Composite Materials Research Progress

Recent Advances in Discontinuously Reinforced Aluminum… 281
Figure 6. Microstructure of Al 5000/20.6vol.% AlN nanocomposite prepared by cryomilling and hot
pressing [13].
Structure-Property Relationship
Aluminum-based nanocomposites can be classified into two categories according to the size
dimensions of reinforcing particle and aluminum matrix employed, i.e. micrograined matrix
composites reinforced with nanoparticles and UFG matrix composites reinforced with
submicron- or nanoparticles. In the former case, ceramic nanoparticles are introduced directly
into aluminum matrix having grain sizes in micrometer level via PM or ingot casting. The
latter relates the use of cryomilling to refine the reinforcing particles and aluminum matrix
down to submicrometer of nanoscale regime. Alternatively, the matrix grains of the
composites can also be refined to submicrometer level using the SPD process.
It is well recognized that the deformation behavior of nanocrystalline metals is quite
different from their micro-grained counterparts. According to the Hall-Petch relation, a
substantial increase in yield strength can be achieved by reducing the grain size of metals
to the submicrometer or nanometer regime. Nanocrystalline metals generally have very low
tensile ductility, and exhibit creep and superplasticity at lower temperatures compared to their
micro-grained counterparts [9]. This is attributed to large volume (more than 50%) of atoms
are located at the grain boundaries or interfacial boundaries of nanometals. Consequently,
grain boundary activity is a dominant factor for controlling the mechanical properties. It is of
practical interest to understand the effect of particle additions on the mechanical properties of
aluminum and its alloys having submicrometer or nanometer grain sizes.
Micro-grained Matrices
Tjong et al. investigated the microstructure and mechanical properties of pure aluminum
reinforced with low loading levels of Si3N4 (15 nm) or Si-N-C (25 nm) nanoparticles. Such