D. DVORSKY et al.: THE EFFECT OF Y, Gd AND Ca ON THE IGNITION TEMPERATURE OF EXTRUDED MAGNESIUM ALLOYS
669–675
THE EFFECT OF Y, Gd AND Ca ON THE IGNITION
TEMPERATURE OF EXTRUDED MAGNESIUM ALLOYS
U^INEK Y, Gd IN Ca NA TEMPERATURO V@IGA
EKSTRUDIRANIH MAGNEZIJEVIH ZLITIN
Drahomir Dvorsky
1*
, Jiri Kubasek
1
, Dalibor Vojtech
1
, Peter Minárik
2
,
Jitka Stráská
2
1
University of Chemistry and Technology, Prague, Faculty of Chemical Technology, Department of Metals and Corrosion Engineering,
Technická 5 166 28 Praha 6 – Dejvice, Czech Republic
2
Department of Physics of Materials, Charles University, Ke Karlovu 5, 12116 Prague 2, Czech Republic
Prejem rokopisa – received: 2019-11-25; sprejem za objavo – accepted for publication: 2020-06-17
doi:10.17222/mit.2019.284
Magnesium, as a lightweight constructional material, is suitable for application in the aviation industry. However, the use of
magnesium in aircraft was banned by the Federal Aviation Administration (FAA) due to the low ignition temperature and
problematic extinguishing. Nevertheless, the ban was lifted under the condition that magnesium alloys have to pass the ignition
tests. Therefore, this paper is focused on the effect of Y, Gd and Ca alloying elements on the mechanical, corrosion and ignition
properties of low-alloyed binary and quaternary magnesium alloys. The highest increase of ignition temperature among binary
alloys (by up to 120 K compared to pure Mg) was achieved for a Mg-0.5Ca alloy. The excellent value of the ignition
temperature (1131 K) was measured for the low-alloyed quaternary alloy Mg-0.5Y-0.5Gd-0.5Ca, thanks to the synergic
protective effect of oxides of all the selected alloying elements.
Keywords: magnesium alloys, ignition temperature, extrusion
Magnezij je kot lahek konstrukcijski material primeren za uporabo v letalski industriji. Vendar pa je uporaba magnezija
prepovedana pri Ameri{ki zvezni upravi za letenje (FAA, Federal Aviation Administration) zaradi nizke temperature v`iga in
problemati~nega samov`iga. Kljub temu je bila prepoved pogojno preklicana, ~e so bili za izbrano Mg zlitino predhodno
izvedeni uspe{ni testi v`iga. Zato so se v ~lanku avtorji osredoto~ili na opis vpliva legirnih elementov (Y, Gd in Ca) na
mehanske in korozijske lastnosti ter temperaturo v`iga malolegiranih binarnih in kvaternarnih magnezijevih zlitin. Najve~je
povi{anje temperature v`iga med binarnimi zlitinami so raziskovalci ugotovili (za 120 K v primerjavi s ~istim Mg) pri zlitini
Mg-0,5Ca. Odli~no temperaturo v`iga (1131 K) so izmerili pri malolegirani kvaternarni zlitini Mg-0,5Y-0,5Gd-0,5Ca,
zahvaljujo~ vzajemnem za{~itnem u~inku izbranih legirnih elementov.
Keywords: magnezijeve zlitine, temperatura v`iga, ekstruzija (iztiskavanje)
1 INTRODUCTION
The low density (1.74 g·cm
–3
) of magnesium, which
is 75 % and 35 % lower than that of iron and aluminum,
respectively, makes it a prospective construction material
in the aviation industry.
1–4
The substitution of aluminum
parts for magnesium would provide up to a 30 %
reduction of weight of the airplane, which would lead to
reduced CO
2
emissions due to the fuel savings.
5,6
The
utilization of magnesium was not possible due to the ban
from the Federal Aviation Administration (FAA).
1,3
The
ban was lifted under the condition that magnesium alloys
used for this application would pass the flammability
tests. It was quickly discovered that widely used AZ
alloys failed flammability tests and WE43 passed the test
due to the formation of a protective layer of Y
2
O
3
.
1,5
Specific magnesium alloys were prepared in order to
make melting easier and without the necessity of
a protective atmosphere as the SF
6
environmental
non-friendly gas is used for melting.
7–10
Moreover, such
alloys exerted increased resistance to oxidation and
suppressed flammability. Alloying elements like Ca, Be,
Gd, Y, and Nd greatly improve the oxidation resistance
due to the creation of protective oxides on the sur-
face.
9,11,12
The effectiveness of the oxide is characterized
by the Pilling-Bedworth ratio (PBR) and the Gibbs free
energy change for oxidation.
9,13
The surface is sufficient-
ly protected if the PBR ratio is between 1 and 2, other-
wise it is porous or with cracks.
13
The Gibbs free energy
for the oxidation of the alloying elements should be
lower than that of the oxidation of magnesium at
elevated temperature. Moreover, the alloying elements
should be soluble in a magnesium solid solution,
otherwise intermetallic phases have limited protective
ability.
14
The positive effect of rare-earth elements (REEs) and
Ca on the ignition temperature was observed for concen-
trations higher than 1 w/%. However, higher concentra-
tions can result in the deterioration of mechanical prop-
erties due to the occurrence of intermetallic phases.
15
On
the other hand, Ca and Y can have a synergic effect on
Materiali in tehnologije / Materials and technology 54 (2020) 5, 669–675 669
UDK 620.1:662.612.12:669.721.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(5)669(2020)
*Corresponding author's e-mail:
dvorskyd@vscht.cz (Drahomir Dvorsky)

the ignition resistance without a deterioration of the me-
chanical properties.
12,16
The ignition temperature could be determined by
continuous heating techniques like DTA (Differential
Thermal Analysis) or TGA (Thermo-Gravimetric
Analysis). Other methods rely on the exposition of the
sample to a direct flame in order to simulate the real
scenario of fire in the aircraft.
15
The ignition temperature
is characterized by a sudden increase of the temperature
during exposition as magnesium has a very high burning
temperature and it is very problematic to extinguish.
17
The instrumentation of the experiment is essential as M.
Liu et al.
18
measured the ignition temperature of the
WE43 alloy to be 917 K, while N. V. R. Kumar et al.
19
measured 1003 K for the same alloy, however, with the
access of air. Therefore, the presence of oxygen is a very
important factor for the oxidation resistance of mag-
nesium alloys. The sole oxygen atoms are absorbed on
the surface, while larger clusters incorporate beneath the
surface in the magnesium matrix.
20,21
This work deals with the influence of the individual
elements of the Mg-Y-Gd-Ca alloy with a low amount of
alloying elements on the mechanical, corrosion and
ignition properties.
2 MATERIALS AND METHODS
2.1 Sample preparation
Samples were prepared by melting the pure elements
in an induction furnace at 1023 K for 0.5 h under a pro-
tective argon atmosphere (99.96 %). The melt was cast
into a cold brass mold of 20 mm in diameter. Cylindrical
samples with a height of 7 mm were extruded at 623 K
with an extrusion ratio of 10 and an extrusion rate of
5 mm·min
–1
. Extruded rods had 6 mm in diameter and
were 120 mm long. For further tests, the surface was
turned on a lathe and the final diameter was 5 mm.
Table 1: The EDS analysis of the composition of prepared alloys
(w/%)
YG dC a
Mg-0.5Y 0.5±0.1 – –
Mg-0.5Gd – 0.4±0.1 –
Mg-0.5Ca – – 0.3±0.1
Mg-0.5Y-0.5Gd-0.5Ca 0.5±0.1 0.5±0.1 0.2±0.1
2.2 Microstructure characterization
Samples were firstly ground on SiC papers
P80–P4000, afterwards they were polished on diamond
pastes D2 and D0.7 (UR-diamant). The final polishing
was performed with an Eposil F suspension. The micro-
structure was characterized with a scanning electron
microscope SEM TescanVEGA3 having energy-dis-
persive spectrometry (Oxford EDS, AZtec). Image
analyses of the phase volume were made with ImageJ
software.
2.3 Mechanical properties
Compressive testing was carried out on cylindrical
samples with a diameter of 5 mm and 7 mm high using
LabTest 5.250SP1-VM at room temperature at a strain
rate of 0.001 s
–1
. Compressive yield strength (CYS),
ultimate compressive strength (UCS) and deformation to
fracture were determined from the compression curves.
2.4 Immersion tests
Cylindrical samples with a diameter of 5 mm and
7mm high were ground on all sides on SiC paper P2500.
Samples were attached in the plastic holders and
immersed in the 3 w/% NaCl solution at room tem-
perature for 7 d. The corrosion rate was calculated from
the weight changes after the removal of the corrosion
products in the solution containing 200 g·L
–1
CrO
3
,
10 g·L
–1
AgNO
3
and 20 g·L
–1
Ba(NO
3
)
2
.
2.5 Ignition temperature measurement
The surfaces of the samples with diameter of 5 mm
and hieght of 15 mm were finished by grinding on the
paper P1200. Subsequently, the samples were put in an
Al
2
O
3
crucible, which was inserted into the resistance
furnace. The instrumentation is similar to the work of
Aydin et al.
17
and it is shown in Figure 1.
One thermocouple was in direct contact with the
sample and the other one was in the middle of the
crucible. There was a tube with an airflow on the other
side of the crucible. Technical air with an airflow of
100 L·h
–1
was used to supply the crucible with oxygen.
The heat rate of the furnace was set to 35 K·min
–1
;
however, due to the heat transmission and the partial
cooling with the technical air, the real heating rate for the
sample was about 25 K·min
–1
. The temperature increased
with time until it rose rapidly due to the ignition of the
sample. Each sample was measured three times. After
the identification of the ignition temperatures, selected
D. DVORSKY et al.: THE EFFECT OF Y, Gd AND Ca ON THE IGNITION TEMPERATURE OF EXTRUDED MAGNESIUM ALLOYS
670 Materiali in tehnologije / Materials and technology 54 (2020) 5, 669–675
Figure 1: Instrumentation for the measurement of ignition tempe-
ratures

samples were exposed at the same furnace temperature
of 923 K. When the temperature on the surface of the
sample surface reached the required value, samples were
immediately removed from the furnace and cooled in the
air. The formed surface oxide layer was studied using
SEM with EDS. For this purpose, samples were mounted
into the epoxy resin and ground and polished under
ethanol in order not to damage the oxide layer.
3 RESULTS AND DISCUSSION
3.1 Microstructure characterization
The microstructures of the extruded ingots are shown
in Figure 2. One can see that the microstructures of all
the samples are very similar and they all contain inter-
metallic phases. Unfortunately, due to the low amount of
intermetallic phases (0.5–1.0 vol.% according to the
image analysis), they could not be detected by XRD
analysis. Therefore, detailed EDS analysis of the phases
was performed at 10 kV in order to reduce the analysed
area. With EDS analysis of the individual phases and
knowledge of the Mg-Y phase diagram, we can presume
that the phase containing  30 w/% of Y in Mg-0.5Y
alloy is Mg
24
Y
5
with typical cubic shape (Figure 2a),
even though a similar-looking cubic-shaped phase con-
taining Y was recently identified as YH
3
.
22,23
The
Mg-0.5Gd alloy contained roughly the same amount of
intermetallic phases as the previous alloy and the phases
were according to the phase diagram and EDS analysis
( 20 w/% of Gd) identified as Mg
5
Gd (Figure 2b).
24
In
the case of the Mg-0.5Ca alloy, there was a slightly
lower amount of intermetallic phases due to the lower
content of Ca. The lower content of Ca was selected due
to the lower solubility of Ca in the magnesium solid
solution. The presented intermetallic phases were Mg
2
Ca
( 0.7 w/% of Ca). Unfortunately, due to the lower
standard reduction potential of alloying elements in these
phases compared to the Mg, those phases are more
noble, and therefore, are preferentially etched from the
matrix (Figure 2c).
25
The quaternary alloy contained all
the phases specified for the binary alloys (Figure 2d).
Nevertheless, the majority of the alloying elements
(0.2–0.4 w/%) was dissolved in the magnesium solid
solution in all cases. Exact values are summarized in
D. DVORSKY et al.: THE EFFECT OF Y, Gd AND Ca ON THE IGNITION TEMPERATURE OF EXTRUDED MAGNESIUM ALLOYS
Materiali in tehnologije / Materials and technology 54 (2020) 5, 669–675 671
Figure 2: Microstructures and details of intermetallic phases of the extruded alloys: a) Mg-0.5Y, b) Mg-0.5Gd, c) Mg-0.5Ca,
d) Mg-0.5Y-0.5Gd-0.5Ca

Table 2. The grains of all the materials were equiaxed
and the size ranged between 5 μm and 15 μm.
3.2 Mechanical properties
Representative compression curves of the extruded
ingots are summarized in Figure 3. One can see that the
binary alloys were characterized by only slightly diffe-
rent mechanical behaviour. This is due to the similar
amount of alloying elements dissolved in the solid
solution, as it is considered as the main increment that
improves the mechanical properties in this case. The
solid-solution strengthening is associated with different
atomic diameters compared to the matrix.
26
The diameter
of magnesium is 0.16 nm, while Y, Gd and Ca have
0.18 nm. The difference of 12 % has an impact on the
mechanical strengthening;
27
however, due to the same
diameter of alloying elements, the compressive yield
strength should be also similar. On the other hand, the
solid-solution strengthening also depends on the shear
modulus of the alloying elements according to Equation
(1),
26
where  is the approximate shear modulus of the
binary alloy, G
1
is the shear modulus of the alloying
element (G
Y
= 26.0 GPa, G
Gd
= 22.0 GPa, G
Ca
= 7.4 GPa)
and G is the shear modulus of magnesium (G
Mg
=
17.0 Pa).
27,28
 =2 I(G
1
– G)I·(G
1
+ G)
–1
(1)
According to Equation (1), the highest shear modulus
of a binary alloy is for the Ca, which is in accordance
with the measured values. Similar compressive proper-
ties were measured for the Mg-0.5Y alloy, even though
the shear modulus for this alloy is almost half of the
Mg-0.5Ca. This might be associated with the amount of
yttrium in the solid solution, which is double the amount
of Ca in the Mg-0.5Ca alloy. The Mg-0.5Gd alloy con-
tained slightly more alloying elements in the solid so-
lution than the Mg-0.5Ca alloy; however, the shear
modulus for this alloy was more than three times lower,
and therefore, the effect of the solid-solution strength-
ening is the lowest among the measured materials. The
quaternary alloy exerted the best compressive properties
as it contained the most alloying elements in the solid
solution (0.7 w/%).
3.3 Immersion tests
Immersion tests were performed in 3 w/% NaCl
solution at room temperature for 7 days. The corrosion
rates calculated from the weight changes after removal of
the corrosion products are summarized in Table 2. One
can see that the binary alloys were characterized by
relatively similar corrosion rates. Even though yttrium
and gadolinium are known to reduce the corrosion rate
through the formation of more stable corrosion products
on the surface, the concentration presented in the pre-
pared alloys was probably too low to take the proper
effect. In contrast, the presented intermetallic phases can
work as cathodes or anodes, depending on their potential
compared to the Mg matrix.
22
Intermetallic phases
containing Y and Gd are characterized by only slightly
higher standard reduction potential compared to the Mg.
Therefore, Mg
24
Y5 and Mg
5
Gd work as cathodic sites
and slightly increase the corrosion rate of the Mg matrix.
In contrast, Mg
2
Ca is characterised with a lower standard
reduction potential than pure Mg, and therefore, the
intermetallic phases corrode preferentially, providing
cathodic protection for the Mg matrix.
29
Nevertheless,
the lowest corrosion rate was measured for the quater-
nary alloy due to the combined effect of Y and Gd,
which provides stable protective oxide layers on the
interface of material and immersion solution.
D. DVORSKY et al.: THE EFFECT OF Y, Gd AND Ca ON THE IGNITION TEMPERATURE OF EXTRUDED MAGNESIUM ALLOYS
672 Materiali in tehnologije / Materials and technology 54 (2020) 5, 669–675
Figure 3: Compressive curves of extruded materials
Table 2: Summarized properties of extruded alloys
Mg-0.5Y Mg-0.5Gd Mg-0.5Ca Mg-0.5Y-0.5Gd-0.5Ca
X in solid solution (w/%) 0.4±0.1 0.3±0.1 0.2±0.1 0.2/0.3/0.2
CYS (MPa) 118±4 108±3 132±2 163±2
UCS (MPa) 384±5 363±5 393±3 433±3
Relative deformation (%) 16±1 18±1 16±1 14±1
Corrosion rate (mm·y
–1
) 0.76±0.32 0.53±0.15 0.63±0.11 0.38±0.02
Ignition temperature (K) 981±4 1041±5 1056±5 1131±6

3.4 Ignition temperature characterization
The measurement of the ignition temperature was
performed three times for each sample, while the differ-
ences between individual measurements were negligible.
Representative curves of the temperatures against the
time for individual specimens are summarized in Fig-
ure 4.
The highest increase of the ignition temperature for
binary alloys was observed after a Ca addition. Even a
small addition of 0.3 w/% was enough to increase the
ignition temperature of the Mg by almost 200 K. The
improved ignition temperature is due to the formation of
CaO on the surface of the sample during heating (Fig-
ure 5c), which effectively protects the bulk Mg. The
CaO is formed on the surface due to the lower free Gibbs
energy of CaO formation compared to the formation of
MgO. An yttrium addition was reported to significantly
improve the ignition properties; however, only in high
contents, which subsequently deteriorate the mechanical
properties due to the formation of intermetallic
phases.
15,30
Y is able to form the layer of Y
2
O
3
on the
surface (Figure 5a), which sufficiently protects the Mg
against burning. The 0.5 w/% of Gd addition was almost
as effective as the addition of Ca. Again, the protective
oxide layer of Gd
2
O
3
(Figure 5b) was responsible for the
improved ignition properties. A relatively high ignition
temperature of 1131 K was measured for the quaternary
alloy. Such a high ignition temperature is achieved due to
the synergic effect of the individual elements (Fig-
D. DVORSKY et al.: THE EFFECT OF Y, Gd AND Ca ON THE IGNITION TEMPERATURE OF EXTRUDED MAGNESIUM ALLOYS
Materiali in tehnologije / Materials and technology 54 (2020) 5, 669–675 673
Figure 5: EDS analysis of the cuts of samples after exposure to 923 K for extruded alloys: a) Mg-0.5Y, b) Mg-0.5Gd, c) Mg-0.5Ca,
d) Mg-0.5Y-0.5Gd-0.5Ca
Figure 4: Increase of temperature with time for measured samples
under an airflow (100 L·h
–1
) with a heating rate of 25 K·min
–1

ure 5d). N. Zhou et al.
7
showed the great synergic effect
of Y and Ca, where the addition of 0.2 w/% of Ca to the
Mg-5Y-0.1Ca alloy improved the ignition temperature to
a value of 1213 K. Nevertheless, the quaternary alloy
presented in this work contains only 1.2 w/% of alloying
elements in sum compared to the 5.3 w/% mentioned in
the previous work. Out of the results, we can deduce that
the synergic effect is very important, as individual
elements make multilayer oxides on the surface, which
work as barriers and protect the material against oxi-
dation.
The measured ignition temperatures are compared
with similar alloy systems or with systems with a low
amount of alloying elements found in literature sources
in Table 2. Based on the comparison with literature
sources
7,10,12,13,15–18,31–33
the highest measured ignition
point (1131 K) is measured for the Mg-0.5Y-0.5Gd-
0.5Ca alloy presented in this study. The Mg-3.5Y-0.5Ca
alloy measured by J. F. Fan et al.
33
was characterized by
a similar ignition temperature of 1120 K. However, the
amount of alloying elements is three times higher, and
therefore, the price for such an alloy would also be
higher.
Table 2: Ignition temperatures of prepared alloys compared with the
literature
Ignition
temper-
ature
Conditions Reference
Mg-0.5Y-0.5Gd-
0.5Ca
1131 K Airflow in furnace This work
Mg-0.5Y 981 K Airflow in furnace This work
Mg-0.5Gd 1041 K Airflow in furnace This work
Mg-0.5Ca 1056 K Airflow in furnace This work
Mg 931 K Airflow in furnace This work
Mg-1.1Y 916 K TG/DTA
10
Mg-2.2Y 914 K TG/DTA
10
Mg1.6Y-1Ca 955 K TG/DTA
33
Mg-2.5Y-0.6Ca 960 K TG/DTA
33
Mg-3Y-0.9Ca 1085 K TG/DTA
33
Mg-3.5Y-0.3Ca 1090 K TG/DTA
33
Mg-3.5Y-0.5Ca 1120 K TG/DTA
33
Mg 933 K Tube furnace on air
7
Mg-1.5Y 977 K Tube furnace on air
7
Mg-3Y 1091 K Tube furnace on air
7
Mg-0.3Nd 933 K Airflow in furnace
17
Mg-0.5Nd 1043 K Airflow in furnace
17
Mg-1Nd 1033 K Airflow in furnace
17
Mg-1.5Nd 1053 K Airflow in furnace
17
Mg-2Nd 1048 K Airflow in furnace
17
Mg-0.3Ca 1007 K Furnace on air
16
Mg-0.9Ca 1007 K Furnace on air
16
Mg-1.2Ca 1093 K Furnace on air
16
Mg-3.5Y-0.3Ca 1090 K Furnace on air
16
Mg-3Gd 980 K Furnace on air
13
4 CONCLUSION
The microstructures of the extruded Mg-0.5X (x =
Ca, Y, Gd) binary alloys and the Mg-0.5Gd-0.5Y-0.5Ca
quaternary alloy were characterized by recrystallised
microstructures with similar grain sizes of about 10 μm
in diameter. Due to the similarities in the material
microstructure conditions, the mechanical properties
differ only slightly; however, higher values of CYS and
UCS were measured for the Mg-0.5Gd-0.5Y-0.5Ca,
especially due to the higher total concentration of
alloying elements in the solid solution. The corrosion
rate of the Mg-0.5Gd-0.5Y-0.5Ca was superior to the
binary systems, which is related to the synergic
protection effect of the oxide layers of Y and Gd on the
surface. The ignition temperature was increased by 50 K
due to the addition of 0.5 w/% of Y. A greater improve-
ment (by up to 120 K) was achieved by the addition of
Ca and Gd. Nevertheless, the best improvement of
ignition temperature (by up to 200 K) was due to the
synergic effect of Y, Gd and Ca in the quaternary alloy.
Acknowledgment
The authors wish to thank the Czech Science
Foundation (project no. GA19-08937S) and specific
university research (A1_FCHT_2020_003 and
A2_FCHT_2020_027) for the financial support of this
research. Peter Minárik acknowledges partial financial
support by ERDF under project No.
CZ.02.1.01/0.0/0.0/15 003/0000485.
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