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Fig. 4: Microstructure of pressurelessly
sintered composite sample with the inital
composition 90 vol. % Mg 2 Si and 10 vol.
% TiB 2
Fig. 4a: XRD of the sample shown in the
Fig. 4
the primary (TiC and TiB 2 ) or secondary
in situ formed (TiSi 2 , Ti 3 SiC 2
and SiC) particulates additionally increased
the brittleness of the composite
species. Furthermore, the fracture
toughness of liquid sintered Mg 2 Si-
TiB 2 samples was slightly higher than
in reactively sintered Mg 2 Si-TiC, most
probably due to the presence of continuous
Mg 0.15 Si 0.85 bonding phase.
However, comparison of the fracture
toughness of sintered and infiltrated
samples, Table 2 and 3, revealed that
toughness was significantly lower in
sintered than in infiltrated counterparts,
Table 2.
A potential mechanism for the observed
behaviour is an increase of the
intrinsic toughness of Mg 2 Si-Mg-TiC
samples mostly caused by the presence
of the continuous, ductile Mg
phase. In that case, toughness was
enhanced by increasing the micro-
ALUMINIUM · 12/2009
structural resistance and changing
the nature of the matrix (from brittle
to ductile) to suppress damage in the
form of microcracking or microvoid
formation ahead of the crack tip. In
addition, TiC particles also toughened
Mg 2 Si-Mg-TiC samples to some
degree by crack bridging. This extrinsic
toughening acts primarily behind
the crack tip to effectively reduce the
crack-driving force actually experienced
at the crack tip.
Analogous toughening in sintered
samples (Mg 2 Si-TiC and Mg 2 Si-TiB 2 )
was considerably less developed, in
part because such composites are designed
with rather strong reinforcement-Mg
2 Si interfaces – and thus do
not develop crack bridging to any significant
degree.
conclusions
1. Depending on the selected fabrication
technique of (i) pressureless infiltration
of porous Mg2Si-TiC and Mg-
2Si-TiB2 preforms with molten magnesium,
or (ii) pressureless sintering
of green Mg2Si-TiC and Mg2Si-TiB2 compacts, Mg2Si-based composites
of different nature, microstructure
and combination of properties were
successfully synthesized. In addition,
the effect of different microstructures
developed in Mg2Si-based composites
on improvement of their mechanical
properties (strength, hardness and
toughness) was investigated.
2. The pressureless infiltration
of porous Mg2Si-TiC preforms with
molten magnesium resulted in dense
(≥ 95% T.D.) Mg-Mg2Si-TiC metal
matrix composites with a metallic matrix
discontinuously reinforced with
Mg2Si and TiC particulates.
3. Under the same experimental
conditions, the pressureless infiltration
of porous Mg2Si-TiB2 preforms
with molten magnesium was found to
be unsuccessful.
research
4. The pressureless sintering of green
Mg2Si-TiC and Mg2Si-TiB2 compacts
lead to the formation of almost fully
dense (≥ 97% T.D.) intermetallic matrix
composites with a Mg2Si matrix
discontinuously reinforced with primarily
introduced particles and different
secondarily formed bonding
phases.
5. In the case of Mg2Si-TiC samples,
the formation of the secondary
bonding phase was caused by chemical
reactions between Mg2Si and TiC.
Furthermore, the phase composition
of the bonding phase obtained was
found to be dependent on the initial
volume fraction of TiC reinforcement.
In samples with a lower TiC concentration
(≤ 10 vol.%), the secondarily
formed bonding phase consisted of
TiSi2 and Si, while in samples with a
higher TiC concentration (≥ 10 vol.%),
the main constituents were TiSi2 and
Ti3SiC2 and SiC.
6. The densification of Mg2Si-TiB2 proceeded via liquid sintering. During
the early stage of densification, Mg2Si particles melted incongruently, forming
a peritectic liquid phase which
then wetted and well impregnated the
individual Mg2Si and TiB2 particles in
porous samples. On cooling, the liquid
phase crystallized in the form of
a continuous bonding phase with an
average composition of Mg0.15Si0.85 ,
causing the complete densification of
Mg2Si-TiB2 species.
7. The Mg-Mg2Si-TiC composite
samples obtained by pressureless
reactive infiltration of molten magnesium
into a porous preform of Mg2Si with TiC ceramic reinforcement were
tailored to consist of a continuous
magnesium matrix discontinuously
reinforced with Mg2Si and TiC. Such
a design was selected in order to improve
the fracture toughness of the
composite, thus creating an ultra-light
structural material with excellent tensile
properties (Young’s modulus ➝
Initial composition (vol. %) Retained porosity (%) Density (g/cm3 ) E (GPa) Tensile strength (MPa) Vickers Hardness (GPa) KIC (MPa m1/2 )
Mg2Si+10%TiC 1.8±0.2 2.25±0.1 132±13 487±49 9.1±1 1.9±0.2
Mg2Si+20%TiC 2.2±0.2 2.53±0.1 141±14 532±53 9.9±1 1.7±0.2
Mg2Si+30%TiC 2.4±0.2 2.78±0.1 149±14 564±53 10.7±1 1.5±0.2
Mg2Si+40%TiC 3.0±0.2 3.03±0.1 157±14 596±53 11.4±1 1.4±0.1
Mg2Si+10%TiB2 2.3±0.2 2.22±0.1 134±13 477±48 9.6±1 2.3±0.2
Mg2Si+20%TiB2 2.9±0.3 2.43±0.2 146±15 528±53 10.3±1 1.8±0.2
Table 3: Average room temperature tensile properties and Vickers hardness of pressureless sintered composite samples
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