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CHAPTER 3
PRODUCTION AND CHARACTERIZATION OF Al-TiB2 MMC
3.1 INTRODUCTION
The development of metal matrix composites (MMC) has reflected the need
to achieve property combinations beyond those attainable in monolithic metals.
Thus, tailored composites resulting from the addition of reinforcements to a metal
may provide enhanced specific stiffness coupled with improved fatigue and wear
resistance, or perhaps increases the specific strength combined with desired thermal
characteristics (for example, reduced thermal expansion coefficient and
conductivity) in the resulting MMC.
The melting point of most aluminium alloys is near that of pure aluminium,
approximately 660°C; this relatively low melting temperature, in comparison to
most other potential matrix metals, facilitates processing of aluminium based
MMCs. Among various processes used for fabrication of Al MMCs, stir casting
process is a highly potential and low cost process. Stir casting is a liquid state
method for fabricating composite materials, in which a dispersed phase (ceramic
particles, short fibres) is mixed with a molten matrix metal by means of mechanical
stirring. The liquid composite material is then cast by conventional casting methods
and could also be processed by conventional metal forming technologies.
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In this chapter, details about the in-situ stir casting process used to produce
the proposed aluminium (Al6061-T6) metal matrix composite reinforced with
titanium boride (TiB2) are presented. The mechanical and metallurgical
characterizations of the cast MMCs are also detailed.
3.2 EXPERIMENTAL PROCEDURE
3.2.1 Production of Al-TiB2 MMC
The matrix material was chosen as Al6061 and the reinforcing material was
chosen as TiB2. K2TiF6 and KBF4 salts were used in a proper proportion to obtain
the required volume fraction of TiB2 in the composite through in situ process. Table
3.1 shows the chemical composition of Al6061-T6 alloy.
Table 3.1Chemical Composition of Al6061-T6 Alloy
Element Mg Si Fe Mn Cu Cr Zn Ni Ti Al
wt.% 0.95 0.54 0.22 0.13 0.17 0.09 0.08 0.02 0.01 Balance
The procedure for deriving the amount of chemical salts required to get the
estimated volume fraction of TiB2 in Al MMC is given below:
1) The atomic weight of each chemical that are in the salts is evaluated.
2) The ratio of the chemicals in the particular composition is calculated.
3) Percentage of chemical composition of the salt is calculated.
4) The percentage of chemical required for producing the required volume of
TiB2 is estimated.
5) The weight of each salt required to obtain the required weight percentage of
TiB2 in the composite is found.
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Using the above procedure, the weight of each salt that is used to extract the
required weight percentage of TiB2 Particles was estimated and presented in Table
3.2.
Table 3.2 Calculated Weights of Salts per Casting
Sl.No. % Reinforcement Weight of Salts
TiB2 (wt. %) K2TiF6 (g) KBF4 (g)
1. 10 34.55 36.24
2. 11 38.00 39.86
3. 12 41.45 43.48
4. 13 44.91 47.11
5. 14 48.36 50.73
The casting was done using the resistance type electric furnace at the
Welding Research Cell, Coimbatore Institute of Technology, Coimbatore (Figure
3.1). The melting was carried out in a graphite crucible. The chemicals used were
highly corrosive in nature. Therefore a primer coating with Wulfrakot was applied
on the crucible and stirrer blade. The crucible was preheated to 200 °C. Six Al6061
T6 treated rods of size 150 mm were placed in the crucible at 450 °C. The weight of
the six rods was 1235 gms. The furnace was heated up to 850 °C. Both the
chemicals as per the quantity mentioned in Table 3.2 were added and mixed
thoroughly.
The following is the chemical reaction taking place in the in-situ process:
3K2TiF6 + 22Al + 6KBF4 3Al3Ti + 3AlB2 + 9KAlF4 + K3AlF6 + Heat (3.1)
3Al3Ti + 3AlB2 12Al + 3TiB2 (3.2)
The salt K2TiF6 reacts with Al to form Al3Ti and gases. The other salt KBF4
reacts with Al to form AlB2 and gases. The formed Al3Ti and AlB2 react together
and yield TiB2 as the product. The theoretical mole ratio of inorganic salts required
to produce TiB2 can be obtained from equations (3.1) and (3.2). One mole of
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K2TiF6 and two moles of KBF4 are required to produce one mole of TiB2. It was
reported in a study that when these salts were added to produce the MMC, a large
amount of Al3Ti was present in MMCs (Zhang et al 2008, Zhao et al 2007).
Figure 3.1 Stir Casting Setup
This might be due to the non-availability of sufficient AlB2 to convert all
Al3Ti into TiB2 as the reaction specified in equation (3.2) was incomplete. The
shape of Al3Ti was reported to be needle like structure which is obviously
detrimental to the strength of the composite (Zhao et al 2007, Zhao et al 2005 and
Varin 2002). Therefore here the salts were added such that KBF4 was slightly in
excess of theoretical mole ratio to eliminate the formation of Al3Ti.
The molten slurry was kept as such at 850 °C for about 10 minutes for the
reaction to take place completely. After the formation of TiB2 the slag in the molten
metal (emerging out of the chemical reaction) was removed completely. Then the
stirrer was switched on and stirred for about 10 minutes so that the TiB2 particles
are completely dispersed into the molten metal. Then the molten metal was brought
to the pouring temperature of around 750 °C. The crucible was taken out and
poured into the preheated die cavity of size 100 mm x 50 mm x 50 mm. Manual
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stirring was done even while pouring using a graphite rod. Composites containing
percentages of Al-TiB2 varying from 10% to 14% reinforcement were fabricated.
Figure 3.2 shows a typical Al-TiB2 composite that was fabricated by in situ stir
casting process. The cast composite is then sliced to a size of 10 mm x 50 mm x 6
mm using a wire cut EDM machine.
Figure 3.2 A Typical In-situ Stir Cast Al-10 wt. % TiB2 MMC
3.2.2 X-ray Diffraction
Five specimens of size 10 mm x 5 mm x 3 mm were prepared from castings
having different content of TiB2 particles. The X-ray diffraction patterns (XRD)
were obtained using Panalytical X-ray diffractometer with Cu- -ray radiation to
confirm the formation of TiB2 particles.
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3.2.3 Evaluation of Tensile Strength
From each sliced plate, three samples were cut according to ASTM E08
Standard as shown in Figure 3.3 for testing the tensile strength of the cast MMC.
Figure 3.4 shows the typical tensile specimen samples. The tensile strength was
evaluated using a computerised universal testing machine (HITECH TUE-C-1000)
for all the three samples and the average UTS and % Elongation were calculated.
Figure 3.3 Dimensions of Tensile Specimen in mm as per ASTM E08 Standard
Figure 3.4 Typical Tensile Test Specimens
3.2.4 Evaluation of Wear Resistance of MMCs
For conducting the wear test on the MMCs, specimens of size 6 mm x 6 mm
x 20 mm were cut from the castings. The wear rate was measured using a pin on
disk (POD) wear and friction monitoring machine (DUCOM TR20) at room
temperature. ASTM G99-05 standard test was conducted on all the samples and the
results were tabulated.
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Figure 3.5 shows the experimental setup which was used for this study. The
polished sample was made to slide on disk made of EN35 steel alloy. Based on trial
experiments, the parameters for evaluating the dry sliding wear were fixed. The
fixed parameters are given below.
1. Sliding velocity = 1.2 m/s
2. Sliding distance = 1500 m
3. Normal load = 15 N
The wear in micron and frictional force in N were calculated using the
sensors and the results were plotted using a computerised data acquisition system.
Figure 3.5 Photograph of Pin-on-disc Apparatus
3.2.5 Evaluation of Corrosion Resistance of MMCs
Electrochemical pitting corrosion test was conducted on the MMCs to
evaluate the corrosion resistance of the MMCs. ACM Gill 5500 Potentiostat &
Galvanostat instrument was used to conduct the tests according to ASTM G5
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standard. The experimental setup has a flat cell in three electrode configuration as
shown in Figure 3.6. The working electrode comprises of the specimen prepared
from the cast MMC. A saturated calomel electrode (SCE) and platinum gauze were
used as reference electrode and counter electrode respectively. The MMC specimen
was held on the flat cell using an adjustable shoe assembly. The shoe made an
electrical contact with the back surface of the specimen and also secured the
specimen against a Teflon gasket. The Teflon gasket exposed 1 cm2 of the specimen
to the cell solution. The test solution was prepared by dissolving 8.75 g of reagent
grade NaCl salt in 250 ml distilled water to form 3.5% (by weight) sodium chloride
solution. The flat cell was filled with 250 ml of this solution and the electrodes were
connected to the respective ports in the corrosion measuring instrument through the
lead provided in the flat cell.
Figure 3.6 ACM Gill 5500 Potentiostat and Galvanostat Instrument
After the specimen had been immersed for 50 minutes to allow for the rest
potential to settle, the test was commenced by measuring the open circuit potential.
A potential scan was applied at -250 mV and scanned in more noble direction to
250 mV at a rate of 100 mV/min. The current density was measured continuously
59
using the data acquisition system connected to the computer using a RS232 port.
The Tafel graph with current density in logarithmic scale along x axis and potential
along the y axis was obtained using the analysis software. The Tafel graph was
further processed using the analysis module in the software and the values of anodic
slope (Ba), cathodic slope (Bc), corrosion current (ICorr), corrosion potential (ECorr),
pitting potential (EP), Polarization resistance (RP) and corrosion rate (VCorr) were
estimated.
3.2.6 Microstructural Analysis
The cast composite plates were cross-sectioned at its mid-point and
specimens of size 20 mm x 10 mm x 6 mm were obtained for metallographic study.
The samples were prepared as per standard metallographic procedure. These
specimens were observed using a scanning electron microscope (SEM) (JEOL-
JSM-6390). The specimens were then etched with a colour etchant containing 2 3 g
sodium molybdate, 5 ml HCl (35%) and 1 2 g ammonium bi fluoride in 100 ml
distilled water for microstructure analysis (Beraha et al 1977). Colour
metallographic study was carried out using a metallurgical microscope (Olympus
Microscope BX51 M).
3.2.7 Microhardness Test
The microhardness of the cast composite was measured using a
microhardness tester (MITUTOYO-MVK-H1) shown in Figure 3.7. The samples
were clamped to the vice available in the machine and using a diamond indenter the
specimen were tested at 50 g load applied for 10 seconds at fifteen different
locations transversely. The test was repeated for three different regions and the
average value of microhardness was calculated and presented in section 3.3.6.
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Figure 3.7 Microhardness Tester
3.3 RESULT AND DISCUSSION
3.3.1 X-ray Diffraction Analysis
Figure 3.8 shows the XRD patterns obtained from the developed composite
revealing the presence of TiB2. The peaks of TiB2 are distinctly clear and they
increase with the increase in percentage reinforcement of TiB2 particles. It is
evident from the XRD pattern that TiB2 is the only phase present without any other
intermetallic phases which indicate the thermodynamic stability of in-situ formed
TiB2. TiB2 did not produce any other inter metallic compounds by reacting with
molten Al6061. Absence of other compounds in significant quantity indicates that
the interface between Al6061 and TiB2 is free from contamination.
3.3.2 Tensile strength
Figure 3.9 shows the effect of the amount of TiB2 particles on the tensile
strength of the produced MMCs. The ultimate tensile strength (UTS) increases as
the amount of TiB2 particles increase. Figure 3.10 shows the effect of TiB2 particles
on the % elongation of MMCs. From the figure it is found that the % elongation
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linearly decreases as the % reinforcement of TiB2 particles increases. Al6061-14 wt.
% TiB2 MMC exhibits 52% higher UTS compared with cast Al6061. The %
elongation of the same is 25% lower than that of cast Al6061.
Figure 3.8 XRD Results of the fabricated composites
Figure 3.9 Effect of % of TiB2 on UTS
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The reason for the increase in UTS in the Al-TiB2 MMC is explained below.
Al6061 and TiB2 has different coefficient of thermal expansion. The average
thermal expansion coefficient of Al6061 is 24 x 10-6/°C while that of TiB2 is 8.1 x
10-6/°C. This difference in thermal expansion between matrix and reinforcement
tends to reduce the dislocations around TiB2 particles during solidification (Han et
al 2002). The interaction between TiB2 and dislocations strengthens the composites
during loading. Increase in content of TiB2 results in increased interaction which
enhances UTS. The uniform distribution of TiB2 particles in the aluminium matrix
also plays a vital role in enhancing UTS. The shape of TiB2 particles which are
formed during the in-situ reaction are mostly spheroid in shape which also
contribute to higher UTS by reducing notch effect. The clear interface between the
matrix and the reinforcement increases the load bearing capacity of the composite
which means higher stress is needed initially to initiate cracking (Kumar et al
2010a). The in-situ formed TiB2 particles are also free from flaws assisting
enhancement of UTS.
Figure 3.10 Effect of % of TiB2 on % Elongation
As the % of reinforcement increases, there is an overall reduction in ductile
matrix content. This leads to the reduction in the % elongation of Al6061-TiB2
MMC when TiB2 content is increased in the MMCs. Figures 3.11 and 3.12 reveal
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the fracture morphology of the Al6061 and Al6061-14wt. % TiB2 composite
respectively. Figure 3.11 shows the presence of larger and uniform distribution of
voids which suggest that the matrix alloy Al6061 has undergone ductile fracture.
On the other hand smaller size voids are seen in Figure 3.12 which indicates
Al6061-14wt. % TiB2 composite underwent brittle fracture macroscopically and
ductile fracture microscopically. The TiB2 particles are intact at many places due to
strong bonding with the matrix.
Figure 3.11 Fracture Morphology of Al6061
Figure 3.12 Fracture Morphology of Al6061-14wt. % TiB2 MMC
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3.3.3 Wear Resistance of MMCs
Figure 3.13 shows the effect of the amount of TiB2 particles on the wear rate
of Al6061-TiB2 MMC. It is evident that as the % reinforcement of TiB2 particle
increases, the wear rate decreases.
Figure 3.13 Effect of % of TiB2 on Wear Rate
The trend shows a non linear curve indicting the complexity in
understanding the sliding wear behaviour of the composite. This nonlinear
behaviour is due to the addition of TiB2 particles in the aluminum alloy. These TiB2
particles tend to increase the hardness of the aluminum alloy which reduces the rate
of material removal.
Figures 3.14 and 3.19 show the worn surface morphology of Al6061 and
Al6061-14 wt. % TiB2 MMC respectively. Figure 3.14 shows that the grooves are
deeper and non-uniform, depicting excess wear. The grooves and pits in the wear
morphology reduce progressively as depicted in Figures 3.15 to 3.19, indicating the
high resistance offered by the MMC to wear when the amount of TiB2 increases.
Presence of TiB2 particles in the aluminium matrix can be attributed to the
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reduction in the depth of the grooves in the wear morphology whereas there is no
such resistance offered by pure AA6016.
Figure 3.14 Worn Surface Morphology of Al6061
Figure 3.15 Worn Surface Morphology of Al6061-10 wt. % of TiB2 MMC
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Figure 3.16 Worn Surface Morphology of Al6061-11 wt. % of TiB2 MMC
Figure 3.17 Worn Surface Morphology of Al6061-12 wt. % of TiB2 MMC
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Figure 3.18 Worn Surface Morphology of Al6061-13 wt. % of TiB2 MMC
Figure 3.19 Worn Surface Morphology of Al6061-14 wt. % of TiB2 MMC
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Figure 3.19 also indicates that the grooves and ridges are parallel and aligned
along the direction of sliding exhibiting lower plastic deformation at the edges of
the grooves. The wear debris is in powder form and not sticking to the surface due
to hard TiB2 particles. The reduction in the plastic deformation and depth of
grooves from the worn surface indicate that the wear rate of the matrix alloy is
reduced when TiB2 particles are reinforced (Kumar et al 2010b, 2010c).
3.3.4 Corrosion Resistant Property
Figure 3.20 shows the effect of TiB2 on the corrosion rate of Al6061-TiB2
MMC. The trend shows that corrosion rate increases with the increase in the wt. %
of TiB2 in the MMC. The reason is attributed to the presence of TiB2 particles
which has dislodged the aluminium matrix with their presence. The TiB2 particles
respond to the chemical reaction of the test solution and get dislodged.
Figure 3.20 Effect of % of TiB2 on Corrosion Rate of Al6061-TiB2 MMC
The interface between aluminium matrix and TiB2 particles gets oxidised and
the particles come out of their position creating voids in the surface thereby
0
1E-05
2E-05
3E-05
4E-05
10 11 12 13 14
Cor
rosi
on R
ate
(mils
/yr)
wt. % of TiB2
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initiating pitting corrosion. As the amount of TiB2 particles increase the pitting
locations increase making the MMC prone to corrosion. Figure 3.21 shows the
corroded surface of MMC with 10 wt. % of TiB2 and it is found that the size of the
pit is very small and it is of the order of about 10 microns. Figure 3.22 shows that
the size of the pit occurred are in the order of about 30 microns and in a number of
places the pitting has been initiated.
Figure 3.21 Corroded Surface of Al6061-10 wt. % TiB2 MMC
Figure 3.22 Corroded Surface of Al6061-14 wt. % TiB2 MMC
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3.3.5 Microstructural Analysis
Figures 3.23 to 3.34 show the optical micrographs and SEM micrographs of
Al6061 and Al6061-TiB2 MMC with various percentage of reinforcement. Figure
3.23 shows the microstructure of cast Al6061 alloy. It exhibits a typical dendritic
Al which is formed due to the super cooling
during solidification. Secondary intermetallic phases are seen around the dendrites
which comprises of the alloying elements like Mg and Si forming Mg2Si (Rao et al
2009). Figures 3.24 to 3.28 show the microstructure of Aluminium MMCs
containing different amount of TiB2. The in-situ formed TiB2 particles have refined
Al grains because TiB2 Al during
solidification. TiB2 particles itself act as nucleus on which the aluminium grains
solidify (Han et al 2002).
Figure 3.23 Optical Micrograph of Al6061 Cast Alloy
Figures show that TiB2 particles are uniformly distributed in the matrix in all
MMCs. The reaction of K2TiF6 and KBF4 with molten aluminium produced TiB2
particles of varying shapes and sizes. Most of the TiB2 particles exhibit spherical
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shape. Some particles show elliptical shape and a few irregular grains are also
visible. The appearance of single particle can be considered as a cluster of particles
because in-situ reaction produces fine particles up to nano scale. The in-situ formed
particles join together to form clusters depending upon synthesis temperature,
holding time, reaction rate and cooling rate (Tjong et al 2000). Kumar et al (2010a)
reported that average reinforcement particle size increased with increased
percentage of reinforcement particles and the coarsening of the particles was due to
high reaction rate. The shape, size and spatial distribution of TiB2 particles
influence the mechanical properties of the composites to a larger extend.
Figure 3.24 Optical Micrograph of Al6061-10wt. %TiB2 MMC
The distribution of TiB2 particles in the matrix is related to the solidification
process of the matrix alloy. The density difference between melt and a particle is
over 1.8 g/cm3. Hence the in-situ formed TiB2 particles will be suspending in the
melt (Han et al 2002). As the wetting effect between particles and melt retards the
movement of the TiB2 particles, it can be distributed in the melt for a long time. The
TiB2 Particle
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Al crystallizes as the primary phase at the beginning of the solidification while
TiB2 particles are extracted to the solidification interface. The microstructure also
reveals some small pits in Figures 3.25 to 3.28. These pits are formed because of the
etchants which have removed the aluminium alloy surrounding the particles thereby
removing the particles from the location leaving a pit on the surface.
Figure 3.25 Optical Micrograph of Al6061-11wt. %TiB2 MMC
Figures 3.24 to 3.28 show that TiB2 particles are well bonded with the matrix
because of the exothermic reaction which induces an increase in local melt
temperature and enhances the wetting of the particles. Neither voids nor reaction
products surround the particle which indicates good interfacial integrity between the
particle and the matrix. Synthesizing TiB2 particles within the melt itself reduces
the opportunity for oxidation thus a cleaner and stronger interface is obtained.
Figures 3.29 to 3.33 show the SEM photomicrographs of Al6061-TiB2 MMCs
containing 10-14 wt. % TiB2.
TiB2 Particle
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Figure 3.26 Optical Micrograph of Al6061-12wt. %TiB2 MMC
Figure 3.27 Optical Micrograph of Al6061-13wt. %TiB2 MMC
TiB2 Particle
TiB2 Particle
Al3Ti
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Figure 3.28 Optical Micrograph of Al6061-14wt. %TiB2 MMC
The micrographs show a uniform distribution of TiB2 particles. The variation
in the amount of reinforcement particles is visible with increased content of TiB2.
SEM micrographs reveal that each single particle present is a cluster of particles
which consists of numerous fine TiB2 particles.
Figure 3.29 SEM Micrograph of Al6061-10wt. %TiB2 MMC
TiB2 Particle
Agglomeration
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Figure 3.30 SEM Micrograph of Al6061-12wt. %TiB2 MMC
Figure 3.31 SEM Micrograph of Al6061-14wt. %TiB2 MMC
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Figure 3.32 SEM Micrograph of Al6061-13wt. %TiB2 MMC
Figure 3.33 SEM Micrograph of Al6061-10wt. %TiB2 MMC
Many particles are in sub-micron size. The Figures 3.29 and 3.30 show
particle of the order of 1 micron to 3 micron in size. Some ultra-fine particles are
also observed in the matrix. The distribution of TiB2 particles in the melt is
influenced by convection current in the melt, movement of solidification front
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against particles and buoyant motion of particles (Bauri et al 2011). It is evident
from SEM micrographs that TiB2 particles are distributed in both transgranular and
intergranular regions in MMCs (Figures 3.29 to 3.32).
3.3.6 Microhardness Survey
Figure 3.34 shows the effect of TiB2 on the microhardness of the MMCs.
The microhardness of the fabricated composites tends to increase with the increase
in TiB2 content. Al6061-10wt. % TiB2 composite exhibits 49.7% higher
microhardness and Al6061-14wt. % TiB2 shows 63.7% higher microhardness when
compared to Al6061 alloy which is attributed to the presence of TiB2 in the
composites. During solidification TiB2 particles increase the dislocation density
which in turn creates more resistance to plastic deformation resulting in enhanced
microhardness.
Figure 3.34 Effect of wt.% of TiB2 on Microhardness
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3.4 SUMMARY
The following are the summary of the results of Chapter 3:
i. Aluminium MMCs containing 10 to 14 wt. % TiB2 were successfully
fabricated by in situ reaction.
ii. The presence of TiB2 and the absence of intermediate phases in the Al
MMCs were confirmed by XRD analysis.
iii. The presence of TiB2 particles increased the tensile strength, wear resistance
and microhardness of the MMCs.
iv. TiB2 particles were found to have uniform distribution, spherical in shape,
good bonding and clear interface resulting in enhanced mechanical and wear
properties.
v. But addition of TiB2 particles reduced the % elongation and corrosion
resistance of the MMCs.
vi. The maximum content of TiB2 was limited to 14wt. % due to the formation
of excessive slag.