncertainty of the sample radius and diagonal length (d) and it can be expressed as
(2)
where λ is the lamellar spacing, Δx is total distance of n lamellar and n is the number of lamella. For the statistical reliability at least 10 measurements for each cross section were made. So, the statistical error in the measurement of the eutectic spacing is about 10% and given in
∣
ΔHB ΔD Δd ∣ = 2∣ ∣ + 2∣ ∣ HB D d
(4)
The values of r, D and d were measured from photographs sample 4
Materials Characterization 161 (2020) 110157
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Fig. 4. Typical optical images of AleCu eutectic alloy solidified under different values of negative static electric fields; (a) none electrical field (b) 7.8 kV/cm (c) 8.9 kV/cm and (d) 10.2 kV/cm.
Table 1 Lamella spacing of AleCu eutectic alloy solidified under different values of static positive electrical field. Positive static electrical field, E+ (kV/cm)
0
8.1
8.8
10.0
Distance from top of sample (mm)
Lamellar spacing, λ+ (μm)
0.837 0.780 0.798 0.805 ± 0.07
1.16 0.81 0.825 0.932 ± 0.09
1.493 1.031 1.006 1.177 ± 0.07
1.938 1.396 1.68 1.671 ± 0.11
5 15 25
Average lamellar spacing, λ+,
Average,
(μm)
Table 2 Lamella spacing of AleCu eutectic alloy solidified under different values of static negative electrical field. Negative static electrical field, E− (kV/cm)
0
7.8
8.9
10.2
Distance from top of sample (mm)
Lamellar spacing, λ− (μm)
0.837 0.780 0.798 0.805 ± 0,07
0.817 0.7711 0.7955 0.795 ± 0.09
0.6126 0.6444 0.5610 0.606 ± 0.06
0.4680 0.4472 0.4778 0.464 ± 0.03
5 15 25
Average lamellar spacing, λ−,
Average
(μm)
Table 3 Eutectic grain size of AleCu eutectic alloy solidified under different values of static positive electrical field. Positive static electrical field, E+ (kV/cm)
0
8.1
8.8
10.0
Distance from top of sample (mm)
Eutectic grain size, EGS+ (μm2)
25,814.08 26,729.27 23,209.07 25,250.81 ± 5050.16
26,998.84 31,007.45 24,847.46 27,617.92 ± 5523.58
31,202.41 33,555.78 25,652.84 30,137.01 ± 6027.40
44,209.35 34,248.14 38,894.1 39,117.2 ± 7823.44
5 15 25
Average eutectic grain size, EGS+, (μm2)
Average
5
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Table 4 Eutectic grain size of AleCu eutectic alloy solidified under different values of static negative electrical field. Negative static electrical field, E− (kV/ cm)
0
7.8
8.9
10.2
Distance from top of sample (mm)
Eutectic grain size, EGS− (μm2)
25,814.08 26,729.27 23,209.07 25,250.81 ± 5050.16
26,713.78 23,957.21 23,820.54 24,830.51 ± 4966.10
20,993.85 22,589.55 22,545.8 22,042.87 ± 4408.57
14,333.82 13,855.05 12,208.20 13,465.49 ± 2693.09
5 15 25
Average eutectic grain size, EGS−, (μm2)
Average
Table 5 Brinell hardness of AleCu eutectic alloy solidified under different values of static positive electrical field. Positive static electrical field, E+ (kV/cm) −2
Brinell hardness, ΗΒ+ (kg mm
Average Brinell hardness, ΗΒ+,
)
Average
(kg mm−2)
0
8.1
8.8
10.0
Distance from top of sample (mm)
160.46 159.48 163.97 161.30 ± 6.45
158.99 158.43 160.91 159.44 ± 6.38
154.846 157.922 159.23 157.33 ± 6.29
116.792 137.32 139.778 131.30 ± 5.25
5 15 25
Table 6 Brinell hardness of AleCu eutectic alloy solidified under different values of static negative electrical field. Negative static electrical field, E− (kV/cm) −2
Brinell hardness, ΗΒ− (kg mm
)
Average Brinell hardness, ΗΒ− (kg mm−2)
0
7.8
8.9
10.2
Distance from top of sample (mm)
160.46 159.48 163.97 161.30 ± 6.45
162.45 163.43 164.14 163.34 ± 6.53
169.424 167.886 175.764 171.02 ± 6.84
182.47 184.638 194.322 187.14 ± 7.49
5 15 25
positively charged liquid atoms reduces the number of liquid atom transfer from liquid phase to solid phase i.e. reduce the mass transfer rate from liquid phase to solid phase as shown in Fig. 2(a). This means the solid liquid interface growth rate decreases and the lamellar spacing increases with increasing the value of F+ effects on positively charged liquid atoms under the positive static electrical field as shown in Fig. 2(a). The value of λ−,Average decreases with increasing the value of E− as can be seen from Fig. 4 and Table 2. In the case of negative electrical field, first the molten AleCu eutectic liquid alloy is connected to earth and thus, the liquid Al and Cu atoms lose the electron, or electrons, in their highest energy level and become positively charged ions (Al+3 and Cu+). Lateral, the required d.c. voltage was applied to the stainless steel disc and the direction of electrical field force (F−) effects on positively charged liquid Al+3 and Cu+ ions is antiparallel to the solidliquid growth direction. In this situation, the negative electrical field force (F−) increases the rate of liquid Al+3 and Cu+ ions transfer from liquid to solid i.e. increases the mass transfer rate from liquid phase to solid phase as shown in Fig. 2(b). Thus, the solid liquid interface growth rate (V) is increased and the lamellar spacing (λ) is decreased by increasing the value of negative static electrical force (F−) effects on positively charged liquid atoms as shown in Fig. 2(b). The variations of λAverage values with E+ and E− values are plotted and it was given in Fig. 5. As shown in Table 1, Table 2 and Fig. 5, the value of λ+, Average increases with increasing the value of E+ while the value of λ−, Average decreases with increasing the value of E−. The λ+, Average value of 1.67 μm with 10.0 kV/cm electrical field is the twice of the λ+, Average value of 0.805 μm with none electric field while the λ−, Average value of 0.464 μm with 10.2 kV/cm electrical field is about half of the λ+, Average value of 0.805 μm with none electric field. As can be seen from Fig. 5, the data form straight lines, the linear regression analysis gives the dependences of λ on E as,
and the indentation traces. The fractional uncertainty for measurements of D and d are about 1%, respectively. Thus, total estimated experimental error in the measurement of microhardness is about 4%. 3. Results and discussions 3.1. Influence of the static electrical field on lamellar spacing and eutectic grain size As mentioned above, the lamellar spacing of AleCu eutectic alloy solidified under none electrical field and different directions and magnitudes of static electrical field were measured from optical images taken from cross sections of samples by linear intercept method [16]. Typical optical images of the AleCu eutectic alloy solidified with different directions and magnitudes of static electrical field are shown in Fig. 3 and Fig. 4. For each step of solidification (under none electrical field and different directions and magnitudes of electrical field), the lamellar spacing measurements were made from three different cross sections of each sample to see whether there is any difference or not on the lamellar spacing along to length of samples? Measurements show there is no apparent difference in the microstructure along the length of rest specimen whether the electric field in negative or positive electric field because of the potential voltages is identic along the sample lengths. The measured values of λ from different the pieces of rest sample and also average value of λAverage for the AleCu eutectic alloy solidified under none electrical field and different directions and magnitudes of static high electrical field are given in Table 1 and Table 2. It is well known that the application of an electric field to conducting liquid solutions can produce substantial changes in composition due to differences in ionic mobility. The value of λ+,Average increases with increasing the value of E+ for the AleCu eutectic alloy as can be seen from Fig. 3 and Table 1. The reason for this can be explained as fallows. The liquid atoms are positively charged with the applied positive high voltages. Thus, the direction of electrical field force effects on the positive charged liquid atom (F+) is parallel to the solid-liquid growth direction and the static electrical field force (F+) effects on the
λ + = k1 × E+n
(5)
m λ − = k2 × E− −
(6)
where k1 and k2 are constants and n and m are exponent values of E+ 6
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Lamellar spacing, λ (μm)
2.0
parameter for directional solidification. The exponent value of E+ and E− are equal or bigger than 2.00 for AleCu eutectic alloy and also the exponent value of E+ is about 40% bigger than the exponent value of E−. Typical optical images of the eutectic grains for the AleCu eutectic alloy solidified with different directions and magnitudes of static electrical fields are shown in Fig. 6 and Fig. 7. The eutectic grain size measurements were also made from the cross sections of three pieces of each sample for each step of solidifications. Eutectic grain sizes measured from optical images for E+ and E− are given in Table 3 and Table 4. The variations of average eutectic grain size (EGSAverage) with the values of E+ and E− are plotted and they are given in Fig. 8. The dependence of the eutectic grain size values (EGSAverage) on the values of E+ and E− were obtained by linear regression analysis and found as
1 0.9 8.8 0.7 0.6 0.5 0.4
λ+ (μm) =1.672 E+2.77
λ− (μm) = 0.482 E−−2.00
0.3 0.9
0.8
1
Electrical Field (V μm−1)
EGS(+,Average) (μm2) = 38618.85 × E1.69 +
(9)
EGS(−,Average) (μm2) = 13978.73 × E−2.66
(10)
and E−, respectively. The relationships between the λAverage and E+ and E− were determined by using linear regression analysis and found as
As shown in Fig. 8, the value of EGS+, Average increases with increasing the value of E+ while the value of EGS−, Average decreases with increasing the value of E−. The exponent value of 1.69 related with E+ is about half of the exponent value of 2.66 related with E−. This means the value of EGS+, Average dependence on the value of E− is stronger than the value of EGS−, Average dependence on the value of E+ in the AleCu eutectic system.
λ (+,Average) (μm) = 1.672 × E+2.77
(7)
3.2. Hardness dependence on directions and magnitudes of electrical fields
2.00 λ (−,average) (μm) = 0.482 × E− −
(8)
The measured values of Brinell hardness (HB) for directionally solidified samples under different directions and magnitudes of static electrical field are given in Table 5 and Table 6. The variations of HB with E+ and E− are plotted and they were shown in Fig. 9. The relationships between them were obtained by linear regression analysis as
Fig. 5. Lamellar spacing variations with the directions and magnitudes of static electrical field in the AleCu eutectic alloy.
The exponent values of E+ and E− related with the values of λ(+,average) and λ(−,average) were found to be 2.77 and 2.00, respectively. Thus, the static electrical field becomes more effective a control
Fig. 6. Typical optical images of AleCu eutectic grains solidified under different values of positive static electric fields; (a) none electrical field (b) 8.1 kV/cm (c) 8.8 kV/cm and (d) 10.0 kV/cm. 7
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Fig. 7. Typical optical images of AleCu eutectic grains solidified under different values of negative static electric fields; (a) none electrical field (b) 7.8 kV/cm (c) 8.9 kV/cm and (d) 10.2 kV/cm. 50000 45000
HB+= 103.83 E+−0.96
Brinell hardness, HB (kg mm−2)
2
Eutectic grain size, EGS (μm )
40000 35000 30000 25000
20000
15000
EGS+= 38618.85 E+1.69 EGS−= 14955.32 E−−2.88
200
HB−= 5.51 E−0.51
150
10000 0.8
0.9
1
800
Electrical field (V μm−1)
900
1000
Electrical field (V mm−1)
Fig. 8. Eutectic grain sizes variations with the directions and magnitudes of static high electrical field in the AleCu eutectic alloy.
HB(+,Average) (kg mm−2) = 103.83 × E+−0.96
(11)
HB(−,Average) (kg mm−2) = 5.51 × E−0.51
(12)
Fig. 9. Brinell hardness variations with the directions and magnitudes of static electrical field in the AleCu eutectic.
4. Conclusions From present results of microstructure and hardness analysis in the AleCu eutectic alloy solidified under different directions and magnitudes of static high electrical field, the conclusions can be made as follows,
As shown in Fig. 9, the value of HB+, Average decreases with increasing the value of E+ while the value of HB−, Average increases with increasing the value of E−. In the hardness case, the exponent value of E+ is about twice of the exponent value of E− i.e. the dependence of HB value on the value of E+ is stronger than the dependence of HB value on the value of E− in the AleCu eutectic alloy.
1. The applied electric field constitutes an electrical force on liquid atoms or molecules and the electrical field force leads an increment or decrement in the number of atoms or molecules transferring from liquid phase to solid phase when its direction is the parallel or 8
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antiparallel to the solid liquid interface growth direction. The negative and positive electric fields decrease and increase the microstructure parameters such as lamellar spacing and grain size and mechanical properties such as hardness of the eutectic AleCu alloy, respectively. 2. The static electric field can be considered an effective control parameter for the directional solidification of the AleCu alloy to affect the growth rate when its direction is the parallel or antiparallel to the solid liquid interface growth direction. 3. The critical value of static high electrical filed in both positive and negative electrical field directions for the AleCu eutectic alloy was found to be 6 kV/cm. 4. There is no apparent difference in the microstructure along the specimen whether the electric field in negative or positive electric field.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was supported financially by the Scientific and Technical Research Council of Turkey (TUBİTAK) under contract no. 118M695. The authors are grateful to the Scientific and Technical Research Council of Turkey (TUBİTAK) for their financial supports. References [1] Ü. Bayram, N. Maraşlı, Thermal conductivity and electrical resistivity dependences on growth rate in the directionally solidified Al–Cu–Ni eutectic alloy, J. Alloys Compd. 753 (2018) 695–702. [2] R. Caram, S. Milenkovic, Microstructure of Ni-Ni3Si eutectic alloy produced by directional solidification, J. Cryst. Growth 198–199 (1999) 844–849. [3] W. Kurz, D.J. Fisher, Fundamentals of Solidification, Third edition, Trans Tech, Publications, Aedermannsdorf, Switzerland, 1989, pp. 94–95.
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