Composite Materials Research Progress

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S.C. Tjong
threshold stress resulting from the interaction between moving dislocations and alumina oxide
nanoparticles (Fig. 13). Such dislocation-particle interaction would impede lattice dislocation
movement, thereby reducing creep rate of the composite. By incorporating the threshold
stress into analysis, plots of creep rate versus effective stress yield straight lines with different
slopes, i.e. n = 3 for low stress region and 5 for high stress regime (Fig. 14). Hence, the creep
behavior of nanocomposite is consistent with the behavior of Al-Cu solid solution alloy (2014
Al) that exhibits a transition from viscous glide (n = 3) to the high-stress region (n = 5) where
dislocations break away from the solute atom atmosphere. They indicated that the true creep
characteristics of PM 2024 Al/Al2O3 nanocomposite are consistent with those reported for
aluminum solid-solution alloys [43]. Therefore, the creep deformation of nanocomposite with
a matrix containing solutes is controlled by a viscous glide slip mechanism.
Ultrafine Grained Matrices
As mentioned above, conventional PM blending method yields Al nanocomposites with
inhomogeneous distribution of reinforcing particles within the metal matrix. Cryomilling can
provide a homogeneous dispersion of reinforcing particles in submicrometer or
nanocrystalline matrix. The subsequent hot consolidation of cryomilled nanopowders into
final bulk products causes the composites to have an UFG structure as a result of grain growth
of the matrix. Schoenung and coworkers investigated the microstructure and tensile behavior
of bulk nanostructures 5083 Al/5 vol.% SiC (25 nm) composite prepared by cryomilling
followed by hot isostatic pressing and hot rolling [48]. They reported that the hot rolled
composite consists of regions dispersed with SiC nanoparticles (100 -200 nm) and regions
free of SiC nanoparticles (~ 700 nm). Fig. 15 shows the TEM micrograph of the SiCdispersed
region in which SiC nanoparticles are distributed homogeneously within the
ultrafine grains of the 5053 al matrix. The tensile properties of such composite from room
temperature to 573 K are shown in Fig. 16. The composite exhibits very high tensile strength
at room temperature but extremely low ductility. The strength decreases but the ductility
increases with increasing test temperatures. In a nanocomposite with an UFG matrix, the
dislocation movement in the matrix is restricted by the high density of grain boundaries.
Consequently, the composite exhibits high tensile strength but very low tensile ductility.
It is well known that nanocrystalline (NC) materials exhibit very low tensile ductility and
toughness due to the lack of strain hardening [9]. The presence of coarser grains within the
nanocrystalline matrix can enhance the ductility of nanostructured materials at the expense of
mechanical strength [49-51]. Different toughening approaches have been proposed to enhance
the ductility of NC materials either via thermomechanical treatment or cryomilling. Recently,
Lavernia and coworkers reported that the UFG Al-Mg alloys with a bimodal microstructure
exhibit a combination of high strength and good ductility [52- 55]. Such alloys were
synthesized by consolidation of a mixture of cryomilled Al-Mg and unmilled powders.
Consequently, strain hardening is regained in CG regions while maintaining high strength in
NC regions. The CG grains can provide more dislocation activity than the NC grains. Ductilephase
toughening in bi-modal structured Al-Mg alloys is attributed to the occurrence of crack
bridging as well as delamination between UFG and CG regions during plastic deformation
[51].