Composite Materials Research Progress

Recent Advances in Discontinuously Reinforced Aluminum… 277
applications. Nanoparticles can be synthesized by several processes such as gas phase
condensation, laser ablation, aerosol route, mechanochemical processing is well established
[5, 7-9]. They reveal unique physical and mechanical properties that are different from those
of bulk solids and microparticles. Due to their high specific surface area, nanoparticles exhibit
a high reactivity and strong tendency towards agglomeration. It is necessary to disperse exsitu
nanoparticles more uniformly in aluminum matrix in order to obtain desired mechanical
properties. In the case of liquid metallurgy processing, high-intensity ultrasonic waves can be
employed to disperse the SiC nanoparticles more uniformly in molten aluminum alloy [10,
11]. In powder metallurgy route, mechanical alloying, particularly cryomilling has been used
to refine and disperse the ceramic phase in the Al matrix [12 -16]. In most cases, ceramic
particles with original sizes of several micrometers can be reduced to nanometer level after
cryomilling [17].
Recently, there has been a growing interest in the application of severe plastic
deformation (SPD) such as high pressure torsion (HPT) and equal channel angular pressing
(ECAP) for producing materials with ultrafine grain structure in submicrometer levels [18 -
29]. ECAP is more attractive for industrial applications because it can be employed to
produce large fully-dense samples or products. It consists of pressing the sample through a
die into an L-shaped channel without changing its cross-section. The sample deforms by
simple shear, thereby inducing a high density of dislocations that are subsequently arranged to
the meta-stable sub-grains of high-angle boundaries. By repeating the pressing process, the
strain is accumulated during each increment cycle. The ultra-fine grained composites
processed by ECAP exhibit high yield strength and good ductility [27].
Agglomeration of Particles
Generally, ceramic particles of micrometer sizes are prone to cluster during the composite
fabrication. Particle clustering is more prevalent in cast than in PM microcomposites [30, 31].
This leads to the mechanical properties of microcomposites are far below the theoretical
values. For the PM microcomposites, the particle size ratio of the matrix and reinforcement is
the main factor controlling the degree of microstructural homogeneity [32-35]. Furthermore,
secondary processing technique such as ECAP and HPT are reported to be very effective to
improve the dispersion of reinforcing ceramic particles in the PM DRA composites [20, 24,
27]. Figs. 2(a) -2(c) show the effect of ECAP extrusion cycles on the particle distribution in
PM 6061 Al/20% Al2O3 composite. The composite in the as-fabricated condition shows
extensive particle clustering as expected. The clusters are aligned along the extrusion
direction (Fig. 2(a)). These clusters begin to dissolve and disperse into individual particles
after four ECAP passes at 370 ºC. The particle distribution appears homogeneous after
pressing for seven passes. In addition to declustering, ECAP treatment also yields grain
refinement of the aluminum alloy matrix.
It is well recognized that nanoparticles tend to agglomerate into large clusters during
composite processing even under low loading levels of reinforcement. In this respect,
appropriate processing procedures are needed to improve the dispersion of nanoparticles in
aluminum matrix. Recently, Yang et al. used high-intensity ultrasonic waves to assist the
dispersion of SiC nanoparticles (average size ≤ 30 nm) in molten aluminum alloy A356 [10,
11]. Fig. 3 shows a typical experimental setup for the ultrasonic assisted melting. The