Department of Mechanical Engineering,National University of Singapore(NUS),9 Engineering Drive 1,Singapore 117 576,Singapore Available online 16 April 2014

Hybridizing micro-Ti with nano-B4C particulates to improve the microstructural and mechanical characteristics of Mg-Ti composite

S.Sankaranarayanan*,S.Jayalakshmi,M.Gupta

Department of Mechanical Engineering,National University of Singapore(NUS),9 Engineering Drive 1,Singapore 117 576,Singapore Available online 16 April 2014

In this study,the effects of hybridizing micron-sized titanium particles with nano-sized boron carbide particles on the microstructural and mechanical properties of Mg-Ti composite were investigated.Microstructural characterization revealed grain ref i nement attributed to the presence of uniformly distributed micro-Ti particles embedded with nano-B4C particulates.Electron back scattered diffraction(EBSD)analyses of the Mg-(Ti+B4C)BMhybrid composite showed relatively more localized recrystallized grains and lesser tensile twin fraction,when compared to Mg-Ti.The evaluation of mechanical properties indicated that the best combination of strength and ductility was observed in the Mg-(Ti+B4C)BMhybrid composite.The superior properties of the Mg-(Ti+B4C)BMhybrid composite when compared to Mg-Ti can be attributed to the presence of nano-reinforcement,the uniform distribution of the hybridized particles and the better interfacial bonding between the matrix and the reinforcement particles achieved by nano-B4C addition.

Composite materials;Electron microscopy(SEM);Electron diffraction(electron back scattered diffraction);Mechanical properties

1.Introduction

Magnesium is ～35%lighter than aluminium and it exhibits properties that are comparable to that of Al.For this reason,the research and development of magnesium(Mg)-based materials are currently gaining momentum to make them the best possible replacement to Aluminium(Al)for eff i cient weight savings.In addition to weight saving capabilities,the advantages of Mg as engineering material also include excellent machinability,damping capacity and castability[1,2].However,the poor mechanical characteristics of Mg such as low strength and ductility restrict its utilization in critical engineering applications.To overcome these limitations,hard ceramic reinforcement such as SiC,Al2O3are often added to Mg which results in extreme brittleness[1,2].In this context,available literature highlights the ef fi ciency of nanoscale particulate reinforcement in simultaneously improving the strength and ductility of Mg materials[1].Similar results were also observed for Mg composite prepared with insoluble metallic reinforcement addition[1].Further,the positive infl uence of addition of hybrid reinforcement on the mechanical properties of the Mg-composite has been recently identi fi ed [3].

In our recent work on the in fl uence of hybrid particulate reinforcement addition(micron-sized metallic Ti+nano-sized ceramic Al2O3)on the mechanical response of Mg[3],we identi fi ed that the method of hybrid reinforcement addition played a major role in determining the mechanical response of the Mg-composite(in addition to the properties of the individual reinforcement and the strengthening mechanisms).When the hybrid reinforcement addition to Mg was carried out after pre-processing of the reinforcement by ball milling (rather than direct addition of the reinforcement to Mg),better ductility along with strength retention was achieved[3].

In the present work,micron-sized Ti particulates are hybridized with nano-sized B4C particulates by ball milling,and the hybrid(Ti+B4C)BMmixture was used as reinforcement in pure Mg.The primary aim of this work is to study the effect of hybrid reinforcement addition on the properties of Mg-MMCs.The microstructural evolution of as-extruded Mg-composite containing(Ti+B4C)BMmixture have been studied in detail in comparison to Mg-Ti using electron back scattered diffraction(EBSD)analysis.The results of mechanical properties characterization are correlated with the microstructural observations to understand the mechanical behaviour of developed Mg materials.

2.Materials and methods

2.1.Materials

Mg turnings of>99.9%purity supplied by ACROS Organics,New Jersey,USA were used as the matrix material. Elemental titanium(Ti)particulates of size<140 μm(98% purity)from Merck and nano-sized boron carbide(B4C)particulates of particle size ～50 nm supplied by Nabond,China were used as particulate reinforcement.The concentration of Ti and B4C are 5.6 wt.%and 2.5 wt.%respectively.

2.2.Hybrid reinforcement preparation

A Retsch PM-400 mechanical alloying machine was used to ball-mill the hybrid(metal-ceramic)particulate mixture(Ti and B4C),herein referred to as(Ti+B4C)BM.Prior to ball milling,the elemental particulates were blended for 1 h(with 0.3 wt.%stearic acid as process control agent)so as to ensure the uniform mixing of powder particulates.After blending, hardened stainless steel balls of diameter 15 mm were added and the blended mixture was ball-milled for 2 h at 200 rpm, with a ball-to-powder ration of 20:1.Ball-milling was carried out to reduce the size of powder particulates and to allow it to hybridize with each other[3,4].

2.3.Melting and casting

Mgmaterialsusedinthisstudyweresynthesizedthroughthe liquid metallurgy route based disintegrated melt deposition technique[3,4].Mg turnings together with either individual Ti particulates or the hybrid(Ti+B4C)BMmixture were heated in a graphite crucible to 800°C in an electrical resistance furnace under inert argon gas protective atmosphere.In order to facilitate the uniform distribution of reinforcement particulates in Mg,the superheated molten slurry was stirred at 465 rpm using a twin blade(pitch 45°)mild steel impeller(coated with Zirtex 25)for 5 min.The composite melt was then bottom poured into the steel mould after disintegration by two jets of argon gas oriented normal to the melt stream.Following deposition,aningot of 40 mm diameter was obtained.The obtained ingots were then machined to 36 mm diameter and soaked at 400°C for 60 min.Hot extrusion was then carried out using a 150 T hydraulic press at 350°C with an extrusion ratio of 20.25:1 to obtain rods of 8 mm in diameter.

Fig.1.Microstructure of(a)as-received Ti,(b)and(c)ball milled(Ti+B4C)BMpowder[4].

2.4.Microstructural characterization

The presence and distribution of second phases in Mg matrix of developed Mg-MMCs were studied using a Hitachi S-4300 f i eld emission scanning electron microscope(FESEM) and a Jeol JXA-8530F Electron Probe Micro analyser (EPMA).An ESEM Quanta 200,FEI Field Emission Gun Scanning Electron Microscope(FEG-SEM)equipped with electron back scattered diffraction(EBSD)detector,was used to study the microstructural evolution of Mg-matrix postextrusion.The grain characteristics of Mg-matrix were studied in terms of grain size and its distribution.The results of microstructural evolution studies based on EBSD scan were obtained in the form of inverse pole f i gure(IPF)maps, misorientation distribution(MD),kernel average misorientation distribution(KAM)and grain orientation spread(GOS) map.The EBSD scans were recorded on the surface parallel to the extrusion direction(ED).TSL software was used for the OIM data acquisition and analyses[5].

Fig.2.Microstructure of(a)Mg-Ti and(b)Mg-(Ti+B4C)BMcomposite.

2.5.Mechanical characterization

The tensile and compressive properties of developed Mg-MMcs in accordance with ASTM test methods E8/E8M-08 and E9-09 respectively were determined using a fully automated servo-hydraulic mechanicaltesting machine, Model-MTS 810[3,4].For each composition,a minimum of 6 tests were conducted to obtain repeatable values.The fractured samples were then analysed using a Hitachi S-4300 FESEM.

3.Results and discussion

3.1.Microstructure

Micrographs showing the microstructural characteristics of the as-received Ti and the ball milled(Ti+B4C)BMpowder particles are shown in Fig.1(a)-(c)[4].It indicates that the large sized(as-received)Ti particulates were f l attened;their sharp edges were rounded off and broken down during the ball milling process,which can be attributed to the repeated loading,f l attening and breakdown of powder particulates during ball milling process[6].Fig.1(c)also indicates that the nano-B4C particles are adhered to the micron-sized Tiparticles.

The distribution of reinforcement/second phases in Mgmatrix of Mg-Ti and Mg-(Ti+B4C)BMare shown in Fig.2(a)and(b).It indicates fairly uniform distribution of Ti particles with less agglomeration in Mg matrix.Further,the interfacial characteristics of Mg-(Ti+B4C)BMcomposite studied using electron probe microscopic analysis is shown in Fig.3[4].It shows increased concentration of B and C around the Ti particles which indicates the combined presence of Ti(B,C)phases on the Ti particle and at the Mg/Ti interface.

The EBSD generated micrographs showing the matrix grain characteristics of Mg-Ti and Mg-(Ti+B4C)BMare shown in Fig.4.It shows unimodal grain size distribution in the extruded Mg-MMCs and signif i cant grain ref i nement in Mg-(Ti+B4C)BMwhen compared to Mg-Ti.Further,the matrix grain characteristics(misorientation distribution,grain boundary character distribution and grain orientation spread) of developed Mg-(Ti+B4C)BMcomposite are studied in detail in comparison to Mg-Ti to identify the inf l uence of hybridizing micro-Ti with nano-B4C on the microstructural evolution.The distribution of misorientation angles(which represents the change in orientation of the specif i c crystal axis (reference)to the orientation of neighbour crystal axis due to deformation,extrusion[5])in Mg-Ti and its hybrid composite is shown in Fig.5(a).In Mg-Ti,a diffused population of(15-180°)high angle grain boundaries(HAGBs)with small peak at ～30°and large peak at ～90°is observed. While,in Mg-(Ti+B4C)BMcomposite,an increased intensity in 30°and decreased intensity in 90°is observed.The peak at ～30°generally attributes to either recrystallization or{10-11}-{10-12}double twinning(38 ± 5°<11-20>) and the peak at ～90°is due to{10-12}tensile twin boundaries(86 ± 5°<11-20>)[5,7].This indicates an increase in recrystallization and decrease in tensile twin fraction due to nano-B4C addition.This can further be verif i ed from the Kernel Average Misorientation(KAM)criterion which shows the distribution of local misorientation within a grain due to the strain imparted in the material during extrusion[7].In the present case,it can be clearly observed that the KAM number fraction increases and its peak value shifts towards left with the addition of nano-B4C particles which conf i rms the presence of more recrystallized grains Mg-(Ti+B4C)BMcomposite in comparison to Mg-Ti,thus corroborating with the misorientation distribution(Fig.5(b)).

Fig.3.Compositional distribution in Mg-(Ti+B4C)BMhybrid composite around Mg/Ti interface[4].

Fig.4.EBSD generated inverse pole f i gure maps of(i)Mg-Ti and(ii)Mg-(Ti+B4C)BMcomposite.

Fig.5.(a)Misorientation angle and(b)kernel average misorientation distribution prof i les of Mg-Ti and Mg-(Ti+B4C)BM.

Further,the EBSD micrographs of Mg-Ti and Mg-(Ti+B4C)BMcomposite were partitioned based on the grain orientation spread(GOS)criterion which separates the DRX grains from the deformed grains as shown in Fig.6(a).In the GOS maps,a high average GOS value correspond to higher geometrically necessary dislocation(GND)content and more deformation in the sample(deformed microstructure); while low GOS value indicates lower dislocation content due to recrystallization(recrystallized microstructure).Here,the recrystallized grains with orientation spread less than the average orientation spread are shown in blue colour.Similarly, the deformed grains with orientation spread greater than the average orientation spread of the total microstructure are shown in red colour indicating the area of larger lattice distortion accompanied by the high dislocation density[7].In both the cases,the combined presence of near equi-axed recrystallized grains and elongated grains(deformed grains) can be seen in the(extrusion)microstructures(Fig.6).However,relatively more localized recrystallized grains are observed in Mg-(Ti+B4C)BMcomposite in comparison to Mg-Ti.This conf i rms the enhanced recrystallization effects due to nano-B4C hybridized micro-Ti addition in Mg-(Ti+B4C)BMcomposite.

3.2.Mechanical properties

The results of mechanical properties measurements conducted on the samples of Mg-Ti and Mg-(Ti+B4C)BMare listed in Table 1.It indicates superior strength properties of Mg-(Ti+B4C)BMcomposite in comparison to Mg-Ti.The f l ow curves as shown in Fig.7(a)and(b)under tensile and compressive loading clearly indicates the strengthening effects arisen from hybrid(Ti+B4C)BMreinforcement addition when compared to individual Ti addition.The improved strengths of Mg-(Ti+B4C)BMcomposite can primarily be ascribed to the presence and relatively uniform distribution ofhybrid (Ti+B4C)BMreinforcement/second phases and the combined presence of Ti(B,C)intermetallic phases at the Mg/Ti interface(Fig.3)[8,9].The improvement in strength properties of Mg-(Ti+B4C)BMcomposite can also be attributed to coupled effects of(a)effective load transfer from matrix to second phases due to the presence of f i ne hybrid reinforcement/intermetallic phases,(b)signif i cant grain ref i nement due to enhanced matrix recrystallization and(c)mismatch in elastic and thermal expansion coeff i cients between matrix andreinforcement (αMg- 28.4 × 10-6°C-1, αTi-9.17 × 10-6°C-1, αB4C-5 × 10-6°C-1)[2,5].Further,it can be seen from Table 1 that when compared to Mg-Ti,the strength improvement in Mg-(Ti+B4C)BMcomposite under both tension and compressive loading occurred without compromising ductility.While the presence of nano-B4C reinforcement in Mg-(Ti+B4C)BMcomposite is expected to improve the strength at the expense of ductility,no signif i cant adverse effect on ductility was observed under both tension and compression loading.This could be attributed to the absence/relatively lower volume fraction of Mg based brittle secondary phases[1,8,9].

Fig.6.Grain orientation spread maps of(i)Mg-Ti and(ii)Mg-(Ti+B4C)BMcomposite.

Table 1Mechanical properties of developed Mg-MMCs under tension and compressive loading.

Fig.7.Flow stress curves of developed Mg-MMCs under(a)tensile and(b) compressive loading.

The fractographic evidences of Mg-Ti and Mg-(Ti+B4C)BMcomposites tested under tensile loading are shown in Figs.8 and 9.It indicates mixed mode fracture with prominent ductile features showing evidence of plastic deformation in contrast to the typical cleavage type brittle fracture as reported generally in the case of pure Mg[3]. Fig.8(a)shows prominent Ti particle debonding in Mg-Ti alongside ductile features,which can be attributed to the lack of chemical bonding at the Mg/Ti interface[10].However,in Mg-(Ti+B4C)BMcomposite,prominent particle crack extending from Mg-matrix was observed without any crackingat the matrix-reinforcement interface.This indicates that the second phase particles had actively involved as a load-bearing member in Mg-(Ti+B4C)BMcomposite due to good interfacial bonding between the Mg-matrix and Ti-reinforcement. This behaviour is unlike those seen in ceramic reinforcement,wherein matrix cracking,interfacecracking and debonding are prominent fracture features[11].

Fig.8.Tensile fractographs showing(a)Ti-particle debonding in Mg-Ti and (b)matrix crack extending into the Ti particle in Mg-(Ti+B4C)BMcomposite.

Fig.9.Shear bands observed in the compression failed(a)Mg-Ti and(b) Mg-(Ti+B4C)BMcomposite samples.

In the compression test fractured samples,the fracture was observed to be initiated near the specimen ends and it was seen at ～45°angle with respect to the compression test axis at the maximum stress.It indicates that the overall failure mechanism in both Mg-Ti and its hybrid composite remain unchanged under compression loading.Further,the fractographic evidencesofMg-Tiand itshybrid compositeunder compressive loading conditions reveal the presence of shear bands as shown in Fig.9.It attributes to the heterogeneous deformation and the work hardening behaviour as the work hardening rate is faster in the case of samples failed by shear bands[12,13].

4.Conclusions

Mg hybrid nanocomposite containing micron-sized Ti, hybridized with nanosized-B4C was successfully synthesized and the effects of hybridizing micro-Ti with nano-B4C particulates on the microstructural and mechanical properties of Mg-Ti composite were studied.Based on the structureproperty correlation,the following conclusions are drawn.

1.Compared to Mg-Ti,nano-B4C hybridized Mg-Ti composite exhibit enhanced localized recrystallization and ref i ned microstructure.

2.Based on EBSD analyses,the Mg-(Ti+B4C)BMhybrid composite,show more recrystallized grains and less tensile twins compared to Mg-Ti.

3.Mg-(Ti+B4C)BMhybrid composite exhibit the best combination of properties with enhanced strength and improved/retained ductility undertensile loads and compressive loads.

4.The strength properties improvement in Mg-(Ti+B4C)BMhybrid composite can be attributed to the presence of nano-reinforcement,the uniform distribution of the hybridized particles and better interfacial bonding between the matrix and the reinforcement particles achieved by nano-B4C addition.

Acknowledgements

One of the authors,S.Sankaranarayanan gratefully acknowledges National University of Singapore for providing a Research Scholarship to support his pursuit of graduate studies.The authors also thank Professor Satyam Suwas and Mr.Rama Krushna Sabat,Department of Materials Engineering,Indian Institute of Science,Bangalore for their help during the course of investigation.

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*Corresponding author.Tel.:+65 93510324.

E-mail address:seetharaman.s@nus.edu.sg(S.Sankaranarayanan).

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http://dx.doi.org/10.1016/j.jma.2014.03.001.

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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.