Development of new routes of severe plastic deformation through cyclic expansion–extrusion process

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Developmentofnewroutesofsevereplastic
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ImpactFactor:2.57·DOI:10.1016/j.msea.2014.06.074

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Elsevier Editorial System(tm) for Materials Science & Engineering A
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Manuscript Number:
Title: Development of new routes of severe plastic deformation through cyclic expansion-extrusion
process
Article Type: Research Paper
Keywords: Severe plastic deformation; Cyclic expansion extrusion; Aluminum; Mechanical properties;
Microstructure; Micro shear band
Corresponding Author: Dr. Ramin Ebrahimi,
Corresponding Author's Institution: Shiraz University
First Author: Nima Pardis, PhD Candidate
Order of Authors: Nima Pardis, PhD Candidate; Cai Chen, PhD Student; Mehrdad Shahbaz, PhD
Candidate; Ramin Ebrahimi; Laszlo Toth, Professor
Abstract: This paper introduces two new processing routes for a recently introduced severe plastic
deformation technique, cyclic expansion extrusion (CEE). Two processing Routes (I and II) were
experimentally performed on aluminum alloy 1050, the processed samples were investigated and
compared in terms of their microstructural and mechanical characteristics. A significant improvement
in mechanical properties was observed after one CEE pass via different processing routes. Different
grain structures were achieved after Routes I and II showing a more homogeneous microstructure and
hardness distribution in Route II compared to Route I. In addition, compression tests of the processed
samples demonstrated that Route II results in a homogeneous compressive strength. Finally,
microstructure evolution during subsequent passes of this process was investigated by electron back
scattered diffraction. Micro shear bands were found as potential sites for accelerating the formation of
new grains which resulted in fragmentation of the initial grains and leading to an ultrafine-grained
(UFG) microstructure.

Cover Letter

April 21 ,2014

Dear Editor
I am sending you our new paper entitled: “Development of new routes of severe plastic deformation
through cyclic expansion-extrusion process” to be considered for publication in Materials Science and

Engineering: A. The manuscript is the authors’ original work and has not been published in any journal
nor has it been simultaneously submitted elsewhere.

Sincerely yours,
Dr. Ramin Ebrahimi,
Associate Professor,
Department of Materials Science and Engineering,
School of Engineering,
Shiraz University,


Manuscript
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Development of new routes of severe plastic deformation through
cyclic expansion-extrusion process
N. Pardis*1, C. Chen2,3, M. Shahbaz1, R. Ebrahimi†1, L.S. Toth‡2,3
1

Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz, Iran

2

Laboratoire d'Etude des Microstructures et de Mécanique des Matériaux (LEM3),
UMR 7239, CNRS / Université de Lorraine, F-57045 Metz, France

3

Laboratory of Excellence on Design of Alloy Metals for low-mAss Structures (DAMAS), Université de Lorraine,
France

Abstract
This paper introduces two new processing routes for a recently introduced severe plastic
deformation technique, cyclic expansion extrusion (CEE). Two processing Routes (I and II) were
experimentally performed on aluminum alloy 1050, the processed samples were investigated and
compared in terms of their microstructural and mechanical characteristics. A significant
improvement in mechanical properties was observed after one CEE pass via different processing
routes. Different grain structures were achieved after Routes I and II showing a more
homogeneous microstructure and hardness distribution in Route II compared to Route I. In
addition, compression tests of the processed samples demonstrated that Route II results in a
homogeneous compressive strength. Finally, microstructure evolution during subsequent passes
of this process was investigated by electron back scattered diffraction. Micro shear bands were
found as potential sites for accelerating the formation of new grains which resulted in
fragmentation of the initial grains and leading to an ultrafine-grained (UFG) microstructure.
Keywords
Severe plastic deformation; Cyclic expansion extrusion; Aluminum; Mechanical properties;
Microstructure; Micro shear band.

*

†

‡

E-mail:
Corresponding Author: (R. Ebrahimi)
E-mail:
Tel.: +98 711 2307293; fax: +98 711 2307293.
E-mail:

1


1. Introduction
Severe plastic deformation is considered as a powerful processing tool for producing bulk
ultrafine grained (UFG) / nanostructured materials. This approach is based on giant straining of
bulk metallic materials [1-4]. Most SPD techniques are classified as batch processing methods in
which strain accumulation is achieved by performing consecutive passes. From an industrial
point of view, batch SPD techniques such as equal channel angular pressing (ECAP) [1,5-7],
might seem less interesting compared to other continuous processes like high pressure torsion
(HPT) [8] or high pressure tube twisting (HPTT) [9], where there is no need to repeat the process
several times to reach the desired amount of accumulated strain. On the other hand, batch
techniques like ECAP may give an opportunity to define different processing routes between
consecutive passes of the process which can be an effective tool in controlling and manipulating
the resulting microstructure [5,6]. Generally, these processing routes are simply performed by
rotation of the sample around its main axis. Such rotation is applicable in some SPD techniques
like ECAP [5,6] and simple shear extrusion (SSE) [10,11], which do not have axisymmetric die
geometry. However, in other techniques with axisymmetric die geometry, like cyclic extrusion
compression (CEC) [12], cyclic expansion extrusion (CEE) [13], or tube channel pressing (TCP)
[14], the processing route would be limited to reversing the sample orientation/pressing direction
with respect to die between consecutive passes [15]. This paper presents a non- axisymmetric

version of the CEE technique which makes it possible to introduce new processing routes of this
technique for processing samples with rectangular cross section. The resulting mechanical and
microstructural evolutions are presented and discussed.

2. Principles of different processing routes in CEE
Two major processing routes are defined for CEE processing of samples with rectangular cross
section. These routes are nominated as Route I and Route II which are illustrated in Fig. 1.

As can be seen in Fig.1, CEE processing in Route I is performed under plane strain conditions
and therefore, both steps (expansion and extrusion) are performed in the same plane (Fig. 1a). On
the other hand, in Route II, expansion and extrusion steps take place on different planes which
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are normal to each other (Fig. 1b). Although each step (expansion or extrusion) is performed in
plane strain condition, the overall process in Route II cannot be considered as a plane strain
operation. Based on Fig. 1, a sample of a×b cross section is expanded in plane strain condition to
a square of b×b , which is subsequently extruded in plane strain condition to a rectangular cross
section of a×b and b×a through processing Routes I and II, respectively. Therefore, the amount
of von Mises accumulated strain in one pass can be calculated for both routes as:

This relation, however, is an average deformation value across the whole section in which shear
components are neglected. Namely, it is not excluded - and will be shown below - that the
deformation can be heterogeneous leading to smaller strains in the center region and larger
strains in the external part of the sample.
3. Experimental procedure
3.1. Material and processing
Samples of 10 mm × 20 mm × 60 mm were machined from a 1050 aluminum alloy strip. They
were annealed at 600˚C for 2 h and furnace cooled to room temperature. A split die configuration
(consisting of two similar extrusion dies) was designed and used for CEE processing, suitable for

both routes (Fig. 2).

Using this die set up (Fig. 2), a sample with 10 mm × 20 mm cross section is expanded in plane
strain condition to a square with 20 mm side length following by an extrusion stage during which
the sample regains its 10 mm × 20 mm initial section. In this way, the process can be repeated
several times. According to Eq. (1), the dimension values of the sample result in an average
imposed strain value of

for each pass of the process. Route I was performed by placing

the two die halves in plane strain configuration (Fig. 1a), while 90 degree rotation of one half
with respect to the other changes the configuration from Route I into Route II (Fig. 1b).
After assembling the die halves in a desired configuration (Route I or II), the process was started
by pressing a "sacrifice" sample to fill the blocked expansion-extrusion chamber. Subsequently,
the exit channel was unblocked and consecutive pressing of other samples was performed. More
details of these processing sequences are described and illustrated in Ref. [13]. To reduce the
3


friction at the die-sample interface, each sample was wrapped with Teflon tape before inserting it
into the die channel. The process was performed with a pressing speed of 0.2 mm/s.
3.2 Microstructural studies
Different planes on the processed samples were polished to a mirror like surface and etched with
modified Poulton’s reagent [16] to reveal the corresponding microstructure using a stereo
microscope. The polished surfaces of samples were also electro-etched with Barker reagent [16]
and subsequently investigated using polarized optical microscopy. Electron back scatter
diffraction (EBSD) technique was used for detailed examination of the microstructure. The
investigations were performed using JEOL 6500F scanning electron microscope (FEG-SEM)
equipped with a field emission gun operating at 15 kV. The steps of the sample preparation for
EBSD were mechanical grinding (using 500-4000 grit size SiC papers) followed by polishing the

surface with diamond compounds of 9 μm, 3μm and 1μm, subsequently. The samples were
further electro-polished in an electrolytic solution composed of 10 ml perchloric acid and 90 ml
ethanol at 263K with a DC voltage of 18V to obtain a mirror-like surface. The EBSD
observations were accomplished with step size of 0.1 μm. For processing the measurements, the
HKL acquisition software was used.
3.3. Mechanical studies
Vickers microhardness measurements were performed by applying 25 g load with a loading rate
of 5 g/s and 15 s dwell time. The tests were carried out across the thickness of the samples on the
ED-ND plane (Fig. 1) with an incremental distance of 0.5 mm. The indentation was repeated
four times at each location and the average microhardness value was calculated. Compression
tests were performed on rectangular parallelepipeds of 10 mm × 10 mm × 15 mm extracted along
the extrusion (ED) and transverse (TD) directions to give compression planes on TD-ND and
ED-ND, respectively. Fig. 3 illustrates the orientations of the compression specimens with
respect to a CEE processed sample. The side length in the cross section of these parallelepipeds
was considered equal to thickness of the CEE processed billet (i.e. 10 mm) to maintain various
grain structures across the thickness of compression sample. Subsequently, the length of main

4


axis in these samples was chosen as 15mm to give the recommended aspect ratio of height/width
equal to 1.5 [17].

4. Results and discussion
Fig. 4 illustrates the in situ geometry of the samples for the two processing Routes I and II.
Geometrical and dimensional examination of these samples approved that for both processing
routes, the expansion-extrusion chamber was satisfactorily filled and therefore, the designed
amount of strain (

) was accumulated in the samples in one pass. The sample, however,

does not recover its rectangular shape at the end of the deformation process because its end
sections become U-shape; see in Fig. 3. This shape is due to strain heterogeneity; there is more
strain in the outer regions along the ND direction which leads to more stretching in the ED
direction and produces the U-shape end in the samples.

Consideration of the deformed geometry of an initially cubic elements (Fig. 1) shows that in
Route I, the material experiences a cyclic deformation path as each element of the material
expands and extrudes in the same plane and by the same amount of strain (Fig. 1a). However, the
situation is different for Route II, as expansion and extrusion steps take place on different planes
(Fig. 1b). Therefore, material elements experience no cyclic deformation after one CEE pass via
Route II. This fact is illustrated in Fig. 1 by considering distortion of cubic elements of a sample
at different stages of deformation for each processing route. Therefore, processing Route II is
expected to be more effective for grain refinement which will be verified below by
microstructural investigations.
The results of microstructural investigations obtained by optical microscopy are illustrated in
Figs. 5 and 6. Fig. 5a shows a typical microstructure of the undeformed sample in TD-ND plane
(the plane normal to the extrusion direction). The microstructures in the same plane after
performing one pass in processing Routes I and II are illustrated in Figs. 5b and 5c, respectively.

It can be seen that the grain size and morphology after both processing routes are significantly
different compared to the annealed structure. For Route I, grains are equiaxed at the vicinity of
the center line, while elongated grains are visible after processing Route II which are oriented
5


along TD (Figs. 5c and 5d). This observation is in good agreement with the predicted
configuration of initially cubic elements in ideal deformation condition after different processing
routes (Fig. 1) which only considers normal strain components. However, a more severely
deformed structure is observed at regions adjacent to the surface of processed samples after both

processing routes. The corresponding microstructure viewed at ED-ND plane is illustrated in Fig.
6.
Similar to Fig. 5b, it is seen that the through thickness structure after Route I is not uniform.
While equiaxed grain structure is observed at the central parts of sample after Route I, grains are
severely elongated adjacent to the lateral boundaries of the die. This excessive deformation is
due to a redundant shear deformation which is dependent on the amount of reduction, semi die
angle as well as the die profile [18].
The excessive shear deformation is more visible in the outer region after processing Route I
compared to Route II (Figs. 6b and 6c). The more uniform structure in Route II can be attributed
to the change in orientation of the extrusion plane with respect to the expansion plane in this
route (Fig. 1). As a result, the through thickness direction of the extruded sample would be the
plane strain direction of its previous expansion step and therefore a more uniform structure is
expected across the thickness. In addition, the differences in orientation and aspect ratio of the
expanded elements with respect to the extrusion direction for Routes I and II (Fig. 1) might be
another reason for such differences in microstructures.
Although redundant strain would intensify the degree of severe plastic deformation, it would
affect structural homogeneity and uniformity of the resulting mechanical properties across the
thickness of samples. This fact was investigated by analyzing the variation of microhardness
values on ED-ND plane and the microstructures which are illustrated in Fig. 7 for the two
processing routes.
It can be seen that Route II results in a more uniform hardness distribution across the thickness
together with a more uniform grain structure on the ED-ND plane. On the other hand, the
hardness value is higher near the surface of the sample after Route I and gradually decreases
toward the center. Such hardness gradient is attributed to the microstructural inhomogeneities
across the thickness resulting from high amount of shear strain near the surface. Despite the
differences in hardness homogeneity after these processing routes, the average hardness values
after these processing routes are close to each other (53 HV and 53.7 HV for Route I and Route
6

II, respectively) and significantly higher than the average value in the annealed condition (29
HV).
Microstructural and microhardness investigations indicated that Route II leads to a more uniform
structure after one pass. This fact was further confirmed by performing compression tests at
different orientations. The compression test results are displayed in Fig. 8 indicating the
uniformity of compressive strength after processing Route II. This might be due to the
uniformity of deformation throughout the cross section as well as the similarity of grain
orientations with respect to the different compression directions (ED and TD) (Figs. 5c and 6c).
After processing Route I, however, the grain structure on the planes normal to ED and TD are
not similar (Figs. 5b and 6b). Despite the equiaxed structure in the central part of this sample, the
majority of grains are elongated in the extrusion direction at the vicinity of central band which is
due to additional shear component of strain imposed by the die geometry at these areas.
Therefore, anisotropy in properties is seen in the compressive strength after processing Route I.

It is also evident from Fig. 8 that after one CEE pass the yield strength is significantly increased
compared to its value in the annealed condition. Such considerable increase in compressive
strength as well as the significant microhardness improvement and structure refinement, even
after one pass of CEE, indicate that the CEE process can be used as an effective SPD technique
for producing bulk ultrafine grained materials.

Due to the observation of a more homogeneous structure and properties after processing Route
II, more detailed microstructure investigations were performed on samples processed by this
route (Route II). As an example, the microstructure of a sample after the first CEE pass via
Route II was investigated by polarized light as well as electron backscatter diffraction (EBSD)
and shown in Fig. 9. These images were taken at the vicinity of the center line on the ED-ND
plane after the first pass through Route II. This position is schematically illustrated in Fig. 9.

Some bands are visible in both images along which crystallites of submicron size are evident.
These bands can be considered as deformation heterogeneities where dislocations form a quasi
periodical structure consisting of geometrical necessary dislocation (GNDs). These GNDs are

suspected to fall into walls and create in this way new grains [19]. Crystallites of low angle
7


boundaries are expected to convert subsequently into ultrafine grains by gradual increase in their
boundary misorientation angle during further passes of the process. Indeed, one can see in Fig.
9c that already after one pass in Route II there is a large proportion of high angle boundaries
between the newly formed grains reaching a fraction of about 67%. It should be noted that in this
work the misorientation distribution was calculated between neighboring grains, not between
neighboring pixels. Such misorientation analysis was introduced by Toth et al. [20] which has
more physical meaning than the usually shown pixel-to-pixel misorientation distribution. In this
method, grains (with grain boundaries of at least 5° misorientation) are identified, then the
misorientation between grains is calculated as the misorientation between the average
orientations of the grains.

Fig. 10 shows Fig. 9b in form of band contrast map. Careful inspection of both figures reveals
the presence of shear bands along which deformation localization can result in enhanced
continuous dynamic recrystallization (CDRX) due to the larger local deformation. Examples of
such shear bands are highlighted in Figs. 10b and b'. This fact is in agreement with a model for
the strain-induced formation of UFGs [21] which states the formation of new grains at micro
shear bands (MSBs) at the initial stages of deformation. It is important to note that the
mechanism is not nucleation and growth of grains (discontinuous dynamic recrystallization,
DDRX) but CDRX by forming new boundaries. Subsequently, the fraction of high angle
boundaries increases with increase in the density of MSBs at higher strain values, examples are
illustrated in Fig. 11. It is clear that increasing the amount of applied strain after the second CEE
pass results in a higher density of MSBs as well as some intersecting MSBs (Fig. 11) which is
believed to help the generation of a more uniformly subgrain structure. As a result, the UFG
structure spreads through the whole volume [22]. Such uniform distribution of UFG grains is
illustrated in Fig. 12 which shows EBSD micrograph after four CEE passes through Route II.

Equiaxed grain structure with high angle boundaries and an average grain size of ~720 nm was
achieved after four extrusion passes (Fig.12). Careful inspection of this image reveals some
intersecting bands along which higher density UFGs are visible. This result, together with the
previous ones (Figs. 9, 10 and 11) confirms the recommended model for strain-induced UFG

8


formation based on the evolution of MSBs [21] as well as occurrence of enhanced CDRX in
these areas.
The next-neighbor misorientation distribution of samples after four passes (Fig. 13) shows a
nearly constant frequency as a function of misorientation angle. It does not mean, however, that
the grain orientation distribution is random. Similar result was also reported for processing of
pure aluminum by HPTT [23] for which the accumulated strain was about the same as the strain
value in this study. As seen in Fig. 13, there is very significant difference of the obtained
distribution from the Mackenzie distribution which corresponds to the perfectly randomly
oriented neighbor case. There is a significant evolution of the misorientation distribution
between the first and fourth passes (see Figs. 9c and 13); in the fourth pass, there is more high
angle boundaries: the more than 15° boundaries are now represent about 84% of the whole
distribution. A further observation from Fig. 13 is that at the very large angles - near 60°- the
distribution follows the Mackenzie one very precisely while at the lower angles, below 30°, the
measured frequency is higher than the Mackenzie one. The latter range of misorientation belongs
to grains that are within larger grains (entirely inside, not at the original grain boundary) meaning
that at this stage there is still a grain fragmentation process going on, the steady state is not yet
reached. It has been shown by Pougis et al. [24] in copper that in the steady state the
misorientation distribution becomes very similar to the random Mackenzie.
5- Summary and conclusions
New processing routes (I and II) were developed for severe plastic deformation of samples with
rectangular cross section by cyclic expansion-extrusion (CEE). The samples were processed
successfully and further studied by conducting mechanical and microstructural investigations.

Mechanical properties were significantly improved in both processing routes due to the
considerable changes observed in grain structure and size. Redundant shear deformation was
more visible near the surface of the samples after processing Route I which resulted in nonuniformity of the mechanical properties. However, a more homogeneous microstructure as well
as uniform compressive strength and hardness distribution were achieved in processing Route II.
Micro shear bands (MSBs) were found as regions where the formation of new grains is
accelerated by CDRX within the original grains. Further CEE processing introduced a high

9


density of intersecting MSBs in the grain interiors and resulted in fragmentation of the original
grains to ultrafine grains with high angle misorentation boundaries.

Acknowledgements
This work was supported by the French State through the program "Investment in the future"
operated by the National Research Agency (ANR) and referenced by ANR-11-LABX-0008-01
(LabEx DAMAS).

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Fig. Captions:
Fig. 1. Schematic illustration of two CEE processing routes and the respective deformed
configuration of cubic elements of a sample for (a) processing Route I, (b) processing Route II.
Fig. 2. CEE split dies, a. expansion die, b. extrusion die, c. punch, d. exit channel blocking insert.
Fig. 3. Orientation of compression specimens with respect to a CEE processed sample.
Fig. 4. Macroscopic view of samples after interrupted CEE processing (Routes I and II).
Fig. 5. Microstructures of samples in the TD-ND plane: (a) Annealed condition, (b) after one
pass Route I, (c) after one pass Route II.
Fig. 6. Microstructure of samples in the ED-ND plane: (a) Annealed condition, (b) after one pass
Route I, (c) after one pass Route II.
Fig. 7. Microhardness profile and the corresponding microstructure on the ED-ND plane, (a)
Route I, (b) Route II.
Fig. 8. Stress-strain curves of compression samples tested in different orientations, before and
after one CEE pass in Routes I and II.
Fig. 9. Microstructures developed after the first CEE pass via Route II, (a) Polarized light
microstructure, (b) Inverse pole figure EBSD map, (c) Grain boundary misorientation
distribution from EBSD in comparison with the random Mackenzie distribution.
Fig. 10. (a) Band contrast EBSD map of the microstructure developed after one pass CEE via
Route II. (b) and (b') are the same enlarged regions of (a).
Fig. 11. Polarized light microstructure of samples after two passes viewed on the ED-ND plane,
(a) Route I, (b) Route II.
Fig. 12. Inverse pole figure EBSD map of a sample processed to four passes via Route II.
Fig. 13. Next-neighbor grain misorientation angle distribution measured in the sample processed
to four passes via Route II. (a) center, (b) edge.

12


Figure(1)

Fig. 1. Schematic illustration of two CEE processing routes and the respective deformed
configuration of cubic elements of a sample for (a) processing Route I, (b) processing
Route II.


Figure(2)

Fig. 2. CEE split dies, a. expansion die, b. extrusion die, c. punch, d. exit channel
blocking insert.


Figure(3)

Fig. 3. Orientation of compression specimens with respect to a CEE processed sample.


Figure(4)

Fig. 4. Macroscopic view of samples after interrupted CEE processing (Routes I and II).


Figure(5)

Fig. 5. Microstructures of samples in the TD-ND plane: (a) Annealed condition, (b) after
one pass Route I, (c) after one pass Route II.


Figure(6)

Fig. 6. Microstructure of samples in the ED-ND plane: (a) Annealed condition, (b) after

one pass Route I, (c) after one pass Route II.


Figure(7)

Fig. 7. Microhardness profile and the corresponding microstructure on the ED-ND plane,
(a) Route I, (b) Route II.


Figure(8)

Fig. 8. Stress-strain curves of compression samples tested in different orientations, before
and after one CEE pass in Routes I and II.


Figure(9)

Fig. 9. Microstructures developed after the first CEE pass via Route II, (a) Polarized light
microstructure, (b) Inverse pole figure EBSD map, (c) Grain boundary misorientation
distribution from EBSD in comparison with the random Mackenzie distribution.


Figure(10)

Fig. 10. (a) Band contrast EBSD map of the microstructure developed after one pass CEE
via Route II. (b) and (b') are the same enlarged regions of (a).