Materials Letters 68 (2012) 255–257

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Materials Letters

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Thermodynamic evaluation of Al–Mg2Si with addition of Ni

Chong Li a, Jing Hou a, Degang Zhao b, Yuying Wu a, Xiangfa Liu a,⁎a Key Laboratory of Liquid–solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, 17923 Jingshi Road, Jinan 250061, Chinab New Materials Research Institute, Shandong Academy of Sciences, Keyuan Road 19, Jinan, China

⁎ Corresponding author. Tel./fax: +86 531 88395414E-mail address: xfliu@sdu.edu.cn (X. Liu).

0167-577X/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.matlet.2011.10.074

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 September 2011Accepted 21 October 2011Available online 29 October 2011

Keywords:Metals and alloysNickel aluminidesThermodynamicsMicrostructure

This paper presents the thermodynamic calculation of Al–Mg2Si alloys with addition of Ni using the Thermal-Calc software. In Al–Mg2Si–NiAl3 pseudo-ternary phase diagram, the composition of ternary eutectic is Al–12.1%Mg2Si–8.4%NiAl3 and the calculated temperature in equilibrium state is 587.05 °C. For Al–15%Mg2Si–NiAl3 system, two critical compositions were detected at 7.9% and 13.1% NiAl3 where the temperatures ofliquidus and start of binary reaction have mutations. These critical compositions show differences in theformation of NiAl3 intermetallic particles and microstructure during the solidification interval. The calculationof formation temperature shows a good agreement with differential scanning calorimetry (DSC) tests.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The growing demand for reducing light and improving vehicle fueleconomy and emission is a challenge for the automotive industry.Mg2Si reinforced Al-based composites possess considerable perfor-mance advantages (low density, excellent castability, good wear re-sistance and excellent mechanical properties) over aluminum alloysand are now considered as ideal candidates to replace steel and ironin the automotive industry [1–3].

J. Zhang et al. [4] calculated that Al–13.9%Mg2Si was thepseudoeutectic composition in pseudobinary Al–Mg2Si phase diagram.In hypereutectic Al–Mg2Si alloys, primary Mg2Si particles first precipi-tate from the melt, and then Al and Mg2Si binary eutectic co-solidifyfrom the liquid alloys [4,5]. The presence of hard primary Mg2Si parti-cles contributes to the alloys' good mechanical properties and highresistance to wear. So many advanced processing techniques havebeen adopted successfully to control the size, morphology and distribu-tion of primary Mg2Si affecting the performance of alloys [6], such asrapid solidification processing [7], superheating treatment [8] and addi-tions of refiners or modifiers [9–13].

Moreover, the addition of transition element Ni can form eutecticintermetallic phase NiAl3 which is effective for increasing the high tem-perature strength of cast Al–Mg2Si alloys, and the morphology and pre-cipitation sequence of NiAl3 is different with the change of Ni content[14]. However, accurate thermodynamic information and solidificationprocesses of the Al–Mg2Si–Ni are deficient for understanding the system.

In the presentwork, the phase diagram and thermodynamic proper-ties in the Al–Mg2Si–Ni are calculated and analyzed. It is useful for

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optimizing alloy composition and understanding alloy behavior. Theseresults can also be as a guide for alloy development and microstructuremanipulation during solidification process and potential heat treatment.

2. Experimental procedures

To better understand the solidification behavior and com-pound formation of Al–Mg2Si alloys with different Ni contents, theThermal-Calc software was used to investigate the thermodynamicproperties and the solidification behavior [15,16]. The calculationsdone in this study correspond to the equilibrium conditions at eachtemperature. Commercial pure Al (99.7%, wt.% in this work), com-mercial pure crystalline Si (98.5%) and commercial pure Mg and Ni(99.9%) were used as starting materials to prepare Al–15%Mg2Si–x%NiAl3 (x=5, 20) alloys in a 25 kW medium frequency induction fur-nace. The alloy was remelted at 800 °C and held at this temperature.After holding 30 min, the melt was poured into a cast iron mold.

Metallographic specimens were cut at the same position of thetabulate samples and polished through standard procedure. The mi-crostructure characteristics of the specimens were analyzed withscanning electron microscopy (SEM) (model JSM-6700F, Japan).

The thermal analysis of solidification process of Al–15%Mg2Si–x%NiAl3 (x=5, 20) was conducted by using differential scanning calo-rimetry (DSC) (model NETZSCH 404, Germany). The DSC samplewas heated to 700 °C and then cooled to 300 °C at a constant rate of10 °C min−1 in an argon atmosphere.

3. Results and discussion

In Al–Mg2Si–Ni alloys, the nickel can form NiAl3 which can improvethe mechanical properties at elevated temperature [14]. So it can beseen as pseudo-ternary Al–Mg2Si–NiAl3 system.

Fig. 2. Calculated vertical section of Al–15%Mg2Si to NiAl3.

Eutectic NiAl3

a

256 C. Li et al. / Materials Letters 68 (2012) 255–257

Fig.1 shows the calculated Al rich part of Al–Mg2Si–NiAl3 pseudo-ternary phase diagram. The point E represents ternary eutectic reac-tion. Its composition is Al–12.1%Mg2Si–8.4%NiAl3 and the calculatedtemperature in equilibrium state is 587.05 °C. The lines P1E, P2E andP3E represent Al–Mg2Si, Al–NiAl3 and Mg2Si–NiAl3 binary reactiontransformation, respectively. The compositions of P1 and P2 are corre-sponding to Al–13.9%Mg2Si and Al–13.4%NiAl3. From Fig. 1, it can beseen that the Al rich part is divided into six zones, marked from I toVI, in which the solidification processes are different. For example,the sequence of solidification in zone II is as follows: L→Mg2Si;L→(Al+Mg2Si)E; L→(Al+Mg2Si+NiAl3)E. However, if alloy com-position falls into region V, the process of solidification changes andcan be express: L→NiAl3; L→(Al+NiAl3) E; L→(Al+Mg2Si+NiAl3) E.

In order to fully display the strength effect of Mg2Si and NiAl3, thealloy design is focused on hypereutectic Al–Mg2Si with addition of Ni.So to better understand the solidification behavior and compoundformation of alloys with different NiAl3 content, an investigation ofAl–15%Mg2Si to NiAl3 was carried out. The composition of researchedalloys ranges from region II to region IV, shown as color shades ofFig. 1.

Fig. 2 graphically demonstrates the vertical section of the Al–Mg2Si–NiAl3 equilibrium diagram at compositions from Al–15%Mg2Si to NiAl3.The variation of the transition temperatures as a function of NiAl3 con-tent illustrates the two critical NiAl3 points at 7.9% and 13.1%. With theaddition of NiAl3, the NiAl3 phase first appears only in the ternary eutec-tic reaction. However, with NiAl3 contents between 7.9% and 13.1%,NiAl3 appears in both the binary and ternary reactions. Above 13.1%NiAl3, it solidifies as a primary phase as well as during the binary andternary reactions. The transition temperatures, including the liquidus,the start of binary and ternary reactions were also calculated andshown intuitively.

The liquidus temperature increases with the increase of NiAl3 con-tent. It needs to be pointed out that binary reaction changes from Al–Mg2Si to Mg2Si–NiAl3 when the content of NiAl3 is greater than 7.9%.Owing to the variation, the binary reaction temperature initially de-creases from 592.9 °C (without NiAl3) to 587.2 °C (with 7.9% NiAl3),and then increases with the increase of NiAl3 content. However, thevariation of the NiAl3 content has no obvious effect on the isothermalternary reaction temperature (587.05 °C).

Fig. 3 shows the microstructures of Al–15%Mg2Si–x%NiAl3 (x=5,20) alloys, using field emission scanning electron microscopy(FESEM). This figure indicates that the structure changes with the in-crease of NiAl3, as predicated in Fig. 2. When x=5%, NiAl3 exists in

587.05 oC

Fig. 1. Al rich part of Al–Mg2Si–NiAl3 pseudo-ternary phase diagram.

the form of eutectic phase (Fig. 3a). However, increasing the contentof NiAl3 up to 20%, primary NiAl3 appears besides eutectic NiAl3(Fig. 3b). Meanwhile, the morphology of NiAl3 also changes significant-ly from strip or rod (eutectic NiAl3) to large block (primary NiAl3).

The results of DSC test carried out for Al–15%Mg2Si–x%NiAl3(x=5, 20) alloys are shown in Fig. 4. From the DSC heating curve(Fig. 4a and c), it can be seen that the start melting temperature iscorresponding to the temperature of ternary reaction (Fig. 2) whichverifies the correctness of the calculated phase diagram. As shownin Fig. 4b and d, the cooling curve shows four exothermic peaks la-beled as A, B, C, D and the measured start temperatures of reactions.The above processes can be explained as:

The exothermic peak A can be ascribed to the precipitation of pri-mary Mg2Si (Fig. 4b) and NiAl3 particles (Fig. 4d), respectively. The

50µm

50µm

Primary NiAl3

b

Eutectic NiAl3

Fig. 3. FESEM micrograph of the microstructures of Al–15%Mg2Si–x%NiAl3 alloys fromconventionally cooled samples: (a) x=5 and (b) x=20.

a

c

b

A

B C

exo

D

d

A B

C

exo

D

5.04.54.03.53.02.52.01.51.00.5

-0.50.0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

100 200 300 400 500 600 700Temperature/°C

100 200 300 400 500 600 700Temperature/°C

DSC

/mw

/mg)

300 400 500 600 700Temperature/°C

300 400 500 600 700Temperature/°C

DSC

/mw

/mg)

DSC

/mw

/mg)

DSC

/mw

/mg)

°C

°C

°C

°C

°C

°C °C0.5

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

-3.5

0.5

0.0

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

Fig. 4. DSC results of Al–15%Mg2Si–x%NiAl3 (x=5, 20) alloys: (a) heating curve (x=5); (b) cooling curve (x=5); (c) heating curve (x=20); (d) cooling curve (x=20).

257C. Li et al. / Materials Letters 68 (2012) 255–257

peaks of B and C are corresponding to the processes of binary reactionand Al–Mg2Si–NiAl3 ternary eutectic, respectively. Due to the increas-ing NiAl3 content, the binary reaction changes from Al–Mg2Si toMg2Si–NiAl3 reaction (Fig. 2) and the corresponding start tempera-ture of binary reaction increases obviously from 584.5 °C (Fig. 4b) to631.2 °C (Fig. 4d). It also needs to be pointed out that there is asmall exothermic peak (marked as D) behind peak C. The peak D isconsidered to be a multi-eutectic reaction due to the existence of im-purities (Fe, Mn) in alloys.

4. Conclusions

In Al–Mg2Si–NiAl3 pseudo-ternary phase diagram, the compositionof ternary eutectic is Al–12.1%Mg2Si–8.4%NiAl3 and the calculatedtemperature in equilibrium state is 587.05 °C. For Al–15%Mg2Si–NiAl3system, two critical compositions were detected at 7.9% and 13.1%NiAl3 where the temperatures of the liquidus and the start of the binaryreaction are changed. The NiAl3 phase first appears only in the ternaryeutectic zone for the composition of NiAl3 up to 7.9%. With NiAl3 con-tents between 7.9% and 13.1%, NiAl3 appears in both the binary andternary reactions. Above 13.1% NiAl3, it solidifies as a primary phase aswell as during the binary and ternary reactions.

Acknowledgments

We gratefully acknowledge National Natural Science Foundationof China under projects 51071097 and 51001065, and the GraduateIndependent Innovation Foundation of Shandong University.

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