Microstructure and Mechanical Properties of Hot-Extruded Mg–Zn–Ga–(Y) Biodegradable Alloys

Abstract

Magnesium alloys are attractive candidates for use as temporary fixation devices in osteosynthesis because they have a density and Young’s modulus similar to those of cortical bone. One of the main requirements for biodegradable implants is its substitution by tissues during the healing process. In this article, the Mg–Zn–Ga–(Y) alloys were investigated that potentially can increase the bone growth rate by release of Ga ions during the degradation process. Previously, the effectiveness of Ga ions on bone tissue regeneration has been proved by clinical tests. This work is the first systematic study on the microstructure and mechanical properties of Mg–Zn–Y alloys containing Ga as an additional major alloying element prepared by the hot-extrusion process. The microstructure and phase composition of the Mg–Zn–Ga–(Y) alloys in as-cast, heat-treated, and extruded conditions were analyzed. In addition, it was shown that the use of hot extrusion produces Mg–Zn–Ga–(Y) alloys with favorable mechanical properties. The tensile yield strength, ultimate tensile strength, and elongation at fracture of the MgZn4Ga4 alloy extruded at 150 °C were 256 MPa, 343 MPa, and 14.2%, respectively. Overall, MgZn4Ga4 alloy is a perspective for applications in implants for osteosynthesis with improved bone regeneration ability.

Full text

Citation: Bazhenov, V.; Li, A.;
Tavolzhanskii, S.; Bazlov, A.;
Tabachkova, N.; Koltygin, A.;
Komissarov, A.; Shin, K.S.
Microstructure and Mechanical
Properties of Hot-Extruded
Mg–Zn–Ga–(Y) Biodegradable
Alloys. Materials 2022,15, 6849.
https://doi.org/10.3390/
ma15196849
Academic Editors: Madlen Ullmann,
Kristina Kittner and Jordi Sort
Received: 30 August 2022
Accepted: 29 September 2022
Published: 2 October 2022
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4.0/).
materials
Article
Microstructure and Mechanical Properties of Hot-Extruded
Mg–Zn–Ga–(Y) Biodegradable Alloys
Viacheslav Bazhenov 1,* , Anna Li 2, Stanislav Tavolzhanskii 1, Andrey Bazlov 3, Natalia Tabachkova 4,5 ,
Andrey Koltygin 1, Alexander Komissarov 2,6 and Kwang Seon Shin 6,7
1Casting Department, National University of Science and Technology “MISiS”, Leninskiy pr. 4,
119049 Moscow, Russia
2Laboratory of Hybrid Nanostructured Materials, National University of Science and Technology “MISiS”,
Leninskiy pr. 4, 119049 Moscow, Russia
3
Laboratory of Advanced Green Materials, National University of Science and Technology “MISiS”, Leninskiy
pr. 4, 119049 Moscow, Russia
4Department of Materials Science of Semiconductors and Dielectrics, National University of Science and
Technology “MISiS”, Leninskiy pr. 4, 119049 Moscow, Russia
5Fianit Laboratory, Laser Materials and Technology Research Center at GPI, Prokhorov General Physics
Institute RAS, Vavilov st. 38, 119991 Moscow, Russia
6Laboratory of Medical Bioresorption and Bioresistance, Moscow State University of Medicine and Dentistry,
Delegatskaya 20/1, 127473 Moscow, Russia
7
Magnesium Technology Innovation Center, Department of Materials Science and Engineering, Seoul National
University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea
*Correspondence: v.e.bagenov@gmail.com; Tel.: +7-(905)-553-55-64
Abstract:
Magnesium alloys are attractive candidates for use as temporary fixation devices in os-
teosynthesis because they have a density and Young’s modulus similar to those of cortical bone.
One of the main requirements for biodegradable implants is its substitution by tissues during the
healing process. In this article, the Mg–Zn–Ga–(Y) alloys were investigated that potentially can
increase the bone growth rate by release of Ga ions during the degradation process. Previously, the
effectiveness of Ga ions on bone tissue regeneration has been proved by clinical tests. This work is the
first systematic study on the microstructure and mechanical properties of Mg–Zn–Y alloys containing
Ga as an additional major alloying element prepared by the hot-extrusion process. The microstructure
and phase composition of the Mg–Zn–Ga–(Y) alloys in as-cast, heat-treated, and extruded conditions
were analyzed. In addition, it was shown that the use of hot extrusion produces Mg–Zn–Ga–(Y)
alloys with favorable mechanical properties. The tensile yield strength, ultimate tensile strength, and
elongation at fracture of the MgZn4Ga4 alloy extruded at 150
◦
C were 256 MPa, 343 MPa, and 14.2%,
respectively. Overall, MgZn4Ga4 alloy is a perspective for applications in implants for osteosynthesis
with improved bone regeneration ability.
Keywords:
biomaterials; gallium; hot extrusion; magnesium; mechanical properties; microstructure
1. Introduction
Permanent bone fixation implants are the gold standard in osteosynthesis and are used
in healthcare systems in many countries. Titanium alloys have been used for bone fixation
implants. However, the titanium systems have disadvantages including temperature
sensitivity, tactile sensation of implants, possible growth restrictions, hampering of imaging
and radiotherapy, presence of titanium particles in surrounding tissue, and potential
mutagenicity [
1
]. These disadvantages result in symptomatic removal in up to 40% of
cases [
1
]. At present, temporary biodegradable implants are gaining popularity, that
gradually dissolve as the healing process progresses [
2
]. This approach helps to minimize
implant-induced inflammation and reduces healthcare costs by eliminating secondary
surgery for implant removal [
3
]. Mg alloys are attractive candidates for the fabrication of
Materials 2022,15, 6849. https://doi.org/10.3390/ma15196849 https://www.mdpi.com/journal/materials

Materials 2022,15, 6849 2 of 17
temporary fixation devices for osteosynthesis. They have good biocompatibility, sufficiently
high mechanical strength, and an acceptable biodegradation rate [
4
–
6
]. Furthermore, unlike
Ti implants for permanent bone fixation, Mg alloys have a similar density and Young’s
modulus to cortical bone [7,8].
At present, the commercial NOVAMag
®
and MAGNEZIX
®
fixation screws produced
by Botiss biomaterials GmbH (Berlin, Germany) and Syntellix AG (Hannover, Germany) are
used in orthopedic practice and showed equal performance in tissue regeneration with Ti
alloys [
9
,
10
]. One of the main capabilities of biodegradable implants is the same rates of im-
plant degradation and bone tissue growth for substituting the voids with new bone tissue.
Several studies revealed Mg has a positive impact on the bone regeneration [
2
], but addi-
tional efforts can increase the Mg implants’ applications. For example, previously, a study
on animals showed that Ga-contained hydroxyapatite coating on Gription
™
implants (West
Chester, PA, USA) increased the bone growth rate by two times [
11
]. However, the coating
used for biodegradable implants will improve osteogenesis only before it dissolves. The
main objective of this work is proposing the new alloy system with the addition of a compo-
nent that can improve the bone tissue growth process. The most appropriate component in
this method is gallium. Gallium is known as a bone resorption inhibitor that increases the
Ca and P content in the developed bone [
12
,
13
]. Thus, it effectively treats osteoporosis [
14
],
hypercalcemia [
15
–
17
], Paget’s disease [
18
,
19
], and multiple myeloma [
20
]. In addition,
Ga has an anti-osteoclastic effect that reduces osteoclastic resorption, differentiation, and
formation without negatively affecting the osteoblast’s viability and proliferation [21–23].
When the release of Ga ions occurs during the degradation of Mg–Zn–Ga–(Y) alloys,
and local Ga delivery to tissues takes place. Thus, the Mg–Zn–Ga–(Y) alloys in bone fixation
implants can improve the bone healing process, which is the benefit in comparison with
other conventional magnesium alloys. The design and introduction of a new implant
into medical practice require deep knowledge of the mechanical, corrosion, and biological
performance of the new material [24].
The effect of Zn addition on Mg alloys properties is well known [
25
,
26
], but the
effect of Ga is currently under investigation. Liu et al. showed that Ga decreased the c/a
proportion of magnesium solid solution (
α
-Mg), leading to the activation of additional slip
planes and improved plastic deformation of Mg alloys [
27
]. Furthermore, the addition of
Ga can reduce the critical strain for dynamic recrystallization (DRX) in Mg alloys, which
is associated with a reduced stacking fault energy, increased twinning density during
deformation, and an increase in the number of DRX nucleation sites [
28
,
29
]. Gallium also
acted as an effective grain refiner [
30
]. It is known that the work hardening ability of
Mg–Ga alloys is higher than that of pure Mg due to a uniform fine equiaxed microstructure
with a low dislocation density [
31
]. Thus, Ga addition to Mg is expected to lead to excellent
mechanical properties [
31
]. Furthermore, the addition of Zn to Mg–Ga alloys reduces the
activation energy barrier for nucleation of the Mg
5
Ga
2
phase, resulting in more and finer
precipitates [32].
Previously, Mg–4Zn–4Ga (wt.%) alloys with small additions of Ca, Y, and Nd were
investigated after equal channel angular pressing (ECAP). It was found that these alloys
had high strength (up to 300 MPa) and a low corrosion rate of ~0.2 mm/year in Hanks’
solution [
33
]. The addition of Y does not decrease the corrosion rate of Mg–Zn–Ga alloys
compared to that of Ca and Nd [
33
]. Further, Y addition contributes to enhanced protective
properties and a higher alloy ignition temperature [
34
]. Therefore, the Mg–Zn–Ga alloy
with Y addition needs to be investigated further.
Despite our previous study on Mg–Zn–Ga–(Y) alloys after ECAP processing, the po-
tential of other deformation processing techniques such as hot extrusion on alloy properties
has not yet been thoroughly examined [
33
]. The hot extrusion process has various advan-
tages compared with ECAP: less limitations in size and shape of the billet, easy control
of microstructure, low processing cost, high yield, etc. Due to this, the hot extrusion is
better fit to high-volume manufacturing. Thus, the aim of the study was to investigate
the effects of the chemical composition and extrusion temperature on the microstructure

Materials 2022,15, 6849 3 of 17
and mechanical properties of Mg–Zn–Ga–(Y) alloys. This work is the first systematic
study on the properties of Mg–Zn–(Y) alloys containing Ga as an additional major alloying
element prepared by the hot-extrusion process to evaluate their potential for application in
orthopedic implants.
The extrusion processing temperature window is usually determined by the possibili-
ties of equipment used (possible lowest extrusion temperature) and limit of hot cracking
(possible highest extrusion temperature) [
35
]. For Mg–Zn–Ga–(Y) alloys, the solidus tem-
perature is close to 300
◦
C, which was shown previously [
33
] and confirmed via CALPHAD
calculation and DSC in this work. Due to this, the upper limit of extrusion temperature
250
◦
C was chosen. The lower limit of 150
◦
C was chosen in accordance with the maximal
pressure of the used press.
2. Materials and Methods
2.1. Alloy Preparation and Hot Extrusion
The scheme of preparing samples for investigation is presented in Figure 1. High-
purity bulk metals, including Mg (99.98 wt.% purity; SOMZ, Solikamsk, Russia), Zn
(99.995 wt.%; UGMK, Verkhnaya Pyshma, Russia), Ga (99.9999 wt.%; Girmet Ltd., Moscow,
Russia), and Mg–20Y (wt.%) master alloy (Metagran, Moscow, Russia) were used as the
starting materials for the preparation of alloys. The melts were prepared using a resistance
furnace with a steel crucible. For melt protection from ignition, an Ar + 2 vol.% SF
6
protective atmosphere was used. The details of the melting procedure can be found
elsewhere [
36
]. Cylindrical ingots with a diameter of 60 mm and a length of 200 mm were
cast into an aluminum permanent mold preheated to 150
◦
C. Five alloys with different
Zn and Ga contents were prepared, as listed in Table 1. In addition, an alloy with the
addition of Y was prepared. The chemical compositions of the alloys were analyzed using
energy-dispersive X-ray spectroscopy (EDS) on the metallographic sections with 0.1 wt.%
accuracy. The three areas with size 1 ×1 mm2were analyzed for each specimen.
Materials 2022, 15, x FOR PEER REVIEW 3 of 17
extrusion is better fit to high-volume manufacturing. Thus, the aim of the study was to
investigate the effects of the chemical composition and extrusion temperature on the
microstructure and mechanical properties of Mg–Zn–Ga–(Y) alloys. This work is the first
systematic study on the properties of Mg–Zn–(Y) alloys containing Ga as an additional
major alloying element prepared by the hot-extrusion process to evaluate their potential
for application in orthopedic implants.
The extrusion processing temperature window is usually determined by the
possibilities of equipment used (possible lowest extrusion temperature) and limit of hot
cracking (possible highest extrusion temperature) [35]. For Mg–Zn–Ga–(Y) alloys, the
solidus temperature is close to 300 °C, which was shown previously [33] and confirmed
via CALPHAD calculation and DSC in this work. Due to this, the upper limit of extrusion
temperature 250 °C was chosen. The lower limit of 150 °C was chosen in accordance with
the maximal pressure of the used press.
2. Materials and Methods
2.1. Alloy Preparation and Hot Extrusion
The scheme of preparing samples for investigation is presented in Figure 1. High-
purity bulk metals, including Mg (99.98 wt.% purity; SOMZ, Solikamsk, Russia), Zn
(99.995 wt.%; UGMK, Verkhnaya Pyshma, Russia), Ga (99.9999 wt.%; Girmet Ltd.,
Moscow, Russia), and Mg–20Y (wt.%) master alloy (Metagran, Moscow, Russia) were
used as the starting materials for the preparation of alloys. The melts were prepared using
a resistance furnace with a steel crucible. For melt protection from ignition, an Ar + 2 vol.%
SF6 protective atmosphere was used. The details of the melting procedure can be found
elsewhere [36]. Cylindrical ingots with a diameter of 60 mm and a length of 200 mm were
cast into an aluminum permanent mold preheated to 150 °C. Five alloys with different Zn
and Ga contents were prepared, as listed in Table 1. In addition, an alloy with the addition
of Y was prepared. The chemical compositions of the alloys were analyzed using energy-
dispersive X-ray spectroscopy (EDS) on the metallographic sections with 0.1 wt.%
accuracy. The three areas with size 1 × 1 mm2 were analyzed for each specimen.
Figure 1. The scheme of extruded bars processing from casting to hot extrusion.
Table 1. Elemental compositions of the prepared alloys.
Alloy
Element Content (wt.%)
Mg
Zn
Ga
Y
MgZn4Ga4
Bal.
4.2
4.1
-
MgZn4Ga4Y0.5
Bal.
4.2
4.1
0.4
MgZn6.5Ga2
Bal.
6.5
2.0
-
MgZn4Ga2
Bal.
4.2
2.2
-
MgZn2Ga2
Bal.
2.3
2.3
-
Figure 1. The scheme of extruded bars processing from casting to hot extrusion.
Table 1. Elemental compositions of the prepared alloys.
Alloy Element Content (wt.%)
Mg Zn Ga Y
MgZn4Ga4 Bal. 4.2 4.1 -
MgZn4Ga4Y0.5 Bal. 4.2 4.1 0.4
MgZn6.5Ga2 Bal. 6.5 2.0 -
MgZn4Ga2 Bal. 4.2 2.2 -
MgZn2Ga2 Bal. 2.3 2.3 -

Materials 2022,15, 6849 4 of 17
To dissolve the eutectic phases and homogenize the ingot composition, a solid solution
heat treatment (HT) at 300
◦
C for 15 h + 400
◦
C for 30 h was applied [
33
]. First, the ingots
were machined into cylindrical billets with a height of 145 mm and diameter of 50 mm.
Next, hot extrusion of the alloys was performed on a 300 ton vertical hydraulic press
PS-300A7 (Gidrosfera, Moscow, Russia) using the direct extrusion method at a ram speed
of 1 mm/s and an extrusion ratio of 6.25 (Figure 1). Finally, cylindrical extruded bars with
a diameter of 20 mm and length of ~1 m were obtained. Before the hot extrusion process,
the die was preheated to 200–250
◦
C. Extrusion was performed at billet temperatures of
150, 200, and 250 ◦C to evaluate the effect of this temperature of the final alloy properties.
2.2. Microstructural Observations, Phase Composition, and Thermal Analysis
The phase composition and phase transition temperatures of the Mg–Zn–Ga–(Y)
alloys were calculated according to the calculation of phase diagram (CALPHAD) method
using FactSage software (GTT-Technologies, Aachen, Germany). Furthermore, the Scheil–
Gulliver solidification of the alloys was calculated [
37
]. The thermodynamic database FTlite
(GTT-Technologies, Aachen, Germany) was used.
The alloy microstructure and elemental content of the phases were investigated using
scanning electron microscopy (SEM; Vega SBH3, Tescan, Brno, Czech Republic) with an
EDS detector (Oxford, UK) and transmission electron microscopy (TEM; JEM-2100, JEOL,
Tokyo, Japan). The accelerating voltage used for TEM investigations was 200 kV. The
samples for TEM were prepared using ion-beam etching, which was performed using a
Precision Ion Polishing System PIPS II (Gatan, Pleasanton, CA, USA) at a voltage of 3 keV.
The phase volume fractions were determined by calculating the area of phases in the SEM
image using Tescan software (Tescan, Brno, Czech Republic). The grain size of the extruded
samples was determined using the linear intercept method, with the assistance of SEM and
an Axio Observer D1m (Carl Zeiss, Oberkochen, Germany) optical microscope (OM). To
reveal the grains, the metallographic cross-sections were etched for 5 s using an etchant
(11 g picric acid, 11 mL acetic acid, and 100 mL ethanol).
The solidus temperatures of the alloys were measured using differential scanning
calorimetry (DSC; Labsys Setaram, Caluire, France) under an Ar gas flow at a heating rate
of 20 ◦C/min in Al2O3crucibles.
The alloy phase compositions were examined using bulk cylindrical specimens with
X-ray diffractometry (XRD) with a D8 ADVANCE diffractometer (Bruker, Billerica, MA,
USA) under monochromatic Cu Kαradiation.
2.3. Mechanical Properties
The mechanical properties of the alloys were investigated using a 5569 universal testing
machine (Instron, Norwood, MA, USA) equipped with an advanced video extensometer.
The tolerance of testing machine was less than 0.5%. Tensile tests were performed using
two types of specimens: standard cylindrical specimens obtained by extruded bar lathe
machining and small flat plate tensile test specimens produced by wire cutting the hot-
extruded bars [
38
]. The compression tests were performed using small cuboid specimens
(3 mm
×
3 mm
×
6 mm). The small flat plate and cuboid specimens were cut along and
perpendicular to the extrusion direction (ED). Three standard cylindrical specimens and six to
eight flat plates and cuboid specimens were tested for each alloy and extrusion temperature.
3. Results and Discussion
3.1. Quality of Extruded Bars
The quality of the extruded bars is shown in Figure 2a. The bars were divided into
three groups by defectiveness: bars with large radial cracks visible by the naked eye
(Figure 2b), bars with surface cracks with a depth of 1
−
2 mm (Figure 2c), and bars of good
quality with no visible cracks on the surface (Figure 2d). Increasing the alloying element
content and extrusion temperature decreased the quality of the bars. During extrusion, the
deformation energy is converted into heat, thereby increasing the billet temperature above

Materials 2022,15, 6849 5 of 17
the extrusion temperature. If the billet temperature is close to the solidus temperature,
crack formation is possible. Therefore, the residual eutectic in the high-alloyed alloys that
melts during extrusion at 250 ◦C results in crack formation (Figure 2b).
Materials 2022, 15, x FOR PEER REVIEW 5 of 17
content and extrusion temperature decreased the quality of the bars. During extrusion,
the deformation energy is converted into heat, thereby increasing the billet temperature
above the extrusion temperature. If the billet temperature is close to the solidus
temperature, crack formation is possible. Therefore, the residual eutectic in the high-
alloyed alloys that melts during extrusion at 250 °C results in crack formation (Figure 2b).
(a)
(b)
(c)
(d)
Figure 2. (a) Graph defining the quality of the Mg–Zn–Ga–(Y) alloy bars as a function of alloy
composition and extrusion temperature. The quality examples are shown in: (b) bar with large
radial cracks (e.g., MgZn4Ga2 alloy extruded at 250 °C), (c) bar with small (~1−2 mm) surface cracks
(e.g., MgZn4Ga4 alloy extruded at 150 °C), and (d) bar without cracks (e.g., MgZn2Ga2 alloy
extruded at 150 °C).
3.2. Microstructure and Phase Composition
Table 2 shows liquidus and solidus temperatures, fraction of phases at room
temperature (RT) and temperatures when secondary phases start to precipitate calculated
with FactSage software. The increase in alloying elements’ content leads to a decrease in
equilibrium solidus and liquidus temperature. In accordance with calculation results, the
RT microstructure of investigated alloys must comprise α-Mg, Mg5Ga2, and Mg12Zn13
(otherwise known as MgZn). When Y is added, the I phase (Mg3Zn6Y) is precipitated.
Furthermore, for high alloyed alloys, the precipitation temperature of Mg5Ga2 and
Mg12Zn13 phases and its fraction at RT are higher than for their low alloyed counterparts.
This means that precipitation hardening must be greater for MgZn4Ga4, MgZn4Ga4Y0.5,
and MgZn6.5Ga2 alloys.
Figure 2.
(
a
) Graph defining the quality of the Mg–Zn–Ga–(Y) alloy bars as a function of alloy
composition and extrusion temperature. The quality examples are shown in: (
b
) bar with large radial
cracks (e.g., MgZn4Ga2 alloy extruded at 250
◦
C), (
c
) bar with small (~1
−
2 mm) surface cracks (e.g.,
MgZn4Ga4 alloy extruded at 150
◦
C), and (
d
) bar without cracks (e.g., MgZn2Ga2 alloy extruded at
150 ◦C).
3.2. Microstructure and Phase Composition
Table 2shows liquidus and solidus temperatures, fraction of phases at room tem-
perature (RT) and temperatures when secondary phases start to precipitate calculated
with FactSage software. The increase in alloying elements’ content leads to a decrease in
equilibrium solidus and liquidus temperature. In accordance with calculation results, the
RT microstructure of investigated alloys must comprise
α
-Mg, Mg
5
Ga
2
, and Mg
12
Zn
13
(otherwise known as MgZn). When Y is added, the I phase (Mg
3
Zn
6
Y) is precipitated. Fur-
thermore, for high alloyed alloys, the precipitation temperature of Mg
5
Ga
2
and Mg
12
Zn
13
phases and its fraction at RT are higher than for their low alloyed counterparts. This
means that precipitation hardening must be greater for MgZn4Ga4, MgZn4Ga4Y0.5, and
MgZn6.5Ga2 alloys.

Materials 2022,15, 6849 6 of 17
Table 2.
The Mg
−
Zn
−
Ga alloys phase composition and phase transition temperatures calculated via
FactSage software.
Alloy Tliq.Sch
(◦C)
Tsol.eq
(◦C)
Meut.Sch
(wt.%)
Eq. Phase Amount at RT (wt.%) Eq. Precipit. Start. Temp. (◦C)
Mg5Ga2Mg12Zn13 I Mg5Ga2Mg12 Zn13 I
MgZn4Ga4 624 370.5 9.82 7.5 5.41 - 266 258 -
MgZn4Ga4Y0.5
623 412 6.58 7.5 3.05 2.49 264 189 434
MgZn6.5Ga2
623 334 8.02 3.57 8.5 - 182 321 -
MgZn4Ga2 630 421 8.70 3.94 5.41 - 188 250 -
MgZn2Ga2 636 497 5.05 4.12 2.85 - 189.5 174 -
The calculation of the solidification pathway using the Scheil–Gulliver solidification
model shows that in all of investigated alloys, the ternary eutectic transition L
→α
-Mg +
Mg
12
Zn
13
+ Mg
5
Ga
2
occurs at 299
◦
C. Therefore, the two-stage solution heat treatment was
used for alloys, where the first stage at 300
◦
C is for dissolution of non-equilibrium eutectic.
The microstructures of the alloys in the as-cast condition are shown in Figure 3.
All alloys contained
α
-Mg dendrites with clear visible micro-segregation and eutectic
intermetallic phases. The microstructure and EDS maps in Figure 4show the Zn, Ga,
and Y distributions for the MgZn4Ga4 and MgZn4Ga4Y0.5 alloys. The microstructure
of MgZn4Ga4 showed two eutectic phases with a composition close to the Mg
51
Zn
20
(otherwise known as Mg
7
Zn
3
) and Mg
5
Ga
2
phases, as determined by EDS point analysis.
According to the phase diagram, when the temperature is lower than 325
◦
C, Mg
51
Zn
20
does not exist, and the Mg
12
Zn
13
phase precipitates [
39
]. However, in magnesium alloys
with Zn addition, the Mg
51
Zn
20
intermetallic phase could be found in the microstructure of
the as-cast alloys. Further, it will be seen that after solution heat treatment and hot extrusion,
the Mg
12
Zn
13
phase is present in microstructures rather than Mg
51
Zn
20
, in accordance with
the Mg–Zn phase diagram.
Materials 2022, 15, x FOR PEER REVIEW 6 of 17
Table 2. The Mg−Zn−Ga alloys phase composition and phase transition temperatures calculated via
FactSage software.
Alloy
Tliq.Sch
(°C)
Tsol.eq
(°C)
Meut.Sch
(wt.%)
Eq. Phase Amount at RT
(wt.%)
Eq. Precipit. Start. Temp.
(°C)
Mg5Ga2
Mg12Zn13
I
Mg5Ga2
Mg12Zn13
I
MgZn4Ga4
624
370.5
9.82
7.5
5.41
-
266
258
-
MgZn4Ga4Y0.5
623
412
6.58
7.5
3.05
2.49
264
189
434
MgZn6.5Ga2
623
334
8.02
3.57
8.5
-
182
321
-
MgZn4Ga2
630
421
8.70
3.94
5.41
-
188
250
-
MgZn2Ga2
636
497
5.05
4.12
2.85
-
189.5
174
-
The calculation of the solidification pathway using the Scheil–Gulliver solidification
model shows that in all of investigated alloys, the ternary eutectic transition L→α-Mg +
Mg12Zn13 + Mg5Ga2 occurs at 299 °C. Therefore, the two-stage solution heat treatment was
used for alloys, where the first stage at 300 °C is for dissolution of non-equilibrium
eutectic.
The microstructures of the alloys in the as-cast condition are shown in Figure 3. All
alloys contained α-Mg dendrites with clear visible micro-segregation and eutectic
intermetallic phases. The microstructure and EDS maps in Figure 4 show the Zn, Ga, and
Y distributions for the MgZn4Ga4 and MgZn4Ga4Y0.5 alloys. The microstructure of
MgZn4Ga4 showed two eutectic phases with a composition close to the Mg51Zn20
(otherwise known as Mg7Zn3) and Mg5Ga2 phases, as determined by EDS point analysis.
According to the phase diagram, when the temperature is lower than 325 °C, Mg51Zn20
does not exist, and the Mg12Zn13 phase precipitates [39]. However, in magnesium alloys
with Zn addition, the Mg51Zn20 intermetallic phase could be found in the microstructure
of the as-cast alloys. Further, it will be seen that after solution heat treatment and hot
extrusion, the Mg12Zn13 phase is present in microstructures rather than Mg51Zn20, in
accordance with the Mg–Zn phase diagram.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Figure 3. Cont.

Materials 2022,15, 6849 7 of 17
Materials 2022, 15, x FOR PEER REVIEW 7 of 17
(i)
(j)
(k)
(l)
(m)
(n)
(o)
(p)
(q)
(r)
(s)
(t)
Figure 3. Microstructure of the (a–d) MgZn4Ga4, (e–h) MgZn4Ga4Y0.5, (i–l) MgZn6.5Ga2, (m–p)
MgZn4Ga2, and (q–t) MgZn2Ga2 alloys in the (a,e,i,m,q) as-cast condition, (b,f,j,n,r) after HT for
15 h at 300 °C + 30 h at 400 °C, and after hot extrusion at (c,g,k,o,s) 150 or (d,h,l,p,t) 200 °C. The
insets with pink boxes show higher-magnification images, while those in blue boxes show the
microstructure after etching (only for extruded alloys).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Figure 4. Microstructure and EDS maps of the (a–c) MgZn4Ga4 and (d–g) MgZn4Ga4Y0.5 alloys in
the as-cast condition. The EDS maps showing the distribution of (b,e) Zn, (c,f) Ga, and (g) Y.
Figure 3.
Microstructure of the (
a
–
d
) MgZn4Ga4, (
e
–
h
) MgZn4Ga4Y0.5, (
i
–
l
) MgZn6.5Ga2,
(
m
–
p
) MgZn4Ga2, and (
q
–
t
) MgZn2Ga2 alloys in the (
a
,
e
,
i
,
m
,
q
) as-cast condition, (
b
,
f
,
j
,
n
,
r
) af-
ter HT for 15 h at 300
◦
C + 30 h at 400
◦
C, and after hot extrusion at (
c
,
g
,
k
,
o
,
s
) 150 or (
d
,
h
,
l
,
p
,
t
)
200 ◦C
.
The insets with pink boxes show higher-magnification images, while those in blue boxes show the
microstructure after etching (only for extruded alloys).
Materials 2022, 15, x FOR PEER REVIEW 7 of 17
(i)
(j)
(k)
(l)
(m)
(n)
(o)
(p)
(q)
(r)
(s)
(t)
Figure 3. Microstructure of the (a–d) MgZn4Ga4, (e–h) MgZn4Ga4Y0.5, (i–l) MgZn6.5Ga2, (m–p)
MgZn4Ga2, and (q–t) MgZn2Ga2 alloys in the (a,e,i,m,q) as-cast condition, (b,f,j,n,r) after HT for
15 h at 300 °C + 30 h at 400 °C, and after hot extrusion at (c,g,k,o,s) 150 or (d,h,l,p,t) 200 °C. The
insets with pink boxes show higher-magnification images, while those in blue boxes show the
microstructure after etching (only for extruded alloys).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Figure 4. Microstructure and EDS maps of the (a–c) MgZn4Ga4 and (d–g) MgZn4Ga4Y0.5 alloys in
the as-cast condition. The EDS maps showing the distribution of (b,e) Zn, (c,f) Ga, and (g) Y.
Figure 4.
Microstructure and EDS maps of the (
a
–
c
) MgZn4Ga4 and (
d
–
g
) MgZn4Ga4Y0.5 alloys in
the as-cast condition. The EDS maps showing the distribution of (b,e) Zn, (c,f) Ga, and (g) Y.

Materials 2022,15, 6849 8 of 17
The addition of Y to MgZn4Ga4 leads to formation of the Ga–Y eutectic phase. The
obtained results were similar to those observed previously [
33
,
39
–
42
], where the Ga–Y
phase observed. Furthermore, this phase is not rich in Zn and consists mostly of Y and
Ga. It differs from the FactSage calculation results, where I phase (Mg
3
Zn
6
Y) rich in Zn
formation is predicted.
The alloy microstructures after HT are also shown in Figure 3. In comparison with as-
cast microstructure, the dissolution of intermetallic phases and changes in their morphology
to spherical structures were observed. The fractions of Mg
12
Zn
13
, Mg
5
Ga
2
, and Ga–Y
intermetallic phases in Mg–Zn–Ga–(Y) alloys in the as-cast condition and after HT are
present in Figure 5a. After HT, a residual intermetallic phase was observed for high-alloyed
alloys (MgZn6.5Ga2, MgZn4Ga4, and MgZn4Ga4Y0.5). For low-alloyed MgZn4Ga2 and
MgZn2Ga2 alloys, a single-phase
α
-Mg microstructure was observed. In the microstructure
of the alloys after HT, Mg
12
Zn
13
, Mg
5
Ga
2
, and Ga–Y intermetallic phases are present.
These phases act as cathodes for
α
-Mg during the corrosion process [
33
]. Therefore, high
volume fractions of such phases in the alloy are undesirable from the corrosion standpoint.
In opposite, the intermetallic phases can act as recrystallization centers according to the
particle-simulated nucleation mechanism during subsequent deformation and lead to a
decrease in grain size [43].
Materials 2022, 15, x FOR PEER REVIEW 8 of 17
The addition of Y to MgZn4Ga4 leads to formation of the Ga–Y eutectic phase. The
obtained results were similar to those observed previously [33,39–42], where the Ga–Y
phase observed. Furthermore, this phase is not rich in Zn and consists mostly of Y and
Ga. It differs from the FactSage calculation results, where I phase (Mg3Zn6Y) rich in Zn
formation is predicted.
The alloy microstructures after HT are also shown in Figure 3. In comparison with
as-cast microstructure, the dissolution of intermetallic phases and changes in their
morphology to spherical structures were observed. The fractions of Mg12Zn13, Mg5Ga2, and
Ga–Y intermetallic phases in Mg–Zn–Ga–(Y) alloys in the as-cast condition and after HT
are present in Figure 5a. After HT, a residual intermetallic phase was observed for high-
alloyed alloys (MgZn6.5Ga2, MgZn4Ga4, and MgZn4Ga4Y0.5). For low-alloyed
MgZn4Ga2 and MgZn2Ga2 alloys, a single-phase α-Mg microstructure was observed. In
the microstructure of the alloys after HT, Mg12Zn13, Mg5Ga2, and Ga–Y intermetallic
phases are present. These phases act as cathodes for α-Mg during the corrosion process
[33]. Therefore, high volume fractions of such phases in the alloy are undesirable from the
corrosion standpoint. In opposite, the intermetallic phases can act as recrystallization
centers according to the particle-simulated nucleation mechanism during subsequent
deformation and lead to a decrease in grain size [43].
(a)
(b)
Figure 5. (a) Total fraction of intermetallic phases Mg51Zn20 (or Mg12Zn13 after HT), Mg5Ga2, and Ga–
Y, and (b) content of Zn and Ga in the α-Mg solid solution for Mg–Zn–Ga–(Y) alloys in as-cast and
HT conditions.
The contents of Zn and Ga in the α-Mg of the as-cast and heat-treated Mg–Zn–Ga–
(Y) alloys obtained using EDS analysis are shown in Figure 5b. The Zn and Ga contents in
the α-Mg after HT were close to the expected nominal contents of these elements in the
alloy. This is in agreement with the near-full dissolution of the intermetallic phases in the
alloys (Figure 5a). In contrast, the Y content in α-Mg for the MgZn4Ga4Y0.5 alloy before
and after HT was around 0.05 wt.% and is not shown in Figure 5b. This is in accordance
with our previous observations and indicates that the Ga–Y phase is not dissolved during
HT [33].
The DSC results for Mg–Zn–Ga–(Y) alloys in the as-cast and HT conditions are shown
in Figure 6. The solidus temperature for all investigated alloys obtained from as-cast
samples was 316 °C, which is associated with the ternary eutectic transition L→α-Mg +
Figure 5.
(
a
) Total fraction of intermetallic phases Mg
51
Zn
20
(or Mg
12
Zn
13
after HT), Mg
5
Ga
2
, and
Ga–Y, and (
b
) content of Zn and Ga in the
α
-Mg solid solution for Mg–Zn–Ga–(Y) alloys in as-cast
and HT conditions.
The contents of Zn and Ga in the
α
-Mg of the as-cast and heat-treated Mg–Zn–Ga–(Y)
alloys obtained using EDS analysis are shown in Figure 5b. The Zn and Ga contents in the
α
-Mg after HT were close to the expected nominal contents of these elements in the alloy.
This is in agreement with the near-full dissolution of the intermetallic phases in the alloys
(Figure 5a). In contrast, the Y content in
α
-Mg for the MgZn4Ga4Y0.5 alloy before and after
HT was around 0.05 wt.% and is not shown in Figure 5b. This is in accordance with our
previous observations and indicates that the Ga–Y phase is not dissolved during HT [33].
The DSC results for Mg–Zn–Ga–(Y) alloys in the as-cast and HT conditions are shown
in Figure 6. The solidus temperature for all investigated alloys obtained from as-cast
samples was 316
◦
C, which is associated with the ternary eutectic transition L
→α
-Mg +
Mg
51
Zn
20
+ Mg
5
Ga
2
[
33
]. This temperature is close to the ternary eutectic transition tem-

Materials 2022,15, 6849 9 of 17
perature of 299
◦
C obtained via FactSage calculation. In the post-HT alloys, the endothermic
effect connected with the eutectic melting was not observed. During HT, the intermetallic
phases formed due to eutectic transition dissolved, and the solidus temperature of the alloys
increased. However, in the alloy microstructure produced under HT conditions (Figure 5a),
complete intermetallic phase dissolution was observed for low-alloyed MgZn4Ga2 and
MgZn2Ga2 alloys only. The high-alloyed alloys after HT still contained intermetallic phases
in their microstructures. The absence of an endothermic peak at 316
◦
C in the DSC curve
for these alloys after HT is associated with a low fraction of intermetallic phases in their
microstructure leading to a low thermal effect of eutectic melting.
Materials 2022, 15, x FOR PEER REVIEW 9 of 17
Mg51Zn20 + Mg5Ga2 [33]. This temperature is close to the ternary eutectic transition
temperature of 299 °C obtained via FactSage calculation. In the post-HT alloys, the
endothermic effect connected with the eutectic melting was not observed. During HT, the
intermetallic phases formed due to eutectic transition dissolved, and the solidus
temperature of the alloys increased. However, in the alloy microstructure produced under
HT conditions (Figure 5a), complete intermetallic phase dissolution was observed for low-
alloyed MgZn4Ga2 and MgZn2Ga2 alloys only. The high-alloyed alloys after HT still
contained intermetallic phases in their microstructures. The absence of an endothermic
peak at 316 °C in the DSC curve for these alloys after HT is associated with a low fraction
of intermetallic phases in their microstructure leading to a low thermal effect of eutectic
melting.
Figure 6. DSC heating curves for Mg–Zn–Ga–(Y) alloys in the as-cast condition and after HT.
The microstructures along the direction parallel to the ED of the Mg–Zn–Ga–(Y)
alloys extruded at 150 and 200 °C are shown in Figure 3. The alloys extruded at 250 °C
had similar microstructures (not shown here). In the extruded alloys, the intermetallic
phases, which did not fully dissolve during HT, were elongated towards the ED and were
significantly fragmented. MgZn2Ga2 and MgZn4Ga2, which had a single-phase
microstructure after HT, were the alloys with no large intermetallic phases observed. The
insets in Figure 3 (pink boxes) show the microstructures of the extruded alloys at high
magnification. Small precipitates of secondary phases were formed during extrusion via
a stress-induced precipitation mechanism in the α-Mg matrix.
TEM images of the hot-extruded Mg–Zn–Ga–(Y) alloys are shown in Figure 7. A large
amount of near-spherical precipitates, 100-200 nm in diameter, were observed in the
MgZn4Ga4 alloy extruded at 200 °C (Figure 7a). The precipitates were located both inside
the grains and at the grain boundaries. According to the EDS results, the compositions of
the precipitates are close to the Mg5Ga2 and Mg12Zn13 phases, which form agglomerates,
and Figure 7a also shows these agglomerates at higher magnification.
Figure 6. DSC heating curves for Mg–Zn–Ga–(Y) alloys in the as-cast condition and after HT.
The microstructures along the direction parallel to the ED of the Mg–Zn–Ga–(Y)
alloys extruded at 150 and 200
◦
C are shown in Figure 3. The alloys extruded at 250
◦
C
had similar microstructures (not shown here). In the extruded alloys, the intermetallic
phases, which did not fully dissolve during HT, were elongated towards the ED and
were significantly fragmented. MgZn2Ga2 and MgZn4Ga2, which had a single-phase
microstructure after HT, were the alloys with no large intermetallic phases observed. The
insets in Figure 3(pink boxes) show the microstructures of the extruded alloys at high
magnification. Small precipitates of secondary phases were formed during extrusion via a
stress-induced precipitation mechanism in the α-Mg matrix.
TEM images of the hot-extruded Mg–Zn–Ga–(Y) alloys are shown in Figure 7. A
large amount of near-spherical precipitates, 100–200 nm in diameter, were observed in the
MgZn4Ga4 alloy extruded at 200
◦
C (Figure 7a). The precipitates were located both inside
the grains and at the grain boundaries. According to the EDS results, the compositions of
the precipitates are close to the Mg
5
Ga
2
and Mg
12
Zn
13
phases, which form agglomerates,
and Figure 7a also shows these agglomerates at higher magnification.
In the MgZn2Ga2 alloy extruded at 150
◦
C, only Mg–Ga precipitates were found, but
their compositions and sizes were different (Figure 7b). Most of the precipitates had a size
of 20–50 nm and their composition was close to that of Mg
5
Ga
2
. However, large precipitates
(up to 1
µ
m) and compositions close to Mg
2
Ga
5
or MgGa were also observed, but their
volume fraction was low. The Mg
5
Ga
2
precipitates also formed agglomerates. Thus, the
large fraction of small precipitates must provide high mechanical properties.

Materials 2022,15, 6849 10 of 17
Materials 2022, 15, x FOR PEER REVIEW 10 of 17
(a)
(b)
Figure 7. TEM images of (a) MgZn4Ga4 alloy extruded at 200 °C and (b) MgZn2Ga2 alloy extruded
at 150 °C.
In the MgZn2Ga2 alloy extruded at 150 °C, only Mg–Ga precipitates were found, but
their compositions and sizes were different (Figure 7b). Most of the precipitates had a size
of 20–50 nm and their composition was close to that of Mg5Ga2. However, large
precipitates (up to 1 μm) and compositions close to Mg2Ga5 or MgGa were also observed,
but their volume fraction was low. The Mg5Ga2 precipitates also formed agglomerates.
Thus, the large fraction of small precipitates must provide high mechanical properties.
The insets in Figure 3 in the blue box show the microstructure of the alloys after
etching. The grain structure was close to a fine DRX one, but a small fraction of coarse
non-DRX grains was also observed. The average grain sizes of the Mg–Zn–Ga–(Y) alloys
extruded at different temperatures are shown in Figure 8. The grain sizes of the
MgZn4Ga4 alloys formed at extrusion temperatures of 150 or 200 °C were not measured
because the samples could not be effectively etched due to high fraction of intermetallic
phases in its structure. However, it can be concluded that the finer grains were observed
at lower extrusion temperatures, and for the MgZn2Ga2 alloy, it is more clearly seen.
Figure 7.
TEM images of (
a
) MgZn4Ga4 alloy extruded at 200
◦
C and (
b
) MgZn2Ga2 alloy extruded
at 150 ◦C.
The insets in Figure 3in the blue box show the microstructure of the alloys after
etching. The grain structure was close to a fine DRX one, but a small fraction of coarse
non-DRX grains was also observed. The average grain sizes of the Mg–Zn–Ga–(Y) alloys
extruded at different temperatures are shown in Figure 8. The grain sizes of the MgZn4Ga4
alloys formed at extrusion temperatures of 150 or 200
◦
C were not measured because the
samples could not be effectively etched due to high fraction of intermetallic phases in
its structure. However, it can be concluded that the finer grains were observed at lower
extrusion temperatures, and for the MgZn2Ga2 alloy, it is more clearly seen.
Materials 2022, 15, x FOR PEER REVIEW 11 of 17
Figure 8. Average grain size vs. extrusion temperature of the Mg–Zn–Ga–(Y) alloys.
Increasing the Zn content from 2 to 6.5 wt.% resulted in a three-fold decrease in the
grain size. In contrast, Ga had little effect on the grain size of the hot-extruded alloys. The
Mg5Ga2 and Mg12Zn13 phases were dynamically precipitated during extrusion and can act
as pinning obstacles in the growth of DRX grains via the Zener drag effect [44,45].
However, Mg12Zn13 precipitates are more effective for forming a fine grain structure.
Previously, it was shown that MgZn4Ga4 after ECAP at 310 °C has a bimodal structure
that comprises large grains surrounded by small grains [33]. In contrast with ECAP,
extrusion of the alloy promoted unimodal microstructure formation.
Figure 9 shows the XRD spectra of the Mg–Zn–Ga–(Y) alloys after extrusion at 150
°C. The extrusion temperature had little influence on the phase composition of the
investigated alloys, and the XRD spectra of the alloys after hot extrusion at 200 or 250 °C,
were the same as those at 150 °C and are not shown. In all alloys, the α-Mg, Mg5Ga2, and
Mg12Zn13 phases were observed, as confirmed by the TEM EDS results of the extruded
alloys (Figure 7). Further, the XRD peaks of the Mg2Ga5 and MgGa phase were not
observed for the extruded alloys because of their low fraction compared to the Mg5Ga2
phase. The alloy with added Y in extruded state contained a Ga–Y phase (the peaks
corresponds to both Ga6Y and Ga3Y5) that confirmed the EDS results of as-cast alloy and
opposite to CALPHAD calculation results via FactSage. This can be connected with FTlite
thermodynamic database limits.
Figure 9. XRD patterns of (a) MgZn4Ga4, (b) MgZn4Ga4Y0.5, (c) MgZn6.5Ga2, (d) MgZn4Ga2, (e)
MgZn2Ga2 after hot extrusion at 150 °C.
Figure 8. Average grain size vs. extrusion temperature of the Mg–Zn–Ga–(Y) alloys.
Increasing the Zn content from 2 to 6.5 wt.% resulted in a three-fold decrease in the
grain size. In contrast, Ga had little effect on the grain size of the hot-extruded alloys. The
Mg
5
Ga
2
and Mg
12
Zn
13
phases were dynamically precipitated during extrusion and can act

Materials 2022,15, 6849 11 of 17
as pinning obstacles in the growth of DRX grains via the Zener drag effect [
44
,
45
]. However,
Mg
12
Zn
13
precipitates are more effective for forming a fine grain structure. Previously, it
was shown that MgZn4Ga4 after ECAP at 310
◦
C has a bimodal structure that comprises
large grains surrounded by small grains [
33
]. In contrast with ECAP, extrusion of the alloy
promoted unimodal microstructure formation.
Figure 9shows the XRD spectra of the Mg–Zn–Ga–(Y) alloys after extrusion at 150
◦
C.
The extrusion temperature had little influence on the phase composition of the investigated
alloys, and the XRD spectra of the alloys after hot extrusion at 200 or 250
◦
C, were the same
as those at 150
◦
C and are not shown. In all alloys, the
α
-Mg, Mg
5
Ga
2
, and Mg
12
Zn
13
phases
were observed, as confirmed by the TEM EDS results of the extruded alloys (Figure 7).
Further, the XRD peaks of the Mg
2
Ga
5
and MgGa phase were not observed for the extruded
alloys because of their low fraction compared to the Mg
5
Ga
2
phase. The alloy with added Y
in extruded state contained a Ga–Y phase (the peaks corresponds to both Ga
6
Y and Ga
3
Y
5
)
that confirmed the EDS results of as-cast alloy and opposite to CALPHAD calculation
results via FactSage. This can be connected with FTlite thermodynamic database limits.
Materials 2022, 15, x FOR PEER REVIEW 11 of 17
Figure 8. Average grain size vs. extrusion temperature of the Mg–Zn–Ga–(Y) alloys.
Increasing the Zn content from 2 to 6.5 wt.% resulted in a three-fold decrease in the
grain size. In contrast, Ga had little effect on the grain size of the hot-extruded alloys. The
Mg5Ga2 and Mg12Zn13 phases were dynamically precipitated during extrusion and can act
as pinning obstacles in the growth of DRX grains via the Zener drag effect [44,45].
However, Mg12Zn13 precipitates are more effective for forming a fine grain structure.
Previously, it was shown that MgZn4Ga4 after ECAP at 310 °C has a bimodal structure
that comprises large grains surrounded by small grains [33]. In contrast with ECAP,
extrusion of the alloy promoted unimodal microstructure formation.
Figure 9 shows the XRD spectra of the Mg–Zn–Ga–(Y) alloys after extrusion at 150
°C. The extrusion temperature had little influence on the phase composition of the
investigated alloys, and the XRD spectra of the alloys after hot extrusion at 200 or 250 °C,
were the same as those at 150 °C and are not shown. In all alloys, the α-Mg, Mg5Ga2, and
Mg12Zn13 phases were observed, as confirmed by the TEM EDS results of the extruded
alloys (Figure 7). Further, the XRD peaks of the Mg2Ga5 and MgGa phase were not
observed for the extruded alloys because of their low fraction compared to the Mg5Ga2
phase. The alloy with added Y in extruded state contained a Ga–Y phase (the peaks
corresponds to both Ga6Y and Ga3Y5) that confirmed the EDS results of as-cast alloy and
opposite to CALPHAD calculation results via FactSage. This can be connected with FTlite
thermodynamic database limits.
Figure 9. XRD patterns of (a) MgZn4Ga4, (b) MgZn4Ga4Y0.5, (c) MgZn6.5Ga2, (d) MgZn4Ga2, (e)
MgZn2Ga2 after hot extrusion at 150 °C.
Figure 9.
XRD patterns of (
a
) MgZn4Ga4, (
b
) MgZn4Ga4Y0.5, (
c
) MgZn6.5Ga2, (
d
) MgZn4Ga2,
(e) MgZn2Ga2 after hot extrusion at 150 ◦C.
3.3. Mechanical Properties
Typical engineering tensile stress–strain curves obtained for large standard cylindrical
specimens of the Mg–Zn–Ga–(Y) alloys after hot extrusion at 150 or 200
◦
C are shown in
Figure 10, while the corresponding tensile properties are shown in Figure 11a–c. From the
tensile stress–strain curves, the slope of the elastic regionwas the same for all investigated alloys.
Materials 2022, 15, x FOR PEER REVIEW 12 of 17
3.3. Mechanical Properties
Typical engineering tensile stress–strain curves obtained for large standard
cylindrical specimens of the Mg–Zn–Ga–(Y) alloys after hot extrusion at 150 or 200 °C are
shown in Figure 10, while the corresponding tensile properties are shown in Figure 11a–
c. From the tensile stress–strain curves, the slope of the elastic region was the same for all
investigated alloys.
Figure 10. Engineering tensile stress–strain curves for the Mg–Zn–Ga–(Y) alloys extruded at 200 or
150 °C.
(a)
(b)
(c)
(d)
(e)
(f)
Figure 10.
Engineering tensile stress–strain curves for the Mg–Zn–Ga–(Y) alloys extruded at 200 or
150 ◦C.

Materials 2022,15, 6849 12 of 17
Materials 2022, 15, x FOR PEER REVIEW 12 of 17
3.3. Mechanical Properties
Typical engineering tensile stress–strain curves obtained for large standard
cylindrical specimens of the Mg–Zn–Ga–(Y) alloys after hot extrusion at 150 or 200 °C are
shown in Figure 10, while the corresponding tensile properties are shown in Figure 11a–
c. From the tensile stress–strain curves, the slope of the elastic region was the same for all
investigated alloys.
Figure 10. Engineering tensile stress–strain curves for the Mg–Zn–Ga–(Y) alloys extruded at 200 or
150 °C.
(a)
(b)
(c)
(d)
(e)
(f)
Materials 2022, 15, x FOR PEER REVIEW 13 of 17
(g)
(h)
(i)
Figure 11. Mechanical properties as a function of the extrusion temperature of the Mg–Zn–Ga–(Y)
alloys evaluated using (a–c) large standard cylindrical tensile-test specimens, (d–f) small flat-plate
tensile-test specimens, or (g,h) small compression-test specimens cut along the directions parallel
and perpendicular to the ED. (a,d) TYS; (b,e) UTS; (c,f) El; (g) CYS; and (h) CS; (i) legend.
For high-alloyed alloys (MgZn4Ga4, MgZn4Ga4Y0.5, MgZn6.5Ga2), the tensile yield
strength (TYS; Figure 11a) increased when the extrusion temperature increased from 150
to 200 °C. We observed that the alloys extruded at 150 °C had smaller grain sizes, and the
higher TYS values could be due to a higher quantity or effectiveness of the Mg12Zn13 and
Mg5Ga2 precipitates formed during extrusion. For low-alloyed alloys (MgZn4Ga2 and
MgZn2Ga2), no difference in the TYS between alloys extruded at 150 and 200 °C was
observed because of a small amount of precipitates and their minimal effect on TYS.
Increasing the extrusion temperature to 250 °C led to a decrease in the TYS, which was
attributed to the large grain size (Figure 8). It can be seen from Figure 11b,c that the
extrusion temperature has little effect on the ultimate tensile strength (UTS) and
elongation at fracture (El). The exceptions were MgZn4Ga4 and MgZn6.5Ga2 alloys,
which have low El (<10%), but show the highest TYS. The addition of Y promoted a
decrease in TYS. As shown previously, Y was not dissolved in α-Mg during HT, and the
Ga–Y phase precipitates remained in the microstructure and decreased TYS.
An increase in the Zn and Ga contents significantly affected the tensile properties of
the alloys. The high-alloyed alloys showed higher TYS and UTS than their low-alloyed
counterparts, but the El curves were very similar for the investigated alloys. For example,
the maximal and minimal mechanical properties were observed for MgZn4Ga4 and
MgZn2Ga2 alloys, respectively. These results are in good agreement with the content of
alloying elements and the grain size of the alloys (Figure 8), where the fine grain structure
corresponded to better tensile properties.
Hot-extruded materials have anisotropic properties in directions parallel and
perpendicular to the ED. It is known that loads can be applied to bone implants in
different directions, and the complex stress–strain distributions can also change during
implant degradation. Therefore, the material properties that are applied to implants
should be investigated in different directions. The tensile properties of the extruded Mg–
Zn–Ga–(Y) alloys as a function of extrusion temperature in directions parallel and
perpendicular to the ED were obtained using small flat-plate specimens, as shown in
Figure 11d–f. Similar TYS and UTS values were obtained from the large standard
cylindrical specimens and the small flat-plate ones cut parallel to the ED. However, El was
lower when small flat-plate specimens are used, which could be due to the higher surface
area to volume ratio for small specimens and the higher density of surface stress
concentrators. The influence of the alloy composition on the anisotropy of the mechanical
properties was negligible. As expected, for all alloys and extrusion temperatures, the
tensile properties obtained for the specimens cut perpendicular to the ED were inferior to
Figure 11.
Mechanical properties as a function of the extrusion temperature of the Mg–Zn–Ga–(Y)
alloys evaluated using (
a
–
c
) large standard cylindrical tensile-test specimens, (
d
–
f
) small flat-plate
tensile-test specimens, or (
g
,
h
) small compression-test specimens cut along the directions parallel
and perpendicular to the ED. (a,d) TYS; (b,e) UTS; (c,f) El; (g) CYS; and (h) CS; (i) legend.
For high-alloyed alloys (MgZn4Ga4, MgZn4Ga4Y0.5, MgZn6.5Ga2), the tensile yield
strength (TYS; Figure 11a) increased when the extrusion temperature increased from 150
to 200
◦
C. We observed that the alloys extruded at 150
◦
C had smaller grain sizes, and
the higher TYS values could be due to a higher quantity or effectiveness of the Mg
12
Zn
13
and Mg
5
Ga
2
precipitates formed during extrusion. For low-alloyed alloys (MgZn4Ga2
and MgZn2Ga2), no difference in the TYS between alloys extruded at 150 and 200
◦
C
was observed because of a small amount of precipitates and their minimal effect on TYS.

Materials 2022,15, 6849 13 of 17
Increasing the extrusion temperature to 250
◦
C led to a decrease in the TYS, which was
attributed to the large grain size (Figure 8). It can be seen from Figure 11b,c that the
extrusion temperature has little effect on the ultimate tensile strength (UTS) and elongation
at fracture (El). The exceptions were MgZn4Ga4 and MgZn6.5Ga2 alloys, which have low
El (<10%), but show the highest TYS. The addition of Y promoted a decrease in TYS. As
shown previously, Y was not dissolved in
α
-Mg during HT, and the Ga–Y phase precipitates
remained in the microstructure and decreased TYS.
An increase in the Zn and Ga contents significantly affected the tensile properties of
the alloys. The high-alloyed alloys showed higher TYS and UTS than their low-alloyed
counterparts, but the El curves were very similar for the investigated alloys. For exam-
ple, the maximal and minimal mechanical properties were observed for MgZn4Ga4 and
MgZn2Ga2 alloys, respectively. These results are in good agreement with the content of
alloying elements and the grain size of the alloys (Figure 8), where the fine grain structure
corresponded to better tensile properties.
Hot-extruded materials have anisotropic properties in directions parallel and per-
pendicular to the ED. It is known that loads can be applied to bone implants in different
directions, and the complex stress–strain distributions can also change during implant
degradation. Therefore, the material properties that are applied to implants should be
investigated in different directions. The tensile properties of the extruded Mg–Zn–Ga–(Y)
alloys as a function of extrusion temperature in directions parallel and perpendicular to
the ED were obtained using small flat-plate specimens, as shown in Figure 11d–f. Similar
TYS and UTS values were obtained from the large standard cylindrical specimens and the
small flat-plate ones cut parallel to the ED. However, El was lower when small flat-plate
specimens are used, which could be due to the higher surface area to volume ratio for
small specimens and the higher density of surface stress concentrators. The influence of
the alloy composition on the anisotropy of the mechanical properties was negligible. As
expected, for all alloys and extrusion temperatures, the tensile properties obtained for
the specimens cut perpendicular to the ED were inferior to those cut along the ED. The
minimal anisotropy of the tensile properties was observed for alloys extruded at the highest
temperature (250
◦
C), while the maximal anisotropy of UTS and El was observed for alloys
extruded at 200
◦
C. This anisotropy in the mechanical properties could be related to the
deformation texture and non-DRX grains in the alloy structure. The maximal difference for
the TYS, UTS, and El values obtained parallel and perpendicular to the ED were 90 MPa,
160 MPa, and 6%, respectively. Hence, the anisotropy of the mechanical properties should
be considered when using Mg–Zn–Ga–(Y) alloys for bone implants.
Implants are generally subjected to alternating loads, and it is essential to investigate
their compressive properties. The compressive yield strength (CYS) and compressive
strength (CS) were investigated as a function of the extrusion temperature (Figure 11g,h,
respectively) using small cuboid specimens of the extruded Mg–Zn–Ga–(Y) alloys with
the long axis cut parallel or perpendicular to the ED direction. With increasing extrusion
temperature from 150 to 200
◦
C, the change in CYS was insignificant, but a further increase
in temperature led to a decrease in CYS. The CS decreased for most alloys with increasing
extrusion temperature. The maximum and minimum values of both CYS and CS were
observed for MgZn4Ga4 and MgZn2Ga2 alloys, respectively. Overall, the effects of the ex-
trusion temperature and alloy composition on the tensile and compression properties were
similar. The maximal difference of CYS measured in the directions parallel and perpendicu-
lar to ED was 50 MPa, which was lower than a difference obtained for the tensile test results.
As expected, the CS of the alloys was higher than that of the UTS for all alloys. The yield
asymmetry (CYS/TYS) was in the range of 0.74–0.99 for the investigated alloys. Typically,
the deformation texture and grain size affect the yield asymmetry [
46
]. We observed that
the yield asymmetry was slightly lower for alloys extruded at
150 ◦C
than those extruded
at 200
◦
C. With increasing extrusion temperature from 150 to 200
◦
C, TYS increased, but
CYS remained the same. The lowest yield asymmetry was observed for MgZn4Ga4 (~0.95),

Materials 2022,15, 6849 14 of 17
that probably connected with the maximal fraction of small recrystallized grains of alloy
and lower texture in ED after deformation processing [35].
In previous work, it was shown that the solution heat treatment of the Mg–Zn–Ga–(Y)
alloys induced a high El (up to 15.2%) and produced a low fraction of intermetallic phases
that provided high corrosion resistance [
36
]. However, aging had a low effect on the alloy
strength and still has a low value. Therefore, the deformation processing is the only way to
make the use of these alloys for biodegradable implants in osteosynthesis possible.
The YS and El of Mg–Zn–Ga magnesium alloys after extrusion are higher than its
binary Mg–Zn and Mg–Ga counterparts [
47
,
48
]. This indicates that addition of both Ga
and Zn has a positive effect on magnesium alloy’s strength. It was shown previously that
the YS, UTS, and El of the MgZn4Ga4 alloy after ECAP at 310 ◦C were 165 MPa, 300 MPa,
and 22%, respectively [
33
]. Based on the results of this work, this alloy, after extrusion at
150
◦
C, showed higher strength (YS = 256 MPa and UTS = 343 MPa) and lower El (14%).
The enhancement in the mechanical properties after hot extrusion occurred due to grain
refinement, which produces a significant effect on the properties of Mg alloys because of
their high Hall–Petch strengthening coefficient (~300 MPa
·µ
m
1/2
) [
49
]. The difference in
the obtained properties is attributed to the lower deformation processing temperature and
lower grain size for the hot extrusion process in comparison with ECAP processing.
This study was the first attempt to investigate the influence of hot extrusion on the
microstructure and mechanical properties of newly developed Mg–Zn–Ga–(Y) alloys. For
better understanding of the mechanical behavior of the mentioned alloys, extensive texture
analysis and Visco-Plastic Self-Consistent (VPSC) simulations are needed. Furthermore,
the implants’ work in the human body and the mechanical properties in the body fluids
corrosive environment such as stress corrosion cracking and corrosion fatigue must be
known for Mg–Zn–Ga–(Y) alloys in order to use them as biodegradable materials.
4. Conclusions
Five Mg–Zn–Ga–(Y) alloys with different Zn and Ga contents were prepared and
subjected to hot extrusion. When appropriate extrusion temperatures (150 and 200
◦
C)
were used, no defects were observed on the bars of low-alloyed alloys, in contrast with
radial cracks observed on high-alloyed ones. The as-cast alloys contained
α
-Mg and two
eutectic phases (Mg
5
Ga
2
and Mg
51
Zn
20
). HT resulted in the near-complete disappearance
of intermetallic phases and α-Mg with Zn and Ga contents close to those of the bulk alloy.
Micro-alloying with Y promoted the formation of Ga–Y phase, which was retained after
HT. The hot extrusion of the alloys resulted in the elongation and fragmentation of residual
intermetallic phases and the formation of precipitates (Mg
5
Ga
2
and Mg
12
Zn