Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

2.5
Magnesium-Based Nanocomposites
Magnesium is the lightest structural metal with a density of 1.74 g cm 3 , which is
about two third of density of aluminum (2.70 g cm 3 ). It also exhibits other advantages
such as good mechanical damping properties, good castability (particularly
suitable for die casting), and abundant supply globally. Compared with aluminum,
magnesium has relatively low strength and ductility, poor creep and corrosion
resistance. Magnesium and its alloys are difficult to deform plastically at room
temperature due to their hexagonal close-packed (HCP) crystal structure and limited
number of slip systems. Despite these deficiencies, magnesium and its alloys have
attracted considerable attention in automotive and aerospace industries where light
weight is an important consideration. The automotive applications include wheels,
steering column lock housings, and manual transmission housings [54]. The low
mechanical strength of Mg can be improved by adding ceramic micro- and nanoparticles
[55–57].
2.5.1
The Liquid Metallurgy Route
2.5 Magnesium-Based Nanocompositesj61
2.5.1.1 Compocasting
Very recently, Honma et al. fabricated high-strength AZ91D magnesium alloy
(Mg-9Al-1Zn-0.3Mn) reinforced with 1.5–7.5 wt% Si-coated carbon nanofibers
(CNF) [58]. The CNFs were coated with Si in order to improve the wettability.
The nanocomposites were prepared using a compocasting method at semi-solid
temperature (585 C) in a mixed gas atmosphere of SF6 and CO2. The resulting
nanocomposites were then subjected to squeeze casting and extrusion. The dispersion
of CNFs in magnesium alloy matrix is quite uniform due to the improvement in
the wettability of CNFs by the Si coating and the break-up of agglomerating clusters
by shear deformation during squeeze casting. Compocasting uses semi-solid stirring
and is effective for the fabrication of magnesium composites due to its ease of
operation and low cost. In the process, reinforcements are introduced into the matrix
alloy and stirred in the semi-solid condition. The slurry is then reheated to a
temperature above the liquidus of the alloy and poured into the molds. As many
pores and segregation are introduced in the composites during compocasting, an
additional squeeze casting process is required to minimize the casting defects. This
process involves the pouring of molten metal into a pre-heated die and subsequent
solidification of the melt under pressure.
2.5.1.2 Disintegrated Melt Deposition
Gupta and coworkers have developed the spraying process termed disintegrated
melt deposition (DMD) to deposit near net shape metal-matrix microcomposites
and nanocomposites [59–61]. The process involves mechanical stirring of molten
metal with an impeller under the addition of ceramic particulates, disintegration of
the composite slurry by an inert gas jet and subsequent deposition on a substrate in