Effect of Y2O3 on microstructure and mechanical properties of hypereutectic Al-20%Si alloy
YANG Ya-feng(杨亚锋), XU Chang-lin(许长林), WANG Hui-yuan(王慧远),
LIU Chang(刘 畅), JIANG Qi-chuan(姜启川)
Key Laboratory of Automobile Materials of Ministry of Education,
Department of Materials Science and Engineering, Jilin University, Changchun 130025, China
Received 28 July 2006; accepted 15 September 2006
Abstract:
The effect of Y2O3 on the microstructure and mechanical properties of the hypereutectic Al-20%Si(mass fraction) alloy was investigated. The results show that, with the addition of Y2O3 into the Al-P-Ti-TiC modifier, the average size of primary silicon in the Al-20%Si alloy modified by Al-P-Ti-TiC-Y2O3 modifier (approximately 15mm or less) is significantly reduced, and the morphology of eutectic silicon changes from coarse acicular and plate like to refined fibrous. The Brinell hardness (HB189) and tensile strength (301 MPa) of Al-20%Si alloy modified by the Al-P-Ti-TiC-Y2O3 increase by 11.6% and 10.7%, respectively, for the alloys after heat treatment.
Key words:
primary silicon; mechanical properties; hypereutectic Al-Si alloy; modification; eutectic silicon; heat treatment;
1 Introduction
Recently, there has been growing interest in the hypereutectic Al-Si alloy as a candidate material because of its properties such as high wear resistance, low thermal-expansion coefficient and good castability [1-3]. However, a conventional casting process of the Al-Si alloy usually produces polygonal, star-like (five-folded) and coarse platelet primary silicons, and acicular and lamellar eutectic silicon phases, which limits the further improvement on the properties of the alloy[4].
In order to achieve the fine silicon phases with beneficial shape and distribution, many studies have been carried out to modify primary and eutectic silicon phases in the hypereutectic Al-Si alloy, and modification is usually adopted by adding phosphor or phosphorous compound to the molten hypereutectic Al-Si alloy[5]. CHANG et al[6] added some rare earth metals into the Al-21%Si alloy, and achieved simultaneous refinement of primary and eutectic silicon. YI and ZHANG[4] reported that La could modify the eutectic silicon in the hypereutectic Al-Si alloy and some La-rich phases were observed to envelop some small polygonal silicon crystals. Recently, GURGU et al [7] studied the effect of P, Ti+B and mixed metals on the microstructure and mechanical properties of the hypereutectic Al-Si alloy, and achieved fine eutectic silicon phase. In our previous study[8], we have successfully designed a new Al-P-Ti-TiC-Y modifier to refine primary silicon in the hypereutectic Al-20%Si and Al-29%Si alloys. It was found that the average sizes of the primary silicons in the modified alloys were decreased to approximately 20 μm and 35 μm, respectively, and the hardness and wear resistance of the alloys were also significantly improved. According to Refs.[9-10], AlP and Si have both diamond cubic structure with very similar lattice parameters, and thus AlP can act as heterogeneous nucleus of primary silicon, which promotes the refinement of primary silicon. On the other hand, the effect of Ti and TiC in the modifier was reported to be significant on the morphology and size of primary silicon in the hypereutectic Al-Si alloy[8]. However, Y is unstable and easy to be oxidated, which results in that the cost of production is too high and the processing condition is too rigorous to be applied extensively. Further-more, the effect of Y2O3 on the primary and eutectic silicons in hypereutectic Al-Si alloy is rarely studied up to now.
Based on the above analyses, therefore, the modifier containing P, Ti, TiC has been designed to modify primary and eutectic silicons. Due to the low cost of preparing Y2O3, an attempt has been made to design a new modifier (containing Y2O3) that can change primary and eutectic silicons in the hypereutectic Al-Si alloy to investigate the effect of Y2O3 on the primary and eutectic silicons. At the same time, the microstructure, hardness and tensile strength of the heat-treated hypereutectic Al-Si alloy are also studied.
2 Experimental
One modifier used in the experiment consisted of Al, P, Ti and TiC (0.4P-10Ti-20TiC-Al, mass fraction, %), and another consisted of Al, P, Ti, TiC and Y2O3 (0.4P-10Ti-20TiC-10Y2O3-Al, mass fraction, %). The base alloy is Al-20%Si alloy, which was made by the addition of solid silicon preheated at 400 ℃ for 2 h to Al-12%Si alloy melt. The chemical composition was measured with an ARL 4460 Metals Analyzer from five measurements, and the result is given in Table 1. Then the base alloy was melted at 850 ℃ in a graphite crucible. Subsequently, the Al-P-Ti-TiC-Y2O3 modifier was added into the molten Al-20%Si alloy, and the addition level of the modifier was 0.5% for the base alloy. It should be noted that the molten metal was stirred at 850 ℃ for approximately 2-5 min to ensure adequate modification. After that, the modified melt was poured into a metal mould (200 mm×150 mm×12 mm) preheated at 200 ℃. In order to provide an insight into the effect of Y2O3 on primary silicon in hypereutectic Al-Si alloy, Al-P-Ti-TiC modifier was added to the melt of the hypereutectic Al-20%Si alloy with the same addition level as the above.
Table 1 Chemical compositions of Al-20%Si base alloys(mass fraction, %)
After T6 heat treatment, the average value of Brinell hardness was taken from five measurements, and the tensile tests were carried out on MTS810. After cutting the samples from the unmodified and modified Al-Si alloys, the samples were etched with 0.5% NaOH solution for 30 s at 25 ℃. Then the morphological features of primary and eutectic silicons in the investigated Al-20%Si alloys were characterized using scanning electron microscope (SEM).
3 Results and discussion 3.1 Microstructure
Fig.1 shows the microstructures of the unmodified and modified Al-20%Si alloys with the Al-P-Ti-TiC and Al-P-Ti-TiC-Y2O3 modifiers, respectively. The primary silicon crystals in Fig.1(a) exhibit coarse platelet and star shapes with average sizes of approximately 150 μm, and the eutectic silicons (approximately 120 μm) are in the form of large plates with sharp sides and edges. In the Al-Ti-TiC-P-modified alloy (Fig.1(b)), the average sizes of primary and eutectic silicons are significantly reduced, which are approximately 40 μm and 50 μm, respectively. Meanwhile, the morphologies of primary and eutectic silicons partially change. While in the Al-Ti-TiC- P-Y2O3-modified alloy (Fig.1(c)), primary and eutectic silicons almost change to fine blocky and refined fibrous shape, with the average sizes of approximately 15 μm and 10 μm, respectively, indicating the significant effect of Y2O3 on the primary and eutectic silicons in the hypereutectic Al-Si alloy.
Fig.1 As-cast microstructures of unmodified and modified Al-20%Si alloys: (a) Unmodified; (b) Modified alloy with Al-P-Ti-TiC modifier; (c) Modified alloy with Al-P-Ti-TiC- Y2O3 modifier
KNUUTINEN et al [11] reported that yttrium could modify the eutectic silicon in Al-7%Si-Mg alloy from the coarse structure to the refined plate-like structure, and argued that chemical modifiers are impurities which poison already grown silicon layers by being adsorbed onto surface and prevent the attachment of silicon atoms to the crystal, based on the impurity induced twinning theory[12]. HAN et al[13] have reported that CeO2 is decomposed into [Ce] and [O] at high temperature. The significant effect of Y2O3 on primary and eutectic silicons may be partially explained that Y2O3 is decomposed into [Y] and [O] at high temperature. According to KNUUTINEN et al[11], during solidifi- cation, [Y] may have a restrained effect on the growth of primary and eutectic silicons. Based on the present experimental observation, the addition of Y2O3 can promote the refinement of primary and eutectic silicons. However, the modification mechanism of Y2O3 requires the further investigation.
In our previous study[8], the average size of primary silicon in the Al-P-Ti-TiC-Y-modified Al-20%Si alloy was decreased to approximately 20mm. In the present study, however, it decreases to 15 μm or less. Due to the low cost and easy processing condition, as well as better modification effect, the new developed Al-Ti-TiC-P-Y2O3 modifier can be applied more extensively.
3.2 Mechanical properties
Fig.2 shows the microstructures of the unmodified and modified Al-20%Si alloys with Al-P-Ti-TiC and Al-P-Ti-TiC-Y2O3 modifiers after the heat treatment. The eutectic silicon fibres are clearly disintegrated, and spheroidisation of the silicon particles is also observed. Therefore, the average sizes of eutectic silicons are much smaller than those in as-cast Al-Si alloys. However, the average sizes of primary silicons in the unmodified and modified Al-Si alloys are hardly changed by heat treatment.
Table 2 lists the hardness and tensile strength of the unmodified and modified Al-20%Si alloys with Al-P-Ti-TiC and Al-P-Ti-TiC-Y2O3 modifiers after the heat treatment. It is worth noting that the Brinell hardness values of the non-heat-treated Al-Si alloys (unmodified and modified) are significantly lower than those of the heat-treated Al-Si alloy (unmodified and modified), which are mainly attributed to the presence of spheroidised and fine eutectic silicon and the precipitation of small intermetallic compounds (Al2Cu) [14].
Compared with the modified Al-Si alloy, the coarse primary silicon in the unmodified alloy aggravates the localized stress concentration at the sharp corner of primary silicon, which promotes cracks to initiate at the sharp corner and propagate along the interface between the primary silicon and matrix. The result causes the coarse primary silicon to separate from the matrix and further reduce the tensile strength of unmodified Al-Si alloy. On the contrary, the finer primary silicon in the modified Al-Si alloy relieves the localized stress concentration at the sharp corner of finer primary silicon, which suppresses cracks to initiate at the sharp corner. As a result, the tensile strength of modified Al-Si alloys is higher than that of the unmodified alloy.
Fig.2 Microstructures of unmodified and modified Al-20%Si alloys after T6 heat treatment: (a) Unmodified; (b) Modified alloy with Al-P-Ti-TiC modifier; (c) Modified alloy with Al-P-Ti-TiC-Y2O3 modifier
Table 2 Brinell hardness and tensile strength of investigated Al-20%Si alloys
During the tensile tests, the plastic deformation causes the concentration of dislocations and forces them to glide. The phase interface suppresses the dislocation movement and the dislocations would be piled up at the phase interface[15]. In comparison with the coarse primary and eutectic silicons in the unmodified Al-20%Si alloy, the finer primary and eutectic silicons in the modified alloy can increase the length of phase interface and prevent the movement of the dislocations greatly. Due to the reduction of the dislocation movement distance and the increase of dislocation wall that prevents the movement of dislocation, the tensile strength of the modified Al-Si alloy is higher than that of the unmodified Al-Si alloy.
According to the Hall-Petch equation:
σs=σ0+KD-1/2 (1)
where σ is the yield strength, σ0 is the yield strength of single crystal, K is a constant, and D is the average grain diameter. Here, it may be supposed that the tensile strength is the function of the average sizes of primary and eutectic silicons similar to the grain of the alloys. As a result, the tensile strength of the modified Al-Si alloy with fine primary and eutectic silicons is higher than that of the unmodified Al-Si alloys with coarse ones.
It is expected that the hardness may depend on the grain size of the silicon phase following the Hall-Petch type equation[15]:
H=H0+KHD-1/2 (2)
where H is the hardness, H0 is the hardness expected at a hypothetical infinite grain size, KH is a constant, and D is the average grain diameter. The result causes the Brinell hardness of the modified Al-Si alloys to be higher than that of the unmodified Al-20%Si alloy.
4 Conclusions
1) After the addition of Y2O3, the morphology of primary silicon changes from star-shape to fine polyhedron, and the morphology of eutectic silicon changes from coarse acicular (plate like) shape to refined fibrous shape obviously. The result shows that the average size of primary silicon in the Al-20%Si alloy modified by Al-P-Ti-TiC-Y2O3 modifier (approximately 15 μm or less) is much smaller than that in the unmodified and modified Al-20%Si alloys modified by Al-P-Ti-TiC modifier (approximately 150 μm and 40 μm, respectively).
2) After heat treatment, compared with Al-20%Si alloy modified by Al-P-Ti-TiC, the Brinell hardness (HB189) and tensile strength (301 MPa) of the modified Al-20%Si alloy with Al-P-Ti-TiC-Y2O3 modifier are increased by 11.6% and 10.7%, respectively.
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(Edited by YANG Bing)
Foundation item: Project(50501010) supported by the National Natural Science Foundation of China; Project supported by 985 Program—Automotive Engineering of Jilin University, China
Corresponding author: JIANG Qi-chuan; Tel/Fax: +86-431-5094699; E-mail: jqc@jlu.edu.cn
