dissertation global and local fracture properties of metal matrix ...

Section 2
high strength, fine reinforcements and a relatively large particle volume fraction are preferred
[2]. However, it is difficult to take advantage of both of these requirements because they are
prone to cause an inhomogeneous particle distribution [23].
A number of studies have shown that during the deformation of particulate reinforced MMCs
there is a progressive development of damage within the material. It has been observed that
the damage tends to originate preferentially in particle clustered regions [24, 25]. Thus, one of
the most important microstructural features of the MMCs is the spatial distribution of the
particles. In [26, 27], an Eshelby based approach was used to create a model of a clustered
composite. A composite with non-uniform particle distribution was considered as a two phase
material comprising regions of higher than average local volume fraction as stiffer regions
within a more compliant dilutely reinforced matrix. This analysis suggests somewhat
surprisingly, that a composite containing clustered particles should have a higher flow stress.
It was also found that larger stresses may be expected in both the matrix and particles in
clustered regions compared with a composite containing a homogeneous particle distribution.
On the other hand, it is suggested in [28] that reinforcement clustering tends to reduce the
composite flow stress, relative to an equi-spaced array. Zhiru et al. [29] have proposed that
the matrix triaxial stresses are generally higher in clustered regions of four or more particles,
thus shielding the center of the cluster from plastic flow due to the higher local levels of
constraint imposed on the matrix. Plastic flow is, thus, only expected in the center of the
clusters once the more dilutely reinforced regions of the matrix have become sufficiently
work hardened. In [30], a decrease of experimentally determined global fracture toughness
with increasing degree of clustering was observed.
2.4. The process of void initiation in materials with particles
As well known, micro-ductile fracture consists of three stages: void nucleation, void growth,
and void coalescence [31]. In ductile materials reinforced with hard second phase particles,
void nucleation occurs either by particle/matrix decohesion or the particle fracture
mechanism. Void nucleation can determine the ductility, since the subsequent stages of void
growth and coalescence can be extremely rapid (especially in the case of large ceramic
particles) once the void initiation has occurred. Therefore, the full understanding of void
initiation process is very important to predict the fracture properties of MMCs.
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