L. W. LU et al.: IMPROVING THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MAGNESIUM-ALLOY ...
1019–1023
IMPROVING THE MICROSTRUCTURE AND MECHANICAL
PROPERTIES OF MAGNESIUM-ALLOY SHEETS WITH A NEW
EXTRUSION METHOD
IZBOLJ[ANJE MIKROSTRUKTURE IN MEHANSKIH LASTNOSTI
PLO^EVINE IZ Mg-ZLITINE Z NOVO METODO IZTISKAVANJA
Liwei Lu1, Zhenru Yin1, Yanfeng Liu1, Dandan Chen1, Chuming Liu2,
Zhiqiang Wu1
1Hunan University of Science and Technology, Hunan Provincial Key Laboratory of High Efficiency and Precision Machining of
Difficult-to-Cut Material, Xiangtan, Hunan 411201, China
2Central South University, School of Materials Science and Engineering, Changsha 410083, Hunan, China
cqullw@163.com, wuzq2009@163.com
Prejem rokopisa – received: 2017-05-19; sprejem za objavo – accepted for publication: 2017-06-22
doi:10.17222/mit.2017.059
A new effective extrusion technique, including integrated direct extrusion and bending-shear deformation, was proposed to
fabricate AZ31-magnesium-alloy sheets with high performance, and the grain refinement, texture and mechanical properties
were further investigated. The results show that the new extrusion technique can noticeably refine the grain size from 240 μm to
3.2 μm at 563 K, and the basal texture is also dramatically weakened, which can be attributed to a fully dynamic recrystalliza-
tion triggered by a large shearing strain accumulated due to a bar-sheet deformation and continuous bending-shear deformation.
Consequently, the ductility significantly increases from 11.67 % to 27.7 %, and the ultimate tensile stress can still keep a high
value of 290 MPa, rendering this new technique an effective processing method for further tailoring of the microstructure and
properties of Mg-based alloys for a wide range of engineering applications.
Keywords: AZ31-Mg alloy, hot extrusion, microstructure, mechanical properties
Avtorji prispevka so raziskovali vpliv nove u~inkovite tehnike iztiskavanja (ekstrudiranja) na lastnosti plo~evine iz Mg zlitine
AZ31. Pri tej tehniki sta zdru`ena postopka direktnega iztiskanja in upogibanja s stri`no deformacijo (angl.: Direct Extrusion
and Bending Shear; DEBS). Avtorji ugotavljajo, da je pri{lo do udrobljenja mikrostrukture, raziskali pa so tudi teksturo in
dolo~ili mehanske lastnosti tako izdelane plo~evine. Rezultati raziskave so pokazali, da je nova tehnika ekstrudiranja opazno
zmanj{ala velikost kristalnih zrn iz 240 μm na 3,2 μm pri 563 K (290 °C), prav tako je ob~utno oslabela bazala tekstura, kar je
pripisati popolni dinami~ni rekristalizaciji zaradi velike stri`ne deformacije nakopi~ene med preoblikovanjem okroglice v
plo~evino in neprekinjene upogibno-stri`ne deformacije. Posledi~no se je ob~utno povi{ala duktilnost iz 11, 67 % na 27, 7 % in
natezna trdnost je {e vedno ostala na visokem nivoju 290 MPa. Zato ta nova tehnika iztiskavanja lahko postane u~inkovita
procesna tehnologija za nadaljnje prilagajanje mikrostrukture in lastnosti Mg zlitin {iroki paleti in`enirskih aplikacij.
Klju~ne besede: AZ31 magnezijeva zlitina, vro~e ekstrudiranje, mikrostruktura, mehanske lastnosti
1 INTRODUCTION
The efforts to improve energy efficiency and develop
low-density and high-specific-strength magnesium alloys
for various structural applications in transportation
vehicles are strongly driven by the development of light-
weight construction. However, its application is still
limited due to the hexagonal close-packed structure with
poor slip systems at room temperature.1–3 To overcome
this problem, many scholars paid more attention to the
grain refinement using severe plastic-deformation (SPD)
techniques.4,5 Nowadays, the most attractive SPD proce-
dures are equal-channel angular pressing (ECAP),6,7
cyclic extrusion compression (CEC),8,9 high-pressure
torsion (HPT)10,11 and accumulative roll bonding
(ARB).12,13 A. Muralidhar et al.14 adopted four passes of
ECAP via the Bc route for a magnesium alloy at a tempe-
rature of 573 K. The average grain size was reduced
from 31.8 μm to 8 μm. J. B. Lin et al.15 used CEC to
extrude the GW102 K alloy at 623 K with 8 passes. The
elongation to failure was dramatically increased by 162.5
%, and the yield strength (YS) and ultimate tensile
strength (UTS) increased by 41.3 % and 16.1 %,
respectively. It should be noted that these techniques
commonly require several passes before fine grains and a
homogeneous strain can be obtained. Therefore, it is
urgent to decrease the SPD passes to reduce the cost and
increase the efficiency of practical processing of Mg
alloys.
In the current study, AZ31-magnesium-alloy sheets
are fabricated using a new extrusion method, integrating
direct extrusion and bending shear (DEBS), which adds a
successive bending-shear deformation to the conven-
tional extrusion for further improving the microstructure
and the basal texture. This deformation process can
accumulate a large and homogeneous strain in the ma-
terials in a single pass, and the grains and basal texture
of the AZ31-Mg-alloy sheets are prominently refined
and weakened, respectively. Moreover, DEBS is proved
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1019–1023 1019
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:620.1:62-42:669.721.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(6)1019(2017)
to be a promising way to produce high-performance
Mg-alloy sheets.
2 EXPERIMENTAL PART
A schematic diagram of the DEBS process is in
Figure 1, which shows the configuration of a designed
and manufactured DEBS die. For the sake of clarity, the
characteristic zones of the extrusion deformation were
divided into stages I, II, III, IV and V, representing the
conical part, the forward-extrusion bar sheet, the first
bending-shear zone, the second bending-shear zone, and
the eventual forming stage, respectively. As for zone I,
the forward-extrusion ratio and conical angle were 7.08°
and 60°, respectively, and the sample shape was trans-
formed from a bar with a Ø25 mm diameter into a sheet
with a width and thickness of 25 mm and 3 mm, respec-
tively. The bending radii R1, R2, the bending angles 
 and
 were set to be 6 mm, 3 mm, 140° and 100°, respec-
tively.
In the present work, the AZ31-magnesium alloy with
a composition of 3 % mass fraction of Al, 1 % mass frac-
tion of Zn, 0.3 % mass fraction of Mn, and the Mg
balance was employed. A graphite + engine oil lubrica-
tion containing a mass fraction of 70 % of cylinder oil
(74#) and 30 % of graphite (400 mesh) was sprayed on
the specimens with a 25-mm diameter and 30-mm
length, made from as-cast ingots and dies, to reduce the
friction. Subsequently, the billet and die were heated to
the target temperature for 10 min, and the heated billet
was promptly installed into the die to conduct the
extrusion process for fabricating AZ31-magnesium-alloy
sheets at 563 K and 643 K.
The extrusion products were cut into a rectangle
shape with dimensions of 6 mm × 3 mm × 8 mm along
the longitudinal plane, as shown in Figure 1. The micro-
structures of the DEBS-ed sheets were examined from
the extrusion-direction (ED), transverse-direction (TD)
and normal-direction (ND) planes by means of optical
microscopy, and the average grain size was measured
using the linear intercept process. The central region of
an extrusion product was chosen for an X-ray diffraction
(XRD) analysis using a Bruker Discover equipment with
Cu-K radiation operated at 40 kV and 40 mA, and
parameters  and 
 were measured from 0° to 75° and
360°, respectively. Tensile tests were carried out using
specimens with a gauge section of 25 mm × 3 mm × 10
mm from the extruded sheets along the ED at room tem-
perature and a constant velocity of 2 mm/min. Finally,
the tensile-fracture surface was further analyzed with a
scanning electron microscope. Besides, the microstruc-
ture and properties of the as-cast specimen were also
tested for comparison.
3 RESULTS AND DISCUSSION
3.1 Microstructure
To examine the microstructure-refinement effect, the
optical microstructures of the AZ31-Mg alloy before and
after the extrusion are shown in Figures 2 and 3. The
grains of the as-cast billet are coarse and the average
grain size is calculated to be about 240 μm (Figure 2a).
Significantly, the DEBS-ed AZ31-magnesium-alloy
sheet extruded at 563 K shows a prominent grain-refine-
ment effect, and the average grain sizes obtained from
the x, y and z planes are about 7.2 μm, 4.5 μm and 3.2 μm,
respectively, as presented in Figures 2b to 2d. After one
pass, the originally coarse grains are refined markedly
during the DEBS process, which is mainly attributed to
the occurrence of DRX, triggered by a large accumulated
strain during the extrusion process. In Figure 2b, the
microstructure consists of some coarse grains (8 μm) in
the local area, being surrounded by much finer grains
(2–4 μm). Even so, from an overall perspective, the
grains are relatively homogeneously distributed and
refined. It is obvious that the average grain size of the
AZ31-magnesium alloy can be only reduced from 31.8
μm to 8 μm after four ECAP passes at 573 K for rout
Bc.14 In contrast, the DEBS process has a unique
advantage and a potential for a grain refinement.
L. W. LU et al.: IMPROVING THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MAGNESIUM-ALLOY ...
1020 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1019–1023
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Optical microstructures of AZ31-Mg alloy before and after
the DEBS process at 563 K: a) as-cast, b) to d) x, y and z planes of the
extruded sample
Figure 1: Sketch of the DEBS die and definition of external orienta-
tions; ED, TD, and ND denote the extrusion, transverse and normal
directions
With an increase in the extrusion temperature
(643 K), many tiny grains emerge (Figures 3a to 3c).
However, as there are still some inhomogeneously
distorted grains in the local area, marked by ellipses
(Figure 3b), there is not enough energy stored to fully
complete DRX during the DEBS process, demonstrating
that the extrusion temperature plays a significant role in
the formation of distorted grains affected by the dislo-
cation density, the evolution of dislocation cells and
sub-structures. The aforementioned result is consistent
with an earlier research result reported by J. F. Jiang et
al.16 They also found that the mean grain size of 27.67
μm can be obtained with ECAP at 623 K, which is much
larger than that obtained at 643 K using DEBS.
3.2 Textural evolution
Figure 4 shows the {0002} and {1010} pole figures
of the DEBS-ed sheets at different temperatures, exhi-
biting a typical {0002} fiber texture after the DEBS
process. However, in comparison with the conventional
extrusion (17.55)17, the texture intensity of the DEBS-ed
sheets is significantly weakened (Figures 4a and 4b) and
the texture component becomes more dispersive along
the ED, which can induce a good deformation capability
due to the <1120> direction in fewer grains orientated
parallel to the ED.
A closer observation reveals that the {0002} pole
figure forms a double-peaked basal texture characterized
by a pronounced {0002} basal plane tilted towards the
ED at about 30° and 13° (Figure 4a), respectively, which
may be a result of a larger shear strain accumulated by
the bar-sheet deformation and continuous bending-shear
deformation, activating more <c+a> slip systems. These
phenomena were previously reported for compressive
shock loading and ARB processes by H. Asgari18 and
H. Chang19, respectively. When the extrusion tempera-
ture increases to 643 K, the {0002} texture intensity de-
creases to 9.54, as shown in Figure 4b, and the {0002}
basal plane tilts towards the ED at an angle of 10°, which
is smaller than the one at 563 K. The decrease in the
basal texture intensity may be attributed to an increased
activation of the non-basal slip at the higher temperature
during the DEBS process.
3.3 Mechanical properties
To shed light on how the grain refinement and texture
evolution affect the mechanical properties, tensile tests
of the as-cast and DEBS-ed AZ31-Mg alloys are carried
out at room temperature, and the tensile stress-strain pro-
L. W. LU et al.: IMPROVING THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MAGNESIUM-ALLOY ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1019–1023 1021
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Light microstructures of DEBS-ed AZ31-Mg alloy at 643K: a)–c) x, y and z planes of the extruded sample
Figure 5: Tensile stress-strain curves at room temperature for the
AZ31 samples processed with DEBS at 563 K and 643 K
Figure 4: {0002} and {1010} pole figures for the DEBS-processed
AZ31 alloys at different temperatures: a) 563 K, b) 643 K
to be a promising way to produce high-performance
Mg-alloy sheets.
2 EXPERIMENTAL PART
A schematic diagram of the DEBS process is in
Figure 1, which shows the configuration of a designed
and manufactured DEBS die. For the sake of clarity, the
characteristic zones of the extrusion deformation were
divided into stages I, II, III, IV and V, representing the
conical part, the forward-extrusion bar sheet, the first
bending-shear zone, the second bending-shear zone, and
the eventual forming stage, respectively. As for zone I,
the forward-extrusion ratio and conical angle were 7.08°
and 60°, respectively, and the sample shape was trans-
formed from a bar with a Ø25 mm diameter into a sheet
with a width and thickness of 25 mm and 3 mm, respec-
tively. The bending radii R1, R2, the bending angles 
 and
 were set to be 6 mm, 3 mm, 140° and 100°, respec-
tively.
In the present work, the AZ31-magnesium alloy with
a composition of 3 % mass fraction of Al, 1 % mass frac-
tion of Zn, 0.3 % mass fraction of Mn, and the Mg
balance was employed. A graphite + engine oil lubrica-
tion containing a mass fraction of 70 % of cylinder oil
(74#) and 30 % of graphite (400 mesh) was sprayed on
the specimens with a 25-mm diameter and 30-mm
length, made from as-cast ingots and dies, to reduce the
friction. Subsequently, the billet and die were heated to
the target temperature for 10 min, and the heated billet
was promptly installed into the die to conduct the
extrusion process for fabricating AZ31-magnesium-alloy
sheets at 563 K and 643 K.
The extrusion products were cut into a rectangle
shape with dimensions of 6 mm × 3 mm × 8 mm along
the longitudinal plane, as shown in Figure 1. The micro-
structures of the DEBS-ed sheets were examined from
the extrusion-direction (ED), transverse-direction (TD)
and normal-direction (ND) planes by means of optical
microscopy, and the average grain size was measured
using the linear intercept process. The central region of
an extrusion product was chosen for an X-ray diffraction
(XRD) analysis using a Bruker Discover equipment with
Cu-K radiation operated at 40 kV and 40 mA, and
parameters  and 
 were measured from 0° to 75° and
360°, respectively. Tensile tests were carried out using
specimens with a gauge section of 25 mm × 3 mm × 10
mm from the extruded sheets along the ED at room tem-
perature and a constant velocity of 2 mm/min. Finally,
the tensile-fracture surface was further analyzed with a
scanning electron microscope. Besides, the microstruc-
ture and properties of the as-cast specimen were also
tested for comparison.
3 RESULTS AND DISCUSSION
3.1 Microstructure
To examine the microstructure-refinement effect, the
optical microstructures of the AZ31-Mg alloy before and
after the extrusion are shown in Figures 2 and 3. The
grains of the as-cast billet are coarse and the average
grain size is calculated to be about 240 μm (Figure 2a).
Significantly, the DEBS-ed AZ31-magnesium-alloy
sheet extruded at 563 K shows a prominent grain-refine-
ment effect, and the average grain sizes obtained from
the x, y and z planes are about 7.2 μm, 4.5 μm and 3.2 μm,
respectively, as presented in Figures 2b to 2d. After one
pass, the originally coarse grains are refined markedly
during the DEBS process, which is mainly attributed to
the occurrence of DRX, triggered by a large accumulated
strain during the extrusion process. In Figure 2b, the
microstructure consists of some coarse grains (8 μm) in
the local area, being surrounded by much finer grains
(2–4 μm). Even so, from an overall perspective, the
grains are relatively homogeneously distributed and
refined. It is obvious that the average grain size of the
AZ31-magnesium alloy can be only reduced from 31.8
μm to 8 μm after four ECAP passes at 573 K for rout
Bc.14 In contrast, the DEBS process has a unique
advantage and a potential for a grain refinement.
L. W. LU et al.: IMPROVING THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MAGNESIUM-ALLOY ...
1020 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1019–1023
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: Optical microstructures of AZ31-Mg alloy before and after
the DEBS process at 563 K: a) as-cast, b) to d) x, y and z planes of the
extruded sample
Figure 1: Sketch of the DEBS die and definition of external orienta-
tions; ED, TD, and ND denote the extrusion, transverse and normal
directions
With an increase in the extrusion temperature
(643 K), many tiny grains emerge (Figures 3a to 3c).
However, as there are still some inhomogeneously
distorted grains in the local area, marked by ellipses
(Figure 3b), there is not enough energy stored to fully
complete DRX during the DEBS process, demonstrating
that the extrusion temperature plays a significant role in
the formation of distorted grains affected by the dislo-
cation density, the evolution of dislocation cells and
sub-structures. The aforementioned result is consistent
with an earlier research result reported by J. F. Jiang et
al.16 They also found that the mean grain size of 27.67
μm can be obtained with ECAP at 623 K, which is much
larger than that obtained at 643 K using DEBS.
3.2 Textural evolution
Figure 4 shows the {0002} and {1010} pole figures
of the DEBS-ed sheets at different temperatures, exhi-
biting a typical {0002} fiber texture after the DEBS
process. However, in comparison with the conventional
extrusion (17.55)17, the texture intensity of the DEBS-ed
sheets is significantly weakened (Figures 4a and 4b) and
the texture component becomes more dispersive along
the ED, which can induce a good deformation capability
due to the <1120> direction in fewer grains orientated
parallel to the ED.
A closer observation reveals that the {0002} pole
figure forms a double-peaked basal texture characterized
by a pronounced {0002} basal plane tilted towards the
ED at about 30° and 13° (Figure 4a), respectively, which
may be a result of a larger shear strain accumulated by
the bar-sheet deformation and continuous bending-shear
deformation, activating more <c+a> slip systems. These
phenomena were previously reported for compressive
shock loading and ARB processes by H. Asgari18 and
H. Chang19, respectively. When the extrusion tempera-
ture increases to 643 K, the {0002} texture intensity de-
creases to 9.54, as shown in Figure 4b, and the {0002}
basal plane tilts towards the ED at an angle of 10°, which
is smaller than the one at 563 K. The decrease in the
basal texture intensity may be attributed to an increased
activation of the non-basal slip at the higher temperature
during the DEBS process.
3.3 Mechanical properties
To shed light on how the grain refinement and texture
evolution affect the mechanical properties, tensile tests
of the as-cast and DEBS-ed AZ31-Mg alloys are carried
out at room temperature, and the tensile stress-strain pro-
L. W. LU et al.: IMPROVING THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MAGNESIUM-ALLOY ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1019–1023 1021
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Light microstructures of DEBS-ed AZ31-Mg alloy at 643K: a)–c) x, y and z planes of the extruded sample
Figure 5: Tensile stress-strain curves at room temperature for the
AZ31 samples processed with DEBS at 563 K and 643 K
Figure 4: {0002} and {1010} pole figures for the DEBS-processed
AZ31 alloys at different temperatures: a) 563 K, b) 643 K
perties are shown in Figure 5. The yield strength (YS),
ultimate tensile strength (UTS) and elongation of the
as-cast AZ31-Mg alloy are 103.3 MPa, 163.6 MPa and
11.67 %, respectively, and they are mainly caused by the
original coarse grains and casting defects.20 However, the
mechanical properties are enormously enhanced with the
DEBS process. As for the extrusion sheet at 563 K, the
YS and UTS are about 203 MPa and 290 MPa, respec-
tively. It should be noted that the elongation (27.7 %) is
two times higher than that of the as-cast sample, which
can be attributed to the combined effect of the weak
basal texture and fine-grain size. It is well known that the
weaker-texture intensity can lead to a larger Schmid
factor of the <a> basal slip, which can improve its duc-
tility.21 When the processing temperature increases from
563 K to 643 K, the YS and UTS are inversely increased
to 220 MPa and 300 MPa, respectively. There are some
distorted grains with a high density of dislocation in the
643 K sample, which play an important role in im-
proving the strength. However, the elongation decreases
from 27.7 %to 25.7 %, indicating that the grain refine-
ment plays a dominant role in the enhancement of
ductility. On the other hand, the tilt angle of the basal
texture at 563 K is larger than that at 643 K, as shown in
Figures 4a and 4b. Besides, the high density of dislo-
cation and stress in the 643 K sample due to the accumu-
lated distorted grains can also decrease its elongation.
The tensile fracture surface of an AZ31-alloy sheet
before and after the DEBS process is presented in Figure
6. There are a large amount of cleavage planes, some
shallow dimples and a few cracks in the fracture surface
of the as-cast AZ31-Mg alloy (Figure 6a), which are
typical characteristics of a quasi-cleavage fracture. By
comparison, many dimples emerge and become much
deeper in the DEBS-ed AZ31-alloy sheet (Figure 6b),
but when the extrusion temperature increases to 643 K,
the depth and number of dimples become shallower and
fewer again (Figure 6d), indicating that the extrusion
temperature plays an important role in tailoring the
ductility.
Anyhow, the DEBS process can dramatically improve
the plasticity and produce typical characteristics of a
ductile fracture, which is mainly attributed to the tilted
weak texture and grain refinement. A small-scale view of
the tensile fracture surface can be seen in Figures 6c and
6e. There is almost no cleavage plane in Figure 6c; how-
ever, some noticeable cleavage traces and inhomoge-
neous dimples are shown in Figure 6e, caused by some
large local and inhomogeneously distributed grains.
Therefore, to obtain refined and homogeneous micro-
structures and a titled weak basal texture, an advanced
processing technology can be used as one of the most
effective ways of modifying the plasticity of the Mg
alloys important for engineering.
4 CONCLUSION
In summary, the DEBS technique was applied to
fabricate an AZ31-magnesium-alloy sheet with high
performance and we systematically investigated the grain
refinement, texture evolution and mechanical properties
of the processed alloy. We find that the average grain
size of the as-cast AZ31 alloy is successfully refined
from 240 μm to 3.2 μm at 563 K; a crystallographic
rotation and a weak texture are yielded after the DEBS
process. Due to the combined effect of the grain
refinement and texture weakening, both the plasticity and
strength are dramatically improved. It is demonstrated
that DEBS is a simple and highly efficient way of pro-
ducing AZ31-alloy sheets with high performance.
Acknowledgments
This work was supported by the National Natural
Science Foundation of China (Grant No. 51505143), the
Scientific Research Fund of Hunan Provincial Education
Department (Grant No. 17B089) and the China Post-
doctoral Science Foundation (Grant No. 2016T90759 &
2014M562128).
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1022 Materiali in tehnologije / Materials and technology 51 (2017) 6, 1019–1023
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
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Ungar, L Balogh, Grain refinement in nanostructured Al-Mg alloys
subjected to high pressure torsion, J. Mater. Sci., 45 (2010),
4659-4664, doi:10.1007/s10853-010-4604-3
12 M. H. Maghsoudi, A. Zarei-Hanzaki, H. R. Abedi, Modification of
the grain structure,  phase morphology and texture in AZ81 Mg
alloy through accumulative back extrusion, Mater. Sci. Eng. A, 595
(2014), 99–108, doi:10.1016/j.msea.2013.11.095
13 G. Faraji, M. M. Mashhadi, H. S. Kim, Microstructure inhomoge-
neity in ultra-fine grained bulk AZ91 produced by accumulative back
extrusion (ABE), Mater. Sci. Eng. A, 528 (2011), 4312–4317,
doi:10.1016/j.msea.2011.02.075
14 A. Muralidhar, S. Narendranath, H. Shivananda Nayaka, Effect of
equal channel angular pressing on AZ31 wrought magnesium alloys,
J. Magnesium Alloys, 1 (2013), 336–340, doi:10.1016/j.jma.2013.
11.007
15 J. B. Lin, X. Y. Wang, W. J. Ren, X. X. Yang, Q. D. Wang, Enhanced
strength and ductility due to microstructure refinement and texture
weakening of the GW102K alloy by cyclic extrusion compression, J.
Mater. Sci. Technol., 32 (2016), 783–789, doi:10.1016/j.jmst.2016.
01.004
16 J. F. Jiang, Y. Wang, J. J. Qu, Microstructure and mechanical pro-
perties of AZ61 alloys with large cross-sectional size fabricated by
multi-pass ECAP, Mater Sci. Eng. A, 560 (2013), 473–480,
doi:10.1016/j.msea.2012.09.092
17 L. W. Lu, C. M. Liu, Z. R. Yin, J. Zhao, L. Gan, Z. C. Wang, Double
extrusion of Mg-Al-Zn alloys, Int. J. Adv. Manuf. Technol., 89
(2017), 869–875, doi:10.1007/s00170-016-9146-7
18 H. Asgari, A. G. Odeshi, J. A. Szpunar, L. J. Zeng, E. Olsson, Grain
size dependence of dynamic mechanical behavior of AZ31B mag-
nesium alloy sheet under compressive shock loading, Mater.
Charact., 106 (2015), 359–367, doi:10.1016/j.matchar.2015.06.030
19 H. Chang, M. Y. Zheng, W. M. Gan, K. Wu, E. Maawad, H. G. Brok-
meier, Texture evolution of the Mg/Al laminated composite fabri-
cated by the accumulative roll bonding, Scripta Mater., 61 (2009) 7,
717–720, doi:10.1016/j.scriptamat.2009.06.014
20 W. Guo, Q. D. Wang , B. Ye, H. Zhou, Microstructure and mecha-
nical properties of AZ31 magnesium alloy processed by cyclic
closed-die forging, J. Alloys Compd., 558 (2013), 164–171,
doi:10.1016/j.jallcom.2013.01.035
21 Q. S. Yang, B. Jiang, H. C. Pan, B. Song, Z. T. Jiang, J. H. Dai, L. F.
Wang, F. S. Pan, Influence of different extrusion processes on
mechanical properties of magnesium alloy, J. Magnesium Alloys, 2
(2014), 220–224, doi:10.1016/j.jma.2014.10.001
L. W. LU et al.: IMPROVING THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MAGNESIUM-ALLOY ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1019–1023 1023
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
perties are shown in Figure 5. The yield strength (YS),
ultimate tensile strength (UTS) and elongation of the
as-cast AZ31-Mg alloy are 103.3 MPa, 163.6 MPa and
11.67 %, respectively, and they are mainly caused by the
original coarse grains and casting defects.20 However, the
mechanical properties are enormously enhanced with the
DEBS process. As for the extrusion sheet at 563 K, the
YS and UTS are about 203 MPa and 290 MPa, respec-
tively. It should be noted that the elongation (27.7 %) is
two times higher than that of the as-cast sample, which
can be attributed to the combined effect of the weak
basal texture and fine-grain size. It is well known that the
weaker-texture intensity can lead to a larger Schmid
factor of the <a> basal slip, which can improve its duc-
tility.21 When the processing temperature increases from
563 K to 643 K, the YS and UTS are inversely increased
to 220 MPa and 300 MPa, respectively. There are some
distorted grains with a high density of dislocation in the
643 K sample, which play an important role in im-
proving the strength. However, the elongation decreases
from 27.7 %to 25.7 %, indicating that the grain refine-
ment plays a dominant role in the enhancement of
ductility. On the other hand, the tilt angle of the basal
texture at 563 K is larger than that at 643 K, as shown in
Figures 4a and 4b. Besides, the high density of dislo-
cation and stress in the 643 K sample due to the accumu-
lated distorted grains can also decrease its elongation.
The tensile fracture surface of an AZ31-alloy sheet
before and after the DEBS process is presented in Figure
6. There are a large amount of cleavage planes, some
shallow dimples and a few cracks in the fracture surface
of the as-cast AZ31-Mg alloy (Figure 6a), which are
typical characteristics of a quasi-cleavage fracture. By
comparison, many dimples emerge and become much
deeper in the DEBS-ed AZ31-alloy sheet (Figure 6b),
but when the extrusion temperature increases to 643 K,
the depth and number of dimples become shallower and
fewer again (Figure 6d), indicating that the extrusion
temperature plays an important role in tailoring the
ductility.
Anyhow, the DEBS process can dramatically improve
the plasticity and produce typical characteristics of a
ductile fracture, which is mainly attributed to the tilted
weak texture and grain refinement. A small-scale view of
the tensile fracture surface can be seen in Figures 6c and
6e. There is almost no cleavage plane in Figure 6c; how-
ever, some noticeable cleavage traces and inhomoge-
neous dimples are shown in Figure 6e, caused by some
large local and inhomogeneously distributed grains.
Therefore, to obtain refined and homogeneous micro-
structures and a titled weak basal texture, an advanced
processing technology can be used as one of the most
effective ways of modifying the plasticity of the Mg
alloys important for engineering.
4 CONCLUSION
In summary, the DEBS technique was applied to
fabricate an AZ31-magnesium-alloy sheet with high
performance and we systematically investigated the grain
refinement, texture evolution and mechanical properties
of the processed alloy. We find that the average grain
size of the as-cast AZ31 alloy is successfully refined
from 240 μm to 3.2 μm at 563 K; a crystallographic
rotation and a weak texture are yielded after the DEBS
process. Due to the combined effect of the grain
refinement and texture weakening, both the plasticity and
strength are dramatically improved. It is demonstrated
that DEBS is a simple and highly efficient way of pro-
ducing AZ31-alloy sheets with high performance.
Acknowledgments
This work was supported by the National Natural
Science Foundation of China (Grant No. 51505143), the
Scientific Research Fund of Hunan Provincial Education
Department (Grant No. 17B089) and the China Post-
doctoral Science Foundation (Grant No. 2016T90759 &
2014M562128).
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20 W. Guo, Q. D. Wang , B. Ye, H. Zhou, Microstructure and mecha-
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L. W. LU et al.: IMPROVING THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF MAGNESIUM-ALLOY ...
Materiali in tehnologije / Materials and technology 51 (2017) 6, 1019–1023 1023
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS