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Effect of Er-Ba Composite Modification on Microstructure and Mechanical Properties of Mg-2.5Si-4Zn Cast Alloy  PDF

  • Tong Wenhui ✉
  • Huang Bonan
  • Sun Bowei
  • Wang Jie
  • Bai Yufei
School of Materials Science and Engineering, Shenyang Aerospace University, Shenyang 110136, China

Updated：2023-08-25

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Abstract

The enhanced phase Mg2Si in the magnesium alloy can significantly improve the hardness, wear resistance, especially high temperature creep resistance. But the Mg2Si phase of the as-cast hyper-eutectic Mg-Si alloy will seriously cut the alloy matrix because of the coarse angular shape of the primary Mg2Si and complex Chinese script morphology of the eutectic Mg2Si. In order to improve the properties of Mg-2.5Si-4Zn alloy, the modification experiments of adding Er/Er-Ba were performed and the effects on the alloy microstructure and Mg2Si phase were investigated by the optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffractometer (XRD). The mechanical properties were measured and analyzed by the computer-aided electric-loaded tensile tester. The results show that with the addition of 0.6wt% Er to the Mg-2.5Si-4Zn alloy, the primary Mg2Si in the microstructure transforms from a coarse dendritic shape into a regular tetragonal block shape, while the eutectic Mg2Si transforms from a coarse Chinese script type to a more complex short rod shape. When adding 0.8wt% Ba subsequently, the primary Mg2Si further transforms from a regular tetragonal bulk shape to an irregular fine bulk shape with grooves and holes, and the eutectic Mg2Si has an obvious refining effect and is diffusely distributed with the dot/short-line shape in the alloy matrix. The best metamorphic effect is obtained by adding 0.6wt% Er and 0.8wt% Ba. The mechanical properties of the Mg-2.5Si-4Zn alloy modified by Er-Ba composite modifier are significantly improved, with the tensile strength σb and elongation δ increasing to 168 MPa and 5.04%, respectively.

Magnesium alloy has a broad development prospect in the fields of automobile production and aerospace because of its low density, high specific strength and good vibration dissipation, etc. However, the wide application of magnesium alloy has been limited due to the lack of alloy strength and thermal stability at high temperature and poor corrosion resistance [

1–3].

In the study of Mg alloying, it is found that the enhanced phase Mg2Si formed by element Si and Mg has high hardness, high melting point and high elastic modulus, which can significantly improve the hardness, wear resistance and high temperature creep resistance of magnesium alloy and refine the alloy matrix to a certain extent, showing better research and application prospects[

4–6].

In spite that the Mg-Al-Si alloy shows better creep resistance than the Mg-Al alloy without Si, its application is limited because the intermetallic Mg17Al12 phase in the alloy's microstructure has poor high-temperature stability[

7]. The Mg-Si-Zn alloy, completely free of Al, can not only avoid the generation of Mg17Al12 phase, but also further reduce the cost[8–9]. But the matrix of the as-cast Mg-Si alloy is cut by the primary and eutectic Mg2Si in the solidified microstructure because of their coarse and complex morphology, which seriously deteriorates the mechanical properties of the magnesium alloy [10]. Therefore, controlling the size and improving morphology of Mg2Si, which can significantly improve the mechanical properties of magnesium alloys, has become one of the hot topics in the research on magnesium alloys containing Si [11].

At present, researchers usually use hot extrusion, rapid solidification, and modification treatment[

12–16] to improve the microstructure of Si-containing Mg alloys, among which the addition of various trace elements (such as Sr[17], Sb[18–19], Ca[20], Er[21], Y[21–22]) to modify Mg2Si has been studied more and more. Srinivasan[18] and Yang[19] et al reported that Sb can transform Mg2Si from Chinese character shapes to fine poly-gonal shapes and the comprehensive mechanical properties of magnesium alloys are significantly improved. Song[20] et al found that Ca has good modification effect on eutectic Mg2Si in Mg-Si-Zn alloy, and makes eutectic Mg2Si diffuse and fine. Jiang[22] et al studied the modification effect of Y in Mg-5Si alloy and found that Y has good modification effect on both primary and eutectic Mg2Si.

Ba also has a good modified effect on Mg2Si[

23–24]. Chen[25] et al investigated the effect of Ba-Sb composite modification on Mg-5Si alloy, and the results showed that with the simul-taneous addition of Ba and Sb modifying agents, the refine-ment of primary Mg2Si is significant and the morphology changes from coarse dendrites to fine polygons, and the Ba2Sb compound is thought to be the heterogeneous nucleation core of primary Mg2Si.

For the Si-containing magnesium alloy, the addition of Er or Ba presents a good modification effect, but the study of Er and Ba composite modification in the hyper-eutectic Mg-Si alloy has been scarcely found to be carried out. In this study, for the Mg-2.5Si-4Zn alloy, the effects of Er-Ba composite modification on the microstructure and mechanical properties were investigated and the modification mechanism of two modifiers and their interaction were also revealed. The content of modifiers was optimized to improve the morphology and to refine the size of the primary phase and eutectic Mg2Si microstructure.

1 Experiment

1.1 Alloy composition design and alloy melting

In order to investigate the effect of Er and Ba elements on the microstructure and mechanical properties, in the experiment, the alloy composition was designed by the addition of different content of Er or Ba, based on the Mg-2.5Si-4Zn alloy, as shown in Table 1.

Table 1  Chemical composition of Mg-2.5Si-4Zn-xEr-yBa alloys (wt%)
Alloy No.SiZnErBaMg
1 2.5 4 0 0 Bal.
2 2.5 4 0.4 0 Bal.
3 2.5 4 0.6 0 Bal.
4 2.5 4 0.8 0 Bal.
5 2.5 4 1.0 0 Bal.
6 2.5 4 1.2 0 Bal.
7 2.5 4 0.6 0.5 Bal.
8 2.5 4 0.6 0.8 Bal.
9 2.5 4 0.6 1.0 Bal.
10 2.5 4 0.6 1.2 Bal.
11 2.5 4 0.6 1.5 Bal.
12 2.5 4 0.6 2.0 Bal.

The alloy specimens were prepared by melting and casting according to the alloy composition ratios shown in Table 1, using industrial pure magnesium (99.9wt%), pure zinc (99.9wt%), Mg-10Si master alloy, Mg-20Er master alloy and Mg-10Ba master alloy as raw materials. The melting and casting experiments were performed using the SG-7.5-12 atmospheric-well type resistance furnace equipped with a homemade device mixing shielding gas as follows.

1) When the resistance furnace was heated up to 500 °C and held, pure magnesium was added to the crucible in the furnace. At the same time, the mixture of CO2 and SF6 gas, with a volume ratio of 100:1, from a homemade gas mixing device, was passed into the crucible for protecting magnesium alloy at high temperatures. And then the resistance furnace was continuously heated.

2) When the furnace temperature was rasied to 750 °C for holding, the pure magnesium in the crucible was gradually melt. After the pure magnesium was completely melted for about 30 min, the weighed Mg-10Si alloy was added to the magnesium liquid in batches and stirring was carried out every 30 min until the Mg-10Si alloy was thoroughly melted to ensure the uniform composition of alloy melt.

3) After above, Zn and the modifiers of Er or Ba were added to the magnesium alloy melt; then the melt was stirred and held for 5‒10 min.

4) Finally, the slag on the surface of the melt was removed and the melt was refined using RJ-5 type refining agent. After 5 min, the melt was poured into a cylindrical graphite casting mold at 150 °C to obtain alloy specimens with dimensions of Φ50 mm×100 mm.

1.2 Analysis and test methods

Metallographic specimens were cut from the middle of the alloy ingot perpendicular to its axis direction, then ground and polished, and etched with 0.4vol% HNO3 alcohol solution, after which the specimen surface was cleaned with anhydrous ethanol.

The microstructure of alloy specimens was observed by metallographic microscopy (Olympus OM) and scanning electron microscopy (SEM), the micro-zone composition was measured and analyzed by energy dispersive spectroscopy (SEM-EDS) and phase discrimination and identification were conducted by X-ray diffractometer (XRD). The room-temperature properties were measured by RG3050 computer-aided electric-loaded tensile tester. The effect of Er, Ba on the modification mechanism of the primary Mg2Si and the eutectic Mg2Si in the Mg-2.5Si-4Zn alloy and the interaction of the compound modification were analyzed and revealed, and the addition of Er and Ba elements in the alloy was optimized according to the obtained results.

2 Results and Discussion

2.1 Effect of Er on solidification structure of Mg-2.5Si-4Zn alloy

Different amounts of Er were added into the Mg-2.5Si-4Zn magnesium alloy melt (0wt%, 0.4wt%, 0.6wt%, 0.8wt%, 1.0wt%, and 1.2wt%), and the obtained microstructure is shown in Fig.1. The average area of primary Mg2Si grains and eutectic Mg2Si with different Er contents is shown in Fig.2. It can be seen from Fig.1 that the metallographic microstructure of the Mg-2.5Si-4Zn alloy specimen is mainly composed of α-Mg, primary Mg2Si, eutectic Mg2Si and eutectic MgZn.

Fig.1  Effect of Er content on solidification microstructure of Mg-2.5Si-4Zn alloy: (a) 0wt%, (b) 0.4wt%, (c) 0.6wt%, (d) 0.8wt%, (e) 1.0wt%, and (f) 1.2wt%

Fig.2  Average area of primary Mg2Si grains and eutectic Mg2Si with different Er contents

As shown in Fig.1a‒1c and Fig.2, the size of the primary and eutectic Mg2Si gradually decreases and the morphology is improved with increasing the content of Er from 0wt% to 0.6wt%. The primary Mg2Si changes from dendrites with 30‒40 μm in length, 10‒20 μm in width and 420 μm2 in the average area to fine quadrilateral blocks with the diagonal length of 10‒30 μm and the average area of about 181 μm2, and the eutectic Mg2Si breaks down from a complex Chinese script shape with the average area of about 390 μm2 into a more complex short-rod shape with the average area of about 157 μm2.

When the addition of Er exceeds 0.6wt% at less than 1.2wt%, the size of primary and eutectic Mg2Si starts to increase, while the primary Mg2Si changes from a regular flat rectangular block to the irregular polygon shape with pores, and the eutectic Mg2Si regrows from the short rod-like microstructure to the more complex Chinese script shape. Therefore, addition of 0.6wt% Er element into the magnesium alloy has the best modification effect on the primary and eutectic Mg2Si.

Further, Fig.3 shows the statistical size distribution of the primary Mg2Si of the magnesium alloy added with 0.6wt% of the Er element, where the histogram shows the frequency of primary Mg2Si grain in each size interval, the red shows the fitted curve, and the blue is the cumulative frequency curve. As shown in Fig.3, the primary Mg2Si grain size is 11.93 μm, with an arithmetic mean of 12.61 μm.

Fig.3  Primary Mg2Si grain size statistics of Mg-2.5Si-4Zn-0.6Er alloy

The modification mechanism of Er element in Mg-2.5Si-4Zn alloy was analyzed by the SEM results (Fig.4) and XRD (Fig.5). Combining the EDS result with XRD patterns, it can be identified that the Mg2Si and MgZn in the alloy appear.

Fig.4  Analysis of modification mechanism of Er element in Mg-2.5Si-4Zn-0.6Er alloy: (a) SEM image; (b‒e) element distributions of Mg, Si, Zn and Er along the line marked in Fig.4a; (f) EDS result of point A; (g) EDS result of point B

Fig.5  XRD pattern of Mg-2.5Si-4Zn-1.2Er alloy

From Fig.4, it can be seen that the Er content is higher near the primary and eutectic Mg2Si. It can be deduced that during the solidification of the alloy melt, Er elements enrich on the surface of the primary Mg2Si and “occupy” the non-close-packed growth interface[

26], which makes the growing atoms of Mg2Si cannot effectively adhere to the atom position of the crystal surface, slowing down the rapid growth of the primary Mg2Si in the preferred growth direction, and turning the dendritic primary Mg2Si into small rectangular lumps.

Similarly, the element of Er can also accumulate at the growth front of eutectic Mg2Si when it grows and inhibits the anisotropic growth of eutectic Mg2Si. So, the eutectic Mg2Si with the complex multi-stroke Chinese script shape is transformed into the more complex short rod-like one.

But Er in the Mg-2.5Si-4Zn alloy melt may react with Mg or Si to form Mg-Er or Er-Si compounds when the adding amount of Er is higher than 0.6wt% due to the high chemical activity of the rare earth element Er and its low solid solubility in magnesium alloys. It is well-known that the smaller the electronegativity difference between the two elements, the easier to form a solid solution or a compound[

27]. The electronegativities of Mg, Si and Er are 1.31, 1.8 and 1.22, respectively. So, Er prefers to react with Si to form ErxSiy compounds[28] because the electronegativity difference between Er and Si (0.58) is much larger than that between Er and Mg (0.09). Combined with Fig.5, it can be identified that the compound between Er and Si is ErSi.

According to the classical theory of nucleation, increasing the concentration of Er in the alloy melt can induce enough constituent fluctuations to generate ErSi nuclei. Once the crystal nuclei appear in the melt, their rapid growth will consume a large number of Er and Si atoms, making the Er modification effect weakened. Therefore, when the addition of Er in the alloy melt exceeds the optimum content of 0.6wt%, an over-modified effect occurs as the Er content continues to increase, i.e. the size of primary Mg2Si phase increases and its morphology turns back to coarse dendrites, while the morphology of the eutectic Mg2Si tends back to complex Chinese script shape.

2.2 Effect of Ba addition on solidified microstructure of Mg-2.5Si-4Zn-0.6Er alloy

2.2.1 Solidified microstructure

On the basis of the optimal result from the previous experiments, the Er-Ba composite modification was performed with different addition amounts of Ba. Fig.6 shows the solidified microstructure of the Mg-2.5Si-4Zn-0.6Er alloy with different addition amounts of Ba. The eutectic Mg2Si in the microstructures of Mg-2.5Si-4Zn-0.6Er alloy is magnified, as shown in Fig.7. Fig.7a presents the details of eutectic Mg2Si at different magnifications in the microstructure of Mg-2.5Si-4Zn-0.6Er alloy and Fig.7b presents the details of eutectic Mg2Si after adding the element Ba to Mg-2.5Si-4Zn-0.6Er alloy. Fig.8 shows the average area of primary Mg2Si grains and eutectic Mg2Si with different Ba contents.

Fig.6  Microstructures of solidified Mg-2.5Si-4Zn-0.6Er alloy with different Ba contents: (a) 0.5wt%, (b) 0.8wt%, (c) 1.0wt%, (d) 1.2wt%, (e) 1.5wt%, and (f) 2.0wt%

Fig.7  Eutectic Mg2Si in the as-cast Mg-2.5Si-4Zn-0.6Er alloy without Ba addition (a) and after Ba addition (b)

Fig.8  Average area of primary Mg2Si grains and eutectic Mg2Si with different Ba contents

Combining Fig.6, Fig.1c and Fig.8, it can be seen that the primary Mg2Si is changed from a regular tetragonal block to an irregular polygonal block with holes and grooves and its size decreases a little with the increase in addition amount of Ba within 0.8wt%, and it gradually changes back to a coarse irregular dendritic shape and increases obviously in size when the Ba addition increases from 0.8wt%.

In addition, from Fig.6, the eutectic Mg2Si is changed from a more complex short rod shape to a fine dot-rod shape with the increase in Ba addition content, which can be seen more clearly from Fig.7, and the grain size gradually decreases, but not obviously.

Thus, when 0.6wt% of Er and 0.8wt% of Ba are added to the Mg-2.5Si-4Zn alloy, the modification effect of primary and eutectic Mg2Si is the best, as seen from Fig.6 and Fig.8, in which the primary Mg2Si becomes an irregular polygonal block shape with an average area of about 124 μm2 and the eutectic Mg2Si has a simple dot-rod shape with an average area of about 69 μm2.

Fig.9 shows the statistical frequency distribution of the primary Mg2Si grain size of the magnesium alloy, in which 0.6wt% of Er element and 0.8wt% of Ba element are added. The histogram shows the frequency of primary Mg2Si grain in each size interval, the red curve is fitted data, and the blue one is the cumulative frequency. The primary Mg2Si grain size is 9.49 μm, with an arithmetic mean of 10.64 μm.

Fig.9  Primary Mg2Si grain size statistics of Mg-2.5Si-4Zn-0.6Er-0.8Ba alloy

2.2.2 Er-Ba composite modification

In order to investigate the mechanism of composite modification of adding Er-Ba to the magnesium alloy, the SEM observation and XRD analysis of Mg-2.5Si-4Zn-0.6Er-1.2Ba alloy were carried out, as shown in Fig.10 and Fig.11. The EDS results of Fig.10b shows that the acicular phase marked in Fig.10a contain different contents of Ba, Mg and Si, indicating that the phase is Ba-Mg-Si compound. Further, the BaMg2Si2 phase is detected in the XRD pattern as shown in Fig.11. Combining the EDS detection result with XRD pattern, it can be inferred that the acicular compound is BaMg2Si2 in the alloy.

Fig.10  SEM image of Mg-2.5Si-4Zn-0.6Er-1.2Ba alloy (a) and EDS result of acicular phase (b)

Fig.11  XRD pattern of Mg-2.5Si-4Zn-0.6Er-1.2Ba alloy

The modification effect intensifies with the addition of the Ba modifier to the Mg-2.5Si-4Zn-0.6Er alloy, presumably because the generated BaMg2Si2 acts as a heterogeneous nucleation core for the primary Mg2Si, which increases the nucleation rate of the primary Mg2Si and suppresses the growth.

In order to confirm this conjecture, i.e. the BaMg2Si2 compounds becomes the cores for Mg2Si nucleation, the low-index plane matching the compounds with Mg2Si can be calculated according to the following Bramfitt's lattice mismatch equation[

29]:

δhklshkln=13∑i=13duvwsicosθ-duvwniduvwni×100% (1)

where (hkl) s is a low-index plane of the substrate, [uvw]s is a low-index direction in (hkl) s, d[uvw]s is the interatomic spacing along [uvw]s; (hkl) n is a low-index plane of the nucleated crystal, [uvw]n is a low-index direction in (hkl)n, d[uvw]n is the interatomic spacing along [uvw]n; θ is the angle between [uvw]s and [uvw]n (θ＜90°)[

30].

According to the Bramfitt's lattice mismatch equation [

29], only when the mismatch between the substrate and the crystalline phase is δ< 15%, it is possible to become a heterogeneous nucleation core of the crystalline phase. If the mismatch is δ< 6%, it can be a more effective heterogeneous nucleation core. The spatial lattice structure of Mg2Si and BaMg2Si2 are face-centered cubic and simple tetragonal, respectively. There are the same atomic arrangement and close atomic spacing between the low-index plane (001) of Mg2Si and the low-index plane (001) of BaMg2Si2, as shown in Fig.12. It is calculated out from the data in Fig.12 according the Bramfitt's lattice mismatch equation that the lattice mis-match of the two is 3.01%[31]. Therefore, the BaMg2Si2 compound can become a heterogeneous nucleation core of Mg2Si.

Fig.12  Schematic diagrams of lattice mismatch between BaMg2Si2 and Mg2Si: (a) atomic arrangement of BaMg2Si2 on (001) lattice plane, (b) atomic arrangement of Mg2Si on (001) lattice plane, and (c) matching relationship between atoms of BaMg2Si2 on (001) lattice plane and Mg2Si on (001) lattice plane

Further, in the experiment, from the line scanning results (Fig.13), it can be found that not only the elements of Mg and Si but also Ba appear near the centers of three crystal blocks of A, B and C (marked in Fig.13a) of Mg2Si (the specific positions pointed by the blue arrows in Fig.13b‒13f), by which it can be proved that BaMg2Si2 compound is positioned at the core of Mg2Si crystal.

Fig.13  SEM images (a) and EDS line scanning results of element Mg (b), Si (c), Zn (d), Er (e) and Ba (f) along marked line in Fig.13a for Mg-2.5Si-4Zn-0.6Er-1.2Ba alloy

Therefore, as shown in Fig.13, it is inferred from the EDS line scan results and the Er-Ba phase diagram that no compounds are generated between Er and Ba, and the composite modification mechanism of Er-Ba is as follows.

At the stage of initiating the solidification of the alloy melt after adding Er-Ba modifier, the BaMg2Si2 compound first formed in the melt acts as a nucleation substrate for the primary Mg2Si to promote the heterogenous nucleation of the primary Mg2Si. Because the precipitation of the BaMg2Si2 compound is fine and dispersed, the nucleation rate of the primary Mg2Si significantly increases, inhibiting the growth of the primary Mg2Si and obtaining grain refinement. At the same time, as the solidification proceeds, the excess unreacted Ba will be enriched at the front of Mg2Si crystals growing together with the element of Er and inhibit the anisotropic growth of Mg2Si crystals [

26], which plays another role in refining Mg2Si crystals.

So, the simultaneous addition of Er and Ba can play a synergistic role in modification of Mg2Si. The element Ba can generate fine and dispersed crystal nuclei of BaMg2Si2 in the Mg-2.5Si-4Zn alloy melt as the core of the incipient Mg2Si at the early stage of solidification of the alloy. In addition, the growth of primary and eutectic Mg2Si is inhibited due to the strong enrichment of both Er and Ba on the surface of Mg2Si crystal, which results in a better modification effect than the single modifier of Er.

In addition, when the Ba content is very high (larger than 0.8wt%), BaMg2Si2 crystal nucleus will undergo the growth under the enough composition condition and gradually grow into needle-like crystal. These needle-like BaMg2Si2 phases cannot become the heterogeneous nucleation core of the incipient Mg2Si. The amount of BaMg2Si2 in the alloy melt as the heterogeneous nucleation core of Mg2Si decreases, resulting in a weakened modification effect as an over-modified phenomenon.

2.3 Effect of Er and Ba modification on mechanical properties of Mg-2.5Si-4Zn alloy

The engineering stress-engineering strain curves of the Mg-2.5Si-4Zn-0.6Er alloys modified with different contents of Ba are shown in Fig.14. It can be seen that the Er-Ba composite modification has a significant effect on the tensile strength and elongation of the Mg-2.5Si-4Zn alloy. The room-temperature mechanical properties of Mg-2.5Si-4Zn-0.6Er alloys with different Ba contents are shown in Fig.15. It can be seen that when 0.8wt% of Ba is added into the Mg-2.5Si-4Zn-0.6Er alloy, its tensile strength σb and elongation δ reach the maximum of 168 MPa and 5.04%, respectively, and the tensile strength σb is increased by 14.3% compared with that of the Mg-2.5Si-4Zn alloy with 0.6wt% Er single modifier.

Fig.14  Stress-strain curves of Mg-2.5Si-4Zn-0.6Er alloys with different Ba content

Fig.15  Tensile strength σb and elongation δ of Mg-2.5Si-4Zn-0.6Er alloys with different Ba contents

Under the condition of Er-Ba composite modification, the morphology of the primary Mg2Si is effectively transformed into complex fine polygonal morphology with holes, while the size of eutectic Mg2Si is also significantly reduced with more uniform and dispersed distribution, which makes the matrix of the magnesium alloy be strengthened by the fine Mg2Si dispersion and the cutting effect of the coarse primary Mg2Si on the alloy matrix weakened or even eliminated. Therefore, the tensile strength of the alloy increases markedly. But when the Ba addition is larger than 0.8wt%, the tensile strength of the alloy is contrarily smaller than that of the alloy only modified by the Er element. According to the above explana-tion, it presumably results from the weakening modification and the matrix cutting by the needle-like BaMg2Si2 phase (as shown in Fig.6f) in the alloy with higher Ba content.

Fig.16 shows the SEM fracture morphology of the tensile specimens of the alloy, where Fig.16a‒16b show the tensile fracture of the specimen added with 0.8wt% Ba and Fig.16c‒16d shows the alloy added with 1.5wt% Ba.

Fig.16  Details of fracture morphologies of Mg-2.5Si-4Zn-0.6Er alloy with different adding contents of Ba: (a‒b) 0.8wt% and (c‒d) 1.5wt%

It can be seen from Fig.16a and 16b that the dark-grey smooth and flat area located in the white dotted ellipse (in Fig.16b) on the fracture surface of the specimen (called “dimple”) is the fracture morphology of the primary Mg2Si, and the lamellar bright area formed by tearing is α-Mg (marked in Fig.16b). The irregular smooth polyhedral or fine granular profile of the primary Mg2Si, which can be considered as a kind of intergranular fracture morphology, is obviously the main component of the fracture morphology. But in Fig.16d, there are some long tears inside the coarser primary Mg2Si phase located in the white dotted ellipse (in Fig.16d), indicating the transgranular fracture. Meanwhile, eutectic Mg2Si cannot be observed on the fracture surface, suggesting that it does not act as a crack source during the fracture of the tensile specimen.

During the tensile process, the stress concentration will occur at the primary Mg2Si due to its characteristics of high hardness and high elastic modulus. If there are some holes and grooves mentioned above on the surface of primary Mg2Si formed during the solidification of the alloy melt, they can be regarded as the defects of the primary Mg2Si. The coarser the primary Mg2Si, the more the above defects. When the stress concentrated on the defect is greater than the cracking condition of the alloy, the source of fracture at this location is the primary Mg2Si itself, that is, transgranular fracture will occur (as shown in Fig.16d). But when the shape of the primary Mg2Si is regular polygonal or the stress at the defect is not sufficient to cause the primary Mg2Si crystal to crack, the intergranular fracture will occur, only depending on a microcrack at the interface between the primary Mg2Si and α-Mg, as shown in Fig.16b. Compared with the fractures shown in Fig.16c and 16d, there are a lot of dimples formed by the intergranular facture, increasing the tensile strength.

3 Conclusions

1) With increasing the addition of Er from 0wt% to 0.6wt% to Mg-2.5Si-4Zn alloy, the primary Mg2Si in the alloy changes from coarse dendritic to regular tetragonal shape and the eutectic Mg2Si changes from Chinese script to more complex short-rod shape. Moreover, the primary and eutectic Mg2Si become small in size. But when the Er addition amount continues to increase to >0.6wt%, the size and morphology of the primary and eutectic Mg2Si change back gradually. The best effect of Er modification is achieved when 0.6wt% of Er is added to the magnesium alloy.

2) With increasing the addition content of Ba within 0.8wt%, the primary Mg2Si in the Mg-2.5Si-4Zn-0.6Er alloy is changed from a regular tetragonal block to an irregular polygonal block with holes and grooves and its size decreases a little, while the primary Mg2Si gradually changes back to a coarse irregular dendritic shape and increases obviously in size when the Ba addition increases to above 0.8wt%. More-over, the eutectic Mg2Si is changed from a more complex short rod shape to a fine dot-rod shape with the increase in Ba addition and its size gradually decreases.

3) The modification of primary and eutectic Mg2Si is the best when the 0.6wt% Er and 0.8wt% Ba are simultaneously added to the Mg-2.5Si-4Zn alloy, by which the primary Mg2Si becomes an irregular polygonal block with the average area of about 124 μm2 and the eutectic Mg2Si has a simple dot-rod shape with an average area about 69 μm2. The mechanical properties of the Mg-2.5Si-4Zn alloy are significantly improved after the Er-Ba composite modification and reach to the best values (tensile strength σb=168 MPa and elongation δ=5.04%, respectively) with the optimal adding contents of Er and Ba.

4) The simultaneous addition of Er and Ba can play a synergistic role in modification of Mg2Si. The fine and dispersed BaMg2Si2 generated in the Mg-2.5Si-4Zn alloy melt at the beginning of solidification acts as the heterogeneous nucleation core for the primary Mg2Si and the growth of primary and eutectic Mg2Si is inhibited due to the strong enrichment of both Er and Ba on the surface of Mg2Si crystal, which results in a better modification effect than the single modifier of Er.

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