Materials Sciences and Applications, 2011, 2, 465-468
doi:10.4236/msa.2011.25062 Published Online May 2011 (
Copyright © 2011 SciRes. MSA
Effects of Solution Atoms and Precipitates on
Isochronal Annealing Behavior of Cold-Rolled
Squeeze-Cast SiCw/Al Composites
Wenglong Zhang, Mingyuan Gu
State Key Lab of MMCs, Shanghai Jiao Tong University, Shanghai, China.
Received October 4th, 2010; revised December 16th, 2010; accepted May 11th, 2011.
SiCw/Al and SiCw/6061Al composites, which contained 15 vol pct of whiskers, were fabricated through a squeeze cast
route followed by cold rolling and isochronal annealing. Effects of whicker, solution atoms and precipitates on isoch-
ronal annealing behavior of cold rolled composites were investigated by comparing the isochronal annealing behavior
between pure Al, SiCw/Al, solution-treated SiCw/6061Al and aged SiCw/6061Al. It was found that the recrystalliza tion
of SiCw/Al compo site occurs earlier than that of p ure Al, indicating SiCw has a role of particle stimulated nucleation.
Solution atoms have no significant influence on the isochronal annealing behavior of SiCw/6061Al composite, while
precipitates have such a strong retarding effect on the recrystallization of SiCw/6061Al composite that the isochronal
annealing curve of aged SiCw/6061 composite loses the definable recrystallization step.
Keywords: Composite Materials, Deformation and Fracture, Isochronal Annealing, Precipitates
1. Introduction
Aluminum metal matrix composites (AMCs) are now
recognized as candidate materials for aerospace and au-
tomotive industries because of their low density, high
stiffness and strength. AMCs containing discontinuous
reinforcement, such as particulate, whisker or short fiber,
are especially attractive because they can be readily
shaped with conventional secondary metal-working tech-
niques [1-10]. During such forming operations, AMCs
may experience an appreciable amount of cold working,
which may both influence their further forming opera-
tions and degrade mechanical properties, such as elonga-
tion to failure and fatigue resistance. Therefore, after
forming operations it may be necessary to anneal the
composites to recover their deformation ability and me-
chanical properties [7,11-18]. To optimize forming oper-
ations and mechanical properties of AMCs, under-
standing the annealing behavior of plastically deformed
AMCs is essential.
Recent studies [8,12-20] have examined the micro-
structural development in AMCs during annealing opera-
tions. These studies focus mostly on AMCs manufac-
tured by powder metallurgy route, including particulate-
and whisker-reinforced composites, in which fine disper-
sions of Al2O3 is present. It was reported that, for powder
metallurgy Al-SiCw composites, the reinforcing whisk-
ers were of a significant influence on the deformation
and recrystallization behavior of the matrix alloy [11-14].
Firstly, when whisker volume fraction was equal to or
less than 8%, the presence of whiskers resulted in a more
inhomogeneous deformed microstructure in the matrix of
AMCs compared with those unreinforced materials. And
the recrystallization rate of AMCs was usually faster than
that of the corresponding unreinforced alloy in this case.
When whisker volume fraction was equal to 10%, the
presence of whiskers led to a delocalized structure with
many highly misorientated equiaxed subgrains, and re-
crystallization as a result of nucleation and growth didn’t
occur. Secondly, the fine dispersions of Al2O3 reduced
recovery reaction and also inhibited the recrystallization.
For the powder metallurgy Al-SiCp composites [4], it
was found that recrystallization resulted in an obvious
hardness drop for Al-18vol. pct SiCp composite, but not
for Al-35vol pct SiCp composite. It was not clear wheth-
er or not recrystallization had occurred in the Al- 35vol
pct SiCp composite [4]. Squeeze cast SiCw/Al com- po-
sites are different in microstructure from the powder
Effects of Solution Atoms and Precipitates on Isochronal Annealing Behavior of Cold-Rolled
Squeeze-Cast SiCw/Al Composites
Copyright © 2011 SciRes. MSA
metallurgy ones. One of the main differences is that
squeeze cast SiCw/Al composites contain no fine disper-
sions of Al2O3. This may lead to an annealing behavior in
squeeze cast composites which is different from that in
powder metallurgy ones. The reason why powder metal-
lurgy composites of Al-10vol. pct SiCw didn’t show re-
crystallization behavior may be due in part to the role of
fine dispersions of Al2O3. In order to both eliminate ef-
fects of solution atoms and precipitates existing in matrix
and make a direct comparison between reinforced and
unreinforced materials, research on recrystallization has
been carried out on a range of MMCs with commercially
pure aluminum as the matrix [11,13, 14]. However,
MMCs with pure aluminum as matrix are rarely used in
engineering applications; therefore it is necessary to in-
vestigate the recrystallization of MMCs with aluminum
alloys as matrix from the point of view of engineering
applications. Because the existence of alloying elements,
the annealing behavior of the composites with aluminum
alloys as matrix may be much different from that of the
composites with pure aluminum as matrix.
In this study, two composites (squeeze cast SiCw/Al
and SiCw/6061Al composites), which contained 15 vol.
pct of whiskers, was cold rolled and annealed. Effects of
solution atoms and precipitates on isochronal annealing
behavior of the cold rolled composite were investigated.
2. Materials and Methods
The composites used in this study were reinforced with
type SiC whiskers, 0.1 - 1.0 m in diameter and 30 - 100
m in length. The matrix is commercially pure aluminum
(99.5 wt pct Al) or 6061 alloy. The composites were
fabricated through a squeeze-cast route and had a SiC
whisker volume fraction, Vf, of 15%. It was difficult to
cold deform the as-cast composites due to its bad ductil-
ity. To obtain a higher cold deformation degree, the
as-cast composites were subjected to a hot extrusion with
an extrusion ratio of 18:1 before cold rolling. It was
found that the capability of cold deformation of SiCw/Al
composites could be greatly improved by the pre-hot-
extrusion. The as-extruded composites were then cold-
rolled along the extrusion direction to about 30 pct re-
duction in thickness. The spatial distribution of SiC
whiskers in cold-rolled sheets was fairly uniform and
highly aligned along the rolling direction i.e. the extru-
sion direction, as shown in Figure 1. For SiCw/6061Al
composites, solution treatment was carried out at 520˚C
and water quenched. Then some of the solution-treated
composites were aged at 170˚C for 10 h. Isochronal an-
nealing was performed for 1 h in a temperature range
from 100 to 500˚C. Vickers macrohardness measurement
Figure 1. Whisker morphology in cold rolled SiCw/Al com-
posites. The reduction in thickness is 30%.
was carried out using 5 kg load on the cold-rolled and
annealed specimens. On an average, ten hardness mea-
surements were performed on each specimen for each
date point. Microstructures of the cold-rolled and aged
samples were studied by scanning electron microscopy
(SEM) and transmission electron microscopy (TEM).
3. Results and Discussion
Figure 2 shows the isochronal annealing curves of the
cold rolled composites and pure Al. It can be seen from
the figure that, for all the materials, hardness decreases
with the increase in annealing temperature. For solu-
tion-treated SiCw/6061Al, SiCw/Al and Al, the isoch-
ronal annealing curves are similar in shape and show
definable transition steps from the higher to the low
hardness values. For aged SiCw/6061, however, the
isochronal annealing curve is nearly linear in shape and
without a definable recrystallization step. Comparing the
isochronal annealing curve of pure Al with that of its
composite (SiCw/Al), it is found that the starting recrys-
tallization temperature of SiCw/Al (about 200˚C) is
lower than that of pure Al (about 240˚C), indicating that
SiCw may play a big role in stimulating recrystallization
nucleation. The similar isochronal annealing curve shape
between solution-treated SiCw/6061Al and SiCw/Al
suggests that the isochronal recrystallization behaviour of
Effects of Solution Atoms and Precipitates on Isochronal Annealing Behavior of Cold-Rolled
Squeeze-Cast SiCw/Al Composites
Copyright © 2011 SciRes. MSA
Figure 2. Isochronal annealing curves for cold rolled SiCw/
Al composites and pure Al. The reduction in thickness is
solution-treated SiCw/6061Al may be similar to that of
SiCw/Al. Because the only difference between solu-
tion-treated SiCw/6061Al and SiCw/Al is that there are
solute atoms in the matrix in the former but not in the
latter, it can be said that solute atoms in solution have no
significant influence on isochronal annealing behaviour
of SiCw/Al. That is to say, putting the solute atoms into
the SiCw/Al and making them in solution would not sig-
nificantly change the isochronal recrystallization beha-
vior of SiCw/Al. The hardness drops of the composites
causing by annealing are shown in Figure 3. It can be
clearly seen that the hardness drop of aged SiCw/6061Al
is much less than those of SiCw/Al and solution-treated
The differences in isochronal annealing behavior be-
tween pure Al, SiCw/Al and aged SiCw/6061Al can be
explained from the viewpoint of particle size. Figure 4
shows precipitates in aged SiCw/6061Al. It can be seen
that the round precipitates exist in the aged SiCw/6061Al.
The average radius and volume fraction of the precipi-
tates were measured to be about 0.05µm and 3%. It is
well known that the kinetics of recrystallization is
strongly affected by a dispersion of particles. Generally,
fine and closely spaced particles inhibit recrystallization,
whereas large and widely spaced particles promote re-
crystallization [21,22]. The transition between accele-
rated and retarded recrystallization is primarily depen-
dent on the dispersion parameter and typically occurs at a
value of f/r ~0.2 to 0.4 µm1 [22] where f is the volume
fraction and r is the particle radius. In the present mate-
rials, SiCw has f/r value of ~0.34 µm1 (f is 15% and r is
about 0.44 µm) and hence should accelerate the recrys-
tallization. However, the precipitates have f/r value of
~0.6 (f is 3% and r is about 0.05 µm) which is more than
0.5 and hence should retard the recrystallization. In the
case there is only SiCw, such as in SiCw/Al, the recrys-
Figure 3. Hardness drop before and after annealing.
Figure 4. Precipitates in the matrix of aged SiCw/6061
tallization is accelerated, and hence SiCw/Al has a lower
starting recrystallization temperature compared with the
pure Al (Figure 2). In the case of aged SiCw/6061Al,
however, in addition to SiCw there are precipitates. The
two types of particles (whiskers and precipitates) will
have an opposing effect on the recrystallization process,
and hence resulting in the isochronal annealing curve
without a definable recrystallization step.
4. Conclusions
1) The existence of SiCw can stimulate recrystallization
nucleation of SiCw/Al composite;
2) Solution atoms have no significant influence on the
isochronal annealing behavior of SiCw/6061Al compo-
Precipitates retard recrystallization of SiCw/6061Al
composite so strongly that the isochronal annealing curve
of aged SiCw/6061 composite loses the definable recrys-
tallization step.
Effects of Solution Atoms and Precipitates on Isochronal Annealing Behavior of Cold-Rolled
Squeeze-Cast SiCw/Al Composites
Copyright © 2011 SciRes. MSA
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