Effects of Sr, Ce and P on the microstructure and mechanical properties of rapidly solidified Al7Si alloys

Accepted Manuscript Effects of Sr, Ce and P on the microstructure and mechanical properties of rapidly solidified Al-7Si alloys

Qing Liu, Maowen Liu, Cong Xu, Wenlong Xiao, Hiroshi Yamagata, Shenghui Xie, Chaoli Ma PII: DOI: Reference:

S1044-5803(18)30328-0 doi:10.1016/j.matchar.2018.04.018 MTL 9158

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

7 February 2018 11 April 2018 12 April 2018

Please cite this article as: Qing Liu, Maowen Liu, Cong Xu, Wenlong Xiao, Hiroshi Yamagata, Shenghui Xie, Chaoli Ma , Effects of Sr, Ce and P on the microstructure and mechanical properties of rapidly solidified Al-7Si alloys. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Mtl(2017), doi:10.1016/j.matchar.2018.04.018

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ACCEPTED MANUSCRIPT Effects of Sr, Ce and P on the microstructure and mechanical properties of rapidly solidified Al-7Si alloys Qing Liu a, Maowen Liu a, Cong Xu a, Wenlong Xiao a,*, Hiroshi Yamagata b,

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Shenghui Xie c, Chaoli Ma a,*

Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education,

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School of Materials Science and Engineering, Beihang University, Beijing 100191, China Center for Advanced Die Engineering and Technology, Gifu University, 1-1 Yanagido, Gifu City,

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School of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China

Email of corresponding author: [email protected]

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Gifu 501-1193, Japan

Abstract

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A series of Al-7Si based alloys with trace addition of Sr, Ce or P were prepared

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using melt-spinning at two different high cooling rates, and the microstructure as well as the hardness of the ribbons was comprehensively studied. The results show that Sr

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or Ce induces the formation of columnar transition zone and modifies the morphology of eutectic Si phase from dispersed particle chains to fine fibers only at extremely

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high cooling rate. However, P has little effect on the microstructure of either -Al or eutectic Si. Compared with conventional casting, the Al-7Si ribbons exhibit extremely high hardness due to the fine dispersion of eutectic Si, the grain refinement and the supersaturation of Si. The hardness is further enhanced by Sr or Ce addition at extremely high cooling rate, especially for columnar and equiaxed zones, attributing to the modification effect on Si phase.

Key words: Al-Si alloy; melt spinning; modification; microstructure; high hardness

ACCEPTED MANUSCRIPT 1. Introduction Hypoeutectic Al-Si cast alloys have been widely used in automotive, aerospace and electronics industries due to their good castability, high specific strength and excellent corrosion resistance [1]. However, the eutectic Si phase tends to exhibit as coarse plates by conventional sand casting and permanent mold casting, resulting in

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low ductility and fracture toughness of alloys [2]. Therefore, many efforts have been devoted to the modification of Al-Si cast alloys in order to obtain refined silicon phase

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with beneficial shapes and distribution. One of the most effective methods is chemical modification. Trace amount of Sr, Na and rare earth elements proven to be effective

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modifiers has been practically used in Al-Si foundry alloys [3]. Phosphorus is a common impurity element in aluminum and generates small AlP particles in the melt,

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which are very effective nucleus for eutectic silicon during solidification [4]. The morphology of Si phase in Al-Si alloys is also sensitive to solidification rate

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[5]. Thus, besides the chemical modification, rapid solidification processing (RSP) technique can also modify the Si phase effectively [6]. In addition, samples prepared

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by RSP technique can yield excellent homogeneity, extremely low level of

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segregation, ultrafine structures and extending solid solubility [7]. Among several RSPs, melt spinning is one of the most commonly used methods to obtain an extremely high cooling rate (105-107 K/s) [8]. Many investigations on Al-Si melt-spun

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ribbons focused on their surface microstructure [9, 10], while the cross-sectional microstructure is still unclear [11-17]. Moreover, most of the modification phenomena

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reported were under low or medium solidification rate (<100 K/s), and the combined effect of chemical modification and rapid solidification has been rarely studied. The objective of this work is to evaluate the effects of conventional modifier (Sr, Ce) and P on the microstructure and hardness of Al-7Si (wt. %, hereafter in weight percentage) ribbons prepared by melt spinning at different high cooling rates.

2. Experimental procedures The Al-7Si base alloys with controlled amount of Sr, Ce and P were prepared from high purity Al and Si (≥ 99.99%), and Al-10Sr, Al-10P and Al-5Ce master alloys

ACCEPTED MANUSCRIPT in a high frequency electric resistance furnace. The content of additions is based on previous experiments, where optimal microstructures and performances of the alloys can be achieved [18-21]. The chemical compositions of ingots are analyzed by Thermo Scientific ARL3460 OES metal analyzer, and the results are listed in Table 1. The ingots were cut into small pieces and loaded into a silica tube for melt-spinning

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process. The pieces were remelted in an induction heating smelter under the protection of Ar atmosphere, and then the feedstock was rapidly solidified to ribbons

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by melt-spinning using Cu wheel with a diameter of 198 mm at a rotating speed of 30 m/s or 50 m/s. The width and thickness of melt-spun ribbons ranged from 2 to 5 mm

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and 22 to 44 μm, respectively.

Microstructural characterizations of the ribbons were performed by using LEICA

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DM4000 optical microscopy (OM), Apollo 300 scanning electron microscopy (SEM)，and JEM-2100F transmission electron microscopy (TEM) equipped with

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energy dispersive spectroscopy (EDS). For cross-sectional observation, the ribbons were cold mounted, and prepared using standard metallographic techniques followed

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by chemical etching in Keller reagent (95 vol. % H2O, 2.5 vol. %HNO3, 1.5 vol. %

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HCl and 1 vol. % HF) for 40 s. Specimens for TEM observation were prepared by ion beam milling using a cold stage. Phase composition was identified by D/MAX-2200 X-ray diffraction (XRD) using Cu-Kα radiation (wavelength λ = 0.15406 nm) and a

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scanning speed of 2°/min. The thermal history of ribbons was evaluated in a STA-449F3 power compensated differential scanning calorimetry (DSC) at a heating

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rate of 10 K/min. The hardness was tested by a CSM-NHT2 nano-indentation, and three different zones along the cross-section of each ribbon were selected with an applied load of 5 mN for 10 s. The hardness value of each zone is an average result of at least 9 measurements. The load and displacement resolutions are 0.02 μN and 0.01 nm, respectively.

ACCEPTED MANUSCRIPT Table. 1 Chemical compositions of the experimental alloys. Alloy designation Al-7Si Al-7Si-0.008P Al-7Si-0.04Sr Al-7Si-0.5Ce

Si (wt.%)

Mg (ppm)

Fe (ppm)

P (ppm)

Sr (ppm)

Ce (wt.%)

Al

6.99 7.09 7.11 7.05

5 7 7 6

95 95 92 92

1 84 0 1

0 0 426 0

0 0 0 0.52

Balance Balance Balance Balance

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3 Results and discussion 3.1 Overview of the microstructure

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3.1.1 -Al grains

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Fig. 1 shows the cross-sectional microstructures of Al-7Si ribbons with different compositions at different rotating speeds. It is clear that samples prepared by rapid

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solidification show extremely fine microstructures. For Al-7Si ribbon (Fig. 1a-b), two distinct regions can be observed from its cross section at two rotating speeds. As

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shown in Fig. 1a, there is a region with fine -Al grains close to the wheel side, which is known as “refined zone” for its extremely fine structure [22]. Close to the refined

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zone, the equiaxed grains are coarser and inhomogeneous, which take up almost 80 %

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of the whole cross-sectional area. When the rotating speed increases to 50 m/s (Fig. 1b), the ribbon as well as refined zone becomes a little thinner. With respect to Al-7Si-0.008P alloys (Fig. 1c and d), the microstructure at two rotating speeds is

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similar to that of Al-7Si alloy, indicating that P has little influence on -Al grains under rapid solidification condition. In the case of alloys modified with Sr or Ce and

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produced at 30 m/s (Fig. 1e and g), the refined zone and inhomogeneous equiaxed zone can also be observed. However, with increasing rotating speed of 50 m/s, an extra columnar transition zone appears between the refined and equiaxed zone (Fig. 1f and h) in both Sr and Ce modified ribbons.

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Fig. 1 Cross-sectional microstructure of melt-spun ribbons: (a, b) Al-7Si; (c, d) Al-7Si-0.008P; (e, f) Al-7Si-0.04Sr; (g, h) Al-7Si-0.5Ce. The rotating speed is 30 m/s for a, c, e, g and 50 m/s for b, d, f, h.

In order to take a close inspection, enlarged images of the Sr-modified alloys are

ACCEPTED MANUSCRIPT displayed in Figs. 2 and 3. The microstructures of regions B, C and D along the cross-section of the ribbon are magnified in detail. Fig. 2 shows the ribbon prepared at 30 m/s in which the equiaxed zone near the air surface has an obvious coarsening tendency and more inhomogeneous microstructure as compared with its wheel side counterpart. As can be seen in Fig. 3, the thickness of the ribbon prepared at 50 m/s is only about half of that in Fig. 2. Between the refined and equiaxed zone, there is a

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columnar transition zone (~10 m), which takes up approximately 50% of the whole

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area.

Fig. 2 Detailed microstructure of Al-7Si-0.04Sr melt-spun ribbon prepared at the rotating speed of 30 m/s. (b), (c) and (d) are the magnification of regions B, C and D along the cross-section, respectively.

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Fig. 3 Detailed microstructure of Al-7Si-0.04Sr melt-spun ribbon prepared at the rotating speed of 50 m/s. (b), (c) and (d) are the magnification of regions B, C and D along the cross-section, respectively.

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The very fine grains in refined layer adjacent to the chilling surface arises from the initial nucleation events which are governed by the extremely high thermal

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undercooling and super rate of heat extraction near the Cu wheel. After the formation of the thin refined layer, the columnar grains develop almost perpendicular to the

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wheel contact surface because of the unidirectional heat flow condition and the thermal gradient in the ribbon has an inclination in the reverse direction [22]. Then, the reduced cooling rate from the wheel side to the air side leads to a decreasing stability of the liquid-solid interface [23], resulting in an equiaxed zone with coarser and inhomogeneous grains (Fig. 3). The results indicate that the effect of Sr and Ce on -Al grains is sensitive to the cooling rate. Under rapid solidification condition, Sr and Ce can significantly change the microstructure of -Al grains, which is mainly embodied in the emergence of the columnar transition zone.

ACCEPTED MANUSCRIPT 3.1.2 Si phase For comparison, the typical microstructures of Al-7Si alloys with P, Sr and Ce addition after conventional casting are shown in Fig. 4. Fig. 4a shows that the eutectic Si phase in Al-7Si alloy exists as coarse plates. In the Al-7Si alloy with 0.008% P (Fig. 4b), the primary reaction between P and Al in the liquid leads to a fine dispersion of

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AlP particles which act as potent nucleation sites for eutectic Si and assist to promote the formation of flaky and coarse Si phase [20]. The coarsening morphology of Si

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phase is mainly attributed to the increased nucleation sites in the vicinity, rather than in direct contact of -Al, which results in blocky Si polyhedrons located in central

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eutectic cells [24]. With respect to Sr modified alloy, the coarse Si plates transform to well-refined fibers (Fig. 4c). The addition of Ce results in a fine coral-like

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morphology, which is intermediate between plates and fibers (Fig. 4d). It has been suggested that the modification effect is attributed to the enrichment of Sr or Ce at the

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reentrant edges or corners of the eutectic Si, leading to frequent twinning [4].

Fig. 4 Morphology of the conventional casting ingots: (a) Al-7Si; (b) Al-7Si -0.008P; (c) Al-7Si-0.04Sr; (d) Al-7Si-0.5Ce.

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Compared with the conventional casting (Fig. 4), rapid solidification significantly changes the microstructure of Si phase (Fig. 1). In Al-7Si ribbon prepared at 30 m/s, nanoscale Si chains uniformly distribute around the -Al, as shown in Fig. 1a. When the rotating speed increases to 50 m/s (Fig. 1b), the morphology of Si particles does not change. With the addition of P, the eutectic Si

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particles at different rotating speeds show similar morphology to that in Al-7Si ribbons, indicating that P has little influence on eutectic Si morphology under rapid

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solidification condition. With respect to the Sr or Ce modified alloys prepared at 30

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m/s (Fig. 1e and g), the morphology of eutectic Si also remains unchanged. Magnification of Si in the Sr modified ribbon prepared at 30 m/s (Fig. 2) reveals that

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the nanoscale faceted eutectic Si particle chains are significantly different from the fibrous Si formed by conventional casting (Fig. 4c). The size of Si particles in the

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ribbon increases from the chilling side to the air side because of the decreasing cooling rate, and the average diameter ranges from 50 nm to 350 nm. The nanoscale

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Si particles in ribbons arise from the extremely high cooling rate, which greatly

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hindered the diffusion of Si atoms and lead to the inadequate growth of Si. For the Sr and Ce modified alloys produced at 50 m/s (Fig. 1f and h), the eutectic Si exhibits a fine dendritic morphology. The magnified image of the Sr modified ribbons is

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presented in Fig. 3, where the spherical Si phase is completely turned to fibrous eutectic Si and uniformly distributes around the -Al grains in both columnar and

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equiaxed zones.

The surface microstructure of Si phase in the ribbons prepared at 50 m/s is characterized by TEM and shown in Fig. 5, and the equiaxed cellular structure can be distinctly observed. In Al-7Si ribbon, silicon particle chains disperse along the -Al grain boundaries, and a few silicon particles embedded inside the -Al grains, as shown in Fig. 5a. In the ribbon with a trace amount of P, the morphology of the eutectic Si phase still remains as particle chains (Fig. 5b), while the particle size is a little larger than that in the Al-7Si ribbon. In the Sr-modified alloy (Fig. 5c), all the fiber-shaped Si phase homogeneously distributes along -Al grain boundaries.

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Fig. 5 Bright-field TEM images of the as-melt spun ribbons produced at 50 m/s: (a, d) Al-7Si; (b, e) Al-7Si -0.008P; (c, f) Al-7Si-0.04Sr.

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Similar with previous work [25], cooling rate plays an important role in determining the morphology of eutectic Si, resulting in the change of eutectic Si from

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plate-like to spherical shape during melt-spinning. However, it can be noted that the

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eutectic Si remains spherical morphology in the Sr and Ce modified alloys prepared at 30 m/s, while becomes fibrous at a relatively higher cooling rate (50 m/s). Therefore, it is reasonable to infer that there would be a critical cooling rate in rapid

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solidification, above which the modifiers can efficiently change the morphology of Si

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phase from particle chains to fibrous ones.

3.2 Columnar transition zone Figs. 1-3 reveal that the addition of Sr or Ce results in columnar transition zone only at extremely high cooling rate (50 m/s). In general, the formation of columnar crystals is affected by many factors such as constitutional undercooling, nucleation potential and liquid convection [26]. 3.2.1 Constitutional undercooling In Al-Si binary system, the solubility of Si or Al in the liquid is much higher than that in the solid solution. During solidification, the negative liquidus slope leads to the

ACCEPTED MANUSCRIPT increase of solute content in remaining liquid, particularly in front of the solid-liquid interface [27]. As a result, the actual liquidus temperature falls below the equilibrium liquidus temperature, and “constitutional supercooling (CS)’’ develops. It is well known that constitutional supercooling zone is extended with the increasing solute content and decreasing thermal gradient [28, 29], and an extended supercooled zone

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implies that more particles will be activated and equiaxed crystals have a greater chance to survive [30, 31]. Schematic diagram of the formation of columnar zone and

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equiaxed zone is shown in Fig. 6. Therefore, CS exerts a significant influence on

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promoting the formation and refinement of equiaxed structures.

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Fig. 6 Columnar to equiaxed transition zone arising from the constitutional undercooling ΔT. T is temperature of the melt and TN represents nucleation temperature.

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In the present work, the cooling rate in melt spinning is calculated as 2×107 K/s at the rotating speed of 30 m/s [32, 33], which is much higher than that of the conventional casting. Rapid solidification would result in an increasing thermal gradient and an extending of solid solubility of Si in the Al matrix [34]. The increasing thermal gradient reduces the degree of CS directly. Additionally, the solidification rate is high enough to retain the alloying elements in solid solution, which can be illustrated by the peak shift of XRD pattern (Fig. 7). A solid solubility of 3 at.% can be obtained using the linear relationship between the atomic fraction of Si

ACCEPTED MANUSCRIPT and the lattice parameter given by Bendjik [35, 36], which is extraordinarily higher than the solid solubility at room temperature (~ 0.048 at.%) [37]. The solid solubility of Si obtained here for Al-7Si melt-spun ribbon is fully consistent with those given by Dong [34] for Al-9Si-1Cu (3.03 at.%) and Karaköse [38] for Al-8Si-1Sb (3.83 at.%) melt-spun ribbons. Therefore, the increased solid solubility leads to a relatively low

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content of Si in the remaining liquid, which also decreases the degree of CS. As a result, the formation of equiaxed Al grains is suppressed by rapid solidification and a

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columnar transition zone appears. It can be inferred that there may exist a critical

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cooling rate that can induce the formation of columnar transition zone.

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Fig. 7 X-ray diffraction pattern of Al-7Si melt-spun ribbon prepared at the rotating speed of 30 m/s. Similar to -Al, columnar transition zone of eutectic Si only appears in ribbons with Sr or Ce at extremely high cooling rate. During the solidification of two-phase eutectic alloys, two kinds of solutes are rejected from the growing tips respectively. The solutes need to diffuse along the solid-liquid interface from one phase to another, thus solutes accumulation depends on the eutectic lamellar spacing [39]. The eutectic Si particle chains in ribbons without modifiers are obviously coarser than the modified fibrous ones. The wide spacing between the eutectic particles gives rise to

ACCEPTED MANUSCRIPT difficult diffusion of solute atoms in the melt [40], which contributes to the accumulation of atoms in front of the solid-liquid interface. Therefore, constitutional undercooling zone is extended which suppresses the formation of columnar grains. When Sr or Ce modifiers are added to the alloy, according to the “restricted growth theory”, the modifiers retard Si growth by selectively adsorbing at re-entrant edges or

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growing surfaces of Si phase [41]. Instead of growing fast in few selected <112> directions, modified growth tends to branching, leading to fibrous structure. The

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refinement of eutectic phase reduces diffusion spacing and thereafter decreases CS.

result, the columnar transition zone forms.

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3.2.2 Nucleation potential

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Besides, rapid solidification increases thermal gradient and also reduces CS. As a

The presence of heterogeneous nuclei facilitates the formation of equiaxed grains

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[26]. Even in high purity Al-Si alloys, there still exists a certain amount of P or Fe elements. P forms small AlP particles in the melt and Fe promotes the formation of

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-(Al, Si, Fe) phase [42], which are effective nuclei for eutectic silicon[43, 44]. These

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are not in conflict with the P content analysis in the present work where the trace amount of P cannot be detected, as P-rich particles can act as nuclei for silicon even in 99.995% purity alloys [45]. For eutectic Al-Si, Si is the leading phase, whose

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formation determines the morphology of eutectic microstructure. However, AlP or -(Al, Si, Fe) phase in the melt would be poisoned by Sr or Ce [46], and thus eutectic

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nucleation is inhibited. Therefore, the addition of modifier Sr or Ce is beneficial to the formation of columnar transition zone rather than equiaxed grains, while P promotes the formation tendency of equiaxed grains.

3.2.3 Liquid convection Apart from CS and nucleation potential, the increasing liquid convection would also be beneficial to the formation of columnar structure [26]. It is well known that solidification in hypoeutectic alloy occurs within a certain temperature range. The primary dendrite leads to a relatively high flow resistance for the alloy melt, which

ACCEPTED MANUSCRIPT decreases the convection ability of the liquid. Therefore, a wide range of the solidification temperature implies low convection ability of the melt. Fig. 8 shows continuous DSC heating curves of melt-spun ribbons and each of them consists of two peaks. The first peak at the range from 540oC to 580oC represents the melting temperature of Al-Si eutectic reaction, and the second one between 580oC and 630oC belongs to the melting of -Al. Table 2 shows the melting points of the eutectic and

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-Al obtained from DSC curves. In the ribbons with Sr or Ce, the endothermic peaks

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of -Al shift closer to the eutectic temperature (Fig. 8a) compared with the ribbons without modifier. Therefore, the solidification temperature range gets smaller, which

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implies that the modifiers would enhance the convection ability of the alloy, and then promotes the formation for columnar zone. However, P has an opposite effect on the

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temperature range (Fig. 8a and Table 2), indicating the decrease of liquid convection, which is probably attributed to its easy reaction with Al at the temperature above the

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formation of -Al [20, 24]. Consequently, no columnar grains are observed in

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Al-7Si-0.008P alloy.

Fig. 8 DSC curves of the ribbons: (a) the ribbons with different addition of impurity elements and produced at 50 m/s, (b) the ribbons without any addition of impurity elements produced at different cooling rates. Peaks A, A’, A’’ and A’’’ represent the melt of Al-Si eutectic; peaks B, B’, B’’ and B’’’ represent the melt of -Al.

ACCEPTED MANUSCRIPT Table. 2 Melting points of the eutectic and -Al obtained from DSC curves. melting point of the eutectic (℃)

compositions -1

-Al (℃)

583.1 582.1 580.6 581.5 582.9

crystallization temperature range (℃)

598.9 607.5 611.6 608.5 611.6

15.8 25.4 31.0 27.0 28.7

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Al-7Si-0.04Sr 50 m.s Al-7Si-0.5Ce 50 m.s-1 Al-7Si-0.008P 50 m.s-1 Al-7Si 50 m.s-1 Al-7Si 30 m.s-1

melting point of

The effects of Sr on the convection of the melt were also demonstrated by

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several researchers through different experimental methods. Shankar et al. [47, 48]

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suggested that addition of Sr in Al-Si alloys changes the interface energy and rheological properties such as melt viscosity, thus influences the nucleation event of

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solidification. Xi [49] and Srirangam [50] pointed out that and even trace level addition of Sr can make the melt disordered and delay the clustering tendency of the

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liquid atoms. Based on DSC results, it can be inferred that Ce has the similar but weaker effect on promoting the liquid convection compared with Sr. As a result, Ce

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and Sr induce the formation of columnar zone.

3.3 Hardness

The hardness of Al-7Si melt-spun is about 93 HV, much higher than that of the

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ingot (48 HV) prepared by conventional casting, which is mainly attributed to the grain refinement, the fine dispersion of eutectic Si, and the supersaturation of Si [10].

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Compared with conventional casting ingot, the melt spun ribbons produced at 30 m/s exhibits a fine structure with dramatically refined grains. For illustration, we choose Al-7Si melt-spun ribbon to calculate the harness increment. The average grain size of Al matrix is 0.36 μm (measured over 500 grains). Therefore, the contribution to hardness caused by grain refinement is about 20 HV according to the Hall–Petch relation [51]. In addition, the hardness increment caused by dispersion strengthening from fine eutectic Si particles can be calculated through the Orowan strengthening mechanism[52]:

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 oro

 2   dp    3  ln    b     0.4 Gmb  M      1 

(1)

where ν is Poisson’s ratio for aluminum (0.34) [53], M is the mean orientation factor

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(3.06), λ is the interparticle spacing, Gm is the shear modulus of Al-matrix (25.4 GPa [54]), b is the Burgers vector of the dislocations (0.286 nm) and dp is the average size

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of Si particles. For Al-7Si ribbon produced at 30 m/s, dp is 0.145 μm, and λ is 0.137

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μm. Thus, the hardness increment stems from Orowan strengthening was ~ 48 HV. Moreover, when Si solid solubility in Al exceeds 1.6%, the hardness enhanced

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by solute atoms is much higher than the calculated one [55]. Compared with pure aluminum, the solid solubility is about 3% in Al-7Si ribbon prepared at 30 m/s, whose

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contribution to hardness was measured to be about 20 HV [55]. Based on the calculation, the enhancement of hardness through melt spinning is

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about 88 HV. It shows that the calculated result is in a good agreement with the

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experimental result as well as the comparison between the other studied melt-spun

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ribbons (not shown here).

Fig.9 Hardness of different cross-sectional zones for individual ribbon: (a) wheel side; (b) center of the ribbons; (c) air side.

The hardness of the ribbons with different compositions and cooling rates at various positions are shown in Fig.9. For the ribbons prepared at 30 m/s, the addition of P, Sr and Ce can hardly change the hardness in the wheel side, central or air side regions. However, the hardness is more or less enhanced in the ribbons with Sr or Ce

ACCEPTED MANUSCRIPT prepared with increasing the cooling rate (from 30 m/s to 50 m/s). In detail, the hardness of ribbon with Sr shows a 2% increasement near the wheel side, and the distinct enhancement is found at central (77%) and near the air side (72%) of the ribbon. For the ribbon with Ce addition, the increase of hardness from wheel side to air side is 36%, 31% and 30%, respectively.

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In Al-Si alloys, hardness of Si phase is as high as 813 HV, which is much higher than that of Al phase (~7 HV) [51]. Hardness of Al-Si alloys is sensitive to the shape,

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size and distribution of Si phase. According to the results, it turns out that the hardness improvement of ribbons is closely related to the morphology transition of Si

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phase. For all the ribbons prepared at 30 m/s and the ribbons prepared at 50 m/s without modifier, the eutectic Si phase is in the shape of dispersed particle chains.

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However, the ribbons with Sr or Ce prepared at 50 m/s contain fine and fibrous Si phase which units to a reticulate structure that can provide larger resistance to plastic

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deformation in contrast to the spherical ones. It can be inferred that the combination of modification and extremely high cooling rate would lead to an enhancement of

4 Conclusions

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hardness arising from the morphology change of Si phase.

The influences of Sr, Ce and P on the microstructure and hardness of melt-spun

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Al-7Si alloys were evaluated, and the following conclusions can be drawn: (1) The microstructure of Al-7Si ribbons produced at two different rotating

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speeds consisted of refined zone near wheel side and equiaxed zone on the air side. P has almost no effect on the microstructure, while the addition of Sr or Ce induce the formation of columnar transition zone at extremely high cooling rate.

(2) The plate-like eutectic Si normally formed under conventional casting is changed to nanoscale Si particle chains by rapid solidification. The eutectic Si chains are modified to fibers at extremely high cooling rate with the addition of Sr or Ce, while remain unchanged in the ribbon with P. (3) The enhancement of hardness through melt spinning is mainly attributed to

ACCEPTED MANUSCRIPT grain refinement, fine dispersion of eutectic Si and the supersaturation of Si. (4) P addition shows little improvement in the hardness of ribbons, while the hardness is significantly enhanced by Sr or Ce addition at extremely high cooling rate, attributing to their modification effect on Si phase.

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Acknowledgements The authors are grateful to the financial support by National Natural Science

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Foundation of China (NSFC, No. 51671007), the financial support by National Key Research and Development Program of China (No. 2016YFB0300901), the 111

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Project (No. B17002), the National 863 Project (No.2013AA031001) and International Science and Technology Cooperation Program of China (No.

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2015DFA51430, 2013DFB70200) to carry out this work.

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Data availability

The processed data required to reproduce these findings cannot be shared at this time

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as the data also forms part of an ongoing study.

References

[1] Xu C, Xiao W, Zheng R, Hanada S, Yamagata H, Ma C, The synergic effects of Sc and Zr on the microstructure and mechanical properties of Al–Si–Mg alloy, Materials & Design 2015;88:485-92.

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[2] Xu C, Xiao W, Hanada S, Yamagata H, Ma C, The effect of scandium addition on microstructure and mechanical properties of Al–Si–Mg alloy: A multi-refinement modifier, Materials Characterization 2015;110:160-9.

AC

[3] Knuutinen A, Nogita K, McDonald S, Dahle A, Modification of Al–Si alloys with Ba, Ca, Y and Yb, Journal of Light Metals 2001;1:229-40. [4] Makhlouf M, Guthy H, The aluminum–silicon eutectic reaction: mechanisms and crystallography, Journal of light metals 2001;1:199-218. [5] Li JH, Zarif MZ, Albu M, McKay BJ, Hofer F, Schumacher P, Nucleation kinetics of entrained eutectic Si in Al–5Si alloys, Acta Materialia 2014;72:80-98. [6] Rios CT, Peres M, Bolfarini C, Botta W, Kiminami C, Microstructure and mechanical properties of Al–Si–Mg ribbons, Journal of Alloys and Compounds 2010;495:386-90. [7] Jacobson LA, McKittrick J, Rapid solidification processing, Materials Science and Engineering: R: Reports 1994;11:355-408. [8] Salehi M, Dehghani K, Structure and properties of nanostructured aluminum A413.1 produced by melt spinning compared with ingot microstructure, Journal of Alloys and Compounds

ACCEPTED MANUSCRIPT 2008;457:357-61. [9] Chen Z, Lei Y, Zhang H, Structure and properties of nanostructured A357 alloy produced by melt spinning compared with direct chill ingot, Journal of Alloys and Compounds 2011;509:7473-7. [10] Karaköse E, Keskin M, Effect of solidification rate on the microstructure and microhardness of a melt-spun Al–8Si–1Sb alloy, Journal of Alloys and Compounds 2009;479:230-6. [11] Salehi M, Dehghani K, Structure and properties of nanostructured aluminum A413.1 produced by melt spinning compared with ingot microstructure, Journal of Alloys & Compounds 2008;457:357-61. [12] Rajabi M, Simchi A, Davami P, Microstructure and mechanical properties of Al–20Si–5Fe–2 X

PT

( X = Cu, Ni, Cr) alloys produced by melt-spinning, Materials Science & Engineering A 2008;492:443-9.

[13] Rios CT, Peres MM, Bolfarini C, Botta WJ, Kiminami CS, Microstructure and mechanical

RI

properties of Al–Si–Mg ribbons, Journal of Alloys & Compounds 2010;495:386-90. [14] Birol Y, Microstructural characterization of a rapidly-solidified Al-12 wt% Si alloy, Journal of

SC

Materials Science 1996;31:2139-43.

[15] Birol Y, Microstructural evolution during annealing of a rapidly solidified Al–12Si alloy, Journal of Alloys & Compounds 2007;439:81-6.

NU

[16] Ünlü N, Genç A, Öveçoğlu ML, Eruslu N, Froes FH, Characterization investigations of melt-spun ternary Al–xSi–3.3Fe (x=10, 20 wt.%) alloys, Journal of Alloys and Compounds 2001;322:249-56. [17] Dong X, He L, Li P, Gradient microstructure and multiple mechanical properties of AlSi9Cu

MA

alloy ribbon produced by melt spinning, Journal of Alloys and Compounds 2014;612:20-5. [18] Srirangam P, Kramer M, Shankar S, Effect of strontium on liquid structure of Al–Si hypoeutectic alloys using high-energy X-ray diffraction, Acta materialia 2011;59:503-13. science 2008;43:3009-27.

D

[19] Hegde S, Prabhu KN, Modification of eutectic silicon in Al–Si alloys, Journal of materials

PT E

[20] Liang S-M, Schmid-Fetzer R, Phosphorus in Al–Si cast alloys: Thermodynamic prediction of the AlP and eutectic (Si) solidification sequence validated by microstructure and nucleation undercooling data, Acta Materialia 2014;72:41-56. [21] Nogita K, McDonald SD, Dahle AK, Eutectic modification of Al-Si alloys with rare earth metals,

CE

Materials Transactions 2004;45:323-6.

[22] Rooyen MV, Pers NMVD, Katgerman L, Keijser THD, Mittemeijer EJ, RELATION BETWEEN SOLIDIFICATION MORPHOLOGY AND TEXTURE OF MELT-SPUN Al AND Al-ALLOYS, Rapidly

AC

Quenched Metals 1985:823-6. [23] Uzun O, Yılmaz F, Kölemen U, Başman N, Sb effect on micro structural and mechanical properties of rapidly solidified Al–12Si alloy, Journal of Alloys and Compounds 2011;509:21-6. [24] Eiken J, Apel M, Liang S-M, Schmid-Fetzer R, Impact of P and Sr on solidification sequence and morphology of hypoeutectic Al–Si alloys: Combined thermodynamic computation and phase-field simulation, Acta Materialia 2015;98:152-63. [25] Uzun O, Karaaslan T, Gogebakan M, Keskin M, Hardness and microstructural characteristics of rapidly solidified Al–8–16 wt.% Si alloys, Journal of Alloys and Compounds 2004;376:149-57. [26] Stefanescu D. Science and engineering of casting solidification: Springer; 2015. [27] Scheil E, Bemerkungen zur schichtkristallbildung, Zeitschrift für Metallkunde 1942;34:70-2. [28] StJohn D, Prasad A, Easton M, Qian M, The contribution of constitutional supercooling to nucleation and grain formation, Metallurgical and Materials Transactions A 2015;46:4868-85.

ACCEPTED MANUSCRIPT [29] Flemings MC. Solidification Processing.

Materials Science and Technology: Wiley-VCH Verlag

GmbH & Co. KGaA; 2006. [30] Easton M, StJohn D, Grain refinement of aluminum alloys: Part I. the nucleant and solute paradigms—a

review

of

the

literature,

Metallurgical

and

Materials

Transactions

A

1999;30:1613-23. [31] Murty BS, Kori SA, Chakraborty M, Grain refinement of aluminium and its alloys by heterogeneous nucleation and alloying, International Materials Reviews 2002;47:3-29. [32] Levy S, HUGHES R, KEMPPINEN A, Quantitative Metallography of As-Cast Aluminum Microstructures, CAST METALS RES J 1969;5:93-6.

PT

[33] Anantharaman T, Suryanarayana C, A decade of quenching from the melt, Journal of Materials Science 1971;6:1111-35.

[34] Dong X, He L, Li P, Gradient microstructure and multiple mechanical properties of AlSi9Cu

RI

alloy ribbon produced by melt spinning, Journal of Alloys and Compounds 2014;612:20-5. [35] Bose SK, Kumar R, Structure of rapidly solidified aluminium-silicon alloys, Journal of Materials

SC

Science 1973;8:1795-9.

[36] Bendijk A, Delhez R, Katgerman L, De Keijser TH, Mittemeijer EJ, Van Der Pers NM, Characterization of Al-Si-alloys rapidly quenched from the melt, Journal of Materials Science

NU

1980;15:2803-10.

[37] Ahmed E, Physical properties of as melt spun Al–7 wt.% Si–x wt.% Ti, Materials Letters 2008;62:960-3.

MA

[38] Karaköse E, Keskin M, Effect of solidification rate on the microstructure and microhardness of a melt-spun Al–8Si–1Sb alloy, Journal of Alloys and Compounds 2009;479:230-6. [39] Hu HQ. The Principle of Metal Solidification. Beijing: China Machine Press; 2000.

D

[40] Stefanescu DM. Science and Engineering of Casting Solidification, Second Edition: Springer US.

PT E

[41] Kobayashi KF, Hogan LM, The crystal growth of silicon in Al-Si alloys, Journal of Materials Science 1985;20:1961-75.

[42] Dinnis CM, Taylor JA, Dahle AK, As-cast morphology of iron-intermetallics in Al–Si foundry alloys, Scripta Materialia 2005;53:955-8.

CE

[43] Makhlouf MM, Guthy HV, The aluminum–silicon eutectic reaction: mechanisms and crystallography, Journal of Light Metals 2001;1:199-218. [44] Cho YH, Dahle AK, Authors’ Reply to Discussion of “The Structure of Al-Si Alloys”, Metallurgical

AC

and Materials Transactions A 2009;40:1011-2. [45] McDonald SD, Nogita K, Dahle AK, Eutectic nucleation in Al–Si alloys, Acta Materialia 2004;52:4273-80.

[46] Crosley P, Mondolfo L, The modification of aluminum-silicon alloys, Mod Cast 1966;49:99-100. [47] Makhlouf M, Shankar S, Riddle Y, 05-088 Mechanisms of Formation and Chemical Modification of the Morphology of the Eutectic Phases in Hypoeutectic Al-Si Alloys, TRANSACTIONS-AMERICAN FOUNDRYMENS SOCIETY 2005;113:145. [48] Tonmukayakul N, Makhlouf M, Shankar S, Characterization of the Flow Behavior of Near Eutectic Composition Aluminum-Silicon Alloys, International Journal of Metalcasting 2009;3:7-16. [49] Li X, Gagnoud A, Fautrelle Y, Moreau R, Du D, Ren Z, et al., Effect of a Transverse Magnetic Field on Solidification Structures in Unmodified and Sr-Modified Al-7wtpctSi Alloys During Directional Solidification, Metallurgical and Materials Transactions A 2015;47:1198-214.

ACCEPTED MANUSCRIPT [50] Srirangam P, Kramer MJ, Shankar S, Effect of strontium on liquid structure of Al–Si hypoeutectic alloys using high-energy X-ray diffraction, Acta Materialia 2011;59:503-13. [51] Hansen N, Hall–Petch relation and boundary strengthening, Scripta Materialia 2004;51:801-6. [52]

Sanaty-Zadeh

A,

Comparison

between

current

models

for

the

strength

of

particulate-reinforced metal matrix nanocomposites with emphasis on consideration of Hall–Petch effect, Materials Science and Engineering: A 2012;531:112-8. [53] Meyers M, Mechanical metallurgy: principles and applications, Prentice-Hall, Inc, Englewood Cliffs, N J 07632, U S A, 1984 761 1984. [54] Ogata S, Li J, Yip S, Ideal pure shear strength of aluminum and copper, Science

PT

2002;298:807-11.

[55] Liu M, Fu H, Xu C, Xiao W, Peng Q, Yamagata H, et al., Precipitation kinetics and hardening mechanism in Al-Si solid solutions processed by high pressure solution treatment, Materials

AC

CE

PT E

D

MA

NU

SC

RI

Science and Engineering: A 2018;712:757-64.

ACCEPTED MANUSCRIPT Highlights 1. Rapid solidification caused formation of nanoscaled Si particle chains. 2. Sr or Ce induced the columnar transition zone only at extremely high cooling rate. 3. Enhancement of hardness is attributed to fine dispersion of eutectic Si, grain refinement and the supersaturation of Si.

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4. Sr or Ce caused further enhancement of hardness, attributing to Si modification.