# CNTs/TiC Reinforced Titanium Matrix  Nanocomposites via Powder Metallurgy and Its Microstructural and Mechanical Properties

Authors: Katsuyoshi Kondoh; Thotsaphon Threrujirapapong; Hisashi Imai; Junko Umeda; Bunshi Fugetsu
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

By using pure titanium powder coated with unbundled multiwall carbon nanotubes (MWCNTs) via wet process, powder metallurgy (P/M) titanium matrix composite (TMC) reinforced with the CNTs was prepared by spark plasma sintering (SPS) and subsequently hot extrusion process. The microstructure and mechanical properties of P/M pure titanium and reinforced with CNTs were evaluated. The distribution of CNTs and in situ formed titanium carbide (TiC) compounds during sintering was investigated by optical and scanning electron microscopy (SEM) equipped with EDS analyzer. The mechanical properties of TMC were significantly improved by the additive of CNTs. For example, when employing the pure titanium composite powder coated with CNTs of 0.35 mass%, the increase of tensile strength and yield stress of the extruded TMC was 157 MPa and 169 MPa, respectively, compared to those of extruded titanium materials with no CNT additive. Fractured surfaces of tensile specimens were analyzed by SEM, and the uniform distribution of CNTs and TiC particles, being effective for the dispersion strengthening, at the surface of the TMC were obviously observed.


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## Body

## 1. Introduction

Carbon nanotubes (CNTs),
found in 1976s by  Oberlin et al. [1], have a potential to be used as attractive
reinforcements of the composite materials because they have a low density of
1.3g/cm3 and  973 GPa Young’s modulus [1–3]. It is also
well known that CNT also has excellent electrical and thermal conductivities
[4]. Previous studies indicated
characteristics of the metal matrix composites (MMCs) reinforced with CNTs, where
industrial metals such as aluminum, magnesium, nickel, copper, and their alloys
were used [5–14]. Titanium (Ti) and titanium alloy are also interested as the
matrix material because they are widely used in various industrial applications,
for example, automotive, motorcycles, and airplane industries due to their high
specific strength and Young’s modulus. Their chemical and petrochemical
applications are also excellent because of their high corrosion resistance [15]. On the
other hand, when adding CNTs into metals as reinforcements, powder metallurgy
(P/M) process is more suitable than ingot metallurgy (I/M) because the
separation of CNTs from the molten metals easily occurs due to their buoyancy
caused by the low density. Furthermore, the agglomeration of the bundled CNTs
always exists in P/M composite materials because of their van der Waals force,
which is attractive or repulsive between carbon atoms [16, 17]. One of the
researchers in the previous study showed an effective preparation of unbundled
CNTs by using the zwitterionic
surfactant solutions via wet process [18]. In this study, the microstructures
and mechanical properties of P/M titanium matrix composite (TMC) reinforced
with unbundled CNTs by using the above wet process. It is
strongly expected that the combination of Ti and CNTs will serve superior tensile
strength to the conventional TMC. The strengthening mechanism of the composites
with CNTs is discussed by their microstructures analysis and identification of
the dispersoids.


## 2. Experimental

Atomized pure Ti powders, having
a mean particle size of 30μm, were used as starting materials. The impurity
contents of raw powder were Fe: 0.03, Si: 0.01, Mg: <0.001, Cl: <0.002, O: 0.21, N: 0.02, C: <0.01 (mass%).  The
zwitterionic surfactant solution with CNT concentration of 3.0 mass% was
prepared. Multiwall CNTs with about 10 nm diameter and 0.5~1μm
length were employed in this experiment. Polar zwitterions generally had a high
solubility in water, but a poor solubility in most organic solvents. 3-(N,
N-dimethylstearylammonio) propanesulfonate, a typical linear zwitterionic
surfactant, was used in this study. It had both hydrophobic and hydrophilic
groups. Electrostatic interactions, having larger attractive forces than the van der
Waals forces between CNTs, occurred at
the hydrophilic because of the positive charge and negative charges on their
headgroups. Therefore, the unbundled CNTs existed in the zwitterionic
surfactant solution [18]. Ti powders were dipped into the solution, and
subsequently dried in oven at 373 K for 10.8 kiloseconds. The composite Ti powders
coated with CNTs were obtained, and the individually independent CNT were
observed on the powder surface as shown in Figure 1. Surfactant solids also
existed, and should be removed before hot consolidation because they changed to
gases at elevated temperature over 773 K [19], and would cause the pores as
material defects. From a viewpoint of the thermal resolution of the surfactant
solids, the Ti composite powders with CNTs were consolidated by spark plasma
sintering (SPS, Syntech Co. SPS-1030S) process with two steps heating; 873 K
for 3.6 kiloseconds for their removal, and 1073 K for 1.8 kiloseconds for the
vacuum sintering. The applied load during the heating was 20 kN and 41.6 kN,
respectively. The sintered Ti composite compact was heated at 1073 K for 180 seconds
in argon atmosphere, and immediately served to hot extrusion. The extrusion
ratio, speed, and die temperature were 37, 3.0 mm/s and 673 K, respectively.
The preheating at 273 K was enough for in situ synthesis of titanium carbides
(TiCs) by the reaction of Ti matrix and CNTs. Microstructure and phase
characterizations were investigated by X-ray diffraction (XRD, Shimadzu
XRD-6100), optical microscope and scanning electron microscope with energy dispersive
spectrometry (SEM-EDS, JOEL, JSM-6500F). P/M extruded TMC materials were
machined to tensile specimen bars with 3.5 mm diameter and 10 mm gauge length,
and evaluated at room temperature. The fractured
surface of the specimens was observed by FE-SEM to investigate the dispersion
strengthening mechanism by CNTs and in situ TiCs.Figure 1
SEM observation on surface of pure titanium
powder coated with unbundled CNTs.


## 3. Results and Discussion

Chemical analysis on P/M
extruded Ti material and reinforced with unbundled CNTs indicates that the
carbon content is<0.01% and 0.351%, respectively. It means the content of
dispersed CNTs of the TMC material is about 0.35%. The oxygen content is 0.21% and 0.27%, respectively. That is, the oxidation with 600 ppm during the
consolidation of the composite Ti powders by the process as mentioned above
slightly occurred. XRD patterns shown in Figure 2 obviously show the peaks of
in situ synthesized TiCs of the composite material at 2θ=36.08°,41.74°, and
60.89°. Additionally, there is no difference of 2θ for
titanium peaks between extruded Ti and reinforced with CNTs. A mean
grain size of the TMC with CNTs and no CNT is around 6 μm and 5 μm, respectively,
and no significant difference between them is observed. The optical and
scanning electron microscope observation results on extruded TMC with CNTs are
shown in Figure 3. TiC particles are uniformly distributed in the matrix, that
is, the original distribution of unbundled CNTs on Ti powder surface is
homogeneous. The image analysis on the optical microstructure of Figure 3(a)
indicates a mean particle size of in situ formed TiC is 1.8 μm. The TiC dispersoids
have two different shapes: elongated and spherical type. Most of the formers, having
a length of about 10 μm or less,
are lined along the extrusion direction because of the severe plastic
deformation of the titanium matrix powder and recombination of CNTs during hot
extrusion. Then, CNTs are also lined along the direction, and reacted with the
titanium matrix, resulting elongated TiCs. The formation of spherical TiC
particles indicates the incompletely disassembled CNTs, causing the coarse TiC
particles, partially exist on the powder surface even after this wet process.
The dark areas around any TiC particle are detected, and the magnified
observation by FE-SEM is shown in Figure 3(b).  It is obviously clarified that
they correspond to the titanium matrix, and have a good bonding with the TiC
particle due to the solid-state reaction in heating at 1273 K. Furthermore, the
titanium matrix seems as the porous lamellar structure in perpendicular
direction with paper, which is presumed corroded by hydrous solution during etching.  This
trend may be due to the effect of TiC particle on the orientation of lamellar
structure at interfaces  during phase transformation in hot extrusion process.Figure 2
XRD patterns of (a) P/M extruded titanium and (b) reinforced with CNTs.Optical microstructure of (a) P/M extruded TMC with CNTs and (b) interface
observation between TiC and titanium matrix by SEM.
(a)(b)The tensile test results are shown in Figure4. The ultimate tensile strength, yield stress and elongation of them are summarized in Table 1. Tensile properties of P/M extruded Ti with no CNT prepared in this study are almost
same as those of the conventional wrought titanium [20]. Comparing the
stress-strain curves of the wrought titanium materials, the increased UTS and
YS are 157 MPa and 169 MPa, respectively. When considering a very small
increase of oxygen with 600 ppm, these mechanical improvements are due to the
dispersion strengthening effect by unbundled CNTs and in situ synthesized TiC
particles. As a remarkably suggested point, in spite of the above increase, the
TMC composite reveals a large elongation of 26%. This is because the total
carbon content of 0.35 mass% is very small, compared to the conventional MMCs,
and significantly effective for strengthening. Furthermore, when adding CNTs to
Ti powder, the microvickers hardness values (HV0.05) increase from 261 to 285. Fractured surface observation of TMC specimen is
shown in Figure 5. CNTs have been remained in matrix, and also
confirmed by EDS analysis. CNTs with the
original size and shape are observed at the surface of the composite. On the
other hand, TiC particles formed by the reaction of CNT with Ti are also
obviously observed. It means that solid-state sintering condition at 1073 K by
SPS and preheating at 1273 K before extrusion are suitable to prepare the Ti
composite including both of CNTs and TiC particles. The remained CNTs
presumably improve the mechanical properties of TMC.Table 1
Tensile properties of P/M extruded titanium and reinforced with CNTs.
UTS/MPaYS/MPaElongation (%)P/M extruded titanium58542329.7P/M extruded TMC with CNTs74259226.0Figure 4
Stress-strain curves of P/M extruded titanium and reinforced with CNTs.SEM observation on fractured surface of tensile specimen of TMC with CNTs.
(a)(b)


## 4. Conclusions

(1) Wet process using the zwitterionic
surfactant solutions was effective to coat unbundled CNTs on Ti powder surface,
and assemble them in P/M extruded titanium composite. (2) In situ formed TiC
particles were also uniformly distributed in the matrix, and had elongated and
spherical shapes. (3) Mechanical properties of tensile strength and microvicker’s
hardness were remarkably improved by the reinforcement of 0.35 mass% CNTs. In
particular, TMC via this process showed a good balance of high UTS of 742 MPa
and large elongation of 26% when using pure titanium powder as input raw
materials. (4) Observation on the fractured surface of tensile specimens
indicated that CNTs were still remained with no morphological change and
promoted their mechanical properties.



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*Source: 127538-2009-03-05.xml*