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Nanocrystalline materials
c. Suryanarayana
The current status of research and development on
the structure and properties of nanocrystalline
materials is reviewed. Nanocrystalline materials
are polycrystalline materials with grain sizes of up
to about 100 nm. Because of the extremely small
dimensions, a large volume fraction of the atoms is
located at the grain boundaries, and this confers
special attributes to these materials.
Nanocrystalline materials can be prepared by inert
gas condensation, mechanical alloying, plasma
deposition, spray conversion processing, and many
other methods. These are briefly reviewed. A clear
picture of the structure of nanocrystalline materials
is only now emerging. Whereas the earlier workers
had concluded that the structure of grain
boundaries in nanocrystalline materials was quite
different from that in coarse grained materials,
recent studies have shown unambiguously that the
structure of the grain boundaries is the same in
both nanocrystalline and coarse grained materials.
The properties of nanocrystalline materials are very
often superior to those of conventional
polycrystalline coarse grained materials.
Nanocrystalline materials exhibit increased
strength/hardness, enhanced diffusivity, improved
ductility/toughness, reduced density, reduced
elastic modulus, higher electrical resistivity,
increased specific heat, higher thermal expansion
coefficient, lower thermal conductivity, and
superior soft magnetic properties in comparison
with conventional coarse grained materials. New
concepts of nanocomposites and nanoglasses are
also being investigated with special emphasis on
ceramic composites to increase their strength and
toughness. There appears to be a great potential
for applications in the near future for
nanocrystalline materials. The extensive
investigations in recent years on structure-property
correlations in nanocrystalline materials have
begun to unravel the complexities of these
materials, and pave the way for successful
exploitation of alloy design principles to synthesise
better materials than hitherto available. IMR/271
© 1995 The Institute of Materials and ASM International.
The author is in the Institute for Materials and Advanced
Processes, University of Idaho, Moscow, 10, USA.
Introduction
Metallurgists and materials scientists have been con-
ducting research investigations for several centuries
to develop materials which are 'stronger, stiffer, and
lighter' than the existing materials and also capable
of use at elevated temperatures ('hotter'). The high
technology industries in the developed countries have
given an added fillip to these efforts. Several novel
and non-equilibrium processing methods have been
developed during the past few decades to improve
the performance of the existing materials; these
include rapid solidification from the liquid state,1,2
mechanical alloying," plasma processing.t'" and
vapour deposition," A central underlying theme in all
these methods is to energise the material to bring it
into a highly non-equilibrium (metastable) state (also
including a possible change of state from the solid
to liquid or gas) through melting, evaporation,
irradiation, application of pressure, storing of
mechanical energy, etc," The material is then brought
to another lower energy metastable state by quench-
ing or related processes when it can exist as a super-
saturated solid solution, metastable crystalline or
quasicrystalline phase, or even in a glassy state,
affording ample opportunities to modify the crystal
structures and/or microstructures. These processes
have led to considerable improvement in the prop-
erties of a number of alloy systems and, consequently,
some industrial applications; these have been
described and fully documented in the references
listed above.
A novel way of transforming a material to a
metastable state is to reduce its grain size to very
small values of a few nanometres when the proportion
of atoms in the grain boundaries is equivalent to or
higher than those inside the grains. This type of
metastability can be classified as morphological meta
(or 'in') stability in the scheme of Turnbull." Materials
with such small grain sizes are now referred to as
nanocrystalline materials (and also as nanocrystals,
nanostructures, nanophase materials, or nanometre
sized crystalline solids), and have been shown to have
properties much improved over those exhibited by
conventional grain sized (> 10 urn) polycrystalline
materials. It is the combination of unique com-
positions and novel microstructures that leads to
the extraordinary potential of the nanocrystalline
materials.
There has been a continued increase in the number
of research investigations in recent years on the
synthesis/processing, characterisation, properties, and
potential applications of these novel materials. A new
journal entitled Nanostructured Materials, * was
started in 1992. In addition to several national confer-
ences and also as part of other symposia, a series of
international conferences is planned exclusively to
discuss the developments in nanostructured materials.
The first of these international conferences was held
in Cancun, Mexico, in September 1992 and the second
one was recently held in Stuttgart, Germany, in
October 1994.
The purpose of the present article is to present a
very broad overview of the structure and properties
of nanocrystalline materials. Potential applications of
these novel materials are also highlighted. This field
has been reviewed earlier with emphasis on some
selected topics.8-24 However, there is no comprehen-
sive review related to the materials aspects; this review
is an attempt to fill that gap.
* Published by Pergamon Press.
International Materials Reviews 1995 Vol. 40 No.2 41

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42 Suryanarayana Nanocrystalline materials
1 Schematic of the four types of nanocrystalline
materials (after Ref. 23)
Classification
Nanocrystalline materials are single phase or multi-
phase polycrystals, the crystal size of which is of the
order of a few (typically 1-100) nanometres in at
least one dimension. Thus, they can be basically
equiaxed in nature and will be termed nanostructure
crystallites (three-dimensional (3D) nanostructures),
or they can consist of a lamellar structure, and will
be termed a layered nanostructure (one-dimensional
(lD) nanostructure), or they can be filamentary in
nature (two-dimensional (2D) nanostructurej.l"
Additionally, Siegel/" considers zero-dimensional
atom clusters and cluster assemblies. Table llists this
classification and Fig. 1 illustrates the four types
of nanostructures schematically. The magnitudes of
length and width are much greater than thickness in
the layered nanocrystals, and length is substantially
larger than width or diameter in filamentary nano-
crystals. The nanocrystalline materials may contain
crystalline, quasicrystalline, or amorphous phases and
can be metals, ceramics, or composites.
Among the above, maximum attention has been
paid to the synthesis, consolidation, and characteris-
ation of the 3D-nanostructured crystallites followed
by the ID-Iayered nanostructures. While the former
are expected to find applications based on their
high strength, improved formability, and a good
combination of soft magnetic properties, the latter
are intended for electronic applications. Relatively
few investigations have been carried out on the
2D-filamentary nanostructures and it is only recently
Table 1 Classification of nanocrystalline materials
Typical method(s)
of synthesisDimensionality Designation
Three dimensional Crystallites
(equiaxed)
Filamentary
Layered
(lamellar)
Clusters
Gas condensation
Mechanical alloying
Chemical vapour deposition
Vapour deposition
Electrodepositi on
Sol-gel method
Two dimensional
One dimensional
Zero dimensional
International Materials Reviews 1995 Vol. 40 No.2
2 Schematic representation of equiaxed
nanocrystalline metal distinguishing between
atoms associated with the individual grains (.)
and those constituting grain boundary
network (0) (after Ref. 11)
that zero-dimensional clusters are being investigated
to 'tailor' the optical properties.
Characteristics
A schematic representation of a hard sphere model
of an equiaxed nanocrystalline metal is shown in
Fig. 2. Two types of atoms can be distinguished:
crystal atoms with nearest neighbour configuration
corresponding to the lattice and boundary atoms with
a variety of interatomic spacings, differing from
boundary to boundary. A nanocrystalline metal
contains typically a high number of interfaces
(-- 6 X 1025 m -3 for a 10 nm grain size) with random
orientation relationships, and consequently, a sub-
stantial fraction of atoms lie in the interfaces.
Assuming that grains have the shape of spheres or
cubes, the volume fraction of nanocrystalline mater-
ials associated with the boundaries can be calculated."
as
C = 311/d
where 11 is the average grain boundary thickness and
d the average grain diameter. Thus, the volume frac-
tion of atoms in the grain boundaries can be as much
as 50% for 5 nm grains and decrease to about 30%
for 10 nm grains and 30/0for 100 nm grains.
In recent years, it has become apparent that it is
the total intercrystalline region (consisting of grain
boundaries and triple junctions, i.e. intersection lines
of three or more adjoining crystals) which is important
since at very small grain sizes triple junctions become
an important component of the microstructure. Since
neither of the above grain shapes (spheres or cubes)
is reasonable nor are they suitable for deriving triple
junction volume fractions, Palumbo et al.26 consid-
ered the grains to have the regular 14-sided tetra-
kaidecahedron shapes, with the hexagonal faces
representing the grain boundaries, and edges corres-
ponding to triple junctions. Assuming the maximum
diameter of an inscribed sphere as the grain size d
and the intercrystalline component as an outer 'skin'
of the tetrakaidecahedron with a thickness of 11/2, the