materials
ArticleCorrosion Behavior and Strength of Dissimilar
Bonding Material between Ti and Mg Alloys
Fabricated by Spark Plasma Sintering
Patchara Pripanapong 1, *, Shota Kariya 1 , Tachai Luangvaranunt 2 , Junko Umeda 3 ,Seiichiro Tsutsumi 3 , Makoto Takahashi 3 and Katsuyoshi Kondoh 3
1
2
3

*

Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan;
kariya-s@jwri.osaka-u.ac.jp
Department of Metallurgical Engineering, Chulalongkorn University, Phayathai, Patumwan,
Bangkok 10330, Thailand; tachai.l@chula.ac.th
Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaragi, Osaka 567-0047, Japan;
umedaj@jwri.osaka-u.ac.jp (J.U.); tsutsumi@jwri.osaka-u.ac.jp (S.T.);
makotot@jwri.osaka-u.ac.jp (M.T.); kondoh@jwri.osaka-u.ac.jp (K.K.)
Correspondence: patchara@jwri.osaka-u.ac.jp; Tel.: +81-06-6879-8669

Academic Editor: Daolun Chen
Received: 11 July 2016; Accepted: 1 August 2016; Published: 6 August 2016

Abstract: Ti and solution treated Mg alloys such as AZ31B (ST), AZ61 (ST), AZ80 (ST) and AZ91 (ST)were successfully bonded at 475 ˝ C by spark plasma sintering, which is a promising new method
in welding field. The formation of Ti3 Al intermetallic compound was found to be an importantfactor in controlling the bonding strength and galvanic corrosion resistance of dissimilar materials.
The maximum bonding strength and bonding efficiency at 193 MPa and 96% were obtained from
Ti/AZ91 (ST), in which a thick and uniform nano-level Ti3 Al layer was observed. This sample also
shows the highest galvanic corrosion resistance with a measured galvanic width and depth of 281
and 19 µm, respectively. The corrosion resistance of the matrix on Mg alloy side was controlled by its
Al content. AZ91 (ST) exhibited the highest corrosion resistance considered from its corrode surface
after corrosion test in Kroll’s etchant. The effect of Al content in Mg alloy on bonding strength andcorrosion behavior of Ti/Mg alloy (ST) dissimilar materials is discussed in this work.
Keywords: Ti; Mg alloys; spark plasma sintering; bonding strength; galvanic corrosion

1. Introduction
The applications of lightweight materials are gradually increased in many industrial fields
recently. Titanium is one of well-known lightweight materials, which is applied in many industrial
fields such as automobile, aerospace, medical prosthesis and chemical industry. This is attributed tothe advantageous properties of Ti such as high specific strength, good biocompatibility and corrosion
resistance [1–4]. In order to reduce the weight of automobile and aerospace component, a new light
dissimilar material between Ti and Mg alloy was fabricated in this research. The application of thislight material will reduce fuel consumption and pollution emitted to the environment from vehicles.
However, reports related to the bonding of Ti and Mg alloy are still scarce because of the difficultyin bonding, as no intermetallic compounds exist between Ti and Mg, referring to the binary phase
diagram [5]. Transient liquid phase bonding between AZ31 (Mg alloy) and Ti64 was reported by
Atieh et al. However, pure Ni coated on Ti64 surface was necessary to obtain a successful bond. The
thick intermetallic compound (IMC) layer that degraded the shear strength of bonding material wasformed due to the eutectic liquid phase formation during bonding [6]. Fouad studied the bonding
between pure Ti and AZ31 with Al inserted sheet fabricated by hot isostatic pressing (HIP), andMaterials 2016, 9, 665; doi:10.3390/ma9080665

www.mdpi.com/journal/materials

Materials 2016, 9, 665

2 of 11subsequent annealing. However, bonding components showed a poor tensile strength due to theformation of thick intermetallic layer at the Al/AZ31 interface [7].
The corrosion resistance, especially the galvanic corrosion of the bonding material, is anotherimportant property similar to the tensile strength as the bonding components are used under corrosive
atmosphere [8–10]. The report related to the effect of intermetallic layer formation at the bonding
interface on the galvanic corrosion resistance of the bonding material was also scarce. Funutsu et al.
fabricated the carbon nanotubes (CNTs) reinforced Mg composites, and studied the galvanic corrosionof this material. They proposed that the galvanic corrosion between α-Mg matrix and CNTs was
relieved by the concentration of Al atoms around CNTs via heat treatment. This phenomenon caused
the reduction of surface potential difference (SPD) between α-Mg matrix and CNTs [11]. Huang et al.
studied the galvanic corrosion behavior of Inconel 718 after electron beam welding. The galvanic
corrosion resistance of solution-annealed Inconel 718 weld was improved by applying post-welding
precipitation hardening treatment. The post-weld heat treatment caused a disappearance of the heataffected zone (HAZ), which existed between fusion zone and base metal, resulting in a decreasingof the corrosion potential difference between fusion zone and base metal [12]. Norouzi studied theeffect of brazing on the corrosion resistance of clad modified Al alloys between AA4xxx/AA3xxx.
The results show that the voltage potential of re-solidified matrix (clad) was increased after brazing,
and the voltage potential difference between re-solidified matrix and Si needle at the bonding interface
was decreased. This resulted in an improvement of the galvanic corrosion resistance [13].
In this research, the effect of intermetallic compound (IMC) layer formation on tensile strengthand galvanic corrosion resistance of the dissimilar bonding materials between Ti and Mg alloys werestudied. The galvanic corrosion in each dissimilar material was predicted by scanning kelvin probe
force microscope (SKPFM), which is used for SPD measurement. The corrosion behavior of dissimilarmaterials that are different for each Mg–Al alloy was elucidated by mass loss measurement and surface
profile analysis after corrosion test.
2. Materials and Methods
Ti rod (16 mm in diameter) with purity of 99.95% and four types of cast Mg alloy rod (16 mm indiameter), AZ31B, AZ61, AZ80 and AZ91, were used in this research. All materials were purchasedfrom Nilaco Co., Ltd. (Tokyo, Japan). The chemical composition of Ti and Mg alloys are listed inTables 1 and 2, respectively. All Mg alloys were solution-treated at 420 ˝ C for 12 h in muffle furnace andsubsequently quenched in water before surface preparation. Pure Ti and Mg alloys rods were cut into
20 mm height, and their surfaces were prepared before bonding process. The surface of Ti was groundby SiC emery paper until 2000# and polished with 0.05 µm Al2 O3 colloidal. On the other hand, thesurfaces of Mg alloys were ground by SiC emery paper until 2000# and polished with 0.25 µm diamondpaste. The polished Ti and Mg alloy rod were inserted in a carbon container with an inner diameterof 16 mm, and the carbon punches were placed at both ends of the sample. Ti and Mg alloys werebonded together by a spark plasma sintering (SPS) machine (SYNTECH Co. SPS103S, Sinterland Inc.,
Niigata, Japan), in which a sample was heated by a flow of current through an electrode and carbon
container. The thermocouple was inserted in carbon container wall with a distance far from a sample
of 1 mm to provide a precise temperature measurement. The schematic drawing of the component
setting in SPS chamber and bonded sample are shown in Figure 1a,b, respectively. Ti and Mg alloys
were bonded at 400 and 475 ˝ C for 1 h under a bonding pressure of 10 MPa in a vacuum atmosphere,
and subsequently cooled in SPS chamber. For microstructure observation, samples were cut at thebonding interface, and their surfaces were prepared by the same method as applied for preparing Mg
alloy surface followed by etching with picric acid. Microstructure observations were performed byscanning electron microscope (JEOL JSM6500F, JEOL Ltd., Tokyo, Japan) and transmission electron
microscope (JEOL JEM-2010, JEOL Ltd., Tokyo, Japan). Three tensile specimens were machined froma bonding sample with a gauge length and diameter of 20 and 3 mm, respectively. Tensile test wasMaterials 2016, 9, 665

3 of 11

testing speed of 0.05 mm/min, and load direction is normal to the bonding interface. The bonding
Materials 2016, 9, 665
3 of 11efficiency was simply calculated by Equation (1).
Bonding efficiency = (/) × 100(1)

performed at room temperature under testing speed of 0.05 mm/min, and load direction is normal tothe σbonding
interface.
The ofbonding
efficiency
simplyby Equation(1).parent Mg alloys
whereb = Bonding
strengthbonding
samplewas(MPa);
andcalculated
σp = Tensile
strength of(MPa).
Bonding
efficiency“ pσb{σpqˆ 100 difference (SPD) at the bonding(1)
The surface potential of Ti and
Mg alloys
and surface
potential
interface were measured by scanning kelvin probe force microscope or SKPFM (WET-SPM,
where σb = Bonding strength of bonding sample (MPa); and σp = Tensile strength of parent Mg alloys (MPa).
SHIMADZU,
Shimadzu Co., Kyoto, Japan) before corrosion test. The scanning area was 30 µm × 30
The surface potential of Ti and Mg alloys and surface potential difference (SPD) at theµmbondingand the interface
scan linewere
at themeasured
bondingby
interfacewas
10 µm.
Theforce
surfacepotential
measuredfromscanning
kelvinprobe
microscopeor that
SKPFM(WET-SPM,
SKPFM
is an absolute
value
with
a platinumreferenceelectrode.
bonding
were
cutµm
to a
SHIMADZU,
Shimadzu
Co.,
Kyoto,
Japan) before
corrosion
test. TheThe
scanning
areasamples
was 30 µm
ˆ 30
sizeandof 8the
mm
×
8
mm
×
4
mm
and
immersedin
Kroll’setchant(HF:HNO
3:H2O = 1:5:100) at roomscan line at the bonding interface was 10 µm. The surface potential that measured fromtemperature.
The
Kroll’svalueetchant
selectedreference
for corrosion
testingbecause samples
it can corrode
SKPFM is anabsolute
withwasa platinum
electrode.
The bonding
were cutbothtotitaniumandMg ˆalloy,
is andsuitablefor studying
behavior3 :H
of2 O
dissimilara size of
8 mm
8 mmwhichˆ 4 mmimmersedin Kroll’scorrosion
etchant (HF:HNO= 1:5:100)materials.
at room
Furthermore,
theThecorrodedsurfaces
ofselecteddissimilarmaterialstesting
were cleaned
corrosion
producttemperature.
Kroll’s etchant
wasfor corrosion
because itwithout
can corrode
both titanium
resultedin aalloy,
precisemeasurement
ofstudying
galvaniccorrosion
width anddepth. of dissimilar materials. Furthermore,and Mgwhich
is suitable forbehavior
thecorrodedof dissimilarmaterials
wereforcleanedcorrosionaThe
samplesurfaces
was immersed
in Kroll’s
etchant
2 and without
10 min forsurfaceproductprofileresulted
analysisinand
precisemeasurementofgalvanic
widthanddepth.
mass loss measurement, respectively. The surface profile analysis after corrosion test was performed
The analysis
sample was
immersed in
Kroll’s etchantfor 2 and10 min forsurface
profileanalysismass
by surface
microscope(KEYENCE
VR-3100,
KeyenceCo.,
Osaka,
Japan).
Theandgalvanic
loss
measurement,
respectively.
The
surface
profileanalysisafter
corrosion
test
wasperformedbywidth and depth were measured to evaluate a severity of galvanic corrosion. The mass of dissimilar
surfacewere
analysis
microscope(KEYENCE
VR-3100,
KeyenceCo., Osaka,
The galvanic
width
and
materials
measured
beforeand after
corrosion
test and
simplyJapan).
calculated
to percent
of mass
depth
were
measuredtoevaluateaseverityofgalvanic
corrosion.
The
mass
of
dissimilarmaterials
loss to study the effect of aluminum content in Mg alloy on corrosion resistance of dissimilar
were measured
beforecorrosion
test andwas
simplyto percent
of mass
to study
materials.
The mass
lossandof afterdissimilarmaterials
alsocalculatedcompared
to a mass
lossloss
of Mg
alloy
the
effectofaluminumcontent
in
Mg
alloy
on
corrosion
resistanceofdissimilarmaterials.
The
mass
parent metal (sample size of 8 mm × 4 mm × 4 mm) to confirm the existence and severity of galvanic
loss of dissimilar materials was also compared to a mass loss of Mg alloy parent metal (sample size ofcorrosion.
8 mm ˆ 4 mm ˆ 4 mm) to confirm the existence and severity of galvanic corrosion.

Table 1. Chemical composition of Pure Ti rod applied in this research (wt %).
Table 1. Chemical composition of Pure Ti rod applied in this research (wt %).

Material
Pure Ti

Ti
Bal.Material

Ti
0.3

Fe
Fe

H
0.013

H
O

N
0.13

O

Pure Ti

Bal.

0.3

0.013

0.13

0.05

N
0.05

Table 2. Chemical composition of cast Mg alloys rod applied in this research (wt %).
Table 2. Chemical composition of cast Mg alloys rod applied in this research (wt %).

Mg Alloys
AZ31B
AZ61
AZ80
AZ91

Mg
Bal.
Mg Alloys
Bal.
AZ31B
AZ61
Bal.
AZ80
Bal.
AZ91

Mg
Bal.
Bal.
Bal.
Bal.

Al
2.8 Al5.5 2.8
7.8 5.5
8.5 7.8
8.5

Zn
0.8
0.7
0.3
0.6

Zn
0.8
0.7
0.3
0.6

Mn
0.3
0.3
0.4
0.4

Mn
0.3
0.3
0.4
0.4

Figure1. Schematic drawing of component setting in SPS chamber (a); and bonded sample (b).
Figure 1. Schematic drawing of component setting in SPS chamber (a); and bonded sample (b).

Materials 2016, 9, 665

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Materials 2016, 9, 665

3. Results
and9,Discussion
Materials 2016,665

4 of 114 of 11

3. Results and Discussion

3. Results and Discussion
3.1. Microstructure
Analysis3.1. Microstructure
AnalysisThe
optical microstructureof pure Ti applied in this research is shown in Figure 2. The pure Ti
3.1.
Microstructure
Analysisconsisted Theof α-Ti
matrixwith no otherphases,and itsingrain
size wasisapproximately
Figureopticalmicrostructureof pureTi applied
this researchshown in Figure30
2.µm.
The pure
Ti 3a–dThe optical microstructure of pure Ti applied in this research is shown
Figure 2. The pure Ti
˝ Cinapproximately
consistedof α-Ti matrix
withno otherits grainatsize
µm.
shows
microstructuresof Mg
alloysafter phases,
solutionand
treatment
420wasfor 12 h and 30
subsequentconsisted of α-Ti matrix with no other phases, and its grain size was approximately 30 µm.
Figure in
3a–dshows microstructuresof Mg
alloys microstructure
after solution treatment
at 420°C for12 h only
and Mgquenchingwater.
Mg alloys show
a similar
and grain
size,
Figure 3a–dshowsAll
microstructuresof Mg
alloys after
solution treatment
at 420°C in
forwhich
12 h and
subsequent
quenchinginwater.
All
Mg
alloysshowasimilarmicrostructure
and
grain
size,
in
matrixand twin
was observed.
Theof solution
treatment
was to dissolve
the size,
brittlesubsequent
quenchingin water.
Allpurpose
Mg alloys
show a similar
microstructureand grainin β-Mg
which only Mg matrix and twin was observed. The purpose of solution treatment was to dissolve(Mg17
Al12 )only
particles,
which
inhibitedbonding
betweenTi andofMg
alloy into
the matrix,
to create awhich
Mg matrix
andtwin wasobserved.
The purpose
solutiontreatment
was toanddissolve
the brittle β-Mg (Mg17Al12) particles, which inhibited bonding between Ti and Mg alloy into theuniform
Al distribution
bondingwhichsurfaceof Mg bonding
alloys [14].
Hereafter,
the brittle
β-Mg (Mg17on
Al12the) particles,
inhibited
betweenTi andthe
Mgsolution-treatedalloy into the Mg
matrix, and to create a uniform Al distribution on the bonding surface of Mg alloys [14]. Hereafter,
and to as
createaalloyuniform
Al distribution on the bonding surface of Mg alloys [14]. Hereafter,
alloymatrix,
wassolution-treated
named
MgMg
(ST).
the
alloy
was named as Mg alloy (ST).
the solution-treated Mg alloy was named as Mg alloy (ST).

Figure 2. Microstructure of pure Ti observed by optical microscope.

Figure2.2.Microstructureof purepureTiTiobserved
observed
opticalmicroscope.
FigureMicrostructure of
byby
opticalmicroscope.

Figure 3. Microstructures of Mg alloys after solution treatment at 420 °C for 12 h and being quenched
Figure 3. Microstructures of Mg alloys after solution treatment at 420 °C for 12 h and being quenchedin 3.
water:(a) AZ31B; (b)ofAZ61;(c) AZ80;
and(d) AZ91.
FigureMicrostructures
Mg alloys
after
solutiontreatment at 420 ˝ C for 12 h and being quenchedin water: (a) AZ31B; (b) AZ61; (c) AZ80; and (d) AZ91.
in water: (a) AZ31B; (b) AZ61; (c) AZ80; and (d) AZ91.
Figure 4 shows the bonding interfaces of Ti bonded to AZ31B (ST) or Ti/AZ31B (ST) and
Figure 4 shows the bonding interfaces of Ti bonded to AZ31B (ST) or Ti/AZ31B (ST) and
Ti/AZ91 (ST) at 475 °C for 1 h under bonding pressure of 10 MPa. Figure 4a shows the bonding
Ti/AZ91
at 475for 1 hinterfaces
under bonding
pressuretoofAZ31B
10 MPa.
Figure4a shows (ST)the bonding
Figure 4(ST)
shows
the°C
bondingof Ti bonded(ST)or Ti/AZ31B
and Ti/AZ91(ST) at 475 ˝ C for 1 h under bonding pressure of 10 MPa. Figure 4a shows the bonding interface ofMaterials 2016, 9, 665

5 of 11

Ti/AZ31B
an existence of discontinuous Ti3Al IMC layer with a thickness of 30
Materials (ST)2016, 9,which
665
5 ofnm
11 wasobserved, and confirmed by diffraction pattern in Figure 4d. This IMC layer was also detected in a darkof which
Ti/AZ31B
discontinuous
Ti3Al(Figure
IMC layer
thicknessinterfaceoffieldinterface
image, inthe(ST)
IMCwhichlayeranis existencebrighter ofthanthe other areas4b).with
Theabonding
30 nm was observed, and confirmed by diffraction pattern in Figure 4d. This IMC layer was alsocharacteristic of Ti/AZ61 (ST) observed by TEM was similar to Ti/AZ31B (ST) but the thickness ofdetected in a dark field image, in which the IMC layer is brighter than the other areas (Figure 4b).
the IMC layer slightly increased to 36 nm. Figure 4c shows the bonding interface of Ti/AZ91 (ST)The bonding˝ interface characteristic of Ti/AZ61 (ST) observed by TEM was similar to Ti/AZ31B (ST)bondedat thickness
475 C for1 hIMC
under
anslightly
appliedpressureof nm.
10 MPa.
brightimageshows thebut theof thelayer
increased
to 36
Figure The
4c shows
thefieldbonding
interfaceexistence
of
a
Ti
Al
IMC
layer
at
the
bonding
interfacewithathicknessof
50
nm(Figure4c),
3 bonded at 475 °C for 1 h under an applied pressure of 10 MPa. The bright field imagewhichof Ti/AZ91 (ST)was showsconfirmed
by a darkoffield
andlayerdiffraction
pattern (FigureThisintermetallic
compound,the existencea Tiimage3Al IMC
at the bonding
interface4d).
with
a thickness
of 50
nm
which
was observed
the confirmed
bonding interface,layer, implying
a reaction
between(Figure4c), whichatwas
by a darkwas
fielda continuous
image and diffraction
patternthat(Figure4d). This
compound,which wasobserved
at simpler
the bonding
interface,
was acompared
continuoustolayer,
Al inintermetallic
Mg alloy and
Ti at the bonding
interface
wasand more
uniform
Ti/AZ31B
that a reaction
between
Al in Mg alloy
andincreased
Ti at the bonding(ST) implying
and Ti/AZ61(ST). Thiswas explainedby thecontentinterfaceof Al inwas
Mgsimpleralloy: andmore Al
more
uniform
comparedto
Ti/AZ31B(ST)
and
Ti/AZ61
(ST).
This
was
explainedbytheatoms cause a strong reaction between Ti and Al and result in an increased thickness ofincreased
the IMC layer.
content of Al in Mg alloy: more Al atoms cause a strong reaction between Ti and Al and result in an
For Ti/AZ80 (ST), the thickness of Ti3 Al IMC layer slightly decreased to 47 nm compared to Ti/AZ91
increased thickness of the IMC layer. For Ti/AZ80 (ST), the thickness of Ti3Al IMC layer slightly(ST). The dark areas in Figure 4b,c on both Ti and Mg alloy matrix are high dislocation density areas,decreased to 47 nm compared to Ti/AZ91 (ST). The dark areas in Figure 4b,c on both Ti and Mg alloy
which
wereareaffectedby a bonding
pressurethat causes
a plastic
in bothmatrixhigh dislocation
densityareas, whichwere affectedby adeformation
bonding pressure
thatmaterials.
causes a The
plastic
deformation
facilitated
two surfaces
to perfectly
contact,facilitated
and this corresponded
wellplastic
deformationin boththematerials.
The plastic
deformation
the two surfaces
to to theTEMperfectly
images contact,that no andcavities
were observed
ontotheinterface[15].
The Aldiffusion
this corresponded
wellthebonding
TEM images
that nocavities
wereobservedlayer
on wasalso theobserved
the bonding
interfacein eachits thickness
bondingon
interface[15]. TheAl diffusion
layerbonding
was alsomaterial
observed byon TEM-EDS
the bondingand
interfacein each was
bonding
material
by
TEM-EDSandits
thicknesswassimilartothethickness
of
the
IMC
layer.
similar to the thickness of the IMC layer.

˝ C for 1 h: (a) bright
Figure4. Bonding
interface(ST)andandTi/AZ91
Ti/AZ91(ST)
bondedat °C
475forFigure4. Bonding
interfaceofofTi/AZ31B
Ti/AZ31B (ST)(ST)
bondedat 475
1 h: (a) bright
Ti/AZ31B(ST);(b)(b)darkdarkfield
field Ti/AZ31B
Ti/AZ31B (ST);field
Ti/AZ91(ST);(ST);and (d)fieldfield
Ti/AZ31B(ST);(ST);(c)(c)brightbrightfield
Ti/AZ91
anddark(d) field
dark field
Ti/AZ91(ST).
Ti/AZ91 (ST).

3.2. Bonding Strength Evaluation

3.2. Bonding Strength Evaluation

Figure 5 shows bonding strength and bonding efficiency of Ti/Mg alloy dissimilar materials
Figure 5 shows bonding strength and bonding efficiency of Ti/Mg alloy dissimilar materials
bonded at 475 °C for 1 h under bonding pressure of 10 MPa. Ti/AZ31B (ST) shows the lowest
˝ C for 1 h under bonding pressure of 10 MPa. Ti/AZ31B (ST) shows the lowest bonding
bondedat 475bonding
strengthamong the bonding materials with the value of 136 ± 9 MPa. This was explained bystrengthamong
the bonding
materials
withof 136˘ 9theMPa.
This Alwas
explaineda degreeof reaction
betweenTi and Al
thatthethisvaluematerialshows
thinnest
diffusion
andby
Ti3aAldegreeof reaction
Ti andthatthis materialshowsinthe
thinnest
diffusionand Ti3 Al(ST)layer oflayer of between
30 nm (Figure4a).AlThe
increasingof Al content
Ti/AZ61(ST)Alcompared
to Ti/AZ31B

30 nm (Figure 4a). The increasing of Al content in Ti/AZ61 (ST) compared to Ti/AZ31B (ST) resulted
in an increasing of bonding strength from 136 ˘ 9 to 157 ˘ 3 MPa. This was attributed to an increase

Materials 2016, 9, 665
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6 of 11

resulted in an increasing of bonding strength from 136 ± 9 to 157 ± 3 MPa. This was attributed to anincreasein theofthicknessof andAl diffusion
3Al
30 tothe36characteristic
nm. However,in
the thickness
Al diffusion
Ti3 Al layerand
fromTi30
to layer
36 nm.from
However,of Ti3theAlcharacteristicof Tito3Al
layer was(ST),similarTi/AZ31B(ST),
as itForwasa discrete
layer.
Ti/AZ80(ST),
layer
was similar
Ti/AZ31B
as ittowasa discrete
layer.
Ti/AZ80(ST),
the Forbonding
strength
the bonding
increased
to increasing
173 ± 15 MPa
with an increasingin thickness
Ti3Al layertowasincreasedstrength
to 173 ˘was
15 MPawith anin thicknessof Ti3 Al layer
to 47 nm.of
Another
reason
47 nm.
Another reason
that contributed
toathisimprovement
was a change
in alayer
characteristicof Ti3Al
that
contributed
to this improvement
waschangein a characteristicof Ti3 Alfrom a discretetolayer from alayer
discreteto continuous
layer4b).theTi/AZ91(ST)
exhibitedthe highest
bonding
continuous(Figure4b). Ti/AZ91(ST)(Figure
exhibitedhighestbonding
strengthof 193 ˘
8 MPa,
strength
ofexpected
193 ± 8 from
MPa,itswhich
wasinterface
expectedcharacteristicfrom its bonding
interface characteristic
that
which
was
bondingthat a formationof a continuous
anda
formationTiof
a continuous and uniform Ti3Al layer with a thickness of 50 nm was observed anduniform
3 Al layer with a thickness of 50 nm was observed and confirmed by dark field image
confirmedbyfield samples
image (Figure4d). All on
bonding
sampleswere
Mg alloy(Figure 4d). Alldark
bonding
were fractured
Mg alloyside near
thefractured
bonding on
interface,andside
the
near
the
bonding
interface,
andthefracture
direction
wasperpendiculartotensiledirection.
fracture direction was perpendicular to tensile direction.
The bonding
be be
simplycalculatedusing
Equation(1). TheThe
bonding efficiency
efficiency shown
shownininFigure
Figure5 can
5 can
simplycalculatedusing
Equation(1).
bonding
efficiency
indicatedthecompatibility
betweentwomaterials
whenitbondedtogether.
The
The bonding efficiency indicated the compatibility between two materials when it bonded together.
resultshowsthat theefficiency
was directly
relatedto aluminum
contentcontent
in Mg alloy.
wasThe
resultshows
thatbonding
the bonding
efficiency
was directly
relatedto aluminumin MgItalloy.
gradually
increasedfrom from67% 67%
to 96%
when
Al content
in in
Mg
alloy
It
was gradually
increased
to 96%
when
Al content
Mg
alloywaswasincreased.
increased. The
The bonding
bonding
efficiency ofofTi/AZ91
Ti/AZ91(ST)
(ST)
was
very
satisfying:
the bonding
strength
thestrengthtensileefficiency
wasvery
satisfying:
the bonding
strengthalmostalmost
reachedreached
the tensile
strengthof AZ91(ST)
parentmetal.
These
results emphasized
the importanceof Alanddiffusion
andof
AZ91 (ST)parentmetal.
These
resultsemphasized
the importanceof Al diffusion
Ti3 Al layer
Ti
3
Al
layer
formationtotheimprovementofbonding
strength.
formation to the improvement of bonding strength.

FigureFigure 5.
5. Effect
Effect ofof Al
Al content
content inin Mg
Mg alloy
alloy onon bonding
bonding strength
strength andand bonding
bonding efficiency.
efficiency.

3.3. Surface
at Bonding
Bonding Interface
Interface ofof Dissimilar
Dissimilar Materials
Materials
3.3.
Surface Potential
Potential Measurement
Measurement at
Figure 66shows
showsa surface
a surfacepotentialof alloys
Mg alloys
after solution
at for42012°Cfor 12 hFigurepotentialof Mg
after solution
treatedtreatedat 420 ˝ Ch measured
measuredby
SKPFM.
These
surfacepotentials
wereanaveragevalue
measuredfrom
fivedifferent
by SKPFM. These surface potentials were an average value measured from five different positions.
positions.
(ST) (ST)and show
AZ61a(ST)a highsurfacepotential
1.731.70± 0.08andV,1.70± 0.09 V,
AZ31B(ST)AZ31B
and AZ61
highshowsurfacepotentialof 1.73
˘ 0.08ofand˘ 0.09respectively.
respectively.
the surface
potential
measured
SKPFM,
which isvalue,
an absolutevalue,
Accordingto According
the surfacetopotential
measuredfrom
SKPFM,fromwhich
is an absolutethe material
thematerialthat
possesses
thehighersurfacepotentialismoresusceptible
tothecorrosionthat possesses the higher surface potential is more susceptible to the corrosion compared to the lower
comparedto the lower
potential.
the other
hand,
AZ80
shows acomparedlower surface
potential
surfacepotential.
On thesurface
other hand,
AZ80Onshows
a lowersurfacepotential
to AZ31B(ST)
compared
to
AZ31B
(ST)
and
AZ61
(ST)
with
a
value
of
1.65
±
0.05
V.
The
incrementinAl
content
inand AZ61 (ST) with a value of 1.65 ˘ 0.05 V. The increment in Al content in Mg alloy was effective in
Mg
alloy
was
effectiveindecreasinginsurfacepotential,
which
was
attributed
toaformationofdecreasing in surface potential, which was attributed to a formation of stable Mg–Al–O film on Mg
stablesurface
Mg–Al–O
filmAZ91
on Mg
alloy
surface[16,17].
AZ91 (ST)showsa surfacepotentialof 1.63among± 0.06alloy[16,17].
(ST)shows
a surfacepotentialof 1.63
˘ 0.06V, whichis the lowest
V,
which
is
the
lowestamong
Mgalloysinthisresearchdue
toits
highestAl
content.
Accordingto
Mg alloys in this research due to its highest Al content. According to this result, AZ91 (ST) should
thisresult,AZ91(ST)shouldpossessthehighestcorrosion
resistanceamong
Mgalloys.
The
surfacepossess the highest corrosion resistance among Mg alloys. The surface potential of pure Ti was also
potential of
purefive
Ti different
was also positions,
measured andfromit fivedifferent
a valuethatof 0.78
measuredfromshows
a valuepositions,of 0.78 ˘ and0.03 itV.shows
This implied
pure±
0.03
V.
This
implied
that
pure
Ti
was
much
less
susceptible
tocorrosioncomparedtoMgalloysTi was much less susceptible to corrosion compared to Mg alloys because of the formation of a TiO2
becausefilmof theformation
a TiO2 surface
film thatwas very effective at preventing corrosion.
surfacethatwas veryofeffectiveat preventing
corrosion.

Materials 2016, 9, 665

7 of 11

Materials 2016, 9, 665

7 of 11

Materials 2016, 9, 665

7 of 11

Figure 6. Effect of Al content on surface potential of Mg alloys.

Figure 6. Effect of Al content on surface potential of Mg alloys.
Figure 7 shows the surface potential differences at the bonding interface of Ti/Mg alloy bonding
materials
bonded475 °Cpotential
for 1 h before
corrosion
From the interface
surface potential
curves,Figure7 shows
the atsurfacedifferencesat test.
the bondingof Ti/Mgalloythebonding
˝
potential
gradients
weremeasuredatthebonding
interface
(at
dot
line
position)inallbonding
materials bonded at 475 C for 1 h before corrosion test. From the surface potential curves, the potential
materials from three different positions. This potential gradient referred to a rate of surface potential
gradients were measured atFigure
the bonding
interface(atsurface
dot line
position)in all bonding materials from
6. Effectof Al
contentonof Mgoralloys.
change at the bonding interface
becausean existence
ofpotential
interaction
diffusion layer [13]. The
three different
This potential
gradient
referred
to aa large
rate ofsurfacechange at theexistencepositions.
of the interaction
or diffusion
layer will
prevent
andsuddenpotential
surface potential
Figure 7becauseshows the surface
potentialat thebonding interface
of
Ti/Mg
alloy
bonding of thebondingchangeinterfaceexistenceof differencesinteraction
or diffusion
layercorrosion
[13].
The
existenceat the bonding aninterface,resultingin a reductionof the galvanic
damages.
Formaterials bonded at 475 °C for 1 h before corrosion test. From the surface potential curves, theTi/AZ31B(ST) dissimilarmaterial,
SPDa between
Ti sudden
and AZ31B(ST) wasmeasured
to beat0.81
interaction
or diffusion
layer will
prevent
large andsurfacepotentialchangethe Vbonding
potential gradients were measured at the bonding interface (at dot line position) in all bonding(Figure7a). Thisvalue waslarge,and maydamages.
result in severe
galvanic corrosion
atinterface,resultingin aSPD
reduction
ofconsiderably
the galvanic
corrosion
For Ti/AZ31B(ST) dissimilarmaterials from three different positions. This potential gradient referred to a rate of surface potential
the bonding interface. The calculated slope, which represented a potential gradient of Ti/AZ31B (ST),material,change
SPD between
Ti andinterface
AZ31B (ST)wasanmeasured
to be0.81 V (Figure7a). layerThis SPDvalue wasat the bonding
becauseexistence ofinteraction
or diffusion[13]. The
was 0.77 V/µm. This value was rather high because of sudden change in surface potential at theconsiderablylarge,
andmay
resultinseveregalvanic
corrosionatthebonding
interface.
Thecalculated
existence of the interaction or diffusion layer will prevent a large and sudden surface potential
bonding interface, and a poor galvanic corrosion resistance of this dissimilar material was predicted.
changeatthebonding
interface,resultinginareductionofthegalvanic
corrosiondamages.
For rather
slope, which
represented
a potential
gradientof bonding
Ti/AZ31B(ST),ofwas
0.77 V/µm.
Thisvalue towas
The suddenchange insurface potential
at theinterface
Ti/AZ31B(ST) wasattributeda
Ti/AZ31B(ST)
dissimilarmaterial,SPD
between
Ti
and
AZ31B
(ST)
was
measured
to
be
0.81
V
high becauseof suddenchangeinAl,surfacepotentialat thelayer
bonding
interface,and a withpoorthegalvanic
poor reaction
betweenTi andas a thin
Al diffusionof 30 nm
was observed(Figure 7a). This SPD value was considerably large, and may result in severe galvanic corrosion atformationof thinTi3Aldissimilar
layer in someareas (Figure4a). Ti/AZ61
also showsa large
SPD andcorrosion
resistanceof thismaterial
was predicted.
The(ST)suddenchangein surface
potential
the bonding interface. The calculated slope, which represented a potential gradient of Ti/AZ31B (ST),potential
gradient
between
Ti
and
Mg
alloy
similarto
Ti/AZ31B(ST)(Figure
7b).
This
was
explainedat the bonding
interfaceof
Ti/AZ31B(ST)
was
attributed
to
a
poor
reaction
betweenTi
and
was 0.77 V/µm. This value was rather high because of sudden change in surface potential at theAl, as a
by a similar characteristic at the bonding interface between Ti/AZ31B (ST) and Ti/AZ61 (ST): a thin
thin Al diffusion
layer of
30anm
observed
withthe formationof thinmaterial
Ti3 Al layer
in some areas
bonding interface,andpoorwasgalvanic
corrosion
resistanceof this dissimilar
was predicted.
Al diffusion and Ti3Al layer were observed. In the case of Ti/AZ80 (ST), the measured SPD value was
The suddenchangesurfacepotentialat theSPD
bondingof Ti/AZ31B
was attributed
a alloy(Figure 0.75
4a).
Ti/AZ61(ST)inalso
shows
a largeandinterface
potential
gradient(ST)betweenTi andtoMg
V (Figure 7c). This indicated that the galvanic corrosion resistance of Ti/AZ80 (ST) was superior
poor
reaction between
Ti and7b).
Al, This
as a thinAl
diffusion bylayer
of 30 nm
was observed atwiththesimilar to
Ti/AZ31B(ST)(Figurewas
explainedasimilarcharacteristic
thebonding
to Ti/AZ31B (ST) and Ti/AZ61 (ST) [18]. The potential gradient of Ti/AZ80 (ST) was also much lower
formation of thin Ti3Al layer in some areas (Figure 4a). Ti/AZ61 (ST) also shows a large SPD andinterfacethan
betweenTi/AZ31B
and Ti/AZ61
a thin
AlV/µm.
diffusion
Ti3 Allayerinwere
observed.
Ti/AZ31B(ST) and(ST)Ti/AZ61(ST) with a(ST):value
of 0.38
The and
gradualchangesurfacepotential gradient between Ti and Mg alloy similar to Ti/AZ31B (ST) (Figure 7b). This was explained
potentialatthebonding
interfaceof
Ti/AZ80(ST)
confirmedthat
thismaterial
possessedagood
In the case of Ti/AZ80 (ST), the measured SPD value was 0.75 V (Figure 7c). This indicated that theby a similar characteristic at the bonding interface between Ti/AZ31B (ST) and Ti/AZ61 (ST): a thincorrosion
resistance.
The SPD andgradienttoofTi/AZ31B
Ti/AZ91 (ST)(ST)
wereandsimilarto that ofgalvanicgalvanic
corrosion
resistanceof Ti/AZ80(ST)potential
was
Ti/AZ61(ST) [18].
Al
diffusion and
Ti3Al layer
were observed.
In
the superior
case of Ti/AZ80(ST), the measuredSPDvalue was
Ti/AZ80 (ST) with a value of 0.77 V and 0.46 V/µm, respectively (Figure 7d). These results suggested
The potential
Ti/AZ80(ST)
also muchlower
than Ti/AZ31B
Ti/AZ61 (ST)0.75 V gradient(Figure 7c).ofThis
indicated
thatwasthe galvanic
corrosion
resistanceof Ti/AZ80(ST)(ST) andwas superior
that Ti/AZ80 (ST) and Ti/AZ91 (ST) should possess similar galvanic corrosion resistances.
to Ti/AZ31B
and TheTi/AZ61(ST) [18].
The potential
gradientof Ti/AZ80(ST) was alsomuch lower
with a valueof 0.38 (ST)V/µm.
gradualchangein surface
potentialat the bonding
interfaceof Ti/AZ80
than Ti/AZ31B
and Ti/AZ61(ST) witha valueof 0.38
V/µm. The
gradual change
in andsurface(ST) confirmedthat this(ST)material
possessed
a goodgalvanic
corrosion
resistance.
The SPD
potential
potential at the bonding interface of Ti/AZ80 (ST) confirmed that this material possessed a good
gradient of Ti/AZ91 (ST) were similar to that of Ti/AZ80 (ST) with a value of 0.77 V and 0.46 V/µm,
galvanic corrosion resistance. The SPD and potential gradient of Ti/AZ91 (ST) were similar to that ofrespectively(Figure 7d). These results suggested that Ti/AZ80 (ST) and Ti/AZ91 (ST) should possess
Ti/AZ80 (ST) with a value of 0.77 V and 0.46 V/µm, respectively (Figure 7d). These results suggested
similar galvanic
corrosionthat Ti/AZ80(ST) andresistances.
Ti/AZ91 (ST) should possess similar galvanic corrosion resistances.

Figure 7. Changes in surface potential measured across the bonding interface of: (a) Ti/AZ31B (ST);(b) Ti/AZ61 (ST); (c) Ti/AZ80 (ST); and (d) Ti/AZ91 (ST) bonded at 475 °C for 1 h.

Figure 7. Changes in surface potential measured across the bonding interface of: (a) Ti/AZ31B (ST);

Figure 7.(b)Changesin surface potential measured across the bonding interface of: (a) Ti/AZ31B (ST);Ti/AZ61 (ST); (c) Ti/AZ80 (ST); and (d) Ti/AZ91 (ST) bonded at 475 °C for 1 h.
(b) Ti/AZ61 (ST); (c) Ti/AZ80 (ST); and (d) Ti/AZ91 (ST) bonded at 475 ˝ C for 1 h.

Materials 2016, 9, 665

8 of 113.4. Corrosion Behavior Analysis of Dissimilar Materials
Figure 8 shows the line profiles of Ti/Mg alloy (ST) dissimilar materials at the bonding interface
after corrosion test in Kroll’s etchant. From these results, a galvanic corrosion between Ti and Mg
alloys (ST) was confirmed by a characteristic of the plot that the surface level of Mg alloys (ST) near
the bonding interface was lower than areas far from the surface. The galvanic corrosion depth (G.D.)
and galvanic corrosion width (G.W.) were measured to evaluate a degree of galvanic corrosion in eachbonding sample. For Ti/AZ31B (ST), high G.W. and G.D. of 510.1 and 29.8 µm, respectively, were
measured. The large gap at the bonding interface between Ti and AZ31B (ST) caused by galvanic
corrosion was clearly observed (Figure 8a). The surface of Ti was not damaged after corrosion test,
which confirmed that corrosion mostly occurred on Mg alloy (ST) surface. This result also corresponded
well to SPD between Ti and AZ31B (ST), which exhibited the highest value among bonding materials.
Ti/AZ61 (ST) showed a similar G.D. compared to Ti/AZ31B (ST) but a large decrease in G.W. witha value of 457.4 µm (Figure 8b). This was explained by an increasing of Al diffusion and Ti3 Al layer
at the bonding interface, which further reduced SPD between Ti and Mg alloys (ST) compared to
Ti/AZ31B (ST). Although thickness of Ti3 Al and Al diffusion layer was at the nano-level, it wasable to inhibit the progress of galvanic corrosion from the bonding interface to further areas in Mg
alloy [19]. The G.W. and G.D. were further decreased in Ti/AZ80 (ST) with values of 346.1 and 26.5 µm,
respectively (Figure 8c). This referred to an improvement of galvanic corrosion resistance of Ti/AZ80(ST) compared to Ti/AZ61 (ST) with a decrease in SPD. Ti/AZ91 (ST) exhibited the lowest G.W. and
G.D. with a value of 281.5 and 19.1µm, respectively (Figure 8d). The gap between Ti and Mg alloy (ST)was very small compared to other bonding materials. The formation of thick and continuous Ti3 Al
layer play an impotant role in the improvement of galvanic corrosion resistance in this sample.
Figure 9 shows the corroded surfaces of Ti/Mg alloy (ST) bonding materials bonded at 475 ˝ C
for 1 h on Mg alloy (ST) side. AZ31B (ST), which possessed the highest surface potential of 1.73 V,
shows much severer corrosion damage compared to other bonding materials (Figure 9a). Many large
pitting corrosion areas were observed throughout the corroded surface. The black area near the pitting
corrosion damage was an area that suffered severe corrosion, and had a lower surface level comparedto the adjacent area (grey area matrix). AZ61 (ST) possessed a lower surface potential compared to
AZ31B (ST), shows less severe damage compared to AZ31B (ST), the pitting corrosion areas are smaller(Figure 9b) and the black areas mostly disappeared. This indicated that the corrosion resistance ofAZ61 (ST) was clearly improved compared to AZ31B (ST). For AZ80 (ST), the amounts of small pitting
corrosion areas were decreased compared to AZ61 (ST) because the surface potential of AZ80 (ST)was lower than AZ61 (ST) (Figure 9c). This indicated that the corrosion resistance of AZ80 (ST) was
further improved compared to AZ61 (ST). AZ91 (ST) shows superior corrosion resistance comparedto other Mg alloys because the pitting corrosion damages were barely observed on the corroded
surface. However, deep corroded areas were observed in many locations because of the strong acidic
characteristic of Kroll’s etchant.
Figure 10 shows a mass loss of Ti/Mg alloy (ST) dissimilar materials and Mg alloys (ST) parent
metal after corrosion test in Kroll etchant for 10 min. The mass loss gradually decreased as Al content
in Mg alloy (ST) increased. This was explained by the decreasing surface potential of Mg alloy when
Al content increased, and the results also corresponded well to the corroded surface on Mg alloy side,in which high Al content Mg alloy showed light corrosion damage (Figure 9). The mass losses of
Mg alloy (ST) parent metal were also measured after corrosion test. The mass losses of AZ31B (ST)and AZ61 (ST) parent metal were slightly lower compared to their bonding materials. This may be
explained by the formation of large gap in Ti/AZ31B (ST) and Ti/AZ61 (ST) due to galvanic corrosion(Figure 9a,b). For Ti/AZ80 (ST) and Ti/AZ91 (ST), the mass losses after corrosion test were similar
to their bonding materials due to smaller galvanic corrosion gaps compared to Ti/AZ31B (ST) and
Ti/AZ61 (ST), which represented the good galvanic corrosion resistance of these materials. In thisstudy, the formation of Ti3 Al layer was found to control the tensile strength and galvanic corrosion
resistance of dissimilar materials.

Materials 2016, 9, 665
Materials
Materials2016,
2016,9,9,665
665

9 of 119 9ofof1111

Figure8.8.8.LineLine
profileanalysisatthe
thebonding
interfaces
after
corrosionin Kroll’s
etchant:
Figurebonding
interfaces
after
corrosion
test
etchant:(a)FigureLineprofile
profileanalysis
analysisatat
thebonding
interfaces
after
corrosion
testintest
inKroll’s
Kroll’setchant:
(a)
(ST);
(b)
Ti/AZ61
(ST);
(c)
(ST);
Ti/AZ91(ST).
(a)Ti/AZ31B
Ti/AZ31B(ST);(b)
Ti/AZ61(ST);(c)
Ti/AZ80(ST);and(d) Ti/AZ91
Ti/AZ31B(ST);(b)
Ti/AZ61(ST);(c)Ti/AZ80
Ti/AZ80(ST);andand(d)(d)Ti/AZ91(ST). (ST).

FigureFigure9.9.Corroded
Corrodedsurfaces
surfaceson
onMg
Mgalloy
alloy(ST)(ST)side
sideofofTi/Mg
Ti/Mgalloys
alloys(ST)(ST)dissimilar
dissimilarmaterial:material:(a)(a)AZ31B
AZ31B
Figure 9. Corroded surfaces on Mg alloy (ST) side of Ti/Mg alloys (ST) dissimilar material: (a) AZ31B
(ST);
(b)
AZ61
(ST);
(c)
AZ80
(ST);
and
(d)
AZ91
(ST).
(ST); (b) AZ61 (ST); (c) AZ80 (ST); and (d) AZ91 (ST).
(ST); (b) AZ61 (ST); (c) AZ80 (ST); and (d) AZ91 (ST).

Materials 2016, 9, 665
Materials 2016, 9, 665

10 of 11
10 of 11

FigureEffectofof
content
mass
loss
Ti/Mgalloy
alloy(ST)(ST)bonding
bondingmaterials
materialsand
andparent
parentMg
Figure10.10.Effect
AlAlcontent
onon
mass
loss
of of
Ti/Mg
Mg
alloys(ST)
after
corrosion
test
Kroll’setchant.
alloys(ST)
after
corrosion
test
in in
Kroll’setchant.

Conclusions
4.4.Conclusions
andMg
Mgalloys
alloys(ST)(ST)were
weresuccessfully
successfullybonded
bondedwith
withananadvanced
advancednew
newmethod,
method,spark
plasma1.1. TiTiand
spark
plasmasintering
without
applying
any
inserted
sheet.
The
surfaces
of
Ti
and
Mg
alloy
(ST)
were
sintering without applying any inserted sheet. The surfaces of Ti and Mg alloy (ST) were perfectly
perfectlywithout
contacted
without
cracksor voids
the highbonding pressure.
contacted
anycracks any
or voids
becauseof because
the high ofbondingpressure.
2. The formation of Al diffusion and Ti3Al layer at the nano-level were proven by controlling the2.
The formation of Al diffusion and Ti3 Al layer at the nano-level were proven by controllingbonding strength of bonding materials. A uniform and thick Ti3Al layer (about 50 nm) was
the bonding strength of bonding materials. A uniform and thick Ti3 Al layer (about 50 nm)
required in order to obtain the maximum bonding strength and bonding efficiency of dissimilar
was required in order to obtain the maximum bonding strength and bonding efficiency ofmaterials.
dissimilar materials.
3. The galvanic corrosion of dissimilar materials could be improved by formation of Ti3Al IMC
3.
The galvanic corrosion of dissimilar materials could be improved by formation of Ti3 Al IMC layer.
layer. This IMC layer has a surface potential between Ti and Mg alloy, which prevents large
This IMC layer has a surface potential between Ti and Mg alloy, which prevents large and abrupt
and abrupt changes in surface potential at the bonding interface, and improves galvanic
changes in surface potential at the bonding interface, and improves galvanic corrosion resistancecorrosion resistance4.
The mass loss of dissimilar materials was controlled by Al content in Mg alloy. The high Al
4. The mass loss of dissimilar materials was controlled by Al content in Mg alloy. The high Al
content in Mg alloy contributed to a formation of a stable Mg–Al–O film, which lowers the surface
content in Mg alloy contributed to a formation of a stable Mg–Al–O film, which lowers thepotential, and resulted in high corrosion resistance on Mg alloy matrix. The galvanic corrosion
surface potential, and resulted in high corrosion resistance on Mg alloy matrix. The galvanic
may contribute to mass loss because of severe galvanic corrosion, such as in Ti/AZ31B (ST).
corrosion may contribute to mass loss because of severe galvanic corrosion, such as in
Ti/AZ31B (ST).
Author Contributions: Patchara Pripanapong designed, conducted the experiments and analyzed the results.
Junko
Umeda
and Katsuyoshi
Kondohsupervised
the experiments.
Kariya helped
surface thepotential
AuthorContributions:
Patchara
Pripanapong
designed,
conductedShota
the experiments
andin
analyzed
results.
measurement
by
SKPFM.
MakotoTakahashi
contributedtoTEM
observation.
Seiichiro
Tsutsumi
conducted
Junko Umeda and Katsuyoshi Kondoh supervised the experiments. Shota Kariya helped in surface potential
the surface profile measurement by surface analysis microscope and Tachai Luangvaranunt helped inmeasurement by SKPFM. Makoto Takahashi contributed to TEM observation. Seiichiro Tsutsumi conductedmanuscript writing.
the surface profile measurement by surface analysis microscope and Tachai Luangvaranunt helped in
Conflicts
of Interest:
manuscript
writing.The authors declare no conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest.
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