Improved mechanical proprieties of“magnesium based composites”with titanium-alum

*cAhsan Ullah

aCollege of Materials Science and Engineering,Chongqing University,Chongqing 400044,China

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

cChongqing Academy of Science and Technology,Chongqing,Chongqing 401123,China

dSchool of Materials Science and Engineering,Dalian University of Technology,Dalian 116024,China

eDepartment of Physics,Quaid-i-Azam University,Islamabad 46000,Pakistan

Improved mechanical proprieties of“magnesium based composites”with titanium-aluminum hybrids

Muhammad Rashada,b,*,Fusheng Pana,b,c,Muhammad Asifd,Jia Shea,b,Ahsan Ullahe

aCollege of Materials Science and Engineering,Chongqing University,Chongqing 400044,China

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

cChongqing Academy of Science and Technology,Chongqing,Chongqing 401123,China

dSchool of Materials Science and Engineering,Dalian University of Technology,Dalian 116024,China

eDepartment of Physics,Quaid-i-Azam University,Islamabad 46000,Pakistan

In this study,the effect of micron-sized titanium and aluminum addition on the microstructural,mechanical and work-hardening behavior of pure Mg is investigated.Pure Mg reinforced with 10%Ti and 10%Ti-1%Al particulates were synthesized through semi-powder metallurgy route followed by hot extrusion.Semi-powder metallurgy appears to be promising approach for the synthesis of Mg based composite,as it is free of ball milling.Tensile results indicate that the direct addition of micron-sized 10wt.%titanium particulates to pure Mg,caused an improvement in elastic modulus,0.2%yield strength,ultimate tensile strength,and failure strain(+72%;+41%;+29%;and+79%respectively).The addition of micron-sized 10wt.%titanium particles along with 1.0wt.%Al particles to pure Mg,resulted in an enhancement in elastic modulus,0.2% yield strength,ultimate tensile strength,and failure strain(+74%;+56%;+45%;and+241%respectively).Besides tensile test,Vickers hardness and work-hardening behavior of prepared composites were also examined.Impressive failure strain of Mg-10Ti-1Al composite can be attributed to the better compatibility of Ti particulates with Mg due to presence of alloying element Al.

Mechanical properties;Microstructure;Powder metallurgy method;Metal matrix composite

1.Introduction

Magnesium alloys are a class of structural materials with increasing industrial interest in automobile service due to their good strength to weight ratio and low density[1].Mg has hexagonal closed-packed(HCP)structure which leads to low ductility and toughness[2].The problem of low ductility and tensile strength of Mg can be overcome by incorporation of different kind of reinforcements in the form of particles or fbers.Literature study reveals that ceramic and intermetallic (SiC,TiC,TiB2,Al2O3,Y2O3,TiO2,Mg2Sietc)reinforcements have been extensively used to increase the strength of monolithic Mg[3-14].But brittle nature of reinforcements leads to limited ductility of Mg composites. During past decade,carbon nanotubes(CNTs)have been extensively used as reinforcement for magnesium composites. Even though CNT/Mg composites have been extensively investigated,but uniform dispersion of CNTs in the matrix is big challenge for researchers which limit its use for practicalapplications.This is caused by agglomerates formation due to its one dimensional structure and strong van der Waal attractions between carbon atoms[15,16].

Metallic reinforcement such as titanium has good ductility, strength,hardness and Young's modulus.The main advantage of Ti based Mg alloys it that there is no formation of any brittle inter-metallic compounds between Ti and Mg as shown in Ti-Mg binary phase diagram[17].The research on hybrid reinforcement is gaining importance in recent years because they have positive infuence on the mechanical properties of the Mg composites[18,19].In 2011,Sankaranarayanan et al. [20]investigated the mechanical behavior of Mg-5.6wt.%Ti-2.5wt.%Al2O3composite.The evaluation of mechanical properties indicated a signifcant enhancement in tensile strength however failure stain was no more than 6.8%.Similar behavior in strength properties were observed when Cu particulates were added to Mg-5.6wt.%Ti alloy[21].Recently, Sankaranarayanan et al.Ref.[22]examined the effect of nano-SiC particles on mechanical behavior of Mg-5.6wt.%Ti composites.Room temperature tensile resultsrevealed animprovement in tensile strength and failure strain(i.e Failure strain was 9.6%).In another report,effect of 9.6wt.%Ti particulates addition on mechanical properties of pure Mg was investigated.Tensile results indicated improvement in tensile strength and ductility(ductility was 9.5%)[23].

Fig.1.Flowchart of semi-powder metallurgy method.

Fig.2.X-ray diffraction spectra of pure Mg and its composites conducted on: (a)powder samples;and(b)extruded samples.

In current work,an attempt have been made to increase the ductility of pure Mg by adding micron-sized Ti(10wt.%)and Al(1.0wt.%)particulates through semi-powder metallurgy technique.Room temperature mechanical testing revealed a signifcant enhancement in tensile strength and failure strain. The failure strain value was higher than that of earlier reports [20-23].Besides tensile strength,microstructure and workhardening behavior of Pure Mg and its composites were also analyzed.

2.Experimental procedures

2.1.Materials

Raw materials,magnesium,aluminum and titanium powders(having particle size of 74,3 and 25 μm respectively) with 99.5%purity were purchased from Shanghai Customs Golden Powder Material Co.Ltd.China.

2.2.Processing

Ball milling is an incompatible technique as it produces heat which can burn Mg powder easily.Therefore,a simple solution based strategy named as semi-powder metallurgy method was adopted to mix the composite powders(Fig.1). Pure Mg powder was mixed in ethanol using a mechanical agitator at the speed of 2000RPM.At the same time reinforcement particles 10wt.%Ti and 1.0wt.%Al were mixed in ethanol using magnetic stirring.Reinforcement particle solution was then added drop wise into the above Mg slurry in ethanol.Mixing process was continued for an hour to obtain the homogeneous mixture.Mechanically agitated mixture was fltered and vacuum dried at 80°C for 12 h to obtain the mixture powder.Samples of pure Mg and Mg-10wt.%Ti composite were prepared using same procedure.Pure Mg, Mg-10wt.%Ti and Mg-10wt.%Ti-1.0wt.%Al composite powders were compacted under 620 MPa pressure to obtain the green billets of 75 mm in diameter and 40 mm in height.The compacted billets were sintered in the box furnace at 630°C for 110 min under argon atmosphere.The sintered billets were preheated to 350°C for an hour and extruded at 1 m/min extrusion speed.Final diameter of the rods obtained after extrusion was 16 mm.Samples from extruded rods were used for further characterization.

2.3.Materials characterization

X-ray diffraction(XRD)analysis on powder mixtures and polished samples from extruded bars were carried out by X-ray diffraction(D/MAX-1200,China),using Cu Kα radiation in the range 10-90°.Raw XRD data were refned and analyzed via MDI Jade 6.0 program(Materials Data Incorporated:Livermore,CA,USA).Samples for microstructural characterization were machined from the extruded bars.Optical microscopy was used to investigate the grain size of pure Mg and Mg-10wt.%Ti,Mg-10wt.%Ti-1.0wt.%Al composites. Scanning electron microscopy(SEM)equipped with energydispersive spectrometer(EDS)was used to analyze the surface morphology and dispersion of reinforcement particles in the matrix.Automatic digital micro hardness tester (SHANGHAI HX-1000TM)was used to measure the Vickers Hardness of monolithic Mg and Mg-10wt.%Ti,Mg-10wt.%Ti-1.0wt.%Al composites.Micro hardness test was carried out on polished samples under a load of 100 g and 15 s dwell time in accordance with the ASTM standard E384-99.For tensile test,round samples with 3 mm diameter and 15 mm gauge length were machined from the extruded rods(Fig.1).Tensile test was carried out at ambient temperature with initial strain speed of 1× 10-3s-1and the tensile direction was parallel to extrusion direction(ED).Three samples were made for each composition to minimize the error.Images of tensile fracture surfaces were taken using SEM.

Fig.3.Optical microscopic images showing the grain characteristic of:(a)Pure Mg;(b)Mg-10Ti;and(c)Mg-10Ti-1Al composites.

Table 1Grain size characteristics of pure Mg and its composites.

3.Results and discussion

3.1.Microstructure

The results of x-ray diffraction analysis conducted on pure Mg,Mg-10Ti and Mg-10Ti-1Al composite powders are depicted in Fig.2(a).In addition to the Mg peaks,Ti peaks were observed in the powder samples.Peaks corresponding to the Al were absent which can be attributed to the low volume fraction of Al in the Mg-10Ti-1Al composite.Fig.2(b) shows the x-ray diffraction patterns of extruded samples.It is clear from the fgure that only peaks corresponding to pure Mg and Ti are observed in Mg-10Ti and Mg-10Ti-1Al extruded composites.This witness that no phase formation occurs between Mg and Ti which is consistent with their binary phase diagram[17].According to phase diagrams of Mg-Al and Ti-Al,the Al can react with both Mg and Ti to form inter metallic phases however in Mg-10Ti-1Al composite its content is too low to form intermetallic phase and was not detected by XRD.For both Mg-10Ti and Mg-10Ti-1Al composites,the intensity of Mg diffraction patterns becomes stronger which may be attributed to the recrystallization and grain refnement during sintering and extrusion process.

The grain characteristics(grain size and morphology)of pure Mg and its composites are depicted in Fig.3(a-c)and Table 1.The pure Mg exhibits largest grain size(29 μm). However,addition of Ti particulates to the pure Mg(Mg-10Ti composite)leads to the refned grain structure.The combined additions of micron-sized Ti and Al particulates to the pure Mg,lead to the effective reduction in grain size which reveals the smallest grain size(about 10 μm)among all the materials.

Fig.4.SEM micrographs showing the surface morphology:(a)Pure Mg;(b)Mg-10Ti;(c)Mg-10Ti-1Al;and(d)micrograph showing the insoluble Ti particulates in the Mg matrix.

Table 2Room temperature mechanical properties of Pure Mg,Mg-10Ti and Mg-10Ti-1Al composites.

The microstructure of pure Mg and synthesized composites is depicted in Fig.4(a-d).It can be seen that pure Mg exhibits large grain size and a lot of micro pores on its surface.On the other hand,Mg-10Ti composite exhibits very smooth surface with few micro pores on its surface.Therefore reveals higher tensile strength as compare to pure Mg(Table 2).The microstructure of Mg-10Ti-1Al composite reveals unclear grain boundaries which may be due to diffusion of Al particulates at grain boundaries.The diffused Al particulates induce the better compatibility between Mg and Ti particles thus resulting in higher tensile strength as compared to the pure Mg and Mg-10Ti composite(Table 2).Moreover it can be seen from Fig.4(d)that Ti particles are insoluble in the Mg matrix.Therefore Mg matrix and Ti particles interface are free of intermetallic phase,as evident from the XRD analysis (Fig.2(b))and phase diagram[17].The insoluble Ti particles act as site for the grain nucleation center during the sintering and extrusion process.Therefore,restricts the grain growth,so resulting in refned structure[24].

Fig.5(a-d) shows x-ray mapping results ofthe Mg-10Ti-1Al composite.It can be seen that reinforcements Ti and Al are uniformly distributed in the matrix.The reasonably uniform distribution of reinforcement particles can be attributed to the effcient strategy adopted while fabrication of the composites.

3.2.Mechanical characterization

Fig.5.X-ray mapping of Mg-10Ti-1Al composite:(a)Mg-10Ti-1Al composite;(b)Magnesium matrix;(c)Titanium;and(d)Aluminum.

The room temperature mechanical properties are depicted in Fig.6 and Table 2.It can be seen that pure Mg reveals very low hardness,tensile strength and failure strain.However addition of micron-sized Tiparticulates to pure Mg (Mg-10Ti)leads to signifcant enhancement in hardness, elastic modulus,yield strength,ultimate tensile strength and failure strain.The improvement in mechanical properties of Mg-10Ti composite can be attributed to the strengthening effects arisen from the(1)Hall-Petch relationship due to refned grains(Fig.2 and Table 1),(2)mismatch in coeffcient of thermal expansion(CTE),elastic modulus and hardness between Mg matrix and Ti particulates,and(c)opposition offered by Ti particulates against the dislocation motion[25-28].

The synergetic effectof10wt% Tiand 1wt%Al (Mg-10Ti-1Al composite)particulates in the Mg matrix revealed the impressive increase in the hardness,tensile strength and failure strain values.The Mg-10Ti-1Al composite displayed the higher mechanical properties than pure Mg and Mg-10Ti composite as shown in Fig.6 and Table 2. The enhancement in the tensile strength is due to the effects similar to that observed with addition of individual microsized Ti particulates,as explained in above paragraph.

It can be seen from Table 2 that an impressive enhancement in failure strain(21.2%)was achieved by the combined addition of Ti and Al particulates.Interestingly such failure stain improvement occurred along with signifcant positive effect on the tensile properties.Since the Ti is more ductile as compare to the Mg,therefore during homogeneous dispersion of Ti particulates in the matrix,the ductile Ti particles can more easily assist the geometrical changes of Mg during tensile loading without rupture.In addition,the absence of intermetallic phases is also advantageous.Thus leads to the higher failure strain(11.1%)of Mg-10Ti composite.The failure strain of the synthesized Mg-10Ti composite is limited up to 11.1%which maybe attribute to the insolubility of Ti in the Mg matrix.Thus leads to poor bonding between matrix Mg and Ti particles.The common alloying element Al has good solubility and bonding with the matrix Mg and Ti particulates.Therefore,synergetic effect of Ti and small fraction of Al is effective to improve the boding between Ti particles and Mg matrix.Thus lead to impressive increase in the failure strain of the Mg-10Ti-1Al composite (Fig.6 and Table 2).The small fraction of alloying element Al was used to prevent the formation of intermetallic phases between Ti/Mg and Al which have adverse effect on the failure strain.

Fig.6.Mechanical behavior of pure Mg,Mg-10Ti and Mg-10Ti-1Al composites:(a)True stress-strain curves;(b)Engineering stress-strain curves;(c)workhardening rate vs strain plots;and(d)Hardening capacities.

Besides tensile test,work-hardening behavior of the samples was also examined as shown in Fig.6(c).The workhardening rate,θ(θ=dσ/dε;where σ and ε are macroscopic stress and strain)[29]verses strain ε curves are shown in Fig.6(c).It can be seen that work-hardening rate θ for pure Mg,Mg-10Ti and Mg-10Ti-1Al composites are 9689, 14,069,and 13,827 MPa respectively.It can be observed from the graph that Mg-10Ti and Mg-10Ti-1Al composites exhibit steeper curves than pure Mg of stage III.The difference in slopes of stage III may be attributed to the difference of the dislocations density.The hardening capacity,Hcof the materials can be defned as Hc=(σUTS-σ0.2)/σ0.2[30]where σUTSand σ0.2are the ultimate tensile stress and 0.2%yield stress.Fig.6(d)revealed that pure Mg exhibit highest hardening capacity(0.58).One the other hand Mg-10Ti and Mg-10Ti-1Al composites revealed hardening capacities of 0.44 and 0.46 respectively.The variation of Hcis related to the grain size and dislocation density of as extruded samples.

3.3.Fractography

The fractographic evidences of pure Mg and its composites under tensile loading are depicted in Fig.7(a-c).The fractograph of pure Mg exhibits brittle fracture as shown in Fig.7(a).Generally,micro-cracks are generated in the composites due to the interfacial stresses between matrix and reinforcements[31].The tensile fracture image of Mg-10Ti composite reveals cleavage planes and tear ridges as shown in Fig.7(b).The fracture image of Mg-10Ti-1Al composite (Fig.7(c))composed of dimples which witness the high elasticity(21.2%)[32-39].

4.Conclusions

The pure Mg and its composites were successfully synthesized through semi-powder metallurgy method followed by hot extrusion technique.Based on microstructural and mechanical characterization following conclusions can be drawn.

1-Semi-powder metallurgy method is an effcient technique to fabricate Mg based composite by excluding the ball milling process.

2-Compare to monolithic Mg,the synthesized composites (Mg-10Ti&Mg-10Ti-1Al)exhibited improved hardness,elastic modulus,0.2% yield strength,ultimate strength and failure strain(%).

3-The impressive increase in failure strain of the Mg-10Ti-1Al composite is due to the better compatibility of Mg matrix with Ti particulates due to presence of small fraction of alloying element Al.

4-Increased hardness and tensile strength of the composites can be attributed to the(a)mismatch in CTE and Elastic modulus;(b)Orowan strengthening;and(c)load transfer mechanism,between Mg matrix and reinforcement.

Fig.7.Tensile fracture images of:(a)Pure Mg;(b)Mg-10Ti;and(c)Mg-10Ti-1Al composite.

Acknowledgment

The present work was supported by the National Natural Science Funds of China(No.50725413),the Ministry of Science and Technology of China (MOST) (No. 2010DFR50010 and 2011FU125Z07),and Chongqing Science and Technology Commission(CSTC2013JCYJC60001).

[1]F.H.Froes,Mater.Sci.Eng.A 184(1994)119-133.

[2]M.Rashad,F.Pan,M.Asif,in:M.S.A.Tiwari(Eds.),Graphene materials:fundamentals and emerging applications,Wiley-Scrivener PublishingLLC,Beverly,MA,2015,pp.153-189.

[3]J.Lan,Y.Yang,X.Li,Mater.Sci.Eng.A 386(2004)284-290.

[4]H.Ferkel,B.L.Mordike,Mater.Sci.Eng.A 298(2001)193-199.

[5]R.A.Saravanan,M.K.Surappa,Mater.Mater.Sci.Eng.A 276(2000) 108-116.

[6]M.Gupta,M.O.Lai,D.Saravanaranganathan,J.Mater.Sci.35(2000) 2155-2165.

[7]Z.Xiuqing,W.Haowei,L.Lihua,T.Xinying,M.Naiheng,Mater.Lett. 59(2005)2105-2109.

[8]G.Garc′es,M.Rodríguez,P.P′erez,P.Adeva,Mater.Mater.Sci.Eng.A 419(2006)357-364.

[9]S.F.Hassan,M.Gupta,Mater.Mater.Sci.Eng.A 392(2005)163-168.

[10]L.Lu,K.K.Thong,M.Gupta,Comp.Sci.Technol.63(2003)627-632.

[11]V.Skleniˇcka,M.Svoboda,M.Pahutov′a,K.Kuchaˇrov′a,T.G.Langdon, Mater.Sci.Eng.A 319-321(2001)741-745.

[12]C.Mayencourt,R.Schaller,Mater.Mater.Sci.Eng.A325(2002)286-291.

[13]Y.Park,K.Terasaki,K.Igarashi,T.Shimizu,Adv.Comp.Mater.10 (2001)17-28.

[14]S.Vaucher,O.Beffort,J.Ku¨bler,F.Lehner,Adv.Eng.Mater.5(2003) 669-672.

[15]T.Hertel,R.E.Walkup,P.Avouris,Phy.Rev.B 58(1998)13870-13873.

[16]L.Y.Jiang,Y.Huang,H.Jiang,G.Ravichandran,H.Gao,K.C.Hwang, B.Liu,J.Mech.Phys.Solids 54(2006)2436-2452.

[17]J.L.Murray,ASM Int.(1998).

[18]M.Rashad,F.Pan,H.Hu,M.Asif,S.Hussain,J.She,Mater Sci Eng A 630(2015)36-44.

[19]M.K.Habibi,S.P.Joshi,M.Gupta,Acta Mater.58(2010)6104-6114.

[20]S.Sankaranarayanan,S.Jayalakshmi,M.Gupta,J.Alloys Compd.509 (2011)7229-7237.

[21]S.Sankaranarayanan,S.Jayalakshmi,M.Gupta,Mater.Des.37(2012) 274-284.

[22]S.Sankaranarayanan,R.K.Sabat,S.Jayalakshmi,S.Suwas,M.Gupta,J. Alloys Compd.575(2013)207-217.

[23]S.F.Hassan,M.Gupta,J.Alloys Compd.345(2002)246-251.

[24]M.Gupta,T.S.Srivatsan,J.Mater.Eng.Perform.8(1999)473-478.

[25]D.J.Lloyd,Int.Mater.Rev.39(1994)1-23.

[26]S.Colin,Metals Reference Book,ffth ed.,Butterworth's&Co.Ltd, London,1976.

[27]G.Meijer,F.Ellyin,Z.Xia,Comp.Part B:Eng.31(2000)29-37.

[28]G.E.Dieter,Mechanical Metallurgy,McGraw-Hill,USA,1986.

[29]U.F.Kocks,H.Mecking,Prog.Mater.Sci.48(2003)171-173.

[30]N.Afrin,D.L.Chen,X.Cao,M.Jahazi,Scr.Mater.57(2007) 1004-1007.

[31]N.M.L.S.,M.Gupta,Magn.Magn.Alloys Magn.Composit.(2011). Wiley.com.

[32]M.Rashad,F.Pan,A.Tang,Y.Lu,M.Asif,S.Hussain,J.She,J.Gou, J.Mao,J.Magn.Alloys 1(2013)242-248.

[33]M.Rashad,F.Pan,M.Asif,A.Tang,J.Indust.Eng.Chem.20(2014) 4250-4255.

[34]M.Rashad,F.Pan,A.Tang,M.Asif,J.She,J.Gou,J.Mao,H.Hu,J Com Mater 49(3)(2015)285-293.

[35]M.Rashad,F.Pan,A.Tang,M.Asif,S.Hussain,J.Gou,J.Mao,J.Ind. Eng.Chem.(2014).http://dx.doi.org/10.1016/j.jiec.2014.08.024.

[36]M.Rashad,F.Pan,A.Tang,M.Asif,M.Aamir,J.Alloys Compd.603 (2014)111-118.

[37]M.Rashad,F.Pan,M.Asif,S.Hussain,M.Saleem,Mater.Charact.95 (2014)140-147.

[38]M.Rashad,F.Pan,A.Tang,M.Asif,Prog.Nat.Sci.24(2014)101-108.

[39]M.Rashad,F.Pan,M.Asif,A.Ullah,Mater.Sci.Technol.(2014),http:// dx.doi.org/10.1179/1743284714Y.0000000726.

Received 25 November 2014;revised 19 December 2014;accepted 25 December 2014 Available online 25 March 2015

*Corresponding author.College of Materials Science and Engineering, Chongqing University,Chongqing 400044,China.

E-mail address:rashadphy87@gmail.com(M.Rashad).

Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China,Chongqing University.

http://dx.doi.org/10.1016/j.jma.2014.12.010.

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Copyright 2015,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.