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Materials Science and Engineering A 456 (2007) 364–367

Influence of rapid solidification on the mechanicalproperties of Mg-Zn-Ce-Ag magnesium alloy

J. Cai a,b, G.C. Ma c, Z. Liu d, H.F. Zhang a,∗, A.M. Wang a, Z.Q. Hu a

a Shenyang National Laboratory for Materials Science, Institute of Metal Research,Chinese Academy of Sciences, Shenyang 110016, China

b Graduate School of Chinese Academy of Sciences, Beijing 100039, Chinac Shenyang R&D Center for Advanced Materials, Institute of Metal Research,

Chinese Academy of Sciences, Shenyang 110016, Chinad Department of Materials Science and Engineering, Shenyang University of Technology,

Shenyang 110023, China

Received 19 September 2006; received in revised form 26 November 2006; accepted 27 November 2006

bstract

This paper presents the differences in microstructure and mechanical properties between the conventional casting and rapidly solidified Mg-Zn-

e-Ag alloy. The experimental results showed that the mechanical properties of rapidly solidified alloy are enhanced, which can be attributed to

he changes of the microstructure significantly. The grain was refined and homogeneously distributed �-Mg12Ce phase was obtained. High coolingate refined the microstructure and changed the morphology of divorce eutectic phase. The mechanical properties of rapidly solidified alloy areigh and variable. Microporosity appeared to degrade affect the mechanical properties of magnesium alloys. 2006 Elsevier B.V. All rights reserved.

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eywords: Magnesium alloy; Mechanical properties; Microporosity; Microstru

. Introduction

Magnesium and its alloys are among the lightest metallictructural materials (ρ = 1.74 g/cm3) and especially attractive fortructural automotive applications where weight saving and con-equent increase in fuel efficiency are important [1–4]. However,nly little research has been carried out on magnesium alloy asompared with aluminum alloy, and its applications are alsoimited [5]. Principal reasons for this situation are the relativelyoor mechanical properties of magnesium alloy at room andlevated temperatures [6]. Therefore, developing magnesiumlloys with excellent mechanical properties are crucial for itspplication.

Recently, it has been reported that the addition of Zr increaseshe strength and ductility of Mg-Zn-Ce alloy, due to the effect

f grain refinement [7]. In current research, rapid solidificationas carried out on Mg-Zn-Ce-Ag alloys. The results indicate

hat high cooling rate has strong effects on both microstruc-

∗ Corresponding author. Fax: +86 24 23971783.E-mail address: hfzhang@imr.ac.cn (H.F. Zhang).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.11.140

ure evolution and mechanical properties of magnesiumlloy.

. Materials and experimental procedure

Nominal chemical composition of the studied alloy is listedn Table 1. For conventional casting (CC) method, the alloy waselted in a graphite crucible by induction in Ar atmosphere.elts were stirred for about 5–10 min and air cooled. For rapidly

olidified (RS) method, the alloy was remelted in quartz tube andnjected into a cylinder-shaped copper mould with a cavity ofmm diameter and 60 mm length under Ar atmosphere.

Alloys were etched by 5% Nital solution. Microstructure wasbserved using an optical microscope (OM, LEICA MEF-4)nd scanning electron microscope (Hitachi S-3400N) with anccelerating voltage of 25 kV. Phase analysis was preformedsing X-ray diffractometer with a resource of Cu K� radiation.icrohardness was measured using Vickers pyramid indentor

ith the load of 100 g. Compression tests were carried outsing a MTS810 Material Testing System at a strain rate of.5 × 10−4 s−1. The compressive specimens were 4 mm in diam-ter and 6.5 mm in length.

J. Cai et al. / Materials Science and Engineering A 456 (2007) 364–367 365

Table 1Nominal chemical composition of Mg-Zn-Ce-Ag

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Zn Ce Ag Mg

t% 1 2 1 96

. Results and discussion

.1. Microstructure

XRD spectrum (Fig. 1) shows that the main phases in Mg-Zn-e-Ag alloy are �-Mg and �-Mg12Ce. It can be clearly seen that

he peaks of rapid solidification samples are broader than that ofhe CC samples. The effect can be attributed to the grain refine-

ent. One finds that the peaks of Mg12Ce are different betweenC and RS alloys. Solid solution of Zn and Ag in Mg12Ce phaseay lead to the change of lattice constant. Although the lattice

onstant is different between the phase of CC and RS alloys,hey belong to the same phase.

Fig. 2 shows the change of microstructure with increasingooling rate. Fig. 2a shows the typical microstructure of CClloy. The black dendrite phase is primary Mg. Bulk eutecticMg + Mg12Ce) phase exists in the grain boundary in the formf network. CC alloy has a typical dendrite structure. Well devel-ped primary �-Mg dendrites with secondary dendrite showingixfold symmetry are clearly visible. Comparing Fig. 2a withig. 2b, it can be inferred that high cooling rates lead to theorphological change of primary phase from well developed

endrite to refined rosette-shaped dendrite. In addition, Mg12Cehase also becomes finer and its distribution is more uniformhan that for conventional casting alloy. The grain size, volumeraction of eutectic structure, and secondary dendrites arm spac-ng (SDAS) of the two alloys are summarized in Table 2. Table 2ndicates that with the increase of cooling rate, the grain size and

DAS decrease accordingly but the volume fraction of the sec-ndary phase increases. Both CC and RS alloy have a very fineutectic structure which can be resolved at low magnificationFig. 2). The eutectics are shown more clearly at higher magni-

Fig. 1. X-ray diffraction patterns: (a) CC alloy and (b) RS alloy.

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Fig. 2. Microstructure of the alloys: (a) CC alloy and (b) RS alloy.

cation (Fig. 3) with SEM. These two eutectics differ in shapend distribution. In CC alloy, the morphology of divorced eutec-ic phase is characterized by ‘islands’ of eutectic �-Mg within

g12Ce, but bulks of �-Mg are outside of Mg12Ce particle. It ispartially divorced eutectic morphology (Fig. 3a). In RS alloy,fully divorced eutectic structure (Fig. 3b) is that two eutectichases are completely separated.

.2. Mechanical properties

The typical stress-strain curves of Mg-Zn-Ce-Ag alloy with

ifferent casting method are shown in Fig. 4. The results of theompression tests are summarized in Table 3 [7,8]. The com-ressive strength of RS alloy is higher than that of CC alloy. Itmproved to values as high as 480 MPa.

able 2omparison of microstruture between CC and RS alloy

Grain size SDAS Volum fraction ofeutectic regions

C alloy 500 �m–2 mm 45–65 �m 0.373S alloy 10–40 �m About 6 �m 0.690

366 J. Cai et al. / Materials Science and Engineering A 456 (2007) 364–367

σ

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dtaRobserved on RS samples should attribute to grain refinementand homogeneous distribution of secondary phase [12,13]. Thehigher strength is also related to other factors, such as thedecrease of SDAS, the increase of volume fraction of eutectics

TM

CRMM(M(

Fig. 3. SEM microstructure of the alloys: (a) CC alloy and (b) RS alloy.

As following is a classical Hall-Petch formula:

0.2 = σ0 + kd−1/2

here d is the grain size in �m. It is widely accepted thathe microstructure of casting can be refined by increasinghe number of potential nuclei in the melt and the thermalnd constitutional under cooling at the advanced solid/liquid

nterface [9,10]. High cooling rate can produce more nucleirequency in the melt, therefore leading to a refined microstruc-ure. Grain refinement is one of the important practices tomprove the properties of casting [11]. It is essential and fun-

able 3echanical properties of CC alloy and RS alloy at ambient temperature

Compressive strength (MPa) Yield stren

C alloy 270 150S alloy 480 250g-1.5%Ce-0.6%Zr (Ref. [8]) 297.6g-2.8%Ce-0.7%Zn-07%Zr

Ref. [7]) as-castg-2.8%Ce-0.7%Zn-07%Zr

Ref. [7]) extrusion

Fig. 4. Strain-stress curves of CC alloy and RS alloy.

amental approach since grain size significantly influenceshe mechanical properties of the casting. Grain refinementccounts for the mechanical properties difference between theS alloy and CC alloy. The increase in mechanical properties

Fig. 5. Strain-stress curves of four RS samples.

gth (MPa) Elongation (%) Hv (MPa) Tesile strength (MPa)

10 72710 923

124.8

257.8

J. Cai et al. / Materials Science and Eng

Fig. 6. Optical micrograph of RS alloy before etching.

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ig. 7. SEM fractograph of the fracture surface of compressive test specimenepicting porosity.

nd the difference of texture tropism. Fine Mg12Ce precipi-ates homogeneously distributes along grain boundaries (G.B.).hese particles would prevent G.B. from sliding and increaseardness.

Fig. 5 shows the stress-strain curves of four RS samples. Theechanical properties of RS alloy are high and significantly vari-

ble. RS alloy contains a considerable amount of microporosity.icroporosity and other casting defects appear to negative affect

he mechanical properties of casting magnesium alloys anday lead to significant variability in their mechanical properties

14].

RS alloy has a significant microporosity as illustrated in Fig. 6

hich shows unetched sample. There were more micro poresnd pores were somewhat larger in the center of samples thanowards the samples surface. Fig. 7 shows a typical SEM image

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ineering A 456 (2007) 364–367 367

f compressive fracture surface. The fracture surface containshrinking porosity as well as gas (air) porosity. In these frac-ure surfaces, the porosity can be detected due to the presencef unfractured dendrites below the pores. The cause of microp-rosity in magnesium alloy has been studied by Baker in 1940s15]. Much debate has occurred concerning which variablesontribute to the formation of microporosity, e.g. solidificationhrinkage, grain size, cooling rate and gas dissolution [15]. Theorosity can be identified in the fracture surface due to the pres-nce of intact dendrites below the pores. Thus, all the regions ofhe fracture surface that contain intact dendrites are essentiallyhe areas occupied by either shrinkage pore or gas pore. Oneuch region is designed by arrow in Fig. 7.

. Conclusions

Rapidly solidified Mg-Zn-Ce-Ag alloy with a diameter ofmm was obtained by copper mould casting. Rapid solidifi-ation refined microstructure. The dendrites can be broken anducleation sites can be increased for primary �-Mg, which refinerain size and decrease the SDAS. The variability of mechanicalroperties in rapidly solidified samples occur with applicationf copper mould, which resulted from the presence of microp-rosity.

cknowledgement

The authors gratefully acknowledge the financial supportrom the Ministry of Science and Technology of China (Grantso. 2006CB65201, 2005DFA50860), the National Natural Sci-

nce Foundation of China (Grant No. 50471077).

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