Effect of calcium addition on microstructure, casting fluidity and mechanical properties of Mg-Zn-Ce-Zr magnesium alloy

JOURNAL OF RARE EARTHS, Vol. 35, No. 5, May 2017, P. 503

Effect of calcium addition on microstructure, casting fluidity and mechanical properties of Mg-Zn-Ce-Zr magnesium alloy FU Yu (付 玉), WANG Han (王 晗), LIU Xiaoteng (刘晓滕), HAO Hai (郝 海)* (Key Laboratory of Solidification Control and Digital Preparation Technology (Liaoning Province), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China) Received 23 August 2016; revised 9 November 2016

Abstract: The influence of Ca addition on the as-cast microstructure, casting fluidity and mechanical properties of the Mg-4.2Zn-1.7Ce-0.5Zr (wt.%) alloy was investigated. The results showed that the as-cast alloys consisted of α-Mg matrix, Ca-contained T-phase and Mg51Zn20 phase. Addition of 0.2 wt.%–0.6 wt.% Ca led to effective grain refinement and enhanced the fluidity of the alloys. When the content of Ca was 0.2 wt.%, the alloy exhibited the finest grain size of 35.9 μm, and the filling length was increased by approximately 55.4% compared with the quaternary alloy. The improvement of the fluidity was attributed to the grain refinement, less energy dissipation and the oxidation resistance of Ce and Ca. With an increase in Ca content, the yield strength increased gradually, whereas the ultimate tensile strength and elongation showed a decreasing tendency. Moreover, the fracture surface mode was quasi-cleavage fracture. Keywords: Mg-Zn-Ce-Zr alloy; Ca; microstructure; casting fluidity; mechanical properties; rare earths

The compelling need for weight reduction of structural elements, particularly in the aerospace and automotive industries is associated with reducing energy consumption policies, has motivated the development of lightweight thin-wall castings over the last years[1,2]. Because of the advantages such as low density, high specific strength and stiffness, and good machinability, magnesium alloys are widely used in thin-wall castings for aero engine industry[3,4]. Despite the above mentioned advantages, the manufacturing of large magnesium thin-wall castings is still challenged by mold filling. The fluidity depends not only on the alloy compositions, solidification type, mold temperature, but also on the grain size and inclusion[5]. Rapid cooling of thin-wall sections of the casting decreases the fluidity of molten metal, resulting in some casting defects, such as misrun, shrinkage and cold shut[6]. Therefore, good fluidity of magnesium alloys is a prerequisite for obtaining high-quality thinwall castings and the improvement of fluidity is a challenging work. During the past decade, Mg-Zn-RE-Zr alloys have garnered increasing attention due to their relatively good strength and heat resistance. Addition of the rare earth (RE) elements is an effective way to enhance the strength, castability and oxidation resistance of magnesium alloys[7–9]. Wang et al.[10] suggested that the optimal mechanical properties were obtained by the addition of 1.25

wt.% RE to Mg-4.2Zn-xRE-Zr alloy. Wang et al.[11] reported that adding 3 wt.% Nd to Mg-4.5Zn-1Y-3Nd-0.5Zr alloy could refine grains and improve yield strength due to second-phase strengthening and grain boundary strengthening. However, the study which focuses on the effects of alloying elements on the fluidity of Mg-Zn-RE-Zr alloys is limited. Yang et al.[12] reported that adding 1.0 wt.% Sn to ZA84 alloy improved casting fluidity. Previous work indicated that alloys solidified with an equiaxed dendrite structure exhibited a relationship with dendrite coherency and fluidity, and adding grain-refining agents to the alloys contributed to improved fluidity[5]. Light rare earth Ce and alkaline earth Ca are both useful alloying elements for possessing high grain refining efficiency and favorable flame retardancy[13,14]. Recently, it was reported that Ca, Ce and Y were effective to improve the oxidation resistance of magnesium alloys due to the compacted oxide films[15]. Furthermore, Wang et al.[16] studied the effect of RE on the fluidity of AZ91 alloys with different section thickness. Therefore, it is reasonable to expect that Ca addition possibly plays a beneficial role in the fluidity for Mg-Zn-Ce-Zr alloys. This present study aimed to investigate the effect of Ca addition on the as-cast microstructure, fluidity and mechanical properties of the Mg-4.2Zn-1.7Ce-0.5Zr alloy. It was hoped that such an investigation would contribute to

Foundation item: Project supported by the National Key Research and Development Program of China (2016YFB0701204) and the Fundamental Research Funds for the Central Universities (DUT15JJ (G) 01) * Corresponding author: HAO Hai (E-mail: [email protected]; Tel.: +86-411-84709458) DOI: 10.1016/S1002-0721(17)60940-2

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the preparation of thin-wall magnesium alloy castings for practical applications.

1 Experimental Generally, fluidity is empirically defined as the ability for a molten metal to flow through and fill a standard mould channel before solidification occurs. Thus it governs the filling of moulds and the sharpness of cast details[17]. Fig. 1 shows the schematic diagram of the fluidity test apparatus used in this work. The size of the cross section of the Archimedian spiral is 5 mm high, 3 and 5 mm wide at top and bottom, respectively. The spiral gives a maximum running length of 750 mm. Small pips on the pattern at a regular spacing of approximately 25 mm along the centerline of the channel assist in the measurement of filling length[18]. The fluidity was evaluated by measuring the running length of the casting after solidification. Each test was performed three times for accuracy. The Mg-4.2Zn-1.7Ce-0.5Zr (all the compositions hereinafter were in mass fraction, % unless otherwise specified) alloys with different Ca additions were processed under the conventional casting condition using Mg ingot, Zn ingot, Mg-20Ca master alloy, Mg-30Zr master alloy and Ce-rich mixed rare-earth with commercial purity. The alloys were melted in an electrical resistance furnace using a boron nitride coated mild steel crucible under the mixed protection gas atmosphere of SF6 (0.3 vol.%) and CO2 (Bal.). The molten alloy was stirred and hold at 750 ºC for 20 min to ensure the alloying elements completely dissolved and diffused. When the furnace temperature decreased to 730 ºC, the melt was poured into a spiral steel mould and cylindrical ingots of 50 mm diameter and 130 mm height. The moulds were preheated to 300 ºC. The chemical compositions of the ex-

perimental alloys determined by inductively coupled plasma (ICP) analysis are listed in Table 1. The microstructures of the alloys were observed using an optical microscopy (OM). The samples were etched by a solution of 5 mL H2O, 2.1 g picric acid, 5 mL acetic acid, and 35 mL ethanol. The average grain sizes (d) were determined by analyzing the optical micrographs with the mean linear intercept method, where d=1.74 L; and L is the linear intercept length. The phases were analyzed by X-ray diffraction (XRD, Empyrean) with Cu Kα radiation. The microstructures and compositions of different phases in the alloys were analyzed by a scanning electron microscope (SEM, SUPARR 55) equipped with an energy-dispersive spectroscopy (EDS) analysis system. Tensile tests were performed with a strain rate of 2 mm/min at room temperature. The tensile specimens were machined out of the bars with a gauge dimension of 30 and 6 mm in diameter. Three specimens were used for test conditions to ensure the reproducibility of data. The schematic illustration of the experimental procedure is shown in Fig. 2.

2 Results and discussion 2.1 Microstructure Fig. 3 shows the XRD results of the as-cast Mg-4.2ZnTable 1 Actual chemical compositions of the experimental alloys (wt.%) Alloys

Experimental alloys

a

Mg-4.2Zn-1.7Ce-0.5Zr

b

Mg-4.2Zn-1.7Ce-0.5Zr-0.2Ca

c

Mg-4.2Zn-1.7Ce-0.5Zr-0.4Ca

4.24 1.43 0.43 0.46 Bal.

d

Mg-4.2Zn-1.7Ce-0.5Zr-0.6Ca

4.34 1.44 0.37 0.64 Bal.

Fig. 1 Schematic diagram of the fluidity test apparatus (a) Fluidity test mould; (b) A typical fluidity spiral; (c) Cross section of the spiral

Fig. 2 Schematic illustration of the experimental procedure

Zn

Ce

Zr

4.01 1.32 0.45

Ca

Mg

–

Bal.

4.45 1.38 0.41 0.25 Bal.

FU Yu et al., Effect of calcium addition on microstructure, casting fluidity and mechanical properties of Mg-Zn-Ce-Zr … 505

Fig. 3 XRD patterns of the as-cast Mg-4.2Zn-1.7Ce-0.5Zr-xCa alloys (1) x=0; (2) x=0.2 wt.%; (3) x=0.4 wt.%; (4) x=0.6 wt.%

1.7Ce-0.5Zr-xCa alloys. As seen in Fig. 3, the as-cast alloys are mainly composed of α-Mg phase, T-phase and Mg51Zn20 phase. T-phase which is rich in Zn and RE forms interdendritically during solidification of the alloy. It is identified with a C-centred orthorhombic crystal structure by Wei et al.[19]. In addition, a small quantity of Ca addition to the quaternary alloy does not result in the formation of a new phase, but mainly dissolves in the α-Mg matrix and eutectic compounds. Fig. 4 shows the optical micrographs of the as-cast Mg-4.2Zn-1.7Ce-0.5Zr-xCa alloys. They all consist of equiaxed α-Mg matrix and eutectic compounds. The average grain size of as-cast alloys is shown in Table 2. It is found that the average grain size of α-Mg phase de-

creases with increasing Ca concentration up to 0.2 wt.%. Above this level, the grain size increases gradually again. It can be indicated that addition of Ca results in effective grain refinement in the quaternary alloy and 0.2 wt.% Ca-contained alloy has the finest grain size of 35.9 μm. Ca has a high growth restriction factor (GRF), which is quantified by Q-value. In a binary system, Q=mC0 (k–1), where m is the slope of the liquidus line, C0 is the initial composition of the alloy, and k is the equilibrium partition coefficient for element[20]. Due to the presence of Ca solute (QCa=11.94[21]), the grain size of the quaternary alloy is effectively refined. The low and high magnification SEM images of the as-cast Mg-4.2Zn-1.7Ce-0.5Zr-xCa alloys are presented in Fig. 5 and the corresponding EDS results are listed in Table 3. As seen in Fig. 5, the microstructure consists of the primary α-Mg phase and eutectic compounds at the grain boundaries with inconsecutive reticular structure distribution. With increasing Ca concentration from 0.2 wt.% to 0.6 wt.%, the eutectic phases tend to exhibit from quasi-continuous net to relatively continuous shapes, and the amount of eutectic compounds increases. In addition, it is found from Fig. 5(c) and (d) that as the Ca content increases from 0.4 wt.% to 0.6 wt.%, a small quantity of globular phases exist inside α-Mg matrix. Table 2 Average grain size of the as-cast experimental alloys (μm) Alloys No.

a

b

c

d

Average grain size

61.8

35.9

41.4

52.6

Fig. 4 Optical microstructure of the Mg-4.2Zn-1.7Ce-0.5Zr-xCa alloys (a) x=0; (b) x=0.2 wt.%; (c) x=0.4 wt.%; (d) x=0.6 wt.%

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Fig. 5 SEM micrographs of the Mg-4.2Zn-1.7Ce-0.5Zr-xCa alloys (a) x=0; (b) x=0.2 wt.%; (c) x=0.4 wt.%; (d) x=0.6 wt.% Table 3 EDS results of corresponding positions in Fig. 5 (b), (c) and (d) (at.%) Position

Mg

Zn

RE

Zr

Ca

Point A

98.7

1.0

–

0.2

0.1

Point B

70.0

21.5

8.0

–

0.5

Point C

74.7

21.3

3.5

–

0.5

Point D

69.0

26.5

4.5

–

–

Point E

61.5

–

–

1.7

36.8

The EDS analysis on the α-Mg matrix (marked by point A) reveals that Zn, Zr and Ca solutionized in it. The EDS results of point B, C and D shown in Fig. 5 indicate that the compositions of the eutectic phases at grain boundary triple junctions are Mg-21.5Zn-8.0RE- 0.5Ca, Mg-21.3Zn-3.5RE-0.5Ca and Mg-26.5Zn-4.5RE (at.%), respectively, and RE contains Ce and trace amount of La which is brought from the raw material of mischmetal alloy. The compounds exhibit a pronounced enrichment of Mg, Zn and RE elements, whereas a small amount of Ca element is also detected in the compounds. With increasing Ca addition, the Ca content of eutectic phases increases. Moreover, Ca possesses high GRF owing to the strong segregation power, thus Ca addition into Mg alloys generates a constitutional supercooled area in front of the solid-liquid interface, resulting in the improvement of Ca concentration along the grain boundaries. Particle marked as E is observed inside α-Mg matrix from SEM image in Fig. 5(d). Its composition is determined to be Mg-1.7Zr-36.8Ca (at.%) by EDS analysis. An enlarged SEM image and EDS composition maps

of the intermetallic compounds in the as-cast Mg-4.2Zn1.7Ce-0.5Zr-0.2Ca (wt.%) alloy are shown in Fig. 6. As depicted in Fig. 6, Mg distributes uniformly in the matrix, while Zn and Ce are rich in grain boundaries. In addition, Zr and Ca distribute at both the grain boundaries and grain interiors. It confirms the presence of the Ca-contained T-phase in the alloy, which is consistent with XRD and EDS results. 2.2 Casting fluidity Fig. 7 shows the effect of Ca content on the fluidity of Mg-4.2Zn-1.7Ce-0.5Zr-xCa alloys. The result indicates that the addition of 0.2 wt.%–0.6 wt.% Ca into the quaternary alloy leads to an increase in filling length. The addition of 0.2 wt.%, 0.4 wt.% and 0.6 wt.% Ca into the quaternary alloy causes 55.4%, 10.9% and 15.8% increase in filling length, respectively. The grain-refined alloy with 0.2 wt.% Ca has a larger filling length than other alloys. Consequently, it is preliminarily conjectured that grain refinement has a significant influence on the fluidity of the alloys. Fig. 8 clearly shows the schematic diagram of flow stoppage in the fluidity test. At an early stage of solidification, as shown in Fig. 8(a), the dendrites nucleate at the mold walls, and grow towards the channel centre. With increasing content of solute, as shown in Fig. 8(b), the constitutional supercooling ahead of the growing dendrites increases, thus the equiaxed dendrites which consist of dendrite fragments or form on substrate particles in the melt survive in the melt flow. The equiaxed

FU Yu et al., Effect of calcium addition on microstructure, casting fluidity and mechanical properties of Mg-Zn-Ce-Zr … 507

Fig. 6 SEM image showing compound particles and EDS maps of the elements Mg, Zn, Ce, Zr and Ca in the Mg-4.2Zn-1.7Ce-0.5Zr0.2Ca (wt.%) alloy

Fig. 7 Effect of Ca content on the fluidity of the Mg-4.2Zn1.7Ce-0.5Zr-xCa alloys

dendrites are transported by the flow tip. Then when the amount of solid in the melt reaches a critical fraction solid or exceeds the dendrite coherency point, as shown in Fig. 8(c), the dendrites at the tip start to interlock and form an interdendritic network, making the flow stop[22]. Finally, as shown in Fig. 8(d), the solidification is completed. The results show that the excellent fluidity corresponds to the finest grain size of the alloy. A likely explanation of the tendency is the relationship between grain refinement and dendrite coherency point. Dendrite

coherency point is the solid fraction in which a continuous three-dimensional dendrite network forms in the melt[23]. Grain refining increases the solid fraction where dendrite coherency occurs, leading to a beneficial impact on the melt flow[22,23]. Therefore, grain-refined alloys possess a favorable fluidity. Another factor which is also interpreted as a reason is the influence of dendrite network breakage. Breaking the dendrite network leads to the energy dissipation, thus reducing the driving force for the melt transport[23]. Since the structure becomes more globular with increasing Ca content, the possibility of dendrite fragmentation decreases. As a consequence, there is sufficient energy available for the melt flow. In addition, during pouring and filling of the mould, oxidation of the metal will take place and oxide inclusions will be mixed with the melt flowing. Due to the oxidation resistance of Ce and Ca, a compact oxide film is formed on the melt surface during melting and casting[24], which inhibits the formation of oxide inclusions in the melt, leading to an increased fluidity of the alloys[25]. However, Dahle et al.[23] reported that the strength of large oxide film enclosing the tip of the flow is an adverse factor that may restrict the movement of the melt. Therefore, the filling length is reduced with increasing Ca content above 0.2 wt.% and the alloy refined with 0.2 wt.% Ca has the optimum fluidity among all the alloys.

Fig. 8 Schematic diagram of flow stoppage in the fluidity test[22]

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2.3 Mechanical properties The results of tensile properties of as-cast experimental alloys at various Ca contents are shown in Fig. 9. The related tensile properties including ultimate tensile strength (UTS), yield strength (YS), elongation (El.) of the as-cast alloys are listed in Table 4. With increasing Ca content, the amount of eutectic compounds distributed along the boundaries increases (Fig. 5). In addition, the orthorhombic crystal Ca-contained T-phase is incoherent with the hcp α-Mg matrix. It leads to severe stress concentration at the interface between α-Mg matrix and eutectic phases, and micro-cracks will initiate in these regions. Consequently, ultimate tensile strength and elongation of the alloys show a decreasing tendency. However, with the increase in Ca content, it is inferred that the major contribution of the enhanced YS results from grain refinement on the basis of microstruc-

Table 4 Room temperature mechanical properties of the magnesium alloys Alloy No.

UTS/MPa

YS/MPa

El./%

a

141.4

82.1

5.1

b

126.5

85.6

4.8

c

122.7

87.3

3.4

d

118.2

90.5

2.7

ture observation (Fig. 4). Fig. 10 shows the SEM images of tensile fractograph morphology for the as-cast alloys at room temperature. It can be seen that a number of cleavage planes and cleavage steps are present and some tear ridges can be observed in the tensile fracture surfaces of the alloys. It is indicated that all the tensile fracture surfaces of the alloys have the characteristic of quasi-cleavage fracture. During plastic deformation, the stress concentration at the interface between α-Mg matrix and eutectic phases causes initiation of micro-cracks. The micro-cracks propagate quickly when the deformation intensifies, leading to intercrystalline rupture. Therefore, there is a resulting low elongation and concomitant low ultimate tensile strength for the Mg-4.2Zn-1.7Ce-0.5Zr-xCa alloys.

3 Conclusions

Fig. 9 Mechanical properties of as-cast experimental alloys at different Ca contents

In this paper, the effects of Ca addition on the as-cast microstructure, fluidity and mechanical properties of the Mg-4.2Zn-1.7Ce-0.5Zr alloy were investigated. The conclusions were as follows: (1) Addition of 0.2-0.6 wt.% Ca led to effective

Fig. 10 SEM fractograph morphology of the as-cast Mg-4.2Zn-1.7Ce-0.5Zr-xCa alloys (a) x=0; (b) x=0.2 wt.%; (c) x=0.4 wt.%; (d) x=0.6 wt.%

FU Yu et al., Effect of calcium addition on microstructure, casting fluidity and mechanical properties of Mg-Zn-Ce-Zr … 509

grain refinement in the Mg-4.2Zn-1.7Ce-0.5Zr-xCa alloys. The alloy containing 0.2 wt.% Ca showed the finest grain size of 35.9 μm. Moreover, the distribution of the eutectic compounds transformed from quasi-continuous net to continuous shapes with increasing Ca content. The existence of Ca-contained T-phase was confirmed by both XRD and EDS. (2) Adding 0.2 wt.%–0.6 wt.% Ca to the Mg-4.2Zn1.7Ce-0.5Zr alloy could improve the fluidity of the alloys. Among the Ca-contained alloys, the Mg-4.2Zn-1.7Ce0.8Zr-0.2Ca alloy exhibited the optimum casting fluidity, which was attributed to the favorable influence of grain refinement on dendrite coherency point as well as the energy dissipation of dendrite network breakage. In addition, the anti-oxidation of Ca and Ce embodied the competing effect on the fluidity of the alloys. (3) With increasing Ca content from 0.2 wt.% to 0.6 wt.%, the yield strength increased gradually, whereas the ultimate tensile strength and elongation of Ca-contained alloys decreased slightly. The addition of Ca to the alloy resulted in cleavage planes and tear ridges, which revealed quasi-cleavage fracture feature.

References: [1] Tresa M P. Weight loss with magnesium alloys. Science, 2010, 328: 986. [2] Liu L Z, Chen X H, Pan F S, Wang Z W, Liu W, Cao P, Yan T, Xu X Y. Effect of Y and Ce additions on microstructure and mechanical properties of Mg-Zn-Zr alloys. Mater. Sci. Eng. A, 2015, 644: 247. [3] Hirai K, Somekawa H, Takigawa Y, Higashi K. Effects of Ca and Sr addition on mechanical properties of a cast AZ91 magnesium alloy at room and elevated temperature. Mater. Sci. Eng. A, 2005, 403(1-2): 276. [4] Zhang G D, Li X G, Ma M L, Li Y J, Shi G L, Yuan J W, Zhang K. Homogenization heat treatment of Mg-7.68Gd4.88Y-1.32Nd-0.63Al-0.05Zr alloy. J. Rare Earths, 2014, 32(5): 445. [5] Kwon Y D, Lee Z H. The effect of grain refining and oxide inclusion on the fluidity of Al-4.5Cu-0.6Mn and A356 alloys. Mater. Sci. Eng. A, 2003, 360: 372. [6] Ravi K R, Pillai R M, Amaranathan K R, Pai B C, Chakraborty M. Fluidity of aluminum alloys and composites: A review. J. Alloys Compd., 2008, 456: 201. [7] Fang X G, Lü S L, Zhao L, Wang J, Liu L F, Wu S S. Microstructure and mechanical properties of a novel Mg-RE-Zn-Y alloy fabricated by rheo-squeeze casting. Mater. Des., 2016, 94: 353. [8] Zhang Z Q, Liu X, Wang Z K, Le Q C, Hu W Y, Bao L, Cui J Z. Effects of phase composition and content on the microstructures and mechanical properties of high strength Mg-Y-Zn-Zr alloys. Mater. Des., 2015, 88: 915. [9] Yang W P, Guo X F, Lu Z X. TEM microstructure of rap-

idly solidified Mg-6Zn-1Y-1Ce alloy. Trans. Nonferrous Met. Soc. China, 2012, 22(4): 786. [10] Wang Y D, Wu G H, Liu W C, Pang S, Zhang Y, Ding W J. Effects of chemical composition on the microstructure and mechanical properties of gravity cast Mg-xZn-yRE-Zr alloy. Mater. Sci. Eng. A, 2014, 594: 52. [11] Wang J, Liu R D, Dong X G, Yang Y S. Microstructure and mechanical properties of Mg-Zn-Y-Nd-Zr alloys. J. Rare Earths, 2013, 31(6): 616. [12] Yang M B, Pan F S. Effects of Sn addition on as-cast microstructure, mechanical properties and casting fluidity of ZA84 magnesium alloy. Mater. Des., 2010, 31: 68. [13] Yang M B, Zhang J, Guo T Z. Effects of Ca addition on as-cast microstructure and mechanical properties of Mg-3Ce-1.2Mn-1Zn (wt.%) magnesium alloy. Mater. Des., 2013, 52: 274. [14] Cai H S, Guo F, Ren X S, Su J, Chen B D. Effects of cerium on as-cast microstructure of AZ91 magnesium alloy under different solidification rates. J. Rare Earths, 2016, 34(7): 736. [15] Zhou N, Zhang Z Y, Dong J, Jin L, Ding W J. Selective oxidation behavior of an ignition-proof Mg-Y-Ca-Ce alloy. J. Rare Earths, 2013, 3(10): 1003. [16] Wang Q D, Lu Y Z, Zeng X Q, Ding W J, Zhu Y P, Li Q H, Lan J. Study on the fluidity of AZ91+xRE magnesium alloy. Mater. Sci. Eng. A, 1999, 271: 109. [17] Sin S L, Dubé D. Influence of process parameters on fluidity of investment-cast AZ91D magnesium alloy. Mater. Sci. Eng. A, 2004, 386: 34. [18] Campbell J. Castings. Oxford: Butterworth-Heinemann Press, 1991. [19] Wei L Y. Crystal symmetry of the pseudo-ternary Tphases in Mg-Zn-rare earth alloys. J. Mater. Sci. Lett., 1996, 15: 4. [20] Ali Y H, Qiu D, Jiang B, Pan F S, Zhang M X. The influence of CaO addition on grain refinement of cast magnesium alloys. Scr. Mater., 2016, 114: 103. [21] Ali Y H, Qiu D, Bin Jiang, Pan F S, Zhang M X. Current research progress in grain refinement of cast magnesium alloys: A review article. J. Alloys Compd., 2015, 619: 639. [22] Dahle A K, Arnberg L. The Effect of grain refinement on the fluidity of aluminium alloys. Mater. Sci. Forum, 1996, 217: 259. [23] Dahle A K, TΦndel P A, Paradies C J, Arnberg L. Effect of grain refinement on the Al-Si foundry alloys. Metall. Mater. Trans. A, 1996, 27A: 2305. [24] Fan J F, Yang G C, Cheng S L, Xie H, Hao W X, Wang M, Zhou Y H. Surface oxidation behavior of Mg-Y-Ce alloys at high temperature. Metall. Mater. Trans. A, 2005, 36A: 235. [25] Hu H, Luo A. Inclusions in molten magnesium and potential assessment techniques. JOM, 1996, 48(10): 47. [26] Jun J H, Park B K, Kim J M, Kim K T, Jung W J. Effects of Ca addition on microstructure and mechanical properties of Mg-RE-Zn casting alloy. Mater. Sci. Forum, 2005, 488: 107.