Journal of Alloys and Compounds 450 (2008) 255–259
Influence of Ti addition on the microstructure and hardness properties of near-eutectic Al–Si alloys Muzaffer Zeren ∗ , Erdem Karakulak The Department of Metallurgy & Materials Engineering, Kocaeli University, Turkey Received 12 September 2006; received in revised form 26 October 2006; accepted 28 October 2006 Available online 28 November 2006
Abstract In this study, the influence of Ti addition on the microstructure and hardness of near-eutectic Al–Si has been investigated. Near-eutectic Al–Si alloys (13.1 wt.% Si) with 0.1, 1, 2, 3, 5, and 10% Ti have been utilized for this purpose. The melting operation was carried out in an electrical furnace, where the charge materials were placed in a graphite crucible. After melting, alloys have been cast in the metal mold at 1100 ◦ C and solidified. © 2006 Elsevier B.V. All rights reserved. Keywords: Near-eutectic Al–Si alloy; Ti content; Hardness
1. Introduction Aluminum–silicon based alloys are well-known casting alloys with high wear resistance, low thermal-expansion coefficient, good corrosion resistance, and improved mechanical properties at a wide range of temperatures. These properties lead to the application of Al–Si alloys in the automotive industry, especially for cylinder blocks, cylinder heads, pistons and valve lifters [1–8]. It is common practice to add Ti to Al–Si foundry alloys because of its potential grain refining effect. However, an excess of Ti may cause problems in the liquid metal process and defects in casting [9], due to precipitation of primary TiAlSi coarse particles above the liquid temperature. Although there is ample information of TiAl intermetalics in the binary Al–Ti system mostly related to the grain refining mechanism, little publication literature exists describing the formation and growth of TiAlSi intermetallics in widely used Al–Si foundry alloys. At the aluminum-rich corner of ternary Al–Si–Ti system, three possible types of titanium aluminides could be present [10].
range of lattice parameters. It can be commonly written as Ti(AlSi)3 . 2. τ 1 . Commonly written as Ti7 Al5 Si12 . This phase is stable below 900 ◦ C. 3. τ 2 . Commonly written as Ti(AlSi)2 . This phase forms at higher amount of silicon and an existence range from 46% to 38% Si. Table 1 summarizes the possible phases existing in aluminum-rich corner. As each ternary phase has a range of chemical compositions and lattice parameters, this causes great difficulties for the identification of these ternary intermetallic phases. The morphology of TiAlSi intermetallics in Al–Si foundry alloys has been reported to be similar to TiAl3 in binary alloys, i.e. flakes and blocks [10,11]. However the favorite conditions under which either of these morphologies would form are still unknown. The aim of the present study is to investigate the effect of Ti addition on the structure and micro-hardness properties of as-cast near-eutectic Al–Si–xTi alloys. 2. Experimental procedure
1. TiAl3 . Up to 15% Al can be replaced by silicon in TiAl3 lattice structures, resulting in various chemical compositions and a ∗
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The Al–Si–xTi alloys were prepared using Al–Si near-eutectic alloys of composition (13.1 wt.% Si) and commercial Ti–6Al–4V. Solidification of neareutectic Al–Si alloys with 0.1, 1, 2, 3, 5 and 10% Ti have been realized by melting in an electrical furnace and casting in the metal mold at 1100 ◦ C. The hardness of as-cast specimens was measured using a microhardness tester (Fischerscope H 100), and each reported value is an average of five measurements.
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Table 1 Possible phases existing in the Al-rich part of the ternary Al–Si–Ti system [10] Common name
Phase
Chemical range
Ti(AlSi)3 τ 1 or Ti7 Al5 Si12 τ 1 or Ti(AlSi)2
Ti(Al1−x Six )3 (Ti1−x Alx )8 (Aly Si1−y )16 Ti(Alx Si1−x )2
0 ≤ x ≤ 0.15 x ≈ 0.12, 0.06 ≤ y ≤ 0.25 0.15 ≤ x ≤ 0.30
Metallographic observations have been made by the combination of optical microscopy (OM, Zeiss) and scanning electron microscopy (SEM-JEOL 6063) with using an energy dispersive X-ray spectroscopy (EDX). Image Analyzer (Quantimet 501) has been used to determine the influence of Ti content on the microstructure of near-eutectic Al–Si alloys. The analyses were performed by
optical emission spectroscopy (OES). Particle size and shape was characterized by an image analysis system (Quantimet 501, Leica).
3. Results 3.1. Microstructural observations Fig. 1 shows the optical micrograph of the as-cast microstructures of Al–Si–xTi alloys. A few petal-like particles were found in the cast Al–Si–1Ti microstructure (as arrowed in Fig. 1b). The flake-like particle morphology was the most common in the cast Al–Si–xTi (x = 2, 3, 5 and 10 wt.%) microstructures.
Fig. 1. Optical micrographs of Al–Si–xTi alloys. (a) Al–Si–0.1Ti, (b) Al–Si–1Ti, (c) Al–Si–2Ti, (d) Al–Si–3Ti, (e) Al–Si–5Ti and (f) Al–Si–10Ti (100×).
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Fig. 2. Optical micrograph of Al–Si–10Ti alloy, as-cast, etched with Keller light grey area: ␣ (aluminium phase), flake-like TiAlSi particle, surrounded by eutectic (a), energy dispersive X-ray tracing of the intermetallic particle (as arrowed) (b).
It is reported that Ti-based intermetallics can have three different morphologies (flakes, petals and blocks) depending on the solidification conditions and the temperature history of the alloys. Slow cooling from high temperature produces flakes. Rapid cooling and high thermal gradients form a petal-like shape. If the alloys is produced at a relatively low temperature and at a high Ti saturation, faceted blocky aluminides form, which may range from nearly cubic to long flat plates [10]. Fig. 2b represents that the energy-dispersive X-ray analysis of the intermetallic particles. EDX result indicates that these intermetallics contain Al, Si and Ti[Ti(AlSi)3 ]. 3.2. Phase morphology and type of TiAlSi intermetallics The petal-like particles were found only in the Al–Si–1Ti cast alloy (Fig. 3). Most TiAlSi intermetallic had flake-like morphology. Fig. 4 shows the SEM micrograph of the as-cast microstructure of Al–Si–2Ti alloys. SEM shows the flake-like particles.
Fig. 4. SEM showing flake-like TiAlSi particles (light grey) in the Al–Si–2Ti cast alloy.
3.3. Microhardness of the alloys The effect of Ti content on microhardness of the as-cast Al–Si–xTi alloys as a function of Ti content is shown in Fig. 5. The increase in Ti content results in a increase in microhard-
Fig. 3. Petal-like TiAlSi intermetallic in the Al–Si–1Ti cast alloy.
Fig. 5. Graphical representation of the microhardness values of Al–Si–xTi alloys in the as-cast condition.
1.9 4.8 4.3 3.8 7.8 11.4 0.1 0.1 0.1 0.1 0.3 0.3 3.1 4.4 6.3 11.6 6.0 6.3 6.6 18.0 42.6 48.0 97.3 115.3 0.6 0.5 1.6 2.0 3.7 3.3 0.1 1.6 4.2 3.0 6.4 10.0 0.2 2.6 4.3 4.5 8.0 12.5 0.1 0.1 0.03 0.3 0.4 0.4 0.1 0.352 0.049 0.626 0.694 0.946 0.1 2.3 4.3 3.7 7.0 11.0 5.9 5.0 8.2 29.8 15.6 15.6 1.5 3.1 10.0 17.2 19.0 29.8 3.1 17.4 48.3 73.0 102.8 168.9 11.0 27.3 65.0 88.9 97.2 116.4 Al–Si–01Ti Al–Si–1Ti Al–Si–2Ti Al–Si–3Ti Al–Si–5Ti Al–Si–10Ti
S.E. S.D. Mean
14.5 84.1 185.7 360.4 503.0 969.2
Maximum S.E. Minimum Maximum S.E. S.D. Mean Minimum
1.6 2.1 2.3 2.4 3.0 2.3 1.3 3.0 8.1 8.8 20.3 18.9 5.1 9.0 16.3 21.3 22.8 19.7
0.2 0.9 0.9 0.5 1.6 1.8
Maximum S.E. S.D. Mean S.D. Mean
Minimum
Roundness
Maximum
ness values. This is due to the increase in the volume fraction of relatively hard-phase Al3 Ti [9] 3.4. Image analysis
Length (m) Area (%) Area (m2 ) Alloy
Table 2 Results of the image analysis performed
1.1 1.2 1.2 1.6 1.3 1.0
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Particle size and shape was characterized by an image analysis system (Quantimet 501, Leica). Depending on the magnification (in this study magnification: 200×), at least 200 intermetallic particles were measured in six optical fields of samples. Results of the image analysis are presented in Table 2. The influence of Ti content on the particle size of the as-cast Al–Si–xTi alloys is given in Table 2. As seen in Table 2, the increase in Ti content results in an increase in the coarsening of the TiAlSi particles. The mean area of the particle with 0.1% Ti is 11.0 m2 , whereas the mean area of the particle is enlarged up to 116.4 m2 with 10% Ti alloying. Also the amount of the TiAlSi phase climbs up to 11.0% at 10% Ti alloying. As the particles have a needle form, also the length and the roundness are measured. Mean value of the lengths show also a strength increase due to Ti-alloying. The roundness shows the same effect without being statistically significant. 4. Conclusion Increase in Ti content leads to an increase in hardness as seen in Fig. 5, which is also in agreement with the literature [9]. By increasing Ti content, hardness increase due to of relatively hard-phase Al3 Ti intermetallics. It is found that increasing Ti content from 0.1% to 10%, hardness increases from 841 to 1543 HV. Ti-based intermetallics can have different morphologies (flakes and petals) depending on the solidification conditions and the temperature history of the alloys. The few petal-like particles were found in the cast Al–Si–1Ti microstructure. The flake-like particles were found in the cast Al–Si–xTi (x = 2, 3, 5 and 10 wt.%) microstructures. The increase in Ti content results in an increase in the coarsening of the TiAlSi particles. The mean area of the particle with 0.1% Ti is 11.0 m2 , whereas the mean area of the particle is enlarged up to 116.4 m2 with 10% Ti alloying. Also the amount of the TiAlSi phase climbs up to 11.0% at 10% Ti alloying. More research work is necessary for better understanding of the morphologies and mechanisms responsible for hardness increase in Al–Si–xTi alloys. References [1] [2] [3] [4] [5]
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[9] N. Saheb, T. Laoui, A.R. Daud, M. Harun, S. Radiman, R. Yahaya, Wear 249 (2001) 656–662. [10] X. Chen, M. Fortier, Mater. Forum 28 (2004) 659–665. [11] S. Gupta, Mater. Charact. 49 (2003) 321–330.
