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

Section 2
2.2. The influence of the global microstructure and the matrix condition on the global
mechanical properties in metal matrix composites
The influence of the global microstructural parameters such as the particle volume fraction,
the average particle size, etc., on the global mechanical properties has been already
investigated in detail. The experimental results can be roughly generalized as follows [7]:
With increasing particle volume fraction, the yield strength and the ultimate tensile strength
increase, the ductility and the fracture toughness decrease, e.g., [8]. For a constant particle
volume fraction, the tensile strength tends to increase with decreasing particle size, e.g.,
[9,10]. No such simple pictures have been found for the more complex properties fracture
toughness and fatigue resistance; ambiguous results have been reported here [7, 11-12].
Stronger matrix alloys produce stronger composites, e. g. [13,14]; the increase in strength due
to the reinforcement decreases with increasing matrix strength; in the case of very high
strength alloys, reinforcements may even lead to a reduction in strength [15]. Therefore, one
of most important factors affecting the mechanical properties of MMCs is the heat treatment.
In [16], the effect of the heat treatment on mechanical properties of the PM-MMC Al7093-
15%SiC was investigated. An increase of the yield strength with increasing aging time up to
the peak aged condition was found. A further increase of aging time led to a slight decrease of
the yield strength. The dependence of the fracture strain and the strain hardening coefficient
on the aging time demonstrated an opposite character. A good correlation between the
fracture strain and the strain hardening coefficient was found. It was shown that the fracture
toughness has an inverse dependence on strength and correlates well to the strain hardening
coefficient. Fracture surface inspection revealed the dominance of the particle fracture for the
solution treated, under-aged, peak-aged, and slightly over-aged conditions of the matrix
whereas, in the highly overaged condition, (near-)interface debonding failure dominated.
In [17], a model was proposed to predict the fracture toughness in this material. This model is
based on a prediction proposed by Hahn and Rosenfield [18],
8
1
2
1 ⎡
⎤ 1
3 −
⎢ ⎛π
⎞
6
= 2σ
E⎜
⎟ d⎥
f . (2.1)
⎢ ⎝ 6 ⎠ ⎥
⎣
⎦
K IC
y
In Eq. 2.1 σy is the yield strength of material, E the Young modulus, d the particle size, and f
the particle volume fraction. It was shown in [17] that Eq. 2.1 significantly overestimates the