Effect of Al-TiO2-C-XY2O3 refiner on grain size and mechanical properties of Al-5Cu alloy

A series of new Al-TiO2-C-Y2O3 aluminum alloy grain refiners with different Y2O3 content was prepared by exothermic dispersion method using Al, TiO2, C, and Y2O3 powders as raw materials. Scanning electron microscopy (SEM), x-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS) were used to investigate the effect of Y2O3 content on the structure of the Al-TiO2-C refining agent as well as the effect of the Al-TiO2-C-Y2O3 refining agent on the grain size and mechanical properties of an Al-5Cu alloy. The results showed that the Al-TiO2-C-Y2O3 refiner was composed of Al2O3, TiC, Al3Ti, Al5Y3O12, and Al20Ti2Y phases. The refiner with 4% Y2O3 content exhibited the best Al-5Cu alloy refining effect, achieving a grain size of about 210.5 μm. This was about 42% of the grain size of the original Al-5Cu alloy. Moreover, this refined Al-5Cu alloy exhibited the best mechanical properties, with a tensile strength and elongation of 173.13 MPa and 9.19% respectively. This was a 20.9% and 83.8% improvement compared with the original Al-5Cu alloy. However, with a further increase in Y2O3 content, an Al5Y3O12 phase was preferentially formed. This led to a decline in Al20Ti2Y phase content and a correspondingly weaker refinement effect.


Introduction
Due to rapid economic development around the world, aluminum alloys are widely used in many fields such as construction, transportation, machinery manufacturing, and the aerospace industry. Many applications have increasingly demanding material requirements regarding the comprehensive properties of aluminum alloys [1][2][3]. The most effective and economical strategy for improving the comprehensive properties of aluminum alloys is to add a grain refiner to the aluminum alloy melt [4]. At present, Al-Ti-B refiners are usually used in industry to refine aluminum alloys. However, these Al-Ti-B refiners lead to a 'poisoning' phenomenon when they are used to refine aluminum alloys containing Cr, Zn, and some other elements [5]. Wang et al [6] reported that an Al-Ti-B refiner had no obvious refining effect on an aluminum alloy with Si content of 3 wt% or higher. This was mainly because the enrichment of Si at the TiB 2 /Al-Si interface caused the Al 3 Ti layer on the TiB 2 surface to dissolve into the Al-Si melt. Thus, the particle nucleation ability of TiB 2 was lost, resulting in a reduction in the number of cores available for heterogeneous nucleation and a corresponding increase in grain size. In recent years, scholars have developed Al-Ti-C refiners to effectively avoid the problems associated with Al-Ti-B refiners. Kumar et al [7] prepared Al-Ti-0.8C and Al-5Ti-1.2C master alloys by reacting K 2 TiF 6 salt and graphite powder with molten Al at 1200°C. They found that the refining ability of these alloys was better than that of the traditional Al-5Ti-B master alloy. At the same time, the growth of Al 3 Ti was completely inhibited in the Al-5Ti-1.2C master alloy, demonstrating its enhanced refining ability. However, during the preparation of Al-Ti-C refiners, TiC easily aggregates, the wettability between C and the Al melt is poor, and the reaction process is difficult. These drawbacks limit the development of Al-Ti-C refiners [8][9][10]. Therefore, Al-TiO 2 -C refiners have been extensively studied as an alternative refiner system. In addition to Al 3 Ti and TiC secondary

Experimental procedure
Al powder (99.9%), TiO 2 powder (99.9%), C powder (99.9%), and rare earth oxide Y 2 O 3 powder (99.9%) were evenly mixed in a certain proportion. Y 2 O 3 mass fractions of 0, 2%, 4%, 6%, and 8% were used. The TiO 2 content was 12% and the Ti:C ratio was 10:1. The mixed powder was ground in a planetary ball mill for about 1 h. After grinding, the ground powder was loaded in a universal testing machine, then pressed into a cylindrical preform. A pressure rise rate of 800 N s −1 was used, and the preform was pressed at 90 kN for 180 s. After pressing, the precast block was placed into a corundum crucible, which was filled with Al 2 O 3 powder around the sample. The corundum crucible was placed in a drying oven for drying at 150°C, then in a high-temperature sintering furnace for sintering. The heating rate for sintering was about 3°C min −1 , the target sintering temperature was 1300°C, and the dwell time was 2 h. After cooling, Al-TiO 2 -C refiners with different Y 2 O 3 content were obtained. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive x-ray spectroscopy (EDS) were used to analyze the phase and microstructure of the refiners.
Refining experiments were carried out in a crucible resistance furnace, in which the amount of refiner was 0.3 wt% (mass fraction). First, the Al-5Cu alloy and Al-TiO 2 -C-Y 2 O 3 refiner were weighed. The crucible was preheated in a drying oven at 200°C for 1 h before casting. Next, the weighed alloy was added to the preheated crucible in batches, and the loaded crucible was placed in a crucible resistance furnace at 750°C for 45 min to melt the alloy. After melting, the refiner was added to the molten alloy, which was stirred with a graphite rod for 15s. The crucible was then placed back in the crucible resistance furnace for 15 min to allow the refiner to fully react. Finally, the melt was cast into a preheated mold at 200°C. Several 10 mm×10 mm×10 mm samples were cut from the same position in the center of the mold, polished step by step with sandpaper, and then mechanically polished. The polished samples were then anodized in HBF 4 solution. The HBF 4 solution concentration was 2%, the coating voltage was 25 V, the coating current was 0.1A, and the coating time was 1-2 min. Grain sizes were observed by a polarizing microscope to determine the average grain size.
Several 10 mm×10 mm×10 mm samples were cut from the same position in the center of the mold, mechanically polished, and corroded with Keller's reagent (1.5% HCl 2.5% HNO 1% HF . The phase composition was analyzed by X-ray diffractometer (XRD, Cu K-alpha, D/Max-2500/ PC), the morphology was observed by scanning electron microscope (SEM, QUANTA FEG 650), and the distribution of elements was observed by energy-dispersive x-ray spectroscopy (EDS).
The formula for calculating the standard Gibbs free energy change ∆G of a reaction is: In the formula, are the sum of the standard Gibbs free energy changes of the products and reactants of the reaction.
Tensile experiments were carried out on a computer-controlled electronic tensile testing machine. Plate samples fabricated according to the national standard GB6397-86 were used in the tensile tests. The sample dimensions are shown in figure 1.

Results and discussion
3.1. Effect of Y 2 O 3 content on microstructure of Al-TiO 2 -C-Y 2 O 3 refiner Figure 2 shows the XRD patterns of the Al-TiO 2 -C-XY 2 O 3 refiners with varying Y 2 O 3 content. The secondary phases of the Y 2 O 3 -free Al-TiO 2 -C refiner are mainly Al 2 O 3 , TiC, and Al 3 Ti. When the rare earth oxide Y 2 O 3 is added, two new rare earth phases are generated: Al 5 Y 3 O 12 and Al 20 Ti 2 Y. The intensity of the Al 5 Y 3 O 12 diffraction peak at 18.05°gradually increases with increasing rare earth oxide content. In contrast, the Al 2 O 3 peak intensity gradually decreases with increasing Y 2 O 3 content. It is speculated that Y 2 O 3 and Al 2 O 3 form the new rare earth oxide phase Al 5 Y 3 O 12 after the addition of Y 2 O 3 to the Al-TiO 2 -C refiner. Unfortunately, the characteristic peaks of the Al 20 Ti 2 Y rare earth phase are superimposed with the characteristic peaks of other phases. Therefore, it is difficult to determine the specific trend of Al 20 Ti 2 Y rare earth phase content with increasing Y 2 O 3 addition. Figure 3 shows SEM images displaying the microstructures of the Al-TiO 2 -C-XY 2 O 3 refiners. Two new phases are visible after the addition of Y 2 O 3 : a bright white bulk phase and a bright white granular phase. EDS analysis was performed to confirm the composition of these phases. Figure 4 shows the EDS analysis area of the Al-TiO 2 -C-4%Y 2 O 3 refiner and the EDS spectra of the corresponding points, it can be determined that the white plate phase (point A) is the Al 3 Ti phase, and the bright white bulk phase (point B) is composed of three elements, Al, Ti and Y. In combination with the XRD results, the phase is identified as Al 20 Ti 2 Y phase. The dark black granular phase dispersed in the matrix (point C) is Al 2 O 3 particles, and the white granular phase (point D) is composed of three elements, Al, Y and O, and the element molar ratio is close to 5:3:12. In combination with the XRD results, the phase is determined to be Al 5 Y 3 O 12 particles.
It can be seen from figure 3(a) that Al-TiO 2 -C refiner is composed of Al matrix, Al 2 O 3 , TiC and Al 3 Ti phase.   The microscopic morphology of the refiners shown in figure 3 demonstrates that the white bulk Al 20 Ti 2 Y phase first increases and then decreases with increasing Y 2 O 3 content. The highest amount of Al 20 Ti 2 Y is achieved with 4% Y 2 O 3 , and less Al 20 Ti 2 Y is generated with the addition of 6%-8% Y 2 O 3 . To determine the relative area ratio of the Al 20 Ti 2 Y phase in each sample, three SEM images of each sample obtained at the same magnification level were analyzed by simulation software. The results of this analysis are displayed in figure 5. As shown, the relative area ratio of the Al 20 Ti 2 Y phase is stable at about 6% when 2-4 Y 2 O 3 is added. However, with the further addition of Y 2 O 3 , the surface proportion of Al 20 Ti 2 Y declines. The Al-TiO 2 -C refiner with 8% Y 2 O 3 has the lowest Al 20 Ti 2 Y relative surface area (only 3.86%). At the same time, figure 3 clearly demonstrates that the bright white granular Al 5 Y 3 O 12 phase gradually increases with the increasing addition of Y 2 O 3 .
EDS maps of the Al-TiO 2 -C refiner with 4% Y 2 O 3 are displayed in figure 6. The gray granular Al 2 O 3 phase and the bright white Al 5 Y 3 O 12 phase are present on the Al matrix or grain boundaries. The diameter of the dispersed phases is between 2 μm and 5 μm. This is because the Al 2 O 3 particles and Al 5 Y 3 O 12 have large particle sizes and poor wettability with Al. These phases are distributed along the grain boundaries during the solidification process. This EDS mapping analysis also demonstrates that another rare earth phase, Al 20 Ti 2 Y, is distributed around Al 3 Ti in a flake shape.

Thermodynamic and kinetic analysis of Al-TiO 2 -C-Y 2 O 3 refiners
As the temperature of the reaction system rises during the melting process, the thermite reaction between Al and TiO 2 occurs first. As shown in Reaction (3-1), Al and TiO 2 react to form active [Ti] atoms and Al 2 O 3 , and this reaction releases a significant amount of heat.   (3-7), and the Al 20 Ti 2 Y phase is distributed around Al 3 Ti in a flake shape.
When Y 2 O 3 is added to the Al-TiO 2 -C refiner, the rare earth phase Al 5 Y 3 O 12 is preferentially formed and a large amount of Y 2 O 3 particles are consumed during the reaction. This inhibits Reaction (3)(4)(5)(6)(7), resulting in lower Al 20 Ti 2 Y generation. Figure 7 shows polarized images of Al-5Cu alloys obtained after adding Al-TiO 2 -C-Y 2 O 3 refiners with varying Y 2 O 3 content. The original coarse grains of Al-5Cu, as shown in figure 7(a), gradually become finer with the addition of refiners with increasing Y 2 O 3 content. The best refining effect is achieved with 4% Y 2 O 3 , as shown in figure 7(c). If the content of Y 2 O 3 is increased above 4%, as shown in figure 7(d)-(e), the grain size becomes coarser again. Figure 7(f) shows the average grain size of these alloys. The grain size of the unrefined Al-5Cu alloy is about 500 μm. The addition of the Al-TiO 2 -C refiner containing 4% Y 2 O 3 leads to an alloy with a grain size of about 210.5 μm. This is only 42% of the grain size of the original Al-5Cu alloy. However, this refining effect is not improved by further increasing the Y 2 O 3 content.

Refining effect of Al-TiO 2 -C-Y 2 O 3 refiner on Al-5Cu alloy
In general, the grain size during solidification dependent mostly on the heterogeneous nucleation of the solid phase and the subsequent growth of the newly formed nuclei [17]. The secondary phases of the Al-TiO 2 -C-XY 2 O 3 alloys prepared in this work are mainly TiC, Al 3 Ti, Al 2 O 3 , Al 20 Ti 2 Y, and Al 5 Y 3 O 12 . The TiC crystals have a face-centered cubic structure, which is the same as the crystal structure of aluminum. Moreover, the lattice constants of these two phases are relatively similar, and they both exhibit good stability at high temperatures and can be used as an excellent substrate for heterogeneous nucleation. However, related studies [18] have found that TiC particles are too small to be used as heterogeneous nucleation cores for effective grain refinement. After the Al-TiO 2 -C-XY 2 O 3 refiner enters the Al-5Cu alloy melt, the unstable Al 20 Ti 2 Y and Al 3 Ti phases dissolve, releasing a large amount of free Ti and Y atoms. The Ti atoms accumulate on the surface of TiC, forming a Ti-rich layer. These particles become heterogeneous nucleation cores that effectively refine the crystal grains and improve the nucleation ability of the alloy. The Ti-rich layer can prevent TiC from being poisoned by the formation of Al 4 C 3 or may form constitutional supercooling, hampering grain growth and playing a wonderful refinement in the solidification processing [15,19]. As active rare earth particles, Y atoms improve the wettability of TiC and Al, and improve the nucleation ability of α-Al [20], enhancing grain refinement.
Al 3 Ti can be directly used as heterogeneous nucleation cores for refining grains. The morphology and quantity of Al 3 Ti particles are key factors that influence the effectiveness of the Al-TiO 2 -C-XY 2 O 3 refiners. When the Y 2 O 3 content is low, Al 3 Ti appears as coarse laths and the nucleated grains are abnormally coarse; With increasing Y 2 O 3 content, the lath-shaped Al 3 Ti gradually transforms into a block shape. The block-shaped Al 3 Ti easily dissolves at the reaction system temperature and releases a large amount of free Ti atoms, leading to the formation of a Ti-rich layer that wraps the TiC particles. At the same time, a small amount of undissolved Al 3 Ti will act as nucleating agent to form α-Al. This promotes the non-uniform nucleation of α-Al and refines the alloy grains [21]. Al 2 O 3 has a large particle size and poor wettability with Al. The Al 2 O 3 particles are distributed along the grain boundary during the solidification process, hindering the growth of grains and enhancing grain refinement [22].
The dissolution of Al 20 Ti 2 Y at high temperature not only releases a large number of free Ti atoms, but also generates a small number of Y atoms. Related studies [23,24] have shown that Y atoms will converge on the solid-liquid interface in the solidification process of the alloy ,which reduces the tension of the solid-liquid interface, affects the solute redistribution in the solid-liquid front, causes the concentration gradient of the liquid phase in the solid-liquid front, forms the component undercooling, improves the nucleation rate, and refines the grain size.
The Al-TiO 2 -C-XY 2 O 3 refiner preferentially generates the Al 5 Y 3 O 12 rare earth phase during the preparation process. This Al 5 Y 3 O 12 phase has a larger particle size. However, this impurity phase has no obvious effect on Al-Cu alloy refinement. The formation of the Al 5 Y 3 O 12 phase consumes a large amount of rare earth Y, resulting in a significant reduction in the content of the Al 20 Ti 2 Y rare earth phase, which has a better refinement effect. When 8% Y 2 O 3 is added, the relative area ratio of the Al 20 Ti 2 Y phase in the refining agent is only 3.8%. This is also the main reason why the excessive addition of Y 2 O 3 inhibits the refining effect.

Conclusion
(1) The secondary phases of the Al-TiO 2 -C refining agent are mainly Al 2 O 3 , TiC, and Al 3 Ti. After adding Y 2 O 3 , an Al 20 Ti 2 Y rare earth phase and an Al 5 Y 3 O 12 rare earth phase are formed. The Al 20 Ti 2 Y rare earth phase has an important role in grain refinement. However, the Al 5 Y 3 O 12 phase, which does not improve grain refinement, is preferentially formed. The excessive addition of Y 2 O 3 reduces the Al 20 Ti 2 Y phase content of the alloy, leading to an inhibited refining effect.
(2) The Al-TiO 2 -C refining agent shows the best aluminum-copper alloy refining effect with 4% Y 2 O 3 . The addition of the Al-TiO 2 -C-4%Y 2 O 3 refiner to an Al-5Cu alloy leads to grain refinement from about 500 μm to about 210.5 μm. This refining effect represents a 42% reduction in grain size compared to the original Al-5Cu alloy. (3) The mechanical properties of the tested aluminum-copper alloys are significantly improved by the addition of the Al-TiO 2 -C-XY 2 O 3 refiners. The addition of the Al-TiO 2 -C-4%Y 2 O 3 to the Al-5Cu alloy leads to tensile strength and elongation values of 173.13 MPa and 9.19%, respectively. This represents a 20.9% and 83.8% enhancement compared with the tensile strength and elongation of the original Al-5Cu alloy.