Morphological Evolution of TiB2 and TiAl3 in Al–Ti–B Master Alloy Using Different Ti Adding Routes

Three different Ti addition routes were used to prepare an Al–5Ti–B Master Alloy: the halide salt route, the Ti-sponge route, and the partial Ti-sponge route. In the halide salt route, the raw materials were Al + KBF4 + K2TiF6; K2TiF6 was completely replaced by pure titanium for the Ti-sponge route versus the halide salt route; in the partial Ti-sponge route, K2TiF6 was partially replaced by pure titanium. Here, 30% Ti-sponge or 60% Ti-sponge route means that 30% or 60% K2TiF6 was replaced by pure titanium, respectively. The above Ti addition routes have a significant influence on the growth pattern and morphological evolution of TiAl3 and TiB2, which greatly affect the refining performance of Al–Ti–B Master Alloy. When using the halide salt route, a streamlined “rich Ti, B area” exists in the aluminum melt, which is a complex compound of (Tix, Al1−x) By. The “rich Ti, B area” is essential for the nucleation and growth of TiAl3 and TiB2. Blocky TiAl3 was obtained and its average size was 4.7 μm based on the halide salt route. In the Ti-sponge route, the nucleation of TiAl3 mainly depends on the mutual diffusion of Al and Ti, and TiAlx forms around pure Ti particles, i.e., the so-called Ti–TiAlx mechanism. The average size of the blocky TiAl3 was 9.8 μm based on the Ti–TiAlx mechanism. For the partial Ti-sponge route, the “rich Ti, B area” gradually decreases with the increase in Ti powder’s contents, and large TiAl3 coexists with the small TiAl3. Compared with the Ti-sponge route, the halide salt route can form smaller TiAl3. In the Ti-sponge route, there is a small amount of “rich Ti, B area” due to the influence of the Ti–TiAlx mechanism, which does not meet the requirements of TiB2 growth. In the halide salt route, there is sufficient “rich Ti, B area”, which is conducive to the formation of TiB2. Both the crystal defects and the crowded growth environment caused by the “rich Ti, B area” are fundamental reasons for the fragility and the irregular shape of the TiB2. The refining effect of the Al–Ti–B Master Alloy prepared by the halide salt route is better than the Ti-sponge route. The refining effect of 30% Ti-sponge route is better than that of Ti-sponge route and worse than that of halide salt route.


Introduction
Al-Ti-B Master Alloy is an essential grain refiner and can strongly refine the microstructure and prevent the formation of coarse and equiaxed grains as well as columnar grains during the casting process of aluminum and aluminum alloy. It can also make the ingot's microstructure become uniform, reduce segregation, and inhibit cracks. This greatly improves the mechanical and physical properties of aluminum and aluminum alloys [1][2][3][4][5]. The refinement mechanisms of Al-Ti-B Master Alloy are complicated and mainly include carbide-boride particle theory [6], peritectic reaction theory [7], peritectic hulk theory [8], and the duplex nucleation mechanism [9]. Of these, the duplex nucleation theory has 6KBF 4 (l) + 3K 2 TiF 6 (l) + 10Al(l) → 3TiB 2 (s) + 9KAlF 4 (l) + K 3 AlF 6 (l) (1) 3K 2 TiF 6 (l) + 13Al(l) → 3TiAl 3 (s) + 3KAlF 4 (l) + K 3 AlF 6 (l) (2) 2KBF 4 (l) + 3Al(l) → AlB 2 (s) + 2KAlF 4 (l) 2KBF 4 (l) + 2Ti + 5Al(l) → TiB 2 (s) + TiAl 3 (s) + 2KAlF 4 (l) (4) 2KBF 4 (l) + 3Al(l) → AlB 2 (s) + 2KAlF 4 (l) Liu [20] studied the relationship between the morphology of TiAl 3 and the smelting temperature for the halide salt route. Low-temperature smelting can form block-shaped TiAl 3 , while high-temperature smelting can quickly form rod-shaped TiAl 3 crystals. Xie [21] also found that TiAl 3 mainly has two morphologies: block-shaped and rod-shaped; the solute Ti grows easily around the agglomerate TiAl 3 , and TiB 2 also has specific morphological evolution. TiB 2 particles are hexagonal and independent when Al-Ti-B Master Alloy was prepared by the halide salt route [22]. The reaction temperature does not influence the morphology of TiB 2 . However, TiB 2 particles showed different morphologies at different reaction temperatures when the Master Alloy was prepared by the Ti-sponge route [12]. The TiB 2 particles are larger than 5 µm when the reaction temperature is 850 • C. When the reaction temperature reached 1200 • C, TiB 2 particles gradually changed into layered stacking morphology and even a dendritic morphology. Zhang [23] confirmed by kinetic analysis that TiB 2 particles are not stable in Al melt and Ti addition can suppress the dissolution of TiB 2 particles. TiB 2 may coarsen during the holding temperature and grow during the cooling of the melt. Wang [24] demonstrates a strong epitaxial growth of TiAl 3 on the surface of TiB 2 particles. Fan et al. also found that a layer of TiAl 3 was formed on the surface of TiB 2 and that TiAl 3 can significantly improve the stability of TiB 2 [25]. TiAl 3 and TiB 2 also interact with each other. Wang et al. also proved that TiB 2 is critical to grain refinement of aluminum and aluminum alloys [26]. TiB 2 particles react with aluminum slowly and release Ti into the melt. The TiAl 3 particles then combine with Ti in the melt to form a dynamic Ti-rich layer on the surface of (Ti, Al) B 2 . This layer offers a low crystal mismatch with α-Al and promotes the nucleation of aluminum grains.
The Ti-sponge route replaces K 2 TiF 6 in the halide salt route with Ti powder. According to the addition amount of raw materials, the amount of K 2 TiF 6 needed, for the same content of Ti, is five times that of Ti powder. The preparation of 1 ton of Al-5Ti-B master alloy needs 0.3665 tons of mixed fluoride salt of K 2 TiF 6 and KBF 4 but only 0.1665 tons of Ti powder and KBF 4 . The liquid salt slags and fluoride gas produced by Ti-sponge route are much lower than that of the halide salt route. Therefore, the fluoride pollution of halide salt route is much more serious than that of the Ti-sponge route. The partial Ti-sponge route of combining the halide salt route and the Ti-sponge route may have a better refining effect and improved environmental effects. It is necessary to explore the refining effect of Al-Ti-B Master Alloy using different Ti adding routes.
The growth pattern and morphology distribution of the TiAl 3 and TiB 2 for different preparation processes have not been thoroughly studied. The difference between the Ti-sponge route and halide salt route is quite rarely analyzed. Here, an Al-Ti-B Master Alloy was prepared using different Ti addition routes including the halide salt route, the Ti-sponge route, and the partial Ti-sponge route. The growth pattern and morphological evolution of the TiAl 3 and TiB 2 were analyzed, and the refining effects using different Ti adding routes were also discussed. Table 1 shows the experimental parameters of Al-5Ti-B Master Alloy prepared by the halide salt route. Pure aluminum, KBF 4 , and K 2 TiF 6 were all added at a nominal composition of the Al-5Ti-B Master Alloy (Al:Ti:B = 100:12.4:26.6, weight ratio). The dried pure aluminum was heated to 800 • C in a graphite crucible until the metal was soft-crushed. A coating agent (10.8% calcium fluoride + 72.8% magnesium chloride + 16.4% chlorinated calcium) was then covered on the surface of the aluminum melt. The slag was removed after a hexachloroethane refiner was added at 720 • C and held for 1200 S. When increasing temperature to the experimental reaction temperature (Table 1), mixed fluoride salt (KBF 4 and K 2 TiF 6 were premixed before adding) was added into the aluminum melt, stirred evenly, and held for 10-1800 S ( Table 1). The melt was then poured into a preheated steel mold to obtain a cubic ingot (160 × 100 × 12 mm).  Table 2 shows the experimental parameters of Al-5Ti-B Master Alloy prepared by the partial Ti-sponge route and Ti-sponge route. Based on the halide salt route, K 2 TiF 6 was gradually replaced by pure Ti. The Ti-sponge route means that K 2 TiF 6 was totally replaced by pure Ti. The 30% Ti-sponge route and 60% Ti-sponge route mean that K 2 TiF 6 was partially replaced by 30% Ti and 60% Ti, respectively. The microstructure samples were cut from the center of the ingots at the center height, mounted, and mechanically polished. The samples were then etched using Keller's reagent for 10 s. The microstructures were characterized using a Leica DMR research optical microscope (OM) and a SU-8020 field emission gun scanning electron microscope (FEG SEM) operating at 45 kV and a tube current of 40 µA. A D/max 2500V X-ray diffractometer equipped with Ni-filtered Cu K α radiation source scanning over a range of 2θ = 20-80 • was used to analyze TiB 2 and TiAl 3 . TiB 2 . Figure 1 shows the microstructure and composition of Al-Ti-B prepared by the halide salt route at 800 • C with different reaction times. There are obvious filamentous areas in Figure 1a. According to the EDS analysis, the filamentous areas were rich in elemental Ti but did not contain element F. This indicates that these areas should be the product areas of the fluoride salt reaction. The ratio of Ti to Al at point 2 is about 34.5%, which is nearly equal to the mass fraction of Ti in TiAl 3 (about 36.96%). Considering that Al may be oxidized, point 2 should be TiAl 3 . Point 1 contains Ti and the energy spectrometer cannot detect the existence of B; therefore, we infer that the filamentous area is at least rich in Ti. The filamentous area gradually disappeared with longer reaction times. The filamentous area greatly reduced at 30 S (Figure 1b), and TiAl 3 of thin rods or small blocks and aggregative TiB 2 appeared. The filamentous area disappeared at 60 S (Figure 1c). At this point, the TiAl 3 became thick rods and large blocks, and TiB 2 dispersed into coarse grains. When the reaction time was extended to 600 s, the TiB 2 further dispersed into fine particles (Figure 1d), thus distributing in the Al matrix. TiB 2 was generated in the incipient filamentous area, which suggests that the filamentous Ti-rich area was also rich in B. Therefore, the filamentous area should be called the "rich Ti, B area".

"Rich Ti, B Area" and Morphology of TiAl3, TiB2 in Halide Salt Route
The possible reactions during the Al-Ti-B Master Alloys prepared by the halide salt route are shown in Formulas (1)-(3). These reactions are the main sources of TiAl3 and TiB2. Figure 1 shows the microstructure and composition of Al-Ti-B prepared by the halide salt route at 800 °C with different reaction times. There are obvious filamentous areas in Figure 1a. According to the EDS analysis, the filamentous areas were rich in elemental Ti but did not contain element F. This indicates that these areas should be the product areas of the fluoride salt reaction. The ratio of Ti to Al at point 2 is about 34.5%, which is nearly equal to the mass fraction of Ti in TiAl3 (about 36.96%). Considering that Al may be oxidized, point 2 should be TiAl3. Point 1 contains Ti and the energy spectrometer cannot detect the existence of B; therefore, we infer that the filamentous area is at least rich in Ti. The filamentous area gradually disappeared with longer reaction times. The filamentous area greatly reduced at 30 S (Figure 1b), and TiAl3 of thin rods or small blocks and aggregative TiB2 appeared. The filamentous area disappeared at 60 S (Figure 1c). At this point, the TiAl3 became thick rods and large blocks, and TiB2 dispersed into coarse grains When the reaction time was extended to 600 s, the TiB2 further dispersed into fine particles (Figure 1d), thus distributing in the Al matrix. TiB2 was generated in the incipient filamentous area, which suggests that the filamentous Ti-rich area was also rich in B. Therefore the filamentous area should be called the "rich Ti, B area". The XRD spectra at 800 °C for different reaction times are shown in Figure 2 to further confirm the "rich Ti, B area". The main phases of all samples were Al, TiAl3, and TiB2, and no prominent excess peaks were observed. The intensity of the TiAl3 peak is basically the same from 10 S to 1800 S, which indicates that TiAl3 generated at the initial stage of the halide salt reaction. The TiB2 peak obviously changed from weak to strong. There is no The XRD spectra at 800 • C for different reaction times are shown in Figure 2 to further confirm the "rich Ti, B area". The main phases of all samples were Al, TiAl 3 , and TiB 2 , and no prominent excess peaks were observed. The intensity of the TiAl 3 peak is basically the same from 10 S to 1800 S, which indicates that TiAl 3 generated at the initial stage of the halide salt reaction. The TiB 2 peak obviously changed from weak to strong. There is no TiB 2 peak in the XRD pattern at 10 S and a weak TiB 2 peak appears at 15 S, thus indicating that the formation of TiB 2 is slower than that of TiAl 3 or it generates too little to detect in the early stage. The TiB 2 and AlB 2 have a similar crystal structure, and thus, diffraction peaks may be broadened or separated in XRD diffraction. If the value of Ti/B is greater than 2.2, then excess B will form AlB 2 [27]. Therefore, the peaks in the two square frames in Area1 were considered to be AlB 2 , which are very close to the strong peak of TiB 2 . A small amount of AlB 2 also exists in the early stage of the reaction. Some studies have reported the nonhomogenous distribution of solutes B and Ti close to the salts/Al melt interface during the reaction of the fluoride salts and Al melt. These form unstable AlB 2 . The AlB 2 transforms to TiB 2 via diffusion of solutes Ti and B in the aluminum melt in the subsequent holding temperature process [28,29]. The essence of filamentous areas ( Figure 1a) is a complex compound (Ti x , Al 1−x )B y ; this area was called the "rich Ti, B area". In the early stage of the halide salt reaction, the streamlined "rich Ti, B area" existed in the aluminum melt ( Figure 1a). The formation of TiAl 3 was very fast (less than 10 S). The "rich Ti, B area" was the birthplace of the nucleation and growth of initial TiAl 3 and TiB 2 for the halide salt route. TiB2 peak in the XRD pattern at 10 S and a weak TiB2 peak appears at 15 S, thus indicating that the formation of TiB2 is slower than that of TiAl3 or it generates too little to detect in the early stage. The TiB2 and AlB2 have a similar crystal structure, and thus, diffraction peaks may be broadened or separated in XRD diffraction. If the value of Ti/B is greater than 2.2, then excess B will form AlB2 [27]. Therefore, the peaks in the two square frames in Area1 were considered to be AlB2, which are very close to the strong peak of TiB2. A small amount of AlB2 also exists in the early stage of the reaction. Some studies have reported the nonhomogenous distribution of solutes B and Ti close to the salts/Al melt interface during the reaction of the fluoride salts and Al melt. These form unstable AlB2. The AlB2 transforms to TiB2 via diffusion of solutes Ti and B in the aluminum melt in the subsequent holding temperature process [28,29]. The essence of filamentous areas ( Figure 1a) is a complex compound (Tix, Al1−x)By; this area was called the "rich Ti, B area". In the early stage of the halide salt reaction, the streamlined "rich Ti, B area" existed in the aluminum melt ( Figure 1a). The formation of TiAl3 was very fast (less than 10 S). The "rich Ti, B area" was the birthplace of the nucleation and growth of initial TiAl3 and TiB2 for the halide salt route.  Figure 3 clearly shows the morphology of the TiB2 in the Al-Ti-B Master Alloy prepared by the halide salt route at 800 °C for 300 S. The white shadow on the particle's edge shows that TiB2 is hexagonal with a certain thickness. The maximum section size of these hexagons is 0.5-1.0 μm. In the statistics of T.E. Quested [28], the size of TiB2 particles in conventional Al-Ti-B Master Alloy is about 5 μm, and the size of most TiB2 is 0.5-1.5 μm.  hexagons is 0.5-1.0 µm. In the statistics of T.E. Quested [28], the size of TiB 2 particles in conventional Al-Ti-B Master Alloy is about 5 µm, and the size of most TiB 2 is 0.5-1.5 µm. Figure 3b shows the irregular TiB 2 ; they may be the side cuts of the hexagonal TiB 2 ; however, the mutual "crashing" during fragile TiB 2 s growth also may lead to the forming of the irregular shape ( Figure 3c). terials 2022, 15, x FOR PEER REVIEW 6 of 1 Figure 3b shows the irregular TiB2; they may be the side cuts of the hexagonal TiB2; how ever, the mutual "crashing" during fragile TiB2′s growth also may lead to the forming o the irregular shape ( Figure 3c).  Figure 4 shows the morphology of TiAl3 prepared by the halide salt route at 800 °C for 15 S, 30 S, or 60 S. At 800 °C for 15 S, the TiAl3 was granular or elongated rod-shaped When the reaction time was extended to 30 S, the strip phase became a thick rod. TiA was blocky when the reaction time was 60 S.  (4) and (5). Reaction (4) occurred spontaneously at the temperature range o  Figure 4 shows the morphology of TiAl 3 prepared by the halide salt route at 800 • C for 15 S, 30 S, or 60 S. At 800 • C for 15 S, the TiAl 3 was granular or elongated rod-shaped. When the reaction time was extended to 30 S, the strip phase became a thick rod. TiAl 3 was blocky when the reaction time was 60 S.  Figure 3b shows the irregular TiB2; they may be the side cuts of the hexagonal TiB2; however, the mutual "crashing" during fragile TiB2′s growth also may lead to the forming of the irregular shape ( Figure 3c).  Figure 4 shows the morphology of TiAl3 prepared by the halide salt route at 800 °C for 15 S, 30 S, or 60 S. At 800 °C for 15 S, the TiAl3 was granular or elongated rod-shaped. When the reaction time was extended to 30 S, the strip phase became a thick rod. TiAl3 was blocky when the reaction time was 60 S.

Morphology of TiAl3, TiB2 in the Ti-Sponge and Partial Ti-Sponge Route
The reactions of preparing Al-Ti-B Master Alloy by the Ti-sponge route are shown in Formulas (4) and (5). Reaction (4) occurred spontaneously at the temperature range of 800-900 °C [19]. The mass ratio of Ti to B is 4.43:1 in reaction (4). The mass ratio of Ti/B added to the reaction is 5:1. Therefore, reaction (5) also occurred. Figure 5 shows the X ray diffraction pattern of the Al-Ti-B Master Alloy prepared by the Ti-sponge route with reaction times of 15 S, 300 S, and 1800 S. The diffraction peaks of the TiB2 were observed at

Morphology of TiAl 3 , TiB 2 in the Ti-Sponge and Partial Ti-Sponge Route
The reactions of preparing Al-Ti-B Master Alloy by the Ti-sponge route are shown in Formulas (4) and (5). Reaction (4) occurred spontaneously at the temperature range of 800-900 • C [19]. The mass ratio of Ti to B is 4.43:1 in reaction (4). The mass ratio of Ti/B added to the reaction is 5:1. Therefore, reaction (5) also occurred. Figure 5 shows the X ray diffraction pattern of the Al-Ti-B Master Alloy prepared by the Ti-sponge route with reaction times of 15 S, 300 S, and 1800 S. The diffraction peaks of the TiB 2 were observed at 15 S. AlB 2 and TiB 2 existed in the matrix simultaneously at 15 S (consistent with Formula (5) (5)) for TiB2 diffraction peaks broadening.  Figure 6a1,a2 shows different parts of the same sample. Owing to the short reaction time of 15 S, some Ti powders did not have enough time to diffuse. Therefore, the irregular white blocky TiAl3 was observed in Figure 6a1. Other Ti powders are relatively dispersed to form fine, rod-shaped TiAl3 (Figure 6a2). At a reaction time of 300 S, the whole block dispersed into small, irregular pieces ( Figure 6b). The original fine, rod-shaped structure disappeared. The energy spectrum analysis indicated the white phase in Figure 6b should be pure Ti, and the surrounding gray phase should be TiAlx. The concentration of Al in the gray phase decreased gradually from inside to outside while Ti increased. The value of Ti/Al at point 3 was closer to 36.96%, which is TiAl3.
There are both blocky and rod-shaped TiAl3 in Figure 8. Further, it was found that TiB2 was embedded in blocky TiAl3 (Figure 7b). The hexagonal holes appeared in the rodshaped TiAl3 and were eroded by pure Al (Figure 7a). If B was added into the Al-Ti alloy melt containing TiAl3, then B could form TiB2 on the TiAl3 and cause the TiAl3 to break or even dissolve [29]. The resulting TiB2 will be agglomerated between the TiAl3 and Al matrix. The reaction equation is as follows: TiAl 3 (s) + 2B → TiB 2 (s) + 3Al(l) The microstructure and EDS of Al-Ti-B Master Alloys prepared by the Ti-sponge route and partial Ti-sponge route at 800 • C for different times are shown in Figures 6 and 7. Figure 6a 1 ,a 2 shows different parts of the same sample. Owing to the short reaction time of 15 S, some Ti powders did not have enough time to diffuse. Therefore, the irregular white blocky TiAl 3 was observed in Figure 6a 1 . Other Ti powders are relatively dispersed to form fine, rod-shaped TiAl 3 (Figure 6a 2 ). At a reaction time of 300 S, the whole block dispersed into small, irregular pieces ( Figure 6b). The original fine, rod-shaped structure disappeared. The energy spectrum analysis indicated the white phase in Figure 6b should be pure Ti, and the surrounding gray phase should be TiAl x . The concentration of Al in the gray phase decreased gradually from inside to outside while Ti increased. The value of Ti/Al at point 3 was closer to 36.96%, which is TiAl 3 .
There are both blocky and rod-shaped TiAl 3 in Figure 8. Further, it was found that TiB 2 was embedded in blocky TiAl 3 (Figure 7b). The hexagonal holes appeared in the rod-shaped TiAl 3 and were eroded by pure Al (Figure 7a). If B was added into the Al-Ti alloy melt containing TiAl 3 , then B could form TiB 2 on the TiAl 3 and cause the TiAl 3 to break or even dissolve [29]. The resulting TiB 2 will be agglomerated between the TiAl 3 and Al matrix. The reaction equation is as follows:

Refining Effect of Al-Ti-B Master Alloy
The Al-Ti-B Master Alloy obtained at 800 °C for 1800 S using different routes was used to refine pure aluminum ingot; the refined microstructure is shown in Figure 8. The average grain sizes of pure aluminum ingot refined by halide salt route, 30% Ti-sponge route, and Ti-sponge route are 98.3 μm, 105.4 μm, and 111.8 μm, respectively. The refining effect of 30% Ti-sponge route is better than that of Ti-sponge route and worse than that of halide salt route.
The size of TiAl3 obtained using the Ti-sponge route was larger than that using the halide salt route; they are 9.83 μm ( Figure 6) and 4.7 μm (Figure 4), respectively. In the 30% Ti-sponge or 60% Ti-sponge route, the large-sized granular TiAl3 was coexistent with the small-sized TiAl3 (Figure 4). The size of TiB2 particles is about 5 μm, and the size of most TiB2 is 0.5-1.5 μm (Figure 3). The refining effect of Al-Ti-B Master Alloy is related

Refining Effect of Al-Ti-B Master Alloy
The Al-Ti-B Master Alloy obtained at 800 °C for 1800 S using different routes was used to refine pure aluminum ingot; the refined microstructure is shown in Figure 8. The average grain sizes of pure aluminum ingot refined by halide salt route, 30% Ti-sponge route, and Ti-sponge route are 98.3 μm, 105.4 μm, and 111.8 μm, respectively. The refining effect of 30% Ti-sponge route is better than that of Ti-sponge route and worse than that of halide salt route.
The size of TiAl3 obtained using the Ti-sponge route was larger than that using the halide salt route; they are 9.83 μm ( Figure 6) and 4.7 μm (Figure 4), respectively. In the 30% Ti-sponge or 60% Ti-sponge route, the large-sized granular TiAl3 was coexistent with the small-sized TiAl3 (Figure 4). The size of TiB2 particles is about 5 μm, and the size of most TiB2 is 0.5-1.5 μm (Figure 3). The refining effect of Al-Ti-B Master Alloy is related

Refining Effect of Al-Ti-B Master Alloy
The Al-Ti-B Master Alloy obtained at 800 • C for 1800 S using different routes was used to refine pure aluminum ingot; the refined microstructure is shown in Figure 8. The average grain sizes of pure aluminum ingot refined by halide salt route, 30% Ti-sponge route, and Ti-sponge route are 98.3 µm, 105.4 µm, and 111.8 µm, respectively. The refining effect of 30% Ti-sponge route is better than that of Ti-sponge route and worse than that of halide salt route.
The size of TiAl 3 obtained using the Ti-sponge route was larger than that using the halide salt route; they are 9.83 µm ( Figure 6) and 4.7 µm (Figure 4), respectively. In the 30% Ti-sponge or 60% Ti-sponge route, the large-sized granular TiAl 3 was coexistent with the small-sized TiAl 3 (Figure 4). The size of TiB 2 particles is about 5 µm, and the size of most TiB 2 is 0.5-1.5 µm (Figure 3). The refining effect of Al-Ti-B Master Alloy is related to the size and distribution of TiAl 3 and TiB 2 . A finer size and more uniform distribution of TiAl 3 and TiB 2 causes the Al-Ti-B Master Alloy to have a better grain-refining effect.

Nucleation and Growth of TiAl 3 and TiB 2 in Different Ti Adding Routes-The Ti-TiAl x Mechanism
The nucleation of TiAl 3 mainly depends on the mutual diffusion of Al and Ti for the Ti-sponge route. This diffusion process makes TiAl x form around the pure Ti particles first, as shown in Figure 6b. As diffusion progresses, the external TiAl x is separated into single TiAl 3 , but the concentration of internal Ti remains high (Figure 6a 1 ). Finally, the TiAl x completely converted into separated TiAl 3 after 300 S reaction at 800 • C (Figure 6b). This mechanism is called the Ti-TiAl x mechanism for the Ti-sponge route, as shown in Figure 9. Sunda [30] proposed that Ti-Al's interdiffusion coefficient (D) is a function of composition and temperature; D increases with increasing temperature when the composition is constant. The diffusion activation energy of aluminum (Q) is different in different phases, but Q is substantially constant and independent of the concentration. We have the following order: Q[TiAl 3 ] > Q[TiAl] > Q[α-Ti(Al)] > Q[β-Ti(Al)]. Obviously, the diffusion coefficient of Al in Ti3Al is smaller than that in Ti particles. Therefore, Al enriched on the Ti particle's surface passed through the TiAl 3 layer (Figure 9) resulting in a high concentration of Al surrounding the surface of the Ti particles. Thus, TiAl x is constantly formed and eventually converted into separate TiAl 3 . to the size and distribution of TiAl3 and TiB2. A finer size and more uniform distribution of TiAl3 and TiB2 causes the Al-Ti-B Master Alloy to have a better grain-refining effect.

Nucleation and Growth of TiAl3 and TiB2 in Different Ti Adding Routes-The Ti-TiAlx Mechanism
The nucleation of TiAl3 mainly depends on the mutual diffusion of Al and Ti for the Ti-sponge route. This diffusion process makes TiAlx form around the pure Ti particles first, as shown in Figure 6b. As diffusion progresses, the external TiAlx is separated into single TiAl3, but the concentration of internal Ti remains high (Figure 6a1). Finally, the TiAlx completely converted into separated TiAl3 after 300 S reaction at 800 °C (Figure 6b). This mechanism is called the Ti-TiAlx mechanism for the Ti-sponge route, as shown in Figure 9. Sunda [30] proposed that Ti-Al's interdiffusion coefficient (D) is a function of composition and temperature; D increases with increasing temperature when the composition is constant. The diffusion activation energy of aluminum (Q) is different in different phases, but Q is substantially constant and independent of the concentration. We have the following order: Obviously, the diffusion coefficient of Al in Ti3Al is smaller than that in Ti particles. Therefore, Al enriched on the Ti particle's surface passed through the TiAl3 layer (Figure 9) resulting in a high concentration of Al surrounding the surface of the Ti particles. Thus, TiAlx is constantly formed and eventually converted into separate TiAl3. The nucleation mechanism of the rod-shaped TiAl3 (Figure 6a2) is different from the Ti-TiAlx mechanism. The atomic ratio of Ti to KBF4 is 1:1 in reaction (4). Actually, the ratio of Ti to KBF4 is about 1.13:1 in this experiment so that KBF4 can react completely. The rodshaped TiAl3 comes from reaction (4) and is easy to dissolve versus blocky TiAl3; therefore, it almost disappeared after 300 S of reaction (4), as shown in Figure 6b. For the Ti-sponge route, Ti is involved in the transition of Ti-TiAlx to produce TiAl3, which leads to the deficiency of Ti and excess of B. The B can form TiB2 on the TiAl3 and cause TiAl3 to break and be dissolved. The long, rod-shaped TiAl3 is more easily dissolved than the blockshaped TiAl3 [31]. Therefore, in the Ti-sponge route, the initial formation of the rodshaped TiAl3 does not stay easily in the melt, which is different from the characteristic of the halide salt route. However, there is still a rod-shaped TiAl3 in the microstructure of the Al-Ti-B Master Alloy prepared by the 30% Ti-sponge route with the reaction temperature of 800 °C for 900 S. The total amount of Ti is constant, and with less free Ti content, there is less of a Ti-TiAlx mechanism. When there is less excess B, some of the rod-shaped TiAl3 are retained and less Ti is consumed in the melt. The nucleation mechanism of the rod-shaped TiAl 3 (Figure 6a 2 ) is different from the Ti-TiAl x mechanism. The atomic ratio of Ti to KBF 4 is 1:1 in reaction (4). Actually, the ratio of Ti to KBF 4 is about 1.13:1 in this experiment so that KBF 4 can react completely. The rod-shaped TiAl 3 comes from reaction (4) and is easy to dissolve versus blocky TiAl 3 ; therefore, it almost disappeared after 300 S of reaction (4), as shown in Figure 6b. For the Ti-sponge route, Ti is involved in the transition of Ti-TiAl x to produce TiAl 3 , which leads to the deficiency of Ti and excess of B. The B can form TiB 2 on the TiAl 3 and cause TiAl 3 to break and be dissolved. The long, rod-shaped TiAl 3 is more easily dissolved than the block-shaped TiAl 3 [31]. Therefore, in the Ti-sponge route, the initial formation of the rodshaped TiAl 3 does not stay easily in the melt, which is different from the characteristic of the halide salt route. However, there is still a rod-shaped TiAl 3 in the microstructure of the Al-Ti-B Master Alloy prepared by the 30% Ti-sponge route with the reaction temperature of 800 • C for 900 S. The total amount of Ti is constant, and with less free Ti content, there is less of a Ti-TiAl x mechanism. When there is less excess B, some of the rod-shaped TiAl 3 are retained and less Ti is consumed in the melt. Section 3.1 shows that the "rich Ti, B area" is the birthplace of the nucleation and growth of initial TiAl 3 and TiB 2 for the halide salt route. Similarly, the "rich Ti, B area" are also found in the partial Ti-sponge route as shown in Figure 10b. The "rich Ti, B area" gradually decreased with increasing Ti powder content; only a small amount of filamentous and small strip regions were formed in the Ti-sponge route. In Ti-sponge route, a lot of Ti powder is directly involved in the formation of TiAl 3 by the Ti-TiAl x mechanism. During the nucleation and growth of TiAl 3 , the blocky Ti powder seriously hindered the diffusion of Ti into the Al melt, thus resulting in insufficient Ti in the melt. The insufficient diffusion of Ti also severely reduced the amount of Ti involved in the reaction (4), thus reducing the "rich Ti, B area" in the melt. Section 3.1 shows that the "rich Ti, B area" is the birthplace of the nucleation and growth of initial TiAl3 and TiB2 for the halide salt route. Similarly, the "rich Ti, B area" are also found in the partial Ti-sponge route as shown in Figure 10b. The "rich Ti, B area" gradually decreased with increasing Ti powder content; only a small amount of filamentous and small strip regions were formed in the Ti-sponge route. In Ti-sponge route, a lot of Ti powder is directly involved in the formation of TiAl3 by the Ti-TiAlx mechanism. During the nucleation and growth of TiAl3, the blocky Ti powder seriously hindered the diffusion of Ti into the Al melt, thus resulting in insufficient Ti in the melt. The insufficient diffusion of Ti also severely reduced the amount of Ti involved in the reaction (4), thus reducing the "rich Ti, B area" in the melt. The Ti addition routes have greatly influenced the morphology and size of TiAl3. The size of TiAl3 formed based on the Ti-TiAlx mechanism in the Ti-sponge route is coarse. Ti and Al diffused mutually in the TiAlx around the Ti particles, and the outer TiAl3 is mainly separated into small blocks. The "rich Ti, B area" in the Ti-sponge route under the influence of the Ti-TiAlx mechanism is a small amount that cannot meet the TiB2 growth well. Due to the Ti-TiAlx mechanism, the diffusion efficiency of Ti in Ti-sponge route is much lower than halide salt route. TiAl3 formed based on the chemical reaction (4) is long and rod-shaped. The long, rod-shaped TiAl3 cannot exist stably in the melt due to the excess B and lack of Ti in the pre-reaction melt. Therefore, it does not appear in the final microstructure.

Evolution Mechanism of TiB2 with Irregular Polygon
In addition to reaction (1), the following reaction may produce TiB2 when the Al-Ti-B Master Alloy is prepared by the halide salt route [32]: AlB 2 (s) + Ti → TiB 2 (s) + Al(l) 2B + Ti → TiB 2 (s) (8) Here, TiB2 is nucleated by the fluorination reaction and grown in the rich Ti and B area based on the above two reactions. Figure 11 shows the microstructure of the TiB2 when the mixed fluoride salt is added into the aluminum melt at 800 °C for 10 S-600 S using the halide salt route.
The TiB2 was an irregular small particle at the initial stage of the reaction (10 S and 15 S). Regular hexagonal TiB2 particles appeared with longer reaction times to 20 S and 30 S, and the size of TiB2 was about 0.5 μm. At 60 S, the size of TiB2 particle continued to increase. At 5 min, some TiB2 collided together and cracks appeared along the junction of particles. At a reaction time of 10 min, the size of TiB2 particles reached 1.5 μm and there were a lot of lug boss' around the TiB2 particles. According to the reaction (7), the filamentous AlB2 (corresponding to (Tix, Al1−x)By in Figure 1a) was more likely to be destroyed The Ti addition routes have greatly influenced the morphology and size of TiAl 3 . The size of TiAl 3 formed based on the Ti-TiAl x mechanism in the Ti-sponge route is coarse. Ti and Al diffused mutually in the TiAl x around the Ti particles, and the outer TiAl 3 is mainly separated into small blocks. The "rich Ti, B area" in the Ti-sponge route under the influence of the Ti-TiAl x mechanism is a small amount that cannot meet the TiB 2 growth well. Due to the Ti-TiAl x mechanism, the diffusion efficiency of Ti in Ti-sponge route is much lower than halide salt route. TiAl 3 formed based on the chemical reaction (4) is long and rod-shaped. The long, rod-shaped TiAl 3 cannot exist stably in the melt due to the excess B and lack of Ti in the pre-reaction melt. Therefore, it does not appear in the final microstructure.

Evolution Mechanism of TiB 2 with Irregular Polygon
In addition to reaction (1), the following reaction may produce TiB 2 when the Al-Ti-B Master Alloy is prepared by the halide salt route [32]: 2B + Ti → TiB 2 (s) (8) Here, TiB 2 is nucleated by the fluorination reaction and grown in the rich Ti and B area based on the above two reactions. Figure 11 shows the microstructure of the TiB 2 when the mixed fluoride salt is added into the aluminum melt at 800 • C for 10 S-600 S using the halide salt route.
The TiB 2 was an irregular small particle at the initial stage of the reaction (10 S and 15 S). Regular hexagonal TiB 2 particles appeared with longer reaction times to 20 S and 30 S, and the size of TiB 2 was about 0.5 µm. At 60 S, the size of TiB 2 particle continued to increase. At 5 min, some TiB 2 collided together and cracks appeared along the junction of particles. At a reaction time of 10 min, the size of TiB 2 particles reached 1.5 µm and there were a lot of lug boss' around the TiB 2 particles. According to the reaction (7), the filamentous AlB 2 (corresponding to (Ti x , Al 1−x )B y in Figure 1a) was more likely to be destroyed when it was transformed into TiB 2 [33]. Therefore, TiB 2 was more easily crushed with increasing reaction time.
tal growth model of TiB2 [34]. TiB2 belongs to hexagonal system (the lattice parameters a = 0.3030 nm, c = 0.3232 nm, c/a = 1.0818). The growth direction extends longitudinally in the direction of <0001> or extends laterally in the direction of <11 _ 00>. The growth rate is the slowest because the {0001} surface has the highest atomic density and the lowest surface energy. Thus, the longitudinal growth rate of TiB2 is lower than the lateral expansion speed. The lug boss in <0001> direction and the projecting lug in <11 _ 00> direction were also observed, as shown in Figure 12b,d,e). Figure 11. Morphology of TiB2 for the halide salt route at 800 °C for different reaction times. Figure 11. Morphology of TiB 2 for the halide salt route at 800 • C for different reaction times.
The crushing of TiB 2 is related to the crystal structure. Figure 12a,c represent the crystal growth model of TiB 2 [34]. TiB 2 belongs to hexagonal system (the lattice parameters a = 0.3030 nm, c = 0.3232 nm, c/a = 1.0818). The growth direction extends longitudinally in the direction of <0001> or extends laterally in the direction of <1 _ 100>. The growth rate is the slowest because the {0001} surface has the highest atomic density and the lowest surface energy. Thus, the longitudinal growth rate of TiB 2 is lower than the lateral expansion speed. The lug boss in <0001> direction and the projecting lug in <1 _ 100> direction were also observed, as shown in Figure 12b,d,e. when it was transformed into TiB2 [33]. Therefore, TiB2 was more easily crushed with increasing reaction time.
The crushing of TiB2 is related to the crystal structure. Figure 12a,c represent the crystal growth model of TiB2 [34]. TiB2 belongs to hexagonal system (the lattice parameters a = 0.3030 nm, c = 0.3232 nm, c/a = 1.0818). The growth direction extends longitudinally in the direction of <0001> or extends laterally in the direction of <11 _ 00>. The growth rate is the slowest because the {0001} surface has the highest atomic density and the lowest surface energy. Thus, the longitudinal growth rate of TiB2 is lower than the lateral expansion speed. The lug boss in <0001> direction and the projecting lug in <11 _ 00> direction were also observed, as shown in Figure 12b,d,e).  In the nucleation of TiB 2 , the generation and expansion mechanism of lug boss plays a significant role. The contents of Ti and B in the "rich Ti, B area" are sufficient, and the TiB 2 (0001) surface is small and provides favorable conditions for the formation of lug boss. When the lug boss is generated in the [1] direction, the lug boss rapidly expands the entire (0001) plane, and the thickness and size of the TiB 2 quickly increase. The nucleation of TiB 2 is similar to the layered stacking pattern according to the above mechanism (Figure 12a,b). There may be parallel surface defects between the new layer formed by the lug boss and the original layer. The formation and expansion mechanism of the projecting lug play a significant role during the TiB 2 growth. The Ti and B in the "rich Ti, B area" were partially consumed at TiB 2 growth stage. The growth of the {0001} plane requires larger surface energy, which makes the formation and expansion of the lug boss difficult. Therefore, the thickness of the TiB 2 does not change. The growth of TiB 2 is the lug generation and expansion in the direction <1 _ 100>. Figure 12c-e show that many lugs in the {1100} plane are prone to producing crystal defects during simultaneous growth. When the two lugs grow in parallel, there may be a planar defect parallel to the {1100} plane between the upper and lower lugs. When the two lugs grow side by side, between the left and right lugs, it is possible to form a planar defect perpendicular to the {1100} plane. The {1100} plane is not flat when the lug stops the growth eventually due to the lack of Ti and B.
The crushing of the TiB 2 is also related to the growth environment. Figure 13 shows the microstructure of the "rich Ti, B area" for the halide salt route at 800 • C for 60 S. The TiB 2 was rich in "rich Ti, B area" and many TiB 2 particles were clustered together during the growth process. This aggregation phenomenon can also crush TiB 2 . It is difficult to ensure uniform growth in all directions; thus, TiB 2 has an irregular morphology. growth model; (b) earlier actual growth of TiB2; (c) later TiB2 crystal growth model; (d,e) later actual growth of TiB2.
In the nucleation of TiB2, the generation and expansion mechanism of lug boss plays a significant role. The contents of Ti and B in the "rich Ti, B area" are sufficient, and the TiB2 (0001) surface is small and provides favorable conditions for the formation of lug boss. When the lug boss is generated in the [0001] direction, the lug boss rapidly expands the entire (0001) plane, and the thickness and size of the TiB2 quickly increase. The nucleation of TiB2 is similar to the layered stacking pattern according to the above mechanism (Figure 12a,b). There may be parallel surface defects between the new layer formed by the lug boss and the original layer. The formation and expansion mechanism of the projecting lug play a significant role during the TiB2 growth. The Ti and B in the "rich Ti, B area" were partially consumed at TiB2 growth stage. The growth of the {0001} plane requires larger surface energy, which makes the formation and expansion of the lug boss difficult. Therefore, the thickness of the TiB2 does not change. The growth of TiB2 is the lug generation and expansion in the direction <11 _ 00>. Figure 12c-e show that many lugs in the {1100} plane are prone to producing crystal defects during simultaneous growth. When the two lugs grow in parallel, there may be a planar defect parallel to the {1100} plane between the upper and lower lugs. When the two lugs grow side by side, between the left and right lugs, it is possible to form a planar defect perpendicular to the {1100} plane. The {1100} plane is not flat when the lug stops the growth eventually due to the lack of Ti and B.
The crushing of the TiB2 is also related to the growth environment. Figure 13 shows the microstructure of the "rich Ti, B area" for the halide salt route at 800 °C for 60 S. The TiB2 was rich in "rich Ti, B area" and many TiB2 particles were clustered together during the growth process. This aggregation phenomenon can also crush TiB2. It is difficult to ensure uniform growth in all directions; thus, TiB2 has an irregular morphology.

Conclusions
(1) The fluorine salt reaction occurred very fast when Al-Ti-B was prepared by the halide salt route (<10 S). A streamlined "rich Ti, B area" exists in the aluminum melt in the initial stage of the reaction ((800 °C, <30 S). This is a complex compound of (Tix, Al1−x) By. The "rich Ti, B area" is essential for the nucleation and growth of TiAl3 and TiB2.
(2) The Ti addition route greatly influences the morphology of TiAl3. The formation of TiAl3 using the Ti-sponge route is based on the Ti-TiAlx mechanism. The nucleation of TiAl3 mainly depends on the mutual diffusion of Al and Ti powder, and TiAlx formed around the Ti particles. The TiAl3 formed based on the Ti-TiAlx mechanism Figure 13. SEM of the "rich Ti, B area" and TiB 2 in the Al-Ti-B Master Alloy prepared by the halide salt route at 800 • C for 60 S.

Conclusions
(1) The fluorine salt reaction occurred very fast when Al-Ti-B was prepared by the halide salt route (<10 S). A streamlined "rich Ti, B area" exists in the aluminum melt in the initial stage of the reaction ((800 • C, <30 S). This is a complex compound of (Ti x , Al 1−x ) B y . The "rich Ti, B area" is essential for the nucleation and growth of TiAl 3 and TiB 2 . (2) The Ti addition route greatly influences the morphology of TiAl 3 . The formation of TiAl 3 using the Ti-sponge route is based on the Ti-TiAl x mechanism. The nucleation of TiAl 3 mainly depends on the mutual diffusion of Al and Ti powder, and TiAl x formed around the Ti particles. The TiAl 3 formed based on the Ti-TiAl x mechanism is mainly block-shaped and the average size is 9.83 µm. In the halide salt route, TiAl 3 formation is based on the reaction of Ti powder and KBF 4 to form a rod-shaped TiAl 3 . The long, rod-shaped TiAl 3 disappeared due to excess B and a lack of Ti in the aluminum melt. Finally, small blocks of TiAl 3 formed with an average size of 4.7 µm.
(3) TiB 2 particles showed different morphologies at different reaction times when the Master Alloy was prepared by the halide salt route. The TiB 2 was an irregular and small particle at the initial stage of the reaction (10 S and 15 S). Regular hexagonal TiB 2 particles appeared as the reaction time increased to 20 S and 30 S. The size of TiB 2 was about 0.5 µm. The size of TiB 2 particle continued to increase at 60 S. Some TiB 2 collided together at 5 min. Both the crystal defects and the crowded growth environment caused by the "rich Ti, B area" are the fundamental reasons for the fragility and irregular shape of the TiB 2 .