Composite Materials Research Progress

294
S.C. Tjong
Apart from forming ultrafine grains in composites, ECAP treatment can also consolidate
ultrafine raw powders to produce fully dense (> 98%) bulk composite materials. Alexandrov
et al. used the HPT technique to consolidate the Al powder (50 μm) and Al2O3 nanoparticle
(50 nm) to form the Al/5 vol.% Al2O3 nanocomposite under a pressure of 1.5 GPa at room
temperature. The powder mixture of nanocomposite was ball-milled for 30 min to ensure a
uniform distribution of ceramic particles [21]. Fig. 20 shows the TEM micrograph of the HPT
consolidated Al/5 vol.% Al2O3 nanocomposite. The nanocomposite exhibits an UFG structure
having an average gain size of 120 nm. Room temperature tensile tests showed that the Al/5
vol.% Al2O3 nanocomposite have limited ductility of 1 to 2%. At 300 ºC, the nanocomposite
tested at a strain rate of 10 -3 s -1 had a plastic flow stress of ~ 66 MPa and a tensile ductility of
~ 20 % (Fig. 21). In contrast, pure Al had a flow stress of ~ 60 MPa and a tensile ductility of
~ 40 % tested at the same strain rate. However, the Al/5 vol.% Al2O3 nanocomposite showed
a high strain-rate sensitivity of flow stress at 400 K; the strain-rate sensitivity (m) was 0.35.
Strain rate sensitivity defined as the slope of logarithmic plot of the flow stress vs. strain rate.
It is an inverse of stress exponent (n) and an important parameter in superplasticity. The Al/5
vol.% Al2O3 nanocomposite exhibited a low flow stress of 20 MPa but a high tensile ductility
of ~ 200 %. The enhanced tensile ductility observed in the HPT consolidated nanocomposite
with a total elongation of ~ 200 % indicating the occurrence of superplastic-like flow
behavior. According to the literature, high strain rate super-plasticity can be achieved in
ECAP processed aluminum alloys with UFG structures [59]. High strain rate superplasticity
in the sub-micron metals is often characterized by very high flow stresses or pronounced
strengthening. Grain boundary sliding is considered to be the dominant deformation mode for
superplasticity in the sub-micron and nanocrystalline metals [60]. Future challenges for
materials scientists are to elucidate the underlying creep and superplastic deformation
mechanisms of aluminum based nanocomposites having UFG and nanocrystalline matrices.
Conclusions
The development of aluminum nanocomposites is still in embryonic stage and there are many
challenges in this field in the years ahead. Considerable progress has been made in the
fabrication, microstructural and mechanical characterization of novel aluminum-based metal
matrix nanocomposites in recent years. The nanocomposites can be simply prepared by
incorporating very low volume contents of ceramic nanoparticles into aluminum matrix via
PM or ingot casting. The nanocomposites thus prepared exhibit excellent mechanical
properties including high yield strength and superior creep resistance. However,
agglomeration of nanoparticles occurs readily during the composite fabrication, leading to
poorer mechanical performance of composites with higher filler content. This problem can be
eliminated in cast nanocomposites by using high-intensity ultrasonic waves to disperse the
nanoparticles in molten aluminum. In the case of PM nanocomposites, cryomilling and severe
plastic deformation processes have emerged as the two major processes to produce aluminum
based composites having ultrafine grained matrix structures and homogeneous dispersion of
reinforcing particles within the matrices.