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PROGRESS ARTICLE
RUSLAN VALIEV
Institute of Physics of Advanced Materials,Ufa State Aviation
Technical University,12 K.Marx str.,Ufa 450000,Russia
e-mail: RZValiev@mail.rb.ru
It has been over 20 years since Herbert Gleiter presented
the first concepts for developing nanocrystalline
materials (that is,ultrafine-grained materials with a
grain size under 100 nm) with special properties1.
Since then, the field of nanostructured materials has
developed rapidly,owing to tremendous interest in this
topic of scientific and technological importance.
Gleiter’s original idea was that owing to a very small
grain size,nanocrystalline materials contained an
extremely large fraction of grain boundaries with a
special atomic structure.As a result,nanomaterials
should have unusual properties2.As regards
mechanical properties,one could expect very high
strength, toughness, fatigue life and wear resistance.
Nanostructuring seemed likely to lead to a
revolutionary use of nanomaterials for many
functional and structural applications.But these
interesting prospects were put in jeopardy.
Numerous investigations3–5 showed that although
nanocrystalline materials did demonstrate very high
strength or hardness, they were of very low ductility or
even brittle,producing insuperable problems for
advanced structural applications.When addressing the
reasons for the low ductility,many researchers point
out drawbacks in their synthesis based on compacting
of nanopowders,obtained using various methods4,5.
Nanomaterials produced by compacting usually have
residual porosity,contaminations and,as a rule, small
geometric dimensions.All this may lead to a decline in
their ductility.Another possible reason is of
fundamental nature: the plastic deformation
mechanism associated with generation and movement
of dislocations may not be effective in ultrafine grains
(see below).In this connection,recent findings of
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Nanostructuring of metals by severe plastic
deformation for advanced properties
Despite rosy prospects, the use of nanostructured metals and alloys as advanced structural and functional
materials has remained controversial until recently. Only in recent years has a breakthrough been outlined in
this area, associated both with development of new routes for the fabrication of bulk nanostructured materials
and with investigation of the fundamental mechanisms that lead to the new properties of these materials.
Although a deep understanding of these mechanisms is still a topic of basic research, pilot commercial
products for medicine and microdevices are coming within reach of the market. This progress article
discusses new concepts and principles of using severe plastic deformation (SPD) to fabricate bulk
nanostructured metals with advanced properties. Special emphasis is laid on the relationship between
microstructural features and properties, as well as the first applications of SPD-produced nanomaterials.
×××××××××××××××××
P
P
Plunger
Work piece
Plunger
Support
Sample
Die
ba
Φ
Figure 1 Principles of severe plastic deformation techniques.a,High-pressure torsion:a sample is held
between anvils and strained in torsion under applied pressure (P ).b,Equal channel angular pressing:
a work-piece is repeatedly pressed through a special die.
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PROGRESS ARTICLE
extraordinarily high strength and ductility in several
bulk ultrafine-grained metals are especially
interesting6–9.However,various nanomaterials possess
microstructural features closely linked to processing
methods and regimes.Therefore we should first briefly
address the principles of processing techniques and the
structural features of processed materials.
BULK NANOSTRUCTURED MATERIALS
Earlier studies focused on fabricating nanostructured
materials by inert gas condensation1,2.This technique
makes it possible to produce ultrafine grains with a size
refined down to 10 nm.On the other hand, it has well-
known limitations — overall sample dimensions are
small (only up to 10 mm in diameter and up to 1 mm in
thickness for a typical disc-shaped sample) and some
porosity remains.High residual porosity and
contaminations are also inherent drawbacks in samples
obtained by consolidation of nanopowders produced
by ball-milling or mechanical alloying5.
Recent years have therefore seen growing interest
in a new approach to fabrication of bulk
nanostructured metals and alloys, as an alternative to
nanopowder compacting. This approach is based on
microstructure refinement in bulk billets using severe
plastic deformation (SPD): that is, heavy straining
under high imposed pressure10. SPD-produced
nanomaterials are fully dense and their large
geometric dimensions make it possible to perform
thorough mechanical tests. Fabrication of bulk
nanostructured materials by severe plastic
deformation is becoming one of the most actively
developing areas in the field of nanomaterials11,12.
Since the pioneering work on tailoring of ultrafine-
grained structures by SPD processing13,14, two SPD
techniques have attracted close attention and have lately
experienced further development.These techniques are
high-pressure torsion (HPT) and equal channel angular
(ECA) pressing (Fig. 1).
Samples processed under HPT are disc-shaped
(Fig.1a). In this process, the sample,with a diameter
ranging from 10 to 20 mm and thickness of
0.2–0.5 mm,is put between anvils and compressed
under an applied pressure (P) of several GPa.The lower
anvil turns,and friction forces result in shear straining
of the sample.As a result of high imposed pressure, the
deforming sample does not break even at high strains10.
Essential structure refinement is observed after
deformation through one-half or one complete (360°)
turn.But to produce a homogeneous nanostructure,
with a typical grain size of about 100 nm or less,
deformation by several turns is necessary (Fig. 2).
The important role of applied pressure in the formation
of a more homogeneous nanostructured state during
HPT is also shown in recent work on nickel15.
During ECA pressing,the ingot is pressed in a 
special die through two channels with equal cross-
section16,17, intersecting usually at an angle Φof 90°
(Fig.1b).Here each pass imparts a supplementary strain
of about 1.Among new trends in ECA pressing,there is
processing of hard-to-deform materials which can be
realized using back pressure or with increased channel
intersection angles, that is,Φ> 90°.Experimental and
theoretical modelling of the mechanics of ECA pressing,
focusing on the stress-deformed state,contact stresses
and friction conditions16,18,has made it possible to
construct ECA pressing dies for processing of larger
billets with uniform ultrafine grains out of various
metals, including hard-to-deform titanium and its
alloys19.Titanium billets up to 60 mm in diameter 
and 200 mm long have been successfully processed.
Another new direction is the fabrication of long-length
semi-products (rods,sheets) using continuous ECA
pressing,or other SPD techniques such as accumulative
roll bonding or repetitive corrugation and
straightening11,12.This is an important step for successful
commercialization of nanostructured metals.
Strong refinement of microstructure by ECA
pressing is fairly easy to achieve both in pure metals and
in alloys,using straining with one pass or a few.But it
remains a special problem to produce ultrafine-grained
structures by this technique.For success,many more
passes should be made (as a rule,eight or more; Fig.2b),
and optimal processing routes and regimes should be
determined.The final size of grains produced by ECA
pressing depends on the material under investigation
and on processing features,but for pure metals it is
typically200–300 nm.
The size and shape of ultrafine grains are important
but not the only features of the structure of SPD-
processed metals.As we demonstrate below,the
structure of grain boundaries is an essential feature for
attaining new properties.Early work on ultrafine-
grained metals produced by SPD13 has aimed at
formation of predominantly high-angle grain
boundaries.This,however, is possible only at large
accumulated strains ≥ 6–8.Modern diffraction
methods,such as orientation imaging microscopy or
back electron scattering diffraction,provide evidence of
the presence of up to 70–80% of high-angle boundaries
in the microstructure of samples subjected to multipass
ECA pressing or HPT with five or more revolutions at
relatively low temperatures (usually less than 0.3Tm;
where Tm is the melting point in K)12,20.
Among other important features of the
microstructure of SPD metals, special emphasis should
be laid on the appearance of a crystallographic texture21
and the presence of high internal stresses caused by high
density of dislocations inside the grains and at their
boundaries10.Formation of non-equilibrium grain
boundaries containing numerous grain-boundary
dislocations is an immediate consequence of severe
straining,but it can be controlled by subsequent
annealings or special thermomechanical treatments,or
both.For instance,observations22 of HPT-processed
titanium by transmission electron microscopy and
512 nature materials | VOL 3 | AUGUST 2004 | www.nature.com/naturematerials
Figure 2TEM images of
ultrafine-grained copper.
a, Copper processed by HPT 
at room temperature
(P = 6 GPa, five turns).
b, Copper processed by ECA
pressing (12 passes).
a b
200 nm200 nm
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PROGRESS ARTICLE
high-resolution electron microscopy revealed (Fig.3)
that the grain boundaries are not well defined,but are
wavy,curved or corrugated (Fig. 3c), indicating the
presence of numerous atomic defects.Moreover, there is
a variation in the misorientation angle along the same
boundary by about 5°,probably as a result of the
presence of disclinations.
However,observations after annealing at
250–300 °C (this is below the temperature at which any
grain growth can start) revealed a rearrangement of
dislocations: they had moved from the grain interiors to
the region near the grain boundaries.A schematic
illustration of the defect rearrangement is presented in
Fig.4.The figure emphasizes that although the total
density of dislocations is decreased during low-
temperature annealing, the local density of dislocations
at grain boundaries can grow,thus increasing their 
non-equilibrium,and this may have a strong effect on
grain-boundary processes such as sliding,diffusion or
interaction with lattice dislocations23.Hence recent
investigations show that ultrafine-grained metals
produced by SPD possess a complex microstructure.
Their microstructural features are conditioned by
processing routes and regimes.These features should be
taken into consideration when developing bulk
nanostructured materials with improved properties.
ENHANCED PROPERTIES IN SPD-PRODUCED 
NANOMATERIALS
It is well known that grain refinement promotes
mechanical strength, and thus one can expect
ultrafine-grained materials to possess very high
strength. Moreover, introduction of a high density of
dislocations in SPD-processed nanometals may result
in even greater hardening. However, all this normally
decreases ductility. Strength and ductility are the key
mechanical properties of any material, but they are
typically opposing characteristics. Materials may be
strong or ductile,but rarely both at once.Recent studies
have shown that material nanostructuring may 
lead to a unique combination of exceptionally high
strength and ductility (Fig. 5), but this task calls for
original approaches6–9.
One such new approach to the problem was
suggested recently by Wang et al.8.They created a
nanostructured copper by rolling the metal at low
temperature — the temperature of liquid nitrogen —
and then heating it to around 450 K.The result was 
a ‘bimodal’structure of micrometre-sized grains (at a
volume fraction of around 25%) embedded in a matrix
of nanocrystalline grains.The material showed
extraordinarily high ductility,but also retained its high
strength.The reason for this behaviour is that,while the
nanocrystalline grains provide strength, the embedded
larger grains stabilize the tensile deformation of the
material.Other evidence for the importance of grain-
size distribution comes from work on zinc24,copper25
and aluminium alloy26.What is more, the investigation
of copper25 has shown that bimodal structures can
increase ductility not only during tensile tests,but also
during cyclic deformation.This observation is
important for improving fatigue properties.
Another approach suggested recently4 is based on
formation of second-phase particles in the
nanostructured metallic matrix,which are to modify
shear-band propagation during straining, thereby
increasing the ductility.A systematic study of both hard
and soft second-phase particles with varying sizes and
distributions is required here, to allow mechanical
properties to be optimized.
A third approach to the problem of strength and
ductility is probably the most universal of the three,
because it can be applied both for metals and for 
alloys.The approach is based on formation of
ultrafine-grained structures with high-angle and 
non-equilibrium grain boundaries capable of grain-
boundary sliding (GBS)6,10. It is well known that sliding,
which increases ductility,normally cannot develop at
low-angle boundaries.The importance of high-angle
grain boundaries was verified in work6 on the
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a
b
c
500 nm
0.295 nm
A
A
B
5° 2 nm
200 nm
Figure 3Typical images of
microstructure of HPT-processed
titanium.a,Bright-field
transmission electron
micrograph (with the selected-
area diffraction pattern as an
inset).b,Dark-field transmission
electron micrograph.c,High-
resolution electron micrograph
demonstrating highly distorted
grain boundaries in this as-
processed metal.This is
associated with the high
distortion of the crystal lattice and
a variation in the misorientation
angle (see the region in the white
rectangle and the position of the
lines A,A and B).
a b Figure 4 Arrangement of grain
boundaries in nanostructured
titanium. a,The dislocation
structure after HPT processing.
b,The dislocation structure 
after HPT and low-temperature
annealing leading to 
formation of non-equilibrium
grain boundaries.
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PROGRESS ARTICLE
mechanical behaviour of metals subjected to different
degrees of severe plastic deformation resulting in
formation of various types of grain boundaries.As was
noted above,sliding can be easier when non-
equilibrium boundaries are present.Another example
of this is the extraordinary influence of annealing
temperature on mechanical behaviour found recently
in nanostructured titanium produced by HPT22.Here a
short annealing at 300 °C results in a noticeable increase
in strength combined with greater ductility than in the
HPT-produced state or after annealing at higher
temperature.The growth of strength and ductility was
associated with higher strain-rate sensitivity of flow
stress.An increased strain-rate sensitivity has also been
reported in other works investigating high strength and
ductility in nanometals6,10,27.High strain-rate sensitivity
indicates viscous flow and plays a key role in
superplasticity in materials28,but on the other hand it is
associated with the development of grain-boundary
sliding,and therefore depends on grain-boundary
structure.Thisfact is in agreement with the recent
results of computer simulation and studies of
deformation mechanisms active in nanostructured
metals.Such molecular dynamics simulations have
provided valuable insight into the deformation
behaviour of nanometals29–31.
For coarse-grained metals,dislocation movement
and twinning are well-known primary deformation
mechanisms.But the results of simulation show that
ultrafine grains may also aid in specific deformation
mechanisms such as grain-boundary sliding or
nucleation of partial dislocations30–33.Moreover, the
sliding may have a cooperative (grouped) character
similar to that observed in earlier studies on superplastic
materials34,35. It should be stressed that recent
experiments investigating deformation mechanisms in
nanostructured materials have confirmed a number of
the results of computer simulation22,36,37.
However, there is a question: why should grain-
boundary sliding in nanostructured materials, in
particular in those produced by SPD,take place at
relatively low temperatures? GBS is a diffusion-
controlled process and usually occurs at high
temperatures.A possible explanation is that diffusion
may be faster in SPD-produced ultrafine-grained
materials with highly non-equilibrium grain
boundaries.Experiments have shown that in SPD-
produced metals the diffusion coefficient grows
considerably (by two or three orders),and this is
associated with non-equilibrium grain boundaries38,39.
So perhaps grain-boundary sliding is easier in these
ultrafine-grained metals and develops during straining
even at lower temperatures,producing increased
ductility. It is well known that enhanced sliding in
nanostructured metals can lead even to superplasticity
at relatively low temperatures40.
Processing of nanomaterials to improve 
both strength and ductility is of primary importance 
for fatigue strength and fracture toughness25,41,42.
An extraordinary increase in both low-cycle and high-
cycle fatigue-strength may take place; there exists a
theoretical explanation and the first experimental
evidence of this interesting phenomenon41,42.
It is interesting that the complex structure of SPD-
processed materials can also result in multifunctional
properties.For instance, the nanostructured TiNi alloy43
demonstrates an extraordinary combination of very
high mechanical and functional properties:
superelasticity and shape-memory effect.Such a
combination makes the nanostructured TiNi alloy
different in principle from its conventional (coarse-
grained) counterpart.Engineering of multifunctional
materials is becoming a new direction in the science of
SPD nanomaterials.
USING SPD-PRODUCED NANOSTRUCTURED METALS
Markets for bulk nanostructured materials exist in
virtually every product sector where superior
mechanical properties (in particular, strength,
strength-to-weight ratio and fatigue life) are critical.
Formal market analyses, conducted by companies
such as Metallicum that specialize in nanostructured
metals, have identified over 100 specific markets for
nanometals in aerospace, transportation, medical
devices, sports products, food and chemical
processing, electronics and conventional defence44.
Among them we can single out the following
directions: (1) development of extra-strong
nanostructured light alloys (Al, Ti, Mg), for example
Al-based commercial alloys with yield strength over
800–900 MPa, for the motor industry and aviation;
(2) development of metals and alloys with ultrafine-
grained structure for use at cryogenic temperatures45;
(3) development of nanostructured ductile refractory
metals and high-strength TiNi alloys with advanced
shape-memory effect for space, medical and other
applications. The applications of nanostructured
materials in engineering of new-generation aviation
engines46 or in high-strain-rate superplastic forming
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Mo
Mo
Co
Re
Ti
Cu
Zr
Ru
Be
61%
37%
21%
11%
0
0
U
Mg 75%
35%
Al Pd Pt
Th Ni
V Ti
Zr
Ni
Au
Nb
W
Nanostructured Ti
Nanostructured Cu
FeTa
0 10 20 30 40 50 60
Elongaion to failure (%)
800
600
400
200
0
Yi
el
d 
st
re
ng
th
 (M
Pa
)
Figure 5 Strength and ductility of
the nanostructured metals
compared with coarse-grained
metals.Conventional cold rolling
of copper and aluminium
increases their yield strength but
decreases their ductility.The two
lines represent this tendency for
Cu and Al and the % markings
indicate a percentage on rolling.
In contrast, the extraordinarily
high strength and ductility of
nanostructured Cu and Ti clearly
set them apart from coarse-
grained metals6.
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PROGRESS ARTICLE
of complex-shaped parts for new automobiles and
planes47 are worth a special mention.
Out of the broad range of possible applications of
advanced nanostructured metals,we focus here on one
that is representative of the high-tech market:
biomedical implants and devices.High mechanical and
fatigue properties are the essential requirements for
metallic biomedical materials, in particular titanium
and its alloys48,which have excellent biological
compatibility and high biomechanical properties.
For example, in trauma cases,plates and screws made 
of new titanium materials are planned to be widely used
for fixing bones.These plates need very high
compressive and bending strength,and sufficient
ductility.Different implant-plate constructions for
osteosynthesis have been analysed,resulting in the
design and processing of a series of nanostructured
titanium plates (Fig. 6a,b). Figure 6c illustrates another
application of nanostructured titanium for a special
conic screw,which requires high fatigue strength as well.
In this case all the advantages of nanostructured
titanium are fully used49 — high static and fatigue
strength (yield tensile strength ≥ 950 MPa at strain rate
10–3 s–1,endurance of more than 500 MPa at 2 × 107
cycles) and excellent biological compatibility.
CONCLUSIONS
Recent progress in fabricating bulk metallic materials
using severe plastic deformation,and in understanding
their fundamental mechanisms,probably brings us
closer to the revolutionary use of nanomaterials for
structural and functional applications.The atomic
defect structure of SPD-processed metals is complex
and is closely connected with the features of processing
routes and regimes.Nevertheless, tailoring specific
nanostructures (for example with bimodal grain-size
distribution or ultrafine grains with high-angle and
non-equilibrium grain boundaries) can produce
unique combinations of properties, such as
extraordinarily high strength and ductility,high fatigue
life and toughness.These properties may be of great
importance if nanostructured metals and alloys are to
form the next generation of advanced structural and
functional materials.
doi:10.1038/nmat1180
Acknowledgements
This work was supported partly by the Department of Energy NIS-IPP program at
Los-Alamos National Laboratory, the Alexander von Humboldt Foundation
research award and the Russian Foundation for Basic Research.
Competing financial interests
The author declares that he has no competing financial interests.
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