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Journal of Alloys and Compounds 661 (2016) 402e410

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Journal of Alloys and Compounds

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Effect of Ce addition on the microstructure, thermal conductivity andmechanical properties of Mge0.5Mn alloys

Liping Zhong a, Jian Peng a, b, *, Min Li a, Yongjian Wang a, Yun Lu c, Fusheng Pan a, b

a State Key Laboratory of Mechanical Transmission, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, Chinab Chongqing Academy of Science and Technology, Chongqing 401123, Chinac Graduate School & Faculty of Engineering, Chiba University, Chiba 263e8522, Japan

a r t i c l e i n f o

Article history:Received 15 October 2015Received in revised form9 November 2015Accepted 17 November 2015Available online 2 December 2015

Keywords:Mg alloysCeriumMicrostructureThermal conductivityMechanical propertiesTexture

* Corresponding author. College of Materials ScienceUniversity, Chongqing 400044, China.

E-mail address: jpeng@cqu.edu.cn (J. Peng).

http://dx.doi.org/10.1016/j.jallcom.2015.11.1070925-8388/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

The effect of Ce addition on the microstructure, thermal conductivity and mechanical properties of Mge0.5Mn alloy was investigated systematically in this study. In addition, the thermal conductivity wasmeasured using flash method in the temperature range of 293e523 K. Results indicate that Ce additionrefined the grain structure of both asecast and aseextruded of Mge0.5Mn alloys. Moreover, theweakening of texture can be observed by Ce addition. The thermal conductivity of asecast Mge0.5MnexCe alloys decreased gradually with the increase of Ce content. Meanwhile, the thermal conductivity ofaseextruded Mge0.5Mne0.3Ce alloy (139.7 W/(m$K)) was higher than that of other as-extruded alloys.Furthermore, the aseextruded Mge0.5MnexCe alloys exhibited higher thermal conductivity than that ofasecast counterparts, except for Mge0.5Mne0.6Ce alloy. The weakening of basal texture resulted in theimprovement of thermal conductivity of wrought products. The yield strength (YS) and ultimate tensilestrength (UTS) of aseextruded Mge0.5Mn alloys were enhanced by Ce addition. The aseextruded Mge0.5Mne0.3Ce alloy showed the best strength and moderate elongation (EL). The UTS, YS and EL of thisalloy were 295.9 MPa, 320.9 MPa and 9.6%, respectively.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Mg alloy as heat dissipation materials used in industries, such asautomation, electronics and aeronautics is attractive because of itsexcellent properties, including high specific strength, good elec-tromagnetic shielding property and high thermal conductivity[1e4]. Thermal conductivity has an important function in the per-formance of alloys and is essential in selecting alloys for certainapplications [5]. High thermal conductivity ensures uniform tem-perature distribution that reduces thermally induced stresses andtherefore prolongs the service life of the alloy [6]. Among variousmetallic materials, pure Mg exhibits the best thermal conductivity.However, the poor mechanical properties of pure Mg haverestricted its practical application to a large extent. To date, manyresearchers have focused on obtaining satisfactory thermal con-ductivity of Mg alloys with improved strength. However, widelyused commercial Mg alloys with high strength, such as

and Engineering, Chongqing

MgeZnerare earth element [7], AM50 [8] and AZ91 [9], exhibitunsatisfactory thermal conductivity. Therefore, the development ofMg alloys with both excellent thermal conductivity andmechanicalproperties is expected.

MgeMn alloy is one of the early commercially used wrought Mgalloys with good extrudability and modest strength [10]. Ce is acommon choice of rare earth element inMg, which can enhance thestrength and ductility of Mg alloys owing to the grain refining ef-ficiency of Ce addition [11e14]. Masoumi et al. [15] emphasized thatCe addition can refine the grain structure of asecast and rolled/annealed ofMge1wt.%Mn alloy, and the overall texture intensity ofbasal pole was also weakened for rolled as well as for rolled/annealed MgeMneCe alloys because of the solid solubility of Ce inthe Mgmatrix. Peng et al. [16] proposed that the addition of a smallamount of Ce accelerated the recovery. Both strength and ductilitywere also significantly improved by Ce addition, which was mostlyrelated to the fine grain size and homogeneous secondary pre-cipitates. However, considerable research mainly focuses on theeffect of Ce on the mechanical properties of MgeMn based alloys,and the information available on the effect of Ce on the thermo-physical properties of MgeMn based alloys is scarce, especially inthe case of thermal conductivity.

Table 2Density of the asecast Mge0.5MnexCe alloys.

Alloys Density (kg/m3)

298 K 373 K 448 K 523 K

Mge0.5Mn 1743 1731 1720 1708Mge0.5Mne0.15Ce 1745 1733 1722 1710Mge0.5Mne0.3Ce 1746 1734 1723 1711Mge0.5Mne0.6Ce 1751 1739 1728 1716

L. Zhong et al. / Journal of Alloys and Compounds 661 (2016) 402e410 403

In this study, the microstructure, thermal conductivity andmechanical properties of Mge0.5MnexCe alloys are systematicallyinvestigated. The thermal conductivity and mechanical propertiesof Mge0.5MnexCe alloys are obtained and the results are corre-lated with their microstructures and textures, to reveal the mech-anism of Ce on thermal conductivity and mechanical properties ofMge0.5Mn alloys. Such an investigation aims to provide animportant basis for developing metallic materials with high ther-mal conductivity and moderate mechanical properties as heat sinkmaterials.

2. Experimental procedures

Alloys with nominal composition ofMge0.5MnexCe (x¼ 0, 0.15,0.3 and 0.6 wt.%) were prepared by melting the ingots of commer-cially pure Mg (99.98 wt.%), MgeMn (4.27 wt.%) and MgeCe(20 wt.%) master alloys in a low carbon steel crucible under theprotection of an SF6 (0.5 vol.%) and CO2 (bal.) mixed gas atmosphereat 750 �C. The chemical composition of the investigated alloys wasmeasured with an XRFe1800 CCDE sequential Xeray fluorescencespectrometer, and the analyzed results are listed in Table 1.

Ingots for uniform homogenization treatment used to eliminatedendritic crystal were held at 420 �C for 12 h followed by aircooling. Then, hot extrusion was conducted under a controlledconstant force by an XJe500 horizontal extrusion machine at CCMgin Chongqing University, China. Solid rods, approximately 16mm indiameter, corresponding to an extrusion ratio of 25:1, wereextruded at 400 �C at a billet speed of 2 mm/s and were then aircooled.

The tensile tests were conducted on a CMT6305e300 KN uni-versal testing machine at a strain rate of 10�3 s�1 at room tem-perature. The tensile samples were machined from the extrudedbars along the extrusion direction (ED) with a crossesectionaldiameter of 8 mm and a gauge length of 56 mm. Obtained tensileelongation (EL), yield strength (YS) and ultimate tensile strength(UTS) of three samples were averaged.

Microstructure characterizations were collected by optical mi-croscopy (Canon A650) and a scanning electron microscope (SEM,Tescan Vega 3 LMH) equipped with an Oxford INCA Energy 350energy dispersive Xeray (EDS) spectrometer. The grain size wasmeasured and calculated by Image ProePlus software. Phase ana-lyses and texture measurements were performed by Xeraydiffraction with a Cu target (Rigaku D/Maxe2500 PC). The samplesfor texture analysis and electron backscattered diffraction (EBSD)analysis were cut parallel to the ED from the extrusion bars andbefore analysis the surface was polished. EBSD analysis was con-ducted using an HKL Channel 5 system equipped with a scanningelectron microscope (JEOL 7800F) operating at 20 KV. After me-chanical polishing, the specimens were prepared for EBSD byelectro polishing at 20 V for approximately 30e60 s in AC II solutionat �10 �C to remove surface strain.

Samples for thermal diffusivity measurement were cut from theasecast and aseextruded alloys in the shape of disks with adiameter of 12.7 mm and a thickness of 3 mm. For the aseextruded

Table 1Chemical composition of the investigated Mge0.5MnexCe alloys.

Nominal alloys Composition (wt.%)

Mn Ce Fe Si Mg

Mge0.5Mn 0.454 e 0.018 0.022 Bal.Mge0.5Mne0.15Ce 0.475 0.156 0.021 0.026 Bal.Mge0.5Mne0.3Ce 0.484 0.301 0.022 0.027 Bal.Mge0.5Mne0.6Ce 0.443 0.588 0.016 0.021 Bal.

alloys, the samples were cut perpendicular to the ED. Thermaldiffusivity was measured on Netzsch 447 apparatus via laser flashmethod in the temperature range of 293e523 K. The step of testingtemperature was 75 K, and at least three shots were performed ateach testing temperature. The room temperature density was ob-tained by Archimedes method. The density at elevated temperaturewas calculated using the following equation [17]:

r ¼ r0 � 0:156ðT� 298Þ (1)

where r0 (kg/m3) is the density at room temperature, and T (K) isthe absolute temperature. The densities of the asecast MgeMneCealloys at elevated temperatures are shown in Table 2.

The specific heat capacity of MgeMneCe alloys in this study canbe obtained by NeumanneKopp rule [9,18,19]. Thermal conduc-tivity (l) is calculated from specific heat capacity (Cp), thermaldiffusivity (a) and density (r) via the following equation:

l ¼ a$r$Cp (2)

where a (m2/s)is the thermal diffusivity, r (g/cm3) is the density, Cp

(J/(g$K)) is the specific heat capacity at constant pressure. The un-certainties for measuring the density, specific heat capacity andthermal diffusivity are estimated to be 2%, 5% and 2%, respectively.The total uncertainty for the thermal conductivity is believed to beless than 9%.

3. Results and discussion

3.1. Microstructure analysis

Macroscopic photos of the specimens shown in Fig. 1 clearlyindicate that the asecast grain structure of Mge0.5Mn alloy wassignificantly refined by Ce addition. Asecast Mge0.5Mn andMge0.5Mne0.15Ce alloys (Fig. 1 (a) and (b)) show coarse columnargrains whereas higher Ce compositions (Fig. 1 (c) and (d)) show amixture of columnar grains, finer semi equiaxed and equiaxedgrains. The grain refinement of Ce is largely explained by growthrestriction factor (GRF) [15]. The solid solution of Ce in Mg solidsolution is relatively limited, i.e. the maximum solubility of Ceriumin Mg is approximately 0.09 at% (0.52 wt.%) at 592 �C, and thusrapid enrichment of Ce in the liquid ahead of the growing interfacerestricts the grain growth during solidification. Therefore, the grainsize decreases gradually by Ce addition.

The SEM microstructures of asecast Mge0.5Mn andMge0.5Mne0.15Ce alloys in Fig. 2 (a) and (b) show coarse grainstructure with a small amount of particles dispersedly distributedin the a-Mg matrix. At 0.3wt.% Ce addition, a few semiecontinuousnet compounds developed, and the amount of which increasedwith the increase in Ce addition. Asecast Mge0.5Mne0.6Ce alloyshows the most extensive network of intermetallic compounds.The second phase network in the grain boundaries and interden-dritic boundaries is composed of MgeCe intermetallics as observedin mapping analysis by EDS (Fig. 3). Mapping shows the secondphase region enriched in Ce. The intermetallic compound is

Fig. 1. Macroscopic photos of asecast Mge0.5MnexCe alloys with different Ce addition: (a) x ¼ 0; (b) x ¼ 0.15; (c) x ¼ 0.3; (d) x ¼ 0.6.

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possibly Mg12Ce (Fig. 2(c) and (d)) [20,21], as suggested by the XRDspectra of asecast Mge0.5MnexCe alloys in Fig. 4.

Fig. 5 shows typical back scattered electron images (BSE) of theas-extruded alloys collected along ED. The original coarse particleswere broken into smaller particles dispersed along the ED in the a-Mg matrix after hot extrusion. The volume fraction of these frag-mented phases increasedwith the increase of Ce content.When theCe content increased to 0.6 wt.%, the volume fraction of precipitatesincreased remarkably.

Fig. 6 shows the EBSD maps from longitudinal sections ofaseextruded Mge0.5MnexCe alloys. Compared with the asecastmicrostructure in Fig. 1, the grains were refined apparently after hotextrusion, which may result from the occurrence of dynamicrecrystallization (DRX) and plastic deformation during extrusion.The as-extruded alloys also show a bimodal grain microstructurealong the ED, which is composed of equiaxed fine DRXed grains andelongated coarse unDRXed grains. The average grain size of DRXedgrains was calculated by using the linear intercept method. Ac-cording to the EBSD maps, the average DRXed grain size ofaseextruded Mge0.5Mn is approximately 4.0 mm. After Ce addi-tion, the grain size decreased to around 2.2 mm. By contrast, uponaddition of 0.6wt.% Ce, the grain size increased slightly to 3.0 mm.

The (0002) pole figures and inverse pole figures of aseextrudedMge0.5MnexCe alloys are shown in Fig. 7. The hot extrusion for theinvestigated alloys shows strong fiber texture that corresponds tothe basal planes being mainly oriented parallel to the ED. Thistexture is known to develop during the uni-axial deformation ofMgalloys [3,22]. The Mge0.5Mn alloy exhibited a strong fiber texture,with the intensity of 4.5 m.r.d (multiples of random distribution).

The addition of a trace amount of Ce (0.15 wt.%) to the Mge0.5Mnalloy increased the intensity of fiber texture to 5.3. Notably, withthe increase in Ce addition of more than 0.3 wt.% to Mge0.5Mnalloy, a significant texture weakening effect was observed, asindicated by the low peak intensity. Ce at 0.3wt.% and 0.6wt.%reduced the texture intensity to 3.5 and 3.3, respectively (Fig. 7cand d). Therefore, Ce addition leads to significantly more randomtexture during the hot extrusion of Mge0.5Mn alloy.

3.2. Thermal conductivity of asecast and aseextruded alloys

The temperature and concentration dependences of thermalconductivity of the asecast alloys are presented in Fig. 8. As shown inFig. 8(a), the thermal conductivity of asecast Mge0.5MnexCe alloysgradually increasedwith temperature.Mn andCe atoms in the alloysexist as heterogeneous solute atoms,whichwould strongly cause thelattice distortion and disturb the lattice periodicity. With the tem-perature increase, the vibration of the lattice intensified. In addition,the collision chances of electronephonon and phononephononincreased, which strongly impeded the free movement of bothelectrons and phonons, and thus, the mean free path of electron andphonon decreased. However the average velocities of electronmovement increased with temperature, which became dominant ata higher temperature range. As a result, the thermal conductivity ofthe investigated alloys increased with temperature.

Moreover, Fig. 8(a) shows the dependence of Ce content onthermal conductivity of asecast Mge0.5MnexCe alloys in thetemperature range of 298e523 K. The thermal conductivity of theasecast samples for Mge0.5Mn, Mge0.5Mne0.15Ce,

Fig. 2. SEM microstructures of asecast Mge0.5MnexCe alloys: (a) x ¼ 0; (b) x ¼ 0.15; (c) x ¼ 0.3; (d) x ¼ 0.6.

L. Zhong et al. / Journal of Alloys and Compounds 661 (2016) 402e410 405

Mge0.5Mne0.3Ce and Mge0.5Mne0.6Ce at room temperature is138.2, 133.8, 131.5 and 129.9 W/(m$K), respectively. Apparently, thethermal conductivity of the asecast samples decreased with Cecontent. The thermal conductivity decreased by 5 W/(m$K) uponaddition of 0.15 wt.% Ce, and decreased to 129.9 W/(m$K) uponaddition of 0.6 wt.% Ce. The reasonwas that the increasing additionof the Ce element, which acted as scattering centers for phononsand electrons, caused large distortion of the a-Mg matrix anddestroyed the periodicity of lattice, which limited the mean freepath of electrons and phonons. As shown in Fig. 1 and Fig. 2, withthe increase of Ce content, the average grain size of the asecastalloys decreased and the second phase network of the intermetalliccompounds increased. These factors reduced the thermal conduc-tivity of alloys together. Accordingly, the thermal conductivitydecreased dramatically with the increase of Ce concentration.

Although the thermal conductivity decreased with the increaseof Ce concentration, the slope of the curve had a slight change. Thechange may be attributed to the precipitation of Mg12Ce. The solidsolubility of Ce in Mg is relatively limited. As the addition of Cecontent is larger than the threshold value, and the excess Ce couldreact with Mg to form the intermetallic of Mg12Ce. According toEivani et al. [23], thermal resistance increment caused by alloyingelement as solute atoms dissolved in Mg matrix is about one orderof magnitude larger than that by intermetallic compounds. There-fore, with the increase of Ce content, the influence of Ce addition onthe thermal conductivity of investigated alloys graduallyweakened.

The variations of the thermal conductivity of Mge0.5MnexCe

alloys after extrusion are shown in Fig. 9. The thermal conductivityof aseextrudedMge0.5Mn, Mge0.5Mne0.15Ce, Mge0.5Mne0.3Ceand Mge0.5Mne0.6Ce alloys at room temperature is 138.4, 137.0,139.7 and 126.9 W/(m$K), respectively. Apparently, the thermalconductivity of aseextruded Mge0.5MnexCe alloys was higherthan that of asecast counterparts, except for Mge0.5Mne0.6Cealloy. The microstructure of the aseextruded Mge0.5MnexCe al-loys obtained from EBSD analyses presented in Fig. 6 shows thatdynamical recrystallization occurred and that the grain size ofMge0.5MnexCe alloys largely decreased after extrusion comparedwith the microstructure of asecast alloys (Fig. 1). This findingsuggests that more defects were induced after extrusion, such aspoints, dislocations and grain boundaries [17,24e26]. These latticedefects would act as the scattering centers that block the freemovement of electrons and phonons, thus, the mean free path ofelectrons and phonons decreased and the thermal conductivity waslower in the aseextruded alloys. By contrast, the concentration ofMn and Ce elements in solid solution decreased after extrusion.Given the very low solubility of Mn and Ce atoms in Mg, the for-mation of a-Mn and Mg12Ce precipitates throughout the grainsduring hot extrusion consumes the solute elements in the aeMgmatrix [4,27]. The lattice distortion was released and the scatteringeffect of the solutes on the electron and phonon transportationwaslargely depressed. Thus, the extrusion alloys exhibited higherthermal conductivity than the asecast alloys. The formation of a-Mn and Mg12Ce precipitates during extrusion can be confirmed byXRD spectra of asecast and aseextruded Mge0.5Mne0.15Ce alloys(Fig. 10). Therefore, the increase in thermal conductivity of the

Fig. 3. EDS mapping analysis for Mge0.5Mne0.6Ce alloy showing Ce enrichment in the interdendritic phase.

Fig. 4. XRD spectra of asecast Mge0.5MnexCe alloys: (a) x ¼ 0; (b) x ¼ 0.15; (c)x ¼ 0.3; (d) x ¼ 0.6.

L. Zhong et al. / Journal of Alloys and Compounds 661 (2016) 402e410406

investigated alloys during extrusion was probably due to thedepletion of Mn and Ce elements in thematrix and the formation ofthe precipitates.

As can be seen from Fig. 2, Fig. 4 and Fig. 8(b), when Ce content is0.6wt.%, most of the Mn and Ce atoms are in the form of secondparticles existing in the a-Mg matrix. Compared with the asecast

microstructure, the content of solute atoms in the a-Mg matrix hasslight change. Meanwhile, as aforementioned, the grain size of theas-extruded Mge0.5Mne0.6Ce alloy was largely decreasedcompared with that of asecast alloy. More lattice defects wereinduced after extrusion, which acted as scattering centers of pho-nons and electrons. The scattering centers limit the mean free pathof the electrons and phonons. They also seriously affect phononsand electrons transfer, which leads to a decrement in the thermalconductivity of alloys. As a result, the thermal conductivity of theaseextruded Mge0.5Mne0.6Ce alloy was slightly lower than thatof asecast alloy.

It is noteworthy that the thermal conductivity of theaseextruded Mge0.5Mne0.3Ce alloy was higher than that of otheraseextruded alloys. Texture is generally known to also influencethe thermal conductivity of wrought Mg alloys because of theanisotropic feature of the Mg alloy with a closeepacked hexagonal(hcp) structure [28]. The basal plane is the most closeepacked,which limits the mean free path of electrons and phonons, andthus, the thermal conductivity along the basal plane is smaller thanthat along the prismatic plane, which is less closeepacked. Penget al. [27] reported that the weakening of the basal texture canimprove the thermal conductivity of extrusion rods of Mg alloywhen the measurement direction is parallel to the ED. As can beseen form Fig. 7, the intensity of the basal texture first increased andthen constantly declinedwith Ce increment. TheMge0.5Mne0.3Cealloy exhibited weaker fiber texture compared with the Mge0.5Mnand Mge0.5Mne0.15Ce alloys, which indicates that more grainswere randomly distributed. Accordingly, the thermal conductivity

Fig. 5. Back scattered electron images (BSE) of as-extruded Mge0.5MnexCe alloys: (a) x ¼ 0; (b) x ¼ 0.15; (c) x ¼ 0.3; (d) x ¼ 0.6.

Fig. 6. EBSD maps of aseextruded Mge0.5MnexCe alloys with different content of Ce: (a) x ¼ 0; (b) x ¼ 0.15; (c) x ¼ 0.3; (d) x ¼ 0.6.

L. Zhong et al. / Journal of Alloys and Compounds 661 (2016) 402e410 407

Fig. 7. (0002) pole figures and inverse pole figures of aseextruded Mge0.5MnexCe alloys: (a) x ¼ 0; (b) x ¼ 0.15; (c) x ¼ 0.3; (d) x ¼ 0.6.

Fig. 8. (a) Temperature and concentration dependences of thermal conductivity of asecast Mge0.5MnexCe alloys (x ¼ 0, 0.15, 0.3, 0.6), (b) The dependence of Ce content onthermal conductivity of asecast Mge0.5MnexCe alloys at room temperature.

L. Zhong et al. / Journal of Alloys and Compounds 661 (2016) 402e410408

of the Mge0.5Mne0.3Ce alloy was higher than that of theMge0.5Mn and Mge0.5Mne0.15Ce alloys. However, for theMge0.5Mne0.6Ce alloy, the fiber texture intensity was the weak-est, and the thermal conductivity was lower than that of the otheraseextruded alloys. At 0.6wt.% Ce addition, the solutes of Ce in a-Mg matrix and second particles significantly increased, whichwould intensify the lattice distortion and hinder the transport ofelectrons and phonons. The increased thermal conductivity as aresult of randomized texture could not compensate for the decreaseof thermal conductivity caused by lattice defects, such as secondparticles and the solutes of Ce in a-Mg matrix. Consequently, thesetwo contradictory factors influenced the thermal conductivitytogether, and the thermal conductivity of the aseextrudedMge0.5Mne0.6Ce alloy was lower than that of the otheraseextruded alloys.

3.3. Mechanical properties of aseextruded alloys

The nominal stressenominal strain curves of the aseextrudedMge0.5MnexCe alloy obtained by tensile test are shown in Fig. 11.

The detailed mechanical properties, including YS, UTS and EL aresummarized in Table 3. This table shows that YS and UTS areapparently improved with Ce addition. For the aseextruded alloys,UTS and YS first increased remarkably and then decreased withfurther Ce addition, and aseextruded Mge0.5Mne0.3Ce alloy hasthe highest UTS and YS. After the addition of Ce is higher than0.6 wt.%, UTS and YS decreased severely. In general, the variation ofEL is contrary to that of strength. The EL first decreased and thenincreased with the increase of Ce content. The EL of theaseextruded Mge0.5Mne0.15Ce alloy decreased to 7.6%. As the Cecontent increased continually, the EL of the investigated alloysincreased gradually. For the aseextruded Mge0.5Mne0.6Ce alloy,the EL reached up to 16.3%. The aseextruded Mge0.5Mne0.3Cealloy showed the best strength and moderate EL, i.e., UTS of295.9 MPa, YS of 320.9 MPa and EL of 9.6%.

For the aseextruded alloys, the strength enhancement wasmainly attributed to fine grain strengthening and secondephasestrengthening. As aforementioned, at Ce content lower than0.3wt.%, the grain size of the aseextruded alloys was significantlyreduced and the volume fraction of second phase increased and led

Fig. 9. Comparison of thermal conductivity of asecast and aseextrudedMge0.5MnexCe alloys (x ¼ 0, 0.15, 0.3, 0.6).

Fig. 10. XRD spectra of asecast and aseextruded Mge0.5Mne0.15Ce alloys.

Fig. 11. The nominal stressenominal strain curves of aseextruded Mge0.5MnexCealloys.

Table 3Mechanical properties of aseextruded Mge0.5MnexCe alloys.

Alloys YS (MPa) UTS (MPa) EL (%)

Mge0.5Mn 150.6 (±7.2) 225.0 (±1.5) 12.8 (±0.9)Mge0.5Mne0.15Ce 292.9 (±5.4) 304.0 (±4.4) 7.6 (±0.5)Mge0.5Mne0.3Ce 295.9 (±4.7) 320.9 (±4.6) 9.6 (±0.5)Mge0.5Mne0.6Ce 220.7 (±4.4) 257.1 (±5.2) 16.3 (±1.3)

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to an improvement in tensile strength. When the Ce addition isover 0.6wt.%, the DRXed grain became coarsen and the sec-ondephase strengthening of Mg12Ce would be slight, and thus, thetensile strength subsequently decreases. As shown in Fig. 7, theintensity of basal texture first increased and then decreased. Thebasal texture, which is parallel to ED, is themain texture inwroughtMg alloy. When the direction is parallel to the ED, the Schimidfactor of the basal plane system of grains is zero, which improvesthe strength of alloys. At 0.6wt.% Ce addition, the texturestrengthening effect on alloys is decreased, because of a decrease inbasal texture intensity. Therefore, the strength the of aseextrudedalloys first increases remarkably and then decreases after theaddition of Ce content higher than 0.6 wt.%.

In general, plasticity is also influenced by several factors. Theplasticity first declines and then constantly increases with Ceincrement. The existence of (0002) basal texture has a significantinfluence on the plasticity of Mg alloys because weakening the(0002) basal texture can improve the plasticity of the alloy at roomtemperature [29]. During tensile testing along ED, some of the basalplane of grains along the ED produces a deflection with a certain

angle. The basal plane lying in soft orientation is good for plasticdeformation. The intensity of the basal texture of theMge0.5Mne0.6Ce alloy was the weakest. Thus, the EL of theMge0.5Mne0.6Ce alloy was the highest.

4. Conclusions

The microstructure, thermal conductivity and mechanicalproperties of Mge0.5MnexCe alloys were investigated. From theresults, the following conclusions can be drawn:

(1) Ce addition is helpful for grain refining of the asecastMge0.5Mn alloy. With the increase in Ce addition, the vol-ume fraction of Mg12Ce increases. After extrusion, the grainsize first decreases and then increases with Ce increment.

(2) The thermal conductivity of asecast alloys decrease gradu-ally with the increase in Ce addition, but the slope of thecurve has a slight change because of the precipitation ofMg12Ce.

(3) The thermal conductivity of aseextruded alloys was higherthan that of the asecast alloys except for theMge0.5Mne0.6Ce alloy. In addition, the thermal conduc-tivity of the aseextrudedMge0.5Mne0.3Ce alloy was higherthan that of the other aseextruded alloys.

(4) The strength of the alloys with Ce addition improvedremarkably. Moreover, the Mge0.5Mne0.3Ce alloy exhibitedthe best comprehensive properties. The thermal conductivityof this alloy was is 139.7 W/(m$K), and its UTS, YS and EL are320.9 MPa, 295.9 MPa and 9.6%, respectively.

Acknowledgments

The present work was supported by the National Natural

L. Zhong et al. / Journal of Alloys and Compounds 661 (2016) 402e410410

Science Foundation of China (Project 51474043), Education Com-mission of Chongqing Municipality (KJZH14101) and ChongqingMunicipal Government (CSTC2013JCYJC60001, Two River ScholarProject and The Chief Scientist Studio Project).

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