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Fig. 2: SEM micrograph of pressurelessly
sintered composite simple with the inital
composition 90 vol. % Mg 2 Si and 10 vol.
% TiC
Fig. 2a: XRD of the sample shown in the
Fig. 2.
900°C, the infiltration was complete
within 1h, resulting in composite samples
with less than 5 vol.% of retained
porosity.
It is also important to note that
at the same time under the applied
experimental conditions, the pressureless
infiltration of Mg 2 Si-TiB 2
preforms was unsuccessful.
The microstructure of the composite
samples obtained is presented in
Fig. 1a, 1b. The Mg-Mg 2 Si-TiC composites
consisted of a Mg 2 Si-Mg matrix
with isolated, large block-shaped
Mg 2 Si grains interpenetrated with a
continuous Mg phase, inside which
TiC particulate reinforcement with
an average particle size of about 1 µm
was homogeneously dispersed.
Fig. 1c shows X-ray diffraction patterns
of the Mg-Mg 2 Si-TiC composite
samples. It can be seen that besides
Mg 2 Si, TiC and Mg no secondary
phases were detected, which indicates
that pressureless infiltration of
Mg 2 SiTiC preforms with molten magnesium
was not chemically assisted.
The interfaces observed in the system
were: Mg 2 Si-Mg, Mg 2 Si-TiC and Mg-
TiC. A detailed SEM examination of
interface regions (Fig.1b) confirmed
in all cases the absence of chemical
ALUMINIUM · 12/2009
reactions between the composite constituent
listed above.
Regarding the mechanical properties
of Mg-Mg 2 Si-TiC composites,
which are summarized in Table 2, an
increase in TiC reinforcement content
was observed to have a detrimental
influence on the tensile properties
and Vickers hardness and, on the
other hand, only a marginal influence
on fracture toughness.
The observed improvement of
tensile properties is most probably
associated with an increased amount
of TiC reinforcement well bonded to
the composite matrix. Thus, the stress
transferred from the composite matrix
to the reinforcing phase is higher
as the volume fraction increases due
to a local increase in interfacial area.
In addition, the enhanced hardness
of the composite material can be
ascribed to the influence of the presence
of hard, brittle and essentially
elastically deforming reinforcing inclusions
(TiC) in the soft, ductile and
predominantly plastically deforming
magnesium matrix.
The fracture toughness of fabricated
Mg-Mg 2 Si-TiC composites was
found to be slightly better than in nonreinforced
magnesium and commercial
Mg alloys, but lower than most
aluminium and titanium alloys. With
doubling of the TiC reinforcement
content from 7 to 14 vol.%, the fracture
toughness of the composite samples
(Table 2) remained practically the
same. In other words, considering that
fracture toughness is a quantitative
way of expressing a material’s resistance
to brittle fracture when a crack is
present, one can conclude that based
on the experimental findings the resistance
to brittle fracture in composites
with a nominal composition of 30
vol.% Mg-63 vol.% Mg 2 Si-7 vol.% TiC
Composite initial
composition
(vol. %)
Unalloyed
magnesium
63%Mg2Si+ 30%Mg+7%TiC
56%Mg2Si+ 30%Mg+14%TiC
Retained
porosity
(%)
Density
(g/cm 3 )
research
and 30 vol.% Mg-56 vol.% Mg2Si-14 vol.% TiC was practically the same. In
explaining the results obtained, one
should note that, from the point of
view of indentation toughness analysis,
both examined samples had the
same amount (30 vol.%) of continuous
Mg phase. Thus, magnesium in
Mg-Mg2Si-TiC composites acted as a
continuous ductile matrix reinforced
in both samples with the same total
amount (70 vol.%) of Mg2Si and TiC
particles.
The possible mechanism of fracture
toughness improvement in Mg-
Mg2Si-TiC composites compared to
non-reinforced magnesium and commercial
magnesium alloys is crack
bridging due to the TiC and Mg2Si particles.
Composites made by pressureless
sintering: Pressureless sintering at
1020 °C for 1h of Mg2Si-TiC and Mg-
2Si-TiB2 samples made from mixtures
A, B, C and D (Table 1) resulted in
dense intermetallic matrix composites
discontinuously reinforced with
TiC and TiB2 particles, with a retained
porosity of less than 3 vol.%.
The microstructure of the sintered
composite samples is presented in
Figs. 2, 3 and 4. In contrast to pressureless
infiltration, during which
the preform skeleton (Mg2Si-TiC) and infiltrant (molten magnesium)
remained chemically inert, resulting
in a relatively simple microstructure
of the composite samples, pressureless
sintering, performed at a higher
temperature than infiltration, proceeded
simultaneously with one or
more chemical reactions between the
Mg2Si matrix and TiC or TiB2 particulate
reinforcements. Thus, the microstructure
of these samples is much
more complex and heterogeneous,
with several secondary phases ➝
E
(GPa)
Tensile
strength
(MPa)
Vickers
Hardness
(GPa)
K IC
(MPa m 1/2 )
1.74±0.1 40±4 90±9 0.5±0.05 /
3.6±0.4 2.03±0.1 88±9 186±19 4.9±0.5 6.5±0.7
4.7±0.5 2.32±0.1 97±10 197±20 5.1±0.5 6.4±0.6
Table 2: Average room temperature tensile properties, Vickers hardness and fracture
toughness from submicron derived indentation cracks of pressurelessly infiltrated composite
samples
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