The new trends in SPD processing to fabricate bulk nanostructured materials
During the last decade severe plastic deformation (SPD) has become a widely known method of materials processing used for fabrication of ultrafine-grained materials with attractive properties. Nowadays SPD processing is rapidly developing and is on the verge of a transition from lab-scale research t...
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Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України
2006
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| Цитувати: | The new trends in SPD processing to fabricate bulk nanostructured materials / R.Z. Valiev // Физика и техника высоких давлений. — 2006. — Т. 16, № 4. — С. 9-22. — Бібліогр.: 60 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859981930300178432 |
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| author | Valiev, R.Z. |
| author_facet | Valiev, R.Z. |
| citation_txt | The new trends in SPD processing to fabricate bulk nanostructured materials / R.Z. Valiev // Физика и техника высоких давлений. — 2006. — Т. 16, № 4. — С. 9-22. — Бібліогр.: 60 назв. — англ. |
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| container_title | Физика и техника высоких давлений |
| description | During the last decade severe plastic deformation (SPD) has become a widely known method of materials processing used for fabrication of ultrafine-grained materials with attractive properties. Nowadays SPD processing is rapidly developing and is on the verge of a transition from lab-scale research to commercial production. This paper focuses on several new trends in the development of SPD techniques for effective grain refinement, including those for commercial alloys, and presents new SPD processing routes to produce bulk nanocrystalline materials.
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Физика и техника высоких давлений 2006, том 16, № 4
9
PACS: 62.72.Bb
R.Z. Valiev
THE NEW TRENDS IN SPD PROCESSING TO FABRICATE
BULK NANOSTRUCTURED MATERIALS
Institute of Physics of Advanced Materials, Ufa State Aviation Technical University
K. Marx Str., 12, Ufa, 450000, Russia
E-mail: rzvaliev@mail.rb.ru
During the last decade severe plastic deformation (SPD) has become a widely known
method of materials processing used for fabrication of ultrafine-grained materials with
attractive properties. Nowadays SPD processing is rapidly developing and is on the
verge of a transition from lab-scale research to commercial production. This paper fo-
cuses on several new trends in the development of SPD techniques for effective grain re-
finement, including those for commercial alloys, and presents new SPD processing routes
to produce bulk nanocrystalline materials.
1. Introduction
Recent years have seen growing interest in developing SPD processing to fab-
ricate bulk nanostructured metals and alloys with unique properties [1−4]. This
approach as an alternative to nanopowder compacting is based on microstructure
refinement in bulk billets using SPD: that is, heavy straining under high imposed
pressure [1]. SPD-produced nanomaterials are fully dense and their large geomet-
ric dimensions make it possible to perform thorough mechanical tests, and this is
attractive for efficient practical applications. Fabrication of bulk nanostructured
materials by severe plastic deformation is becoming one of the most actively de-
veloping areas in the field of nanomaterials [5,6]. SPD materials are viewed as
advanced structural and functional materials of the next generation of metals and
alloys [7].
Today, SPD techniques are emerging from the domain of laboratory-scale re-
search into commercial production of various ultrafine-grained materials. This
change is manifested in several ways. First, it is characterized by the fact that not
only pure metals are investigated, but also commercial alloys for special applica-
tions; second, by the requirements of economically feasible production of ultra-
fine-grained metals and alloys. This paper considers these new trends in SPD
processing and highlights some recent results on the development of the pilot
Физика и техника высоких давлений 2006, том 16, № 4
10
commercial production of Ti materials for medical use. We also report here new
results on finding novel SPD processing routes used to produce bulk ultrafine-
grained materials with a small grain size refined down to a typical nanorange of
40−50 nm and less.
2. 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. Moreo-
ver, introduction of a high density of dislocations in SPD-processed nanometals
may result in even greater hardening. However, all this normally decreases ductil-
ity. 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. 1),
but this task calls for original approaches [8−11].
One such new approach to the problem was suggested recently by Wang et al.
[10]. 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 re-
sult 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 behavior 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 zinc [12], copper [13], and aluminium
alloy [14]. What is more, the investiga-
tion of copper [13] has shown that bi-
modal structures can increase ductility
not only during tensile tests, but also
during cyclic deformation. This obser-
vation is important for improving fatigue
properties.
Another approach suggested recently
[15] is based on formation of second-
phase particles in the nanostructured
metallic matrix, which modify shear-
band propagation during straining,
thereby increasing the ductility. A sys-
Fig. 1. Strength and ductility of the na-
nostructured metals compared with
coarse-grained metals. Conventional cold
rolling of copper and aluminium in-
creases their yield strength but decreases
their ductility. The two lines represent
this tendency for Cu and Al and the %
markings indicate a percentage on roll-
ing. In contrast, the extraordinarily high
strength and ductility of nanostructured
Cu and Ti clearly set them apart from
coarse-grained metals [8]
Физика и техника высоких давлений 2006, том 16, № 4
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tematic 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 introduced in [1,8] is based on formation of ultrafine-grained
structures with high-angle and non-equilibrium grain boundaries capable of grain-
boundary sliding (GBS). It is well known that sliding, which increases ducility,
normally cannot develop at low-angle boundaries. The importance of high-angle
grain boundaries was verified in work [8] on the 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 ex-
traordinary influence of annealing temperature on mechanical behavior found re-
cently in nanostructured titanium produced by high-pressure torsion (HPT) [16].
Here, a short annealing at 300°C results in a noticeable increase in strength com-
bined 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
nanometals [1,8,17]. High strain-rate sensitivity indicates viscous flow and plays a
key role in superplasticity in materials [18], but on the other hand it is associated
with the development of grain-boundary sliding, and therefore depends on grain-
boundary structure. This fact is in agreement with the recent results of computer
simulation and studies of deformation mechanisms active in nanostructured met-
als. Such molecular dynamics simulations have provided valuable insight into the
deformation behaviour of nanometals [19−21].
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 dislocations [20−23]. Moreover, the
sliding may have a co-operative (grouped) character similar to that observed in
earlier studies on superplastic materials [24,25]. It should be stressed that recent
experiments investigating deformation mechanisms in nanostructured materials
have confirmed a number of the results of computer simulation [16,26,27].
However, there is a question: why should grain-boundary sliding in nanos-
tructured materials, in particular in those produced by SPD, take place at rela-
tively 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 boundaries [28,29]. So perhaps grain-boundary sliding is
easier in these ultrafine-grained metals and develops during straining even at
Физика и техника высоких давлений 2006, том 16, № 4
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lower temperatures, producing increased ductility. It is well known that enhanced
sliding in nanostructured metals can lead even to superplasticity at relatively low
temperatures [30].
Processing of nanomaterials to improve both strength and ductility is of pri-
mary importance for fatigue strength and fracture toughness [13,31,32]. An ex-
traordinary 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 phenomenon [31,32].
3. Developing SPD techniques for effective grain refinement
HPT and equal-channel angular pressing (ECAP) are the SPD techniques that
were first used to produce nanostructured metals and alloys possessing submi-
cron- or even nano-sized grains [33,34]. Since the time of the earliest experi-
ments, processing regimes and routes have been established for many metallic
materials, including some low-ductility and hard-to-deform materials. HPT and
ECAP die sets have also been essentially modernized [2,4]. Moreover, in recent
years new SPD techniques have been developed, first of all twist extrusion [42],
accumulative roll-bonding [43] and some others [2].
However to date, these techniques have been usually used for laboratory-scale
research. The requirement of economically feasible production of ultrafine-
grained metals and alloys that is necessary for successful commercialization raises
several new problems in the SPD techniques development. The most topical tasks
are to reduce the material waste, to obtain uniform microstructure and properties
in bulk billets and products, and to increase the efficiency of SPD processing.
We solve these tasks by developing continuous ECA-pressing [35] and multi-
step combined SPD processing [36] for fabrication of long-sized rods aimed at
setting up commercial production of nanostructured Ti materials for medical ap-
plications. Some new results of these works are presented below.
3.1. Continuous ECA-pressing
So far, among all SPD techniques, ECAP, also known as equal-channel angu-
lar extrusion (ECAE) [37], has attracted most attention, because it is very effec-
tive in producing UFG structures and can be used to produce UFG billets suffi-
ciently large for various structural applications [1,2,4].
However, the ECAP technique in its original design has some limitations, in
particular, a relatively short length of the workpiece that makes ECAP a discon-
tinuous process with low production efficiency and high cost. In addition, the ends
of a workpiece usually contain non-uniform microstructure or macro-cracks and
have to be thrown away, thus a significant portion of the workpiece is wasted and
the cost of the UFG materials produced by ECAP is further increased. The key to
wide commercialization of UFG materials is to lower their processing cost and
waste through continuous processing. Several attempts have been made to this
end. For example, repetitive corrugation and straightening (RCS) [38,39] has been
Физика и техника высоких давлений 2006, том 16, № 4
13
recently developed to process metal
sheets and rods in a continuous manner.
The co-shearing process [40] and the
continuous constrained strip shearing
(C2S2) process [41] were recently also
reported for continuously processing thin
strips and sheets to produce UFG struc-
tures. However, the question of further
improvement of microstructure uniform-
ity and properties remains topical in the
development of these techniques.
In our recent studies, we have
worked on combining the Conform pro-
cess with ECAP to continuously process
UFG materials for large-scale commer-
cial production [35]. In this invention, the principle used to generate frictional
force to push a workpiece through an ECAP die is similar to the Conform process,
while a modified ECAP die design is used so that the workpiece can be repeti-
tively processed to produce UFG structures.
We have designed and constructed an ECAP-Conform set-up which is sche-
matically illustrated in Fig. 2. As shown in this figure, a rotating shaft in the cen-
ter contains a groove, into which the workpiece is fed. The workpiece is driven
forward by frictional forces on the three contact interfaces with the groove, which
makes the workpiece rotate with the shaft. The workpiece is constrained to the
groove by a stationary constraint die. The stationary constraint die also stops the
workpiece and forces it to turn an angle by shear as in a regular ECAP process. In
the current set-up, the angle is about 90°, which is the most commonly used chan-
nel intersection angle in ECAP. This set-up effectively makes ECAP continuous.
Other ECAP parameters (die angle, strain rate, etc.) can also be used.
In our work [35] we used commercially pure (99.95%) coarse-grained long Al
wire with a diameter of 3.4 mm and more than 1 m in length for processing at
room temperature with 1−4 passes using
ECAP route C, i.e. the sample was rotated
180° between ECAP passes. The starting Al
wire had a grain size of 5−7 µm. Presently
we are working on processing similar rods
from CP Ti (Grade 2).
Fig. 3 shows an Al workpiece at each
stage of the ECAP-Conform process, from
the initial round feeding stock to rectangu-
lar Al rod after the first ECAP pass. As
shown, the rectangular cross-section was
formed shortly after the wire entered the
Fig. 2. Schematic illustration of an
ECAP-Conform set-up
Fig. 3. Al workpiece in the process of
ECAP-Conform
Физика и техника высоких давлений 2006, том 16, № 4
14
groove (see the arrow mark). The
change was driven by the frictional
force between the groove wall and the
Al workpiece. The frictional force
pushed the wire forward, deformed the
wire to make it conform to the groove
shape. After the wire cross-section
changed to the square shape, the fric-
tional force per unit of wire length be-
came larger because of larger contact
area between the groove and the wire.
The total frictional force pushed the
wire forward from the groove into the
stationary die channel, which intersects
the groove at a 90° angle. This part of
the straining process is similar to that
in the conventional ECAP process.
TEM observations showed that the ECAP-Conform led to microstructure
evolution typical of the ECAP process [43,44]. Fig. 4 clearly indicates that the
ECAP-Conform process can effectively refine grains and produce UFG structures
in Al and now in CP Ti. The tensile mechanical properties of the as-processed Al
samples after 1 to 4 passes are listed in Table 1. It is obvious that the ECAP-
Conform process has significantly increased the yield strength (σ0.2) and the ulti-
mate tensile strength (σu), while preserving a high elongation to failure (ductility)
of 12−14%. These results are consistent with those for Al processed by conven-
tional ECAP. We also found that for CP Ti there is strength growth by more than
2 times after the processing as compared with the initial material, and this fact is
also consistent with Ti subjected to conventional ECAP.
Thus, the newly developed continuous SPD technique, ECAP-Conform can
successfully produce UFG materials. The continuous nature of the process makes
it promising for production of UFG materials on a large scale, in efficient and cost
effective manner. However, further study is needed to investigate its ability with
respect to grain refinement and properties improvement of various UFG materials.
Table 1
Yield strength σσσσ0.2, ultimate tensile strength σσσσu, elongation to failure δδδδ,
and cross-section reduction (necking) ψψψψ of Al samples processed with 1 to 4 passes
Processing state σ0.2, MPa σu, MPa δ, % ψ, %
Initial Al rod 47 71 28 86
After 1 pass 130 160 13 73
After 2 passes 140 170 12 72
After 3 passes 130 160 14 76
After 4 passes 140 180 14 76
Fig. 4. TEM micrograph from the longitu-
dinal section of Al wire processed by
ECAP-Conform with four passes
Физика и техника высоких давлений 2006, том 16, № 4
15
3.2. Combined SPD processing
While solving the problem of fabrication of nanostructured Ti materials for
medical applications we showed the advantage of combining ECAP with other
techniques of metal forming such as rolling, forging or extrusion [45,46]. These
advantages are connected with effective shaping of long-sized semiproducts
(sheets, rods) as well as further enhancement of properties of UFG materials. For
example, in Grade 2 CP Ti high strength (YS = 980 MPa, UTS = 1100 MPa) with
elongation to failure δ = 12% was attained using ECAP and extrusion. Also the
results of investigations on processing of Ti rods of over 800 mm in length and
6.5 mm in diameter by a combination of ECAP and thermomechanical treatment
(TMT) including forging and rolling are very impressive [36].
Fig. 5 presents TEM micrographs of CP Ti subjected to ECAP + TMT, 80%. It
can be seen that the combined processing results in significant additional grain
refinement down to 100 nm in comparison with 30−400 nm after ECAP; however,
a considerable elongation of grains takes place. Mechanical testing has shown
(Table 2) that TMT after ECAP results in strength growth of CP Ti and the record
values of σ0.2 and σu are observed; at the same time, sufficient ductility is pre-
served. It is important that these strength values of nanostructured CP Ti are visi-
bly higher than those of the Ti−6%Al−4%V alloy that is presently widely used in
medicine and engineering.
It is also interesting that the microstructure and properties of the obtained rods
are rather uniform, the dispersion of mechanical properties along the rod length
does not exceed ±5% [11]; at the same time material waste totals 0.65. This shows
great prospects for the use of combined SPD processing for commercial produc-
tion of semi-products from Ti for medical application.
a b
Fig. 5. TEM micrographs displaying the microstructure of Grade 2 Ti after ECAP +
TMT, 80%: a − cross-section; b − longitudinal section
Table 2
Mechanical properties of Grade 2 Ti billets at different stages of processing
State σu, MPa σ0.2, MPa δ, % ψ, %
Initial 440 370 38 60
ECAP, 4 passes 630 545 22 51
ECAP, 4 passes + TMT, ε = 80% 1150 1100 11 56
Физика и техника высоких давлений 2006, том 16, № 4
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4. 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 design parameters. Formal market analyses, conducted by
companies such as Metallicum, that specialize in nanostructured materials, have
identified over 100 specific markets for nanometals in aerospace, transportation,
medical devices, sports products, food and chemical processing, electronics and con-
ventional defense [47]. 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 temperatures [48]; 3) develop-
ment of nanostructured ductile refractory
metals and high-strength TiNi alloys with
advanced shape-memory effect for space,
medical and other applications. The applica-
tions of nanostructured materials in engi-
neering new-generation aviation engines [49]
or in high-strain-rate superplastic forming of
complex-shaped parts for new automobiles
and planes [50] are worth a special mention.
Out of the broad range of possible ap-
plications of advanced nanostructured met-
als, we focus here on the one that is repre-
sentative of the high-tech market: biomedi-
cal implants and devices. High mechanical
and fatigue properties are the essential re-
quirements for metallic biomedical materi-
als, in particular titanium and its alloys
[51], which have excellent biological com-
patibility and high biomechanical proper-
ties. 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 com-
pressive and bending strength, and suffi-
cient ductility. Different implant-plate con-
structions for osteosynthesis have been
analysed, resulting in the design and proc-
essing of a series of nanostructured titanium
plates (Fig. 6,a,b). Fig. 6,c illustrates an-
Fig. 6. Medical implants made of
nanostructured titanium: a, b − plate
implants for osteosynthesis; c − conic
screw for spine fixation; d − device for
correction and fixation of spinal column
a
b
c
d
Физика и техника высоких давлений 2006, том 16, № 4
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other application of nanostructured titanium for a special conic screw, which re-
quires high fatigue strength as well. In this case all the advantages of nanostruc-
tured titanium are fully used [46] − 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.
5. The new SPD processing of bulk nanocrystalline materials
Since the first works dating back to the early 1990s [33,34], SPD techniques
have been used mostly because of their ability to produce ultrafine-grained mate-
rials through microstructure refinement in initially coarse-grained metals [2]. The
final grain size produced depends strongly on both processing regimes and the
type of material. For pure metals the mean grain size is typically about 100−200
nm after processing by HPT and about 200−300 nm after processing by ECAP.
For alloys and intermetallics the grain size is usually less and in some cases it
equals 50−100 nm. However, it is very important for fundamental tasks and many
advanced applications to have bulk nanocrystalline materials with a mean grain
size less than 30−50 nm. Is it possible to produce such materials using SPD tech-
niques? In recent years this problem has become the object of special investiga-
tions in our laboratory where we are developing two approaches: SPD consolida-
tion of powders and SPD-induced nanocrystallization of amorphous alloys.
5.1. SPD consolidation
Already in the early work on consolidation of powders [52,53] it was revealed
that during HPT high pressures of several GPa can provide a rather high density
close to 100% in the processed disc-type nanostructured samples. For fabrication
of such samples via severe torsion straining consolidation not only conventional
powders but also powders prepared by ball milling can be used.
HPT consolidation of nanostructured Ni and Fe powders prepared by ball
milling [52,53] can be taken as an example. The conducted investigations showed
that the density of the samples processed at room temperature is very high and
close to 95% of the theoretical density of bulk coarse-grained metals. After HPT
consolidation at 200 or 400°C the samples density is even higher and reaches
98%. TEM examinations showed the absence of porosity. The mean grain size is
very small; it is equal to 17 nm and 20 nm for Ni and Fe, respectively. It is also
very interesting that the value of microhardness of the Ni samples produced by
HPT consolidation was 8.60 ± 0.17 GPa, the highest value of microhardness
mentioned in literature for nanocrystalline Ni.
5.2. SPD-induced nanocrystallization
Recent investigations also show that SPD processing can control crystalliza-
tion of initially amorphous alloys that may result in the formation of bulk nano-
crystalline alloys with a very small grain size and new properties [54,55]. In the
present paper this approach is used to produce and to investigate nanocrystalline
Физика и техника высоких давлений 2006, том 16, № 4
18
a b
c d
Fig. 7. TEM image of rapidly-quenched alloy Ti50Ni25Cu25: a − initial state (dark field); b −
after annealing at 450°С for 10 min; c − after HPT (dark field); d − after HPT and an-
nealing at 390°С for 10 min
Ti−Ni alloys widely known as alloys with shape memory effects. As the material
for this investigation, two alloys of the Ti−Ni system were used: melt-spun
Ti50Ni25Cu25 alloy [55,56] and cast Ti49.4Ni50.6 alloy [57,58].
The amorphous structure of Ti50Ni25Cu25 alloy was confirmed by TEM and
X-ray investigations (Figs 7, 8) [55,56]. However, after HPT at room temperature,
although the diffraction methods still indicated the amorphous structure of the al-
loy, TEM studies showed the appearance of many nanocrystals with very small
sizes of about 2−3 nm (Fig. 7,c).
The essential difference in behaviour of this alloy in the amorphous state and
after HPT was revealed during subsequent annealing. As it can be seen in Fig. 8,
the amorphous alloy was crystallized at 450°C, then, while cooling, a martensite
phase B19 was forming. According to TEM, the microstructure of the alloy after
annealing is rather non-uniform and together with small grains it contains large
grains with a size of almost about 1 micron (Fig. 8,c). At the same time after HPT
crystallization occurs below 390°C and it appears possible to produce a uniform
nanocrystalline structure with a grain size of under 50 nm (Fig. 7,d). It is rather
Физика и техника высоких давлений 2006, том 16, № 4
19
a b
Fig. 8. X-ray diffraction patterns of the Ti50(Ni,Cu)50 alloy: a − initial rapidly-quenched
alloy (1), after annealing at 300°C for 5 min (2), after annealing at 450°C for 5 min (3)
with the phase В19; b − alloy after HPT (1), after HPT and annealing at 300°C for 5 min
(2), after HPT and annealing at 400°С for 5 min, with the phase B2 (3)
interesting that the structure after cooling is an austenitic B2-phase; in other
words, imposing severe plastic deformation on the amorphous alloy has effected
the alloy crystallization during the heating process and changed its phase compo-
sition after the annealing and further cooling to the room temperature.
In the coarse-grained alloy Ti50Ni25Cu25, the temperature of martensite trans-
formation upon cooling equals ~ 80°С, that is why there is a martensite phase in
the alloy at room temperature. In this connection, the existence of only austenitic
phase after HPT and nanocrystallization can be related to the martensite trans-
formation retard in the alloy with a nanocrystalline grain size. This fact was
previously reported in the literature for ultrafine-grained Ti−Ni alloys [59].
Speaking about the alloy Ti50Ni25Cu25, the critical point is the grain size of
about 100 nm. Martensite transformation does not take place at room tem-
perature below this size.
The amorphous state in the Ti49.4Ni50.6 alloy can be obtained directly as a re-
sult of HPT processing (P = 6 GPa, n = 5 revolutions) [57,58]. Then the homoge-
neous nanocrystalline structure was produced by annealing of the HPT material
(Fig. 9). For instance, after annealing at 400°C for 0.5 h the mean grain size is
about 20 nm (Fig. 9,a,b), and after annealing at 500°C it is about 40 nm (Fig.
9,c,d). It is worth to mention that according to HREM observations after such an-
nealing there are no regions of amorphous phase and grain boundaries are well
defined, although there are still small distortions of the crystal lattice near some of
the boundaries.
k = 4π sin θ/λ k = 4π sin θ/λ
Физика и техника высоких давлений 2006, том 16, № 4
20
a b
c d
Fig. 9. TEM micrographs of Ti49.4Ni50.6 alloy after HPT and annealing at 400°C (a, b)
and at 500°C (c, d) for 0.5 h: a, c − bright field images; b, d − dark field images
Tensile mechanical tests showed that the amorphous nitinol produced by HPT had
much higher strength in comparison with the initial microcrystalline state [57], but it
was essentially brittle. Nanocrystallization results in the record value of strength for
this material equal to 2650 MPa with an elongation to failure of about 5%.
Thus, SPD consolidation of powders and SPD-induced nanocrystallization can be
considered as new SPD processing routes for fabrication of bulk nanocrystalline ma-
terials. One of the advantages of this technique is the possibility of producing fully
dense samples with a uniform ultrafine-grained structure having a grain size less than
40−50 nm. Studies of the properties of these materials are of great interest for ongo-
ing research because deformation mechanisms and, as mentioned above, phase trans-
formations can basically change in materials with a small grain size [7,60].
6. Conclusions
Several new trends in SPD processing for fabrication of bulk nanostructured
materials have been presented in this article, based on recent results of our works
on the development of commercial technology of nanostructured Ti materials pro-
duction for medical and some other applications. We demonstrate how new tasks,
Физика и техника высоких давлений 2006, том 16, № 4
21
connected with economically feasible production of UFG metals and alloys, can
be solved by decreasing the material waste, obtaining homogeneous structure and
advanced properties in bulk billets and products.
From the fundamental point of view, investigations focused on fabrication of
bulk nanocrystalline materials using SPD techniques are of continuous interest.
This paper also presented the two new approaches – SPD consolidation of pow-
ders and SPD-induced crystallization − both rising hopes for a successful resolu-
tion of this important manufacturing problem.
The present paper was supported in part by the NIS-IPP Program of DOE
(USA) and the Russian Foundation for Basic Research. Cooperation with co-
authors mentioned in references is gratefully acknowledged as well.
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|
| id | nasplib_isofts_kiev_ua-123456789-70254 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0868-5924 |
| language | English |
| last_indexed | 2025-12-07T16:26:33Z |
| publishDate | 2006 |
| publisher | Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України |
| record_format | dspace |
| spelling | Valiev, R.Z. 2014-11-01T15:57:36Z 2014-11-01T15:57:36Z 2006 The new trends in SPD processing to fabricate bulk nanostructured materials / R.Z. Valiev // Физика и техника высоких давлений. — 2006. — Т. 16, № 4. — С. 9-22. — Бібліогр.: 60 назв. — англ. 0868-5924 PACS: 62.72.Bb https://nasplib.isofts.kiev.ua/handle/123456789/70254 During the last decade severe plastic deformation (SPD) has become a widely known method of materials processing used for fabrication of ultrafine-grained materials with attractive properties. Nowadays SPD processing is rapidly developing and is on the verge of a transition from lab-scale research to commercial production. This paper focuses on several new trends in the development of SPD techniques for effective grain refinement, including those for commercial alloys, and presents new SPD processing routes to produce bulk nanocrystalline materials. en Донецький фізико-технічний інститут ім. О.О. Галкіна НАН України Физика и техника высоких давлений The new trends in SPD processing to fabricate bulk nanostructured materials Article published earlier |
| spellingShingle | The new trends in SPD processing to fabricate bulk nanostructured materials Valiev, R.Z. |
| title | The new trends in SPD processing to fabricate bulk nanostructured materials |
| title_full | The new trends in SPD processing to fabricate bulk nanostructured materials |
| title_fullStr | The new trends in SPD processing to fabricate bulk nanostructured materials |
| title_full_unstemmed | The new trends in SPD processing to fabricate bulk nanostructured materials |
| title_short | The new trends in SPD processing to fabricate bulk nanostructured materials |
| title_sort | new trends in spd processing to fabricate bulk nanostructured materials |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/70254 |
| work_keys_str_mv | AT valievrz thenewtrendsinspdprocessingtofabricatebulknanostructuredmaterials AT valievrz newtrendsinspdprocessingtofabricatebulknanostructuredmaterials |