Composite Materials Research Progress

Recent Advances in Discontinuously Reinforced Aluminum… 283
Table 1. Tensile properties of Al-based micro- and nanocomposites [41].
Specimen
Tensile Strength,
MPa
Yield Strength,
MPa
Elongation at
Break, %
Pure Al 70 30 ---
Al/15 vol.% SiC (3.5 μm) 176 94 14.5
Al/1 vol.% Si3N4 (15 nm) 180 144 17.4
Al/1 vol.% Si-N-C (25 nm) 178 134 19.7
Al/5 vol.% Si-N-C (25 nm) 153 114 6.2
It is widely known that DRA microcomposites exhibit higher creep resistance than their
unreinforced matrix materials because the particulates acting as barriers to dislocation
movement. There is no plastic flow occurs within ceramic reinforcing particles. Accordingly,
plastic deformation of DRA composites is controlled exclusively by flow in the metallic
matrices. The high temperature creep behavior of coarse-grained Al and its alloys reinforced
with microparticles is characterized by high values of n and Q. The creep activation energy of
microcomposites is often much larger than that for aluminum lattice self-diffusion (142
kJ/mol) [42-45]. Such anomalous behavior can be rationalized by introducing a threshold
stress (σo) opposing creep flow. In this respect, the observed creep deformation is not driven
by the applied stress σ but rather by an effective stress σc (σc = σ -σo). The threshold stress
may originate from several sources such as Orowan bowing between particles, attractive
attraction between dislocations and particles as well as back-stress associated with local
dislocation climb [5, 42]. The rate controlling equation can be written as follows:
σ −σ
o n Q
ε& = A(
) exp( − )
[1]
G RT
where ε& is the creep rate, A is a constant, G is shear modulus, R is Universal gas constant
and T is absolute temperature. The creep behavior of Al-based microcomposites is related to
modified creep behavior of aluminum solid solution alloys, and the equations developed for
solid solution alloys can be used to described the creep behavior of composites provided that
the applied stress is replaced by an effective stress. Thus, the threshold stress for creep in Albased
microcomposites is associated with interactions between dislocations and fine
dispersion of particles. These particles may be fine oxides in PM MMCs or precipitates in the
matrix alloy of cast composites [42-44]. Introducing a threshold stress and its temperature
dependence into the creep rate analysis yields a true stress exponent, n, of 3, 5 or 8, and true
creep activation energy. For composites with a true exponent close to 3, dislocation viscous
glide is rate controlling with an activation energy for creep is associated with interdiffusion of
the solute atoms. On the other hand, dislocation climb process predominates for n = 5 in
which the activation energy for creep is associated with aluminum lattice self-diffusion. For
the composites with a true exponent close to 8, creep deformation is controlled by the lattice
diffusion and its rate is proportional to the third power of substructure grain size λ [45, 46].
Mathematically, the phenomenological creep rate equation for n = 8 can be written as:
ε& = S (DL/b 2 ) (λ/b) 3 [(σ- σo)/E] 8 [2]