materials
Article
Rare Earth Element Yttrium Modified Mg-Al-Zn
Alloy: Microstructure, Degradation Properties
and Hardness
Long Liu 1,† , Fulai Yuan 2,† , Mingchun Zhao 3 , Chengde Gao 1 , Pei Feng 1 , Youwen Yang 1 ,
Sheng Yang 4 and Cijun Shuai 1,5, *
1
2
3
4
5
*
†
State Key Laboratory of High Performance Complex Manufacturing, Central South University,
Changsha 410083, China; [email protected] (L.L.); [email protected] (C.G.);
[email protected] (P.F.); [email protected] (Y.Y.)
Health Management Center, Xiangya Hospital, Central South University, Changsha 410008, China;
[email protected]
School of Material Science and Engineering, Central South University, Changsha 410083, China;
[email protected]
Human Reproduction Center, Shenzhen Hospital of Hongkong University, Shenzhen 518053, China;
[email protected]
Key Laboratory of Organ Injury, Aging and Regenerative Medicine of Hunan Province,
Changsha 410008, China
Correspondence: [email protected]; Tel.: +86-731-8480-5412; Fax: +86-731-8887-9044
These authors contributed equally to this work.
Academic Editor: Javier Narciso
Received: 15 March 2017; Accepted: 26 April 2017; Published: 28 April 2017
Abstract: The overly-fast degradation rates of magnesium-based alloys in the biological environment
have limited their applications as biodegradable bone implants. In this study, rare earth element
yttrium (Y) was introduced into AZ61 magnesium alloy (Mg-6Al-1Zn wt %) to control the degradation
rate by laser rapid melting. The results showed that the degradation rate of AZ61 magnesium alloy
was slowed down by adding Y. This was attributed to the reduction of Mg17 Al12 phase and the
formation of Al2 Y phase that has a more active potential, which decreased galvanic corrosion
resulting from its coupling with the anodic matrix phase. Meanwhile, the hardness increased as Y
contents increased due to the uniform distribution of the Al2 Y and Mg17 Al12 phases. However, as
the Y contents increased further, the formation of excessive Al2 Y phase resulted in the increasing of
degradation rate and the decreasing of hardness due to its agglomeration.
Keywords: AZ61 magnesium alloy; microstructure; degradation properties; hardness
1. Introduction
Magnesium-based alloys have aroused keen attention as biodegradable bone implants due to their
unique biodegradable characteristics, proper mechanical properties and favorable biocompatibility [1–6].
The widely used Mg-Al-Zn (AZ series) alloys belong to a magnesium-based alloy, which exhibits
high strength and certain degradation resistance [7–10]. Nevertheless, it still needs to further enhance
degradation resistance in order to have biological applications [11–13]. The rare earth elements such as
neodymium (Nd), gadolinium (Gd) and yttrium (Y) have a beneficial effect in increasing degradation
resistance and enhance the mechanical properties of magnesium alloys [14–18].
Many research works have been carried out on magnesium alloys with rare earth elements.
Zhang et al. [19] reported that alloying cerium (Ce) could improve mechanical properties and corrosion
resistance of cast Mg-4Al-based alloy. Liu et al. [20] reported that the addition of Lanthanum (La)
Materials 2017, 10, 477; doi:10.3390/ma10050477
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Materials 2017, 10, 477
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Lanthanum (La) could enhance the corrosion properties of AM60 alloy. Yttrium (Y), a rare earth
element,
has hexagonal
close-packed
structure
which is
the
sameearth
as that
of magnesium,
and
could
enhance
the corrosion
propertiescrystal
of AM60
alloy. Yttrium
(Y),
a rare
element,
has hexagonal
the atomic radius
of Ystructure
(0.18 nm)which
is close
of magnesium
(0.16 nm) [21].
its solid
solubility
close-packed
crystal
is to
thethat
same
as that of magnesium,
andThus,
the atomic
radius
of Y
limit nm)
in magnesium
alloys
can reach up
to nm)
11.4 [21].
wt %
[22].its
Moreover,
Y has limit
the same
standard
(0.18
is close to that
of magnesium
(0.16
Thus,
solid solubility
in magnesium
electrochemical
magnesium
(−2.372YV)
[23,24].
Qi et
al. investigated
the effects
of Y on
alloys
can reachpotential
up to 11.4with
wt %
[22]. Moreover,
has
the same
standard
electrochemical
potential
the microstructure
mechanical
of as-cast Mg-6Zn-1Mn
showed and
that
with
magnesium (−and
2.372
V) [23,24].properties
Qi et al. investigated
the effects ofalloy.
Y onThe
the results
microstructure
Y
could
improve
its
mechanical
properties
significantly,
and
the
alloy
with
Y
content
of
6.09
wt
%
mechanical properties of as-cast Mg-6Zn-1Mn alloy. The results showed that Y could improve its
has the bestproperties
mechanical
properties [25].
Luoalloy
et al.with
studied
the corrosion
and the
mechanical
significantly,
and the
Y content
of 6.09 wtresistance
% has theproperty
best mechanical
corrosion
evolution
of
as-cast
AZ91
alloy
with
rare
earth
Y.
They
found
that
the
proper
amount
ofof
Y
properties [25]. Luo et al. studied the corrosion resistance property and the corrosion evolution
addition
could
improve
the corrosion
resistance
of as-cast
AZ91amount
alloys effectively
[26].could improve
as-cast
AZ91
alloy
with rare
earth Y. They
found that
the proper
of Y addition
Laser rapid
meltingofhas
the characteristic
of rapid solidification.
The cooling rate during laser
the corrosion
resistance
as-cast
AZ91 alloys effectively
[26].
5 K/s, which can inhibit grain growth and refine the grains [27].
melting
usually
Laser
rapid reaches
melting up
hasto
the10characteristic
of rapid solidification. The cooling rate during laser
5
Meanwhile,
laserreaches
rapid melting
reduce
thecan
composition
segregation.
Furthermore,
laser rapid
melting
usually
up to 10canK/s,
which
inhibit grain
growth and
refine the grains
[27].
melting is a laser
non-equilibrium
process
whichthe
cancomposition
increase thesegregation.
solid solubility
of the alloy
elements
Meanwhile,
rapid melting
can reduce
Furthermore,
laser
rapid
[28].
melting
is a non-equilibrium process which can increase the solid solubility of the alloy elements [28].
In this
this work,
work, the
the AZ61
AZ61 magnesium
magnesium alloys
alloys (Mg-6Al-1Zn
(Mg-6Al-1Zn wt
wt %)
%) with
with different
differentYY contents
contents(0,
(0,1,
1, 2,
2, 3,
3,
In
4
wt
%)
were
prepared
using
laser
rapid
melting.
The
microstructure
was
studied
by
optical
4 wt %) were prepared using laser rapid melting. The microstructure was studied by optical microscopy
microscopy
X-ray (XRD)
diffraction
(XRD) andelectron
scanning
electron microscopy
with
energy
(OM),
X-ray(OM),
diffraction
and scanning
microscopy
(SEM) with(SEM)
energy
dispersed
dispersed spectroscopy
The degradation
properties
were
by theexperiments.
immersion
spectroscopy
(EDS). The (EDS).
degradation
properties were
analyzed
by analyzed
the immersion
experiments.
In hardness
addition, was
the hardness
measured
by Vickers
In
addition, the
measuredwas
by Vickers
hardness
tests.hardness tests.
2. Results and Discussion
2.1.
2.1. Microstructure
Microstructure
The
The microstructures
microstructures of
of the
the AZ61
AZ61 magnesium
magnesium alloys
alloys with
with different
different amounts
amounts of
of Y
Y added
added were
were
examined
examined by
by optical
optical microscopy
microscopy (Figure
(Figure 1)
1) and
and scanning
scanning electron
electron microscopy
microscopy (Figure
(Figure 2)
2) respectively.
respectively.
AZ61
of of
thethe
magnesium
matrix
andand
network
precipitates
(pointed
by black
AZ61 magnesium
magnesiumalloy
alloyconsisted
consisted
magnesium
matrix
network
precipitates
(pointed
by
arrows)
which
were
distributed
mainly
at
grain
boundaries
(Figures
1a
and
2a).
After
adding
Y, the
black arrows) which were distributed mainly at grain boundaries (Figures 1a and 2a). After adding
network
precipitates
decreased
and a small
of amount
granulous
(pointed by black
arrows
Y, the network
precipitates
decreased
andamount
a small
of precipitates
granulous precipitates
(pointed
by
with
a
round
tail)
appeared
(Figures
1b
and
2b).
The
granulous
precipitates
increased
as
the
Y
contents
black arrows with a round tail) appeared (Figures 1b and 2b). The granulous precipitates increased
increased
and were
distributed
as Y reached
2 wtas
%Y(Figures
and%2c).
However,
as 2c).
the
as the Y contents
increased
and uniformly
were distributed
uniformly
reached1c
2 wt
(Figures
1c and
Y
contents as
further
the granulous
precipitates
tended
to be predominant
agglomerate
However,
the Yincreased,
contents further
increased,
the granulous
precipitates
tended toand
be predominant
(Figures
1d,e
and
2d,e).
and agglomerate (Figures 1d,e and 2d,e).
Figure 1.
magnesium
alloys
with
different
Y contents:
(a) 0 (a)
wt 0%;wt
(b)%;
1
Figure
1. Optical
Opticalmicrographs
micrographsofofAZ61
AZ61
magnesium
alloys
with
different
Y contents:
wt %;
(c)%;
2 wt
(d)%;3 (d)
wt 3%wt
and
wt 4%.wt %.
(b)
1 wt
(c) %;
2 wt
% (e)
and4 (e)
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Figure 2. SEM micrographs of AZ61 magnesium alloys with different Y contents: (a) 0 wt %;
Figure 2. SEM micrographs of AZ61 magnesium alloys with different Y contents: (a) 0 wt %; (b) 1 wt %;
(b) 1 wt %; (c) 2 wt %; (d) 3 wt % and (e) 4 wt %.
(c) 2 wt %; (d) 3 wt % and (e) 4 wt %.
The XRD patterns of the AZ61 magnesium alloys with different Y contents were exhibited in
The
XRD the
patterns
theMg
AZ61
magnesium alloys with different Y contents were exhibited in
Figure 3. Only
α-Mgof
and
17Al12 phase were detected in the AZ61 magnesium alloy (Figure 3a).
Figure
3. Only Y,
theaα-Mg
phase
were and
detected
in the AZ61 magnesium
alloy (Figure
3a).
17 Al12
After adding
new and
Al2YMg
phase
was
formed
the corresponding
peak intensity
gradually
After
adding
Y,
a
new
Al
Y
phase
was
formed
and
the
corresponding
peak
intensity
gradually
increased as Y increased. 2Meanwhile, the peak intensity of the Mg17Al12 phase decreased. This
increased
asreduction
Y increased.
Meanwhile,
the peak
intensity
of of
thethe
Mg
Alphase.
decreased.
This
12 phase
implied the
of the
Mg17Al12 phase
and the
increase
Al172Y
Reference
intensity
implied
themethod
reduction
the Mg
and thepercent
increase
the
Al172Al
Y 12phase.
17 Al12 phase
ratio (RIR)
wasofused
to quantify
the weight
ofof
the
Mg
phaseReference
and Al2Y intensity
phase. A
ratio
(RIR)
method
was
used
to
quantify
the
weight
percent
of
the
Mg
Al
phase
Al2 Y phase.
172Y 12
more complete view on the weight percent of the Mg17Al12 phase and Al
phase as and
influenced
by Y
A
more
complete
view
on
the
weight
percent
of
the
Mg
Al
phase
and
Al
Y
phase
as
influenced
17
12
2
content was given in Figure 4. It could be observed that the weight percent of the Mg17Al12 phase
by
Y content
was
given
in Figure
4. It could alloy
be observed
the
weight
percent of
the with
Mg174Al
12 %
decreased
from
7.5%
in the
AZ61 magnesium
to 4.1% that
in the
AZ61
magnesium
alloy
wt
phase
decreased
from
7.5%
in
the
AZ61
magnesium
alloy
to
4.1%
in
the
AZ61
magnesium
alloy
with
Y, while that of Al2Y increased from 0% in the AZ61 magnesium alloy to 2.5% in the AZ61
4magnesium
wt % Y, while
of4Al
fromthe
0%tendency
in the AZ61
magnesium
alloy
to 2.5%
in the AZ61
2 Y%increased
alloythat
with
wt
Y. In general,
of elements
to form
stable
compounds
was
magnesium
alloy
with
4
wt
%
Y.
In
general,
the
tendency
of
elements
to
form
stable
compounds
was
in positive correlation with the electronegativity difference between elements. The electronegativity
in
positive
electronegativity
difference between
elements.
Thebeelectronegativity
values
of Y,correlation
Mg and Alwith
werethe
1.22,
1.31 and 1.61 respectively,
from which
it could
deduced that Y
values
of
Y,
Mg
and
Al
were
1.22,
1.31
and
1.61
respectively,
from
which
it
could
be deduced
thatas
Y
was prone to react with Al to form Al-Y compound [29]. Thus, the Al2Y phase increased
gradually
was
prone
to
react
with
Al
to
form
Al-Y
compound
[29].
Thus,
the
Al
Y
phase
increased
gradually
as
2
Y increased, while the Mg17Al12 phase decreased.
Y increased,
while
the
Mg
Al
phase
decreased.
17
12
The morphology and compositions of the second phases in AZ61 magnesium alloy with 2 wt %
Thestudied
morphology
and
compositions
themagnesium
second phases
in AZ61
magnesium
2 wt %
Y were
by EDS.
It was
presentedof
that
content
reduced
while Alalloy
and with
Y increased
Y
were
studied
by
EDS.
It
was
presented
that
magnesium
content
reduced
while
Al
and
Y
increased
across the granulous particles (Figure 5b). Thus, it was reasonable to conclude that the granulous
across
granulous
(Figure
5b). Thus,
it was reasonable
to conclude
granulous
particlethe
was
in the Alparticles
2Y phase.
Meanwhile,
the rod-shaped
phase had
a Mg/Althat
ratiothe
(78.78/21.22)
particle
was
in
the
Al
Y
phase.
Meanwhile,
the
rod-shaped
phase
had
a
Mg/Al
ratio
(78.78/21.22)
(Figure 5c) which was2close to that of Mg17Al12, and was thereby identified as Mg17Al12 phase.
(Figure
which
was close
to Al
that
MgMg
wasexactly
thereby
identified as
Al12 phase.
The5c)
three
phases
(α-Mg,
2Y of
and
17Al
) were
determined
in Mg
the17AZ61
magnesium
17 Al
12 ,12and
phases
(α-Mg,diagram
Al2 Y and
Al12 ) formation
were exactly
determined
the AZ61
magnesium
alloyThe
withthree
Y. The
schematic
of Mg
the 17
phase
in the
alloy wasinshown
in Figure
6. The
alloy
with Y. The schematic
diagram
of the phase
formation
in the alloyliquid
was shown
in Figure
6.2Y
The
first
first precipitated
phase in the
solidification
of the
high-temperature
phase was
the Al
phase.
precipitated
phase
in the solidification
of the high-temperature
liquid
phase
was(1485
the Al°C)
This could be
explained
by the Al2Y phase
having the highest
melting
point
amongThis
the
2 Y phase.
◦ C) of
three phases
(Figureby
6b)
[30].
the α-Mg
phasemelting
with thepoint
melting
point
650 °C
could
be explained
the
Al2Afterwards,
Y phase having
the highest
(1485
among
thestarted
three
◦ C12started
nucleation
(Figure
6c).Afterwards,
Then, the the
remained
Al atoms
precipitated
in ofMg
17Al
(437 °C)
which
phases
(Figure
6b) [30].
α-Mg phase
with the
melting point
650
nucleation
◦
distributed
on
the
α-Mg
grain
boundaries
(Figure
6d).
(Figure 6c). Then, the remained Al atoms precipitated in Mg17 Al12 (437 C) which distributed on the
α-Mg grain boundaries (Figure 6d).
Materials 2017, 10, 477
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Figure 3.Figure
X-ray3.diffraction
(XRD)(XRD)
patterns
ofofAZ61
magnesium
alloys
with
different
Y contents:
(a) 0
X-raydiffraction
diffraction(XRD)
patterns
of
AZ61
magnesium
alloys
different
Y contents:
X-ray
patterns
AZ61
magnesium
alloys
withwith
different
Y contents:
(a) 0
wt %; (b)(a)
wt
%;
(c)
2wt
wt
3(d)%;
wt
%3%
and
44 (e)
wt
%.
wt(b)
%;1 (b)
2%;wt
wt
%(e)
and
wt %.
wt1 0%;
wt 1%;
(c)%;%;
2 (c)
wt(d)
3(d)
wt
and
(e)
wt 4%.
Figure 4. The weight percent of Mg17Al12 and Al2Y phases in AZ61 magnesium alloys with Y
addition.
Figure
The weight
percent of
of Mg
andand
Al2 YAl
phases
in AZ61 magnesium
alloys with Y addition.
Figure 4.
The4.weight
percent
Mg
17Al
2Y phases
in AZ61 magnesium
alloys with Y
17 Al
12 12
addition.
Materials 2017, 10, 477
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Figure 5. SEM image of (a) AZ61 magnesium alloy with 2 wt % Y and energy dispersive
Figure 5. SEM image of (a) AZ61 magnesium alloy with 2 wt % Y and energy dispersive spectroscopy
Figure 5. SEM
image
of (a)
AZ61
magnesium
alloyinwith
spectroscopy
(EDS)
patterns
of (b)
A line
and (c) B point
(a). 2 wt % Y and energy dispersive
(EDS) patterns of (b) A line and (c) B point in (a).
spectroscopy (EDS) patterns of (b) A line and (c) B point in (a).
Figure 6. Schematic diagram of phase formation in the AZ61 magnesium alloy with Y (a) liquid
Figure(b)
6. Schematic
diagram
of phase
formation
in the(d)
AZ61
alloy with Y (a) liquid
phase;
2Y phasediagram
precipitation;
(c)formation
α-Mg
nucleation;
Mg17magnesium
Al12 formation.
Figure 6. Al
Schematic
of phase
in the AZ61
magnesium
alloy with Y (a) liquid phase;
phase; (b) Al2Y phase precipitation; (c) α-Mg nucleation; (d) Mg17Al12 formation.
(b) Al2 Y phase precipitation; (c) α-Mg nucleation; (d) Mg17 Al12 formation.
2.2. Hardness
2.2. Hardness
The hardness of AZ61 magnesium alloys with different Y contents was shown in Figure 7. The
The hardness
AZ61 magnesium
alloys
with
different
Y contents
was shown increased
in Figure 7.asThe
hardness
of AZ61 ofmagnesium
alloy was
90.9
Hv.
The hardness
continuously
Y
hardness
of
AZ61
magnesium
alloy
was
90.9
Hv.
The
hardness
continuously
increased
as
Y
increased from 0 wt % to 2 wt %. The optimal hardness was 104.9 Hv when the Y content was 2 wt %.
increased from 0 wt % to 2 wt %. The optimal hardness was 104.9 Hv when the Y content was 2 wt %.
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2.2. Hardness
The hardness of AZ61 magnesium alloys with different Y contents was shown in Figure 7.
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The hardness of AZ61 magnesium alloy was 90.9 Hv. The hardness continuously increased as Y
increased from 0 wt % to 2 wt %. The optimal hardness was 104.9 Hv when the Y content was 2 wt %.
However, it decreased as the Y contents further decreased. The increase of hardness was attributed
However, it decreased as the Y contents further decreased. The increase of hardness was attributed to
to the uniform distribution of Al2Y and Mg17Al12 phases which acted as a second phase strengthening
the uniform distribution of Al2 Y and Mg17 Al12 phases which acted as a second phase strengthening
agent
in the
matrix.
As As
thethe
Y contents
further
decreased,
phase formed
formedinin the
agent
in alloy
the alloy
matrix.
Y contents
further
decreased,the
theexcessive
excessive Al
Al22Y
Y phase
alloythe
tended
to
aggregate.
Thus,
the
structure
of
the
alloy
became
uneven
and
the
hardness
of the
alloy tended to aggregate. Thus, the structure of the alloy became uneven and the hardness of the
alloyalloy
decreased.
decreased.
Materials 2017, 10, 477
Figure
7. Hardness
alloyswith
withdifferent
different
Y contents.
Figure
7. HardnessofofAZ61
AZ61magnesium
magnesium alloys
Y contents.
2.3. Degradation
Properties
2.3. Degradation
Properties
Immersion
tests
were
appliedtotostudy
study the degradation
of of
AZ61
magnesium
alloys
Immersion
tests
were
applied
degradationproperties
properties
AZ61
magnesium
alloys
different
Y contents.The
Thehydrogen
hydrogen evolution
evolution volume
thethe
immersion
time,time,
as as
withwith
different
Y contents.
volumevaried
variedwith
with
immersion
shown
in Figure
8. The
hydrogen
evolution
volume
of the
alloys
increased
rapidly
early
shown
in Figure
8. The
hydrogen
evolution
volume
of the
alloys
increased
rapidly
in in
thethe
early
stages
stages
of
immersion
and
then
increased
slowly,
which
indicated
a
reduction
of
degradation
rate.
of immersion and then increased slowly, which indicated a reduction of degradation rate. The
The hydrogen evolution volume of AZ61 magnesium alloys with Y contents of 0, 1, 2, 3 and 4 wt %
hydrogen evolution volume
of AZ61 magnesium
alloys with
Y contents of 0,2 1, 2, 3 and 4 wt % were
2 , 10.5
2 and
were 30.1
mL/cm2 , 13.32mL/cm2 , 6.1 mL/cm
mL/cm
18.1 mL/cm after immersion for
2
2
2
30.1 mL/cm , 13.3 mL/cm , 6.1 mL/cm , 10.5 mL/cm and 18.1 mL/cm2 after immersion for 360 h,
360 h, respectively. It was observed that the degradation rate reduced remarkably with adding Y up
respectively.
wasaobserved
that the
reduced
remarkably
adding Y up to
to 2 wt %,Itwhile
further increase
of Ydegradation
resulted in anrate
increased
degradation
rate.with
The degradation
2 wt rates
%, while
further
increase alloys
of Y resulted
in an Y
increased
degradation
rate.
The degradation
of theaAZ61
magnesium
with different
contents were
calculated
according
to mass lossrates
of the
AZ61
magnesium
alloys
with
different
Y
contents
were
calculated
according
to
mass
loss test
test (Figure 9). The results were consistent with that of hydrogen evolution analysis, which
showed
(Figure
Themagnesium
results were
consistent
with
of hydrogen
evolution
analysis,
which
showed that
that9).
AZ61
alloy
with 2 wt
% Ythat
exhibited
the lowest
degradation
rate (0.28
mm/year).
The
degradationalloy
rate of
AZ61
alloy with
wt % Y was
lower thanrate
the reported
value of The
AZ61
magnesium
with
2 magnesium
wt % Y exhibited
the2 lowest
degradation
(0.28 mm/year).
WE43 magnesium
alloy (0.85
mm/year)alloy
[31]. with 2 wt % Y was lower than the reported value of
degradation
rate of AZ61
magnesium
WE43 magnesium alloy (0.85 mm/year) [31].
The degradation morphology of the AZ61 magnesium alloys with different Y contents after
immersion for 120 h was shown in Figure 10. Obviously, the alloys were covered with a degradation
product film which presented some cracks. The appearance of cracks was believed to be caused by
the dehydration of the degradation product film after drying in ambient atmosphere. The AZ61
magnesium alloy exhibited a severely corroded surface with many cracks (Figure 10a). After adding
1 wt % Y, the surface of the alloy presented relatively shallow cracks. When the Y contents was
2 wt %, the integrated degradation film without a crack formed, which implied that the degradation
degree of the alloy was relatively low (Figure 10c). As the Y contents further increased, the cracks of
the degradation product film gradually increased (Figure 10d,e).
Materials 2017, 10, 477
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7 of 11
Figure
AZ61 magnesium
magnesiumalloys
alloyswith
withdifferent
different
contents
Figure8.8.The
Thehydrogen
hydrogenevolution
evolution volume
volume of AZ61
YY
contents
immersedininthe
thesimulated
simulated body
360
h: h:
(a)(a)
0 wt
wt1%;
2 wt
(d)%;
3 wt
immersed
bodyfluid
fluid(SBF)
(SBF)forfor
360
0 %;
wt (b)
%; 1(b)
wt(c)%;
(c) %;
2 wt
(d)%3and
wt %
(e)(e)
4 wt
%. %.
and
4 wt
Figure
9. Degradation rates of AZ61 magnesium alloys with different Y contents after immersion in
Figure 9. Degradation rates of AZ61 magnesium alloys with different Y contents after immersion in
SBF
solution
SBF solutionfor
for7 7days.
days.
It could be concluded that the appropriate addition of rare earth element Y could enhance
The degradation morphology of the AZ61 magnesium alloys with different Y contents after
degradation resistance of the AZ61 magnesium alloy. In general, the Mg17Al12 phase acted as the
immersion for 120 h was shown in Figure 10. Obviously, the alloys were covered with a degradation
cathode
with respect to the magnesium matrix, which facilitated the degradation of the AZ61
product film which presented some cracks. The appearance of cracks was believed to be caused by
magnesium
alloy [32].
After
adding Y,product
Y reacted
to form
Al2Y phase,
which The
reduced
the dehydration
of the
degradation
filmwith
afterAldrying
in the
ambient
atmosphere.
AZ61the
amount
of
Mg
17Al12 phase on the grain boundaries. Furthermore, the Al2Y phase has more active
magnesium alloy exhibited a severely corroded surface with many cracks (Figure 10a). After adding
potential
[33].
Y could
suppress
the galvanic
corrosion
of the
alloys,
which
1 wt % Y,
the Thus,
surfaceadding
of the alloy
presented
relatively
shallow cracks.
When
the Y
contents
was enhanced
2 wt %,
thethe
degradation
resistance.
However,
as
the
Y
contents
further
increased,
the
excessive
Aldegree
2Y phase
integrated degradation film without a crack formed, which implied that the degradation
accelerated
galvanic
corrosion,
in theAs
increase
of degradation
rate.
of the alloy
was relatively
lowresulting
(Figure 10c).
the Y contents
further increased,
the cracks of the
degradation product film gradually increased (Figure 10d,e).
Materials 2017, 10, 477
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8 of 11
Figure 10. SEM degradation morphology of AZ61 magnesium alloys with different Y contents after
Figure 10. SEM degradation morphology of AZ61 magnesium alloys with different Y contents after
immersion for 120 h: (a) 0 wt %; (b) 1 wt %; (c) 2 wt %; (d) 3 wt % and (e) 4 wt %.
immersion for 120 h: (a) 0 wt %; (b) 1 wt %; (c) 2 wt %; (d) 3 wt % and (e) 4 wt %.
3. Experimental Procedure
It could be concluded that the appropriate addition of rare earth element Y could enhance
degradation
resistance of the AZ61 magnesium alloy. In general, the Mg17 Al12 phase acted as
3.1. Materials Preparation
the cathode with respect to the magnesium matrix, which facilitated the degradation of the AZ61
The spherical
AZ61
powders
purchased
from Tangshan
Weihao
magnesium
alloy [32].
Aftermagnesium
adding Y, Yalloy
reacted
with Al were
to form
the Al2 Y phase,
which reduced
the
Materials
Co.,
Ltd.
(Tangshan,
China,
average
particle
size
70
µ
m)
and
irregular-shaped
Y
powders
amount of Mg17 Al12 phase on the grain boundaries. Furthermore, the Al2 Y phase has more active
were
obtained
from Shanghai
Nano technology
Co.,corrosion
Ltd. (Shanghai,
China, which
average
particle
potential
[33]. Thus,
adding Y Naiou
could suppress
the galvanic
of the alloys,
enhanced
size
20
µ
m).
The
powder
mixtures
with
different
Y
contents
(0,
1,
2,
3
and
4
wt
%)
were
prepared
the degradation resistance. However, as the Y contents further increased, the excessive Al2 Y phase
through
ballgalvanic
milling in
a mixedresulting
gas environment
(1 vol of
% SF
6 and 99 vol % CO2). The rotation speed
accelerated
corrosion,
in the increase
degradation
rate.
was fixed at 450 rpm (revolution per minute) in the course of ball milling and the milling time was 2
h.
3. Experimental Procedure
The AZ61 magnesium alloys with different Y contents (0, 1, 2, 3, 4 wt %) were prepared using a
3.1. Materialslaser
Preparation
homemade
rapid melting system. It consists of a fiber laser, a focus system, a gas protection
device
and
a
computer
system.
The
fiber laser
has
a maximum
power
of 110
W. The
The spherical AZ61control
magnesium
alloy
powders
were
purchased
fromoutput
Tangshan
Weihao
Materials
minimum
diameter
of theaverage
laser beams
is 50size
μm.70More
of the system areY available
the
Co., Ltd. spot
(Tangshan,
China,
particle
µm)details
and irregular-shaped
powdersinwere
reference
[34].
The
processing
parameters
were
as
follows:
laser
scanning
rate
200
mm/min,
laser
obtained from Shanghai Naiou Nano technology Co., Ltd. (Shanghai, China, average particle size
spot
150The
μm powder
and laser
power 80
W.different
The powder
mixtures
melted
by layer
in thethrough
sealed
20 µm).
mixtures
with
Y contents
(0, 1,were
2, 3 and
4 wtlayer
%) were
prepared
building
chamber
protected
by argon gas.
of alloys
mmrotation
× 10 mm
× 5 mm)
ball milling
in a mixed
gas environment
(1 Then,
vol % the
SF6 samples
and 99 vol
% CO2(10
). The
speed
was were
fixed
built
up.
at 450 rpm (revolution per minute) in the course of ball milling and the milling time was 2 h.
The AZ61 magnesium alloys with different Y contents (0, 1, 2, 3, 4 wt %) were prepared using a
3.2.
Materials
Characterization
homemade laser rapid melting system. It consists of a fiber laser, a focus system, a gas protection device
and aThe
computer
control
Thewere
fiber ground
laser haswith
a maximum
offrom
110 W.
Thetominimum
prepared
alloysystem.
specimens
abrasive output
papers power
grading
1000
2000 grit
spot mechanically
diameter of the
laser beams
is 50 µm.
the system
are followed
available in
reference
and
polished
on cotton
clothMore
withdetails
0.5 μmofdiamond
paste,
bythe
being
etched[34].
for
Thes processing
parameters
were as(10
follows:
200ethanol
mm/min,
laser
spot
µm
and
10
with the acetic
picral solution
mL oflaser
aceticscanning
acid, 70 rate
mL of
(99.8%
v/v),
4.2150
g of
picric
laser power
W.ofThe
powder
mixtures
layer by
layer in of
thealloys
sealedwas
building
chamber
acid,
and 1080
mL
distilled
water),
thenwere
the melted
metallurgical
structure
studied
by an
protected
by argon gas.
the samples
of alloys
(10 mm
× composition
10 mm × 5 mm)
were built
optical
microscopy
(OM,Then,
Olympus
BHM, Osaka,
Japan).
The
distribution
ofup.
the alloys
was studied using scanning electron microscopy (SEM, QUANTA FEG250, FEI Company, Hillsboro,
3.2. Materials
OR,
USA) andCharacterization
energy dispersive spectroscopy (EDS, JSM-5910LV, JEOL, Tokyo, Japan). The phase
compositions
werealloy
analyzed
through
X-ray
diffraction
(XRD, papers
D8 Advance,
Inc., to
Karlsruhe,
The prepared
specimens
were
ground
with abrasive
gradingBruker
from 1000
2000 grit
Germany)
using
Cu-Kα
radiation
at
15
mA
and
30
kV
with
scattering
angles
ranging
from
10°
to 80°,
and mechanically polished on cotton cloth with 0.5 µm diamond paste, followed by being etched
step
size
0.02°the
and
scanning
8°/min.
X-ray
diffraction
identified
comparing
for 10
s with
acetic
picralspeed
solution
(10 mL
of acetic
acid, patterns
70 mL ofwere
ethanol
(99.8%by
v/v),
4.2 g of
the
diffraction
the water),
standard
ICDD-PDF
cards. The
quantitative
phase
analysisbywas
picric
acid, and patterns
10 mL ofwith
distilled
then
the metallurgical
structure
of alloys
was studied
an
conducted
by means(OM,
of the
referenceBHM,
intensity
ratio
method
[35].
optical microscopy
Olympus
Osaka,
Japan).
The
composition distribution of the alloys
Vickers hardness tests were performed by a Vickers microindenter (HXD-1000TM/LCD, Digital
Micro Hardness Tester, Shanghai Taiming Optical Instrument Co. Ltd, Shanghai, China) with a load
Materials 2017, 10, 477
9 of 11
was studied using scanning electron microscopy (SEM, QUANTA FEG250, FEI Company, Hillsboro,
OR, USA) and energy dispersive spectroscopy (EDS, JSM-5910LV, JEOL, Tokyo, Japan). The phase
compositions were analyzed through X-ray diffraction (XRD, D8 Advance, Bruker Inc., Karlsruhe,
Germany) using Cu-Kα radiation at 15 mA and 30 kV with scattering angles ranging from 10◦ to 80◦ ,
step size 0.02◦ and scanning speed 8◦ /min. X-ray diffraction patterns were identified by comparing
the diffraction patterns with the standard ICDD-PDF cards. The quantitative phase analysis was
conducted by means of the reference intensity ratio method [35].
Vickers hardness tests were performed by a Vickers microindenter (HXD-1000TM/LCD, Digital
Micro Hardness Tester, Shanghai Taiming Optical Instrument Co. Ltd, Shanghai, China) with a load
of 2.45 N and loading time of 15 s. Ten indents were made for each sample. The hardness was
expressed as a mean and standard deviation of these 10 readings. The dimensions of alloys used for
the immersion tests were 10 mm × 10 mm × 5 mm. Immersion tests were operated at 37 ± 0.5 ◦ C in
simulated body fluid (SBF) (the ratio of the surface area to solution volume was 1 cm2 :100 mL). The
SBF that has similar ion concentrations to those of human blood plasma was prepared according to
the protocol described by Kokubo et al. [36]. In short, the relevant reagent grade chemicals (CaCl2 ,
K2 HPO4 ·3H2 O, KCl, NaCl, MgCl2 ·6H2 O, NaHCO3 and Na2 SO4 ) were dissolved in distilled water
at the appropriate amounts. The pH of the solution was buffered to physiological pH (pH = 7.4) by
adding tri-hydroxymethyl-aminomethane and hydrochloric acid. The hydrogen evolution volume
was monitored during the immersion. After immersion for 7 days, each sample was removed from the
solution and washed with distilled water. A chromic acid solution (200 g/L Cr2 O3 + 10 g/L AgNO3 )
was used to remove the degradation products on the sample surface before mass loss measurement.
Five samples were measured for each group to obtain reproducible results. The degradation rates
(mm/year) were calculated according to mass loss test. After immersion for 120 h, the samples were
taken out from SBF and then blown dry with air at room temperature. The degraded surfaces were
observed by SEM. Before the SEM observations, the samples were coated with gold by using a sputter
coater (Leica EM SCD005, Leica Microsystems GmbH, Wetzlar, Germany).
3.3. Statistical Analysis
The experimental data of mechanical and degradation properties were expressed as mean ±
standard deviation. Statistical analysis was performed to assess the difference by the analysis of
variance. The difference was considered to be significant when p < 0.05.
4. Conclusions
The microstructure, degradation properties and hardness of the AZ61 magnesium alloys with
different Y contents (0, 1, 2, 3, 4 wt %) prepared by laser rapid melting were investigated. Adding
Y to AZ61 magnesium alloy could lead to the formation of Al2 Y phase and reduce the amount of
Mg17 Al12 phase. The degradation resistance of the AZ61 magnesium alloy was enhanced with Y
addition. The AZ61 magnesium alloy with 2 wt % Y exhibited an optimal degradation resistance.
Furthermore, the hardness increased as Y contents increased from 0 wt % to 2 wt %, and then decreased
when Y contents decreased further. In conclusion, laser rapid melting AZ61 magnesium alloy with
2 wt % Y exhibit prospects for future bone implants.
Acknowledgments: This work was supported by the following funds: (1) The Natural Science Foundation of
China (51575537, 81572577); (2) Hunan Provincial Natural Science Foundation of China (14JJ1006, 2016JJ1027);
(3) The Project of Innovation-driven Plan of Central South University (2015CXS008, 2016CX023); (4) The Open-End
Fund for the Valuable and Precision Instruments of Central South University; (5) The fund of the State Key
Laboratory of Solidification Processing in NWPU (SKLSP201605); (6) The fund of the State Key Laboratory for
Powder Metallurgy; (7)The Project of State Key Laboratory of High Performance Complex Manufacturing, Central
South University.
Author Contributions: Long Liu, Fulai Yuan and Cijun Shuai conceived and designed the experiments; Long Liu,
Chengde Gao and Youwen Yang performed the experiments; Pei Feng and Sheng Yang analyzed the data;
Materials 2017, 10, 477
10 of 11
Mingchun Zhao contributed analysis tools; Long Liu and Fulai Yuan wrote the paper. All authors reviewed the
final manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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