APPLICATIONS OF NANOCOMPOSITES
B. Cantor*, C.M. Allen*, R. Dunin-Burkowski*, M.H. Green**, J.L. Hutchinson*,
K.A.Q. O’Reilly*, A.K. Petford-Long*, P. Schumacher*, J. Sloan**
and P.J. Warren*
*Department of Materials and **Department of Chemistry, Oxford University, Oxford, UK
(Received August 25, 2000)
(Accepted in revised form December 27, 2000)
Keywords: Magnetic multilayers; Devitrified amorphous alloys; Carbon nanotubes; Secondary
phases; Grain refiners
1. Introduction
This paper considers examples of how a material nanostructure can influence its properties. The
examples covered are grain refiners and secondary phases in commercial wrought Al alloys, devitrified
amorphous magnetic Fe alloys, magnetic multilayers for storage media and read/write devices, and 1-D
crystals in single walled C nanotubes.
2. Grain Refinement in Al Alloys
Wrought Al alloys are used in applications such as beverage cans, lithographic sheet and automotive
panels. During casting, primary Al grain sizes must be 100m to ensure isotropic properties. Grain
refinement is achieved by adding TiB
2
and Al
3
Ti in a master alloy, but the nucleation mechanism is not
well understood. Neither TiB
2
nor Al
3
Ti work alone. TiB
2
is stable in liquid Al, but does not react to
form solid Al; Al
3
Ti reacts peritectically but is not stable at grain refinement addition levels (1).
Observing nucleation is difficult, as it is obscured by subsequent crystal growth. A glass forming alloy
can be used as a slow motion analogue of an undercooled liquid, allowing TEM study of the nucleation
mechanism. Fig 1 shows a TEM image of a 100 hexagonal TiB
2
plate viewed edge-on along its
basal plane. The particle is embedded in amorphous Al
85
Ni
8
Y
7
Co
2
, with the basal faces covered by a
few monolayers thick adsorbed Al
3
Ti, which has nucleated crystals of Al. The fcc Al crystals,
tetragonal DO
22
Al
3
Ti layer and hexagonal TiB
2
plate have close-packed directions and planes parallel,
with the pseudo close-packed (112) planes in the thin Al
3
Ti layer stretched to match TiB
2
, giving large
strains in the nucleated Al. The 2–5nm Al crystal size is much smaller than in undercooled Al melts,
where the critical nucleus size is -1m at 0.5K undercooling. Mismatch dislocations are required to
maintain coherency in Al crystals larger than 5nm.
3. Secondary Phases in Al Alloys
Ppm impurity levels in wrought Al alloys influence the formation of nm-scale aluminide secondary
phases, which control final properties such as surface finish, corrosion and strength. These effects are
Scripta mater. 44 (2001) 2055–2059
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