Page 140 - Zmist-n4-2015

Structural characteristic of Al + 0% TiO 2 (1), Al + 5% TiO 2 (2),
Al + 5% TiO 2 + 2% Gr (3), Al + 5% TiO 2 + 4% Gr (4)
and Al + 5% TiO 2 + 6% Gr (5) powders
Lattice, Å Stress Dislocation Unit cell
D (Scherrer), e, D (WH), 10 , density, volume
10
–29
–3
space constant nm ´10 nm dyne/cm lines/m 10 ,
2
2
Å
14
1.75 4.043 109.63±0.13 0.336 142.733±0.21 0.191 1.182×10 6.607
14
3.05 5.919 168.07±0.11 0.672 178.471±0.14 1.857 5.762×10 13.306
18
3.93 5.923 149.75±0.17 0.884 188.764±0.19 4.805 3.316×10 13.342
17
3.45 4.967 136.56±0.08 0.672 174.843±0.23 5.483 8.251×10 9.879
17
3.46 4.987 179.98±0.16 0.538 224.624±0.13 4.253 1.504×10 9.999
Fig. 2 shows the effect of reinforcement addition on the grain size which is calcu-
lated by using Williamson-Hall and Scherrer Equations. It is observed from Fig. 2 that
the increase in grain size has been observed for the addition of 5 weight percentage of
TiO 2 to the Al matrix. The grain size increases with the addition of increasing weight
percentage of graphite due to the agglomeration of the particles. The powder particle
size is changing with milling time, as a result of the two opposing factors of cold wel-
ding and fracturing of powder particles. While cold welding increases the particle size,
fracturing reduces the particle size. Hence, under continued milling with increasing
weight percentage of graphite powders the particle size increases. It is expected that the
addition of hard nature of TiO 2 powders will decrease the grain size. But here the
increase in the grain size observed in the present study could be because of minimum
milling time and energy. However the similar results were obtained in [14], the authors
reported for the 2024 aluminum composites reinforced with various weight percentages
of TiO 2 nanoparticles in the early stage of the milling, the A2024 powders are flattened
by the collisions of ball–powder–ball. After this, TiO 2 particles are embedded into the
A2024 powders and progressively dispersed in the matrix. Increased average particle
size of the 12 h milled powder confirms that the A2024 powders undergo repeating
plastic deformation, fracturing, and cold welding process [13]. The maximum stress
strain values are obtained for the Al + 5% TiO 2 + 6% Gr composite powders. In [2] it
was reported that when comparing with nano Al–TiO 2 composite the grain size of mic-
rocomposite is higher due to the more agglomeration of TiO 2 with aluminium matrix.
Thus the agglomeration of reinforcement powders plays vital role during mecahnical
milling process.
Compressibility of Al–TiO 2–Gr mixture powders. The experimental procedure of
Al–TiO 2–Gr mixture powders compaction and the densification curves were presented
in previous works [13]. In this study the Al+5%TiO 2+6%Gr powder mixture is compa-
red with other composition of mixtures. The correlation between TiO 2 and Gr amount
and relative density is shown in Fig. 3. It is noted that the maximum densification (98.4)
was obtained for the unreinforced aluminium under the pressure of 500 MPa. However
for the same compaction pressure the densification obtained for the Al + 5% TiO 2 + 6% Gr
hybrid composites is 93.2%. The similar results were also reported in [14] for the
Al–SiC composites. The authors of [15] explained that the reason for the decrease in
densification could be that the ceramic reinforcement particles are harder than the base
soft Al matrix powder and thus during compaction will not be extruded into the pore
space.

139