,

aChongqing Academy of Science and Technology,Chongqing 401123,China

bNational Engineering Research Center for Magnesium Alloys,Chongqing University,Chongqing 400044,China

Microstructure and mechanical behavior of the Mg-Mn-Ce magnesium alloy sheets

Qingshan Yanga,b,Bin Jianga,b,*,Xin Lib,Hanwu Donga,b,Wenjun Liua,b,Fusheng Pana,b

aChongqing Academy of Science and Technology,Chongqing 401123,China

bNational Engineering Research Center for Magnesium Alloys,Chongqing University,Chongqing 400044,China

The microstructural evolution and mechanical behavior of Mg-Mn-Ce magnesium alloy were investigated in the present study.Mg alloy was prepared with metal model casting method and subsequently hot extruded at 703 K with the reduction ratio of 101:1.The grains were dynamically recrystallized after the extrusion process.Moreover,the(0002)pole f i gure of Mg-Mn-Ce alloy developed a splitting of pronounced basal texture.The mechanical properties were different due to different angles between c-axis and loading direction(0°,45°and 90°)in the tensile tests.This signif i cantly induces an asymmetry in the yield behavior.The Mg-Mn-Ce alloy exhibits a classical dimple structure as a result of slip accumulation and ductile tear.

Mg alloy;Microstructure;Mechanical properties;Texture;Extrusion

1.Introduction

The applications of magnesium alloys are being increasingly evaluated for aerospace and automotive components due to their high specif i c strength,low density and excellent machinability[1-4].Compared with casting counterparts, wrought magnesium alloys have better mechanical properties [5].However,most wrought magnesium alloys usually bring about a strong texture during the primary processing such as the extrusion and the rolling[6],which is related to their limited number of active deformation systems in hexagonal close-packed(hcp)crystal structure[7,8].This leads to a high directional anisotropy of mechanical properties.Basal slip,prismaticslip and pyramidalslip provide only four independent slip systems[9].In general,basal slip systems can easily activate in Mg alloys at room temperature,while the prismaticand the pyramidal slip systems can operate only if a higher driving force is applied or deformed at elevated temperature because of their high critical resolved shear stress(CRSS)[10,11].This can further limit the applications of magnesium alloys.

Mg-Mn-Ce system constitutes a promising alloy base for expanded components[12-14].The addition of cerium is known to improve the strength and elevated temperature properties of magnesium alloys.It also results in the grain ref i ning eff i ciency[15].The well-dispersed second phase particles restrict grain growth and postpone cavitation to higher strain levels to obtain the enhancement in the hotformability[16,17].On the other hand,Manganese is used in magnesium alloys to lower the iron content and improve the corrosion resistance.It is also known that magnesium alloys containing manganese exhibit good ductility,making it possible to form sheet and tubes[12].The peritectic reaction results in the formation of Mg solid solution(α-Mg)and α-Mn in Mg-Mn alloys[18].It can be assumed that Mg alloys containing both Ce and Mn show a good base for the development of casting and wrought alloys.In present work,desired Mg-Mn-Ce alloy was prepared with metal model casting method and subsequently extruded.The microstructure and mechanical behavior were investigated.

Fig.1.X-ray diffraction pattern of the Mg-1.5Mn-0.5Ce alloy.

2.Experimental procedure

Pure Mg(99.9 wt.%),Mg-4.27 wt.%Mn and Mg-20 wt.%Ce master alloys were used to prepare the desired Mg-1.5Mn-0.5Ce alloy.Melt was prepared in an electrical resistance furnace protected by mixture gas of 1 wt.%SF6-50 wt.%CO2-49 wt.%dry-air.The alloy was isothermally held at 993 K for 20 min and poured into a mild steel crucible and air cooled.The cast Mg-Mn-Ce billet was homogenized at 703 K for 2 h.The extrusion was conducted with an extrusion ratio of 101:1 at 20 mm/s extrusion rate.The extrusion temperature was 703 K.The extruded sheets were 56 mm in width(transverse direction,TD)and 1 mm in thickness(normal direction,ND).

Dog-bone tensile samples of 12 mm in gage length,6 mm in width and 1 mm in thickness were machined from the sheets with various directions tilting 0°,45°and 90°to the extrusion direction(ED),respectively.Tensile tests were conducted on a CMT6305-300KN universal testing machine at the initial strain rate of 10-3s-1at room temperature.The microstructures of the alloys were examined using optical microscopy (OM)and scanning electron microscope(SEM).The phase analysis and(0002)pole f i gures of Mg-1.5Mn-0.5Ce Mg alloy were carried out by X-ray diffractometer(XRD,Rigaku D/Max 2500)with Cu Kα radiation,a voltage of 40 kV and a current of 100 mA.The crystal orientation and misorientation angle distribution were examined by electron backscattered diffraction(EBSD).Following mechanical polishing,the specimens were prepared for EBSD by electro polishing at 20 V for about 150 s in the AC2 solution at-10°C in order to remove surface strain.EBSD data was acquired using HKL Channel 5 System equipped FEI Nova 400 FEG-SEM.For orientation mapping the scan step size was set at 1 μm.

3.Results and discussion

To identify the presence of phases in the studied alloy,X-ray diffraction analysis was conducted.Fig.1 shows the XRD pattern taken from the sheet.In addition to the Mg peaks,the ref l ection of Mg12Ce phase can also be indexed.Such an addition of cerium was helpful in the grain ref i ning eff i ciency, and excess cerium reacted with magnesium to form the intermetallic.

Fig.2 shows secondary,the backscatter SEM micrographs and EDS results of as-cast Mg-1.5Mn-0.5Ce alloy.It can be seen that Mg12Ce intermetallics locate in the grain boundary for the as-cast Mg alloy.The large number of second phases was well-distributed in a particle or rod-shaped in the Mgmatrix. Fig. 3 shows the microstructures of Mg-1.5Mn-0.5Ce alloy sheet observed from the longitudinal section(ND-ED plane).The alloys are characterized by inhomogeneous microstructure with fi ner equiaxed dynamically recrystallized(DRX)grains of about 14 μm.

Fig.2.The secondary,the backscatter SEM images and EDS results of as-cast Mg alloy.

Fig.3.(a)OM micrographs and(b)the secondary and backscatter SEM images of Mg-Mn-Ce sheet.

Fig.4.The pole f i gures of the Mg-Mn-Ce sheet.

Fig.4 shows the pole fi gures of the Mg-Mn-Ce sheet. Anothertextureevolution [Prismatic (1010),Pyramidal (1011),(1012)]was also shown in Fig.4.It was found that the Ce addition strongly brought out different texture features, including inclination direction,the rotated angle of the basal pole,and texture intensity[12,19].The sheets developed a splitting of pronounced(0002)basal texture after the hotextrusion.<1120>direction in most grains orientated parallel to the extrusion direction.

Fig.5(a)shows the inverse pole f i gure(IPF)map of Mg-1.5Mn-0.5Ce alloy specimens revealed by EBSD data. The red color indicates(0002)basal plane while the blue color represents the plane lying at 90°away from(0002)basal plane.To understand the micro-texture in detail,the fraction of boundaries as a function of the misorientation angle for adjacent grains was measured,and the resulting misorientation distribution histogram is shown in Fig.5(b).The misorientation axis is mostly close to the c-axis of the hexagonal lattice [20].In the IPF maps,grain boundaries are indicated by various lines depending upon the grain-to-grain misorientation angles:gray for 2°<θ <15°(low-angle boundaries,LABs) and black for 15°<θ <90°(high-angle boundaries,HABs).It was observed that a higher density of high angle misorientation can be seen in this alloy.It suggested that the grains have fully dynamically recrystallized during the extrusion process.

Fig.6 shows mechanical responses(at least three samples were tested in each group)measured at different directions tilting 0°,45°and 90°to ED at room temperature.It reveals the strong anisotropy of stress-strain behavior exhibited by the Mg-1.5Mn-0.5Ce alloy sheet with respect to the initial texture.The detailed mechanical properties i.e.ultimate tensile strength(UTS),the 0.2%proof stress(YS)and theuniform elongation(Eu)are summarized in Table 1.The true stress-strain curves show high yield stress and low strain hardening rate for the 0°sample,low yield stress and high strain hardening rate for the 45°sample,whereas the ductility of the 45°samples is superior.It is expected that the Schmid factorms,

Fig.5.EBSD results showing(a)IPF map and(b)misorientation angle distribution of Mg-1.5Mn-0.5Ce alloy.The EBSD data were obtained on the ED-TD plane.

Fig.6.True stress-strain curves during tensile tests in different directions.

Table 1Results of the tensile tests carried out in the tensile directions of 0°,45°and 90°.

where λ is the angle between the stress axis and the slip direction,and φ is that between the stress axis and the normal direction of the slip plane,respectively.The maximummsis 0.5(λ=45°, φ =45°)[21,22].The dominant slip system of Mg alloy at room temperature is slip in the close packed direction<11 20>oron the basal(0002)plane.The critical resolved shear stress(CRSS)for a basal slip is far below that for a non-basal slip.Furthermore,conventionally extruded Mg alloy sheets generally exhibit a strong basal texture where thec-axis of grains is predominantly aligned parallel to the sheet normal.This strong texture induces a stronger anisotropy[23].

Fig.7 shows the SEM fracture images obtained by both secondary and backscattered electron patterns.The notable feature is a large amount of plastic dimples and cleavage planes.Some dimples are inside the cleavage planes,which belong to the ductile failure[24,25].The fracture surface of 0°sample has less and shallower dimples,while those of 45°sample show cruder,which obviously reveals the good plasticity.It was consistent with mechanical responses as above mentioned.Further investigation of deformed microstructural structure is required to clarify the deformation mechanism of the present extruded Mg alloys.

4.Conclusions

Fig.7.The secondary and the backscatter SEM fracture images of Mg-1.5Ce-0.5Mn sheets:(a)0°,(b)45°and(c)90°.

The present investigation shows that texture has a great inf l uenceon the mechanicalanisotropy and theworkhardening of Mg-Mn-Ce magnesium alloy sheets.The extruded Mg sheets were fully dynamically recrystallized.The mechanical responses were different due to different angles betweenc-axis and loading direction during tensile tests.This signif i cantly induces an asymmetry in the yield behavior.The 90°sample possessed a higher strength,while the 45°sample presented the prior elongation.Mg-Mn-Ce alloy sheet exhibits a classical dimple structure as a result of slip accumulation and ductile tear.

Acknowledgments

The authors are grateful for the f i nancial supports from National Natural Science Foundation of China(51171212), Chongqing Science and Technology Commission (CSTC2012JJJQ50001, CSTC2013jcyjC60001, cstc2012ggB50003),and The National Science and Technology Program of China(2013DFA71070),and the Fundamental Research Funds for the Central Universities (CDJZR13138801).

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Received 10 November 2013;accepted 5 January 2014 Available online 17 March 2014

*Corresponding author.Sha Zheng Jie 174#,Sha Ping Ba District, Chongqing,China.Tel./fax:+86 023 65111140.

E-mail addresses:jiangbinrong@cqu.edu.cn,cquyqs@163.com(B.Jiang). Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China,Chongqing University

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Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.