The nano-sized TiC particle reinforced Al–Cu matrix compositewith superior tensile ductility

Dongshuai Zhou, Feng Qiu n, Qichuan Jiang n

Key Laboratory of Automobile Materials, Ministry of Education, and Department of Materials Science and Engineering, Jilin University, No. 5988 RenminStreet, Changchun 130025, PR China

a r t i c l e i n f o

Article history:Received 14 May 2014Received in revised form31 October 2014Accepted 3 November 2014Available online 12 November 2014

Keywords:Nano-sized TiC particleCompositeDuctilityCasting

a b s t r a c t

The high mechanical properties of the TiC particle reinforced Al–Cu matrix composites are highlydesirable for a wide range of critical applications. However, a long-standing problem for thesecomposites is that they suffer from low ductility and limited formability. Here we fabricated thenano-sized TiC particle reinforced Al–Cu matrix composites by dispersing the nano-sized TiC particlesinto molten Al–Cu alloy. The tensile strength and ductility were significantly improved with the additionof the nano-sized TiC particles. The tensile strength and elongation of the 0.5 wt% nano-sized TiC particlereinforce Al–Cu matrix composite can reach to 540 MPa and 19.0%, increased by 11.08% and 187.9%respectively, than those of the Al–Cu matrix alloy (485 MPa and 6.6%).

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Al alloys have been attracted considerable attentions over the pastdecades, particularly due to their great weight reduction potential inthe realms of aviation, aerospace and automotive [1–3]. Moreover,the strength of the Al alloy was improved through being reinforcedwith the ceramic particles. However, the particle reinforced Al matrixcomposites often process high tensile strength with low ductilitywhich limited their widespread engineering applications [4–8]. So, itis important to simultaneously improve the strength and ductility ofthe Al matrix composites.

Simultaneously improving the strength and ductility has been amajor goal over recent decades for the particle reinforced metalmatrix composites. One approach is to decrease the size of thereinforcing particles. Some researches indicated that when the sizeof the reinforcing particles decreases below 100 nm, the strengthand ductility of the composites could be improved simultaneously[9–11]. But this approach causes non-homogeneous particle dis-persion and poor interface bonding. In our previous research, byusing the carbon nano-tube (CNT) of high chemical activity, nano-sized TiC particles were synthesized by self-propagating hightemperature synthesis (SHS) in the Al–Ti–CNT systems [12]. Onthe other hand, micro-sized TiC particle (1–5 μm) reinforced Almatrix composites were fabricated by adding TiC–Al master alloy,

which was made by reaction of Al, Ti and C powders, into themolten Al matrix. It was indicated that the micro-sized TiCparticles individually dispersed in the Al matrix. The strengthimprovement (from 280 MPa to 328 MPa) is attributed to the goodwetting and hence strong interfacial bonding between Al and TiC[13]. They provide thoughts and guidance for manufacturing thenano-sized TiC particle reinforced metal matrix composites viaSHS and stir casting.

In this process, the nano-sized TiC reinforcing phase is formedin situ through the SHS reaction. Unlike the conventional metalmatrix composites produced by ex situ methods, the in situcomposites exhibit the following advantages: (a) the in situformed reinforcements are thermodynamically stable in thematrix, leading to less degradation in elevated temperatureservice; (b) the in situ formed reinforcements tend to be fineand well distributed; (c) the interface between the reinforcingphase and the matrix is clean, resulting in a strong interfacialbonding [5,14,15]. Therefore, to investigate the in situ nano-sizedparticle reinforced metal matrix composites are very important. Inthe past decade, much of the research has been focused on in situmicro-sized particle reinforced aluminum metal matrix compo-sites due to their potentially low fabrication cost, while less workhas been carried out on in situ nano-sized particle reinforcedaluminum metal matrix composites.

On the other hand, In the Al–Cu alloy, θʹ (Al2Cu) is one of theprimary strengthening precipitates. In the Al–Cu alloy, the pre-cipitation sequence was previously accepted as supersaturatedsolid solution-G.P.(I)-G.P.(II)(or θ″)-θʹ [16]. The θʹ-plates aredistributed over all three {100}-plane variants in aged Al–Cu

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Materials Science & Engineering A

http://dx.doi.org/10.1016/j.msea.2014.11.0060921-5093/& 2014 Elsevier B.V. All rights reserved.

n Corresponding authors at: Key Laboratory of Automobile Materials, Ministry ofEducation, Jilin University, No. 5988 Renmin Street, Changchun 130025, PR China.Tel./fax: þ86 431 8509 4699.

E-mail address: jqc@jlu.edu.cn (Q. Jiang).

Materials Science & Engineering A 622 (2015) 189–193

alloys. During plastic deformation, the semi-coherent θʹ precipi-tates would restrict by-passed by dislocations [17]. Therefore, thesize and spacing of the θʹ precipitates have a strong influence onthe plastic behavior of Al–Cu alloys. It is very important to studythe θʹ precipitates in the Al–Cu matrix composites. However, theinvestigation on the effect of the nano-sized TiC particles is absent.

In this paper, an alternative technique has been employedwhere nano-sized TiC particles are formed in situ, from elementalpowder mixtures, to make a master alloy which is subsequentlyadded to molten Al–Cu. The structure and mechanical propertiesof the resulting composite are reported.

2. Experimental

The composites were fabricated by dispersion the nano-sizedTiC–Al master alloy into molten Al–Cu alloy. The master alloy wasprepared by the following steps. The powders of Al, Ti and carbonnano-tube (CNT) were mixed sufficiently by ball milling at a lowspeed (approximately 70 rpm) for 24 h and then pressed into thecylindrical compacts of approximately 28 mm in diameter andapproximately 25 mm in height with green densities of approxi-mately 65% of theoretical. The ball to powder mass ratio is 20:1.The SHS reactions of the compacts were conducted in a vacuum at1173 K for 10 min. The phase compositions in the master alloyswere identified by X-ray diffraction (XRD, Rigaku D/Max 2500PC)with CuKα radiation using a scanning speed of 4 1/min.

The chemical composition in mass% of the casting Al–Cu alloyused in the present experiment was 5.00 Cu, 0.45 Mn, 0.30 Ti, 0.20Cd, 0.20 V, 0.15 Zr, 0.04 B, and balanced Al. The nano-sized TiC–Almaster alloy was then added to the molten Al–Cu alloy which washeld at 1073 K. The contents were controlled to produce thecomposites containing 0.1, 0.3, 0.5, 0.7 and 1.0 wt% nano-sizedTiC particles, respectively. Then, the molten melt was poured intopre-heated steel die of 200�600�12 mm. Before the tensile test,all the samples underwent the T6 heat treatment (solution at808 K for 12 h and aging at 438 K for 10 h).

After T6 heat treatment, all the samples were machined to thetensile dog-bone shaped samples with a gauge cross section of5.0�2.5 mm and a gauge length of 30.0 mm. Tensile tests wereconducted at room temperatures using a servohydraulic materialstesting system (MTS, MTS 810) at a constant strain rate of1�10�4 s�1. The microstructure was examined by Olypus opticalmicroscope (OM, Olympus PMG3,) and high resolution transmis-sion electron microscope (HRTEM, JEM-2100F).

3. Results

Fig. 1 shows the XRD patterns of the master alloys. As indicated,the master alloy contains only Al and TiC phases without inter-mediate phase Al3Ti was found.

Fig. 2 shows the typical as-cast microstructures of the Al–Cualloy and the composites. The dendritic microstructure resultingfrom the casting solidification process is clearly revealed. Fig. 3shows that the nano-sized TiC particles predominantly dispersedin the dendritic interior. It is assumed that the uniform dispersionof the nano-sized particles provides some heterogeneous nuclea-tion sties of the α-Al crystal during solidification, resulting in amore refined microstructure. As indicated, the microstructure inthe Al–Cu alloy is constituted by the coarse α-Al grains withuneven sizes about 100–180 μm and the Al2Cu phase at the grainboundaries (Fig. 2a); while in the composites, the average size ofthe α-Al dendrites decreases with the addition of the nano-sizedTiC particles increasing. That means the TiC particles successfullyrefined the dendrite of the Al–Cu alloy. Furthermore, compared to

the Al–Cu alloy, the morphologies of the α-Al grains in thecomposites changed from cystiform to dendrite with finer sizes.

Fig. 4(a) and (b) shows the microstructure of the θʹ precipitatesin the Al–Cu alloy and the composite after the T6 heat treatment,respectively. The plate-shaped θʹ precipitates form on the {001}planes of the α-Al matrix, and the dislocation loops are foundaround the precipitates. It can be seen that in the Al–Cu alloy,there are only a few amount of the nano-sized θʹ precipitates with awidth of 10–15 nm and length of 180 nm. However, there are largenumbers of nano-sized θʹ precipitates with a width of 2–5 nm andlength of 80 nm in the composite. At same time, the distribution ofthe θʹ precipitates were more homogeneous.

Fig. 5 shows the typical engineering/true stress–strain curves ofall the samples and Table 1 lists the detail data of the engineeringtensile strength and elongation. As indicated, compared with theAl–Cu alloy, the ultimate tensile strength and elongation of thecomposites were significantly improved by the addition of thenano-sized TiC particles. The tensile strength of the compositesincreases with the increase of the TiC, while the elongationincreases firstly and then decreases. The 0.5 wt% nano-sized TiCparticle reinforced Al–Cu matrix composite possesses the highestductility. The tensile strength and the elongation of the 0.5 wt%TiCp/Al–Cu composite can reach to 540 MPa and 19.0%, respectively.

4. Discussion

The mechanisms contributing to the good tensile property ofthe composites are now analyzed and discussed. Considering allthe microstructural features, the improvement of the tensilestrength and ductility should be attributed to the finer α-Aldendrites and θʹ participates introduced by the addition of thenano-sized TiC particles.

As mentioned above, the nano-sized TiC particles acted asheterogeneous nucleation sties of the α-Al crystal during solidifi-cation, resulting in a more refined microstructure. As known, therefined α-Al dendrite reduces the size of the nucleating flaws andincreases the resistance to crack propagation, leading to a higherfracture stress and ductility. More boundaries are formed becauseof the refined dendrites in the composite. The high boundaryconcentration and the rosebush-like dendrites play an importantrole as barriers to the enablement and transmission of thedislocation, which is helpful to improve the tensile strength andductility. On the other hand, the refined dendrite can also offer ahigher resistance to shear localization and shear fracture, and thusstabilize the hydrostatic triaxial stress. Then, the ductile fracturethrough microvoid nucleation and coalescence can be promoted

Fig. 1. The XRD patterns of the master alloy.

D. Zhou et al. / Materials Science & Engineering A 622 (2015) 189–193190

by this [18,19]. The rosebush-like dendrites microstructure makesit harder for the transgranular fractures which need more energythan the intergranular fracture. Even when the fracture mode isintergranular, the rosebush-like dendrites microstructure can also

improve the strength and ductility by extending the crackpropagation path.

As mentioned above, the size and spacing of the θʹ precipitateshave a strong influence on the plastic behavior of Al–Cu alloys.

Fig. 2. Typical microstructures of the samples: (a) Al–Cu alloy and (b–f) the TiCp/Al–Cu composites with different mass fraction of TiC: (b) 0.1%; (c) 0.3%; (d) 0.5%; (e) 0.7%and (f) 1.0%.

Fig. 3. (a) TEM micrographs of the nano-sized TiC particles in the TiCp/Al–Cu composite; (b) HRFEM images of the interface between the nano-sized TiC and the α-Al matrix;and (c) corresponding SAED pattern.

Fig. 4. TEM micrographs of the θ0 precipitates in (a) Al–Cu matrix alloy sample and (b) nano-size TiCp/Al–Cu composite sample and (c) and (d) are the corresponding SAEDpatterns.

D. Zhou et al. / Materials Science & Engineering A 622 (2015) 189–193 191

It was suggested that Cu is likely to segregate to relieve the stressfield around the edge components of the dislocation and hencethat dislocations can act as nuclei for θʹ formation [20]. Forexample, the acceleration of θʹ formation in fast quenched crystalsis due to dislocations formed on quenching [21]. In our experi-ment, the addition of the nano-sized TiC particles would increasethe dislocations in the composites. Then, more θʹ precipitates wereobtained in the composites. Furthermore, because the α-Al grainsin the composites are finer than those in the Al–Cu alloy, thediffusion distance of the Cu atoms becomes shorter during thesolution process [22]. Therefore, the finer and more uniformlydistributed θʹ precipitates were formed in the α-Al grains duringthe aging process. It was suggested that the precipitation strength-ening, which results from the ability of the nano-scale second-phase precipitates to restrict and impede the dislocation actuationand movement by forcing dislocations to circumvent the nano-scale precipitates, which makes a significant contribution to thestrength enhancement.

As known, due to the presence of highly-dispersed nano-sizedreinforcement (smaller than �100 nm) in a metal matrix, Orowanstrengthening becomes more favorable in the materials [23]. Orowanstrengthening results from the interaction between dislocations andthe dispersed reinforcemnt. In this paper, the dispersed nano-sizedTiC particles act as obstacles to hinder the motion of dislocations andthen enhanced tensile strength of the composites.

In particular, dispersing nano-sized TiC particles and θʹ pre-cipitates in the grain interior is an effective approach to increasethe strength and simultaneously improve the ductility of thecomposites, because they can generate, pin down and thusaccumulate dislocations within the grains. During tensile defor-mation the retention of an increasing number of dislocations inthe grain interior is helpful to improve the tensile strength andelongation. On the other hand, the composite failure is associatedwith particle cracking and void formation in the matrix withinclusters of the particles. Particles cracking by catastrophic

propagation of an internal defect is given by the Griffith equation[24]

σf ¼2EγπC

� �1=2

ð1Þ

where σf is the stress on the particle, γ is the fracture surface energy, Eis Young's modules of the particle, and C is the internal crack length.In our study, because the nano-sized TiC particle is fine anddistributed uniformly at first, so the internal crack length C is shortand the stress on the particle is high. The ductility would beimproved by the decrease of the particles cracking. With the increaseof the nano-sized TiC particles, some agglomerations would be exitedin the composites. The cracks tend to propagate in the agglomerationzone, which would reduce the tensile elongation.

5. Conclusions

In this work we have designed, and successfully fabricated thenano-sized TiC particle reinforced Al–Cu matrix composites via thedispersion the nano-sized TiC–Al master alloy into the moltenAl–Cu alloy. A good tensile ductility together with the highstrength has been derived from the resulting nano-sized TiCparticle reinforced Al–Cu matrix composites. The tensile strengthand elongation of the 0.5 wt% nano-sized TiC particle reinforceAl–Cu matrix composite can reach to 540 MPa and 19.0%,increased by 11.08% and 187.9% respectively, than those of theAl–Cu matrix alloy (485 MPa and 6.6%). The significant improve-ment of the ductility could be attributed to the finer α-Al dendriteand θʹ participates introduced by the addition of the nano-sizedTiC particles. Our finding will help guide endeavors to architectureother nano-sized TiC particle reinforced metal matrix compositesto simultaneously elevate strength and ductility.

Acknowledgment

This work is supported by the National Natural Science Founda-tion of China (Nos. 51171071, 50971065 and 50531030), NationalBasic Research Program of China (973 Program) (No.2012CB619600),the Research Fund for the Doctoral Program of High Education ofChina (No. 20130061110037) and the Project 985-High PerformanceMaterials of Jilin University.

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Fig. 5. Typical (a) engineering and (b) true stress–strain curves of the Al–Cu alloy and the nano-sized TiCp/Al–Cu composites reinforced by different contents of nano-sizedTiC particles.

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TiC (wt%) σb (MPa) δ (%)

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