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

Section 7
particle cluster area area without particle clusters
CODi = 0.2...0.5µm
Fig. 7.9. Schematic view of the fracture surface of the the PM-MMC Al6061-20%Al2O3 in as-
fabricated condition.
local CODi-values is found. In front of the particle cluster, they have near-zero values
0.2…0.5 µm. In Figure 7.10, such a profile is presented. In front of the region with
homogeneous particle distribution on the fracture surface, the CODi-values lie in the range
2…3 µm (Fig. 8.11). On the contrary, in the specimen from the MMC after 7 ECAP passes,
no significant scatter of the CODi-values is observed: the local CODi-values measured at six
different positions vary in the range of 2…3 µm (Table 7.3).
An improvement of the global fracture toughness and the total crack growth resistance of the
MMC with increasing homogeneity of the particle distribution might be explained by results
of estimation of the local fracture properties. Such a difference in local fracture properties at
regions with different local particle distribution can be caused by a different amount of energy
required for crack propagation in this region. It is clear that at particle clusters, where no
matrix between the particles is observed (Fig. 7.7b), no energy is needed for the void
initiation and void growth: the local values of the crack growth resistance are extremely low,
since the local values of the void initiation fracture toughness is almost equal zero. At the
same time, in regions with homogeneous particle distribution, some amount of energy is
required for the void initiation and following void growth that results in relative high values
of the local fracture toughness.
CODi = 2...3 µm
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fracture surface
crack tip
pre-fatigued
region