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Mg 2 Si-TiC and Mg 2 Si-TiB 2 composites
as a promising class of Mg-based
materials, with particular emphasis
on their possible applications in
lightweight structural parts fabricated
by cost-effective procedures such as
pressureless infiltration or pressureless
sintering.
experimental
In the first set of experiments, Mg 2 Si-
Mg-TiB 2 and Mg 2 Si-Mg-TiC composite
samples were fabricated by
pressureless infiltration of porous
preforms with molten magnesium. As
sources of molten magnesium (infiltrant)
Mg plates machined from an
ingot of unalloyed magnesium (ASTM
B92/B92M 9980; supplier: Dead Sea
Magnesium, Israel) were applied.
Preforms were isostatically pressed
from various mixtures of commercial
Mg 2 Si (99.5%, 30µm) and TiC
(99.5%, 30µm) or TiB 2 (99.5%, 30µm)
powders, as listed in Table 1. Samples
were cylinders 30 mm high and
20 mm in diameter. Infiltration was
performed in a vacuum furnace in an
argon atmosphere at temperatures of
700, 800 and 900°C for 1 h.
In the second set of experiments,
composite samples were prepared
by pressureless sintering of isostatically
pressed tablets made from the
same Mg 2 Si-TiC and Mg 2 Si-TiB 2 mixtures
listed in Table 1. Sintering was
performed at 1020°C, for 0.5-1 h in a
protective argon atmosphere.
The as-synthesized composite
samples were cut, machined and polished
in accordance with standard
procedures.
Microstructural characterization
of fabricated composites was performed
by scanning electron micro-
scopy (SEM), while X-ray diffraction
(XRD) measurements were applied
to the samples to identify the phases
present and their crystal structure.
Quantitative determination of the
volume percentage of Mg 2 Si, secondary
phases and ceramic particles in
the matrix, as well as the retained
porosity, was performed by analyzing
the optical and scanning electron micrographs
of as-polished composite
bars using the point counting method
and image analysis and processing
software.
The Archimedes’ principle method
was used to measure the density
of samples utilizing a precision microbalance.
The initial density of the green
compacts (preforms and tablets) was
calculated from the mass and geometry
of the samples.
Tensile tests were conducted on
cylindrical tension-test specimens
3.5 mm in diameter and 16 mm gauge
length using an automated servo-hydraulic
tensile testing machine with a
crosshead speed of 0.254mm/60 s.
Vickers hardness (HV) measurements
were performed at room temperature
on polished composite samples
and calculated as an average of
6 indentations. These measurements
were made with a conventional Vickers
tester (load: 9.8-24.5 N; residence
time: 15 s).
Due to their small dimensions and
high brittleness, the fracture toughness
of the specimens obtained was
determined by applying the indentation
method [19]. K IC of the composite
samples was determined from submicron
derived indentation cracks and
calculated according to the following
equations proposed by Niihara et al.
[10]:
Fig. 1a, 1b: SEM micrograph of a pressurelessly infiltrated preform with the initial composition
of the preform skeleton of 70 vol. % Mg 2 Si-20 vol. % TiC and an initial porosity of
30±5 vol. %. The phases detected are Mg, Mg 2 Si and TiC.
((K ICΦ)/(Ha) 1/2 ) (H/(EΦ)) 2/5 =
0.035(L/a) -(1/2) (1.25 ≤ c/a ≤ 2.5) (1)
((K ICΦ)/(Ha) 1/2 ) (H/(EΦ)) 2/5 =
0.129(c/a) -(3/2) (c/a ≥ 2.5) (2)
where H is the Vickers hardness, a
the length of the half diagonal of the
indent, c the length of the half indentation
crack, L = c-a, E is the Young’s
modulus, and Φ is a constant with a
magnitude of about 3.
results and discussion
In general, two different classes of
composites with a Mg 2 Si-based matrix
discontinuously reinforced with
ceramic particulates were engineered
in this work to provide a combined
improvement in strength, hardness
and fracture toughness: (1) Mg-Mg 2 Sibased
composites, mostly designed to
improve fracture toughness in combination
with additionally enhanced
strength and hardness above the average
for Mg alloys and (2) intermetallic
matrix composites based on a single
phase Mg 2 Si matrix for notable improvement
in strength and hardness,
in combination with a fracture toughness
higher than in as-cast Mg 2 Si.
Composites made by pressureless
infiltration: Composites with a
Mg 2 Si-Mg matrix reinforced with TiC
particulates were fabricated by pressureless
infiltration of porous Mg 2 Si-
TiC preforms. The calculated porosity
of the preforms used was within the
range of 30-35±5 vol.%.
Based on the experimental findings,
pressureless infiltration of
Mg 2 Si-TiC preforms with molten magnesium
did not occur below 900°C. At
Fig 1c: XRD of the sample shown in the
Fig. 1a, 1b
54 ALUMINIUM · 12/2009