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Materials Science and Engineering A 499 (2009) 404–410
Contents lists available at ScienceDirect
Materials Science and Engineering A
journal homepage: www.elsevier.com/locate/msea
Effects of ECAE temperature and billet orientation on the microstructure, texture
evolution and mechanical properties of a Mg–Zn–Y–Zr alloy
W.N. Tang a,c , R.S. Chen a,∗ , J. Zhou b , E.H. Han a
a
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China
Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands
c
Graduate School of the Chinese Academy of Sciences, PR China
b
a r t i c l e
i n f o
Article history:
Received 17 July 2007
Received in revised form 20 August 2008
Accepted 15 September 2008
Keywords:
Magnesium
Mg–Zn–Y–Zr alloy
Equal channel angular extrusion (ECAE)
Texture
Mechanical property
a b s t r a c t
Single-pass equal channel angular extrusion (ECAE) experiments of an extruded Mg–Zn–Y–Zr alloy with
an intense initial basal texture were performed in two inter-perpendicular billet orientations and at 473
and 623 K. The study was aimed to determine the effects of ECAE temperature and billet orientation on
the microstructure, texture evolution and mechanical properties of the ECAEed alloy. It was found that
the grain refinement achieved through the single-pass ECAE in the Orient-I billet orientation (the normal
direction (ND) of the extruded plate parallel with the ECAE exit direction) was more effective than that
in the Orient-II billet orientation (the ND of the extruded plate perpendicular to the ECAE exit direction).
The average grain sizes after ECAE at 473 K were much smaller than those after ECAE at 623 K. The pole
figures of the alloy ECAEed at 473 K showed that most of the basal planes in the Orient-I and Orient-II
samples were inclined about 40◦ and 35◦ , respectively, with respect to the longitudinal direction of the
ECAE extrudate. However, for the alloy ECAEed at 623 K, most of the basal planes were parallel with the
longitudinal direction of the ECAE extrudate. It was remarkable that the yield strengths of the alloy ECAEed
at 473 K were lower than those at 623 K. The peculiar relationship between ECAE temperature and the
mechanical properties of the alloy was ascribed to the texture evolution during ECAE.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Low density, high specific strength and good damping characteristics are attractive attributes of magnesium alloys. In recent
years, these attributes have been increasingly utilized in transport,
electronic and consumer products. Although wrought magnesium
alloys are known for possessing better mechanical properties than
the cast counterparts, the structural applications of extruded,
forged and rolled magnesium alloys are yet quite limited, mainly
because of their poor deformability at room and moderately
elevated temperatures—an intrinsic characteristic of a metallic
material with a hexagonal close packed (HCP) crystal structure
[1,2].
The limited number of activated slip systems in an HCPstructured alloy results in the formation of a strong crystallographic
texture during thermomechanical processing [2]. In the cases of
pure magnesium and its alloys with an initial texture, a number of
studies [1–4] were conducted to investigate their deformation characteristics under the conditions where the tensile or compressive
∗ Corresponding author. Tel.: +86 24 23926646; fax: +86 24 23894149.
E-mail addresses: rschen@imr.ac.cn, rongshichen@yahoo.com (R.S. Chen).
0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2008.09.048
stress axis relative to the initial texture was specially arranged at
various angles. Discrepancies in mechanical behaviour were found
and explained in terms of the orientation relationship between the
loading axis and the texture in the deformed materials.
The application of equal channel angular extrusion (ECAE) as an
effective method for grain refining [5,6] has been extended from
aluminium alloys to magnesium alloys to improve their strength
and ductility [7–9]. Liu et al. [7] found significant improvements
in the yield strength and ductility of the conventionally extruded
Mg–3.3%Li alloy after 4-pass ECAE at 523 K with Route A and Route
Bc (Route A is defined as that when the billet is extruded without rotation between passes; route Bc is defined as a rotation of
90◦ in the same direction between passes), and the improvements
with Route A were greater than those with Route Bc. In contrast
to these findings, Mukai et al. [8] found that, for the AZ31 alloy
after 8-pass ECAE at 473 K with Route Bc, its tensile yield strength
was slightly lower than the conventionally extruded counterpart,
although its elongation to failure was twice as large as that of the
conventionally extruded alloy. Kim et al. [9] obtained similar results
from the tensile tests of the AZ61 alloy after the conventional extrusion and then ECAE at 548 K with Route Bc. The peculiar mechanical
behaviour of these magnesium alloys was attributed to the strong
texture developed during the ECAE process.
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W.N. Tang et al. / Materials Science and Engineering A 499 (2009) 404–410
In recent years, the interest in Mg–Zn–Y(–Zr) alloys with an
icosahedral quasicrystal phase, i.e. the I-phase formed upon solidification, has been growing, because these alloys, after going
through thermomechanical processing such as hot extrusion, possess desired yield and ultimate tensile strengths both at room
temperature and in a low temperature range, typically up to 473 K
[10]. To improve the strength and ductility of Mg–Zn–Y(–Zr) alloys
further, ECAE has been tried [11,12]. Zheng et al. [11] reported that,
while the conventionally extruded Mg–11Zn–0.9Y (wt%) alloy did
not exhibit a marked improvement in ductility after ECAE with
Route Bc at 523–473 K, the yield strength of the ECAEed alloy was
significantly higher than that of the conventionally extruded alloy,
and the yield strength increased as the number of ECAE passes
increased. The authors [12] also reported that, after ECAE under
the same condition, the ductility of the Mg–5.9Zn–0.9Y–0.2Zr
(wt%) alloy increased with increasing ECAE passes, while its yield
strength was lower than that of the conventionally extruded
alloy and decreased significantly as the number of ECAE passes
increased.
It appears that the mechanical properties of extruded magnesium alloys, especially those of extruded Mg–Zn–Y–Zr alloys, after
multi-pass ECAE, do not necessarily exhibit improved strength and
ductility as expected. It is likely that ECAE process parameters, such
as temperature, and the initial crystal orientation of the billet with
respect to the shear stresses imposed during ECAE are influential
on the evolution of texture and thus on the mechanical properties
of the alloys tested uni-axially under tensile or compressive loading
[1–4]. In the open literature, there are few reports on the relationship between ECAE process parameters, billet orientation and the
resultant mechanical properties of magnesium alloys.
The present study was aimed at determining the effects of
ECAE temperature and the orientation of the billet with an initial texture on the microstructure, texture evolution and resultant
mechanical properties of magnesium alloys. To reach this aim, a
conventionally extruded Mg–Zn–Y–Zr plate with an intense initial
(0 0 0 2) basal texture was subjected to single-pass ECAE experiments at 473 and 623 K and in two inter-perpendicular billet
orientations. Microstructure and texture analyses were performed
and the results were used to explain the mechanical behaviour of
the alloy exhibited during tensile testing.
2. Experimental details
The alloy with a chemical composition of Mg–6.43%Zn–
1.0%Y–0.48%Zr (wt%) was prepared from pure magnesium (99.9%),
pure zinc (99.99%), Mg–25%Y and Mg–33%Zr master alloys using an
electric resistance heating furnace in an SF6 and CO2 atmosphere.
The molten alloy was poured into a cylindrical metal mould with a
diameter of 100 mm. The as-cast ingot was machined into extrusion
blocks and extruded at 663 K in the conventional manner into plates
with a rectangular cross-section of 14 mm × 60 mm. The extrusion
ratio applied was about 10:1.
The die used in the ECAE experiments had two equal channels
with a square cross-section of 12 mm × 12 mm and an intersecting angle of 90◦ , as illustrated in Fig. 1. With such an ECAE die
setup, an equivalent strain of 1.05 per pass could be applied to the
billet [5]. The ECAE billets with a length of 100 mm and a square
cross-section of 12 mm × 12 mm were cut from the middle part of
the extruded plate with the cross-section of 14 mm × 60 mm, using
electro-discharge machining. The orientations of the billets for the
ECAE experiments with respect to the plate were divided into two
groups, one with the normal direction (ND) of the extruded plate
parallel with the X direction of ECAE (designated as Orient-I) and
another with the ND of the plate parallel with the Y direction of
405
Fig. 1. Schematic of the ECAE die setup used for the ECAE experiments on the
conventionally extruded Mg–Zn–Y–Zr alloy in the Orient-I and Orient-II billet orientations. The ECAE billets were machined from the extruded plate along the extrusion
direction (ED).
ECAE (designated Orient-II), as shown in Fig. 1. Before a billet was
inserted into the ECAE entry channel, lubrication was applied to the
billet to decrease its friction with the channel inner wall. The billet
was held in the entry channel at test temperature for 15 min before
ECAE started. Single-pass ECAE experiments were performed at 473
and 623 K and at a constant ram speed of 5 mm/min. After ECAE, the
extrudate was taken out from the exit die and quenched in water
immediately.
The ECAEed extrudate was sectioned on the X–Z plane (see Fig. 1)
at the center for metallographic examination. Metallographic samples were polished to a mirror finish, etched in a glycol-diluted
nitric acid solution, and examined using an optical microscope. The
grain size d was estimated using the linear intercept method. Crystallographic texture measurements were made on the ED–TD plane
in the conventionally extruded plate, and on the X–Z plane (not
given in the paper) and the X–Y plane of the ECAE extrudate. The
pole figures of {0 0 0 2} were measured up to a reflection angle of
70◦ using an X-ray diffractometer.
Tensile specimens with a gauge length of 5 mm and a rectangular
cross-section of 2 mm × 3 mm were machined from the extrudate
with their longitudinal axes in parallel with the X direction of ECAE
samples. Tensile tests were performed at room temperature and at
an initial strain rate of 1 × 10−3 s−1 .
3. Results
3.1. Microstructural characteristics
3.1.1. Initial microstructure and texture
The original microstructure of the as-cast ingots before conventional extrusion was shown in Fig. 2a, with a small number of
secondary I-phases distributed in the matrix [10–12]. The initial
microstructure of the conventionally extruded plate before ECAE
is shown in Fig. 2b. It can be seen that the microstructure is inhomogeneous with some extrusion strips and small grains. The strips,
which are some elongated grains oriented in the extrusion direction (ED as shown in Fig. 1), are dispersed in the matrix with small
grains of about 10 ␮m. In addition, the broken second-phase particles (i.e. the black particles in Fig. 2b) scatter in the matrix. The
pole figures of the extruded plate on the ED–TD plane are given in
Fig. 2c, showing a typical texture of an HCP-structured metal after
extrusion deformation, i.e. the c-axes of most crystals being approximately parallel with the normal direction (ND) of the extruded
plate.
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lower temperature (473 K), the microstructures of the material
ECAEed at 623 K are also inhomogeneous, with some elongated
coarse grains oriented at about 45◦ to the X direction of ECAE samples. Although some original large grains remain in both of the
Orient-I and Orient-II samples, a number of initial grains have been
refined (Fig. 4a and c), with the fine recrystallized grains of about
5 ␮m (Fig. 4b and d). However, the average grain sizes of their new
grain structures are obviously greater as compared with those of
the alloys ECAEed at the lower temperature of 473 K.
3.2. Mechanical behaviour
Fig. 2. Microstructure and texture of the Mg–Zn–Y–Zr alloy: (a) optical microstructure of the as-cast ingots; (b) optical microstructure on the ED-ND plane of the
conventionally extruded alloy before ECAE, and (c) (0 0 0 2) and (1 0 1 0) pole figures
on the ED–TD plane of the conventionally extruded alloy before ECAE.
3.1.2. Microstructures after single-pass ECAE
The microstructures of the Orient-I and Orient-II samples after
the single-pass ECAE experiments at 473 K are shown in Fig. 3. In
the microstructure of the Orient-I and Orient-II samples (Fig. 3a
and b), a large number of fine recrystallized grains with sizes about
1 ␮m appeared in the deformation regions (Fig. 3b and d). Some
elongated coarse grains remain in the Orient-I and Orient-II samples (Fig. 3a and c), whose elongated direction inclined about 45◦
with respect to the X direction of ECAE samples, being about parallel with the shear direction of the ECAE die (Fig. 1). However, the
volume fraction of un-recrystallized coarse grains in the Orient-II
sample is a little more than that in the Orient-I sample. Accordingly,
the average grain size of the Orient-II sample is a little coarser than
that of the Orient-I sample.
The microstructures of the Orient-I and Orient-II samples after
the single-pass ECAE experiments at a higher temperature (623 K)
are shown in Fig. 4. In comparison with the alloys ECAEed at the
Tensile tests in parallel with the X direction of ECAEed samples
were carried out at room temperature. The true stress–strain curves
of the specimens after single-pass ECAE under the experimental
conditions applied are shown in Fig. 5 and the corresponding tensile
property data are given in Table 1.
The results demonstrate that ECAE temperature has indeed
strong influences on the mechanical properties of the Mg–Zn–Y–Zr
alloy and the correlations of strengths with ECAE temperature do
not appear to be all consistent with the expectations based on
the average grain sizes. As shown in Fig. 5 and Table 1, the yield
strengths of the material ECAEed at 473 K are lower than those of
the alloy ECAEed at 623 K in both of the Orient-I and Orient-II billet
orientations. In the case of the Orient-I billet orientation, the elongation of the alloy ECAEed at 473 K is nearly twice as high as that
ECAEed at 623 K. However, in the case of the Orient-II billet orientation, the effect of ECAE temperature on elongation to fracture is
the other way around, elongation being higher at the higher ECAE
temperature.
As expected, the mechanical properties of the ECAEed
Mg–Zn–Y–Zr alloy vary with the billet orientation. For the alloy
ECAEed at 473 K, by turning the billet orientation from Orient-I
to Orient-II, the yield strength can be increased from 160 to 178
MPa, while elongation is decreased from 25.8 to 19.8%. For the
alloy ECAEed at 623 K, however, the same billet orientation change
leads to a decrease in yield strength and a significant increase in
elongation.
In comparison with the mechanical properties of the conventionally extruded plate, the single-pass ECAE affects the mechanical
properties of the Mg–Zn–Y–Zr alloy and the extent depends on
ECAE temperature and billet orientation. In all the cases, the
improvements in strength expected from the grain refinement have
not really been realised. In this sense, the present findings are
inconsistent with the general recognition of the ECAE process as
an effective grain refining and strengthening method as in the case
of aluminium alloys [13].
3.3. Texture evolution
The (0 0 0 2) pole figures of the samples ECAEed in the Orient-I
and Orient-II billet orientations and at 473 and 623 K are shown in
Table 1
Tensile properties of the alloy after ECAE under the experimental conditions applied
(TYS, tensile yield strength; UTS, ultimate tensile strength).
Processing condition
TYS (MPa)
UTS (MPa)
Elongation (%)
ECAE at 473 K
Orient-I
Orient-II
160
178
322
313
25.8
19.8
ECAE at 623 K
Orient-I
Orient-II
242
227
347
342
12.6
22.9
The original hot-extruded plate
221
356
18.9
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407
Fig. 3. Optical microstructures of the Mg–Zn–Y–Zr alloy after single-pass ECAE at 473 K: (a, b) in the Orient-I billet orientation; and (c, d) in the Orient-II billet orientation.
Fig. 6. In the alloy ECAEed at 473 K (Fig. 6a and b), a texture with
most of the basal planes nearly parallel with the shear plane of
ECAE has been formed. The c-axes of most crystals in the OrientI sample are inclined about 40◦ with respect to the Z direction of
ECAE and the texture has a maximum intensity of more than 10.01,
while these in the Orient-II sample are inclined about 35◦ and the
texture has a smaller maximum intensity than that in the Orient-I
sample.
The textures of the alloy ECAEed at 623 K (Fig. 6c and d) are
very different from those of the alloy ECAEed at 473 K. A texture
with most crystals whose c-axes are approximately parallel to the
Z direction has been formed in the Orient-I sample and with a
maximum intensity of 6.09. It means that the initial texture in the
conventionally extruded plate has not been changed much after
the single-pass ECAE process, except a slight increase in its peak
intensity. However, the texture formed in the Orient-II sample is
characterised by the basal planes of most crystals being parallel with the X direction of ECAE sample. In comparison with the
textures in the conventionally extruded plate and in the OrientI sample, the intensity of the basal plane texture in the Orient-II
sample is apparently dispersed and the c-axes of more crystals are
distributed nearby the YD (Y direction).
4. Discussion
It has been found from a number of previous studies [14,15] that
the microstructure of a magnesium alloy after single-pass ECAE
can be refined to a great extent through dynamical recrystallization (DRX). The results of the microstructure examinations in the
present study show that recrystallization indeed occurred to different degrees to the Mg–Zn–Y–Zr alloy during single-pass ECAE at
473 and 673 K and in the two different billet orientations. However,
the refined grains are not uniform in size and mixed with the coarse
original grains. This bimodal grain structure comprising the coarse
original grains and newly formed finer grains was also observed by
other researchers [16]. Although the grain structure after the singlepass ECAE under each of the conditions applied is non-uniform, the
average grain size after ECAE is smaller than that after the conventional extrusion. It is apparent that the recrystallized grain sizes are
strongly influenced by ECAE temperature. At the lower ECAE temperature (473 K), the recrystallized grain structures are much finer
than those at the higher ECAE temperature (623 K). According to
the Hall–Petch relationship, the yield strength of the present magnesium alloy after ECAE would be expected to improve from that of
the conventionally extruded plate, as a result of grain refinement
due to DRX. The alloy ECAEed at the lower temperature would be
expected to possess higher yield strength than that ECAEed at the
higher temperature. In addition, a finer grain structure should lead
to a greater ductility.
However, the present results show that the changes in yield
strength and elongation from the original hot-extruded plate do not
meet these expectations. The as-ECAEed yield strength and elongation may be either higher or lower than those of the original
extruded plate, see Table 1. Moreover, the yield strength of the alloy
ECAEed at the lower temperature (473 K) and having a smaller average grain size appears to be lower than that of the alloy ECAEed at
the higher temperature (623 K) and having a greater average grain
size. Obviously, there are other metallurgical factors that disturb
the expected dependence of mechanical properties on the average
grain size, leading to the peculiar mechanical behaviour observed.
The slip systems in the magnesium crystal with an HCP lattice include the basal plane {0 0 0 2} 1 1 2 0, the prismatic system
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W.N. Tang et al. / Materials Science and Engineering A 499 (2009) 404–410
Fig. 4. Optical microstructures of the Mg–Zn–Y–Zr alloy after single-pass ECAE at 623 K: (a, b) in the Orient-I billet orientation; and (c, d) in the Orient-II billet orientation.
{1 0 1 0} 1 1 2 0, the first-order pyramidal systems {1 0 1 1} 1 1 2 0
or {1 0 1 2} 1 1 2 0 and the secondary-order pyramidal system
{1 1 2 2} 1 1 2 3 [17]. It is known that, at room temperature, the
critical resolved shear stress (CRSS) of the basal slip system is as low
as 0.5 MPa, being about 100 times lower than the CRSS values of the
non-basal slip systems in pure magnesium [18]. Consequently, the
non-basal slip systems can hardly be activated and the basal slip
system plays a dominant role in plastic deformation at room temperature and at moderately elevated temperatures [19]. According
to the Schmid’s law, the orientation of a slip plane and the slip direction with respect to the stress axis, as well as the CRSS value of the
slip system, decide whether a slip system can be activated. When
Fig. 5. True stress–true strain curves showing the mechanical behaviour of the
conventional extruded plate and that of the specimens after ECAE under the experimental conditions applied.
the slip plane and the slip direction are at 45◦ relative to the stress
axis, a maximum Schmid factor occurs and the resolved shear stress
on a slip system reaches its maximum value. Therefore, during the
tensile deformation of HCP-structured magnesium at room temperature, the orientation relationship between the direction of tensile
stress applied and the {0 0 0 2} basal plane has a strong influence
on its mechanical properties, notably yield strength and elongation
to fracture.
In this study, most of the {0 0 0 2} basal planes in the alloy
ECAEed at 473 K are inclined with respect to the X direction of
ECAE with an incline angle of about 40◦ in the Orient-I sample
and 35◦ in the Orient-II sample. After ECAE at the higher temperature of 623 K, however, the textures are characterised by most
of the {0 0 0 2} basal planes being parallel with the X direction of
ECAE. The dependence of the texture on ECAE temperature found
in the present study is essentially similar to the results reported by
Yoshida et al. [20] who performed ECAE experiments on the conventionally extruded AZ31 rods at 523 and 573 K and found most
of the basal planes either inclined about 30◦ with respect to the X
direction of ECAE and parallel with the X direction of ECAE, respectively. Furthermore, in a study carried out by Agnew et al. [4], it
was concluded that crystallographic orientation had a profound
effect on the tensile properties of the ECAEed AZ31 alloy, while
grain size had a relatively little effect. For the present Mg–Zn–Y–Zr
alloy, the single-pass ECAE under each of the experimental conditions applied leads to a decrease in the average grain size as a result
of partial recrystallization, whereas the resultant yield strength and
elongation do not all increase as expected. This suggests that the
influences of both texture and average grain size on the mechanical properties of the ECAEed magnesium alloy should be taken into
account.
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409
Fig. 6. The (0 0 0 2) pole figures showing the textures of the Mg–Zn–Y–Zr alloy ECAEed: (a) in the Orient-I billet orientation and at 473 K, (b) in the Orient-II billet orientation
at 473 K, (c) in the Orient-I billet orientation and at 623 K, and (d) in the Orient-II billet orientation and at 623 K. The tables on the right show the intensity levels.
For the alloy ECAEed at 623 K, improvements in both yield
strength and elongation would be expected, because of the grain
refinement brought about by ECAE. For the Orient-I specimen, an
improvement in yield strength is indeed achieved, but its elongation is significantly reduced, as compared with that of the
conventional extruded alloy (Table 1). It is important to note that
the tensile tests at room temperature were performed in the same
direction as ECAE (the X direction). The pole figures show that most
of the basal planes in these specimens are parallel with the tensile
direction (Fig. 6c) and its basal texture intensity is slightly higher
as compared with that of the conventionally extruded plate. In this
case, the basal slip of most crystals in the material has a small
Schmid factor and, in general, is hard to operate, leading to the
marked decrease in tensile ductility [21]. Therefore, at the ECAE
temperature of 623 K and in the billet orientation of Orient-I, it is
the texture, rather than the grain refining, that mainly determines
the mechanical properties of the ECAEed alloy, especially its ductility. However, after ECAE at the same temperature but in the other
billet orientation (Orient-II), the peak intensity of the texture with
most of the basal planes being parallel with the X direction of ECAE
is relatively low (Fig. 6d), and more basal planes tilt an angel of a
few degrees away from the tensile direction. As a result, the yield
strength of the Orient-II specimen is slightly lower than that of
the Orient-I specimen, while the elongation of the former is much
higher.
For the magnesium alloy ECAEed at 473 K, most of the basal
planes tilt about 30–45◦ relative to the tensile direction (Fig. 6a
and b), and thus the basal slip can easily be activated, resulting
in a relatively low work hardening rate [21]. The alloy ECAEed at
this temperature exhibits significant decreases in yield strength
and increases in ductility, as compared with the conventionally
extruded plate. Furthermore, the texture with the basal planes
tilting 40◦ relative to the tensile direction and a stronger intensity in the Orient-I specimen indicate that the basal slip is more
favourable, as compared with that in the Orient-II specimen with a
tilt angle of 35◦ and a lower texture intensity. Therefore, the yield
strength of the Orient-I specimen is lower and the elongation is
higher, in comparison with the Orient-II specimen.
From the preceding discussion, it is clear that ECAE temperature and the orientation of the billet with an initial texture have
significant influences on the microstructure, texture evolution and
resultant mechanical properties. The peculiar relationship between
the yield strength of the Mg–Zn–Y–Zr alloy and the average grain
size indicates that the texture gains an upper hand in determining
if the alloy is strengthened or softened, depending on ECAE temperature and billet orientation, at least in the case of single-pass
ECAE.
5. Conclusions
A conventionally extruded Mg–Zn–Y–Zr alloy plate with an
intense basal texture was subjected to single-pass ECAE experiments at 473 and 623 K in two inter-perpendicular billet
orientations. The as-ECAEed microstructures and textures were
analysed, and tensile properties were determined. The following
conclusions may be drawn.
1. Single-pass ECAE at 473 and 623 K and in both of the billet orientations leads to grain refinement in the Mg–Zn–Y–Zr alloy
through dynamic recrystallization. ECAE at the higher temperature of 623 K results in a larger average grain size than at 473 K.
2. The textures developed during ECAE at 473 K are characterised
by most of the basal planes in the Orient-I and Orient-II samples
being inclined about 40◦ and 35◦ , respectively, with respect to the
X direction of ECAE samples. However, at the higher ECAE temperature (623 K), most of the basal planes are parallel with the X
direction of ECAE samples. After ECAE at 623 K, most crystals in
the Orient-I sample are aligned with their c-axes being parallel
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with the Z direction, but more crystals in the Orient-II sample
are inclined with their c-axes deviated from the Z direction.
3. The yield strength of the magnesium alloy after ECAE at 473 K
is much lower than that of the conventionally extruded plate
and that of the alloy ECAEed at 623 K. The peculiar relationship
between yield strength and ECAE temperature is mainly due to
the basal textures affected by the ECAE processing conditions.
4. The billet orientation has also an influence on the yield strength
and ductility of the ECAEed Mg–Zn–Y–Zr alloy. At a given ECAE
temperature, turning the billet orientation 90◦ may lead to an
increase in yield strength and a decrease in elongation, or a
decrease in yield strength and an increase in elongation.
Acknowledgement
Thanks should be given to National Basic Research Program of China (973 Program) and Natural Science Foundation
of China (NSFC) for their financial supports through projects no.
2007CB613704 and no. 50874100, respectively. .
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