SCIENCE CHINA
Technological Sciences
• RESEARCH PAPER •
September 2012 Vol.55 No.9: 2377–2390
doi: 10.1007/s11431-012-4954-y
Review on modified and novel techniques of severe
plastic deformation
WANG ChengPeng1,2, LI FuGuo1,2*, WANG Lei2 & QIAO HuiJuan2
1
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China;
2
School of Materials Science and Engineering, Northwestern Ploytechnical University, Xi’an 710072, China
Received January 13, 2012; accepted April 11, 2012; published online July 10, 2012
This review highlights very recent achievements and new developments of severe plastic deformation (SPD) technology for
producing bulk ultrafine-grain (UFG) and even nanocrystalline (nc) materials. These numerous modified and novel SPD
methods include cyclic forward-backward extrusion, axi-symmetric forward spiral extrusion, vortex extrusion, simple shear
extrusion, planar twist extrusion, tubular channel angular pressing, cone-cone method, high-pressure tube twisting, tube channel pressing and elliptical cross-section spiral equal-channel extrusion. According to classification, these new methods are categorized into the extension of equal-channel angle pressing (ECAP), high-pressure torsion (HPT), twist extrusion (TE) and
constrained groove pressing (CGP), respectively. The principles of various new SPD technologies are described in detail. In
addition, the microstructure revolution characteristics and mechanical properties of materials produced by SPD process, as well
as the applications of SPD techniques to UFG materials, are also reported. Furthermore, this article reviews recent progresses
in determining the refinement and/or deformation mechanisms, e.g. dislocation deformation mechanism, twin deformation
mechanism and grain boundary sliding and torsional deformation mechanism, and further orientation of SPD technology.
severe plastic deformation, ultrafine-grain, refinement
Citation:
Wang C P, Li F G, Wang L, et al. Review on modified and novel techniques of severe plastic deformation. Sci China Tech Sci, 2012, 55: 23772390,
doi: 10.1007/s11431-012-4954-y
1 Introduction
To pursue the excellent mechanical properties of high
strength, good ductility, superior superplasticity, low friction coefficient, high wear resistance, enhanced highcycle
fatigue life, and good corrosion resistance, the bulk ultrafine-grain (UFG) and even nanocrystalline (nc) materials
fabricated by severe plastic deformation (SPD) have grown
significantly over the past decades [1]. The most familiar
SPD methods [1, 2], such as equal-channel angle pressing
(ECAP), high-pressure torsion (HPT), multi-directional
forging (MDF), sandglass extrusion (SE), repetitive corru*Corresponding author (email: fuguolx@nwpu.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2012
gation and straightening (RCS), twist extrusion (TE), constrained groove pressing (CGP) and accumulated
roll-bonding (ARB), are attracting more and more scholars
to devote their energy to this area. Since the materials effectively produced by SPD technology are porosity-free and
contamination-free, they are potential for both fundamental
theory and industrial application. Although SPD technology
has already significantly demonstrated the ability to refine
microstructure, it has not efficiently produced specific desirable uniform refined structure or favorite material properties. In addition, current SPD methods have some limits,
e.g., rigorous equipment, high processing cost, complex
process, and the difficulty of industrialization and automation for mass production. Nowadays, SPD techniques are
emerging from the domain of laboratory-scale research into
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commercial production for various UFG materials [1, 2]. So
simplifying these processing operations and obtaining
unique properties of materials are the topics of further development of SPD techniques. Some efforts have been
made on the modifications and innovation of SPD methods,
such as multi-pass coin-forging (MCF) [3], conshearing
pressing [4], continuous confined strip shearing (C2S2) [5],
equal channel multi-angular pressing (ECMAP) [6], rotary-die ECAP [7], cross-ECAP [8], T-shaped ECAP [9, 10],
CONFORM process [11], multi-pass ECAP [12], torsional-equal channel angular pressing (T-ECAP) [13], incremental ECAP (I-ECAP) [14] and repetitive side extrusion
process (RSEP) [15].
This review has three main objectives: first, to review
modified and novel SPD techniques during the last several
years, then to summarize the microstructure revolution
characteristics and mechanical properties of UGF materials
produced by SPD, and current research issues on refinement
mechanisms of SPD techniques, and finally to discuss the
further developments of SPD techniques in terms of grain
refinement, enhancement of properties, and potential for
commercialization.
2 SPD Techniques
2.1
Extension of ECAP
2.1.1 Tubular channel angular pressing
Tubular channel angular pressing (TCAP) [16] is suitable
for cylindrical tubes due to its extremely large strains without changing the dimensions of samples. A schematic of
TCAP is shown in Figure 1(a). The tube, constrained by the
inner and outer dies, is pressed by a hollow cylindrical
punch into a tubular angular channel with four flat regions
(a, b, c, and d) and three axisymmetric shear zones (I, II,
and III) (shown in Figure 1(b)). In TCAP processing, there
are some additional radial and circumferential tensile and
compressive strains in region b and region c, respectively,
while the strain state of ECAP [1, 2] could be considered as
the simple shear. The tube-sample with unchanged crosssection allows repeating TCAP process to achieve distinct
strains.
The total effective strain after N passes of TCAP processing can be expressed by the following relationship [16]:
September (2012) Vol.55 No.9
Figure 1 (a) A schematic of TCAP and (b) processing parameters, three
shear zones (hatched zone) during TCAP processing [16].
Thanks to the outstanding capabilities of ECAP, TCAP
method as an extension of ECAP is suitable for processing
tubes material. AZ91 magnesium alloy is processed by single cycle of TCAP. The experimental results revealed reasonably good hardness homogeneity along the length and
thickness directions of the tube-sample. Moreover, the microstructure was refined from the initial grain size of 150 to
1.5 m. This new SPD process is promising for future industrial applications.
2.1.2 Tube channel pressing
Similar to TCAP, another new SPD method entitled tube
channel pressing (TCP) [17] was proposed for long tubular
sample (shown in Figure 2). In TCP processing, the tube is
pressed by the ram into the tubular channel of die with a
neck zone. The crumpling of tube is prevented by a fitted
mandrel. The ram pulls out of the bottom of die, and the
next pass of TCP process continues when the tube passes
the neck zone.
The deformation of TCP-ed tube-sample in the neck zone
resembles that in the angular region of ECAP [1]. Therefore,
the effective strain is imitated from that of ECAP die with
an equal fillet radius. Moreover, the additional strain obtains
from the neck zone deformation, where the cross-section of
tube is constant, while the tube diameter changes in the
neck zone of the channel. Hence, the total effective strain
 achieved after one pass of TCP process is divided into
two parts [17]: the Von-Misses one  Von-Misses and the Hom
 2cot(i /2   i /2)   i csc(i /2   i /2) 

3

 i 1 

3
  N  

4 3 R 
ln  ,
3
R0 
(1)
where i and i are the channel angle and corner angle, respectively. R and R0 are the radius of the tube in the channel
region and final (=initial) tube, respectively.
Figure 2 Schematic of the TCP process [17].
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one  Hom . The total effective strain is obtained by the following expression:
   Von-Misses   Hom 
 Dm  
2   
cot    2 ln 
 ,
3  2
 dm 
(2)
where  is the angle of shearing in each curvature, Dm and
dm are the average diameters of inner and outer sides of the
channel and the neck zone, respectively.
Analytical calculation and simulation have demonstrated
that the average effective strain of 1.2 is imposed on each
pass. Furthermore, the TCP experiment for commercial purity aluminum (AA1050) is successfully carried out in five
passes without any cracks. After TCP processing, the yield
stress, ultimate tensile stress and hardness are increased by
two times of those for the annealed tube-sample. The average cell size of sample is decreased from 1000 m to 360
nm. The hardness distribution through tube thickness has a
maximum at mid thickness and two minimums at the inner
and outer sides of a tube wall. The accumulation of strains
in TCP process is mainly the result of shear deformations.
The inventor of TCP deemed that by comparing this
process with the fixed HPTT (high-pressure tube twisting)
method (mentioned below) [18], some advantages, e.g,
more homogeneous strain, simple tools, low cost and longer
tubes produced, make TCP have a reasonable ability in
strength improvement and grain refinement.
2.2
Extension of HPT
2.2.1 High-pressure tube twisting
Recently, high-pressure tube twisting (HPTT) [18, 19] has
been proposed for cylindrical tubes. More work on HPTT
has been done for the development of tubular UFG materials, which are the most practical essentials in aerospace,
automobile, building construction, petroleum industries, etc.
Two types of HPTT, the free HPTT and the fixed HPTT,
will be described in the following sections.
1) Free HPTT
As shown in Figure 3, in the free HPTT process [19] a
tube sample is pressed against a rotating die under the pressure of a compressed mandrel. This type of HPTT is different from the fixed one described subsequently. In contrast,
here the edges of the sample are not confined and the material could flow to the free surface at the edges of the tube.
This makes it possible to apply much longer tubes.
The effective strain in free HPTT process is [19]
 
2N  R
3 t
,
Figure 3
FE model of the free HPTT process [19].
tube, similar to that found in HPT processing. The process
relies on shear applied by both friction forces acting on the
large surfaces and a high hydrostatic pressure within the
deformation zone. The experiments have demonstrated that
the free HPTT technique can realize to refine microstructure
in practice. However, a uniform UFG microstructure is difficult, although not impossible, to obtain. Further improvements may realize the potential of producing a uniform microstructure.
2) Fixed HPTT
A fixed HPTT [18] is shown schematically in Figure 4. A
high hydrostatic pressure is achieved by the axial compression of a fixed cylindrical mandrel, which is compressed
with a compression machine in its elastic regime. The tube
is twisted by an external torque with the help of friction
forces generated by the hydrostatic pressure. The material
flow is all around the upper and lower ends of the tube (labeled by “ears” in the schematic). The purpose of ears is to
benefit the tube constrained and the building up of a large
gradient in the hydrostatic stress.
During twisting, the deformation mode is locally simple
shear, where the normal direction of the shear plane is the
radial direction of the tube and the shearing direction is parallel to the circumferential direction. For a thin-walled tube,
the amount of shear can be estimated from the geometry as
follows [18]:

r0 
,
t
(3)
where N is the number of turns, R is the tube radius and t is
the wall thickness.
During free HPTT processing, the sample is subjected to
the non-homogeneity of strain in the cross-section of the
Figure 4
Schematic of fixed HPTT [18].
(4)
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where r0 is the average radius of the tube,  is the angle of
twist and t is the thickness of the tube.
By assuming that the friction stress is equal to the flow
stress τf of the material according to Tresca’s friction law,
the following formula can be obtained for the hydrostatic
stress p within the sample [18]:
p
2 f s
w
(5)
,
where s is the length of the “ear” and w is its thickness.
In the fixed HPTT experiment, the deformation leads to
the Al and Cu samples in full plastification and partial plastification within a surface layer, respectively. Further analysis of the Al tube has shown a shear gradient within the tube
wall. The strain gradient may be related to the thickness of
the tube. For a thin-walled tube, the stress state is nearly
constant within the wall. The uniform shear is expected.
Conversely, the thickness is relatively lager; the stress state
is not uniform but depends on the radial position within the
tube wall, leading to a strain gradient. As shown above,
there are large differences in the strain gradient between the
Al and Cu samples.
2.3 Cone-cone method
Cone-cone method (CCM) [20, 21] is suitable for thinwalled cone-shaped sample. A finite element (FE) model of
the CCM process is shown in Figure 5. A conical ring is
applied by the deformation of a conical plunger and a conical die with the pressure and torque, which impose hydrostatic pressure and shear strain, respectively. If the sticking
between the workpiece and the tooling can be established,
the extreme large plastic shear strains and the concomitant
grain refinement can be achieved.
The effective strain in the geometry of the CCM process
is given as follows [21]:
 
2πN  R (x )
3 t
，
Figure 5 FE model of the CCM process [19].
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where N is the number of turns of the ram, t is the thickness
of the sample and R(x) is the (external) radius of a circular
cross-section normal to the cone axis at a distance x from
the apex.
According to the current CCM researches [21], the strain
decreases towards the apex of the cone linearly with the
distance along the central axis. The CCM process relies on
shear strain applied by friction forces acting on the large
surfaces and a high hydrostatic pressure within the deformation zone. The results of OM images have shown the
inhomogeneous microstructure near the cone base, which is
distinct from that in the middle section of the cone. This
observation could be explained by the variation of the normal stress along the contact surfaces between the sample
and the tooling. Moreover, there is a free edge and unconstrained plastic flow along the lateral surface of the cone
that occurs. The lower friction in free region allows slipping
between the sample and the die/the punch, leading to a significant reduction of strain in the region close to the sample
edge.
2.4
Extension of TE
2.4.1 Axi-symmetric forward spiral extrusion
A modified axi-symmetric forward spiral extrusion (AFSE)
[22, 23] method was proposed with a designed die. A half
section of three-dimensional schematic of the AFSE die is
shown in Figure 6. A number of engraved spiral grooves are
responsible for guiding material flow along the groove (
direction) as the material enters the chamfer and the twist
die. The novel structure of the die can avoid the slippage
[24] between die and material, and improve the efficiency
(6)
Figure 6
Different zones in AFSE die [22].
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of the strain accumulation.
The induced effective strain during AFSE can be represented as
 
r tan 
3r0
.
(7)
The helix angle  can be expressed by the formula of
=arctanL/2r0 [23], where L and r0 are the lead and radius
of the helix, respectively. The formula (7) implies that the
total effective strain seems to depend on the radius and helix
angle only and therefore is independent of the chamfer geometry.
Due to the rotation of the material during the process,
there is no need for a route change, unlike ECAP, to ensure
a good aspect ratio of the deformed grains. In addition, the
kinematic and experimental studies have shown that the
deformation develops away from the AFSE die-sample interface. A linear gradient of velocity mainly exists in the
radial direction, and has nearly no change in the sample
cross-section during AFSE processing. Furthermore, the
gradient produces a pure shear deformation, which is active
on a plane normal to the extrusion axis. This new SPD process seems to have good potential for application in both
continuous and batch modes [22].
2.4.2 Vortex extrusion
Shahbaz deemed that the single-pass SPD process with the
change of geometrical dimensions and shape of samples
was better than the general multi-pass SPD process with
unchanged sample. Therefore, his team proposed a single-pass SPD technique named vortex extrusion (VE) [25]
via a special torsional extrusion die (in Figure 7). In the VE
processs, the sample passes throughout three different zones:
Zone I, Zone II and Zone III. Along Zone I, the circular
cross-section gradually changes into a dented pattern
(shown in Figure 8). In Zone II, this pattern gradually rotates along the center axis with the twist angle . Finally,
the dented section comes back to the circular shape in Zone
III. In general, conventional extrusion and AFSE mentioned
above [22, 23] can be considered special forms of VE with
zero twist angles and no reduction in the area, respectively.
Compared with conventional extrusion, the FE analysis
has revealed that VE can produce uniform strain distribution
along diameters of rod-sample. Therefore, VE, as a promising single pass SPD technique, can be easily installed on
Figure 7
A Scheme of the VE die [25].
Figure 8 Gradual change in VE die transverse section along deformation
zone (zone I, II and III) [25].
any standard extrusion equipment with no need of any additional facilities.
2.4.3 Simple shear extrusion
Based on pressing the material through a designed direct
extrusion die, simple shear extrusion (SSE) [26, 27] was
introduced by Pardis and Ebrahimi. As illustrated in Figure 9, the material processed by SSE undergoes shear deformation of square cross-section shape at the channel inlet
to a parallelogram shape with the maximum distortion angle
of  in the middle of the channel, and backs to the same
square at the outlet. Since the shape and geometrical dimensions of the deformed sample remain constant, the SSE
process allows the operation to repeat successively.
The shear strain  and effective strain value  for SSE
processing can be calculated as follows [26]:
  2 tan  ,
 

3

2 3 tan 
.
3
(8)
(9)
The experimental results of SSE process for aluminum
samples have shown that SSE method can accumulate great
strain by repeating the process. A symmetrical distribution
of strain in sample’s cross-section is superior to those of
other SPD techniques.
Figure 9
Schematic illustration of the SSE process [26].
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2.4.4 Planar twist extrusion
The die channel of planar twist extrusion (PTE) [28] has a
middle part, whose cross-section varys from rectangular to
parallelogram and then backs to rectangular along the die
axis (Figure 10(c)). That is different from the principle of
TE [29] (Figures 10(b) and (d)). The curved surfaces of
PTE die are ruled surfaces formed by moving a straight line
along two directrixes created by two straight segments conjugated with arcs (Figure 10(a)).
The schematics of PTE equipment are shown in Figure 11,
where numbers from one to five represent the sample, plunger, curved stationary walls, backpressure plunger and channel-filling sample, respectively. The sample is pushed
through the die channel with two curved and two flat walls.
The plunger feature of the h-beam cross-section has many
benefits, i.e., improving the stability of plunger, increasing
the sample length and reducing the friction forces. The load
is transferred from the main and backpressure plungers to
the sample itself through two channel-filling samples. The
backpressure applied assures the material to fill in the die
Figure 10 Schematics of (a) PTE and (b) TE die channels; gradual
cross-sections change of the die channel for (c) PTE and (d) TE [28].
Figure 11
PTE die with two flat movable walls [28].
September (2012) Vol.55 No.9
channel. Moreover, the pressure also raises the hydrostatic
pressure to benefit improving metal plasticity and grain refinement.
Both experiments for aluminum samples and FEM analysis have shown that the required minimum backforce of
PTE is 100 kN, which is high than that of TE processing.
For characteristics of the strain distribution of PTE, the
isostrain contours are elongated along the shorter side of the
rectangular cross-section of the sample. Furthermore, microstructure examinations of samples have revealed an intensive microscale mixing (about 10 m) and no any macro-scale mixing. The mixing can be attributed to the vortex-like character of metal plastic flow during simple shear
deformation.
2.4.5 Elliptical cross-section spiral equal-channel extrusion
A new SPD method, named elliptical cross-section spiral
equal-channel extrusion (ECSEE) [30], can aggregate the
torsion shear, extrusion and upsetting deformation. The
schematic diagram of the sample via ECSEE deformation is
shown in Figure 12. A round-bar sample is extruded out
through the die with three channel regions: round-ellipse
cross-section transitional channel L1, elliptical cross-section
torsion transitional channel L2 and ellipse-round crosssection transitional channel L3. During the ECSEE process,
the sample undergoes severe deformation while maintaining
its initial cross-section. The diameter D1 is the round crosssection of the sample before and after deformation. This
feature allows the sample to extrude repeatedly for deformation accumulation, which is essential for refining microstructure and improving properties of the material.
The simplified slice-plain-strain method and incorporating incremental superposition theory have been adopted to
calculate the cumulative effective strain of ECSEE process.
The ECSEE deformation is divided into two basic deformation modes: round-ellipse/ellipse-round cross-section
transitional channel deformation and elliptical cross-section
torsion transitional channel deformation, respectively.
Through tracking a particle of the cross-section, the cumu-
Figure 12
Schematic diagram of the blank via ECSEE deformation.
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lative effective strain of these two deformation modes could
be calculated respectively as
1 
2 
i
1
3
 Dk
3L2
 ln
k 1
=
mk (1  mk21 tan 2  )
,
mk 1 (1  mk2 tan 2  )
 R1
3L2 cos 
1  m
mi
2
i
tan 2 
(10)

,
2.5
Extension of CGP
Cyclic forward-backward extrusion (CFBE) [31], as a novel
SPD method, is classified into the extension of CGP because of their similar shear deformation modes. The CFBE
process applied to blocks and bars is different from the CGP
process mainly used for plates. The installation consists of
four parts: die, forward punch, outer punch and backward
punch (shown in Figure 13). One cycle of the CFBE process
includes two steps. In the first step, the forward punch and
backward punch extrude into the bottom small chamber,
while the outer punch moves up. In the second step, the
sample is pressed back by the backward punch and outer
punch, while the forward punch is loosely lifted up. Since
Figure 13
The principle of CFBE process [31].
the initial shape of the sample is reproduced at the end of
each cycle. The performance allows repeating, and a large
strain is accumulated in the sample ultimately.
The accumulated effective strain after N cycles of the
CFBE process is given as follows [31], with some parameters shown in Figure 13.
(11)
where m is the ratio of major-axis and minor-axis length for
the ellipse-cross-section of rod-sample,  is the torsion angle,  is the angle of the line connecting the center and a
particle with the line of elliptic major-axis. R1 is the radius
of the cross-section of initial blank. Subscripts k, k1 and i
identify the states of random time.
The simulated results have shown that the ECSEE accumulation torsion strain is greater than other deformation
forms, and the shear deformation is dominant. The strain
gradient of the cross-section shows the decreasing trend
from the periphery to the center. It is believed that the prospects of ECSEE technology for industrial applications will
be broader with further research.
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2
 2

  2 N ln  D  42 d  4 Dd  .

5d  4 Dd

(12)
Figure 14 shows the shearing patterns of CFBE schematically. The shearing deformation, important to the grain refinement of SPD-ed material, takes place near the forward
punch tip and the corner of bottom die. During each cycle of
CFBE process, the material passes through these channels
twice, i.e., in the first-half cycle and the second-half cycle,
respectively. Therefore, the CFBE shearing deformation is
effectual in the grain refinement. In addition, the CFBE
process for AA1050 aluminum alloy has revealed that a fine
grain with the size below 1 m and a significant increase in
the hardness, by a factor of 2, are obtained after a single
cycle of CFBE process.
3 Microstructure characteristic of SPD materials
3.1
High-angle grain boundary
The high-angle grain boundary (HAGB) is the misorientation of more than 15° between adjacent grains [32]. Many
studies show that the grains of material in SPD continuously
incline, rotate and gradually grow to be HAGDs [32–34].
The SPD process refines grains by introducing large numbers of dislocations, which arrange themselves in low-energy configurations and in the form of low-angle GBs.
Subsequently, the grains evolve, with additional strain, into
Figure 14 Deformation channels and shearing patterns during CFBE
processing [31].
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reasonably homogeneous arrays of ultrafine grains, which
are separated by boundaries with high angles of misorientation. The evolution of microstructure during ECAP of Fe
Grade 1 is illustrated in Figure 15 [35]. The grains are
elongated along the pressing direction with an angle of
about 27.6° after the first pass (Figure 15(b)). After the
eighth pass, the perfect circular rings in selected-area electron diffraction patterns reveal that the misorientation of the
GBs is of a high angle (Figure 15(d)).
3.2
Deformation texture
In the SPD process, the polycrystalline grains will gradually
turn into more consistent orientation grains, and then form
the deformation texture. Texture type and intension have a
direct relationship with the specific processing technology.
For instance, HPT-ed material has fibrous texture [33].
ECAP-ed material has a multiple texture, while the ARB-ed
board material has a distinct sheet texture [36]. Texture in
subsequent recrystallization annealing process would
transmit the recrystallization formation texture to new ones.
The drastic deformation textures in bulk UFG materials
greatly influence the microstructure and property of materials. Currently, researches on deformation texture of material
by SPD process are relatively scarce; therefore, it is necessary to do further work in this area [33]. As illustrated in
Figure 15, the grade has a ferrite grain without any apparent
preferred orientation (Figure 15(a)). After the first pass, the
grains are elongated along the pressing direction after severe shear deformation (Figure 15(b)). With the increase of
deformation, the deformation texture is more obvious as
shown in Figure 15(c).
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4 Mechanical properties of SPD materials
4.1
Strength and ductility
According to the Hall-Petch relationship, UFG materials
processed by SPD have extremely high strength and low
ductility compared with conventional coarse counterparts.
Many researches have found that the strength of the material deformed by SPD process increases gradually with the
strain increasing, while its ductility decreases slightly under
the same condition [17, 37, 38]. However, in the Armcosteel ECAP experiments by Segal [38], the results showed
that compared with the counterparts of conventional deformation processes, such as rolling, stamping and extrusion,
the reducing degree of ductility is relatively small. In addition, recent studies of a series of aluminum alloys (1100,
2024, 3004, 5083, 6061, and 7075) demonstrated that the
ductility of these alloys had been enhanced by 10%–25%
after ECAP [39]. Horita et al. [39] also found that the stability of material ductility would be superior to the counterparts of conventional cold rolling.
In general, the desired and expected materials are anticipated to have both good ductility and high strength. Suprisingly, some specific UFG materials obtained by the SPD
process have not only the strength enhancing but also the
ductility improving. This phenomenon is so-called inverse
Hall-Petch effect [40]. As shown in Figure 16 [41], the extraordinary combination of both high strength and high ductility in nanostructured Cu and Ti processed by SPD clearly
sets them apart from coarse-grained metals. Wang et al. [42]
deduced that in cryorolled Cu sample, micrometer-sized
grains embedded inside a matrix of nanocrystalline and ultrafine grains, which led to an improved combination of
Figure 15 Microstructure of Fe grade 1 (a) after annealing at 1203 K for 1 h before ECAP, (b) after the first pass, (c) after the second pass along the longitudinal direction, and TEM after eight passes [35].
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cess. The content of HAGBs increases the participation of
grain boundary (GB) sliding and superplastic flow [49]. In
the main trend of researches on the superplasticity of UFG
materials at present, GB sliding is the mainly recognized
and accepted deformation mechanism [50]. The fine grains
of UFG materials benefit the GB sliding, so the material has
better superplasticity.
4.3
Figure 16 Yield strength and elongation to failure for different materials
processed by cold rolling or SPD [41].
strength and ductility. Valiev et al. [43] gave the reason that
the generation and movement of dislocations cannot be
produced in UFG structure of materials via SPD.
4.2
Superplasticity
Superplasticity of materials processed via SPD has very
high ductility providing hundreds and thousands of percentage elongation during tension. Recently, SPD methods
have been widely used in obtaining good superplasticity for
materials of Al, Mg, Cu and Ti based alloys [44–50]. In
addition, the UFG materials obtained by SPD process not
only get good superplasticity under appropriate conditions
but also achieve high strain rate superplasticity and/or low
temperature superplasticity.
Many explainations try to unveil the mysterious puzzles
of the relationship between the superplasticity and SPD deformation mechanism. A majority of scholars affirm that the
good superplasticity is related to the ultrafine grains of materials obtained by SPD, and the grains are small enough to
ensure the superplasticity [44, 45]. Garcĺa-infanta et al. [45]
found that the grains of 7075 Al were refined to 100–150
nm, and attained high strain-rate superplasticity in the range
of 250–300°C in the HPT process.
Some researchers believed that the alloying elements of
Sc and Zr, in Al and Cu materials, could generate Al3Zr and
CuZr5 deposition or additional phases to prevent the grain
growth. Therefore, the materials have the thermal stability
of grain sizes [46–48], which greatly affects the superplasticity of materials. This phenomenon has been seen in
the Al-3% Mg-0.2% Sc alloy [46], Cu-Zn alloy [47] and
Al-7034 [48] by ECAP.
In addition, the characteristics of special HAGBs and
microstructures of materials fabricated by SPD technology
are the main reason of superplasticity obtained in SPD pro-
Applications
UFG metals processed by SPD are used as a structural material due to the properties of high strength, ductility and
fatigue characteristics. Several metals and alloys have been
processed by SPD for structural applications (some pictures
shown in Figure 17 [51]). In Russian, the titanium screws of
ECAP preparations have been applied to automotive and
aerospace industries [51, 52]. In addition, some SPD micro-nano-materials were applied to surgery device for fixing
broken-bones [53]. Roven [54] predicted the main areas of
industrial applications of SPD metallic materials include
biomedicine (especially high-strength titanium), aerospace,
defense, sports, energy, electronic equipment (the splash
shooting target of the LCD groups manufacture), micro-wire (aluminum alloys) and especial materials used in
ultimate low temperature condition. The excellent strength
and good ductility of UFG material produced by SPD
methods are becoming especially attractive in many applications.
5 SPD deformation/refinement mechanism
Valiev et al. [33] believed that non-equilibrium GBs and
subgrain boundaries existed in UFG/nano-structured metals
fabricated by SPD process, due to the presence of a high
density of extrinsic defects in their structure. Non-equilibrium GBs are characterized by excess energy and long
range elastic stresses. These stresses result in significant
distortions and dilatations of crystal lattices near GBs. These several types of extrinsic defects are assumed to present
in non-equilibrium GBs with Burgers vectors, normal to a
boundary plane. Moreover, the severe lattice distortions,
extremely high-density dislocations and energy exist in these grains or subgrain boundaries. Therefore, the boundaries
are very unstable, and then trend to be of low energy and
balance GBs [55, 56]. The structural model of non-equilibrium grains with grain size of about 100 nm, and the distortions and dilatations of the crystal lattice can be described
by Figure 18. The elastic distorted region with a width of
several nanometers near the crystal GBs, as well as the central portion of grain with perfect crystal lattices coexists in a
limited range of grain size. If the grain size is reduced to a
critical value, then the distortions and dilatations of the
crystal lattice will embrace the whole grain. In this case, the
lattice lost its strict periodicity.
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Sci China Tech Sci
September (2012) Vol.55 No.9
Figure 17 Application of productions processed by SPD [51]: (a) Bolts manufactured with SPD Ti alloys; (b) micro bolts manufactured UFG Carbon steel
processed by ECAP; (c) worn out UFG 300 mm sputtering target; (d) “piston” type fabricated from nanostructured Al1420; (e) a micro-bulged sheet made of
CG and UFG Al 1070; (f) plate implants made of nanostructured titanium; (g) UFG C-Mn steel sheet forming; (h) hollow blades made of UFG Ti-6Al-4V
sheet.
Figure 19
Figure 18 Schematic representation of UFG structure (a) and nanocrystal
with dislocations disturbed GBs and highly distorted crystal lattice (b) [33].
5.1
Dislocation deformation mechanism
Ref. [57] proposed the following model of evolution of the
defect structure of materials during SPD process (Figure 19).
The main idea is based on the transformation of a cellular
structure to a granular one. High-density dislocations based
on different slip system movements, is impelled by the intense shear stress. When the dislocation density in the cell
walls achieves a certain critical value, part of dislocations
with different signs can annihilate at the cell boundaries. As
a result, excess dislocations of single sign remain and form
a Y-shaped dislocation wall. The dislocations with Burgers
Schematic of dislocation evolution model [57].
vectors, perpendicular to the boundary, lead to the increase
of misorientation and cause the transformation to a granular
structure when their density increases. The shear zones discovered in ECAP, HPT, SE, ARB and RCS processes, increase cross and proliferation, resulting in the microstructure to break. The coarse grains gradually evolve into equiaxed cells and subgrain structures. The shear-stress dislocation model is recognized as the SPD refinement mechanism.
5.2
Twin deformation mechanism
Some researchers found twins as well as dislocations in
some early SPD process simulations [58], and believed that
the deformation of nano-metal with high stacking fault energy was mainly caused by twinning mechanism [59]. The
twins associated with the slip are found in slip deformation
Wang C P, et al.
Sci China Tech Sci
process of face-centered cubic (FCC) metals within some
simulation models. Some experiments also validated that
twinning is an important mechanism in the UFG copper
with grain size less than 300 nm. Yamakov et al. [59] and
Zhu et al. [60] further studied the interaction between twin
GBs and dislocation slip in the FCC cobalt nanocrystals.
They discovered that twin deformation had a dual role in the
mechanical properties of nano-materials. In the early deformation stage, it could increase additional slip system, or
promote slip system migration with the interaction of twins
and dislocations, and thus conducted the deformation. In the
later deformation stage, twin GBs could hinder certain types
of slip dislocations to accumulate dislocations, and caused
the material strain hardening.
5.3 Grain boundary sliding and torsional deformation
mechanism
Gutkin et al. [61, 62] deemed that the deformation of
nano-metal crystals was due to the role of GB sliding and
torsional deformation mode. The slip dislocations in the
trigeminal GBs repeatedly break down into the climbing
ones along the adjacent GBs. This process causes the GB
dislocation walls to climb and the lattice in metal nanograins to deform by rotation transmission.
A model describing the combined action of GB sliding
and rotational deformation on the crossover from GB sliding to rotation deformation occurring at triple junctions of
GBs is shown in Figure 20. In the non-deformed state of the
nanocrystalline specimen, GB sliding occurs via motion of
gliding GB dislocations under shear stress action. Then the
gliding dislocations split at triple junction of GBs into
climbing dislocations. The splitting of gliding GB dislocations repeatedly occurs to cause the formation of walls of
GB dislocations whose climb is accompanied by crystal
lattice rotation in a grain. Ultimately, the climbing dislocations reach triple junction, where they converge into gliding
dislocations to cause further GB sliding.
5.4
Other mechanisms
1) Thermo-mechanical deformation mechanism
In the MDF process [63], the thermo-mechanical deformation refinement is the main deformation mechanism.
Though the deformation temperature of MF process is lower
Figure 20
September (2012) Vol.55 No.9
2387
than 0.5Tm, the cumulative plastic deformation leads to the
decrease of dynamic recrystallization temperature; the actual MDF processing is performed in the range of dynamic
recrystallization.
2) Particle refinement mechanism
The extrusion and upsetting process of SPD make the
material particles flow along the axis and plane, similar to
“kneading” in our life [64]. The reciprocating upsetting and
squeezing urges the particles of material to mix and reallocate. The grains and particles will be collided and broken,
the holes will be closed, and the particle size in materials
becomes smaller.
3) Hypothesis of simple shear double-stage mechanism
Simple shear [65] lies in some effective SPD processes
of HPT, ECAP and TE. Beygelzimer proposed a hypothesis
of two-stage deformation via the simple shear (Figure 21)
[66]. At the first stage (Figure 21(b)) (the shear strain range,
0 <  < c, where c is the critical shear strain), the metal
microstructure changes in the way similar to that during
elongation. At the second stage with  > c (Figure 21(c)),
accidental multi-scale rotative motions, similar to turbulent
motions in liquids, take place in the metal. The stationary
turbulent motion rapidly blurs the boundaries of the initial
grains. Though the total length of boundaries does not increase, the fragments get free of dislocations by the way of
rotations. The movement can cause rapid and mass transport.
Therefore, there is no strain hardening observed at this stage.
The authors supposed that the stationary turbulent motion
during simple shear distinguishes SPD from large plastic
deformation.
6 Current research issues
Since the 1980s when Segel proposed ECAP [33], SPD
technique as the preparation method of UFG/nc materials
has become the research focus in the material science area.
Through nearly three decades development, especially the
intensive research in the last decade, SPD techniques have
involved the material preparation, macro-micro simulation,
structural characterization, microstructure evolution mechanism, texture analysis, physical and mechanical properties,
etc. [1, 2, 33, 52]. Figure 22 illustrates the SPD development from the sprouting to the future.
The leading representatives of SPD research are in
Grain-boundary sliding and torsional deformation model [61].
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Figure 21
Sci China Tech Sci
Scheme of simple shear double-stage model [66].
September (2012) Vol.55 No.9
stride out of the laboratory, and into the mass production
and commercialization [1, 2]. Conventional SPD techniques
are progressively improved into the modified manufacturing
processes for sheets, rods, wires and other kinds of
semi-manufactured blocks. In all the SPD methods, ECAP
may be the most successful SPD technique for the significant potential of commercializing [68]. With some continuous SPD processes progressively proposed [2, 38], it is
believed that the future SPD-ed material will have a finer
grain with more stable and uniform organization, and the
prepared material shape will be larger for applications.
7 Ongoing research and future directions
Figure 22
The SPD development stage.
Russia, Japan and the U.S. and other developed countries,
such as, RValiev of Russia, Langdon and Los Angeles team
of USA, Zehetbauer in Austria, Maier and Paderborn in
Germany, and Zenji Horita in Japan. SPD techniques are
attracting more and more experts and scholars all over the
world. China commenced its study on SPD in the early 21st
century. Many Chinese universities and institutes are mainly
engaged in imitation investigations, e.g., the relationship
between SPD process parameters and microstructural performance. The gap of SPD study includes less original innovation, more follow-up study, and the insufficient basic
research on related engineering applications. For the
short-term SPD development, China should seize the opportunity to promote SPD research following the worldwide
advanced level.
In the laboratory, SPD techniques have successfully prepared some UFG/nc pure metals, alloys and intermetallic
compounds [33]. It is necessary to expand the additional
performances and features of commercial alloys and composite materials, e.g., high strength and ductility, superior
fatigue performance and superplasticity [67]. Recent SPD
techniques try to prepare materials from single-crystal to
polycrystalline, single-phase to multiphase, easy-to-deformed to difficult-to-deformation, conventional metallic
material to the powder material.
Recently, SPD techniques of UFG materials gradually
The current SPD study focuses on the process design, microstructure evolution and mechanical properties. Whereas
the material deformation mechanisms, grain refinement
mechanism and factors affecting technique are dazed in the
SPD research area. Ongoing research and future directions
of SPD technology mainly have three objectives as follows.
1) Methods engineering
The special SPD equipment and settings lead to higher
cost due to the strict SPD process conditions. Though the
current SPD method can produce UFG materials, the bulk is
small and the grain size is large. Moreover, the homogeneity of structure and the stability of performance for SPD materials are unsatisfactory, such as UFG materials prepared
by TE, the refinement in the central area and the surrounding area are inconsistent [1]. The continuous production of
large-scale industrialization is difficult to achieve, hence
SPD techniques are expected to be simple, to have high
efficiency, economic feasibility, and wide applicability, and
to be able to obtain a finer grain size (<50 nm), more stable
structure and properties of bulk UFG materials for industrial
applications.
2) Further mechanism study
The SPD refinement mechanism is so complicated that
no a unified theory system is formed. Three kinds of grain
refinement mechanisms, strain-induced grain refinement
[57, 59, 69], thermal mechanical deformation refinement
[63, 70] and particle refinement [64, 71], are popular in
current SPD research. Further investigations should be
made to the mechanism. Focus on the motion characteristics
of metal atoms and crystal defects (dislocations and vacancies), the deformation model and theory of micro-nano materials are established by means of the advanced computer
simulation techniques and mocro-nano-scopic scale analysis
methods, combined with the mechanical properties and microstructure characteristic parameters of materials. In the
SPD process, non-equilibrium GB, the space distribution of
deformation textures and recrystallization genetic behavior
are the future research directions. In addition, the stability
of structures (grain, phase, particle and structures), and GBs
and crystal structure defects influencing the mechanics
Wang C P, et al.
Sci China Tech Sci
properties are noteworthy, too.
3) Performance integration
Since UFG/nc materials prepared by SPD have a unique
excellent performance, the traditional laws of material
properties also face a great challenge, e.g., for nc materials
obtained by some SPD methods, when the grains are less
than some critical value [72, 73], the mechanical properties
would deviate from the Hall-Petch relationship. At present,
the performance and measurement of SPD materials don’t
have a quantitative standard requirement, so the performance
of prepared materials can not be effectively controlled.
Some SPD-produced UFG/nc materials with a grain size
about 100 nm or less exhibit very high hardness and
strength but low ductility. However, the recent findings of
extraordinarily high strength and ductility in a number of
SPD-produced nanostructured metals have a special interest.
Hence, investigating the relationship of the deformation
mechanisms, microstructure evolution and processing routes
and regimes is necessary to the considerable enhancement
of properties in several SPD-produced metals and alloys.
8 Conclusions
SPD technology is a satisfying and promising method for
fabricating bulk UFG materials. However, the SPD technology is in the early stage of development and a scenario
of competition and consolidation. The recent developments
of UFG/nc materials by SPD depend upon the concurrent
evolution of new characterization techniques, novel metal-forming methods, and increasingly sophisticated models
for interpreting measured properties. Considering the commercial application of UFG/nc materials, one prerequisite
task is the development of new SPD techniques. Furthermore, the establishment of SPD process guidelines should
become an urgent topic in order to obtain new advanced
properties of materials in commercial application.
This work was supported by the Aeronautical Science Foundation of China
(Grant No. 2011ZE53059) and the Graduate Starting Seed Fund of
Northwestern Polytechnical University (Grant No. Z2011006).
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