Directional solidification of Al–Cu–Si–Mg quaternary eutectic alloy

Accepted Manuscript Directional solidification of Al–Cu–Si–Mg quaternary eutectic alloy Yusuf Kaygisiz, Necmettin Maraşli PII:

S0925-8388(17)32003-0

DOI:

10.1016/j.jallcom.2017.06.027

Reference:

JALCOM 42095

To appear in:

Journal of Alloys and Compounds

Received Date: 2 February 2017 Revised Date:

1 June 2017

Accepted Date: 3 June 2017

Please cite this article as: Y. Kaygisiz, N. Maraşli, Directional solidification of Al–Cu–Si–Mg quaternary eutectic alloy, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

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Directional solidification of Al–Cu–Si–Mg quaternary eutectic alloy Yusuf KAYGISIZ,1* Necmettin MARAŞLI2 1

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Department of Energy Systems Engineering, Ereğli Faculty of Engineering and Natural Sciences, Necmettin Erbakan University, Konya, 42310, Turkey 2 Department of Metallurgical and Materials Engineering, Faculty of Chemistry and Metallurgical Engineering, Yıldız Technical University, Davutpaşa-Esenler, İstanbul, 34210, Turkey

ABSTRACT

The effects of growth rates on the microstructure, microhardness, tensile strength, and electrical

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resistivity were studied in directionally solidified Al–Cu–Si–Mg (Al–28wt.%Cu–6wt.%Si– 2.2wt.%Mg) quaternary eutectic alloy. The directional solidification process was carried out at

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five different growth rates (V = 9.63–173.5 µm/s) at a constant temperature gradient (G = 6.88 K/mm). The microstructure of the directionally solidified Al–Cu–Si–Mg quaternary eutectic alloy consists of Al solid solution, irregular Si plates, and intermetallic Al2Cu (θ) and Cu2Mg8Si6Al5 (Q) phases with honeycomb morphology. The dependencies of lamellar spacing, microhardness, tensile strength, and electrical resistivity on growth rates were found to be λCuAl2 = 19.05 V −0.41 ,

λ Si = 8.74 V −0.46 ,

λCu Mg Si Al = 51.28 V −0.43 , 8

6

5

HV = 237.68 (V ) 0.043 ,

HV = 170.7 + 79.08V 0.25 ,

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2

σ = 343.45 (V ) 0.15 , and ρ = 3.42×10-8 (V)0.10 , respectively, for the Al–Cu–Si–Mg quaternary 2 V = 669.2 , and eutectic alloy. The bulk growth rates were also determined as λ2SiV = 92.24 , λCuAl 2

3 2 2 λCu Mg Si Al V = 4205.5 µm /s by using the measured values of λ Mg Si , λ CuAl , λCu Mg Si Al , and V for 8

6

5

2

2

2

8

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5

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the Si plates and intermetallic Al2Cu (θ) and Cu2Mg8Si6Al5 (Q) phases in the Al–Cu–Si–Mg

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eutectic alloy, respectively.

Keyword: Directional solidification; Quaternary eutectic alloy; Aluminium alloy; Microstructure;

Electrical properties

______________________________________________________________ * Corresponding author: Y. Kaygısız E-mail: [email protected]; Tel: +90 332 777 00 30; Fax: +90 332 777 00 40

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Introduction The solidification of industrial alloys generally proceeds through the formation of single-phase primary crystals such as dendrites or poly-phase structures such as eutectics [1]. Eutectic alloys

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have a low melting point and excellent casting behaviour [2], and hence casting alloys often have eutectic or near-eutectic composition. Directional solidification of eutectic alloys is a selfassembling procedure that allows fabrication from the melt of fine homogeneous microstructures

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(eutectic spacings) controlled by the solidification parameters (temperature gradient and growth rate) [3]. The solidification behaviour and microstructural characteristics of eutectic alloys in

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many systems continue to attract interest because of their influence on the properties and performance of materials containing eutectic constituents.

The directional solidification technique by Bridgman-type equipment is one of the most important techniques for achieving crystal growth and is widely utilized in engineering.

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Aluminium and its alloys are prone to having possible casting defects, such as cracks and distortions, because the volume shrinkage causes stresses. These defects can be minimized by using Bridgman's technique, known as the controlled solidification technique. When eutectic

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alloys are directionally solidified in Bridgman-type equipment, the solidification results in the

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formation of two or more phases that are aligned in the growth direction. This microstructural morphology of eutectic alloys affects their mechanical and thermophysical properties.

Aluminium alloys are produced and used in many forms such as sheets, plates, bars, rods, channels, and forging in various areas of industry, especially the aerospace industry. The advantages of these alloys over traditional iron-based alloys are their light weight, corrosion resistance, and very good thermal and electrical conductivity [4, 5].

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Thus, the aims of this work were to experimentally investigate the dependencies of eutectic spacing (λ), microhardness (HV), tensile strength (σ), and electrical resistivity (ρ) on the growth rate (V) at a constant temperature gradient (G) in Al–28wt.%Cu–6wt.%Si–2.2wt.%Mg

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quaternary eutectic alloy and to compare the present experimental results with the results of the previous experimental studies.

2.1. Alloy preparation and directional solidification

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1. Experimental procedure

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The Al–Cu–Si–Mg phase diagram is given in Fig. 1 [6]. The phase diagram consists of three quaternary eutectic reactions: L → (Al)+Al8Mg5 + Mg2Si + Al6CuMg4 (E1), L → (Al) + Al2Cu + Al2CuMg + Mg2Si (E2), and L → (Al) + (Si) + Al2Cu + Cu2Mg8Si6Al5(Q) (E3) [6], but the present work was limited to the (E2) quaternary eutectic reaction.

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In the present study, the composition of quaternary alloy in the Al–Cu–Si–Mg system was chosen to be Al–28wt.%Cu–6wt.%Si–2.2wt.%Mg for the growth of eutectic phases from

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quaternary liquid according to the phase diagram given in Fig. 1. Thus, the Al–Cu–Si–Mg molten alloy was prepared under vacuum using aluminium,

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copper, silicon, and magnesium, all of 99.99% purity, poured into 13 graphite crucibles (length = 200 mm, inner diameter = 4 mm, outer diameter = 6.35 mm) held in a specially constructed casting furnace (hot filling furnace) preheated to a temperature above the eutectic melting point of 507 oC. The molten alloy was directionally solidified and then each sample was positioned in a Bridgman type furnace. Solidification of the samples was carried out at different growth rates of 9.63–173.5 µm/s at a constant temperature gradient of 6.88 K/mm by using synchronous motors

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running at different speeds. A Bridgman-type directional solidification furnace and detailed information related to the experimental procedure are described in Refs. [8, 9].

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The quenched sample was removed from the graphite crucible and typically cut into lengths of 10 mm each. After each sample was moulded, it was sanded and polished. The polished samples were etched with 2 ml of hydrofluoric acid, 3 ml of hydrochloric acid, and 5 ml

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of nitric acid in 190 ml of distilled water for 25–30 seconds in order to reveal their microstructures.

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2.2. Identification of phases and measurement of solidification parameters

The microstructures of both transverse and longitudinal sections of samples were photographed with a LEO model Scanning Electron Microscope (SEM). The typical SEM images of growth morphologies for directionally solidified Al–28wt.%Cu–6wt.%Si–2.2wt.%Mg quaternary

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eutectic alloy are shown in Fig. 2. According to Fig. 2, regular Al and Al2Cu phases seem to grow together while irregular Si phases appear to grow between the Al and Al2Cu phases, but Cu2Mg8Si6Al5(Q) phases are randomly distributed within the honeycomb structure.

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Quantitative chemical composition analysis of the phases in the samples was carried out using energy dispersive X-ray (EDX) analysis, as shown in Fig. 3. According to the EDX results,

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fine grey, thick grey, white, and black phases were identified as irregular Si plates, quaternary Cu2Mg8Si6Al5 with honeycomb structure, Al2Cu, and α–Al matrix, respectively. The quaternary phase (Cu2Mg8Si6Al5) with the honeycomb morphology structure in this reaction has a hexagonal structure with lattice parameters of fl : 1.032 nm and c =: 0.405 nm [6, 10] and its density is 2.79 g/cm3 [11]. The presence of the quaternary phase is very important for the analysis of both the phase composition and decomposition after solidification and a super-saturated solid solution.

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According to the EDX results, the quantitative chemical composition of the quaternary phase (Cu2Mg8Si6Al5), with the honeycomb morphology structure is 23.29 wt.% Cu, 31.57 wt.% Si, and 30.14 wt.% Mg.

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The Al2Cu phase has a tetragonal structure (space group I4/mmm, 12 atoms per unit cell) with lattice parameters of a = 0.6063 nm, c = 0.4872 nm, and a density of 4.34 g/cm3 [6].

Mg has very high solid solubility in solid (Al) and the solid solubilities of Si, Cu, and Mg

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in solid Al are about 0.77, 4.0, and 12 wt.%, respectively, at the eutectic temperature of 507 oC [5, 7].

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As shown in Fig. 2, the eutectic spacing seems to be different in each grain because they were cut at different angles to the polished surface in a longitudinal view. For that reason, longitudinal sections are inadequate for evaluation of the eutectic spacing without the geometrical correction. It was observed that the values of λ measured on the transverse section are more

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reliable than the values of λ measured on the longitudinal section. The details of the measurements of G, V, and λ are given in Refs. [12, 13].

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2.3. Measurement of microhardness and tensile strength

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In general, the hardness of metallic alloys is greater than the hardness of their individual components. This is because the bonding forces between molecules that differ from each other are larger than those between molecules that are similar to each other. This is the reason why the addition of foreign elements to a metal also leads to an increase in the hardness [14]. The hardness of a metal or alloy also depends on the grain size or lamellar spacing (λ); the smaller the lamellar spacing, the higher the hardness.

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One of the goals of this research is to investigate the relationships between the growth rate and the microhardness and tensile strength of Al–28wt.%Cu–6wt.%Si–2.2wt.%Mg quaternary eutectic alloy.

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The microhardness measurement was done using the Vickers hardness measurement method and the uniaxial tensile test was performed at room temperature with a strain rate of 10-3susing a Shimadzu Universal Testing Instrument (Type AG-10KNG).

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2.4. Measurement of electrical resistivity

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A four-point probe measurement is made by applying four electrical probes to the specimen. The contacts of two of the probes are used to measure the current flowing through the sample and the other probes are used to measure the potential difference between any two points. The constant current and the potential difference on the sample were measured by a Keithley 2400 source

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meter and a Keithley 2700 multimeter, respectively. Platinum probes were used to measure the current and potential. The temperature of the specimen was measured by using a K-type thermocouple with a diameter of 0.5 mm in a

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Nabertherm type P320 furnace. The dimensions of the cylindrical sample were a length of 20 mm

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and radius of 2 mm. In the present work, the electrical resistivity of directionally solidified Al– 28wt.%Cu–6wt.%Si–2.2wt.%Mg quaternary eutectic alloy was measured by the d.c. four-point probe method at room temperature. The measured values of growth rate (V), eutectic spacing (λ), microhardness (HV), tensile

strength (σ), and electrical resistivity (ρ) for Al–28wt.%Cu–6wt.%Si–2.2wt.%Mg quaternary eutectic alloy are given in Table 1.

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3. Results and Discussion 3.1. The effect of growth rate on eutectic spacing

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As expected, the formation of the microstructure varied with growth rate at a constant temperature gradient. As the growth rate increased, the eutectic spacing decreased for all phases at a constant temperature gradient as shown in Table 1. The highest eutectic spacing was obtained

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when the minimum growth rate of 9.63 µm/s was applied, as shown in Fig. 2a, while the smallest eutectic spacing was obtained when the maximum growth rate of 173.5 µm/s was applied, as

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shown in Fig. 2c, at a constant temperature gradient of 6.88 K/mm.

The measured values of lamellar spacing in the Al–Cu–Si–Mg alloy as a function of growth rate at a constant temperature gradient are given in Fig. 2d. A comparison of the present results with the previous experimental results for Al-based eutectic alloys [15–18] is also given in Fig. 4.

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The variation of λ versus V is essentially linear on the logarithmic scale. As can be seen from Fig. 2d, the data form straight lines and the linear regression analysis gives the

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λ = k1 V − n

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proportionality equation for the constant temperature gradient (G) as: (1)

where k1 is a constant and n is an exponent value of the growth rate. The relationships between eutectic spacing and growth rate for the Al2Cu,Si and Cu2Mg8Si6Al5 phases were determined as

λ Al Cu = 19 .05 V −0.41 , λ Si = 8.74 V −0.46 , and λCu2 Mg8 Si6 Al5 = 51.28 V −0.43 , respectively, by using linear 2

regression analysis.

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The exponent value of 0.46 for the growth rates of the Si phase in the Al–Cu–Si–Mg eutectic alloy obtained in the present work is very close to the exponent values of 0.45 and 0.46 for the growth rates in Al–Si–Mg and Al–Si eutectic alloys, respectively, obtained in previous

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works [15–16]. Similarly, the exponent value of 0.41 for the growth rate of the Al2Cu phase in the Al–Cu–Si–Mg eutectic alloy obtained in the present work is slightly higher than the exponent values of 0.35 and 0.39 for the growth rates of Al2Cu phase in Al–Si–Mg and Al–Cu eutectic

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alloys obtained in previous works [17, 18]. The exponent values of 0.46, 0.41, and 0.43 for the growth rates of the Si, Al2Cu, and Cu2Mg8Si6Al5(Q) phases in the Al–Cu–Si–Mg eutectic alloy

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obtained in the present work are also very close the exponent value of 0.50 for the growth rates predicted by Jackson–Hunt eutectic theory [19].

However, the correlation coefficient value of 8.74 for the growth rate of the Si phase in the Al–Cu–Si–Mg eutectic alloy obtained in the present work is about three times smaller than

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both correlation coefficient values of 29.32 and 23.44 for the growth rates of Si phase in Al–Si– Mg and Al–Si eutectic alloys, respectively, obtained in previous works [15–16]. The correlation coefficient value of 19.05 for the growth rate of Al2Cu phase in Al–Cu–Si–Mg eutectic alloy

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obtained in the present work is about 15% smaller than the exponent value of 22.1 for the growth

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rate of Al2Cu in Al–Cu eutectic alloy obtained in a previous work [18], while the correlation coefficient value of 19.05 for the growth rate of Al2Cu phase in Al–Cu–Si–Mg eutectic alloy obtained in the present work is three times bigger than the exponent value of 6.35 for the growth rate of Al2Cu in Al–Cu–Mg eutectic alloy obtained in a previous work [17]. These disparities might be due to the existence of Cu2Mg8Si6Al5 intermetallic phase as an extra phase in the Al– Cu–Si–Mg eutectic alloy.

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The bulk growth rates in the Al–Cu–Si–Mg quaternary eutectic alloy were also 2 determined as λ2SiV = 92.24 , λCuAl V = 669.2 , and λ2Cu2 Mg8 Si6 Al5 V = 4205.5 µm3/s by using the 2

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2 measured values of λ Mg 2Si , λCuAl2 , λCu , and V. 2 Mg8 Si6 Al5

3.2. The effect of growth rate on microhardness

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The prime objective in producing metals and alloys is to obtain materials with optimized properties. These properties are related to structure, and thus physical as well as mechanical

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properties form essential parts of metallurgy. The properties of metals and alloys enable the choice of materials in metallurgical engineering [20].

A dislocation that reaches the boundary again cannot continue its slip motion into another grain because of the difference in orientation of the slip planes and directions of the two

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neighbouring grains. Hence grain boundaries serve as obstacles to the movement of dislocations, which pile up near boundaries. Therefore, with decreases grain size, less distance can be travelled by a glissile dislocation before reaching a grain boundary, resulting in higher strength. This

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strengthening mechanism is called grain size hardening [21]. Strengthening due to grain size effects can be described by the Hall–Petch equation [22, 23]:

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σ y = σ 0 + kd − m

(2)

where σ0 and k are the coefficients and are affected by the alloy content, grain size uniformity, and shape as well as crystallographic texture and the theoretical exponent value of m is 0.5 for grain size hardening. For example, the value of σ0 is 10 MPa [24] and increases with increasing alloy content and the value of K is 0.065 [25] for pure aluminium.

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In the present work, the lamellar distance (λ) is related to the mean grain size and λ is inversely proportional to the square root of the growth rate (V) according to the eutectic growth theory. Thus, the Hall–Petch type relationship between the microhardness and growth rate can be

HV = HV 0 + K 2 V

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expressed as follows: 1 4

(3)

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where HV0 is the initial microhardnes of equilibrated solid phase with liquid at the melting

can be experimentally determined [13].

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temperature and K2 is a constant that depends on the type of material. The values of HV0, and K2

The variation of microhardness (HV) as a function of growth rate (V) at a constant temperature gradient of 6.88 K/mm for Al–28wt.%Cu–6wt.%Si–2.2wt.%Mg quaternary eutectic alloy is plotted in the Hall–Petch type and exponential forms in Fig. 5. A comparison of the

is also shown in Fig. 5.

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present results with previous experimental results [9, 15, 17] for binary or ternary eutectic alloys

The Hall–Petch type relationship between the microhardness and growth rate for

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directionally solidified Al–Cu–Si–Mg quaternary eutectic alloy was found to be:

(4)

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HV = 170.7 + 79.08V 0.25

The value of 170.7 kg/mm2 given in Eq. (4) might be the initial micro-indentation

hardness value for equilibrated solid eutectic phases in equilibrium with liquid in the Al–Cu–Si– Mg alloy at its melting temperature and the value of 79.08 is the hardness against deformation with respect to growth rate for the directionally solidified Al–Cu–Si–Mg eutectic alloy. As can be seen from Eq. (4), the initial micro-indentation hardness value of 170.7 for the Al–Cu–Si–Mg

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quaternary eutectic alloy obtained in the present work is about twice the values of 65.77 and 65.37 for the Al–Si–Mg [15] and Al–Si–Fe [9] eutectic alloys, respectively, but about 30% smaller than the value of 242.32 for the Al–Cu–Mg [17] eutectic alloy. These disparities might be

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due to the difference between the alloying elements Si, Cu, and Mg and the existence of Cu2Mg8Si6Al5 intermetallic phase as an extra phase in the Al–Cu–Si–Mg eutectic alloy.

It can be also seen from Fig. 5b that an incensement in the solidification parameters leads

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to an increase in the HV values. The dependence of HV on growth rate was also determined as an

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order of growth rate by linear regression analysis and the exponential relationship between them can be expressed as:

HV = k 2 (V) a

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where k2 is a constant and a is the exponent value for the growth rate. Experimentally, the

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measured microhardness values for directionally solidified Al–Cu–Si–Mg eutectic alloy are also given Table 1. As shown in Table 1, when the solidification rate is increased, the microhardness value increases too. Figure 5b shows the variation of HV as a function of growth rate (V) in the

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exponential form for Al-based binary or multicomponent alloys [8, 9, 15, 17, 26].

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The exponential relationship between HV and V for directionally solidified Al–Cu–Si–Mg eutectic alloy was determined to be:

HV = 237.68(V ) 0.043

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The exponent value of 0.043 for the growth rate of Al–Cu–Si–Mg obtained in the present work agrees with the exponent values of 0.032 and 0.03 for the growth rates of Al–Cu–Mg [17] and Al–Si–Fe [9] eutectic alloys obtained in previous works, respectively, but is about half the exponent values of 0.07 and 0.08 for the growth rates of Al–Si–Mg [15], Al–Si–Ni [26], and Al–

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Cu [27] eutectic alloys obtained in previous works, respectively. The reason for this might be the existence of Cu2Mg8Si6Al5 intermetallic phase as an extra phase in the Al–Cu–Si–Mg eutectic alloy.

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The coefficient value of 237.68 for the quaternary Al–Cu–Si–Mg eutectic alloy is somewhat larger than the coefficient values of 119.9, 72.44, 66.6, and 82.79 for Al–Si–Mg [15], Al–Si–Ni [26], Al–Si–Fe [9], and Al–Si [8] eutectic alloys, respectively, obtained in previous

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works, but smaller than the coefficient values of 308.3 and 323.6 obtained for Al–Cu–Mg [17] and Al–Cu [27] eutectic alloys, respectively. The metastable Al2Cu and Cu2Mg8Si6Al5

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intermetallic phases ensure a substantial effect of dispersion hardening in the course of decomposition of supersaturated solid solutions.

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3.3. The effect of growth rate on tensile strength

The variation of tensile strength with growth rate for quaternary Al–Cu–Si–Mg eutectic alloy is plotted in Fig. 6 and the experimental results for tensile strength are given in Table 1. A

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comparison of the present results with the experimental results obtained in the previous work for Al–Si–Mg [15], Al–Cu–Mg [17], Al–Si–Fe [9], and Al–Si–Ni [26] eutectic alloys is also given in

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Fig. 6. It can be seen from Fig. 6 and Table 1 that the value of tensile strength for quaternary Al– Cu–Si–Mg eutectic alloy increases with increases in the value of V, and the dependence of σt on V can be expressed as:

σ t = k 3 (V) m

where k3 is a constant and m is the exponent value for the growth rate.

(7)

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From Fig. 6, the value of tensile strength also increases from 172.0 to 260.0 MPa with increasing growth rate from 9.63 to 173.5 µm/s and the relationship between σt and V was found to be:

σ t = 343.45(V) 0.15

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(8)

The exponent value of 0.15 for quaternary Al–Cu–Si–Mg eutectic alloy agrees with the exponent value of 0.14 obtained for Al–Cu–Mg [17] and Al–Si–Fe [9] but is approximately 30% lower

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than the exponent values of 0.20 and 0.19 obtained for Al–Si–Mg [15] and Al–Si–Ni [26] ternary eutectic alloys, respectively, and 35% higher than the exponent value of 0.10 obtained for Al–Cu

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[27] binary eutectic alloy . The coefficient value of 343.45 MPa K−0.15 s0.15 for quaternary Al–Cu– Si–Mg eutectic alloy is close to the coefficient value of 300.6 MPa K−0.10 s0.10 obtained for Al–Cu [27] binary eutectic alloy but about 55% bigger than the coefficient values of 222.84 MPa K−0.20 s0.20 and 119.62 MPa K−0.19 s0.19 obtained for Al–Si–Mg [15] and Al–Si–Ni [26] ternary eutectic

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alloys, respectively, and 15% smaller than the coefficient value of 408.60 MPa K−0.14 s0.14 obtained for Al–Cu–Mg [17] eutectic alloy. These disagreements might be due to the different

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alloying elements and existence of different phases.

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3.4. The effect of growth rate on electrical resistivity The variation of electrical resistivity with growth rate for quaternary Al–Cu–Si–Mg eutectic alloy is plotted in Fig. 7. A comparison of the present results with the experimental results obtained in previous work for Al–Si–Mg [15], Al–Cu–Mg [17], Al–Si–Fe [9], and Al–Si–Ni [26] eutectic alloys is also shown in Fig. 7. The experimental results for the tensile strength of quaternary Al– Cu–Si–Mg eutectic alloy are given Table 1. It can be seen from Fig. 7 that the value of electrical

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resistivity of the directionally solidified Al–Cu–Si–Mg eutectic alloy increases with increases in the value of V. The dependence of ρ on V can be expressed as:

ρ = k 4 (V) e

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(9)

where k4 is a constant and e is the exponent value for the growth rate.

From Fig. 7, the relationships between ρ and V were found to be:

ρ = 3.42× 10 -8 (V) 0.10

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(10)

The exponent and coefficient values for quaternary Al–Cu–Si–Mg eutectic alloy were determined

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experimentally to be 0.10 and 3.42, respectively. The values of the exponent and coefficient obtained in the present work are in good agreement with the exponent and coefficient values of 0.08 and 0.12 for ternary Al–Si–Ni [26] eutectic alloy and 4.57 and 5.22 for ternary Al–Si–Fe [9] eutectic alloy, respectively, but the coefficient value is somewhat smaller than the coefficient

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values of 28.82 and 32.31 for ternary Al–Cu–Mg [17] and Al–Si–Mg [15] eutectic alloys. Thus,

4. Conclusions

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the disparity might be due to the difference in alloying elements.

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In this work, we experimentally examined the dependency of the eutectic spacings (λ), microhardness (HV), tensile strength (σ), and electrical resistivity (ρ) on growth rate (V) at constant temperature gradient (G) in Al–28wt.%Cu–6wt.%Si–2.2wt.%Mg quaternary eutectic alloy. The results obtained in the present work for the quaternary Al−Cu−Si–Mg eutectic alloy are summarized below:

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a. The relationships between the eutectic spacings and growth rates were determined to be

λ Al Cu = 19 .05 V −0.41 , λ Si = 8.74 V −0.46 , and λCu 2 Mg8 Si6 Al5 = 51.28 V −0.43 by using linear 2

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regression. b. The dependency of HV on growth rate in the Hall–Petch and exponential forms was determined to be HV = 170.7 + 79.08V 0.25 and HV = 237.68(V ) 0.043 , respectively, by

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using linear regression analysis.

c. The initial micro-indentation hardness value for equilibrated solid eutectic phases in

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equilibrium with liquid in this alloy system at its melting temperature was determined to be 170.7 kg/mm2.

2 d. The value of microhardness increases from 195 ± 3.0 to 223 ± 5.0 kg/mm as the growth

rate increases from 9.63 to 173.5 µm/s.

e. The value of tensile strength also increases from 172.0 to 260.0 MPa as the growth rate

σ t = 343.45(V) 0.15 .

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increases from 9.63 to 173.5 µm/s and the relationship between σt and V was found to be

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f. The relationships between ρ and V were found to be ρ = 3.42× 10 -8 (V) 0.10 . g. As mentioned above, the existence of Cu2Mg8Si6Al5 intermetallic phase in the quaternary

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Al−Cu−Si–Mg eutectic alloy is the cause of the disparity between the present results and

those obtained in previous works for Al-based binary or ternary eutectic alloys.

Acknowledgements

This project was supported by Erciyes University Scientific Research Project Unit under Contract No. FBD-12-4122. The authors are grateful to Erciyes University Scientific Research Project Unit for financial support.

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alloy. J. Mater. Eng. Perform., 12 (5) (2003) 544–551. S. Engin, U. Büyük, N. Maraşlı, The effects of microstructure and growth rate on microhardness, tensile strength, and electrical resistivity for directionally solidified Al-NiFe alloys, J. Alloys Compd., 660 (2016) 23–31. [10] M.E. Drits, N. R. Bochvar, I dr./. Diagrammy sostoyaniya system na osnove alyuminiya I magniya: Spravoch.izd. Nauka, Moscow (1977), p. 228. [11] J.E. Hatch, Ed. Aluminum: Properties and Physical Metallurgy, ASM (1984) Metals park.

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solidified Al-Rich Al-Cu Alloys, Met. Mater. Int., 19 (3) (2013) 411–422.

ACCEPTED MANUSCRIPT Fig.1. Thermal equilibria phase diagram of Al-Cu-Mg-Si alloy system [6]. Fig.2. Typical SEM images of growth morphologies of directionally solidified the Al-Cu-SiMg ternary eutectic alloy at a constant temperature gradient of 6.88 K/mm with growth rate of (a) 9.63 µm/s (b) 46.1 µm/s, and (c) 173.5 µm/s, and (d) the variation of eutectic spacings as a function of growth rate at a constant temperature gradient of 6.88 K/mm.

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Fig.3.The chemical composition analysis of Al-28wt.%Cu-6wt.%Si-2.2wt.%Mg eutectic alloy by using SEM-EDX; The black, white, fine grey and thick grey phases are the α(Al) matrix, Al2Cu, Si and Cu2Mg8Si6Al5 phases, respectively.

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Fig.4. Variations of eutectic spacings as a function of growth rates in the Al based binary and multicomponent eutectic alloys at a constant temperature gradient obtained in present work and previous works.

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Fig.5. Variations of microhardness with growth rates in the Al based binary and multicomponent eutectic alloys at constant temperature gradients in the forms of: a) HallPetch type and b) Exponential. Fig.6. Variations of tensile strength as a function of growth rates in the Al based multicomponent eutectic alloys at constant temperature gradients obtained in present work and previous works.

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Fig.7. Variations of electrical resistivity as a function of growth rates in the Al based multicomponent eutectic alloys at a constant temperature gradient obtained in present work and previous works.

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Alloy

Solidification

(% wt.)

Parameters

λ( Al Cu )

(µ µm)

(µ µm)

9.63

3.07 ± 0.18

7.60 ± 0.39

19.7

2.10 ± 0.21

46.1

1.64 ± 0.16

88.5

1.08 ± 0.20

173.5

0.79 ± 0.14

λ(Q−Cu Mg Si Al )

172

6.84

5.65 ± 0.41

14.88 ± 0.59

202 ± 3.0

183

7.33

3.96 ± 0.41

10.83 ± 0.65

207 ± 4.0

217

8.15

2.90 ± 0.37

7.75 ± 0.64

212 ± 4.0

242

8.50

2.35 ± 0.33

5.26 ± 0.58

223 ± 5.0

260

9.16

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5

ρ × 10-8

195 ± 3.0

5

(µ µm)

λ(Q−Cu Mg Si Al ) : Eutectic spacings values measured from transverse section of the samples for Cu5Mg8Si6Al5 phases. 8

σt

Electrical resistivity

18.35 ± 0.68

6

λ( Al Cu ) : Eutectic spacings values measured from transverse section of the samples for Al2Cu phases. 2

Strength

(Ωm)

8

λ( Si ) : Eutectic spacings values measured from transverse section of the samples for Si phases. 2

Tensile-

(N/mm )

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Microhardness

HV (kg/mm2)

2

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Eutectic Spacings

λ( Si )

G V (K/mm) (µ µm/s)

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Table 1. Experimental measured values of solidification parameters, eutectic spacings, microhardness, tensile-strength and electrical resistivity for directionally solidified Al-Cu-Si-Mg quaternary eutectic alloy.

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ACCEPTED MANUSCRIPT • The dependency of HV on V was obtained as HV = 237.68(V ) 0.043 for Al-Cu-Si-Mg eutectic alloy. • The dependency of σ t on V was obtained as σ t = 343.45(V) 0.15 for Al-Cu-Si-Mg eutectic alloy

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• The dependency of ρ on V was obtained as ρ = 3.42× 10-8 (V)0.10 for Al-Cu-Si-Mg eutectic alloy.

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• Phases were identified as Si, Cu2Mg8Si6Al5 (Q), Al2Cu and α–Al.