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Journal of Magnesium and Alloys 3 (2015) 1e9www.elsevier.com/journals/journal-of-magnesium-and-alloys/2213-9567

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Improved mechanical proprieties of “magnesium based composites” withtitaniumealuminum hybrids

Muhammad Rashad a,b,*, Fusheng Pan a,b,c, Muhammad Asif d, Jia She a,b, Ahsan Ullah e

a College of Materials Science and Engineering, Chongqing University, Chongqing 400044, Chinab National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China

c Chongqing Academy of Science and Technology, Chongqing, Chongqing 401123, Chinad School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China

e Department of Physics, Quaid-i-Azam University, Islamabad 46000, Pakistan

Received 25 November 2014; revised 19 December 2014; accepted 25 December 2014

Available online 25 March 2015

Abstract

In this study, the effect of micron-sized titanium and aluminum addition on the microstructural, mechanical and work-hardening behavior ofpure Mg is investigated. Pure Mg reinforced with 10%Ti and 10%Tie1%Al particulates were synthesized through semi-powder metallurgy routefollowed by hot extrusion. Semi-powder metallurgy appears to be promising approach for the synthesis of Mg based composite, as it is free ofball milling. Tensile results indicate that the direct addition of micron-sized 10wt.% titanium particulates to pure Mg, caused an improvement inelastic modulus, 0.2% yield strength, ultimate tensile strength, and failure strain (þ72%; þ41%; þ29%; and þ79% respectively). The additionof 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 (þ74%; þ56%; þ45%; and þ241% respectively). Besides tensile test, Vickershardness and work-hardening behavior of prepared composites were also examined. Impressive failure strain of Mge10Tie1Al composite canbe attributed to the better compatibility of Ti particulates with Mg due to presence of alloying element Al.Copyright 2015, National Engineering Research Center for Magnesium Alloys of China, Chongqing University. Production and hosting byElsevier B.V. All rights reserved.

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

1. Introduction

Magnesium alloys are a class of structural materials withincreasing industrial interest in automobile service due to theirgood strength to weight ratio and low density [1]. Mg hashexagonal closed-packed (HCP) structure which leads to low

* Corresponding author. College of Materials Science and Engineering,

Chongqing University, Chongqing 400044, China.

E-mail address: [email protected] (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.

2213-9567/Copyright 2015, National Engineering Research Center for Magnesium Alloys of China, Cho

ductility and toughness [2]. The problem of low ductility andtensile strength of Mg can be overcome by incorporation ofdifferent kind of reinforcements in the form of particles orfibers. Literature study reveals that ceramic and intermetallic(SiC, TiC, TiB2, Al2O3, Y2O3, TiO2, Mg2Si etc) re-inforcements have been extensively used to increase thestrength of monolithic Mg [3e14]. But brittle nature of re-inforcements leads to limited ductility of Mg composites.During past decade, carbon nanotubes (CNTs) have beenextensively used as reinforcement for magnesium composites.Even though CNT/Mg composites have been extensivelyinvestigated, but uniform dispersion of CNTs in the matrix isbig challenge for researchers which limit its use for practical

ngqing University. Production and hosting by Elsevier B.V. All rights reserved.

2 M. Rashad et al. / Journal of Magnesium and Alloys 3 (2015) 1e9

applications. This is caused by agglomerates formation due toits one dimensional structure and strong van der Waal attrac-tions between carbon atoms [15,16].

Metallic reinforcement such as titanium has good ductility,strength, hardness and Young's modulus. The main advantageof Ti based Mg alloys it that there is no formation of anybrittle inter-metallic compounds between Ti and Mg as shownin TieMg binary phase diagram [17]. The research on hybridreinforcement is gaining importance in recent years becausethey have positive influence on the mechanical properties of

Fig. 1. Flowchart of semi-pow

the Mg composites [18,19]. In 2011, Sankaranarayanan et al.[20] investigated the mechanical behavior of Mg-5.6wt.%Ti-2.5wt.%Al2O3 composite. The evaluation of mechanicalproperties indicated a significant enhancement in tensilestrength however failure stain was no more than 6.8%. Similarbehavior in strength properties were observed when Cu par-ticulates 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 com-posites. Room temperature tensile results revealed an

der metallurgy method.

Fig. 2. X-ray diffraction spectra of pure Mg and its composites conducted on:

(a) powder samples; and (b) extruded samples.

3M. Rashad et al. / Journal of Magnesium and Alloys 3 (2015) 1e9

improvement in tensile strength and failure strain (i.e Failurestrain was 9.6%). In another report, effect of 9.6wt.%Ti par-ticulates addition on mechanical properties of pure Mg wasinvestigated. Tensile results indicated improvement in tensilestrength and ductility (ductility was 9.5%) [23].

In current work, an attempt have been made to increase theductility of pure Mg by adding micron-sized Ti (10wt.%) andAl (1.0wt.%) particulates through semi-powder metallurgytechnique. Room temperature mechanical testing revealed asignificant enhancement in tensile strength and failure strain.The failure strain value was higher than that of earlier reports[20e23]. Besides tensile strength, microstructure and work-hardening behavior of Pure Mg and its composites were alsoanalyzed.

2. Experimental procedures

2.1. Materials

Raw materials, magnesium, aluminum and titanium pow-ders (having particle size of 74, 3 and 25 mm respectively)with 99.5% purity were purchased from Shanghai CustomsGolden Powder Material Co. Ltd. China.

2.2. Processing

Ball milling is an incompatible technique as it producesheat which can burn Mg powder easily. Therefore, a simplesolution based strategy named as semi-powder metallurgymethod was adopted to mix the composite powders (Fig. 1).Pure Mg powder was mixed in ethanol using a mechanicalagitator at the speed of 2000RPM. At the same time rein-forcement particles 10wt.%Ti and 1.0wt.% Al were mixed inethanol using magnetic stirring. Reinforcement particle solu-tion was then added drop wise into the above Mg slurry inethanol. Mixing process was continued for an hour to obtainthe homogeneous mixture. Mechanically agitated mixture wasfiltered and vacuum dried at 80 �C for 12 h to obtain themixture powder. Samples of pure Mg and Mg-10wt.%Ticomposite were prepared using same procedure. Pure Mg,Mg-10wt.%Ti and Mg-10wt.%Ti-1.0wt.%Al composite pow-ders were compacted under 620 MPa pressure to obtain thegreen billets of 75 mm in diameter and 40 mm in height. Thecompacted billets were sintered in the box furnace at 630 �Cfor 110 min under argon atmosphere. The sintered billets werepreheated to 350 �C for an hour and extruded at 1 m/minextrusion speed. Final diameter of the rods obtained afterextrusion was 16 mm. Samples from extruded rods were usedfor further characterization.

2.3. Materials characterization

X-ray diffraction (XRD) analysis on powder mixtures andpolished samples from extruded bars were carried out by X-ray diffraction (D/MAX-1200, China), using Cu Ka radiationin the range 10e90�. Raw XRD data were refined andanalyzed via MDI Jade 6.0 program (Materials Data Incor-porated: Livermore, CA, USA). Samples for microstructuralcharacterization were machined from the extruded bars. Op-tical microscopy was used to investigate the grain size of pureMg and Mg-10wt.%Ti, Mg-10wt.%Ti-1.0wt.%Al composites.Scanning electron microscopy (SEM) equipped with energy-dispersive spectrometer (EDS) was used to analyze the sur-face morphology and dispersion of reinforcement particles inthe matrix. Automatic digital micro hardness tester(SHANGHAI HX-1000TM) was used to measure the VickersHardness of monolithic Mg and Mg-10wt.%Ti, Mg-10wt.%Ti-1.0wt.%Al composites. Micro hardness test was carried out onpolished samples under a load of 100 g and 15 s dwell time inaccordance with the ASTM standard E384-99. For tensile test,

Fig. 3. Optical microscopic images showing the grain characteristic of: (a) Pure Mg; (b) Mge10Ti; and (c) Mge10Tie1Al composites.

Table 1

Grain size characteristics of pure Mg and its composites.

Materials Grain size (mm) Aspect ratio

Pure Mg 29 ± 3.5 1.62 ± 0.32

Mge10Ti 20 ± 4.1 1.58 ± 0.40

Mge10Tie1Al 10 ± 3.2 1.60 ± 0.25

4 M. Rashad et al. / Journal of Magnesium and Alloys 3 (2015) 1e9

round samples with 3 mm diameter and 15 mm gauge lengthwere machined from the extruded rods (Fig. 1). Tensile testwas carried out at ambient temperature with initial strain speedof 1 � 10-3 s�1 and the tensile direction was parallel toextrusion direction (ED). Three samples were made for eachcomposition to minimize the error. Images of tensile fracturesurfaces were taken using SEM.

3. Results and discussion

3.1. Microstructure

The results of x-ray diffraction analysis conducted on pureMg, Mge10Ti and Mge10Tie1Al composite powders aredepicted in Fig. 2(a). In addition to the Mg peaks, Ti peakswere observed in the powder samples. Peaks corresponding to

the Al were absent which can be attributed to the low volumefraction of Al in the Mge10Tie1Al composite. Fig. 2(b)shows the x-ray diffraction patterns of extruded samples. It isclear from the figure that only peaks corresponding to pure Mgand Ti are observed in Mge10Ti and Mge10Tie1Al extrudedcomposites. This witness that no phase formation occurs be-tween Mg and Ti which is consistent with their binary phasediagram [17]. According to phase diagrams of MgeAl andTieAl, the Al can react with both Mg and Ti to form intermetallic phases however in Mge10Tie1Al composite itscontent is too low to form intermetallic phase and was notdetected by XRD. For both Mge10Ti and Mge10Tie1Alcomposites, the intensity of Mg diffraction patterns becomesstronger which may be attributed to the recrystallization andgrain refinement during sintering and extrusion process.

The grain characteristics (grain size and morphology) ofpure Mg and its composites are depicted in Fig. 3(aec) andTable 1. The pure Mg exhibits largest grain size (29 mm).However, addition of Ti particulates to the pure Mg (Mge10Ticomposite) leads to the refined grain structure. The combinedadditions of micron-sized Ti and Al particulates to the pureMg, lead to the effective reduction in grain size which revealsthe smallest grain size (about 10 mm) among all the materials.

The microstructure of pure Mg and synthesized compositesis depicted in Fig. 4(aed). It can be seen that pure Mg exhibitslarge grain size and a lot of micro pores on its surface. On the

Fig. 4. SEM micrographs showing the surface morphology: (a) Pure Mg; (b) Mge10Ti; (c) Mge10Tie1Al; and (d) micrograph showing the insoluble Ti par-

ticulates in the Mg matrix.

Table 2

Room temperature mechanical properties of Pure Mg, Mge10Ti and Mge10Tie1Al composites.

Materials E (GPa) 0.2%YS (MPa) UTS (MPa) d (%) Vickers hardness (HV)

Pure Mg ~7.0 ~104 ~164 ~6.2 ~40

Mge10Ti ~12.1 ~147 ~212 ~11.1 ~46

Mge10Tie1Al ~12.2 ~163 ~238 ~21.2 ~55

E: Elastic modulus; YS: yield stress; UTS: ultimate tensile stress; d: strain to failure.

5M. Rashad et al. / Journal of Magnesium and Alloys 3 (2015) 1e9

other hand, Mge10Ti composite exhibits very smooth surfacewith few micro pores on its surface. Therefore reveals highertensile strength as compare to pure Mg (Table 2). Themicrostructure of Mge10Tie1Al composite reveals uncleargrain boundaries which may be due to diffusion of Al par-ticulates at grain boundaries. The diffused Al particulatesinduce the better compatibility between Mg and Ti particlesthus resulting in higher tensile strength as compared to thepure Mg and Mge10Ti composite (Table 2). Moreover it canbe seen from Fig. 4(d) that Ti particles are insoluble in the Mgmatrix. Therefore Mg matrix and Ti particles interface are freeof intermetallic phase, as evident from the XRD analysis(Fig. 2(b)) and phase diagram [17]. The insoluble Ti particlesact as site for the grain nucleation center during the sintering

and extrusion process. Therefore, restricts the grain growth, soresulting in refined structure [24].

Fig. 5(aed) shows x-ray mapping results of theMge10Tie1Al composite. It can be seen that reinforcementsTi and Al are uniformly distributed in the matrix. Thereasonably uniform distribution of reinforcement particles canbe attributed to the efficient strategy adopted while fabricationof the composites.

3.2. Mechanical characterization

The room temperature mechanical properties are depictedin Fig. 6 and Table 2. It can be seen that pure Mg reveals verylow hardness, tensile strength and failure strain. However

Fig. 5. X-ray mapping of Mge10Tie1Al composite: (a) Mge10Tie1Al composite; (b) Magnesium matrix; (c) Titanium; and (d) Aluminum.

6 M. Rashad et al. / Journal of Magnesium and Alloys 3 (2015) 1e9

addition of micron-sized Ti particulates to pure Mg(Mge10Ti) leads to significant enhancement in hardness,elastic modulus, yield strength, ultimate tensile strength andfailure strain. The improvement in mechanical properties ofMge10Ti composite can be attributed to the strengtheningeffects arisen from the (1) Hall-Petch relationship due torefined grains (Fig. 2 and Table 1), (2) mismatch in co-efficient of thermal expansion (CTE), elastic modulus andhardness between Mg matrix and Ti particulates, and (c) op-position offered by Ti particulates against the dislocationmotion [25e28].

The synergetic effect of 10wt% Ti and 1wt%Al(Mge10Tie1Al composite) particulates in the Mg matrixrevealed the impressive increase in the hardness, tensilestrength and failure strain values. The Mge10Tie1Al com-posite displayed the higher mechanical properties than pureMg and Mge10Ti composite as shown in Fig. 6 and Table 2.The enhancement in the tensile strength is due to the effectssimilar to that observed with addition of individual micro-sized Ti particulates, as explained in above paragraph.

It can be seen from Table 2 that an impressive enhance-ment in failure strain (21.2%) was achieved by the combinedaddition of Ti and Al particulates. Interestingly such failurestain improvement occurred along with significant positiveeffect on the tensile properties. Since the Ti is more ductileas compare to the Mg, therefore during homogeneous

dispersion of Ti particulates in the matrix, the ductile Tiparticles can more easily assist the geometrical changes ofMg during tensile loading without rupture. In addition, theabsence of intermetallic phases is also advantageous. Thusleads to the higher failure strain (11.1%) of Mge10Ti com-posite. The failure strain of the synthesized Mge10Ti com-posite is limited up to 11.1% which maybe attribute to theinsolubility of Ti in the Mg matrix. Thus leads to poorbonding between matrix Mg and Ti particles. The commonalloying element Al has good solubility and bonding with thematrix Mg and Ti particulates. Therefore, synergetic effect ofTi and small fraction of Al is effective to improve the bodingbetween Ti particles and Mg matrix. Thus lead to impressiveincrease in the failure strain of the Mge10Tie1Al composite(Fig. 6 and Table 2). The small fraction of alloying elementAl was used to prevent the formation of intermetallic phasesbetween Ti/Mg and Al which have adverse effect on thefailure strain.

Besides tensile test, work-hardening behavior of the sam-ples was also examined as shown in Fig. 6(c). The work-hardening rate, q (q ¼ ds/dε; where s and ε are macro-scopic stress and strain) [29] verses strain ε curves are shownin Fig. 6(c). It can be seen that work-hardening rate q for pureMg, Mge10Ti and Mge10Tie1Al composites are 9689,14,069, and 13,827 MPa respectively. It can be observed fromthe graph that Mge10Ti and Mge10Tie1Al composites

Fig. 6. Mechanical behavior of pure Mg, Mge10Ti and Mge10Tie1Al composites: (a) True stressestrain curves; (b) Engineering stressestrain curves; (c) work-

hardening rate vs strain plots; and (d) Hardening capacities.

7M. Rashad et al. / Journal of Magnesium and Alloys 3 (2015) 1e9

exhibit steeper curves than pure Mg of stage III. The differ-ence in slopes of stage III may be attributed to the differenceof the dislocations density. The hardening capacity, Hc of thematerials can be defined as Hc¼(sUTS � s0.2)/s0.2 [30] wheresUTS and s0.2 are the ultimate tensile stress and 0.2% yieldstress. Fig. 6(d) revealed that pure Mg exhibit highest hard-ening capacity (0.58). One the other hand Mge10Ti andMge10Tie1Al composites revealed hardening capacities of0.44 and 0.46 respectively. The variation of Hc is related to thegrain size and dislocation density of as extruded samples.

3.3. Fractography

The fractographic evidences of pure Mg and its compositesunder tensile loading are depicted in Fig. 7(aec). The frac-tograph of pure Mg exhibits brittle fracture as shown inFig. 7(a). Generally, micro-cracks are generated in the com-posites due to the interfacial stresses between matrix and re-inforcements [31]. The tensile fracture image of Mge10Ticomposite reveals cleavage planes and tear ridges as shown inFig. 7(b). The fracture image of Mge10Tie1Al composite(Fig. 7(c)) composed of dimples which witness the highelasticity (21.2%) [32e39].

4. Conclusions

The pure Mg and its composites were successfully syn-thesized through semi-powder metallurgy method followed byhot extrusion technique. Based on microstructural and me-chanical characterization following conclusions can be drawn.

1- Semi-powder metallurgy method is an efficient techniqueto fabricate Mg based composite by excluding the ballmilling process.

2- Compare to monolithic Mg, the synthesized composites(Mge10Ti & Mge10Tie1Al) exhibited improved hard-ness, elastic modulus, 0.2% yield strength, ultimatestrength and failure strain (%).

3- The impressive increase in failure strain of theMge10Tie1Al composite is due to the better compatibilityof Mg matrix with Ti particulates due to presence of smallfraction of alloying element Al.

4- Increased hardness and tensile strength of the compositescan be attributed to the (a) mismatch in CTE and Elasticmodulus; (b) Orowan strengthening; and (c) load transfermechanism, between Mg matrix and reinforcement.

Fig. 7. Tensile fracture images of: (a) Pure Mg; (b) Mge10Ti; and (c) Mge10Tie1Al composite.

8 M. Rashad et al. / Journal of Magnesium and Alloys 3 (2015) 1e9

Acknowledgment

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

References

[1] F.H. Froes, Mater. Sci. Eng. A 184 (1994) 119e133.

[2] M. Rashad, F. Pan, M. Asif, in: M. S. A. Tiwari (Eds.), Graphene ma-

terials: fundamentals and emerging applications, Wiley-Scrivener Pub-

lishing LLC, Beverly, MA, 2015, pp. 153e189.

[3] J. Lan, Y. Yang, X. Li, Mater. Sci. Eng. A 386 (2004) 284e290.[4] H. Ferkel, B.L. Mordike, Mater. Sci. Eng. A 298 (2001) 193e199.

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

108e116.[6] M. Gupta, M.O. Lai, D. Saravanaranganathan, J. Mater. Sci. 35 (2000)

2155e2165.

[7] Z. Xiuqing, W. Haowei, L. Lihua, T. Xinying, M. Naiheng, Mater. Lett.

59 (2005) 2105e2109.[8] G. Garc�es, M. Rodríguez, P. P�erez, P. Adeva, Mater. Mater. Sci. Eng. A

419 (2006) 357e364.

[9] S.F. Hassan, M. Gupta, Mater. Mater. Sci. Eng. A 392 (2005) 163e168.

[10] L. Lu, K.K. Thong, M. Gupta, Comp. Sci. Technol. 63 (2003) 627e632.[11] V. Skleni�cka, M. Svoboda, M. Pahutov�a, K. Kucha�rov�a, T.G. Langdon,

Mater. Sci. Eng. A 319e321 (2001) 741e745.

[12] C.Mayencourt, R. Schaller,Mater.Mater. Sci. Eng. A 325 (2002) 286e291.

[13] Y. Park, K. Terasaki, K. Igarashi, T. Shimizu, Adv. Comp. Mater. 10

(2001) 17e28.

[14] S. Vaucher, O. Beffort, J. Kubler, F. Lehner, Adv. Eng. Mater. 5 (2003)

669e672.

[15] T. Hertel, R.E. Walkup, P. Avouris, Phy. Rev. B 58 (1998) 13870e13873.

[16] L.Y. Jiang, Y. Huang, H. Jiang, G. Ravichandran, H. Gao, K.C. Hwang,

B. Liu, J. Mech. Phys. Solids 54 (2006) 2436e2452.

[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) 36e44.

[19] M.K. Habibi, S.P. Joshi, M. Gupta, Acta Mater. 58 (2010) 6104e6114.

[20] S. Sankaranarayanan, S. Jayalakshmi, M. Gupta, J. Alloys Compd. 509

(2011) 7229e7237.[21] S. Sankaranarayanan, S. Jayalakshmi, M. Gupta, Mater. Des. 37 (2012)

274e284.

[22] S. Sankaranarayanan, R.K. Sabat, S. Jayalakshmi, S. Suwas, M. Gupta, J.

Alloys Compd. 575 (2013) 207e217.

[23] S.F. Hassan, M. Gupta, J. Alloys Compd. 345 (2002) 246e251.

[24] M. Gupta, T.S. Srivatsan, J. Mater. Eng. Perform. 8 (1999) 473e478.

[25] D.J. Lloyd, Int. Mater. Rev. 39 (1994) 1e23.[26] S. Colin, Metals Reference Book, fifth ed., Butterworth's & Co. Ltd,

London, 1976.

[27] G. Meijer, F. Ellyin, Z. Xia, Comp. Part B: Eng. 31 (2000) 29e37.

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

[29] U.F. Kocks, H. Mecking, Prog. Mater. Sci. 48 (2003) 171e173.

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

1004e1007.

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

Wiley.com.

9M. Rashad et al. / Journal of Magnesium and Alloys 3 (2015) 1e9

[32] M. Rashad, F. Pan, A. Tang, Y. Lu, M. Asif, S. Hussain, J. She, J. Gou,

J. Mao, J. Magn. Alloys 1 (2013) 242e248.

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

4250e4255.

[34] M. Rashad, F. Pan, A. Tang, M. Asif, J. She, J. Gou, J. Mao, H. Hu, J

Com Mater 49 (3) (2015) 285e293.

[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) 111e118.

[37] M. Rashad, F. Pan, M. Asif, S. Hussain, M. Saleem, Mater. Charact. 95

(2014) 140e147.

[38] M. Rashad, F. Pan, A. Tang, M. Asif, Prog. Nat. Sci. 24 (2014) 101e108.[39] M. Rashad, F. Pan, M. Asif, A. Ullah, Mater. Sci. Technol. (2014), http://

dx.doi.org/10.1179/1743284714Y.0000000726.