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Materials Science and Engineering A 520 (2009) 197–201

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

hort communication

n situ observation of tensile deformation of high-pressure die-cast specimens ofM50 alloy

ie Songa, Shou-Mei Xionga,∗, Mei Lib, John Allisonb

Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR ChinaMaterials Research & Advance Engineering Department, Research and Advanced Engineering Laboratory, Ford Motor Company, Dearborn, MI 48121, USA

r t i c l e i n f o

rticle history:eceived 6 October 2008

a b s t r a c t

In situ observation of HPDC AM50 alloy in the SEM chamber was performed to study the changes of themicro-voids and the � phase during tensile deformation. The results suggested that micro-voids had little

eceived in revised form 15 May 2009ccepted 18 May 2009

eywords:M50 magnesium alloyigh-pressure die-cast

n situ observation

change in the elastic region, opened linearly with increasing load in the plastic region and led to finalfracture. The detachment of � phase from the interface was also observed in the plastic region and therewas no evidence to suggest that the detachment led to the final fracture.

© 2009 Elsevier B.V. All rights reserved.

ensile deformation

. Introduction

Magnesium die-castings are very attractive for various indus-rial applications due to their light weight and good elongation,uch as in the automobile, aeronautical and aerospace industries1,2]. The typical room temperature mechanical properties of high-ressure die-cast (HPDC) AM50 alloy are 200 MPa (ultimate tensiletrength), 110 MPa (yield strength) and 10% (elongation) accordingo ASTM B94-94. The microstructure of HPDC AM50 magnesiumlloy has been studied by Gertsman and Wang, and their resultshowed that �-matrix and �-Mg17Al12 are the main phases [1,3].he relationship between casting parameters (the wall thickness,he melt and the die temperatures) and the properties of HPDC

agnesium alloys has been established by Aghion [4]. In addition,he porosity distribution and its effect on the mechanical propertiesere analyzed by Lee and Tiryakioglu [5–7]. Many previous stud-

es have focused on the mechanical properties of HPDC AM50 alloynd the processing parameters. However, understanding the mech-nism of tensile deformation is critical in optimizing the processingechniques to achieve the desired microstructures and the prop-rties. In this study, the deformation mechanism and mechanical

roperties of HPDC AM50 alloy were examined using in situ obser-ation on the surface of the specimens in the scanning electronicroscope (SEM) chamber.

∗ Corresponding author. Tel.: +86 10 62773793; fax: +86 10 62770190.E-mail address: smxiong@tsinghua.edu.cn (S.-M. Xiong).

921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2009.05.042

2. Experimental procedure

The test material for this study was AM50 alloy and the chem-ical composition of the test material was shown in Table 1 [8].Step-shape castings were produced by a TOYO 650t cold chamberdie-casting machine with the casting process parameters given inTable 2. The step-shape casting had five different step thicknesses2, 5, 8, 11 and 14 mm. The in situ observation was conducted on theflat dog-bone test specimens extracted from the 11 mm step. Thedimension of the specimens is shown in Fig. 1 with 1 mm samplethickness. Four specimens parallel to the casting surface were pre-pared by electrical discharge machining (EDM) and the distancesfrom the center of each specimen to the surface were 0.75, 2, 3.25and 4.5 mm (labeled numbers 1–4), respectively. The in situ tensiletests were carried out in the SEM chamber at a deformation speedof 1 �m/s. The metallography preparation for the SEM and the insitu observation were performed followed the standard metallo-graphic procedures [9]. The fracture surfaces and microstructure ofthe in situ test specimens were examined using a Hitachi S-4500SEM equipped with an energy dispersive X-ray analyzer (EDS) [10].After the in situ tensile tests, disks of 3 mm in diameter and 0.2 mmin thickness were cut from the specimens by EDM. Then thin foilspecimens for TEM were prepared by twin jet electropolishing thedisks using an electrolyte solution of 5.3 g LiCl, 11.16 g Mg(ClO4)2,

500 ml methanol and 100 ml 2-butoxy-ethanol at 218 K and 90 V.TEM was conducted using on a JEOL 2010 microscope operated at200 kV with a goniometer capable of ±45◦ of tilt about two orthog-onal axes [11]. Scanning acoustic microscopy (SAM) was used toobtain the micro-void distribution of each in situ specimen (Fig. 1).

198 J. Song et al. / Materials Science and Engineering A 520 (2009) 197–201

Table 1Chemical composition of an alloy (wt%).

Al Mn Zn Si Fe Mg

5.11 0.27 0.02 0.014 0.0074 Bal.

Table 2Process parameters for HPDC.

Meltingtemperature

Die temperature Castingpressure

Fast shotspeed

Pressurizationtime

680 ◦C 150 ◦C 67 MPa 2 m/s 40 ms

3

maa

iwvttoAd

ivcro

FA

Fig. 3. Typical stress–strain curve of die-cast AM50 alloy and void opening displace-ment changes VS strain of specimen no. 4.

Fig. 1. Dimensions of the in situ specimen.

. Results and discussion

The typical microstructure of HPDC AM50 alloy contains �-Mgatrix and �-Mg17Al12 (network shape around the grain bound-

ries shown in Fig. 2 and coarse morphology near the micro-voids)nd micro-voids (Fig. 2).

A typical stress–strain curve for the in situ tensile test was shownn Fig. 3. The curve had two regions (elastic and plastic regions),

hich were separated by a knee point. Fig. 4 showed that the micro-oid gradually became detached as the load increased. In addition,he micro-void distribution of the in situ specimen obtained withhe SAM was shown in Fig. 5. Figs. 6 and 7 illustrated the morphol-gy change of the � phase and � matrix during the tensile process.ll the pictures from the in situ observation had the same tensileirection as shown in Fig. 4a.

The micro-void changes during tensile deformation were shownn Fig. 4. Nine stages in the tensile process with stress and strain

alues shown in Fig. 4j were selected to observe the micro-voidhanges. Points a–d represent the different stages in the elasticegion and points e–i represent those in the plastic region. The voidpening displacement (VOD) was used to describe the micro-void

ig. 2. Microstructure of die-cast AM50 alloy with �-dendrites, Mg17Al12 phase,lMn phase and shrinkage porosity.

Fig. 4. Micro-void changes during in situ tensile process at stress level of (a) 6 MPa,(b) 32 MPa, (c) 63 MPa, (d) 87 MPa, (e) 107 MPa, (f) 142 MPa, (g) 172 MPa, (h) 196 MPa,(i) 215 MPa, and (j) the stress–strain curve of specimen no. 4.

J. Song et al. / Materials Science and Engineering A 520 (2009) 197–201 199

Fig. 5. Scanning acoustic microscopy picture of no. 4 specimen. (a) Time of flightanalysis and (b) C-scan mode with fracture pass and initiation site.

Fig. 6. �-Mg17Al12 changes during in situ tensile process at stress level of (a) 110 MPa,(b) 131 MPa, (c) 144 MPa, (d) 158 MPa, (e) 170 MPa, (f) 184 MPa, (g) 189 MPa, and (h)the stress–strain curve of specimen no. 3.

Fig. 7. Slip line changes during in situ tensile process at stress level of (a) 32 MPa,

(b) 109 MPa, (c) 132 MPa, (d) 167 MPa, (e) 206 MPa, (f) 231 MPa, (g) 258 MPa, and (h)the stress–strain curve of specimen no. 1.

changes in different stages (Fig. 4g). The relationship between VODand strain at different stages was displayed in Fig. 3 and a non-linearrelationship was observed. The VOD value did not change uniformlywith the increasing strain value. The knee point separated the curveinto two parts. In the elastic region, the VOD increased very slowlywith the increasing strain up to the knee points indicted by thearrow shown in Fig. 3. Then with the subsequent deformation afterthe knee point in the plastic stage, a sharp increase in the VODand a linear relationship between the strain and VOD were found.This demonstrated that the strain or tensile displacement had littleeffect on the micro-void in the elastic stage, and the deformationmainly happened in the matrix structure.

With the 3D distribution of the in situ specimen (Fig. 5a)obtained with the time of flight (TOF) analyses (illustrated in about10 pictures of the specimen) of the SAM technique, the relation-ship between the micro-void and fracture process can be betterestablished (Fig. 5b). The projected densities of the micro-voids ofthe specimen acquired under the C-scan mode in the SAM testingwere used to establish the fracture process (Fig. 5b). The fracturetended to initiate at the larger micro-voids and in the micro-voids

condensed by adding the fracture path and fracture initiation siteadded to Fig. 5b. The detachment of the micro-voids led to crackpropagation and finally formed the fracture path.

2 nd Engineering A 520 (2009) 197–201

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The surface microcracks in the coarse �-Mg17Al12 phase and theetachment between � phase and � matrix were observed in thelastic deformation region as shown in Fig. 6. With increasing stress,he number of microcracks increased and the width of the detach-

ent became larger. Since no obvious changes were observed in thelastic region, seven stress stages in the plastic region (Fig. 6h) werehosen to observe the changes in the coarse �-Mg17Al12 phases. Athe early stages of the plastic deformation (Fig. 6a and b), thereere no microcracks or detachments in the coarse � phase. When

he stress value was relatively high (Fig. 6c), the microcracks andetachments emerged. The gaps of the detachments were shown

n Fig. 6c–g. This phenomenon resulted from the different physi-al properties of the two main phases (�-Mg and �-Mg17Al12). The-Mg matrix had good plasticity while the �-Mg17Al12 phase wasore brittle. In the early stage of the plastic deformation, the strainas not large enough to cause deformation incompatibility i.e., the

imited deformation ability of the � phase could still adapt to theeformation of the � matrix. Therefore, there were no microcracksr detachments in the coarse � phase at the early stages of plas-ic deformation. However, it is hard for the two phases to deformompatibly at large tensile deformation, resulting in the interfaceetachments of the �-Mg17Al12 phase from the matrix as well asracks in the �-Mg17Al12 phase. In addition, it was observed thathe �-Mg17Al12 cracking could not result in final fracture duringhe tensile test. The main reason could be that little �-Mg17Al12xisted in the microstructure.

Fig. 7 showed the evolution of the slip lines in the � matrixith increasing stress. Seven stages containing both the elastic

nd plastic deformation periods were chosen to illustrate a clearerxplanation of the mechanics of the deformation. The slip lines wererst observed just after the knee point of the stress–strain curve.ith the increasing of the strain, the number of slip lines increased

Fig. 7c–g). The large extent of matrix slips demonstrated that thelloy could bear large plastic deformation before it finally fractured.

The fracture fractography of the in situ specimen was shown inig. 8. The main characteristic of the fractography was dimpling,emonstrating the ductile fracture mechanics of HPDC AM50 alloy.his characteristic also indicates a large final plastic deformation.he elongation of the HPDC AM50 alloy was about 7–12%, which isery high for a magnesium alloy. The two main deformation mecha-isms of HPDC AM50 alloy are slipping and twinning. The presence

f dislocations and deformation twinning was observed in the spec-mens after in situ tensile test (Fig. 9). There were more dislocationsresent at the grain boundaries. The reason was that dislocationsropagated and glided in the � matrix, and then condensed at the

ig. 8. Fracture fractography of the in situ specimen with dimple characteristic.

Fig. 9. Dislocation and twinning of specimen after in situ tensile test. (a) dislocationdisplay after tensile deformation and (b) twinning deformation.

grain boundaries as the deformation proceeded. The twinning sys-tem {1 0 −1 1} 〈1 0 −1 −2〉 [12] was determined from the selectedarea diffraction (SAD) pattern shown in the right bottom of Fig. 9b.

4. Conclusions

1. During the in situ observation of tensile deformation process ofhigh pressure die cast AM50 alloy, little change was detected onthe micro voids in the elastic region. The micro-void changesemerged in the plastic region and opened linearly with theincreasing load. Final fracture tended to initiate at larger micro-voids or in the micro-void condensed area according to thefracture path analysis and SAM results for the void distributionof the specimens.

2. The coarse � phase cracked and detached from the � matrixin the later plastic deformation and there was no direct evi-dence that the final fracture initiated from the detachment ofthe �-phase from the interface under the present experimentalconditions.

3. The great volume of slip lines and fractography characteristicsdemonstrated that the HPDC AM50 alloy could withstand a largeamount of elongation before fracture.

Acknowledgements

This research was financially supported by the National BasicResearch Program of China (2006CB605208-2) and the FordMotor Company. The experimental works were conducted at theTsinghua-TOYO R&D Center of Magnesium and Aluminum Alloys

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[10] S. Lun Sin, D. Dube, R. Tremblay, Materials Characterization 59 (2008) 178–

J. Song et al. / Materials Science a

rocessing Technology with the help of engineers from TOYOachinery & Metal Co., Ltd.

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