Effect of Al-Sr-Y intermediate alloy on microstructure and mechanical properties of A356 alloy

In this experiment, an intermediate alloy (Al-3Sr-8Y) that can refine and modify A356 alloy simultaneously was developed, and the synergistic effect of Sr and Y on the microstructure and mechanical properties of A356 alloy was investigated. The results show that when the content of Al-3Sr-8Y intermediate alloy reaches 0.3 wt%, the morphology of α-Al dendrites is significantly refined and the secondary dendrite arm spacing (SDAS) was significantly reduced. Moreover, the morphology of eutectic Si transforms from acicular to small fibrous, and the average area and aspect ratio of eutectic Si decrease to 0.81 μm2, and 2.01. This change is caused by the twin plane re-entrant edge (TPRE) poisoning mechanism, where the addition of Al-3Sr-8Y can poison the intrinsic growth position of the Si phase, reduce the growth rate of the Si phase, promote isotropic growth, and achieve a highly branched morphology of the Si phase to form a fibrous structure. Due to the excellent synergistic effect of Sr and Y, the tensile strength and elongation of the alloy reached the maximum values of 303.5 MPa and 9.5% when 0.3 wt% Al-3Sr-8Y was added after heat treatment, which is an increase of 18.2% and 86.3% compared with the untreated A356 alloy, respectively.


Introduction
Aluminum alloys, known for superior castability, lightweight, low thermal expansion coefficient, and excellent corrosion resistance, are widely used in the automotive industry [1][2][3].Among them, A356 alloy is a commonly used material for wheel manufacturing.However, its as-cast state exhibits limitations in mechanical properties due to the presence of coarse primary α-Al and plate-like eutectic Si [4,5].Various refinement and modification treatments have been proposed to improve microstructure and enhance properties of A356 alloy.
Numerous grain refiners have been developed for hypoeutectic Al-Si alloys, such as Al-B [6], Al-Ti-B [7], and Al-Ti-C [8].In terms of modifiers, the morphology of eutectic Si can be changed from coarse plate-like to finer fibrous structure with the aid of chemical modification.Reported modification elements can be classified into two categories: one group, including Na [9], Sr [10], and Eu [11], can modify eutectic Si from coarse plate-like into fine fibrous-like.Nowadays, the main metamorphic mechanisms widely accepted by researchers are the twin plane re-entrant edge (TPRE) poisoning mechanism and the impurity induced twinning (IIT) mechanism.The TPRE poisoning mechanism suggests that, after the addition of metamorphic agents, the metastable atoms selectively adsorb and enrich at the locations of the twinning grooves, which hinders the growth of eutectic Si in the mechanism of twinning grooves, reduces the growth rate of the Si phase, promotes the isotropic growth, and realizes the highly branching of the Si morphology [12].The IIT mechanism suggests that the attachment of metastable atoms at the Si phase growth steps causes a change in the stacking sequence of Si atoms, which promotes the generation of high-density twins in the crystal.The high number of constantly alternating twins promotes the formation of Si phase branching, and eventually the eutectic Si morphology undergoes a fibrous transition [13].The other group of elements can only refine eutectic Si, such as Y [14] and Yb [15].Sr is extensively used in the foundry industry because of its excellent and stable modification capacity; however, it can raise porosity and sharply decrease mechanical properties [16].Furthermore, the existence of a mutual poisoning effect between B and Sr [17], Ti and Si [18], which reduces the efficiency of grain refinement and modification.
Researchers find that rare earth elements are not only effective in degassing and removing slag, but also in refining grain [19,20].Therefore, it is important to investigate how modifier Sr and rare earth elements work synergistically.Tang et al [21] studied the effects of Sr and La on Al-Si-Cu-Fe alloys and discovered that these elements changed morphology of eutectic Si from coarse plate-like to finer fibrous-like.Moreover, they had an obvious refining effect on primary α-Al and needle-like β-phase Fe.Dong et al [22] investigated the effects of Sr and Yb on the refinement of eutectic Si in Al-7% Si alloys, revealing that 0.03 wt% Sr and 0.6 wt% Yb transformed the morphology of eutectic Si from a plate-like form to a fine fibrous structure.Wu et al [23] found that Sr and Ce modification could not only modify and refine Al-Si alloys but also enhance the modification capability of Sr According to literature reports, Y can decrease the size of α-Al dendrite grains [24], and change Si phase from acicular to short rod-shaped [25].It is important that the number and size of the blowholes shrank noticeably after adding Y into the Al-Si alloy [26].Dong et al [27] investigated the effects of a Sr-Y composite modifier on the microstructure of A356 alloy, and the results showed that the Si phase in Sr-Y compositemodified A356 alloys at the time of casting was in the form of grains or flakes, and no large flakes of eutectic Si were detected, and the roundness and homogeneity of the particles of Si crystals were significantly improved after the T6 heat treatment.
However, there are few studies on the synergistic effect of Sr and Y on the mechanical properties of A356 alloy.Therefore, we prepared the Al-3Sr-8Y intermediate alloy using alloy melting method by mixing Al-10Sr, Al-20Y, and pure Al.XRD observation revealed that Al-3Sr-8Y is mainly composed of Al 3 Y, Al 4 Sr, and Al phases.Subsequently, Al-3Sr-8Y with mass fractions of 0.1, 0.3, 0.5, and 0.7 was added to A356 alloy, and its effects on grain refinement, eutectic Si modification, and mechanical properties of A356 alloy were studied.Finally, the mechanisms of the synergistic effects of Sr and Y addition on A356 alloy were also discussed.

Experimental procedures 2.1. Melting process
For the preparation of Al-3Sr-8Y intermediate alloy, pure Al (with a purity of 99.99 wt%), Al-10Sr, and Al-20Y intermediate alloys were used.The specific preparation process was as follows: Firstly, pure Al ingot was melted at 735 °C and held for 15 min.Secondly, the melt was heated to 790 °C, and the Al-20Y intermediate alloy was added to the aluminum liquid.The melt was then stirred for 15 s and held for 25 min.When the temperature of the aluminum liquid dropped to 740 °C, Al-Sr intermediate alloy was added into the melt, then stirred for 15 s and held for 20 min, the slag was removed subsequently.Thirdly, the melt was kept for 5 min and then the slag was removed again.Finally, the metal liquid was poured into a permanent mold, the dimensions of the mold are shown in figure 1, and Al-Sr-Y intermediate alloy was obtained.It should be noted that high-purity nitrogen gas was applied to avoid oxidation of the raw materials during the whole procedure.
Commercial A356 alloy was selected as the base alloy (with the compositions shown in table 1).Melting processes were carried out in an electric resistance furnace.The specific preparation process was as follows: Firstly, the A356 alloy was melted at 735 °C and held for 10 min.After nitrogen refining for 8 min, the slag was removed.Subsequently, Al-Sr-Y intermediate alloy was added and held for 15 min, the melt was stirred for 15 s during the holding process.The melt was nitrogen refined again for 8 min and the slag was removed.When the metal liquid temperature dropped to 715 °C, remove the slag and pour the metal liquid into the permanent mold (pre-heated to 200 °C).The flowchart of the melting process is shown in figure 2. The heat treatment conditions used in this experiment were solid solution at 540 °C for 4 h, quenching with water at 40 °C, followed by artificial aging at 175 °C for 6 h.

Microstructure characterization
Specimens for microstructure observation were cut into dimensions of 10 mm × 10 mm × 6 mm by wire electrical discharge machining (DK7735) from the same position of different castings.Sequentially use #400, #600, #800 sandpaper to mechanically grind the surface of the specimens, and then sequentially use 0.5 μm, 0.15 μm diamond polishing paste to polish the surface of the specimens until the surface of the specimens forms a bright mirror.The specimens were mechanically ground with sandpaper, and polished until the surface was bright.Thereafter, the specimens were corroded with 0.5 vol% HF solution for 10-12s.The microstructures of the specimens were then observed using an optical microscope (Axio Lab.A1) and scanning electron microscope (Gemini SEM 300) equipped with energy-dispersive spectra (Inca X-Max 80).The phase compositions were determined using x-ray diffraction (X-Pert PRO MPD).The specimens were mechanically ground as thin foils into 80 μm thickness and then polished by ion milling for transmission electron microscopy measurement (Talos F200X).

Properties testing
The tensile specimens were designed according to the standard of GB/T228.1-2010, the dimensional parameters are shown in figure 3. Tensile properties of the specimens were obtained at room temperature by using a testing machine (AG-100KNXplus).The tests were controlled by a displacement speed of 1.2 mm min -1 .

Microstructure and phase composition of Al-3Sr-8Y intermediate alloy
The chemical composition of the Al-3Sr-8Y intermediate alloy was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-OES, AGILENT 730), and the chemical compositions are listed in table 2.
The XRD analysis results show diffraction peaks of Al 4 Sr and Al 3 Y phases in addition to the α-Al phase diffraction peaks in figure 4, which determines that the main components of Al-3Sr-8Y intermediate alloy are

Modification performance provided by the addition of different Al-3Sr-8Y contents on eutectic-Si
To investigate the effect of Al-3Sr-8Y content on eutectic Si in A356 alloy, a series of detailed detections and analyses are conducted.The evolution of eutectic Si morphology at different Al-3Sr-8Y contents is presented in figure 6, while the corresponding size changes are demonstrated in figure 7. Acicular eutectic Si can be observed in the untreated A356 alloy (figure 6(a)), with an average area of 16.14 μm 2 and an aspect ratio of 5.68.However, as the Al-3Sr-8Y content increases, the size of the needle-like eutectic Si gradually decreases.When the Al-3Sr-8Y content reaches 0.3 wt%, most of the eutectic Si transform   into short fibrous or fine particles that homogeneously disperse along the grain boundaries, as shown in figure 6(c).At this point, the average area and aspect ratio of eutectic Si are only 0.81 μm 2 and 2.01, respectively.These values are 95.0%and 64.6% lower than those of the untreated A356 alloy, indicating a significant improvement in modification.With further increase of Al-3Sr-8Y content to 0.7 wt%, the ability to modify the Si phase begins to decline (figure 6(e)).The average area and aspect ratio increase to 1.47 μm 2 and 2.24, respectively.In order to investigate the impact of Al-3Sr-8Y content on the stereoscopic morphology of eutectic Si, specimens were deeply etched in 20 vol% HF solution for 20 min to dissolve the aluminum matrix, and then the stereoscopic morphology of Si was captured by SEM (as shown in figure 8).
In the untreated alloy, the eutectic Si appears slab-like and is embedded inside the aluminum matrix, as shown in figure 8(a).Upon closer inspection (in the rectangular box), the slab-like Si phase is thicker in size and sharp at the edges.However, as the Al-3Sr-8Y content increases to 0.1 wt%, the slab-like eutectic Si starts to crack and transform into smaller slabs, as demonstrated in figure 8(b).When the content reaches 0.3 wt%, the eutectic Si transforms into a coral-like structure with thin, long branches and large distances between them (figure 8(c)).However, when the Al-3Sr-8Y content increases to 0.7 wt%, the branches of eutectic Si become thicker, shorter, and with significant adhesions between the branches.This eventually promotes the formation of coarse massive eutectic Si, as highlighted by the red circle in figure 8(e).
For the Al-3Sr-8Y-modified alloy, two possible co-zonal twin systems can be seen in the eutectic Si phase (figure 9(a)), which form an angle of 70.5°to each other.Many nanoscaled particles can be also observed within the modified eutectic Si.EDS analysis indicates that the nanoscaled spherical particles can be identified to be the Al (Si, Sr) phase (figure 9(b)).It was determined that the nanoscaled particles were Sr-Al-Si co-segregations [28].This type of co-zonal twin system has been observed in many Sr-modified Al-Si alloys [28,29], and the formation of twins is caused by the addition of Sr [30].The TPRE poisoning mechanism suggests that Si crystals grow in the <211> direction preferentially when the outer surface is distributed with notches (angle 141°) and convex ridges (angle 219°) formed by the intersection of their densely arranged surfaces, and the low energy at the notches is more favorable for the attachment of atoms, if no metastable agent is added.In this experiment, after adding 0.3 wt% Al-3Sr-8Y to A356, the appearance of Sr-Al-Si co-segregations in the Si crystals can poison the intrinsic growth position of the Si phase, reduce the growth rate of the Si phase, promote the isotropic growth, and achieve the highly branched morphology of the Si phase, thus forming a fiber structure.
To investigate the effect of Sr and Y elements on eutectic Si modification, an area scan is conducted on A356 alloy containing 0.3 wt% Al-3Sr-8Y.The distribution of relevant elements is shown in figure 10, wherein Si is primarily distributed between α-Al dendrites, and the distribution path of Sr element is similar to that of Si.Therefore, it can be deduced that Sr mainly biases towards the Si phase, while Y is evenly distributed throughout the alloy, thus indicating that the effects of these two modifying elements are not identical [31].
During solidification, Sr atoms accumulate at the interface between the solid and liquid phases, adsorbing onto the Si lattice and poisoning {111} Si closely packed planes.This enables twin to be induced, leading to the growth of eutectic Si starting from another {111} Si plane, thereby increasing the flexibility of Si branch growth.Bending and splitting can thus occur in eutectic Si, ultimately resulting in the formation of coral-like eutectic Si.Y, on the other hand, promotes constitutional undercooling since its atoms are less soluble in Si and tend to concentrate at the interface between solid and liquid phases during solidification.Additionally, Y can decrease the diffusion rate of Sr atoms from Si to Al and enhance the modification effect of Sr by enabling greater retention of Sr atoms in the Si lattice [23].
In summary, Sr predominantly induces the generation of twin and acts as a modifier on the Si, whereas Y plays the role of triggering twin and promotion of constitutional undercooling.The modification efficiency of Sr can furthermore be enhanced through the addition of Y.Under the combination effect of Sr and Y, the growth of Si phase is tended to shift toward favorable directions and branching structures.

Grain refinement by adding different contents of Al-3Sr-8Y
Figures 11 and 12 respectively display the grain refinement and SDAS evolution in A356 alloy with varying amounts of Al-3Sr-8Y.In untreated A356 alloy, the α-Al phase typically exhibits coarse dendrites that are unevenly distributed, as observed in figure 11(a).Correspondingly, the SDAS reaches approximately 36.5 μm.However, upon an increase in the percentage of Al-3Sr-8Y from 0 wt% to 0.3 wt%, the SDAS decreases from 36.5 μm to 20.6 μm, with evident refinement in the morphology of α-Al dendrites (figures 11(b)-(c)).With a further rise in Al-3Sr-8Y content, the SDAS value starts to gradually increase, and the refinement effect on the   Previous research [32] has revealed that Sr has low ability to induce grain refinement.Hence, the role of Y should be taken into account.When Y is added to aluminum melt, a large number of Al 3 Y second-phase particles form, which can play a role in heterogeneous nucleation process to improve the nucleation rate and refine grains [33].
It is well known that the atomic radius of Y and Al are 0.182 and 0.147 nm, respectively, and the difference between Y and Al atomic radius is above 15% [34].As a result, the Y atoms are difficult to enter the crystal lattice of the α-Al, instead, they tend to concentrate at the front of the solid-liquid interface, causing significant constitutional undercooling [35].This will restrict the growth of α-Al and refine its dendritic grains.SDAS is one of the critical parameters of dendrites, which reflects the effect of a refiner.According to the diffusion ripening model [32,36], the SDAS can predicted by equation (1):   where M is the constant related to the composition of the alloys and t f is the solidification time, which is mainly determined by the solid/liquid temperature gradient ( T D ) and the cooling rate (C), and t f can be expressed as equation (2) [37]: With increasing solute content, the constitutional undercooling zone will be extended, and the temperature gradient ( T D ) will be reduced [38].Sr, Si, and Y solutes will accumulate at the solid/liquid interface and mitigate the diffusion of the solutes during solidification.Therefore, the constitutional undercooling will increase and the temperature gradient will decrease.Consequently, the local solidification time will be shortened, leading to the decrease of SDAS.However, excessive Al-3Sr-8Y will reduce the refining effect of SDAS, which can be attributed to the fact that the surplus Y atoms will combine with other atoms in the metal liquid (as shown in figure 13), consuming Mg, Si and other atoms in the metal liquid, and the reduction of solute atoms will lead to the weakening of the composition undercooling, prolonging the solidification time, and ultimately making the SDAS refinement effect decline.

Microstructure and tensile properties after heat treatment
Figure 14 shows the two-dimensional and three-dimensional morphology of eutectic Si in A356 alloy after solid solution treatment at 540 °C for 4 h with the addition of Al-3Sr-8Y at different contents.After the solid solution treatment of A356 alloy without Al-3Sr-8Y, the long needle-like Si phase in the as-cast microstructure fuses into rods, and its length decreases.At the same time, the sharp edges of the Si phase are passivated, which reduces the cutting effect on the matrix.In addition to the rod-like eutectic Si, a small amount of spherical Si phase also appeared in the organization, as shown in figure 14(a).The three-dimensional morphology of eutectic Si is flaky and the edge positions are no longer sharp.In addition to the flaky Si phase, there are also localized regions of rod-like eutectic Si, as shown in figure 14(b).The addition of 0.1 wt% Al-3Sr-8Y to alloy A356 and solid solution treatment transformed the eutectic Si into spherical grains and short rods as shown in figure 14(c).From the 3D morphology of figure 14(d), it can be seen that the Si phase morphology basically exhibits granular and rod-like shape, indicating that the alloy fusion and spheroidization effects are enhanced.Adding 0.3 wt% Al-3Sr-8Y to A356 alloy, the alloy is in a fully densified state and solution treated for 4 h.The eutectic Si is basically transformed into spherical grains as shown in figures 14(e)-(f).Add 0.7 wt% Al-3Sr-8Y to A356 alloy, the alloy is in the over-denatured state, and the Si phase is still more spherical after solution treatment, as shown in figures 14(g)-(h).Figure 15 displays the mechanical properties of A356 alloy treated with T6 heat treatment (540 °C × 4 h + 175 °C × 6 h) after the addition of varying amounts of Al-3Sr-8Y.The mechanical properties sharply increase with the addition of Al-3Sr-8Y to 0.3wt%, then decline slightly.The untreated A356 alloy has the tensile strength and elongation of 256.7 MPa, and 5.1%, respectively.For the alloy with 0.3 wt% Al-3Sr-8Y, the values of the tensile strength and elongation reach the maximum of 303.5 MPa and 9.5%, and increase by 18.2%, and 86.3%, respectively, in comparison to the untreated A356 alloy.With further increase to 0.7 wt% Al-3Sr-8Y content, the values for tensile strength and elongation become 292.7 MPa and 8.8%, respectively.
The A356 alloy can generate fine diffuse β″-phase after heat treatment, which can significantly improve the mechanical properties of the alloy [39].After the introduction of Al-3Sr-8Y intermediate alloy, the Si phase morphology in the alloy changes from needle-like to fibrous, and the fibrous eutectic Si is more easily spheroidized during heat treatment, which can significantly reduce the cutting effect on the matrix and thus improve the mechanical properties of the alloy.From the aforementioned SDAS decreases and then increases, but is overall smaller than that of the untreated A356 alloy, and the change in SDAS implies a change in grain size.According to Hall-Petch theory [40], the grain size is negatively correlated with the mechanical properties of the alloy, so the mechanical properties of the alloy first increase and then decrease.

Conclusions
A new type of refining-modifying agent (Al-3Sr-8Y intermediate alloy) was developed to investigate the synergistic effect of Sr and Y on the microstructure and mechanical properties of A356 alloy.The results show that the addition of Al-3Sr-8Y can not only modify the eutectic Si from acicular-like to fibrous, but also refine the morphology of α-Al dendrites and decrease SDAS.When Al-3Sr-8Y content reaches 0.3 wt%, the microstructure of the alloy is significantly improved, and with further heat treatment, the mechanical properties of the alloy reach the maximum of 303.5 MPa and 9.5%, and increase by 18.2%, and 86.3%, respectively, in comparison to the untreated A356 alloy.

Figure 1 .
Figure 1.Mold and casting: (a) Mold dimension diagram; (b) Schematic diagram of the casting.
α-Al, Al 4 Sr, and Al 3 Y phases.The microstructure of Al-3Sr-8Y (as shown in figure5) reveals acicular, reticulated, and blocky phases that are embedded in the aluminum matrix.To investigate the composition of different phases, we selected several representative points for EDS testing.The obtained results confirmed that the acicular phase is composed of Al and Sr with an atomic ratio close to 4:1, indicating that the phase is Al 4 Sr.Additionally, the reticulated and irregular blocky phases are made up of Al and Y atoms, and the quantitative analysis shows comparable results.The Al:Y atom ratio approaches 3:1, implying that the phase is Al 3 Y.

Figure 5 .
Figure 5. EDS results of the second phases in Al-3Sr-8Y intermediate alloy.

Figure 7 .
Figure 7.The average area and aspect ratio of eutectic Si of A356 alloys with different contents of Al-3Sr-8Y.

Figure 10 .
Figure 10.Surface scan of major elements in A356 alloy with the addition of 0.3 wt% Al-3Sr-8Y.
α-Al dendrites begins to deteriorate (figures 11(d)-(e)).These results suggest that a small amount of Al-3Sr-8Y can remarkably refine α-Al dendrites and decrease the SDAS.

Figure 12 .
Figure 12.Variation of the average SDAS with different Al-3Sr-8Y contents in the experimental alloys.