Grain refinement and eutectic modification of A356 casting alloy by adding Al-B-Sr master alloys and its effect on tensile properties

The effects of the grain refinement and eutectic Si modification on tensile properties by using Al-B-Sr master alloys in A356 cast alloy were investigated. Two different master alloys, Al-4%B-1%Sr (4B1Sr) and Al-3%B-3%Sr (3B3Sr), were developed. In microstructure observation, the master alloys were consisted of AlB2, SrB6, AlSrF, KAlF4 and Al4Sr compounds. It should be noted that the α-Al phase also exists in two master alloy. The result showed that the AlB2 area fraction in the 4B1Sr alloy was higher than the 3B3Sr alloy. The 4B1Sr alloy has better efficiencies of α-Al grain refinement and eutectic Si modification than the 3B3Sr alloy, and α-Al grains are refined from 3035 to 312 μm and the eutectic Si is modified from the acicular to fibrous morphology. The tensile properties of 4 wt% treated with 4B1Sr alloy has higher UTS and elongation compared to the treated with 3B3Sr alloy.


Introduction
Al-Si cast alloys are widely used in automotive parts and construction applications because of their high castability, high-quality surface finish and low density. In addition, the addition of Mg (Al-Si-Mg) and Cu (Al-Si-Cu) can improve mechanical properties through Mg 2 Si and Al 2 Cu nanoparticle precipitation during the aging process [1]. Several refinement methods have been developed, such as direct-current pulsed magnetic field, Severe plastic deformation (SPD), rapid solidification and master alloy addition [2][3][4]. Currently, the addition of master alloys is widely used because of its ease of use and low cost compared with other methods of grain refinement.
The grain refinement with Al-Ti, Al-Fe, Al-Ti-B, Al-B, Al-Ti-B-Y and new Al-V-B master alloys occurs through their addition into the molten metal to form intermetallic compounds which act as nucleation sites such as Al 3 Ti, Al 3 Fe, TiB 2 , AlB 2 , TiC and VB 2 particles [5][6][7][8][9][10][11]. The smaller α-Al grain size provides increased tensile and impact properties [12,13]and improves casting quality, such as feeding, fluidity, surface finish, and machinability. Al-Fe and Al-Ti alloys have been widely used to refine pure aluminum [6,14]. Researchers have found that (AlSi) 3 Ti intermetallic compounds are present in the microstructure [15]. Therefore, Al-5%Ti-1%B and Al-3%Ti-1%B master alloys have been used for grain refinement [16,17]. The mechanism of grain refinement can be explained through AlTi 3 and TiB 2 promotion of α-Al grain refinement. However, prolonging the holding time reduces the efficiency owing to the agglomeration and sedimentation of AlTi 3 and TiB 2 on the surface silicon [18]. Many researchers have studied the effect of grain refining with an Al-B master alloy on Al-Si alloys. An Al-Si cast alloys (0%-10%Si) revealed that Al-B master alloys showed excellent grain refinement to smaller grains, which caused AlB 2 formation in the melt over the liquidus line and acted as nucleation sites, whereas pure aluminum did not affect grain refinement [19][20][21][22]. When comparing the efficiency of grain refinement in Al-5%Ti, Al-5%Ti-1%B, and Al-4%B master alloys, it was found that Al-4%B had a higher grain refinement efficiency [23]. Many research works, development the Al-B master alloy by using the KBF 4 powder. The melting temperatures of pure Al melt are in the range of 750°C to 1000°C [19,21,24,25], after that adding the KBF 4 powder. They found that the microstructure of lower temperature melting has higher amount of AlB 2 and KAlF 4 compounds compared to high temperature melting because of lower free energy change of reaction of AlB 2 phase [24]. Thus, AlB 2 compound is stable phase at lower temperatures 975°C in melting process. Recently, Al-Ti-C was developed and used for grain refinement [14,[26][27][28]. When added to pure aluminum, Al-Ti-C resulted in excellent refinement of α-Al. However, the grain size increased with prolonged holding time. Similarly, the grain refinement efficiency test in Al-Si cast alloys found that grain size decreased with short holding time but increased with prolonged holding time. A similar efficiency was also observed for Al-Ti-B. In some cases, refinement can be improved by stirring the liquid metal with a casting tool over a holding time of 120 min [18,28]. The mechanism for grain refinement in Al-Ti-C master alloys involves TiC particles, which act as nucleation sites during solidification. The grain refinement efficiency depended on the TiC particle content [29]. Caution in producing Al-Ti-C master alloys is the participation of TiC particles, which can agglomerate both during production and after addition into the melt, leading to a reduced efficiency [26].
The Si element is important in Al casting, providing increased hardness and strength, increased fluidity of the melt, increased hot tear resistance, improved feeding characteristics, reduced specific gravity and coefficient of thermal expansion [30]. One important factor for the mechanical properties is the size and morphology of the eutectic Si phase [31][32][33]. The advantages of changing eutectic Si include improved wear resistance, improved bendability, [34,35] and reduced solution heat treatment time. Naturally, the silicon had an acicular morphology and a large size because of the crystal growth direction. The silicon morphology was improved by the disguised atom, which inhibited the growth of silicon in the <112>direction and many directions of growth. Currently, Na and Sr are the major elements used for the modification of silicon [36][37][38]. When comparing the efficiency of silicon modification, it was found that Na was better than Sr. However, Na was more difficult to use and store. Therefore, Sr is widely used in casting. Recently, several researchers have modified eutectic Si using other rare elements such as Cr, Er, Bi, Gd, Zr, Sm and Y [39][40][41][42][43][44][45]. These elements can modify acicular Si into both fibrous and laminar morphologies. Moreover, Sato reported that K in the form of 60% KCl-40% KF modifies the coarser eutectic Si into a fibrous morphology [46].
The combination of grain refinement and modification processes have been explored to improve various properties such as mechanical properties, wear resistance, toughness, and elimination of hot tears [47][48][49][50][51]. The combination of grain refiners mixed with modifier elements can be added during the molten stage, as with master alloys such as Al-B-Sr and Al-Ti-C-Sr [52][53][54]. These two methods have similar efficiency to refine α-Al and acicular silicon for grain refinement and modification, but the master alloy method is very easy to use and control the level of addition compared to the combined method between grain refiners mixed with modifier elements. The important caution with B and Sr addition comes in the form of SrB 6 , which decreases the concentration of the grain refiner and modified element in the melt and decreases both the grain refinement and eutectic modification efficiency, especially over a prolonged time [55][56][57][58]. The density of the SrB 6 compound was 3.422 g cm −3 which decreased the concentration of solute Sr and silt in the melt [55].
However, the influence of prolonged exposure to AlB 2 , SrB 6 , KAlF 4 , AlSrF and Al 4 Sr containing compound in Al-B-Sr master alloys on solidification behavior and mechanical properties have not been extensively investigated. Therefore, in this study the effects of the grain refinement and eutectic Si modification on tensile properties by using Al-B-Sr master alloys in A356 cast alloy were investigated.

Development of Al-B-Sr master alloy
Firstly, potassium tetrafluoroborate (KBF 4 ) was used by addition into pure Al melt at 800°C in order to produce the Al-6%B (atomic percentage) alloy as a primary master alloy. When the KBF 4 powders were added into Al melted and held for 30 min, the chemical reaction between KBF4 and Al melt occurred. The melting process was carried out in a SiC crucible by using the low-frequency induction furnace. Then, the Al-10%Sr alloy was added to the melt with a holding time of 30 min. Before pouring a melt into the mold, it was stirred in order to uniform chemical composition. The melt was poured into a stainless-steel mold and the control cooling rate was measured by a thermal data logger. The Al-4%B-1%Sr and Al-3%B-3%Sr (atomic percentage) master alloy specimens were prepared from the bottom of ingot cast specimen. The specimens were ground and polished using alumina powder and silicon suspension. Finally, the samples were etched using Ticker's solution for 5-10 s. It should be noted that the microstructures were observed with an optical microscope (OM) and a scanning electron microscopy (SEM). Compositions of each compound were analyzed using the SEM with the energy dispersive spectroscopy (EDS).

Grain refinement and eutectic modification efficiency tests
The Al-7%Si-0.3%Mg alloy (A356) was used to study the grain refinement and eutectic modification efficiency tests. Its chemical composition is showed in table 1. The A356 was heated and melted at 800°C. In melt treatment, addition of master alloys was prepared with 4 wt% (weight percentage) and hold in a furnace before pouring for 10, 30, 60 and 120 min, respectively. Ar gas was used for hydrogen degassing and the removal of inclusions during the casting process. The melt was poured into a steel mold (wall thickness 1 mm.) with installed thermocouple at the center of mold and control a cooling rate for approximate 0.2°C s −1 . A slow cooling rate was used to show the better investigation of the eutectic reaction [59]. Table 2. Shows marked specimens of the modified A356 treated with 4 wt% master alloys. The macrostructure specimens were ground with 120-800 grit SiC papers and etched using Ticker's solution. The microstructure specimen was ground with of 120-1200 grit and polished with alumina powder and silicon suspensions. The grain size was measured according to ASTM E112. The tensile specimens were poured using a gravity permanent mold according to ASTM B108-03a.

Microstructure of developed Al-B-Sr master alloys
In development of master alloys, it is found that the microstructure of the 4B1Sr alloy consisted of SrB 6 , AlB 2 , AlSrF, KAlF 4 and Al 4 Sr compounds, as shown in figure 1. The AlB 2 compound forms as a cluster and surrounding with the KAlF 4 compound. The AlSrF compound has a nodular morphology. The SrB 6 compound has a blocky morphology, whereas Al 4 Sr has a lamina morphology. Thus, when addition KBF 4 into the melt, a chemical reaction between KBF 4 and molten aluminum leads to the formation of AlB 2 and KAlF 4 compounds [60].  Figure 3 shows the microstructure of the 3B3Sr alloy which consisted of SrB 6 , AlB 2 and Al 4 Sr compounds. The small AlB 2 particles are surrounded by SrB 6 compound. The SrB 6 has a cluster cubic morphology. Al 4 Sr has a lamina morphology. Its microstructure is identified using SEM-EDS, as shown in figure 4 and summarized in table 4. From the EDS results, spectrum 1 position is identified as the AlB 2 (black) compound surrounded by a SrB 6 (white) compound. Spectrum 2 position is identified as the KAlF 4 , Spectrum 3 position is the SrB 6 compound. Spectrum 4 position is identified as the Al 4 Sr compound. Spectrum 5 position is the AlSrF compound (white).
Area fraction of compound was measured by the image analyzer, as shown in table 5. In is clearly seen that the AlB 2 and KAlF 4 of the 4B1Sr alloy has higher amount than those of the 3B3Sr alloy. The SrB 6 and AlSrF  Table 2. Marked specimens of the modified A356 treated with 4 wt% master alloys.
A2-RC-120-10 compounds are found higher area fraction in the 3B3Sr alloy. This is because of different ratio of chemical composition of the master alloys.

Conventional cast of unmodified A356 alloy
The macrostructure and microstructure of unmodified A356 alloys are observed in order to use as a reference of this work. Coarse grain size of the unmodified A356 alloys can be observed and eutectic Si has an acicular      morphology in the α-Al matrix, as shown in figures 5(a) and (b). These typical macrostructure and microstructure exhibit poor tensile properties which are unsuitable for engineering applications.

Thermal analysis measurement of A356 with treated 4B1Sr alloy
In this experiment, thermal analysis was used to analyze the solidification behavior during casting of A356 treated with 4 wt% 4B1Sr alloy. It can be seen that the nucleation process of α-Al formation strongly depends on the chemical composition of master alloys. In figure 6, the unmodified A356 shows higher undercooling because it has fewer heterogeneous nucleation sites. The undercooling is reduced after the addition of the 4B1Sr alloy with short holding times before pouring. The lower undercooling of cooling curve causes by the AlB 2 particle acted as heterogeneous nucleation sites. For longer holding time of 120 min, solidification behavior and undercooling are similar to the unmodified A356, which is well known as the fading effect [18].
The effect of the holding times on the α-Al grain size of treated 4B1Sr alloy is observed by macrostructures as shown in figure 7. Refinement of α-Al to smaller grain size is achieved in the ranges of 10-60 min compared to the unmodified A356 alloy. With a prolonged holding time in the furnace, the grain size is increased. The treated 4B1Sr alloy specimen consists of large amounts of AlB 2 compound which excellent promotes a heterogeneous nucleation site of α-Al grain during solidification.
From the experimental results, the undercooling of eutectic Si of unmodified A356 occurs at 577.9°C. When addition of the 4B1Sr alloy decreases the undercooling of eutectic Si to a range of 572.5°C-576°C after holding times of 10-60 min. A prolonged holding time of 120 min increases the undercooling of eutectic Si similar to the unmodified A356 alloy. Moreover, the solidification time is extended and shifted to the right-hand side, as shown in figure 8.       figure 9(d). Therefore, fully refined microstructure of the treated 4B1Sr alloy is maintained up to 120 min.
The formation of SrB 6 compound in the microstructure in the 4B1Sr alloy generally decreases the eutectic Si modification efficiency. But in this result indicated that eutectic Si is significantly modified throughout the holding time during melting. Because the AlKF 4 compound in the 4B1Sr master alloy is involved in modification process. From a previous work, they found that a present of K in the melt is effect to modify the eutectic Si morphology [46].

Thermal analysis measurement of A356 with treated 3B3Sr alloy
The unmodified A356 shows higher undercooling of cooling curve compared to the addition of 3B3Sr alloy with a short holding time (10-30 min). However, when prolonged time (60-120 min), undercooling is increased which caused by loss of modifying efficiency, as shown in figure 10.
A comparison of the macrostructure, which was treated with 4 wt% of 3B3Sr alloy. It can be seen that very fine grain sizes (figures 11(a) and (b)) are achieved form both specimens with holding times for 10-30 min. The grain size is coarser after a prolonged time from 60 to 120 min, as shown in figures 11(c) and (d). From this result, it can be concluded that the Al-B-Sr master alloy contains low AlB 2 and high SrB 6 area fractions have adverse effects to the grain refinement efficiency in during solidification [55,56,58].
In figure 12, the unmodified A356 is showing as acicular Si line. After addition of the 3B3Sr alloy can decreases the undercooling of eutectic Si to ∼572°C after holding times for 10 to 30 min. When the holding time is increased to 60 to 120 min, the cooling curve shows higher undercooling of eutectic Si compared to 10-30 min. In addition, when holding time of 120 min the undercooling was similar to that of the unmodified A356 alloy. Moreover, the solidification time increased with the addition of 3B3Sr alloy.   In order to clearly explain the effect of eutectic Si modification of A356 alloy by addition Al-B-Sr master alloy, we tried to compare the eutectic Si morphology at holding time for 60 min with high resolution (500x). In 4B1Sr alloys, it can be seen that eutectic Si has a fibrous morphology, as shown in figure 14(a). While the morphology of eutectic Si of treated 3B3Sr is an acicular morphology surrounded by a fibrous morphology. This phenomenon can be explain by the small amount of Al 4 Sr and KAlF 4 compounds in the 3B3Sr alloys (Referred to table 5), which is not sufficient to fully modify acicular Si into a fibrous morphology. During solidification, when α-Al forms as a solid. Then, Sr is rejected into the melt and prohibits the growth of Si during eutectic reaction. However, in the specimen treated with 3B3Sr alloy most Sr atoms completely formed SrB 6 compound during the master alloy production. Therefore, it has not sufficient amount Sr to modify the eutectic Si, as shown in figure 14(b).

Effect of holding time on grain size
In figure 15, the average grain size of the unmodified A356 is 3120 μm. This measured grain size value is applied from our previous work [58]. In this work, after addition the 4B1Sr alloy for the grain refinement, the average grain sizes are decreased to 309, 311, and 326 μm of holding times for 10, 30, and 60 min, respectively. In a case of treated with 3B3Sr alloy, short holding times for 10 and 30 min have smaller grain sizes compared to prolonged holding time for 60 and 120 min. The fading phenomenon strongly affects to the addition 3B3Sr alloy during holding the melt in a furnace for 30 to 120 min. For treated with 4B1Sr alloy, fading phenomenon occurs after holding longer than 60 min therefore, it can be concluded that addition of the 4B1Sr alloy has higher grain refinement efficiencies compared to the 3B3Sr alloy.

Solidification temperature formation with treated master alloys
Thermal analysis technique is widely used in order to predict the solidification behavior of casting process. A change of reaction temperature after treatment of the master alloys can be used to confirm the grain refinement and eutectic Si modification efficiencies. Figure 16(a) shows a schematic illustration of the effect of treated with master alloys on solidification in the Al-Si phase diagram. The α-Al formation temperature of both addition master alloy shows high amount of AlB 2 in the 4B1Sr alloy has lower undercooling compared to the combined AlB 2 and SrB 6 compounds in the 3B3Sr alloy, as shown in figures 16(b) and (c). Thus, high amount of AlB 2 compound in the 4B1Sr alloy promotes higher nucleation sites in the melt. When addition 4B1Sr alloy into a furnace at 800°C, the AlB 2 compound is completely dissolved into melt. While SrB 6 compounds (T m = 2500°C) has higher melting point than AlB 2 compound (T m = 640°C) [56,57]. Therefore, the SrB 6 compound still distributes as a solid particle in the melt [58]. Thus, SrB 6 compound has lower effect on grain refinement efficiency compared to AlB 2 compound.  The equilibrium eutectic Si temperature of Al-Si binary system is 577°C. When addition the 4B1Sr or 3B3Sr alloys change the undercooling of eutectic Si to lower temperature as shown in figures 16(d) and (e). Form a data of table 5, area fraction of Al 4 Sr compound is in a range 0.19%-0.21%. When addition master alloys into furnace the Al 4 Sr compound is dissolved into the melt together with master alloy, simultaneously. Sr atom in the melt acts as nucleation sites of eutectic Si modification and reduction the undercooling of eutectic Si to lower temperature than equilibrium.

Effect of cooling rate on macrostructure and Si morphology
In all case of this work, both grain refinement and eutectic Si modification efficiencies are reduced after prolonged holding time because slow cooling during solidification in a mold. Therefore, in this section, the effect of the cooling rate is studied in order to improve the grain size and eutectic Si morphology [61,62]. The melt was poured into the steel molds with very high cooling rate of 10°C s −1 (Mold wall thickness 10 mm.) compared to previous results of 0.2°C s −1 . (Mold wall thickness 1 mm.) By increase the cooling rate, the grain size becomes smaller such as treated with 4B1Sr alloy and holding time for 120 min has 1072 μm and treated with 3B3Sr alloy has 1203 μm, as shown in figure 17. However, a reduction of grain size by increasing a cooling rate of this work can reduce about 49% compared to the slow cooling rate. Because, the α-Al grain size can be controlled by rapid cooling rate which increase the amount nucleation sites and reduce the radius of nuclei (r * Hetarogeneous ) [63]. By increasing cooling rate, the modification of eutectic Si is affected for both treated 4B1Sr and 3B3Sr alloys, as shown in figure 18. A very fine fibrous Si morphology is observed in the treated with 4B1Sr alloy (figures 18(a) and (c)), while a coarser Si particle is observed in the treated with 3B3Sr alloy (figures 18(b) and (d)). It can be indicated that the rapid cooling rate significantly affects the eutectic Si modification.

Tensile properties of added master alloy specimens
In this experiment, 2 and 4 wt% additions of both master alloys were treated into the melt. Tensile test results reveal that the UTS of the unmodified A356 is 148 MPa, while the elongation is 4.7%, as shown in figure 19. With the addition of the 2 wt% 4B1Sr alloy, UTS increases to 164, 162 and 164 MPa while the elongation increases marginally to 14.4, 14.5 and 14.3% for holding times of 30, 60 and 120 min, respectively. When consideration of yield strength, unmodified A356 is 95 MPa and 2 wt% treated 4B1Sr alloy is 99 MPa. Thus, yield strength is increased by addition of 4B1Sr alloy because their smaller grain sizes. The addition of 4 wt% 4B1Sr alloy further increases both UTS to 165.5, 167.0 and 168.2 MPa and elongation to 18.4, 17.4 and 18.2% for holding times of 30, 60 and 120 min, respectively. In all the cases, the holding time has little effect on the UTS and elongation. Therefore, level of additional master alloy is important factor to the tensile properties. In case of 4 wt% treated 4B1Sr alloy has higher strength because of higher amount of B (grain refiner) and K (modifier) content compared with addition of 2 wt%.
For addition 2 wt% of 3B3Sr alloy increases the UTS to 163, 161 and 156 MPa while the elongation increased to 12.9, 12.9 and 11.9% after 30, 60 and 120 min, as shown in figure 20. High amount of SrB 6 compound in the addition 4 wt% increased the UTS to 163, 155 and 148 MPa while the elongation increased to 12.6, 12.5 and 13.2% for 30, 60 and 120 holding times, respectively. It can be seen that prolonged holding times of 60 to 120 min decrease the UTS to lower strength. This is because fading phenomenon of melt treatment due to high density of the SrB 6 compound. Therefore, it is agglomerated at the bottom of the furnace during casting. Thus, in the melt, it has low solute concentrations of B and Sr, which leads to decrease the mechanical properties.  3.9. Macrostructure and microstructure of tensile specimens In order clearly explain a relationship between tensile properties and structures of unmodified A356 compared to addition of Al-B-Sr master alloys. Thus, macro-and micro-tensile specimens were examined. The macrostructure and microstructure of the unmodified A356 show in figure 21. It has a large grain size and acicular morphology of eutectic Si, which is harmful to the UTS and elongation properties.
The macrostructure in figure 22 shows the grain size of the A356 alloy after 4 wt% addition of the master alloys. Because high amount of B element in the 4B1Sr alloy, after treatment of for 30 min holding time the melt has a high B solute concentration, which leads formed AlB 2 during solidification and resulted of small equiaxed grains at the center of the casting specimen. With prolonged holding time, the equiaxed grain area is decreased, while the columnar zone region increased from the outer to the inner of the specimen, as shown in figure 22(b). A coarser grain size is observed in the treated with 3B3Sr alloy both holding time for 30 and 120 min, as shown in figures 22(c) and (d), respectively. Coarser grain size is a reason of lower strength of treated with 3B3Sr alloy specimen compared to treated with 4B1Sr alloy specimen, as shown in figures 19 and 20.
In hypoeutectic Al-Si cast alloys, the eutectic Si area fraction depends on the Si content and addition level of modifier elements. The microstructure of the tensile specimens shows in figure 23. In the 4 wt% treated with 4B1Sr alloy results in a smaller fibrous Si morphology both short and long holding time specimens, as shown in figures 23(a) and (b). It can be observed that eutectic Si area fraction increases after treatment with the master alloy. Thus, modified Si morphology is effects to the elongation of the casting specimen.
In 4 wt% treated with 3B3Sr alloy, the Si morphology with high Sr content from the SrB 6 compound. When holding time for 30 min, acicular eutectic Si is refined to fibrous Si morphology, as presented in figure 23(c). With prolonged holding time, the eutectic Si appears coarse acicular morphology, as shown in figure 23(d). Table 6 shows comparison of mechanical properties of modified A356 treated with 4 wt% master alloys to clarify effect of the grain refinement and eutectic Si modification efficiencies. It can be seen that A1-30-0.2 and A1-60-0.2 specimens have smallest grain size and fibrous Si particle. They have higher yield strength and

Discussion on grain refinement and modification mechanisms of the treated Al-B-Sr alloy
In this work, design and development of Al-B-Sr master alloys, the B:Sr ratios are 4:1 and 3:3 in order to produce different compounds in microstructure of the master alloys. From the grain refinement and modification efficiency tests, the specimen treated with 4B1Sr alloy results in the excellent refined α-Al and eutectic Si  compared to the treated with 3B3Sr alloy. According to compound area fractions in table 5, the 4B1Sr alloy consists of the AlB 2 , SrB 6 , Al 4 Sr, KAlF 4 and AlSrF compounds for 3.40, 0.91, 0.19, 2.23 and 0.15%, respectively. While the 3B3Sr alloy consists of the AlB 2 , SrB 6 and Al 4 Sr, KAlF and AlSrF compounds for 1.28, 3.48, 0.21, 0.56 and 0.21, respectively. Form experimental results, AlB 2 and SrB 6 compounds should be useful for grain refinement. However, in solidification, formed AlB 2 in the melt has higher effect than SrB 6 . Figure 24 shows schematic illustrations of grain refinement mechanism of treated with 4B1Sr alloy and treated with 3B3Sr alloy. It is clearly seen that the excellent grain size refinement is achieved in the modified A356 with treated by the 4B1Sr alloy.
The Al4Sr, KAlF and AlSrF compounds should be useful for the eutectic Si modification. In the casting process, these compounds are dissolved into melt after adding 4B1Sr alloy into the furnace. In solidification process, Sr and K can inhibit the Si transformation in the <112>direction and leads to change a shape form acicular to a fibrous eutectic Si morphology. Figure 25 shows schematic illustrations of eutectic Si modification mechanism during solidification of this work. After adding the master alloys at 800°C, the Al 4 Sr, AlSrF and KAlF 4 are dissolved in the melt. The melting points of the KAlF 4 is 574°C [60]. In the subsequent solidification process, the growth of α-Al phase leads to repelling the K, Sr, O and Si atom to the solid-liquid interface. When the solute concentration of Si in melt increases to 11.7%Si at ∼577°C. The eutectic reaction will be taken place. Then, Sr and K modifier adsorb on the silicon-liquid interface which decrease the undercooling and enhance the twinning of the eutectic Si growth according to the impurity induced twinning (IIT) model. The K modifying element has larger atom than Si  atom. It can promotes a change of acicular eutectic Si morphology to smaller particles [64]. Figure 26 and table 7 show the SEM-EDS analytical result of elemental concentrations of AlSiFeOKF and AlSrO compounds. The x-ray mapping examination in figure 27 clearly shows concentrated K and O elements contained in eutectic Si region. Therefore, the Sr and K in the 4B1Sr master alloy are highly effect to modify the eutectic Si.

Conclusions
1. In microstructural development of Al-B-Sr master alloys, their consist of AlB 2 , SrB 6 , AlSrF, Al4Sr and KAlF 4 compounds. High amounts of AlB 2 and KAlF 4 compounds are found in the 4B1Sr alloy, while high amount of SrB 6 compound is found in the 3B3Sr alloy.
2. In the 4B1Sr alloy, high AlB 2 compound promotes the α-Al grain refinement and KAlF 4 compound contributes eutectic Si modification. Their works as heterogenous nucleation sites during solidification.
3. In the 3B3Sr alloy, high SrB 6 compound reduces both grain refinement and modification efficiencies. Fading phenomenon is main predominant because high density of SrB 6 compound.  4. Completely dissolved Sr in the melt reduced the eutectic Si temperature to lower and shift eutectic Si composition to right-hand side. Thus, high amount of dissolved Sr in melt promotes high modification efficiency.
5. The mechanical properties of the tensile specimens depend on addition level and type of master alloys. Form tensile test result, addition of 4 wt% of 4B1Sr alloy has higher UTS and elongation compared to the addition of 3B3Sr alloy.