D. DVORSKÝ et al.: EFFECT OF HEAT PRE-TREATMENT AND EXTRUSION ON THE STRUCTURE ...
499–505
EFFECT OF HEAT PRE-TREATMENT AND EXTRUSION ON THE
STRUCTURE AND MECHANICAL PROPERTIES OF WZ21
MAGNESIUM ALLOY
VPLIV TOPLOTNE OBDELAVE IN IZTISKOVANJA NA
STRUKTURO IN MEHANSKE LASTNOSTI MAGNEZIJEVE
ZLITINE WZ21
Drahomír Dvorský
1
,Ji øí Kubásek
1
, Dalibor Vojtìch
1
, Miroslav ^avojský
2
, Peter Minárik
3
1
Faculty of chemical technology, Department of metals and corrosion engineering, University of chemistry and technology Prague,
Technická 5, 166 28 Praha 6 – Dejvice, Czech Republic
2
Institute of Materials and Machine Mechanics SAS, Slovak Academy of Sciences, Dúbravská cesta 9, 845 13 Bratislava, Slovak Republic
3
Charles University, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 16 Praha 2, Czech Republic
dvorskyd@vscht.cz
Prejem rokopisa – received: 2017-12-12; sprejem za objavo – accepted for publication: 2018-02-15
doi:10.17222/mit.2017.214
The magnesium alloy WZ21 was prepared by a combination of conventional casting, extrusion and heat treatment. The effect of
various processing conditions on the final structure, mechanical and corrosion properties were investigated. Long-period
stacking ordered (LPSO) phases were observed in the as-cast structure; however, they were at least partially dissolved during the
extrusion or heat treatment. Thermally stable Mg24Y5 was observed in the structure as another dominant phase. The application
of heat treatment to extruded products causes significant grain growth, and, therefore, the loss of mechanical properties. On the
contrary, heat treatment performed before extrusion has a strong positive effect on the observed mechanical properties. In both
cases, the thermal treatment positively affected the corrosion resistance of the prepared materials although the effect is more
significant for material annealed before extrusion. After appropriate processing, the WZ21 alloy was characterized by the high
value of the tensile yield strength of 337 MPa and really promising corrosion rate in simulated body fluid (SBF)
(0.75 mm/year).
Keywords: magnesium, extrusion, mechanical properties, grain size, corrosion
Avtorji ~lanka so magnezijevo zlitino WZ21 pripravljali s kombinacijo konvencionalnega litja, iztiskovanja (ekstruzije) in to-
plotne obdelave. Raziskovali so vpliv razli~nih procesnih pogojev na kon~no mikrostrukturo, mehanske in korozijske lastnosti.
Opazili so urejenost dolgega dosega (LPSO; angl.: Long-Period Stacking Order) faz v liti strukturi, ki pa so bile vsaj delno
raztopljene (razgrajene) med ekstruzijo ali toplotno obdelavo. Mg24Y5 je druga dominantna termi~no stabilna faza v strukturi.
Uporaba toplotne obdelave na ekstrudiranih proizvodih povzro~i znatno rast kristalnih zrn in zato poslab{anje mehanskih
lastnosti. Nasprotno ima toplotna obdelava pred ekstruzijo mo~an pozitivni u~inek na opazovane mehanske lastnosti. V obeh
primerih ima toplotna obdelava pozitiven u~inek na korozijsko odpornost pripravljenih materialov. Vendar je u~inek bolj izrazit
pri materialih, `arjenih pred ekstruzijo. Po primernem procesiranju so Mg zlitino WZ21 analizirali in ugotovili, da ima visoko
mejo te~enja (337 MPa) in zelo obetavno korozijsko hitrost v simulirani telesni teko~ini (0,75 mm/leto).
Klju~ne besede: magnezij, ekstruzija, mehanske lastnosti, velikost zrn, korozija
1 INTRODUCTION
Magnesium alloys offer advantages in the form of
low density, relatively good mechanical properties and
good stiffness.
1
Those benefits are attractive, especially
for the automotive and aviation industries.
2
However
inferior mechanical properties, low ductility and low heat
stability limits its spread of use.
2,3
Therefore, many
studies have been done to improve the mechanical pro-
perties of magnesium alloys. Magnesium is the eighth
most abundant element on Earth and is easily recyclable,
which contributes to environmental protection.
3
The bio-
compatibility and degradation performance of magne-
sium and its alloys makes them suitable materials for
biodegradable implants.
4
The idea of biodegradable im-
plants is based on the gradual degradation of the implant,
while the bone heals and grows in the area of the
degraded implant.
1
This means that only one operation is
necessary. It leads to a decrease in the price of the ope-
ration as well as to increased comfort of the patient.
5
The
main advantage of magnesium is in non-toxicity and the
low Young’s modulus, which is much closer to the corti-
cal bone than other permanent biomaterials like stainless
steel or Ti and its alloys.
4
The stress-shielding effect
occurs in materials with a high Young’s modulus and
complicates the bone ingrowth.
The biodegradable implant should sustain its mecha-
nical properties for at least 8 weeks with ultimate disso-
lution in 12–15 months; therefore, the optimal corrosion
resistance is necessary.
5
Magnesium and most of its
alloys have a rapid corrosion rate, which generates
hydrogen. Hydrogen bubbles complicate bone ingrowth;
however, if the hydrogen is released slowly and
uniformly, then it is easily taken away. Another danger is
the great gradient of pH nearby the corroding implant,
Materiali in tehnologije / Materials and technology 52 (2018) 4, 499–505 499
UDK 669-15:621.983.4:669.721.5:67.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(4)499(2018)

which is lethal to the bone cells. Slow and uniform
corrosion is the key to the usability of the biodegradable
implant. The great danger lies in more noble elements
insoluble in a magnesium matrix, especially Fe, Ni and
Cu.
6
Those elements have to be held beneath a certain
level of mg/kg, otherwise their presence has fatal con-
sequences on the corrosion performance. Pure magne-
sium is more prone to impurities then alloys because
some alloying elements can absorb them. However,
high-purity magnesium exerts better corrosion resistance
than most magnesium alloys, especially thanks to the
absence of secondary phases.
7
Different magnesium-based alloys usually derived
from magnesium alloys used in industry were investi-
gated as possible biodegradable materials. However, the
toxicity of alloying elements has to be taken into
consideration. For example, the presence of aluminum in
alloys from the AZ series contributes to Alzheimer’s
disease.
8
Rare-earth elements seem to be a good choice
for alloying of magnesium as they improve the mecha-
nical and corrosion properties. The cytocompatibility of
rare-earth elements was studied on human cells with
positive results. Moreover, they are used as an anti-carci-
nogenic reagent in small doses. However, the long-term
toxicity has to be investigated as rare-earth elements
concentrate in large bones. Still, the usability of WE
alloys (such as WE43) is in fact really close to the final
commercial application in medicine.
Research is nowadays focused on the reduction or
substitution of RE alloying elements with more
compatible ones. In this case, Zn is often considered as a
suitable alloying element because it is essential for the
human organism and in many cases improves the me-
chanical properties of magnesium-based alloys by solid
solution strengthening.
9
The easy absorption of zinc
in-vivo was observed for alloys with the zinc mass con-
tent of about 1 %.
Based on these characteristics the WZ21 alloy, con-
taining Y and Zn as the main alloying elements, is
considered as a possible biodegradable material. This
alloy was reported to have promising results of low and
uniform corrosion rate, and good cytocompatibility.
4
The
Mg-Y-Zn system has also been recently studied for its
excellent mechanical properties and unique microstruc-
tures.
3
In the Mg-Y-Zn system, several secondary phases
(quasicrystalline I-phase (Mg
3
Zn
6
Y), W-phase
(Mg
3
Zn
3
Y
2
), and long period stacking ordered (LPSO)
phases) are formed depending on the Y/Zn ratio.
4
The
Mg
24
Y
5
cubic phase is also present in Mg-Y-Zn alloys.
10
The icosahedral I-phase (Mg
3
Zn
6
Y) is observed in alloys
with high Zn content. It is relatively thermally stable, it
causes an increase of hardness and corrosion resistance.
4
Its presence in the structure effectively suppresses the
grain growth.
11
The higher amount of this phase is
known to improve the strength of Mg-Y-Zn alloys.
12
On
the contrary, the cubic incoherent W-phase ineffectively
pins the grain boundaries during thermomechanical
processing and negatively affects the mechanical proper-
ties.
4,13
It can be at least partially removed from the
material by proper annealing.
14
LPSO phase occurs in the
alloys with lower Zn content. Different kinds of LPSO
phase exist (6H, 10H, 14H, 18R, 24R, H = hexagonal,
R = rhombohedral).
15
The presence of this phase positi-
vely affects strength and plasticity of the alloy.
2
Kawa-
mura et al.
16
prepared WZ21 alloy containing LPSO
phases with the incredible yield strength of 610 MPa and
elongation of5%atroom temperature.
Extrusion is a common processing method for mag-
nesium alloys. Extruded magnesium alloys are generally
characterized by a specific texture, which causes the
anisotropy of the mechanical properties and deteriorates
the plasticity of the material. However, recent studies
have shown that ductility, tensile yield strength (TYS)
and ultimate tensile strength (UTS) can be spontaneously
improved by the formation of a microstructure with a
low grain size and the suppression of grain growth
during processing by small secondary phases, which pin
the grain boundaries. In such a case, pyramidal c+a slip
systems are activated more easily in fine-grained ma-
terials, which partially prevents the formation of specific
textures by recrystallization processes during extrusion,
and therefore, improve the ductility.
1
The texture of a
magnesium alloy is strongly affected, especially by
rare-earth (RE) elements, which are known to form a
so-called "RE texture".
1
This paper is focused on the preparation of Mg-2Y-
1Zn alloy by extrusion. This alloy may offer a good
combination of mechanical properties and corrosion
resistance depending on the structure of the material. It is
also considered as a promising candidate for application
in medicine. The presented alloy was prepared by the
conventionally available extrusion process and the effect
of heat treatment applied before and after extrusion pro-
cess on the structure, mechanical and corrosion proper-
ties are studied. The presented results confirm the good
mechanical properties of this alloy, which are associated
with the full recrystallization and the related small grain
size. The corrosion tests reveal the positive effect of the
heat treatment on the corrosion resistance. Although a
solution treatment of this alloy causes a significant
decrease in the corrosion rate, our results confirmed that
it should be performed predominantly before extrusion,
not after in order to sustain small grains and good
mechanical properties.
2 EXPERIMENTAL PART
2.1 Materials
The WZ21 magnesium alloy was prepared by melting
pure elements of Mg, Y and Zn in the induction furnace
at 750 °C for 15 min under the protective argon atmo-
sphere. The as-cast ingot was then heat treated at 525 °C
for 8 h followed by quenching in water. Cast and heat
treated ingots were then extruded at 350 °C with
D. DVORSKÝ et al.: EFFECT OF HEAT PRE-TREATMENT AND EXTRUSION ON THE STRUCTURE ...
500 Materiali in tehnologije / Materials and technology 52 (2018) 4, 499–505

extrusion ratio 16 and extrusion speed 0.3 mm/s. Final
extruded products were rods with a 7.5 mm diameter.
2.2 Microstructure
Samples were ground on SiC grinding papers
(P80–P4000) and subsequently polished on diamond
paste. The final polishing for structure observation by
scanning electron microscopy (SEM) was performed on
Topol 2 with fine particles of Al
2
O
3
. The final mecha-
nical polishing for EBSD analysis was done on Etosil E
and neoprene cloth without subsequent etching. Samples
for microstructure analysis were etched in a solution
containing 10 mL of acetic acid, 4.2 g picric acid, 10 mL
distilled water and 70 mL ethanol. The microstructure
was studied by scanning electron microscopy SEM
(TescanVEGA3 equipped with energy dispersion spec-
trometry – EDS, Oxford instruments and NordlysMax
electron back-scattered diffraction (EBSD) detector, and
AZtecHKL software). Subsequent parameters were
selected for EBSD analysis: working distance = 15 mm,
accelerating voltage = 20 kV, dwell-exposure time =
0.26 s/pt, step size = 0.25 μm, binning mode4×4,frame
averaging = 3 times).
2.3 Mechanical properties
Compression tests were performed on LabTest
5.250SP1-VM at room temperature on cylindrical sam-
ples with 6 mm in diameter and 9 mm high. A constant
deformation speed of 0.001 s
–1
was selected. Com-
pressive yield strength (CYS), ultimate compressive
strength (UCS) and total deformation were determined.
Tensile properties were measured on the same machine
at room temperature on samples with 3.5 mm in diameter
in the constricted area and 25 mm in length. A constant
deformation speed of 0.001 s
–1
was selected. The tensile
yield strength (TYS), ultimate tensile strength (UTS) and
elongation were determined.
2.4 Corrosion behavior
Corrosion tests were carried out in simulated body
fluid (SBF) (Table 1). The temperature was set to 37 °C
and time of exposure was two weeks. Samples were
ground on the SiC grinding paper (P2500), rinsed with
distilled water and ethanol before the immersion tests.
Exposure was performed in closed plastic containers in
100 mL of SBF per 1 cm
2
of surface area. The specific
concentrations of the ions in SBF are shown in Table 1.
The SBF was prepared by mixing solutions of KCl,
NaCl, NaHCO
3
, MgSO
4
·7H
2
O, CaCl
2
, TRIS, (tris(hydro-
xymethyl)aminomethane), KH
2
PO
4
in that order. An
experiment was carried out without changing of the SBF.
The pH of the fluid rose from pH 7.6 to 8.2. After the
immersion tests, the samples were rinsed in distilled
water and the corrosion products were removed by the
solution of 200 g·L
–1
CrO
3
, 10 g·L
–1
AgNO
3
, 20 g·L
–1
Ba(NO
3
)
2
. The corrosion rate was calculated from weight
changes and from magnesium ions released to the
solution during the immersion tests.
3 RESULTS AND DISCUSSION
3.1 Microstructure
The structure of the as-cast Mg-2Y-1Zn (AC) is dis-
played in Figure 1a. It is characterized by a dendritic
structure, where the alloying elements are segregated
especially at the dendrite edges. The dendrites consist of
the  -Mg matrix and intermetallic phases. Intermetallic
phases were elongated and narrow or round shaped.
According to the EDS analysis, the atomic ratio between
Y and Zn in the intermetallic phases was 1:1.
3
Such a
ratio indicates the presence of the LPSO phase
(Mg
12
ZnY), which has already been observed in alloys
with Y/Zn weight ratio equal to 2:1, which is in good
accordance with the presented results.
17
On the contrary,
Cabeza et al.
4
investigated the presence of two morpho-
logies – binary eutectic pockets (Y/Zn ratio of 1:2) and
LPSO phase (Y/Zn ratio of 1:1). The  -Mg solid solu-
tion contained about 0.3 % of mass fractions of Y and
0.3 % of mass fractions of Zn. Therefore, the majority of
the alloying elements are contained in intermetallic
phases.
Extrusion of the as-casted ingot (Ex) results in strong
specific orientation of the microstructure constituents,
(Figure 1b) such as intermetallic phases ordered in rows
parallel to the extrusion direction. A small fraction of
intermetallic phases dissolved in the matrix during
extrusion and dendritic microsegregation was almost
removed. As a consequence, the solid solution of  -Mg
consists of 1.3 % of mass fractions of Y and 0.8 % of
mass fractions of Zn. Still, there exists areas of residues
of dendritic microsegregation coming from the as-cast
state, which contains about 5.6 % of mass fractions of Y
and 3.4 % of mass fractions of Zn. Similar behavior
including the partial dissolution of LPSO phase during
was observed by Zhang et al.
13
The grain size of ex-
truded ingot ranged from 0.5 μm to 7 μm. The presented
intermetallic phases pin the grain boundaries and work
as obstacles for grain growth during dynamic recrys-
tallization accompanying the extrusion process at high
temperature.
12
After proper annealing (525 °C for 8 h followed by
quenching) of the as-extruded sample (ExT4), the resi-
dues of dendritic microsegregation and some interme-
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Materiali in tehnologije / Materials and technology 52 (2018) 4, 499–505 501
Table 1: Composition of simulated body fluid
(mmol/L) Na
+
K
+
Mg
2+
Ca
2+
Cl
–
HCO
3
–
HPO
4
2–
SO 4
2–
SBF 142.0 5.0 1.5 2.5 148.5 4.2 1.0 0.5

tallic phases were dissolved in the matrix (Figure 1c).
Only the heat stable binary Mg
24
Y
5
phase remained in
the structure. Zinc is completely dissolved in the matrix.
This annealing also causes strong coarsening of the
structure of the alloy, as can be seen in Figure 1c. The
grain size of material ranged between 50 μm and
300 μm. The solid solution contained about 1.2 % of
mass fractions of Y and 1.0 % of mass fractions of Zn.
The solid solution was homogenized during processing.
Subsequently, the global concentration of Zn and Y in
the solid solution is similar to the extruded material.
The extrusion process was performed alternatively on
the as-cast sample thermally heat pre-treated (525 °C for
8 h followed by quenching) (T4Ex) (Figure 1d). In this
case, the majority of the eutectic phases were dissolved
and only the thermally stable Mg
24
Y
5
phase stays in the
structure during annealing. The solid solution of
extruded specimen consists of 1.8 % of mass fractions of
Y and 1.1 % of mass fractions of Zn. It is worth men-
tioning that the concentration of alloying elements in
solid solution was the highest in this case. If the thermal
treatment is applied on the as-cast state, the majority of
the eutectic phases can be dissolved and the concen-
tration of alloying elements in the solid solution is in-
creased. On the contrary, during extrusion of the as-cast
material, precipitation can partially take place in areas
with an increased concentration of Y and Zn. These
precipitates are stable or even coarsen during subsequent
annealing, and therefore the concentration of Y in solid
solution remains low (1.2 % of mass fractions). The
alloy extruded after thermal pre-treatment was characte-
rized by small equiaxed grains with the grain size similar
as in the case of directly extruded as-cast state (0.5–7
μm). Such an observed structure is the result of dynamic
recrystallization during extrusion.
Both extruded alloys were analyzed using electron-
backscatter diffraction (EBSD) (Figure 2). Due to the
similarities in results obtained for both states, only the
EBSD map and pole figures for Mg-2Y-1Zn are shown.
It is evident from Figure 2 that the majority of grains are
oriented with the basal planes parallel to the extrusion
direction. The maximum texture intensity of 3.8 was
measured. Such texture can subsequently affect the me-
chanical properties. The results of the EBSD analysis for
the extruded sample were similar as for the sample
prepared by the extrusion of a heat-treated ingot. Based
on the obtained results (Figure 2), the material is recrys-
tallized completely and no strongly deformed elongated
areas were observed.
3.2 Mechanical properties
Compressive and tensile properties of the prepared
samples were measured and the results are summarized
in two diagrams in Figures 3 and 4. Similar properties
were observed for the as-cast ingot (AC) and for the
extruded sample processed by subsequent annealing
(ExT4). The similarity was also observed for samples
extruded directly from the as-cast state (Ex) and from the
heat-treated ingot (T4Ex). Weaker mechanical properties
such as CYS, TYS, UCS, and UTS of AC and ExT4 are
mainly contributed to the large grains. Large grains also
increase the plasticity of the material in this case. Large
grain size and specific grain orientation after extrusion
support the twinning mechanism.
18
Such behavior can be
recognized in Figure 5 in some plateau after yielding.
Beyond the yield strength there exists in all cases an area
of lower strengthening (plateau), where the twinning
mechanism occurs. In the last part of the compressive
curve (Ex, T4Ex), the slope of stress-strain dependence
is again increased as a result of higher activation ener-
gies for advanced slip systems. Slightly higher concen-
tration of alloying elements in the solid solution and
D. DVORSKÝ et al.: EFFECT OF HEAT PRE-TREATMENT AND EXTRUSION ON THE STRUCTURE ...
502 Materiali in tehnologije / Materials and technology 52 (2018) 4, 499–505
Figure 2: EBSD orientation map of Mg-2Y-1Zn alloy prepared by ex-
trusion of as-cast state: EBSD map and pole figures of the extruded
ingot (Y0 reflects the extrusion direction (ED))
Figure 1: Microstructure of Mg-2Y-1Zn alloy after different process-
ing (SEM): a) As-cast state (AC), b) Extruded state (Ex), c) Extruded
state subsequently heat treated (ExT4), d) Heat-treated state sub-
sequently extruded (T4Ex)

homogeneous structure in ExT4 sample cause increase in
the ultimate compressive strength.
19
Just a slight improvement of the mechanical pro-
perties can be observed for T4Ex compared with the just
extruded sample. This minor improvement is also con-
tributed to the solid-solution strengthening as well as by
the more homogeneous structure. However, both ma-
terials had strongly reduced plasticity as a consequence
of the texture of the material.
20
This fact is supported by
the anisotropy of the mechanical properties in tension
and compression, which was observed for the extruded
samples (Figure 2). The measured compressive yield
strengths were lower than the tensile yield strengths,
which is associated with the easy activation of the
twinning mechanism during the compression testing.
21
The winning is directionally dependent. The typical
magnesium basal texture means that most of the grains
orient their basal planes (0001) parallel with the extru-
sion direction. If such a structure is pressed in parallel to
the extrusion direction then the twinning mechanism
occurs easily. On the contrary, if the tensile force is
applied in parallel to the extrusion direction, the twinn-
ing mechanism is suppressed and the basal slip is diffi-
cult due to preferential texture and grain orientation.
22
The highest value of the tensile yield strength reached up
to 337 MPa, which is higher than was achieved by
Cabeza et al.
4
(211 MPa). They prepared samples by
extrusion at the same temperature but with extrusion
ratio of 32. Huan et al.
2
reached lower ultimate tensile
strength (320 MPa) and elongation of 11.2 % however
they used higher temperature of 450 °C and extrusion
ratio 9.
3.3 Corrosion
Corrosion behavior was studied on immersion tests in
simulated body fluid (SBF) at 37 °C for two weeks. Each
sample was immersed in the closed container in approxi-
mately 300 ml of SBF. The final corrosion rates were
calculated from weight changes and concentration of
released magnesium ions in the corrosion environment.
The highest corrosion resistance was obtained for mate-
rials that were exposed to proper annealing (Figure 6).
Increased corrosion resistance is caused by different
effects. Firstly, heat-treated materials are characterized
by more homogeneous structures with lower content of
intermetallic phases and also absence of dendritic micro-
segregation. Both these effects have a negative impact on
the corrosion rate of the studied material because they
accelerate the dissolution of magnesium alloy and
support the existence of localized corrosion.
23
A slightly
lower corrosion rate of alloy processed by heat pre-treat-
ment and subsequent extrusion is associated with the
higher amount of alloying elements (especially Y) in
solid solution, which is known to improve the corrosion
resistance by incorporating Y in protective oxide layers
which are formed during corrosion.
24
The corrosion rate
of the sample extruded from the as-cast state is characte-
D. DVORSKÝ et al.: EFFECT OF HEAT PRE-TREATMENT AND EXTRUSION ON THE STRUCTURE ...
Materiali in tehnologije / Materials and technology 52 (2018) 4, 499–505 503
Figure 5: Compressive curves of Mg-2Y-1Zn processed in different
ways
Figure 3: Compressive properties of Mg-2Y-1Zn alloy (CYS, UCS)
Figure 4: Tensile properties of Mg-2Y-1Zn alloy (TYS, UTS)

rized by highest corrosion rate, almost one-third higher
compared to the as-cast state. In both states, micro-
segregation is observed in the microstructure. However,
in the case of as-cast specimen, dendrites and its boun-
daries enriched by Y and Zn are distributed more
homogeneously. Such areas may partially work as a
barrier for spreading of the corrosion, like in the case of
Mg
17
Al
12
phase in AZ31 alloys.
25
This is quite different
in the case of extruded specimen, because here alternate
rows enriched and depleted by Y and Zn. As a conse-
quence, corrosion may be spread more easily in
continuous rows depleted by Y and Zn. The obtained
corrosion rates are much lower compared to the work of
Cabeza et al.
4
(6.8 mm/year). They calculated the
corrosion rate out of released hydrogen for the first 144 h
with the tendency to slow down the corrosion, so the
final corrosion rate might be lower after two weeks.
Nevertheless, a high corrosion rate was caused by the
numerous intermetallic phases which worked as cathodic
areas and accelerate corrosion by micro-galvanic cells.
This is in accordance with our observations. The absence
of intermetallic phases in heat-treated samples resulted
in the significant improvement of corrosion resistance.
4 CONCLUSIONS
The Mg-2Y-1Zn magnesium alloy was prepared by
conventional methods including casting, thermal treat-
ment (525 °C/8 h), and extrusion at 350 °C. The
as-casted ingot was characterized by the presence of a
dendritic structure and LPSO phases. The extrusion
process provides a fine-grained structure with an average
grain size ranging from 0.5 μm to 7 μm. The LPSO
phases partly disappeared after the extrusion of the
as-cast state and the residual intermetallic phases were
ordered in rows parallel to the extrusion direction. A
variation in the processing related to thermal treatment
seriously affects the mechanical and corrosion behavior.
Heat treatment is responsible for the dissolution of the
LPSO phases, the structure homogenization, and there-
fore a decrease of corrosion rate, which was lowered
from 2.5 mm/year to 1 mm/year in the case of the
extruded sample and extruded sample with subsequent
thermal treatment, respectively. The presented results
also showed that thermal treatment is more favorable
before extrusion. In this case, the tensile yield strength
reached 337 MPa, whereas such a value was 5 times
lower for the extruded and the subsequently heat-treated
sample.
Acknowledgment
We wish to thank Czech Science Foundation for
financial support of this research by project No. GA16-
08963S.
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