Improving mechanical performance of Al by using.pdf

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Composites: Part A 38 (2007) 1010–1018

Improving mechanical performance of Al by usingTi as reinforcement

Sanjay Kumar Thakur a,*, Manoj Gupta b

a Delphi Automotive Systems Singapore Pte Ltd., 501 Ang Mo Kio Industrial Park 1, Singapore 569621, Singaporeb National University of Singapore, Department of Mechanical Engineering, Singapore 117576, Singapore

Received 14 March 2006; received in revised form 3 June 2006; accepted 13 June 2006

Abstract

In the present study, Al based composite reinforced with Ti particulates was fabricated by using the disintegrated melt deposition(DMD) processing technique followed by hot extrusion. Microstructural characterization of the as-extruded composite samples revealeda near uniform distribution of the Ti particulates in the Al matrix, good interfacial integrity between the Ti particulates and the Al matrixand minimal presence of porosity. Mechanical properties characterization revealed that the addition of Ti particulates resulted in anincrease in macrohardness, 0.2% YS, UTS and elastic modulus. However, the ductility of the composite was found to be decreasedby the addition of Ti particulates in the Al matrix. The fractured samples of the composite showed the ductile mode of fracture inthe case of Al matrix whilst particle fracture and debonding were observed as the failure mode of the Ti reinforcement.� 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Aluminum; A. Titanium; A. Composite; B. Microstructure; B. Mechanical properties

1. Introduction

In past years, there has been significant interest in par-ticulate and fiber reinforced composite materials, whichhas largely been stimulated by a push to exploit metalmatrix composite (MMC) systems [1–8]. Part of the reasonfor the popularity of particulate and short fiber reinforcedMMCs are due to their lower cost compared to the contin-uous fibers. Meanwhile, most of the emphasis has beenplaced on the matrix of aluminum alloy because of thewidespread availability of processing techniques. Metalmatrices offer not only high-temperature resistance but alsostrength and ductility that increase toughness. Amongst thepopular choices for reinforcement materials are SiC, Al2O3,TiO2 and AlN [9]. However, the beneficial effect of theceramic reinforcements usually results in an increase inthe yield strength, accompanied by a considerable decreasein the elongation to failure [2,5,6,10,11]. This reduction in

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doi:10.1016/j.compositesa.2006.06.014

* Corresponding author.E-mail address: [email protected] (S.K. Thakur).

ductility is generally associated with different parameterssuch as the presence of porosity at the matrix–reinforce-ment interface, chemical reaction between the matrix andthe reinforcement phase that results in the formation ofbrittle phases at the matrix–reinforcement interface, ordamage generation during deformation. An alternative inthe design of composites could be the use of hard metallicphases, as reinforcements [12–16]. Titanium is especiallyinteresting because of its low density, high elastic modulusand high specific strength with good fatigue properties.Additional beneficial effect of titanium addition couldresult from its plastic deformation in such a way that partof the load might be transferred plastically to the titaniumreinforcement [17]. Titanium has been extensively used inaerospace and sports industries. However, Ti has not beenused as a particulate reinforcing material in the Al matrix,although there are examples of using Ti particulate as areinforcing agent in the Mg matrix [13,17].

Earlier, the use of Ti in Al has been explored as an alloy-ing element so as to synthesize high performance materialsfor advanced engineering applications. Processing

mailto:[email protected]

Argon Gas TankResistance Furnace

Thermocouple

Mild steel cover

Stirrer

Graphite Crucible

Nozzle

Molten Slurry

Deposited Ingot Substrate

Motor

750 oC

Insulator

Ar Ar

Argon-filled Chamber

Fig. 1. Schematic diagram of the experimental set up used for DMDprocess.

S.K. Thakur, M. Gupta / Composites: Part A 38 (2007) 1010–1018 1011

techniques based on molten metals (such as conventionalcasting and spray atomization and deposition) and metallicpowders (such as powder metallurgy and mechanical alloy-ing) have been investigated [18–21]. Amongst these tech-niques, the synthesis of Al–Ti materials can be carriedout more cost effectively by the methods based on moltenmetals. The studies conducted so far have shown that theaddition of Ti in the liquid Al leads to: (a) an increase inthe melting temperature of Al significantly and (b) reactionbetween Ti and Al to form Al3Ti through peritectic reac-tion [22]. This necessitates the use of more expensive andspecialized furnaces capable of heating to higher tempera-tures and crucibles made of highly inert materials thusincreasing the overall cost of synthesized material. Theinteraction between Ti and Al in the molten conditionresults in either a solid solution of Ti in Al (with a very lim-ited solid solubility) or the formation of Al3Ti throughperitectic reaction or primary solidification [22]. The micro-structures of Al–Ti synthesized by using conventional cast-ing with slow cooling rate and spray atomization anddeposition with reasonably high cooling rates revealedthe as-expected existence of Al3Ti intermetallic phase andequilibrium/extended solid solubility of Ti in Al. Neitherof these techniques has shown the capability of synthesiz-ing Al–Ti materials at temperatures lower than that exhib-ited by equilibrium Al–Ti phase diagram and in retainingthe Ti as Ti in its elemental form following solidificationby controlling the reaction between the molten Al and Ti.It is conjectured that the reaction between molten Al andTi can be controlled by an appropriate heat treatment ofthe Ti particulate resulting in the formation of TiO2 layeraround Ti particulate [23]. This TiO2 layer would act as adiffusion barrier for molten Al to react with Ti particulateand thus prevent formation of unwanted interfacial reac-tion products, resulting in higher performance of the AlMMCs.

Most of the research studies so far have investigatedceramic particulates as a viable reinforcement as statedbefore. Hence, there has been a lack of extensive studyon the effect of reinforcing metallic particulates into theAl matrix by innovative techniques producing higher per-formance Al MMCs and understanding the interdependentfactors that help correlate mechanical properties withmicrostructure. In the present study, an attempt has beenmade to melt Al and heat treated Ti together in a cruciblefollowed by mixing, disintegrating and spraying the moltenmixture on a metallic substrate such that the molten mix-ture is deposited and solidified on the metallic substrate.This technique is called as disintegrated melt depositiontechnique (DMD). Previously, Ti was incorporated in theAl melt after the latter had reached the designated super-heat temperature because of the highly reactive nature ofTi. The two materials have never been mixed togetherbefore heating. However, in the present study, Ti wasmixed with Al before heating. Hence, the present studyenvisages a novel technique to produce Al–Ti MMCs hav-ing excess amount of Ti in Al than that would have been

allowed by equilibrium phase diagram and characterizingthe extruded MMCs thus produced for interfacial integrity,microstructure, thermo-mechanical and mechanical prop-erties. Particular emphasis has been placed on the effectof varying amount of Ti reinforcement on the microstruc-tural and mechanical properties.

2. Experimental details

2.1. Materials

In the present study, aluminum chips with an averagethickness of 425 lm and a purity of 99.7% were used asthe matrix material. Titanium was used as the reinforce-ment to the aluminum. It may be noted that before beingused, the titanium powder was preheated at 400 �C foran hour in a carbolite furnace. To prevent the titaniumfrom reacting, it was placed in a ceramic container whileheating. These pretreated titanium powders were then usedas the reinforcing agent in the aluminum matrix.

2.2. Primary processing

Synthesis of monolithic Al and Al/Ti composites con-taining three different weight percentage of titanium wascarried out by using a DMD technique as shown inFig. 1. Synthesis of the composites involved heating thealuminum chips and the titanium powder by placing themat alternate levels (in a multi-layered sandwich structure) ina graphite crucible in an inert Ar gas atmosphere to 750 �C

1012 S.K. Thakur, M. Gupta / Composites: Part A 38 (2007) 1010–1018

by using an electrical resistance furnace. The crucible wasequipped with an arrangement for bottom pouring by pro-viding a graphite nozzle (having a 10 mm diameter throughhole) fitted in the 25 mm hole at the bottom of the crucible.While melting the mixture of Al and Ti, the nozzle wasplugged with a graphite rod. Upon reaching the superheattemperature, the molten slurry was stirred for 2 min at 450RPM by using a mild steel twin blade stirrer placed 15 mmabove the base of the crucible to accomplish the incorpora-tion and distribution of the titanium particulates into thealuminum melt through vortex method [7]. The diameterof the stirrer was chosen to be 75 mm conforming to therecommendation that the ratio of the stirrer diameter tothat of the melt surface diameter be 0.6 for optimal incor-poration and distribution of the reinforcement particles[24]. The stirrer was coated with Zirtex 25 (86% ZrO2,8.8% Y2O3, 3.6% SiO2, 1.2% K2O and Na2O, 0.3% traceinorganics) so as to prevent the melt from contamination.The molten slurry was allowed to flow down from the cru-cible through the nozzle at the bottom of the crucible bypulling out the plug from the nozzle. The vertically downflowing slurry was then disintegrated by using two jets ofAr gas normal to the melt stream and located at 215 mmfrom the pouring point. Further, the disintegrated meltstream was deposited onto a stainless steel substrate asshown in Fig. 1 to produce the cast ingots of Al/Ti compos-ites. Synthesis of monolithic aluminum was carried out byusing an identical procedure.

2.3. Secondary processing

The as-processed ingots were machined to a size of35 mm diameter and were homogenized at a soaking tem-perature of 400 �C for 30 min. The homogenized ingotswere subsequently extruded at 350 �C, using an extrusionratio of 19.14:1 with the help of a 410 MPa hydraulic press.

2.4. Physical property measurement

The densities of the monolithic and Al/Ti compositespecimens were determined by using the Archimedes’ prin-ciple [25]. Distilled water was used as the immersion fluid.The density measurements involved weighing polisheddiscs of the extruded samples in air and when immersedin distilled water. The samples were precision weighed onan electronic balance (Model: A&D ER-182A) having anaccuracy of ±0.0001 g. The densities, derived from therecorded weights, were then compared to the rule-of-mix-tures density. The theoretical density of each sample wascalculated by using rule-of-mixture method based on theamount of aluminum and titanium present. The extrudedsamples were subjected to chemical analysis to determinethe exact quantity of the reinforcement and the matrixmaterial in terms of actual weight and volume percentage.The porosity of the samples was calculated by using theweight fraction of Ti, and theoretical and experimentaldensities of the composite samples.

2.5. Microstructural characterization

Microstructural characterization was performed onmetallographically polished samples of the extruded com-posites with the purpose of investigating: (a) the reinforce-ment distribution, (b) reinforcement morphology and (c)interfacial integrity between the matrix and the reinforce-ment phase. The samples were examined by using a scan-ning electron microscope (JEOL JSM-8500LV) equippedwith energy dispersive spectroscopy (EDX). The EDXanalysis was carried out to detect: (a) the presence of oxidelayer on the heat treated titanium powder, (b) the presenceof the reinforcement and (c) the extent of interfacial reac-tion. Further, the Leica Quantimet 520 image analysis sys-tem was utilized to determine the characteristics such asreinforcement size, roundness and aspect ratio.

2.6. X-ray diffraction analysis

X-ray diffraction analysis was performed to verify thepresence of the reinforcement and other phases in the spec-imens by using Shimadzu LabX XRD-6000 X-ray diffrac-tometer. The as-received and preheated titanium anddiscs of the extruded rods were subjected to CuKa radia-tion with a scanning speed of 1.5�/min. The value of Braggangle and interplanar spacing (d) were matched with thestandard values for aluminum, titanium and other expectedphases.

2.7. Hardness measurement

Microhardness measurements were conducted todetermine the hardness of the matrix and the matrix–rein-forcement interface. A Matsuzawa MXT50 digital micro-hardness tester, equipped with an indenter having 136�facing angle, was used at a loading speed of 50 lm/s fora holding period of 15 s at 25 gf load to obtain Vickersmicrohardness. The macrohardness of the composite andthe monolithic material was determined by using a FutureTech Rockwell type hardness tester. The measurement wasmade by using the Rockwell 15T superficial scale having a1.58 mm diameter steel ball indenter by applying a test loadof 15 kgf for 2 s on the polished and flat specimens accord-ing to ASTM Standard E18-94.

2.8. Mechanical behavior

Tensile properties of the extruded and machined rods ofcomposites and monolithic materials were obtained byusing an automated servo-hydraulic Instron 8501 machineaccording to ASTM Standard E8M-96. The specimens of5 mm nominal diameter and 25 mm gauge length weretested at a strain rate of 0.01 s�1. The mechanical proper-ties obtained were: (a) 0.2% yield stress, (b) ultimate tensilestrength (UTS) and (c) ductility. Dynamic elastic modulusof the extruded monolithic and composite specimens wasdetermined by using the free–free beam method according


to ASTM Standard C1259-96. The procedure essentiallyinvolved the following steps: (a) suspending the rods atits nodal points (0.22L and 0.78L where L is the lengthof the rod) by using nylon threads, (b) providing excitationusing a modally tuned hammer equipped with a load sensorat its tip and (c) capturing the first mode of flexural vibra-tion by using an accelerometer positioned at the end of therod. The frequency response function (Bode plot) was usedto determine the natural frequency (xn). The natural fre-quency was used to determine the dynamic elastic modulususing the relationship expressed in Eq. (1) [26]:

E ¼xn=ðbnLÞ2� �2

qL4

Ið1Þ

where L represents length of the beam, I the moment ofinertia of the beam cross-section, q the density of the mate-rial, and bn a constant. The numerical values of (bnL)2 fortypical end conditions are described elsewhere [26].

Fracture surface characterization studies were con-ducted on the failed tensile samples to provide an insightinto the macroscopic fracture mode and intrinsic micro-scopic fracture mechanisms governing tensile deformationand fracture. The samples were examined in a JEOLJSM-5900 LV scanning electron microscope. EDX wasperformed on the failed specimens to confirm the identityof the broken/debonded reinforcement particles.

3. Results and discussion

3.1. Processing and macrostructure

Synthesis of monolithic aluminum and aluminum basedcomposites containing 3.3 wt.%, 6 wt.% and 7.5 wt.% tita-nium was successfully accomplished by using the process-ing technique of disintegrated melt deposition, followedby hot extrusion. The amount of Ti reinforcement thatcould be successfully incorporated into the Al matrix waslimited to 7.5 wt.%. Beyond this wt.% of Ti, the compositeslurry became extremely viscous and upon reaching thedesignated temperature, casting was not possible due tothe fact that the slurry could not flow. However, the reten-tion of the reinforcement for the synthesized compositeswas good and close to their nominal amount determinedby carrying out the chemical elemental analysis as shownin Table 1. Porosity in the synthesized composites was cal-culated based on the data obtained from the chemical anal-ysis and the density measurement results. The results

Table 1Results of density and porosity measurements

Materials ActualTi (wt.%)

EquivalentTi (vol.%)

Experimentaldensity (g/cm3)

Porosity(vol.%)

Pure Al – – 2.701 ± 0.003 0.16Al + 3.3%Ti 3.27 1.8 2.732 ± 0.022 0.32Al + 6%Ti 6.01 3.7 2.756 ± 0.013 0.55Al + 7.5%Ti 7.50 5.0 2.767 ± 0.022 0.78

showed a minimal presence of porosity in the monolithicand composite samples. The volume percentage of porosityincreased with increase in the amount of the Ti reinforce-ment in the Al matrix as shown in Table 1. The similartrend was also observed by earlier investigators for conven-tional composites [27,28]. The results also showed anincrease in the density with increase in the titanium contentin the aluminum matrix. This can be attributed to higherdensity of Ti (4.507 g/cm3) particulates as compared tothe aluminum (2.7 g/cm3) matrix [29]. The minimal pres-ence of defects coupled with a lack of aggregation of thereinforcing titanium particulates due to gravity suggeststhe beneficial influence of optimized flow of argon duringmelting, mechanical stirring, disintegration, depositionand solidification process in the DMD technique.

3.2. Microstructure

SEM micrographs were obtained for the as received andpreheated titanium powders as well as the extruded speci-mens. The micrographs for the as received and preheatedtitanium powder as presented in Fig. 2(a) and (b) showsno significant difference in terms of grain size and appear-ance. Micrographs of the extruded specimens generallyshowed good interfacial bonding between the matrix andthe reinforcement and almost no reaction at the interface.Clustering of the reinforcing particles existed in all the

Fig. 2. SEM micrographs of Ti powder in (a) as received and (b)preheated condition.


composites. However, as can be seen in the attached SEMmicrographs in Fig. 3(a)–(c), clustering only occurred inlimited areas of the composites and there was generally uni-form reinforcement distribution throughout the compositematerial. All the composites exhibited similar size ratio ofcluster to individual particulate as approximately 3.3. Thelower size ratio of cluster to particle can be attributed tothe good wetting between the reinforcement and the liquidmetal [30]. The uniform distribution of titanium particles inthe aluminum matrix can be attributed to: (a) minimalagglomeration of titanium particles during melting of thematrix due to layered arrangement of raw materials inthe crucible, (b) minimal settling of titanium due to appro-

Fig. 3. SEM micrographs showing the uniform distribution of Ti particlereinforcement in (a) Al–3.3%Ti, (b) Al–6%Ti and (c) Al–7.5%Ticomposites.

priate choice of stirring parameters, (c) disintegration ofthe composite slurry by the argon jets and its subsequentdeposition on the substrate and (d) redistribution ofthe particulates during extrusion as a result of differentdeformation modes in operation across the cross-sectionof the extruding billet [31,32]. From SEM micrographs inFig. 4(a)–(c), it is clear that all the composites exhibitedextremely good interfacial integrity. The absence of deb-onding or discontinuity at the interface indicated superiorfluid flow characteristics during solidification. SEM micro-graphs also revealed minimal interfacial reaction in all thecomposites. This can be attributed to the fact that some

Fig. 4. SEM micrographs showing good interfacial integrity between Tireinforcement and Al matrix of (a) Al–3.3%Ti, (b) Al–6%Ti and (c) Al–7.5%Ti composites.


titanium particles could not be completely oxidized duringthe preheating stage. Otherwise, this method of preheatingthe titanium powder at 400 �C for 1 h is extremely effectivein providing a diffusion barrier at the interface to avoidinterfacial reaction despite the reasonable contact timebetween the titanium and the aluminum during heatingand melting. The presence of oxide layer on the titaniumparticles was predicted to slow down or move the Al–Tireaction to a higher temperature or cause longer reactiontime [33]. Aluminum reacts with titanium by first dissolvingthe natural oxide layer on the surface of titanium. By pre-heating the titanium powder, a thicker oxide layer can beachieved [34]. In this way, the highly reactive nature of tita-nium could be overcome and composites of superior prop-erties can be realized. It may be noted that the titanium hasa high affinity for oxygen and forms a very stable andadherent oxide layer. The presence of oxide layer on thetitanium particles was confirmed by the results obtainedfrom the X-ray diffraction analysis, which showed the pres-ence of oxide layer as compared to the XRD spectrum ofthe as received titanium.

3.3. Mechanical behavior

In order to understand the effect of amount of reinforce-ment on the matrix properties, microhardness and macro-hardness measurements on the extruded Al and Al–Ticomposite samples were carried out and summarized inTable 2. From the results, it is clear that there is an increasein the matrix microhardness, interfacial microhardness andbulk macrohardness of all the reinforced composites ascompared to that of the unreinforced aluminum. This isdue to the fact that the presence of stronger and stiffer rein-forcement together with the ability to introduce higher dis-location density contributes to the higher microhardnessvalues [8]. The presence of harder particles also causedthe bulk macrohardness value to increase due to the factthat the average hardness over an area of the compositehas increased.

It may be noted that the interfacial microhardness wasfound to be higher than the corresponding matrix micro-hardness in all the composites, however, the difference ininterfacial microhardness and matrix microhardness wasnot significant. The higher interfacial microhardness maybe due to the higher density of dislocations in the immedi-ate vicinity of the reinforcement particulates as a result ofsignificant difference in the CTE values of Al and Ti partic-

Table 2Results of macrohardness and microhardness measurements

Material Macrohardness (HR 15T) Microhardness (HV)

Matrix Interfacial

Pure Al 31 ± 1 32 ± 0 –Al + 3.3%Ti 37 ± 1 34 ± 0 40 ± 1Al + 6.0%Ti 48 ± 1 43 ± 2 50 ± 1Al + 7.5%Ti 52 ± 1 40 ± 1 50 ± 0

ulates (ratio of CTE value of Al and Ti = 3.12:1). ThisCTE mismatch between the reinforcement and the matrixmay lead to the generation of residual stresses [5,35,36].However, the smaller difference in the microhardness val-ues at interface and matrix may be due to the lack of inter-facial reaction products because the interfacial reactionwould make the interfacial region harder. The lack of inter-facial reaction product was also corroborated by the X-raydiffraction results as discussed before.

The results of the tensile tests performed on extruded Aland Al–Ti composite samples revealed an increase in the0.2% YS, UTS and elastic modulus of the composites ascompared to that of pure Al. It is interesting to note thatthere has been a significant improvement in the 0.2% YSand UTS values of the composites with increase in theamount of Ti reinforcement. The 0.2% YS has increasedby 43.38%, 57.85% and 26.70% with an addition of 3.3%Ti, 6.0% Ti and 7.5% Ti to the Al matrix, respectively. Sim-ilarly, the UTS value of Al has increased by 11.35%,36.52% and 23.90% by the addition of 3.3% Ti, 6.0% Tiand 7.5% Ti to the Al matrix, respectively. The increasein the 0.2% YS and UTS of Al by the addition of Ti parti-cles can be attributed to the combination of: (a) good inter-facial integrity enabling efficient load transfer from thematrix to the reinforcement, (b) the lack of interfacial reac-tion that would create the brittle phases and reduce thecomposite’s overall load carrying capacity, (c) uniform dis-tribution of Ti reinforcements in the Al matrix and (d)smaller size of the reinforcing particles [37]. The resultsof ductility measurement of the extruded pure Al and Al–Ti composites showed that all the reinforced compositeswere less ductile than the unreinforced Al and there wasa gradual decrease in ductility with an increase in theamount of Ti in the aluminum matrix (see Table 3). Thedecrease in ductility with increasing amount of Ti particlesin the Al matrix can be attributed to the reduced cavitationof the aluminum matrix. However, the strength and ductil-ity combination of the Al–Ti composite materials investi-gated in the present study has been found to be superiorto those reported by earlier investigators in Al–SiC com-posite systems [38,39]. The earlier investigators [38]reported the values of 105 MPa for 0.2% YS, 141 MPafor UTS and 14.4% for ductility in Al + 17.7%SiC compos-ite system prepared by mechanical disintegration and depo-sition technique. In an another study, earlier investigators[39] obtained an UTS value of 110 MPa and a ductilityof 8% for Al + 22%SiC MMC produced by using sprayco-deposition technique followed by hot rolling to 50%,annealing for 30 min at 500 �C, cold rolling to 50% andthen annealing for 30 min at 500 �C in the sequencedescribed.

The dynamic elastic modulus determined by free–freebeam method exhibited an increase with increasing amountof Ti particles in the Al matrix. The increase in the elasticmodulus of the Al matrix can be attributed to: (a) higherelastic modulus of Ti (116 GPa) particulates as comparedto that of Al (69 GPa) [35], (b) a uniform distribution of

Table 3Results of tensile tests

Material Process Dynamic modulus (GPa) 0.2% YS (MPa) UTS (MPa) Ductility (%)

Pure Al DMD 68.39 82.81 ± 4.21 118.0 ± 4.9 35.41 ± 0.75Al + 3.3%Ti DMD 74.75 118.74 ± 5.68 131.4 ± 4.8 25.07 ± 2.29Al + 6.0%Ti DMD 80.09 130.72 ± 6.63 161.1 ± 2.5 17.92 ± 3.61Al + 7.5%Ti DMD 84.66 104.92 ± 5.64 146.2 ± 5.5 15.76 ± 3.03Al + 17.7%SiC MDD [38] 105 141 14.4Al + 22%SiC Spray co-deposition [39] 110 8


Ti particulates in the Al matrix and (c) good interfacialintegrity of the Ti particulates with the Al matrix. It isinteresting to note that the elastic modulus values exhibiteda constant increase with increase in the amount of Ti rein-forcement in the Al matrix. However, this trend was notreflected in the case of the 0.2% YS and UTS values ofthe composites. It can be seen in Table 3 that the increasingtrend for YS and UTS values is not continued with thecomposite with the highest percentage of reinforcement.This can be attributed to the presence of fractured particlesin the matrix of the most highly reinforced sample havingincreased interaction of the reinforcing particles and there-fore increased likelihood of particle fracture during second-ary processing, like extrusion in the present case. It may bepossible that a threshold in terms of the successful amountof reinforcement that could be incorporated might havebeen surpassed that could lead to the degradation ofmechanical properties such as 0.2% YS and UTS values.The fracture of the reinforcing particles has detrimental

Fig. 5. SEM fractographs showing the presence of dimples in the fractured s7.5%Ti materials.

effects on the overall load bearing capacity of the compos-ite [40]. Moreover, the sharp micro-cracks that occur canenhance the local plastic flow within the ductile matrixand aid in such failure phenomenon as ductile separationby void growth.

The fractured surfaces of all the tensile specimens ofextruded Al and Al–Ti composites were analyzed to deter-mine the mode of failure. Typically, all the tensile speci-mens exhibited a cup-and-cone failure mode which isindicative of the ductile fracture mode for the Al matrix.The presence of dimples on the fracture surface clearlyindicates that necking had occurred prior to matrix frac-ture (see Fig. 5(a)–(d)). The fractographs of all the compos-ites showed the particulate fracture and the particulatedebonding at the interface (see Fig. 6(a)–(f)). The extensiveparticulate breakage, which is indicative of the successfultransfer of the load from the matrix to the reinforcement,was also observed in the fractographs. This also indicatesthe excellent interfacial bonding obtained between the Al

urface exhibited by (a) pure Al, (b) Al–3.3%Ti, (c) Al–6%Ti and (d) Al–

Fig. 6. SEM micrographs showing particulate breakage and particulate debonding exhibited by (a and b) Al–3.3%Ti, (c and d) Al–6%Ti and (e and f)Al–7.5%Ti materials, respectively.


matrix and the Ti particulate reinforcements in the com-posites prepared by using DMD technique in the presentinvestigation. The oxide layer on the surface of the particledid not have any detrimental effect on the tensile propertiesin the present study.

4. Conclusions

1. The DMD technique can be successfully employed tosynthesize both the monolithic and Ti reinforced Almaterials.

2. A uniform distribution of the Ti reinforcing particles inall the composites materials coupled with good interfa-cial integrity and minimal presence of porosity wereobtained that indicates the suitability of the primaryand secondary processing parameters chosen in the pres-ent investigation.

3. The addition of Ti to Al matrix resulted in improvedCTE, 0.2% YS, UTS and elastic modulus; however,the ductility of the composite was adversely affected.

Acknowledgements

The authors thank L.K. Hoong for his assistance duringthe course of this investigation. The authors also thank Na-tional University of Singapore and Delphi AutomotiveSystems Singapore Pte Ltd for providing assistance whilepreparing the manuscript.

References

[1] Ibrahim IA, Mohamed FA, Lavernia EJ. J Mater Sci 1991;26:1137–56.


[2] Fridlyander JN, editor. Metal matrix composites. London: Chap-man & Hall; 1995. p. 51–60.

[3] Everett RK, Arsenault RJ, editors. Metal matrix composites:processing and interfaces. Academic Press, Harcourt Brace Jovano-vich; 1992. p. 6.

[4] Kjar AR. Particle reinforced aluminum based composites, in: (Eds.),AIME Annual Meeting, Las Vegas, 1989.

[5] Lloyd DJ. Int Mater Rev 1994;39:1–23.[6] Midling OT, Grong O. Key Eng Mater 1995;104–107:329–54.[7] Rohatgi PK, Asthana R, Das S. Int Metals Rev 1986;31:115–39.[8] Walker AL, Walker JA. JOM 1991;43:8–15.[9] ASM metals handbook, casting, ASM International, Metals Park,

Ohio. 1988. p. 44073.[10] Everette RK, Arsenault RJ, editors. Metal matrix composites:

processing and interfaces. Academic Press, Inc.; 1991.[11] Akio K, Atsushi O, Toshiro K, Hiroyuki T. J Jpn Inst Light Metals

1999;49:149–54.[12] Hassan SF, Gupta M. Mater Res Bull 2002;37:377–89.[13] Hassan SF, Gupta M. J Alloys Compd 2002;345:246–51.[14] Hwang S, Nishimura C, McCormick PG. Scr Mater 2001;44:2457–62.[15] Lu L, Lai MO, Toh YH, Froyen L. Mater Sci Eng A 2002;334:

163–72.[16] Hassan SF, Gupta M. J Alloys Compd 2002;335:L10–5.[17] Perez P, Garces G, Adeva P. Compos Sci Technol 2004;64:145–51.[18] St John DH, Hogan LM. J Cryst Growth 1979;46:387.[19] Bletry J. J Phys Chem Solids 1970;31:1263.[20] Ohashi T, Ichikawa R. Z Metall 1973;64:517.[21] Hori S, Tai H, Narita Y. In: Steeb S, Warlimont H, editors. Rapidly

quenched metals. Amsterdam: Elsevier; 1985. p. 911.[22] Majumdar A, Muddle BC. Mater Sci Eng A 1993;169:135–47.[23] Gupta M. US 6,623,571 B1, 2003.

[24] Ghosh PK, Ray S. Trans JIM 1988;29:502–8.[25] Tham LM, Gupta M, Cheng L. Mater Sci Technol 1999;15:1139–46.[26] Thomson W. Theory of vibrations with applications. Eaglewood

Cliffs, NJ: Prentice Hall; 1993.[27] Perez RJ, Zhang J, Gungor MN, Lavernia EJ. Metall Trans A

1993;24:701.[28] Gupta M, Mohamed F, Lavernia EJ, Srivatsan TS. J Mater Sci

1993;28:2245.[29] ASM Handbook, Engineering Materials, 6th edition, Budinski,

Specification sheet, ASM, Metals Park, OH. 1999.[30] Suresh S, Mortensen A, Needleman A. Fundamentals of metal matrix

composites. Boston: Butterworth-Heinemann; 1993.[31] Gupta M, Sikand R, Gupta AK. Scr Metall Mater 1994;30:1343–8.[32] Surappa MK. J Mater Sci Lett 1993;12:1272.[33] Heald SM, Barrera EV. J Mater Res 1991;6:935–42.[34] Donachie Jr MJ. Titanium: a technical guide. ASM International;

1988.[35] Smithells CJ. Metals reference book. London: Butterworths & Co.

Ltd.; 1976.[36] Shackelford JF, Alexander W, Park JS. CRC materials science and

engineering handbook. CRC Press Inc.; 1994.[37] Sritharan T, Xia K, Heathcock J, Mihelich J. In: Bhagat RB, Clauer

AH, Kumar P, Ritter AM, editors. Proceedings of internationalconference on metal and ceramic matrix composites processing,modelling and mechanical behaviour. Anaheim, CA. Warrendale,PA: TMS; 1990. p. 13–22.

[38] Thakur SK, Gupta M, Materials science and technology, in press.[39] Singer ARE, Ozbek S. Powder Metall 1985;28:72.[40] Suresh S, Mortensen A, Needleman A, editors. Fundamentals of

metal matrix composites. Boston: Butterworth-Heinemann; 1993.p. 51, 240.

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