Influence of Li2Sb Additions on Microstructure and Mechanical Properties of Al-20Mg2Si Alloy

It is found that Li2Sb compound can act as the nucleus of primary Mg2Si during solidification, by which the particle size of primary Mg2Si decreased from ~300 to ~15–25 μm. Owing to the synergistic effect of the Li2Sb nucleus and adsorption-poisoning of Li atoms, the effect of complex modification of Li-Sb on primary Mg2Si was better than that of single modification of Li or Sb. When Li-Sb content increased from 0 to 0.2 and further to 0.5 wt.%, coarse dendrite changed to defective truncated octahedron and finally to perfect truncated octahedral shape. With the addition of Li and Sb, ultimate compression strength (UCS) of Al-20Mg2Si alloys increased from ~283 to ~341 MPa and the yield strength (YS) at 0.2% offset increased from ~112 to ~179 MPa while almost no change was seen in the uniform elongation. Our study offers a simple method to control the morphology and size of primary Mg2Si, which will inspire developing new Al-Mg-Si alloys with improved mechanical properties.


Introduction
The as-cast microstructure has a strong influence on mechanical properties of castings [1]. For Al-high Mg 2 Si alloy, the formation of primary Mg 2 Si reinforcement with small grain size and regular morphology is necessary to improve the mechanical properties of alloys and thus is has become the main issue when preparing the materials with excellent properties [2][3][4][5]. Intermetallic compound Mg 2 Si, which exhibits low density (1.99ˆ10 3 kg m´3), high melting temperature (1085˝C), high elastic modulus (120 GPa) and high hardness (4.5ˆ10 9 N m´2) as well as a low thermal expansion coefficient (TEC) (7.5ˆ10´6 K´1), has been widely used as a reinforced phase to prepare Al/Mg 2 Si alloys [6][7][8][9]. The excellent properties of Mg 2 Si can make Al/Mg 2 Si alloys suitable for widespread use in automobile and aerospace fields [10][11][12][13]. However, under equilibrium solidification condition, primary Mg 2 Si tends to form coarse dendrite, which is harmful to the mechanical property of Al-Mg 2 Si alloys and limits their development and application [14][15][16]. Therefore, controlling the morphology and size of primary Mg 2 Si is a great challenge to material scientists [17].
As far as we know, modification treatment is the most effective method to control morphologies and sizes of primary and eutectic Mg 2 Si, which is readily available for commercial applications [18]. Among all kinds of modifiers, Sb has been widely used for modification treatment of primary and eutectic Mg 2 Si [19,20]. The reason is that Mg 3 Sb 2 formed during solidification can act as the nucleus of primary and eutectic Mg 2 Si, refining the size of Mg 2 Si and improving mechanical properties of Al-Mg-Si alloys [19,20]. Alizadeh et al. [21] reported that with the addition of 0.2 wt.% Sb into the Mg-4Zn-2Si melt, flake-like eutectic Mg 2 Si changed into fine polygons, and the mechanical properties such as impression creep and hot hardness were improved significantly. In our previous study [22], we found that with the content of Sb addition increasing from 0 to 0.2 and to 0.5 and finally to 2 wt.%, the morphology of primary Mg 2 Si in Mg-4Si, alloys transformed from coarse dendrite to equiaxed-dendrite and to defective octahedron and finally to perfect octahedron; meanwhile, the morphology of eutectic Mg 2 Si transformed from flake-like to fine polygonal shapes. Based on the above research, one can see that the modification effect of Sb is more effective to eutectic Mg 2 Si than to primary Mg 2 Si. Therefore, how to enhance the modification effect of Sb on primary Mg 2 Si is the key to improving mechanical properties of Al-high Mg 2 Si alloy. However, only limited research has been reported regarding this issue.
Because the electronegativity difference between Li and Sb is relatively large, they could form compounds with thermodynamic stability such as Li 2 Sb and Li 3 Sb during solidification process. The calculated disregistry is 4.0% at the orientation relationship of p1010q Li 2 Sb {{p111q Mg 2 Si for Li 2 Sb while 5.8% at that of p001q Li 3 Sb {{p001q Mg 2 Si for Li 3 Sb, which are both less than 6.0% and may act as the nucleation substrate for primary Mg 2 Si [23]. To change morphologies and refine the size of primary Mg 2 Si during solidification and finally to improve the mechanical properties of Al-20Mg 2 Si alloy, we added Li and Sb simultaneously to the Al-Mg-Si melt. The mechanism of primary Mg 2 Si co-modified with Li and Sb was revealed in this research. The compression property and microhardness of Al-20Mg 2 Si alloys modified with 0, 0.2 and 0.5 wt.% Li-Sb were also tested. The results achieved will be a big step forward in realizing the artificial manipulation of grain refinement and morphology transformation of primary Mg 2 Si in Al alloys, which plays an important role in improving physical and mechanical properties of Al-Mg-Si alloys.

Preparation of Al-20Mg 2 Si Alloy Modified with Various Contents of Li-Sb
In order to prepare Al-20Mg 2 Si alloy, where the unit of "20" is "wt.%" and the unit of "2" is the number of Mg atom in intermetallic compound Mg 2 Si, the contents of the Al ingot (99.98 wt.% purity), Mg ingot (99.85 wt.% purity) and Al-24.4Si master alloy are~57.2 wt.%,~12.6 wt.% and~30 wt.%, respectively. The modifiers are pure Sb ingot (98.00 wt.% purity) and Mg-13.5Li master alloy. Pure Al and Al-24.4Si master alloy were melted at 750˝C in a graphite crucible in an electric resistance furnace of 5 kW; then pure Mg, Sb and Mg-13.5Li master alloy preheated at 150˝C in a vacuum oven were added to the melts together. The designed compositions of Li-Sb in melts were 0, 0.2 and 0.5 wt.%, with an atomic ratio of Li:Sb of 3:1. Manual agitation was conducted in the Al-Mg-Si melts for about 1 min and held at 750˝C for 20 min. Finally, the melts was poured into a steel mold preheated at 150˝C to produce Al-20Mg 2 Si alloy co-modified with various contents of Li and Sb.

Characterization
Metallographic samples with a size of 10 mmˆ10 mmˆ13 mm were cut at the bottom of the ingots. Metallographic samples were prepared by a standard procedure and etched with 0.5 vol.% HF-distilled water solution for about 30 s at room temperature. To observe the 3-D morphologies of primary Mg 2 Si, samples with the size of 1.2 mmˆ12 mmˆ13 mm were put into a 20 vol.% HNO 3 -distilled water solution to dissolve the Al covering on the surface of the primary Mg 2 Si. The samples for compression test were processed into cylinders of which the diameter is 3 mm and the height is 6 mm. X-ray diffraction (XRD) (D/Max 2500PC, Rigaku, Tokyo, Japan) was used to characterize phase constitutions of the samples, using CuK α radiation in step modes from 20˝to 80w ith a scanning speed of 4˝min´1 and an acquisition step of 0.02˝(2θ). As-cast microstructures of Al-20Mg 2 Si alloy were investigated using optical microscopy (OM) (Carl Zeiss-Axio Imager A2m, Gottingen, Germany). The 3-D morphologies of the extracted primary Mg 2 Si were observed using a

Microstructure of Al-20Mg2Si Alloy Modified with Li and Sb Simultaneously
According to the XRD results ( Figure    According to the literature [22,24], Li or Sb can restrict the growth of Mg 2 Si crystal by adsorbing on the growth sites of primary Mg 2 Si particles, and hence refine their size. For comparison, 0.2 wt.% Li and 0.2 wt.% Sb were separately added to Al-20Mg 2 Si alloys. As-cast microstructure of primary Mg 2 Si modified with 0.2 wt.% Li or Sb is shown in Figure 3a,b, respectively. Clearly, the grain refinement effect of 0.2 wt.% Li or Sb is relatively weaker than that of the combined addition of 0.2 wt.% Li-Sb ( Figure 3c). Moreover, the 3-D morphologies of primary Mg 2 Si modified with 0.2 wt.% Li or Sb are also given (Figure 3d-g). As we can see, perfect octahedrons and equiaxed-dendrites were obtained in Al-20Mg 2 Si alloy modified with 0.2 wt.% Li (Figure 3d,e). Similar morphologies were also observed in the alloy modified with 0.2 wt.% Sb (Figure 3f,g). Meanwhile, truncated octahedral primary Mg 2 Si was formed when modified with 0.2 wt.% Li-Sb ( Figure 3h). Apparently, compared with the modification effect of Li or Sb on primary Mg 2 Si, the co-modification effect of Li-Sb was enhanced significantly. According to the literature [22,24], Li or Sb can restrict the growth of Mg2Si crystal by adsorbing on the growth sites of primary Mg2Si particles, and hence refine their size. For comparison, 0.2 wt.% Li and 0.2 wt.% Sb were separately added to Al-20Mg2Si alloys. As-cast microstructure of primary Mg2Si modified with 0.2 wt.% Li or Sb is shown in Figure 3a,b, respectively. Clearly, the grain refinement effect of 0.2 wt.% Li or Sb is relatively weaker than that of the combined addition of 0.

Characterization of Nucleus in Primary Mg2Si
To identify the composition of the nucleus, shown in Figure 1b,c, elemental mapping scanning According to the literature [22,24], Li or Sb can restrict the growth of Mg2Si crystal by adsorbing on the growth sites of primary Mg2Si particles, and hence refine their size. For comparison, 0.2 wt.% Li and 0.2 wt.% Sb were separately added to Al-20Mg2Si alloys. As-cast microstructure of primary Mg2Si modified with 0.2 wt.% Li or Sb is shown in Figure 3a,b, respectively. Clearly, the grain refinement effect of 0.2 wt.% Li or Sb is relatively weaker than that of the combined addition of 0.2 wt.% Li-Sb ( Figure 3c). Moreover, the 3-D morphologies of primary Mg2Si modified with 0.2 wt.% Li or Sb are also given (Figure 3d-g). As we can see, perfect octahedrons and equiaxed-dendrites were obtained in Al-20Mg2Si alloy modified with 0.2 wt.% Li (Figure 3d,e). Similar morphologies were also observed in the alloy modified with 0.2 wt.% Sb (Figure 3f,g). Meanwhile, truncated octahedral primary Mg2Si was formed when modified with 0.2 wt.% Li-Sb (Figure 3h). Apparently, compared with the modification effect of Li or Sb on primary Mg2Si, the co-modification effect of Li-Sb was enhanced significantly.

Characterization of Nucleus in Primary Mg2Si
To identify the composition of the nucleus, shown in Figure 1b,c, elemental mapping scanning analysis was conducted. Note that the distribution of Li was not given because Li is a light element, which is difficult to be detected by EDS. As we can see, the Al atoms were mostly around the primary Mg2Si crystal (

Characterization of Nucleus in Primary Mg 2 Si
To identify the composition of the nucleus, shown in Figure 1b,c, elemental mapping scanning analysis was conducted. Note that the distribution of Li was not given because Li is a light element, which is difficult to be detected by EDS. As we can see, the Al atoms were mostly around the primary Mg 2 Si crystal ( Figure 4b); Mg (Figure 4c) and Si (Figure 4d) atoms were detected in the crystal, while Sb atoms were mainly found inside the nucleus and the intensity of Sb (Figure 4e). Therefore, it is rational to say that the nucleus is a kind of antimony compound. crystal, while Sb atoms were mainly found inside the nucleus and the intensity of Sb (Figure 4e). Therefore, it is rational to say that the nucleus is a kind of antimony compound. Further investigation on the nature of nucleus was carried out by TEM and EDS. A nucleus located in the center of primary Mg2Si co-modified with Li-Sb is shown in Figure 5a. According to the double selected-area diffraction (SAD) pattern of nucleus (Figure 5b), the antimony-containing compound is Li2Sb, which has a hexagonal structure (P-62m) with the lattice constant of a = 0.7947 nm, b = 0.7947 nm, c = 0.3260 nm, α = β = 90° and γ = 120° [23]. In our previous study, we have confirmed that the Si sites in Mg2Si lattice can be substituted by Sb atoms when Sb was added into the Mg-4Si alloy [22], while no substitution occurred when Ca and Sb were simultaneously added to the Al-20Mg2Si alloy [22,25]. To investigate whether substitution occurred in the present case, the EDS analysis for the modified Mg2Si crystal and the nucleus is given in Figure 5c  Note that, in our experiment, the designed atomic ratio of Li:Sb is 3:1, while the nucleus is Li2Sb, so that slight substitution of Sb atoms in Mg2Si lattice may also occur. In general, with the growth of crystal, the crystal facets with high growth rates will shrink gradually, while the facets with low growth rates will be reserved as crystal surfaces [26]. This suggests that some Li atoms did not react Further investigation on the nature of nucleus was carried out by TEM and EDS. A nucleus located in the center of primary Mg 2 Si co-modified with Li-Sb is shown in Figure 5a. According to the double selected-area diffraction (SAD) pattern of nucleus (Figure 5b), the antimony-containing compound is Li 2 Sb, which has a hexagonal structure (P-62m) with the lattice constant of a = 0.7947 nm, b = 0.7947 nm, c = 0.3260 nm, α = β = 90˝and γ = 120˝ [23]. In our previous study, we have confirmed that the Si sites in Mg 2 Si lattice can be substituted by Sb atoms when Sb was added into the Mg-4Si alloy [22], while no substitution occurred when Ca and Sb were simultaneously added to the Al-20Mg 2 Si alloy [22,25]. To investigate whether substitution occurred in the present case, the EDS analysis for the modified Mg 2 Si crystal and the nucleus is given in Figure 5c crystal, while Sb atoms were mainly found inside the nucleus and the intensity of Sb (Figure 4e). Therefore, it is rational to say that the nucleus is a kind of antimony compound. Further investigation on the nature of nucleus was carried out by TEM and EDS. A nucleus located in the center of primary Mg2Si co-modified with Li-Sb is shown in Figure 5a. According to the double selected-area diffraction (SAD) pattern of nucleus (Figure 5b), the antimony-containing compound is Li2Sb, which has a hexagonal structure (P-62m) with the lattice constant of a = 0.7947 nm, b = 0.7947 nm, c = 0.3260 nm, α = β = 90° and γ = 120° [23]. In our previous study, we have confirmed that the Si sites in Mg2Si lattice can be substituted by Sb atoms when Sb was added into the Mg-4Si alloy [22], while no substitution occurred when Ca and Sb were simultaneously added to the Al-20Mg2Si alloy [22,25]. To investigate whether substitution occurred in the present case, the EDS analysis for the modified Mg2Si crystal and the nucleus is given in Figure 5c,d, respectively. According to the result, the EDS collected from the modified Mg2Si crystal contains mainly Mg, Si and Al peaks; only a few (0.09 at.%) Sb atoms were detected in the Mg2Si crystal (Figure 5c), while the EDS obtained from nucleus contains Mg, Sb (31.3 at.%), Si and Al peaks (Figure 5d). Thus, it can be concluded that most of the Sb atoms reacted with Li atoms to form Li2Sb compounds, acting as nucleus for Mg2Si crystals. Note that, in our experiment, the designed atomic ratio of Li:Sb is 3:1, while the nucleus is Li2Sb, so that slight substitution of Sb atoms in Mg2Si lattice may also occur. In general, with the growth of crystal, the crystal facets with high growth rates will shrink gradually, while the facets with low growth rates will be reserved as crystal surfaces [26]. This suggests that some Li atoms did not react Note that, in our experiment, the designed atomic ratio of Li:Sb is 3:1, while the nucleus is Li 2 Sb, so that slight substitution of Sb atoms in Mg 2 Si lattice may also occur. In general, with the growth of crystal, the crystal facets with high growth rates will shrink gradually, while the facets with low growth rates will be reserved as crystal surfaces [26]. This suggests that some Li atoms did not react with Sb and they might be absorbed on the {100} facets. According to Figure 3a,d,e, sub-modification occurred in Al-20Mg 2 Si alloy with 0.2 wt.% Li added. Therefore, as for the primary Mg 2 Si modified with 0.2 wt.% Li-Sb, in addition to that Li 2 Sb nucleus can promote the nucleation of primary Mg 2 Si, additional Li atoms absorbed on {100} facets led to the exposure of {100} facets, and thus truncated octahedral primary Mg 2 Si formed, as shown in Figure 3h.

Effect of Li 2 Sb Nucleus on Mechanical Properties of Al-20Mg 2 Si Alloy
The mechanical properties of Al-20Mg 2 Si alloys with 0, 0.2 and 0.5 wt.% Li-Sb addition are given in Figure 6 and Table 1. With the addition of Li and Sb, ultimate compression strength (UCS) of Al-20Mg 2 Si alloys increased from~283 to~341 MPa and the yield strength (YS) at 0.2% offset increased from~112 to~179 MPa, while almost no change was seen in the uniform elongation. The addition of Li and Sb also resulted in the increase in microhardness of α-Al matrix from~91 to~104 Hv. For a particle reinforced alloy, mechanical property is influenced by the reinforcement to a significant extent [10]. It is well known that primary Mg 2 Si is the reinforced phase in Al-20Mg 2 Si alloys and dendritic primary Mg 2 Si with a large size is harmful to mechanical properties [14][15][16]. Decreasing particle size usually leads to an increase in strength according to the Hall-Petch effect: [10].
where ∆σ YS is the increment of yield strength; D is the size of reinforcement phase; and V m and V γ are the volume fraction of matrix and reinforcement, respectively. Thus, with the addition of Li-Sb, the size of primary Mg 2 Si decreases from~300 to~15-25 µm (Figure 2a-c), leading to improved UCS, YS, and microhardness of Al-20Mg 2 Si alloys. with Sb and they might be absorbed on the {100} facets. According to Figure 3a,d,e, sub-modification occurred in Al-20Mg2Si alloy with 0.2 wt.% Li added. Therefore, as for the primary Mg2Si modified with 0.2 wt.% Li-Sb, in addition to that Li2Sb nucleus can promote the nucleation of primary Mg2Si, additional Li atoms absorbed on {100} facets led to the exposure of {100} facets, and thus truncated octahedral primary Mg2Si formed, as shown in Figure 3h.

Effect of Li2Sb Nucleus on Mechanical Properties of Al-20Mg2Si Alloy
The mechanical properties of Al-20Mg2Si alloys with 0, 0.2 and 0.5 wt.% Li-Sb addition are given in Figure 6 and Table. 1. With the addition of Li and Sb, ultimate compression strength (UCS) of Al-20Mg2Si alloys increased from ~283 to ~341 MPa and the yield strength (YS) at 0.2% offset increased from ~112 to ~179 MPa, while almost no change was seen in the uniform elongation. The addition of Li and Sb also resulted in the increase in microhardness of α-Al matrix from ~91 to ~104 Hv. For a particle reinforced alloy, mechanical property is influenced by the reinforcement to a significant extent [10]. It is well known that primary Mg2Si is the reinforced phase in Al-20Mg2Si alloys and dendritic primary Mg2Si with a large size is harmful to mechanical properties [14][15][16]. Decreasing particle size usually leads to an increase in strength according to the Hall-Petch effect: [10].
where ΔσYS is the increment of yield strength; D is the size of reinforcement phase; and Vm and Vγ are the volume fraction of matrix and reinforcement, respectively. Thus, with the addition of Li-Sb, the size of primary Mg2Si decreases from ~300 to ~15-25 μm (Figure 2a-c), leading to improved UCS, YS, and microhardness of Al-20Mg2Si alloys.  However, it is worth to noting that with Li-Sb content increasing from 0.2 to 0.5 wt.%, similar microstructure features were observed and the size of primary Mg2Si still kept within the range of ~15-25 μm (Figure 2b,c), while the UCS increases significantly (from 306 to 341 MPa). Moreover, except Al and Mg2Si, no other phases that are beneficial to the mechanical properties of the alloy  However, it is worth to noting that with Li-Sb content increasing from 0.2 to 0.5 wt.%, similar microstructure features were observed and the size of primary Mg 2 Si still kept within the range of 15-25 µm (Figure 2b,c), while the UCS increases significantly (from 306 to 341 MPa). Moreover, except Al and Mg 2 Si, no other phases that are beneficial to the mechanical properties of the alloy were detected (Figure 1a-c). Therefore, other factors, like the morphology of primary Mg 2 Si, may also influence mechanical properties of the Al-20Mg 2 Si alloy.
Typical 3-D morphologies of primary Mg 2 Si in Al-20Mg 2 Si alloys without and with various Li-Sb additions are given in Figure 7a-d. As we can see, with the content of Li and Sb increasing from 0 to 0.2 and then to 0.5 wt.%, the morphology of primary Mg 2 Si transformed from coarse dendrite (Figure 7a) to coexistence of defective truncated octahedron and perfect truncated octahedron (Figure 7b-c) and finally to a perfect truncated octahedral shape (Figure 7d). According to the literature, defective truncated octahedron can separate the α-Al matrix in the growth defect to some extent [3], leading to lower UCS of the alloy modified with 0.2 wt.% Li-Sb as compared to the alloy modified with 0.5 wt.% Li-Sb (Table 1). In addition, with the content of Li-Sb increasing from 0.2 to 0.5 wt.%, the size of eutectic Mg 2 Si decreased slightly (Figure 7e-g), which agrees well with the OM observations (Figure 2d-f). The refined size of eutectic phase is propitious to the improvement in the microhardness in modified alloys. Unfortunately, because Mg 2 Si particles are brittle, their existence is harmful to the plasticity of Al-20Mg 2 Si alloy [5,27]. Thus, controlling the morphology and size of primary Mg 2 Si has little effect on improving plasticity of the modified Al-20Mg 2 Si alloy. were detected (Figure 1a-c). Therefore, other factors, like the morphology of primary Mg2Si, may also influence mechanical properties of the Al-20Mg2Si alloy. Typical 3-D morphologies of primary Mg2Si in Al-20Mg2Si alloys without and with various Li-Sb additions are given in Figure 7a-d. As we can see, with the content of Li and Sb increasing from 0 to 0.2 and then to 0.5 wt.%, the morphology of primary Mg2Si transformed from coarse dendrite (Figure 7a) to coexistence of defective truncated octahedron and perfect truncated octahedron (Figure 7b-c) and finally to a perfect truncated octahedral shape (Figure 7d). According to the literature, defective truncated octahedron can separate the α-Al matrix in the growth defect to some extent [3], leading to lower UCS of the alloy modified with 0.2 wt.% Li-Sb as compared to the alloy modified with 0.5 wt.% Li-Sb (Table 1). In addition, with the content of Li-Sb increasing from 0.2 to 0.5 wt.%, the size of eutectic Mg2Si decreased slightly (Figure 7e-g), which agrees well with the OM observations (Figure 2d-f). The refined size of eutectic phase is propitious to the improvement in the microhardness in modified alloys. Unfortunately, because Mg2Si particles are brittle, their existence is harmful to the plasticity of Al-20Mg2Si alloy [5,27]. Thus, controlling the morphology and size of primary Mg2Si has little effect on improving plasticity of the modified Al-20Mg2Si alloy.

Conclusions
In this paper, the effect of Li2Sb nucleus on microstructure and mechanical properties of Al-20Mg2Si alloys was investigated and the main conclusions are drawn as following: (1) The 3-D morphology of primary Mg2Si was observed by extracting the Mg2Si crystals from Al-20Mg2Si alloys. With the addition of Li-Sb, the size of primary Mg2Si decreased from ~300 to ~15-25 μm and the morphology changed from coarse dendrite to defective truncated octahedron and finally to perfect truncated octahedral shape. (2) The modification mechanism of Li-Sb can be concluded as follows: Li2Sb can act as better substrates to enhance the heterogeneous nucleation rate of primary Mg2Si; meanwhile, excess Li atoms were absorbed on and restricted the growth of {100} facets. The modification effect of Li-Sb was better than that of either Li or Sb, respectively. (3) Influence of Li2Sb on mechanical properties of Al-20Mg2Si alloys was also investigated. With the addition of Li and Sb, ultimate compression strength (UCS) of Al-20Mg2Si alloys increased from ~283 to ~341 MPa and the yield strength (YS) at 0.2% offset increased from ~112 to ~179 MPa, while almost no change was seen in the uniform elongation. The addition of Li and Sb also led to the increase in microhardness of α-Al matrix from ~91 to ~104 Hv.

Conclusions
In this paper, the effect of Li 2 Sb nucleus on microstructure and mechanical properties of Al-20Mg 2 Si alloys was investigated and the main conclusions are drawn as following: (1) The 3-D morphology of primary Mg 2 Si was observed by extracting the Mg 2 Si crystals from Al-20Mg 2 Si alloys. With the addition of Li-Sb, the size of primary Mg 2 Si decreased from~300 tõ 15-25 µm and the morphology changed from coarse dendrite to defective truncated octahedron and finally to perfect truncated octahedral shape. (2) The modification mechanism of Li-Sb can be concluded as follows: Li 2 Sb can act as better substrates to enhance the heterogeneous nucleation rate of primary Mg 2 Si; meanwhile, excess Li atoms were absorbed on and restricted the growth of {100} facets. The modification effect of Li-Sb was better than that of either Li or Sb, respectively.
Influence of Li 2 Sb on mechanical properties of Al-20Mg 2 Si alloys was also investigated. With the addition of Li and Sb, ultimate compression strength (UCS) of Al-20Mg 2 Si alloys increased from 283 to~341 MPa and the yield strength (YS) at 0.2% offset increased from~112 to~179 MPa, while almost no change was seen in the uniform elongation. The addition of Li and Sb also led to the increase in microhardness of α-Al matrix from~91 to~104 Hv.