Structural evolution of Al–8%Si hypoeutectic alloy by ultrasonic processing

Abstract

High intensity power ultrasound was respectively introduced into three different solidification stages of Al–8%Si hypoeutectic alloy, including the fully liquid state before nucleation, the nucleation and growth process of primary α(Al) phase and L → (Al) + (Si) eutectic transformation period. It is found that both the primary α(Al) phase and (Al + Si) eutectic structure were refined by different degrees with various growth morphologies depending on the ultrasonic treatment stage. Based on the experimental results, the cavitation-induced nucleation due to the high undercooling caused by the collapse of tiny cavities was proposed as the major reason for refining the primary α(Al) phase. Meanwhile, obvious eutectic morphological change was observed only when ultrasound was directly introduced in the eutectic transformation stage, in which typical divorced eutectics and (Al + Si) eutectic cells with symmetrical flower shape were formed at the top of the alloy sample. The introduction of ultrasound in each solidification stage also improves the yield strength of Al–8% Si alloy to a diverse extent.

Full text

Journal
of
Materials
Science
&
Technology
33
(2017)
1235–1239
Contents
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at
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Journal
of
Materials
Science
&
Technology
j
o
ur
nal
homepage:
www.jm
st.org
Structural
evolution
of
Al–8%Si
hypoeutectic
alloy
by
ultrasonic
processing
J.Y.
Wang∗,
B.J.
Wang,
L.F.
Huang
Department
of
Applied
Physics,
Northwestern
Polytechnical
University,
Xi’an
710072,
China
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
5
June
2017
Received
in
revised
form
10
July
2017
Accepted
18
July
2017
Available
online
29
July
2017
Keywords:
Solidification
Microstructure
Metals
and
alloys
Power
ultrasound
Refinement
a
b
s
t
r
a
c
t
High
intensity
power
ultrasound
was
respectively
introduced
into
three
different
solidification
stages
of
Al–8%Si
hypoeutectic
alloy,
including
the
fully
liquid
state
before
nucleation,
the
nucleation
and
growth
process
of
primary
␣(Al)
phase
and
L
→
(Al)
+
(Si)
eutectic
transformation
period.
It
is
found
that
both
the
primary
␣(Al)
phase
and
(Al
+
Si)
eutectic
structure
were
refined
by
different
degrees
with
various
growth
morphologies
depending
on
the
ultrasonic
treatment
stage.
Based
on
the
experimental
results,
the
cavitation-induced
nucleation
due
to
the
high
undercooling
caused
by
the
collapse
of
tiny
cavities
was
proposed
as
the
major
reason
for
refining
the
primary
␣(Al)
phase.
Meanwhile,
obvious
eutectic
morphological
change
was
observed
only
when
ultrasound
was
directly
introduced
in
the
eutectic
trans-
formation
stage,
in
which
typical
divorced
eutectics
and
(Al
+
Si)
eutectic
cells
with
symmetrical
flower
shape
were
formed
at
the
top
of
the
alloy
sample.
The
introduction
of
ultrasound
in
each
solidification
stage
also
improves
the
yield
strength
of
Al–8%
Si
alloy
to
a
diverse
extent.
©
2017
Published
by
Elsevier
Ltd
on
behalf
of
The
editorial
office
of
Journal
of
Materials
Science
&
Technology.
1.
Introduction
Al–Si
hypoeutectic
alloys
are
widely
used
in
aluminum
alloys
due
to
their
good
mechanical
properties.
The
modification
of
primary
(Al)
phase
and
(Al
+
Si)
eutectic
in
these
alloys
is
of
great
interest
since
their
refinement
can
significantly
promote
the
strength
and
ductility
[1–4].
In
recent
years,
various
modification
techniques
[5–8]
have
been
proposed
to
modify
the
microstruc-
tures
of
Al–Si
alloys,
such
as
thermomechanical
processing
[7]
and
severe
plastic
deformation
(SPD)
[8].
Among
these
technologies,
the
application
of
ultrasound
to
liquid
or
semi-liquid
alloys
is
an
effective
way
to
improve
the
solidification
microstructure
[9–15].
For
Al–Si
hypoeutectic
alloys,
the
most
beneficial
effect
of
ultra-
sound
on
solidification
is
grain
refinement
of
primary
(Al)
phase
through
suppressing
columnar
dendritic
growth
and
promoting
formation
of
equiaxed
grains
[16–18].
Up
to
now,
two
possible
grain
refinement
mechanisms
by
ultrasound
have
been
proposed,
which
are
cavitation
induced
heterogeneous
nucleation
and
cav-
itation
induced
fragmentation
[19–26].
Nevertheless,
there
is
no
clear
evidence
as
which
mechanism
plays
the
most
important
role
in
different
stages
of
crystal
growth
process.
As
for
binary
∗Corresponding
author.
E-mail
address:
wangjy@nwpu.edu.cn
(J.Y.
Wang).
(Al
+
Si)
eutectic
structure
formed
within
ultrasonic
field,
however,
the
results
obtained
by
different
investigators
seem
to
be
con-
tradictory.
Some
found
that
the
eutectic
(Si)
phase
can
also
be
significantly
refined
under
ultrasonic
vibration
[17,18],
whereas
others
reported
the
coarsening
of
eutectic
structure
after
ultrasonic
treatment
[27,28].
In
this
sense,
the
eutectic
growth
characteristics
of
Al–Si
alloys
with
ultrasonic
field
are
still
not
well
understood.
In
the
present
work,
in
order
to
investigate
the
effect
of
ultra-
sound
on
microstructural
evolution
of
Al–8%Si
hypoeutectic
alloy,
high
intensity
ultrasound
was
respectively
introduced
into
its
dif-
ferent
solidification
stages,
including
the
fully
liquid
state
before
nucleation,
the
nucleation
and
growth
process
for
primary
␣(Al)
phase
and
L
→
(Al)
+
(Si)
eutectic
transformation
period,
respec-
tively.
The
effect
of
ultrasound
on
the
primary
␣(Al)
phase
and
(Al
+
Si)
eutectic
structure
during
each
solidification
stage
was
clarified,
and
the
microstructural
evolution
mechanism
within
ultrasonic
field
was
further
proposed
on
the
basis
of
experimental
results.
2.
Experimental
The
experiments
were
performed
with
a
solidification
appara-
tus
incorporated
with
an
ultrasonic
generator.
Al–8%Si
hypoeutec-
tic
alloy
was
prepared
from
pure
Al
(99.9%)
and
Si
(99.9%).
The
ultrasonic
generator
consists
of
two
parts:
a
KNbO3piezoelectric
http://dx.doi.org/10.1016/j.jmst.2017.07.018
1005-0302/©
2017
Published
by
Elsevier
Ltd
on
behalf
of
The
editorial
office
of
Journal
of
Materials
Science
&
Technology.

1236
J.Y.
Wang
et
al.
/
Journal
of
Materials
Science
&
Technology
33
(2017)
1235–1239
transducer
with
a
resonant
frequency
of
20
kHz
and
a
horn
with
an
end
diameter
of

22
mm.
During
experiment,
the
alloy
sample
in
size
of

25
mm
×
25
mm
was
contained
in
a
graphic
crucible
with
a
size
of

25
mm
×
40
mm,
and
melted
with
a
resistance
furnace.
The
alloy
temperature
was
monitored
by
positioning
a
NiCr–NiSi
ther-
mocouple
at
the
sample
top.
When
the
alloy
temperature
reached
about
100
K
higher
than
its
liquidus
temperature,
the
ultrasonic
horn
preheated
to
573
K
was
inserted
into
the
liquid
alloy.
After
this,
the
furnace
was
removed,
and
the
liquid
alloy
was
cooled
nat-
urally
in
air.
During
the
cooling
process,
the
transducer
was
turned
on
and
500
W
ultrasound
was
introduced
into
the
alloy
melt
at
dif-
ferent
solidification
stages
indicated
by
the
real
time
cooling
curves,
which
are
stage
I
(the
alloy
fully
keeps
in
liquid
state),
stage
II
(the
nucleation
and
growth
process
of
primary
␣(Al)
phase),
and
stage
III
(L
→
(Al)
+
(Si)
eutectic
transformation
process).
After
the
experiments
were
conducted,
the
solidified
samples
were
sectioned
longitudinally,
mounted
in
epoxy
resin,
and
pol-
ished.
The
microstructures
were
analyzed
with
a
Zeiss
Axiovert
200
MAT
optical
microscope.
The
alloy
samples
were
then
lightly
etched
by
0.5%
HF–water
solution
and
further
examined
by
using
an
FEI
Sirion
200
scanning
electron
microscope
(SEM).
Finally,
the
samples
were
deeply
etched
in
a
0.5%
HF–water
solution
for
3
h
in
order
to
reveal
the
three-dimensional
morphology
of
the
eutectic
(Si)
phase.
In
order
to
check
the
mechanical
property
of
Al–8%Si
alloy
with
different
ultrasonic
treatment
stages,
specimens
in
a
size
of

6.0
mm
×
10.0
mm
were
cut
from
the
top
part
of
each
alloy
sample,
and
the
static
compression
test
was
performed
by
using
an
Instron
5969
universal
electronic
testing
machine
with
a
loading
speed
of
0.5
mm/min.
To
ensure
the
accuracy
of
test
results,
the
compression
without
samples
was
also
conducted
to
make
a
baseline
correction
of
machine-stiffness.
3.
Results
and
discussion
Fig.
1(a)–(d)
show
the
growth
morphologies
of
primary
␣(Al)
phase
with
different
ultrasonic
treatment
stages,
and
the
corre-
sponding
cooling
curves
are
presented
as
the
insets.
During
static
solidification,
primary
␣(Al)
phase
nucleates
at
849
K,
which
grows
into
very
coarse
dendrites.
As
plotted
in
Fig.
1(e),
the
maximum
length
of
these
␣(Al)
dendrites
reaches
1100
␮m,
and
the
average
length
is
706
␮m.
When
ultrasound
is
applied
during
stage
I,
the
nucleation
temperature
rises
up
to
853
K,
and
the
primary
␣(Al)
phase
turns
in
to
relatively
small
equiaxed
dendrites,
whose
aver-
age
length
is
354
␮m,
i.e.
half
of
that
under
static
condition.
Once
the
ultrasound
is
employed
in
stage
II,
the
resultant
primary
␣(Al)
phase
is
featured
by
tiny
globular
grains
with
a
mean
size
of
90
␮m,
which
is
almost
one
order
of
magnitude
lower
than
that
during
static
solidification.
This
“dendritic–globular”
morphological
tran-
sition
and
size
reduction
of
primary
(Al)
grains
are
also
found
in
aluminum
A356
alloy
after
introducing
ultrasound
in
its
whole
solidification
process
[16,17].
However,
if
ultrasound
is
introduced
during
stage
III,
the
alloy
shows
two-layer
structure.
The
top
layer
is
only
about
6
mm
in
height
and
is
made
up
of
eutectic
structure,
which
will
be
mentioned
in
the
following
section.
In
the
main
bot-
tom
layer,
as
seen
in
Fig.
2(d),
there
is
no
obvious
morphological
change
to
coarse
␣(Al)
dendrites,
though
some
dendritic
fragments
are
observed.
The
measured
average
size
is
554
␮m,
which
is
close
to
that
formed
under
static
condition.
As
reported
by
many
investigators
[20–24,29],
there
are
possibly
two
main
refining
mechanisms
by
ultrasound,
which
are
cavitation
induced
nucleation
and
cavitation
induced
fragmentation.
The
for-
mer
one
can
be
further
explained
by
(i)
cavitation
improves
wetting
between
impurities
and
alloy
melt
[20,23].
(ii)
cavitation
induces
local
high
undercooling
and
stimulates
nucleation
[24,29].
Here,
as
compared
with
other
three
conditions,
introducing
ultrasound
into
fully
liquid
alloy
exhibits
the
highest
nucleation
temperature
of
853
K
(see
insets
in
Fig.
1(b))
indicating
that
ultrasound
pro-
motes
nucleation.
Combining
this
with
the
resultant
half-reduced
grain
size
of
primary
␣(Al)
phase,
we
claim
that
cavitation-induced
wetting
before
nucleation
plays
a
role
in
refining
in
the
present
work.
Similar
results
were
also
reported
in
Ref.
[20],
in
which
Cavitation-enhanced
nucleation
via
improving
wetting
of
native
particles
is
suggested
to
play
the
dominant
role
in
refining
the
pri-
mary
Al3Ti
intermetallic
compound
when
ultrasound
is
applied
in
the
fully
liquid
state
of
Al–0.4%Ti
alloy
[20].
However,
it
also
should
be
mentioned
that
the
improvement
in
wetting
status
does
not
always
occur
in
any
liquid
alloys.
One
exceptional
case
is
that
when
ultrasound
was
applied
above
the
liquidus
temperature
and
termi-
nated
just
before
the
liquidus
temperature
of
Al–2%Cu
alloy,
no
refinement
of
primary
solid
phase
occurred
[19].
The
authors
thus
suggested
that
ultrasound
does
not
contribute
to
the
wetting
of
inclusions
or
the
conditions
for
their
activation
did
not
meet
their
experimental
conditions
[19].
More
importantly,
the
most
promi-
nent
refinement
effect
occurs
when
ultrasound
is
applied
in
the
nucleation
and
growth
process
of
primary
␣(Al)
phase,
implying
that
the
cavitation-induced
high
undercooling
and
or
cavitation-
induced
fragmentation
are
the
dominant
refinement
mechanisms
in
the
present
study.
However,
we
also
see
that
if
ultrasound
is
only
applied
in
the
eutectic
transformation
stage,
there
are
only
a
small
amount
of
fragmented
primary
␣(Al)
dendrites.
This
excludes
the
possibilities
that
cavitation-induced
fragmentation
is
the
main
rea-
son
for
refining
primary
␣(Al)
phase.
Consequently,
it
is
proposed
that
the
cavitation-induced
local
high
undercooling
plays
the
most
important
role
in
refinement
of
primary
␣(Al)
phase.
The
collapse
of
tiny
bubble
continuously
takes
place
in
a
large
number
of
micro-
zones,
leading
to
these
zones
as
nucleation
sites.
The
relationship
between
pressure
P
and
melting
temperature
Tmcan
be
expressed
by
the
Clausius–Clapeyron
equation
[30]:
Tm=
Tm0
+
TEV
Hm
(P
−
P0)(1)
where
Tm0 is
liquidus
temperature
of
Al–8%Si
alloy
at
the
atmo-
spheric
pressure
P0,
V
and
Hmare
the
volume
change
and
enthalpy
change
during
the
liquid-to-solid
transformation.
Assum-
ing
the
cavitation
produces
high
pressure
up
to
5
GPa,
the
local
undercooling
of
Al–8%Si
alloy
is
raised
by
95
K.
This
results
in
much
more
nucleation
sites
with
local
high
undercooling
in
the
alloy
melt.
Therefore,
the
primary
␣(Al)
phase
grows
into
tiny
globular
grains.
Here,
one
necessary
condition
for
refinement
is
the
pre-
heating
of
the
ultrasonic
horn.
It
has
been
reported
that
no
grain
refinement
takes
place
if
the
ultrasound
is
introduced
by
an
unpre-
heated
ultrasonic
horn
in
the
nucleation
process
[19].
This
is
due
to
the
formation
of
a
strong
solidified
layer
on
the
ultrasonic
horn
that
dampens
the
ultrasonic
effect
in
the
surrounding
liquid
[19].
Fig.
2
illustrates
the
eutectic
structural
characteristics
under
static
and
ultrasonic
conditions.
The
static
eutectic
structure
shown
in
Fig.
2(a)
and
(b)
is
featured
by
coarse
(Si)
plates
distributed
on
␣(Al)
matrix.
As
presented
in
Fig.
2(g)
and
(h),
the
average
eutectic
spacing,

and
average
size
of
eutectic
(Si)
phase,
L
are
10.14
and
54.62
␮m,
respectively.
When
ultrasonic
wave
is
introduced
into
stages
I
and
II,
this
eutectic
growth
morphology
is
reserved,
and
both
the
average
eutectic
spacing
and
size
for
eutectic
(Si)
phase
formed
in
these
two
stages
are
decreased
compared
with
those
during
static
solidification.
This
indicates
that
applying
ultrasound
before
the
eutectic
transformation
stage
can
lead
to
the
refinement
of
(Al
+
Si)
eutectic
structure,
maybe
owing
to
the
refinement
of
pri-
mary
␣(Al)
phase
by
ultrasound
in
stages
I
and
II.
As
shown
in
Fig.
1,
the
eutectic
structure
forms
within
the
interdendritic
regions
of
pri-
mary
␣(Al)
dendrites.
In
such
a
case,
the
refinement
of
the
primary
␣(Al)
phase
could
induce
the
refinement
of
the
eutectic
structure.

J.Y.
Wang
et
al.
/
Journal
of
Materials
Science
&
Technology
33
(2017)
1235–1239
1237
Fig.
1.
Microstructure
and
size
distribution
of
primary
␣(Al)
phase
in
solidified
Al–8%Si
alloy
samples:
(a)
static;
ultrasound
applied
in
(b)
stage
I,
(c)
stage
II
and
(d)
stage
III.
The
insets
in
(a),
(b),
(c)
and
(d)
are
the
corresponding
cooling
curves;
(e)
size
distribution
of
primary
␣(Al)
phase.
ds,
dI,
dII and
dIII represent
the
grain
size
of
primary
␣(Al)
phase
in
Al–8%Si
alloy
samples
under
static
condition,
with
ultrasonic
treatment
in
stages
I,
II
and
III,
respectively.
f
is
the
distribution
probability
of
size
distribution
for
primary
␣(Al)
phase.
Nevertheless,
remarkable
morphology
change
takes
place
at
the
top
of
alloy
sample
when
ultrasound
is
applied
in
stage
III.
As
seen
in
Fig.
2(c)
and
(d),
divorced
(Al
+
Si)
eutectic
structure
is
formed
at
the
very
top
of
the
alloy
sample
(about
0–3
mm
from
the
top
sur-
face,
as
shown
in
the
inset
in
Fig.
2(c)),
where
small
globular
␣(Al)
eutectic
phase
and
blocky
(Si)
eutectic
phase
grow
individually.
This
is
because
the
local
high
undercooling
induced
by
cavitation
promotes
the
two
eutectic
phases
to
nucleate
independently,
and
the
strong
micro-jet
efficiently
suppresses
their
coupled
growth.
As
shown
in
the
inset
of
Fig.
2(e),
in
the
region
about
3–6
mm
far
from
the
top
surface
of
the
alloy
sample,
where
the
cavitation
inten-
sity
is
not
as
strong
as
the
sample
top,
some
(Al
+
Si)
eutectic
cells
are
found
to
originate
from
the
center
and
grow
epitaxially
into
a
symmetrical
flower-like
shape
(Fig.
2
(e)
and
(f)).
Similar
eutectic
structure
is
observed
in
alminum
A356
alloy
[17]
and
binary
Sn–Pb
eutectic
alloy
[29].
The
formation
of
such
kind
eutectic
structure
results
from
acoustic
streaming,
which
ensures
the
radial
symme-
try
of
both
the
local
concentration
and
temperature
fields,
and
thus
the
solid-liquid
interface
is
locally
symmetric
in
three
dimensions.
In
the
remaining
lower
part,
as
seen
in
Fig.
1(d),
the
(Al
+
Si)
eutec-
tic
structure
is
similar
to
that
during
static
solidification
but
the
eutectic
(Si)
phase
shows
decreased
spacing
and
smaller
size.

1238
J.Y.
Wang
et
al.
/
Journal
of
Materials
Science
&
Technology
33
(2017)
1235–1239
Fig.
2.
SEM
images
of
(Al
+
Si)
eutectic
structure
in
Al–8%Si
alloy:
(a)
eutectic
morphology
formed
during
static
solidification;
(b)
enlarged
view
of
zone
A
shown
in
(a);
(c)
eutectic
microstructure
formed
at
the
sample
top
(0–3
mm)
when
ultrasound
was
applied
in
stage
III.
The
inset
shows
the
location
of
this
morphology;
(d)
enlarged
view
of
zone
B
shown
in
(d);
(e)
eutectic
structure
formed
at
the
sample
top
(3–6
mm)
of
the
sample
when
ultrasound
was
applied
in
stage
III.
The
inset
shows
the
location
of
this
morphology;
(f)
enlarged
view
of
zone
C
shown
in
(e);
(g)
spacing
of
eutectic
(Si)
phase
;
(h)
size
distribution
of
eutectic
(Si)
phase
L.
The
subscripts
S,
I,
II
and
III
stand
for
the
solidified
alloy
samples
under
static
condition
and
with
ultrasonic
treatment
in
stages
I,
II
and
III,
respectively.
f
is
the
distribution
probability
of
size
distribution
for
primary
␣(Al)
phase.

J.Y.
Wang
et
al.
/
Journal
of
Materials
Science
&
Technology
33
(2017)
1235–1239
1239
Fig.
3.
Compressive
stress–strain
curves
of
Al–8%Si
hypoeutectic
alloy
samples
with
different
ultrasonic
treatment
stages.
The
inset
shows
the
change
of
yield
strength.
Fig.
3
shows
the
compressive
stress–strain
curves
for
Al–8%Si
hypoeutectic
alloy
samples
solidified
under
different
conditions,
where
␴
stands
for
stress
and
␧
denotes
strain.
There
are
four
stress–strain
curves
in
Fig.
3,
marked
as
S
(dot
line),
I
(dash
dot
line),
II
(solid
line)
and
III
(dash
line).
S
denotes
the
stress–strain
curve
for
the
alloy
sample
solidified
under
static
condition,
I,
II
and
III
stand
for
the
stress–strain
curves
of
the
alloy
samples
with
ultrasonic
treatment
in
stages
I,
II
and
III,
respectively.
With
the
increase
of
applied
loading
stress,
the
diameter
of
all
alloy
sam-
ples
increases
and
height
decreases,
exhibiting
obvious
yielding
point,
which
relates
to
the
transition
from
elastic
deformation
to
plastic
deformation.
The
yield
strength
for
the
sample
solidified
under
static
condition
is
42.24
MPa.
For
the
sample
solidified
with
the
presence
of
ultrasound
in
stages
I,
II
and
III,
the
yield
strength
elevates
to
47.69,
61.17
and
91.42
MPa,
respectively
(the
inset
of
Fig.
3).
This
indicates
that
the
extent
of
yield
strength
promotion
is
also
largely
dependent
on
the
ultrasonic
treatment
stage.
Applying
ultrasound
in
eutectic
transformation
process
strikingly
improves
the
yield
strength
by
2.16
times,
whereas
the
refining
in
primary
␣(Al)
phase
does
not
contribute
significantly.
The
reason
may
be
that
solidification
microstructure
is
composed
of
about
40
vol.%
primary
phase
and
about
60
vol.%
(Al
+
Si)
eutectic
structure.
The
remarkable
growth
morphological
change
of
main
(Al
+
Si)
eutectic
structure
induced
by
applying
ultrasound
in
stage
III
results
in
this
significant
yield
strength
promotion.
In
a
comparison,
this
mechan-
ical
promotion
is
more
prominent
than
other
modification
methods
[31,32].
By
adding
1.0
wt%
Ce
into
the
Al–8%Si
alloy,
the
ultimate
tensile
strength
is
raised
by
1.60
times
[31].
By
vacuum
assisted
high
pressure
die
casting,
the
ultimate
tensile
strength
is
increased
by
17%
[32].
4.
Conclusion
High
intensity
power
ultrasound
was
respectively
introduced
into
different
stages
of
solidifying
Al–8%Si
hypoeutectic
alloy
to
clarify
the
refining
mechanism.
It
is
found
that
the
cavitation-
induced
nucleation
due
to
the
high
undercooling
caused
by
the
collapse
of
tiny
cavities
is
the
major
reason
for
refining
primary
␣(Al)
phase,
while
cavitation-induced
wetting
is
also
proven
to
be
one
important
refining
factor.
Besides,
cavitation
induced
dendritic
fragmentation
plays
some
role
in
refining
primary
␣(Al)
phase.
The
eutectic
structure
is
also
refined
when
ultrasound
is
applied
in
each
solidification
stage.
Particularly,
when
power
ultrasound
is
introduced
to
the
eutectic
transformation
stage,
divorced
eutec-
tic
structure
and
eutectic
cell
with
symmetrical
flower
shape
are
both
formed
at
the
top
of
the
alloy
samples.
The
introduction
of
ultrasound
in
each
solidification
stage
improves
the
mechani-
cal
property
of
Al–8%Si
hypoeutectic
alloy
by
increasing
the
yield
strength.
Acknowledgements
This
work
was
financially
supported
by
the
National
Natural
Sci-
ence
Foundation
of
China
(Nos.
51471134
and
51402240),
the
Fund
of
the
State
Key
Laboratory
of
Solidification
Processing
in
NWPU
(No.
SKLSP201735)
and
Ao
Xiang
Xin
Xing
Foundation
of
NWPU.
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