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

Section 4
4.2. The effect of the heat treatment on the mechanism of void initiation
The fracture surfaces of the MMCs are studied in detail in the SEM LEO 440. The regions
near the crack front in the mid-section of the specimens, where plane-strain condition
prevails, are investigated. Ductile fracture by micro-void coalescence is observed in all
materials.
The images of the analysed regions for all investigated materials are given in Appendix 9
(Figs. 9.1-9.25). For each investigated material, from 10 to 20 particles located closely in
front of the crack tip are investigated. For each particle, the mechanism of void initiation
either particle fracture or particle/matrix decohesion is determined from stereo image pairs.
For instance, when the particle is observed only on a single half of the broken specimen, it is
probable that the particle/matrix decohesion has occurred. On the contrary, if broken parts of
a particle are found on both specimen halves, particle fracture can be assumed.
The study of the fracture surfaces of the casdt MMCs reveals a change of the mechanism of
void nucleation with increasing aging condition: a clear tendency to increasing probability of
particle/matrix decohesion mechanism can be noted (see Table 4.8 on p. 65). For instance, in
Specimen Al2O3-10%-200h, 12 of 15 considered particles formed dimple by particle/matrix
decohesion, whereas in Specimen Al2O3-10%-RT, 13 of 16 considered particles are fractured.
Although only particles near the crack front are considered in this statistic, these data reflect
the situation on whole fracture surface, as well.
In Fig. 4.10a, a typical broken alumina particle is shown (Particle 4 of Specimen Al2O3-10-RT
from Fig. 9.1 taken at higher magnification). It is seen that the particle fracture surface has a
smooth view. Fig. 4.10b shows a debonded particle 13 in Specimen Al2O3-10-200h. The
particle/matrix interface is rough and bumpy: very small dimples having a size less than 1 µm
are observed at the debonded interface. These small dimples are initiated by the MgAl2O4
spinel phases formed at the alumina particle/matrix interface [21, 71-72]. It has been
suggested that the spinel phase can be formed at the interface both during the fabrication of
MMCs and during the following heat treatment. The following possible reactions with large
enough thermodynamic driving forces to form the MgAl2O4 phase have been proposed in
[71]:
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