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

Section 4
Table 4.1. Chemical composition of the Al-6061 alloy
Si Fe Cu Mn Mg Zn Cr Ti
0.4÷0.8 0.7 0.15÷0.4 0.15 0.8÷1.2 0.25 0.04÷0.35 0.15
chemical composition of the matrix is given in Table 4.1. The mean alumina particle size is
about 10 µm. Metallographic sectioning shows that the particles are distributed quite
homogeneously in all composites, but a few particle clusters are observed, as well (Fig. 4.1).
The particles have a shape of spheroids. The MMCs were supplied in the shape of bars with a
section of 40x12.5 mm by AMAG (Austria).
The materials were annealed at 560°C for 30 minutes, quenched in water, and kept at room
temperature for 1 week [69]. To study the effect of the matrix properties on the composite
behavior, the materials were subjected to different heat treatments:
(1.) aging at room temperature;
(2.) aging at 160°C for 8h;
(3.) aging at 160°C for 24h;
(4.) aging at 160°C for 200h.
In the following, the investigated specimens are referred to by their volume percentage and
the heat treatment, e.g., Specimen Al2O3-10-RT or Specimen Al2O3-15-8h. The term
“Increasing aging condition” will be used when specimens with the conditions RT, 160°C/8h,
160°C/24h, 160°C/200h are compared.
4.1.1.2. Tensile tests
To determine the global material parameters, conventional tensile mechanical tests are
performed. The tensile specimens have a cylindrical shape with a diameter of 3 mm and a
gage length of 15 mm (Fig. 4.2). The tensile tests are conducted on a mechanical testing
machine “ZWICK” at a loading rate of 5.6·10 -4 s -1 . The materials are assumed to follow a
standard power-law work hardening behavior (Eq. 3.3). The strain hardening coefficient, N,
and the coefficient, α, are determined from the log(ε/ε0) vs. log(σ/σ0) curves. The fit was
taken so that it covers the better part of the stress-strain behavior (Fig. 4.3a). The results of the
tensile tests are collected in Table 4.2. The mechanical properties of the matrix material in
each considered aging condition are given in Table 3.3, as well.
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