Effect of Zr content on microstructure and tensile properties of cast and T6 heat-treated A356-0.3Yb alloy

In this paper,the influence of different Zr addition amounts on the microstructure and tensile properties of A356 alloy modified by 0.3 wt%Yb in cast state and T6 heat-treated state were systematically investigated. The results show that adding Zr on the basis of 0.3 wt%Yb can further refine α-Al and eutectic Si branches. When the content of Zr is 0.25 wt%, α-Al has the smallest grain size of 133 μm, and the aspect ratio of eutectic Si is 1.6. However, the addition of excessive Zr will weaken the modified effect of Yb and make the eutectic Si start coarsening. The addition of Zr forms the (Al,Si)3(Zr,Ti) phase with Ti in the alloy, provides the heterogeneous nucleation base, increases the number of α-Al grains, and refines the α-Al matrix. After T6 heat treatment, the eutectic Si is further spheroidized and the aspect ratio decreases to 1.3. The tensile strength and elongation of A356 alloy with Yb and Zr under cast condition are 187.4 MPa and 5.6% respectively, which are 10.4% and 36.6% higher than those without Zr. The tensile strength and elongation of the alloy after T6 heat treatment are 296.3 MPa and 9.2% respectively, which are 8.2% and 27.7% higher than those of the alloy without Zr after the same heat treatment.


Introduction
A356 alloy has excellent casting properties such as good fluidity, low density and high specific strength, and has been widely used in automobile manufacturing, aircraft parts, rail transit and other industrial fields [1,2]. However, there are α-Al with coarse dendrites and eutectic Si with needle sheets in the microstructure of A356 alloy without refining modification, which breaks the continuity of the matrix and seriously affects the strength and plasticity of the alloy [3,4]. The research [5][6][7][8] shows that the addition of rare Earth elements to A356 aluminum alloy has a good refining effect on α-Al, which is mainly reflected in the refining of secondary dendrite arm spacing and grain size, and also has a modified effect on eutectic Si. Therefore, researchers have shifted their attention to the complex refinement and modification of rare Earth and transition elements [9]. Shou-Peng Xu found that the composite addition of Sc and Zr in A356 alloy can greatly refine the grain, change the harmful coarse eutectic Si structure and the harmful acicular Fe-containing phase morphology.In the heat treatment state, the strength (298 MPa) and plasticity (8.4%) of the alloy containing Sc(0.19 wt%) and Zr(0.22 wt%) reached the maximum [10], and Al 3 (Sc, Zr) strengthened particles were found. Marco Colombo [11] et al found that when 0.25 wt%Er and 0.6 wt%Zr were added to A356 alloy, Al 3 (Er, Zr) formed by the addition of Zr not only transformed α-Al from dendrites to spherical shape, which reduced the grain size, but also the eutectic Si was further refined.The mechanical properties of the alloys with Er and Zr are better than those with Er alone [12,13]. Yb, Sc and Er are rare Earth elements with similar chemical properties. It had been found that adding Yb and Zr to Al can form high density and uniform Al 3 (Zr, Yb) dispersion with cubic L12 structure [14]. At present, there are few reports on the composite addition of Yb and Zr on A356 aluminum alloy. Therefore, In this paper, the effects of Yb and Zr compound addition on the microstructure and mechanical properties of A356 aluminum alloy are studied by means of metallographic Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. microanalysis, SEM, EDS and tensile test. In order to avoid the formation of excessive Yb-rich intermetallic compounds [15], a fixed Yb addition of 0.3 wt% was selected, and the effects of different Zr contents (0.05 wt%, 0.15 wt%, 0.25 wt%, 0.35 wt%, 0.45 wt%, 0.55 wt%) on the microstructure and tensile properties of the alloy in cast and T6 heat treatment states were investigated.

Experimental methods
The raw materials of this experiment are A356 alloy (composition is shown in table 1), Al-10Yb and Al-10Zr intermediate alloys. To compensate for the dilution of Si and Mg contents due to the addition of Yb and Zr, Al-26Si intermediate alloy and pure magnesium were added according to the calculated amount. After the raw materials are dried in a vacuum drying oven, A356 is melted in a resistive crucible furnace at 750°C ∼ 800°C to form liquid aluminum. It is degassed and impurity removed by using high-purity inert gas and sodium free refining agent. Then, Al-10Yb and Al-10Zr intermediate alloys are added to keep the temperature for 15 min, and degassed and impurity removed again. Finally, the alloy melt is poured into a 250°C pre-heated steel mold

Results and discussion
3.1. Effect of Zr content on microstructure of cast A356-0.3Yb alloy 3.1.1. Effect on α-Al Figure 1 shows the cast optical microstructure of A356 alloy with 0.3 wt%Yb as base and different Zr additions.
Since it is found that the alloy tends to change from dendritic crystals to petal-like equiaxial crystals after Zr addition, Image Pro Plus software is used to calculate the α-Al grain sizes with different Zr contents, and the number of statistics exceeds 100. The average value method is used to calculate the equivalent grain diameter, as shown in figure 2. The statistical results are shown in figure 3. As can be seen from figure 1(a), α-Al in the alloy appears as dendritic crystal with a grain size of about 255 μm without Zr. When 0.15 wt% Zr is added, the coarse dendrites show an obvious trend of equiaxial grain variation with the addition of Zr, and the grain size is about 187 μm. The Zr accumulated during solidification at the front of the dendrite is actually the cause of the structure supercooling at the tip of the dendrite, which delays the growth of dendrites and thus makes the    α-Al is about 131 μm, and the refining effect is weakened. When the Zr content is further increased to 0.45 wt% and 0.55 wt%, it can be found that the grain size changes little and the refining effect is not obvious. Figure 4 shows the metallographic morphology of eutectic Si in A356 alloy modified by 0.3 wt%Yb with different Zr contents, and figure 5 shows the statistical results of eutectic Si aspect ratio by Image Pro Plus software. It can be seen from figure 4(b) that when 0.15 wt%Zr is added on the basis of 0.3 wt%Yb, the eutectic Si size of the alloy has little change, and the aspect ratio is 1.7. Figure 4(c) shows that eutectic Si becomes finer and tends to be spherical with an aspect ratio of 1.6 when Zr is further added to 0.25 wt%. When Zr content is 0.35 wt%, the morphology of eutectic Si changes little, and the size of alloy eutectic Si is similar to that of 0.25 wt% Zr. However, when the Zr content continues to increase to 0.45 wt%, the eutectic Si size gradually increases, the aspect ratio increases to 2, and the morphology begins to coarsen. When Zr is increased to 0.55 wt%, it can be seen from figure 4(f) that the needle sheet eutectic Si begins to precipitate again, and the morphology and size further increase, with the aspect ratio also rising to 2.5. The reason for the increase is mainly due to excessive refining and modified elements leading to the formation of excessive intermetallic compounds in the alloy, thus weakening the modified effect. The alloys with different Zr contents were deeply corroded and the three-dimensional morphology of eutectic Si in A356-0.3Yb alloy was analyzed by SEM, as shown in figure 6. It can be seen that when 0.15 wt%Zr is added, there is little difference in the morphology of eutectic Si, which is coral-like.When 0.25 wt%Zr is added, the eutectic Si branches become finer. When Zr content is increased, the eutectic Si begins to grow larger, and the eutectic Si branches become less, and the morphology of the eutectic Si changes from coral shape to coarse keel shape. This indicates that Zr and Yb in Al-Si alloys have refinement and modification on eutectic Si. Bing-Yi Liu et al believed that modified treatment could improve the necessary dynamic conditions for Si phase to transform from non-equilibrium state to equilibrium state, and the mechanism of dendrite fusion to promote spheroidization was similar, because the fusion mainly occurred at the position with high energy state, such as branch, twin, dislocation and other crystal defects [16]. With the addition of Yb and Zr elements, eutectic Si was in a deviated equilibrium state due to its growth mode changed because of sufficient modification. This state leads to the increasing of crystal defects such as twins and dislocations in Si crystals, and finally promotes the melting and spheroidization of eutectic Si [17]. However, the solubility of Si phase in alloys is limited. With the  increase of Zr content, once the overmodified Si crystal reaches saturation after heat treatment, its morphology begins to coarsen . Figure 7 shows the SEM image of the morphology and distribution of intermetallic compounds in A356-0.3Yb-0.25Zr sample, and table 2 is the chemical composition of the second phase in figure 7. Combined with the morphology and composition analysis of intermetallic compounds, the bright white intermetallic compounds with a large contrast with the matrix are rare Earth phases containing Yb, as shown in point A in figure 7. In addition, long rod-like Zr-Ti-Al-Si compounds were also found, as shown at point B in figure 7, indicating the precipitation of (Al, Si) 3 (Zr, Ti) phase with the addition of Zr. Zr generates Al 3 Zr precipitated phase in binary Al- Zr system, but (Al, Si) 3 Zr phase is formed due to the large amount of Si in A356 alloy. In the Ti-rich and Zr-rich phases of Al-Si-Ti-Zr alloys, Ti and Zr can be substituted with each other. With the increase of Zr content, more (Al, Si) 3 (Zr, Ti) phases are generated in peritectic reaction before the formation of α-Al phase, providing a heterogeneous nucleation base and increasing the number of α-Al grains [18]. Iron is an unavoidable harmful element in A356 casting alloy. During the solidification process, iron can react with silicon to form a robust Ferich phase which is densely distributed in the alloy. This acicular fe-rich intermetallic compound mismatches the lattice of the matrix interface, resulting in the decrease of the mechanical properties of the alloy. After refining and metamorphism, the morphology of Fe-rich intermetallic compounds was improved, showing small size fishbone or needle shape, as shown in point C and D in the figure.

Effect of Yb and Zr on the second phase of A356 alloy
3.3. Effect of Zr content on microstructure of T6 heat treated A356-0.3Yb alloy Figures 8 and 9 respectively show the optical microstructure of α-Al and eutectic Si in the T6 heat treatment state (4 h solution at 545°C + 8 h aging at 170°C) of A356 alloy combined with Yb and Zr. Figure 10 shows the statistical results of aspect ratio of eutectic Si. As can be seen from figure 8, with the increase of Zr content, the size of α-Al decreases further. After adding 0.25 wt%Zr, the grain remains basically unchanged, and the overall effect of alloy refinement is similar. It can be seen from figures 9 and 10 that Zr has little effect on the morphology and size of eutectic Si in alloys containing Yb, but only further weak refinement.When Zr content is 0.25 wt%, eutectic Si has the smallest size, elliptic or granular morphology, and the aspect ratio is 1.3. However, when Zr content increases to 0.55 wt%, it can be clearly seen from figure 9(f) that part of eutectic Si begins to show secondary growth, and the morphology no longer tends to spheroidize, but coarsens to a short rod-like shape, resulting in the aspect ratio rising to 1.8.

3.4.
Effect of Zr content on tensile properties of A356-0.3Yb alloy 3.4.1. Tensile mechanical properties as cast Figure 11 shows the variation of tensile strength and elongation of cast A356-0.3Yb alloy with Zr addition respectively. It can be seen from the figure that the addition of Zr on the basis of 0.3 wt%Yb can further improve the tensile property of the cast alloy. When the Zr content is 0.25 wt%, the tensile property of the alloy reaches the peak, and the tensile strength and elongation of the alloy are 187.4 MPa and 5.6% respectively, which are 8.7% and 30% higher than that of the alloy without Zr. This indicates that Yb and Zr have a synergistic effect, and the compound addition of Yb and Zr has an obvious optimization effect on the strength of the alloy, because the addition of Zr to the alloy containing Yb not only refines the α-Al grain, but also further improves the morphology of eutectic Si, and thus increases the strength of the alloy. On this basis, it is found that the tensile properties of the alloy begin to decline by increasing the Zr content, which indicates that excessive Zr can not replace the effect of Yb, but will coarsen eutectic Si and damage the mechanical properties of the alloy. Figure 12 shows the SEM image of tensile fracture morphology of cast A356-0.3Yb alloy with different Zr content. It can be found that the fracture of the alloy containing trace Zr shows typical cleavage fracture behavior, and the eutectic Si phase forms some thick and smooth cleavage steps and river lines, and only a few dimples exist. With the excessive content of Zr, eutectic Si with coarse morphology and long rod-like shape begins to precipitate at the grain boundary, as shown in figures 12(e), (f), stress concentration is easily formed here, leading to brittle fracture. After tensile failure, obvious bright intergranular fracture zone and intergranular crack are visible on the fracture surface, which is intergranular fracture on the whole. Figure 13 shows the tensile properties of alloy T6 in the heat treatment state. It can be seen from the figure that, compared with the alloy containing Yb, the composite alloy containing Yb and Zr has better tensile properties, and is better than the alloy containing Yb and Zr under the cast condition. When the Zr content is 0.25 wt%, the mechanical properties are the best, and the tensile strength and elongation are 296.3 MPa and 9.2% respectively, which are 6.3% and 18% higher than those without Zr. The reason why the mechanical properties of the alloy after heat treatment are higher than those of the as-cast alloy is that the T6 heat treatment not only spheroidizes eutectic Si, but also precipitates a large number of second phases during the aging process. These secondary precipitated phases are dispersed in the alloy, forming more effective heterogeneous nucleation points, which play the role of pinning dislocations and effectively hinder the movement of dislocations and grain boundaries. However, when the Zr content continues to increase, the mechanical properties begin to decline. This is because when the addition of excessive refinement modification, more and more compounds containing Yb and Zr elements aggregate in the structure, the dendrite segregation degree increases, the matrix is separated, and the phenomenon of stress concentration is prone to occur, which leads to the deterioration of mechanical properties. Figure 14 shows the SEM analysis of the fracture of T6 heat-treated A356 alloy with compound addition of Yb and Zr. It can be seen that the addition of trace Zr has no significant change in the fracture morphology of the alloy containing 0.3 wt%Yb. After the T6 heat treatment, eutectic Si is fused and spherodized, which reduces the stress concentration and makes the fracture appear ductile fracture characteristics, and more dimples are generated. However, when Zr content increases to 0.55 wt%, dimples at the fracture are reduced, the opening is larger, the depth is shallower, and some cleavage planes, cleavage steps and tear edges appear again. The fracture mode is quasi-cleavage fracture.

Conclusion
(1)The addition of Zr can significantly refine the α-Al in A356-0.3Yb alloy, transforming it from a coarse dendritic crystal to a petal like equiaxial crystal; The addition of Zr can also refine and modify the eutectic Si, and then spheroidize it. When Zr content is 0.25 wt%, α-Al is more significant, α-Al refining effect is the most significant, the minimum grain size is about 133 μm, The modified effect of eutectic Si is the most significant, the aspect ratio drops to 1.6. When Zr content exceeds 0.25 wt%, the refining effect of α-Al is weakened and eutectic Si is coarsened instead.
(2)When Zr is added to A356-0.3Yb alloy, not only Yb-containing intermetallic compounds but also Zr-Ti-Al-Si compounds are precipitated, both of which could promote matrix nucleation.
(3)After T6 heat treatment, the refining effect of α-Al and eutectic Si in A356 alloy with composite addition of Yb and Zr is the best when the content of Zr is 0.25%, and the aspect ratio of eutectic Si is 1.3.
(4)The tensile properties of the alloys after T6 heat treatment are generally better than those in cast condition.
With the increase of Zr content, the tensile strength and elongation of the alloys in both cast and T6 heat treatment states increase first and then decrease. When the Zr content is 0.25%, the tensile strength and elongation of A356-0.3Yb reach the maximum value. The tensile strength and elongation of A356-0.3Yb in cast condition are 187.4 MPa and 5.6%, respectively, which are 10.4% and 36.6% higher than those without Zr addition, respectively. The tensile strength and elongation of alloy after T6 treatment are 296.3 MPa and 9.2%, respectively, which are 6.3% and 18% higher than those without Zr. The fracture mode of the alloy changes from brittle fracture to ductile fracture with the increase of Zr content, and then changes to brittle fracture when the Zr content is too much.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).