J. KUBÁSEK et al.: THE EFFECT OF THERMO-MECHANICAL PROCESSING ON THE STRUCTURE, STATIC ...
289–296
THE EFFECT OF THERMO-MECHANICAL PROCESSING ON THE
STRUCTURE, STATIC MECHANICAL PROPERTIES AND
FATIGUE BEHAVIOUR OF PURE Mg
VPLIV TERMOMEHANSKE PREDELAVE ^ISTEGA MAGNEZIJA
NA STRUKTURO, STATI^NE MEHANSKE LASTNOSTI IN
OBNA[ANJE PRI UTRUJANJU
Jiøí Kubásek, Dalibor Vojtìch, Drahomír Dvorský
University of Chemistry and Technology, Department of Metals and Corrosion Engineering, Prague, Technická 5,
166 28 Prague 6, Czech Republic
kubasek.jiri@gmail.com
Prejem rokopisa – received: 2016-02-03; sprejem za objavo – accepted for publication: 2016-04-06
doi:10.17222/mit.2016.029
Magnesium alloys find important applications in automotive, aircraft and space industries. Magnesium itself is also a key
component in biodegradable metallic materials considered for applications in medicine. Although there are many studies that
include the mechanical behaviour of pure Mg and its alloys after different thermomechanical processing, there is a huge impact
of the processing method on the observed properties. Based on these facts, predicting the final properties is difficult. This paper
deals with the preparation of pure Mg using a classic extrusion method with a low extrusion ratio, slow extrusion rate and low
temperature. These parameters were selected to prevent an excessive grain size and to obtain optimal mechanical properties. The
prepared materials were subsequently heat treated under different conditions and the relations between the grain size, texture
strength and mechanical properties were studied. As well as static mechanical tests in tension and compression, fatigue tests
were also performed. Finally, the obtained data were fitted using the Hall-Petch relation and the results were compared with the
literature. The observed results confirmed the significant effect of processing on the mechanical properties with a strong impact
on the Hall-Petch behaviour.
Keywords: magnesium, mechanical properties, texture, Hall-Petch relation
Magnezijeve zlitine so pomembne za uporabo v avtomobilski, letalski in vesoljski industriji. Magnezij je tudi klju~na kompo-
nenta v biorazgradljivih kovinskih materialih za uporabo v medicini. ^eprav obstaja mnogo {tudij, ki vklju~ujejo mehansko
obna{anje ~istega Mg in njegovih zlitin po razli~nih termomehanskih obdelavah, je velik vpliv metode obdelovanja na opazo-
vane lastnosti. Na osnovi tega je te`ko napovedovati kon~ne lastnosti. Ta ~lanek obravnava pripravo ~istega Mg s klasi~no
metodo ekstruzije z majhnim razmerjem ekstruzije, majhno hitrostjo ekstruzije pri nizki temperaturi. Ti parametri so bili izbrani,
da se prepre~i prekomerna rast zrn in da se dose`e optimalne mehanske lastnosti. Pripravljeni material je bil pozneje toplotno
obdelan pri razli~nih pogojih in {tudirane so bile odvisnosti med velikostjo zrn, teksturo in mehanskimi lastnostmi. Razen
stati~nih mehanskih preizkusov, nateznih in tla~nih, so bili izvedeni tudi preizkusi utrujanja. Na koncu so bili dobljeni podatki
primerjani s Hall-Petchevim razmerjem in dobljeni rezultati so bili primerjani s podatki iz literature. Dobljeni rezultati so
potrdili mo~an vpliv obdelave na mehanske lastnosti, z mo~nim vplivom na Hall-Petchevo razmerje.
Klju~ne besede: magnezij, mehanske lastnosti, Hall-Petchevo razmerje
1 INTRODUCTION
Magnesium is an essential matrix element of many
materials used in the traffic industry and also of some
biodegradable materials that can be used for medical
applications. Both these groups are characterized by high
demands on the mechanical properties. These properties
are strongly affected by many material factors. One of
the very well-known factors is strengthening by grain
boundaries, which is simply described by the Hall-Petch
Equation (1):
 = 0 + k · d
–1/2 (1)
In this equation "0" and "k" are the constants
representing the intercept and the slope of the Hall-Petch
line, respectively. Magnesium as well as other metals
with hcp structures have already been the subjects of
many studies which showed that "0" is related to the
critical resolved shear stress (CRSS) for the easy slip
system operating within the grain volume or in other
words it represents the friction stress of mobile dislo-
cations on the slip plane1,2, and "k" is related to the CRSS
for the more difficult slip/twinning systems required to
operate near grain boundaries in order to maintain the
continuity of strain3 or in other words "k" represents the
stress-concentration factor.2 Both these constants ("0"
and "k") are texture dependent.
Material texture can be significantly affected by
different processing methods such as rolling, extrusion,
forging and of course by the parameters of those pro-
cesses.
Another Equation (2), which characterizes the depen-
dence of polycrystalline strength "" on the critical shear
strength "0" and the stress concentration required to
propagate slip across a grain boundary "ks" has to be
Materiali in tehnologije / Materials and technology 51 (2017) 2, 289–296 289
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
67.017:669.721.5:621.78 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(2)289(2017)
modified if we consider textured materials since the
orientation constants relating "
0" and "k" to the corres-
ponding single-crystal resolved shear stresses will be
different and depend on the intensity of the respective
textures. In such case, Equation (2) can be modified to
Equation (3), where "M" is the orientation factor relating
"
0" to the CRSS for an easy slip, and "N" is the orien-
tation factor relating "k" to "ks":

 = m · (0 + ks · l
-1/2) (2)

 = M · 0 + N · ks · l
-1/2 (3)
On the basis of the pile-up model, the Hall-Petch
slope "k" is related to the resolved shear stress "c" for
the difficult slip system controlling plastic flow in the
grain-boundary regions, Equation (4):
k = N · kS = C · N · [m · Gb/2a]
l/2 · c
1/2 (4)
In this Equation (4) "C" is a constant, "m*" is the
Sach’s orientation factor that takes into account the
differences in the grain-to-grain orientation, "a" is a
numerical constant depending on the screw or edge
character of the dislocation pile-up, "G" is the shear
modulus, and "b" is the Burgers vector. Therefore, "k" is
related to the CRSS for prismatic slip and the {10-10}
texture can be expected to affect the value of the slope
"k". It is evident that the material properties are strongly
texture dependent and the Hall-Petch behaviour can be
different for materials prepared by different processing
routes.
Due to the limited number of slip systems in hcp
structures, other mechanisms such as twinning also play
an important role in the deformation behaviour of
magnesium and magnesium-based alloys. It has been
shown that twin boundaries act as strong barriers to
dislocation motion.4 Therefore, in practice, the mean free
path of the dislocations should incorporate terms repre-
senting the effects of both the grain size and the presence
of any twins. Several types of single- and double-
twinning such as {1011}, {1012}, {1013}, {1011}–
{1012} and {1013}–{1012} are formed in magnesium
and its alloys,5–7 and the crystallographic relation of the
lattice rotates around the ‹1120› direction by 56°, 86°
and 64° for these types of single twinning formation.
Among these types of twinning, the {10–12}-type
(‹c›-axis tension) twins form easily at the beginning of
plastic deformation, since its CRSS is the lowest and has
a similar order as that in the basal plane.5,6 The twinning
is formed by the concentrated stress due to the dislo-
cation pile-up, as reported in 3. When the magnitude of
the stress concentration is larger than the nucleation
stress, twinning is thought to form at an adjacent grain.8
It has also been reported that twinning tends to form at
grain boundaries with lower misorientation angles,
which are characterized by a higher grain-boundary
energy in magnesium.9,10
Different methods can be used for the manufacture of
magnesium alloys with a low grain size, and different
textures are prepared during these processes. The main
examples of such methods are extrusion, ECAP (equal
channel angular pressing) and HPT (high pressure
torsion). Material prepared by original straight extrusion
has a typical texture consisting of ‹1010› and ‹1120›
fibres (i.e., basal poles perpendicular to the extrusion
axis). ECAP textures and its strength are dependent on
ECAP routes and also on the number of ECAP passes.
Final texture affects measured mechanical properties.
After extrusion, if the tensile axis lies within the basal
plane of the majority of grains, the grains are forced to
deform by either non-basal cross-glide of basal disloca-
tions or pyramidal slip of large Burgers vector ‹c+a›
dislocations.11 These hard deformation modes possess a
high critical resolved flow stress, and ultimately result in
the premature failure of properly oriented samples. In
contrast, if the basal axes are oriented close to the tensile
stress axis, there is very strong signature of the {1012}
tensile twinning [1113]. Twinning re-orients a portion of
the crystallites, and only a small plastic strain is required
to ensure that a major volume fraction of grains becomes
poorly oriented for slip or twinning. Therefore, the
anisotropy of the mechanical properties observed in
magnesium alloys with specific texture and both tensile
and compressive properties are a distinctive base on the
tests’ orientation.
The present work is focused on a classic extrusion
process with a low extrusion ratio equal to 10 and an
extrusion rate only 5 mm/min. Together with a low tem-
perature (200 °C), such conditions were selected to
obtain fine-grained structures and proper mechanical
properties.
2 MATERIALS AND METHODS
2.1 Materials
In this study, pure Mg (99.9 %) in an extruded state
was investigated. Cylindrical ingots of Mg were prepared
by melting pure Mg (99.9 %) in an induction furnace
under a protective argon atmosphere (99.996 %). The
melt was homogenised at 750 °C for 15 min and cast
into a brass mould of 100 mm in length and 20 mm in
diameter. Magnesium was directly extruded at a
temperature of 200 °C, an extrusion rate of 5 mm/min,
and an extrusion ratio of 10:1 to prepare rods of 6 mm in
a diameter. Such specimens were annealed at tempera-
tures of (200, 300 and 400) °C for different times to
prepare magnesium with different grain sizes and
properties.
2.2 Structure
The microstructure of the Mg was studied by optical
microscopy (Olympus PME3) and scanning electron
microscopy (SEM – TescanVega3 LMU) equipped with
an energy-dispersive spectrometer (EDS, Oxford Instru-
ments Inca 350) and an EBSD detector – NordlysMax
and AZtecHKL software. For SEM analysis, the samples
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were mechanically grinded using SiC abrasive papers
P180–P4000, polished by diamond pastes with 2 μm and
0.7 μm particles and etched in a 2 % Nital solution. For
EBSD additional mechanical polishing was performed
on a neutral alumina suspension (OP-AN Suspension).
Finally, samples were prepared by electro-polishing in
15 % nitric acid in ethanol at 12 V for 15 s.
2.3 Mechanical properties
The mechanical studies were performed using
Vickers hardness measurements with a loading of 5 kg
(HV5). The mechanical properties were also characte-
rised by tensile tests and compression tests (LabTest
5.250SP1-VM). For the tensile tests of the as-cast alloys,
rod samples of 4 mm in diameter and 60 mm in length
were used. The strain rate during the tensile tests was set
to 0.001 s–1. Compression tests were performed on rod
samples of 6 mm in a diameter and 12 mm height at a
strain rate of 0.001 s–1 as was used for the tensile tests.
Fatigue tests were performed on a device SCHNCK
PHG. Smooth fatigue specimens with a diameter of
6 mm and a gauge length of 18.7 mm were machined
from the extruded materials. Before the fatigue test they
were mechanically polished using progressively finer
grades of emery paper and buff-finished. The fatigue
tests were performed at a frequency of 50 Hz in labora-
tory air at ambient temperature.
3 RESULTS AND DISCUSSION
3.1 Structure
Typical examples of microstructures of pure Mg
obtained by different thermomechanical processing are
given in Figure 1. Magnesium after the extrusion at
200 °C (Figure 1a) was fully recrystallized with the
grain size ranging from 2 μm to 18 μm and with the
dominant equivalent grain diameter of 4 μm. No elon-
gated grains were observed in the structure. Different
thermal treatment leads to an obvious grain coarsening.
Figure 2 shows the distribution of grain sizes for pure
Mg treated at different conditions – extruded material
and extruded material exposed to the subsequent thermal
treatment at 200, 300 and 400 °C for 1 h and 2 h. It is
clear that the coarsening of Mg is strongly dependent on
both temperature and time. However, it seems that at
lower temperatures such as 200 °C coarsening is not so
significant with prolonged time on the temperature. At
higher temperatures (300 °C, 400 °C) the grain size is
increased significantly, also with a prolonged exposure
time. Figure 3a shows EBSD maps of pure Mg extruded
at 200 °C. The analysis was performed on the surface
parallel to the extrusion direction. As can also be seen in
Figure 4, the majority of grains is oriented in the struc-
ture with the basal planes parallel to the extrusion direc-
tion, which is a known effect of the extrusion process on
magnesium-based alloys. Texture orientation and
strength are evident in Figure 4a. A specimen subjected
to the subsequent annealing at 400 °C for 2 h was anal-
ysed using EBSD (Figures 3b and 4b) for a comparison
with the extruded Mg. The grain size ranged from 20 μm
to 160 μm and the dominant equivalent grain diameter
reached about 40 μm. It can be clearly seen that texture
was maintained similar to the extruded condition and
only texture strength was slightly reduced from 10.2 to
10 for extruded Mg and extruded + annealed Mg, res-
pectively (Figure 4). This confirms that the texture
created during thermomechanical treatment is main-
tained also during the subsequent thermal treatment.
Dynamic recrystallization is supposed to manage the
recrystallization process during the hot extrusion. In this
case, new fine grains are formed at the original grain
boundaries and subsequent nucleation occurs at the
recrystallized grains.
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Structure of Mg after different thermo-mechanical pro-
cessing: a) extruded at 200 °C, b) extruded at 200 °C and annealed at
200 °C for 1 h, refer to extrusion direction, c) extruded at 200 °C and
annealed at 200 °C for 2 h, refer to extrusion direction, d) extruded at
200 °C and annealed at 400 °C for 2 h, refer to extrusion direction
Slika 1: Mikrostruktura v smeri ekstrudiranja Mg po razli~nih termo-
mehanskih obdelavah: a) ekstrudirano pri 200 °C, b) ekstrudirano pri
200 °C in 1 h `arjeno na 200 °C, c) ekstrudirano pri 200 °C in `arjeno
2 h na 200 °C, d) ekstrudirano pri 200 °C in `arjeno 2 h na 400 °C
Figure 2: Grain size distribution in the Mg treated under different
conditions
Slika 2: Razporeditev velikosti zrn v Mg, obdelanem pri razli~nih
pogojih
Discontinuous and continuous dynamic recrystalliza-
tion (DDRX, CDRX) are considered for magnesium
depending on the initial grain size and also the tempe-
rature. DDRX is operative when the initial grain size of
magnesium alloys is large enough for the crystallo-
graphic slip to be heterogeneous, which is the case of the
as-cast Mg. Pure magnesium in the as-cast sate was
characterized by the grains with about 100 μm in
thickness and nearly 1 mm in length. It is known that
extrusion ratio (ER) has a significant effect on the grain
size. Although an ER equal to 10 seems to be low, it is
possible to reach a very fine-grained structure after
extrusion at 200 °C. According to the empirical Equation
(5), strain during extrusion process corresponds only to
2.3:
 = ln(ER)14 (5)
Therefore, other important factors, such as tempe-
rature and extrusion velocity, have to affect the final
grain size significantly. In this case the extrusion velocity
is only 2 mm/min. At a lower extrusion velocity, a lower
strain rate can lead according to the Zener–Hollomon
parametre to the coarsening of the structure. Moreover, it
means prolonged times at temperature during the
extrusion process and subsequent cooling, which can
also lead to the coarsening. However, a lower strain rate
also means only a smaller temperature increase in the
material during deformation, which prevents an
excessive grain growth.15
Texture development is global process that includes
slip in the basal system and also other slip systems and
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292 Materiali in tehnologije / Materials and technology 51 (2017) 2, 289–296
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Figure 3: IPF maps of pure Mg: a) extruded at 200 °C, b) extruded at 200 °C and annealed at 400 °C for 2 h, refer to extrusion direction
Slika 3: IPF-mape pri ~istem Mg v smeri ekstruzije: a) ekstrudirano pri 200 °C, b) ekstrudirano pri 200 °C in 2 h `arjeno na 400 °C
Figure 4: Pole figures of pure Mg: a) extruded at 200 °C (max mud = 10.14), b) extruded at 200 °C and annealed at 400 °C for 2 h (max mud =
9.91), X0–Y0 area is parallel to the extrusion direction and Y0 represents the extrusion direction
Slika 4: Slike polov za ~isti Mg: a) ekstrudirano pri 200 °C (max mud = 10,14), b) ekstrudirano pri 200 °C in 2 h `arjeno na 400 °C (max mud =
9,91), X0-Y0 je podro~je, ki je vzporedno s smerjo ekstruzije, Y0 predstavlja smer ekstruzije
twinning. At low temperatures, the dominate slip system
is basal slip. Tensile twinning also plays an important
part in the accommodation of strain during the deforma-
tion, especially in the coarse-grained magnesium alloys.
Due to the fact that coarse grains are presented in the
structure especially at the beginning of deformation
process, tensile twining is favoured at this stage and
helps to reorient the grains to a basal orientation. It was
also proved that other mechanisms, such as double
twinning and shear and kink banding, also operate to
accommodate c-axis compression when non-basal slip is
limited at low temperature.16 Texture can be partially
suppressed by the slip in non-basal slip systems; how-
ever, the critical resolved shear stresses (CRSS) of
non-basal slip systems for Mg are significantly higher
compared with the basal slip system. This can be
partially compensated by stress concentration at the
grain boundaries, which is effective for the activation of
non-basal slip. A higher extrusion ratio can be partially
responsible for the activation of non-basal slip and
finally also a weaker texture. Also, a higher temperature
can lead to an easier activation of non-basal slip, which
can significantly affect the texture evolution.
The DRX process itself does not lead to the texture
change in the case of magnesium-based alloys exposed
to the extrusion or a combination of extrusion and
annealing (Figure 4); conversely, it serves to strengthen
the basal texture. Due to the low temperature of the
extrusion process, the low extrusion velocity and the
extrusion ratio, it is really difficult for non-basal slip to
be activated. There are only signs of twins in the struc-
ture, which can be responsible for a slight texture
weakening. Overall, all the samples were characterized
by similar and strong textures.
3.2 Mechanical properties
Mechanical properties in both compression and
tension were measured. Based on the conditions of
thermal treatment, magnesium is characterized by
different compressive yield strength (CYS) and ultimate
compressive strength (UCS) and also different behaviour
after the onset of plastic deformation (Figure 5). The
CYS of extruded Mg reached the maximum value of
nearly 130 MPa. After that, the deformation strength-
ening is obvious; however, the ultimate tensile strength
reaches a value as low as 190 MPa. On the contrary,
magnesium exposed to the subsequent thermal treatment
is characterized by lower values of CYS. Moreover, CYS
is decreased with increased temperatures and prolonged
times at these temperatures. When the CYS is overcome,
strong deformation strengthening take place and finally,
UCS reaches values between 240 MPa and 270 MPa
(Figure 5).
The significant strain hardening in compression for
materials with a coarse-grained structure is known and
can be ascribed to the formation of {1012} twins.8,17 The
formation of deformation twins causes a significant
strain-hardening tendency, because the twins block the
propagation of dislocations. With a decrease in the grain
size the decrease in the strain hardening is caused
because the twinning cannot develop in the same manner
as for materials with larger grains.18
The TYS and UTS increased gradually with grain
refinement (Table 1), because magnesium has a large
Taylor factor due to the lack of slip systems.19 It is also
clear that uniform elongation is decreased with sub-
sequent thermal treatment, in other words with increased
grain size.
Table 1: Mechanical properties of magnesium in different states
estimated from the tensile tests
Tabela 1: Z nateznimi preizkusi dolo~ene mehanske lastnosti magne-
zija v razli~nih stanjih
Proof stress
0.2 % (TYS)
(MPa)
UTS
(MPa)
Elongation
(%)
Mg Ex 200 °C 108±10 173±10 6.7±0.8
Mg Ex 200 °C +
200/2 h 91±8 168±7 6.2±0.8
Mg Ex 200 °C +
300/2 h 79±5 160±5 5.3±1.1
Mg Ex 200 °C +
400/2 h 72±5 150±7 3.6±0.9
Although there are many studies about the effect of
texture and related effect on the Hall Petch behaviour,
the obtained values were properly fitted by linear func-
tion, because all the samples seems to be characterized
by a very similar texture and also its strength. Figures 6
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Materiali in tehnologije / Materials and technology 51 (2017) 2, 289–296 293
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Compressive curves of Mg processed by extrusion and
different subsequent heat treatments
Slika 5: Tla~ne krivulje za Mg izdelan z ekstruzijo in z razli~nimi
toplotnimi obdelavami po njej
to 8 show the Hall-Petch relations based on the com-
pressive tests, tensile tests and hardness measurements
for pure Mg. It is worth mentioning that in all cases the
deviation of different points from linear behaviour was
negligible. Based on these facts it seems that the
measured mechanical properties are dependent on the
grain size according to the Hall-Petch relation. The
obvious differences in the estimated constants for
different measurement methods are connected with
different types of mechanical loading and also texture of
the material. It is known that during the extrusion or
hot-rolling process, the basal planes are strongly oriented
parallel to the rolling direction. In the case of extrusion,
the texture strength is often weaker compared to the
rolling; however, it is still there, as can be seen from
Figure 4. The observed mechanical properties are then
strongly dependent on the direction of loading during the
mechanical testing. If the basal planes are parallel to the
axis of tension, so the directions (1010) are parallel to
the extrusion direction, the slip on basal planes is com-
plicated. Therefore, a high tensile strength is observed
along the extrusion direction.20
The flow stress of Mg strongly depends on the grain
size; although there is also a strong texture dependence
due to the large difference in critical shear stress between
the basal planes and the non-basal planes. Such diffe-
rences are decreased with increased temperature. The
effect of texture on the Hall-Petch slope or in other
words the grain size dependence of "K" is significant.21 It
is known that the misorientation of grain boundaries
affects the grain size dependence of the flow stress. As a
result, the value of "K" is lower for low-angle grain
boundaries. Based on these facts, extruded Mg is
characterized by a stronger dependence of the yield
strength on the grain size compared to the Mg with a
weaker texture.
Different works study the Hall-Petch behaviour of
Mg after different thermomechanical processing. Some
results are indicated in Figure 9. N. Ono et al.1 used
rolled Mg sheets as a starting material for a determi-
nation of the Hall-Petch relation using tensile tests. G. S.
Rao et al.3 studied the Hall-Petch behaviour on tensile
specimens from a hot rolled sheet in the longitudinal and
transverse directions and also from hot-rolled square
rods. H. Somekawa et al.8 prepared magnesium plates
using extrusion with an extrusion ratio of 10 at 200 °C.
The compression tests on these samples were performed
with the compression axis parallel to the extrusion
direction, which is the same as in the present research.
A. Yamashita et al.22 studied magnesium and magnesium
alloys prepared by ECAP with an internal angle of 90° at
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Figure 7: Experimental change of the tensile yield strength (TYS)
with the grain size
Slika 7: Spreminjanje natezne meje plasti~nosti z velikostjo zrn
Figure 8: Experimental change of the Vickers hardness (HV5) with
the grain size
Slika 8: Spreminjanje trdote po Vickersu (HV5) z velikostjo zrn
Figure 6: Experimental change of the compressive yield strength
(CYS) with the grain size
Slika 6: Spreminjanje tla~ne meje plasti~nosti z velikostjo zrn
200–400 °C. The Hall-Petch behaviour was subsequently
studied using tensile tests. Compared to our experiments,
it is seen that Mg prepared by rolling is characterized by
a higher value of 
0 (Figure 9). Moreover, this value is
dependent on the intensity of the rolling, which has a
direct effect on texture strength.1, 3 The same reason can
be considered in the case of the difference between the
properties of extruded Mg. From all these studies it is
clear that there is no general Hall-Petch relation, which
could cover different conditions. Instead, it is necessary
to include grain size, texture and also other phenomena
(basal, non-basal slip and twinning), which are affected
by the grain size and texture in a general consideration
for the final behaviour of the Mg material.
In Figure 10, stress versus the number of cycles
fatigue curves are shown for two extreme states of the
studied Mg. Magnesium extruded at 200 °C is charac-
terized by an improved fatigue life. The maximum value
of the stress amplitude for a service life longer than
8×107 cycles is 30 MPa for extruded Mg and about 25
MPa for Mg with a subsequent thermal treatment at 400
°C. Such a difference is connected with grain refinement,
which is very effective for improving the fatigue
strength.23 Another fact that plays an important role
during fatigue testing is the texture. Due to the pre-
ferential orientation of the basal planes parallel to the
extrusion direction, there is an anisotropy between
hardening for tensile and compressive loading.24 During
tensile loading, twinning is very challenging due to the
initial orientation. Pyramidal ‹a› and ‹c+a› pyramidal
slips are characterized by higher values of the Schmidt
factors (0.33 and 0.49 respectively24). Also, ‹a› prismatic
slip is problematic and a value of Schmidt factor equal to
0.35 has been postulated. As a consequence, especially
basal slip with a low Schmidt factor of 0.17 takes place
during this stage. In contrast, during the compressive
step, (1012) ‹1101› twinning and also basal slip are
active. Different twin types have been observed during
cycling, although some types are presented in the struc-
ture after a number of cycles. During cycling, a harden-
ing effect was observed and more stress is concentrated
in the structure. As a consequence, more complicated
twinning processes and also non-basal slip take place
with an increasing number of cycles.25
Tension twinning occurs when the resolved shear
stress exceeds the critical resolved shear stress (CRSS).
The values of 2.2–3.0 MPa were measured for such
behaviour in a single crystal by Q. Yu et al.25 and also the
value of 2.4 MPa was published by the same author for a
different monocrystal orientation.26
Due to such limitations, complex dislocation struc-
tures are rather unlikely in magnesium under cyclic
loading.24,27 In the case of pure Mg, cyclic hardening is
expected as has already been proved in 28.
4 CONCLUSION
The present paper is focused on a study of the mecha-
nical properties of pure Mg prepared by an extrusion
process at 200 °C with an extrusion ratio equal to 10.
Our results confirmed the significant effect of both grain
size and texture on the mechanical properties. Both these
main characteristics affect the slip and twinning activity.
The anisotropy in mechanical properties is documented
by different Hall-Petch behaviour based on compressive
and tensile testing in the same direction and also by a
comparison with literature. It is shown that both 
0 and k
from the Hall-Petch relation are strongly dependent on
the mechanical processing. Fatigue tests performed at
laboratory temperature confirm the increase in the
fatigue life with a decreased grain size. The effect of
J. KUBÁSEK et al.: THE EFFECT OF THERMO-MECHANICAL PROCESSING ON THE STRUCTURE, STATIC ...
Materiali in tehnologije / Materials and technology 51 (2017) 2, 289–296 295
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 9: Hall Petch of pure magnesium after different preparation
conditions
Slika 9: Hall Petch relacija pri ~istem magneziju po razli~nih pogojih
priprave
Figure 10: Fatigue behaviour of magnesium extruded at 200 °C and
subsequently annealed at 400 °C for 2 h
Slika 10: Utrujanje magnezija, ekstrudiranega pri 200 °C in nato
`arjenega 2 h na 400 °C
texture strength can be, in this case, neglected due to the
similar texture strength of the measured samples.
Acknowledgement
Jiøí Kubásek wishes to thank for financial support of
this research (project no. GA16-08963S) and Drahomír
Dvorský wishes to thank the Czech Science Foundation
for financial support (project no. P108/12/G043).
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