S.-H. CHANG et al.: SELECTIVE LEACHING AND SURFACE PROPERTIES OF TiNiFe SHAPE-MEMORY ALLOYS
251–257
SELECTIVE LEACHING AND SURFACE PROPERTIES OF TiNiFe
SHAPE-MEMORY ALLOYS
SELEKTIVNO IZPIRANJE IN POVR[INSKE LASTNOSTI ZLITIN
TiNiFe S SPOMINOM
Shih-Hang Chang, Jyun-Sian Liou, Bo-Yen Huang
National I-Lan University, Department of Chemical and Materials Engineering, I-Lan 260, Taiwan
shchang@niu.edu.tw
Prejem rokopisa – received: 2015-09-02; sprejem za objavo – accepted for publication: 2016-04-21
doi:10.17222/mit.2015.269
This study investigated the selective leaching and surface characteristics of Ti50Ni50–xFex (x = 1, 2, and 3) shape-memory alloys
using inductively coupled plasma-mass spectrometry, X-ray diffractometry, electrochemical tests and X-ray photoelectron
spectroscopy. According to our results, the concentrations of Ni and Fe ions selectively leached from each specimen were
considerably higher than that of Ti ions. Electrochemical tests revealed a gradual deterioration in the corrosion resistance of the
Ti50Ni50–xFex SMAs as the Fe content in the alloys was increased. X-ray photoelectron-spectroscopy results indicate that the
surface of each specimen is primarily made up of a passive TiO2 film. NiO and Fe2O3 oxides, which also formed on the surfaces
of the Ti50Ni50–xFex SMAs, caused a deterioration of the uniformity, undermining the protective effect of the TiO2 films,
resulting in a highly selective leaching of the Ni and Fe ions. The Ti50Ni50–xFex SMAs exhibit a number of favourable properties
compared to the other SMAs; however, high concentrations of selectively leached Ni and Fe ions may pose a risk in biomedical
applications, particularly when used as implant materials.
Keywords: TiNiFe shape-memory alloys, biomaterials, selective leaching, corrosion, X-ray photoelectron spectroscopy
V {tudiji je prou~evano selektivno izpiranje in zna~ilnosti povr{ine zlitine s spominom Ti50Ni50–xFex (x = 1, 2, in 3), z uporabo
masne spektrometrije z induktivno sklopljeno plazmo, rentgensko difrakcijo, elektrokemijskimi preizkusi in rentgensko
fotoelektronsko spektroskopijo. Rezultati ka`ejo, da je bila koncentracija selektivno izpranih Ni in Fe ionov iz vsakega vzorca
veliko ve~ja kot pa Ti ionov. Elektrokemijski preizkusi so pokazali postopno zmanj{anje korozijske odpornosti Ti50Ni50–xFex
SMA, ko je vsebvnost Fe v zlitinah nara{~ala. Rezultati rentgenske fotoelektronske spektroskopije ka`ejo, da je povr{ina vseh
vzorcev predvsem sestavljena iz pasivnih TiO2 plasti. NiO in Fe2O3 oksidi, ki so tudi nastali na povr{ini Ti50Ni50–xFex SMAs,
poslab{ajo enotnost in ogro`ajo varovalno plast iz TiO2 prevlek, kar ima za posledico bolj selektivno izpiranje ionov Ni in Fe.
Ti50Ni50–xFex SMAs vsebuje {tevilne ugodne lastnosti v primerjavi z drugimi SMA; vendar pa velika koncentracija selektivno
izpranih Ni in Fe ionov lahko predstavlja tveganje pri biomedicinski uporabi, posebno pri implantiranih materialih.
Klju~ne besede: TiNiFe zlitine s spominom, biomateriali, selektivno izpiranje, korozija, rentgenska fotoelektronska
spektroskopija
1 INTRODUCTION
Nickel-titanium shape-memory alloys (TiNi SMAs)
are widely used in advanced engineering applications
due to their favourable shape memory and superelastic
properties.1 Most TiNi SMAs further exhibit a low
cytotoxicity and a good biocompatibility2–4 and thus they
are also suitable for biomedical applications, such as
laparoscopic surgery, stents, shape-memory microvalves,
and osteosynthesis devices.5–7 It was reported that sub-
stituting Fe for Ni in TiNi SMAs induces the formation
of the R-phase during the martensitic transformation and
leads to some advanced mechanical properties superior
to those of typical TiNi SMAs.8,9
W. J. Moberly et al.10 investigated the deformation,
twinning and thermo-mechanical strengthening of
Ti50Ni47Fe3 SMAs. Their results demonstrated that cold-
worked and annealed Ti50Ni47Fe3 SMAs had a refined
subgrain size, a high yield strength, and a good ductility.
In addition, several studies reported on TiNiFe SMAs
with ultra-high internal-friction properties, which are
excellent candidates for high-damping applications.11–13
Recently, D. Wang et al.14 established a complete tempe-
rature-composition phase diagram that included the
pre-martensitic state, martensite and strain glass. They
also reported that strain glass forms in TiNiFe SMAs
when the Fe doping exceeds the critical value.
Several articles reveal that TiNiFe SMAs are
candidate materials for biomedical applications. C. Li
and Y. F. Zheng15 investigated the electrochemical
behaviours of Ti50Ni47Fe3 SMAs and found that the
surface of a Ti50Ni47Fe3 SMA mainly consists of TiO2,
which is responsible for the good biocompatibility and
anti-corrosion properties of the alloys. T. A. Tabish et
al.16 further performed an in-vivo cytotoxic evaluation of
TiNiFe SMAs and found that they do not exhibit any
appreciable cytotoxic or systematic reactions to living
systems. However, when SMAs are used as implant
materials, interactions between the alloys and the living
tissue can lead to the corrosion of the surface oxide
layer, thereby increasing the risk of metal ions being
released into the body. These metallic ions pose a
Materiali in tehnologije / Materials and technology 51 (2017) 2, 251–257 251
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 669.017.13:620.164:543.428.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(2)251(2017)
potential health hazard capable of inducing allergic
reactions or even promoting the onset of cancer.17–19
Several researches had previously investigated the
selective leaching behaviour of TiNi and TiNiCu
SMAs20–23; however, the selective leaching of Ti, Ni, and
Fe ions released from TiNiFe SMAs has not been
studied. Therefore, this study investigated the selective
leaching and surface properties of Ti50Ni50–xFex (x = 1, 2,
and 3) SMAs using inductively coupled plasma mass
spectrometry (ICP-MS), an X-ray diffraction (XRD)
analysis, electrochemical tests, and X-ray photoelectron
spectroscopy (XPS).
2 EXPERIMENTAL PART
The Ti50Ni50–xFex (x = 1, 2, and 3) SMAs used in this
study were prepared from pure raw titanium (a purity of
99.9 % mass fraction), nickel (a purity of 99.9 % mass
fraction), and iron (a purity of 99.98 % mass fraction).
The raw materials were re-melted using conventional
vacuum arc remelting to form ingots in an argon atmo-
sphere. Each ingot was hot-rolled at 900 °C using a
rolling machine (DBR150x200 2HI-MILL, Daito Seiki
Co, Japan) to form a 2 mm plates, which were then
solution-heat-treated at 900 °C for 1 h and quenched in
water. The surface oxide layer of a plate was removed
using an etching solution of HF:HNO3:H2O at a volume
ratio of 1:5:20. Each plate was then cut into bulk samples
with dimensions of (30.0  4.0  2.0) mm for charac-
terization.
The crystallographic features of each Ti50Ni50–xFex
SMA were determined using a Rigaku IV XRD instru-
ment with Cu-K radiation ( = 0.154 nm) at room
temperature. The selective leaching properties of
Ti50Ni50–xFex SMAs were evaluated by immersing
samples in test flasks containing 500 mL of Ringer’s
solution. Ringer’s solution was used in selective-leaching
tests because it is an isotonic solution similar to bodily
fluids and is widely used in in-vitro experiments. Each
test flask was maintained at 37 °C in an orbital shaker
incubator for 80 d. We then measured the concentrations
of the released Ti, Ni, and Fe ions in Ringer’s solution
using ICP-MS (Agilent 7500ce).
Electrochemical measurements of the Ti50Ni50–xFex
SMAs were performed using an electrochemical work-
station (ECW-5600, Jiehan) to determine the cathodic
and anodic polarization Tafel curves, where a platinum
plate was used as the counter electrode, a saturated
calomel electrode (SCE) was used as the reference
electrode and Ringer’s solution was used as the test solu-
tion. The average corrosion potential (Ecorr) and the
average corrosion current density (icorr) values of each
specimen were calculated from seven Tafel curves, for
which the maximum and minimum values were deleted.
The surface chemical composition of the Ti50Ni50–xFex
SMAs was analysed using an XPS device (Thermo
Scientific (VGS) K-Alpha) with a monochromatic Al-K
radiation source of 1468.6 eV. The survey spectrum of
each specimen was measured over a range of 200–1200
eV in 1-eV steps. High-resolution Ti, Ni, and Fe 2p
spectra for each specimen were determined in 0.05-eV
steps. The XPSPeak 4.1 software was used for the anal-
ysis of XPS spectra.
3 RESULTS
3.1 XRD results
Figure 1 presents the XRD results of the
Ti50Ni50–xFex SMAs, where each Ti50Ni50–xFex SMA
sample exhibits diffraction peaks related to (110)B2,
(200)B2, and (211)B2 at approximately 2 = 42.2°, 61.3°,
and 77.5°, respectively. Figure 1 further reveals that all
of the Ti50Ni50–xFex SMAs used in this study were in the
parent phase at room temperature, indicating that the
surface relief, which is normally observed at the
R-phase, or the B19’ martensite of TiNi-based SMAs,
had no influence on the selective leaching.
3.2 Selective leaching behaviours
Figures 2a to 2c present the concentrations of the Ti,
Ni, and Fe ions, respectively, which were selectively
leached from the Ti50Ni50–xFex SMAs in Ringer’s solution
as a function of the immersion time. Figure 2a shows
that the concentration of the Ti ions selectively leached
from the specimens was extremely low (< 1·10–8)
throughout the 80 d of the immersion. Figure 2b shows
that the concentrations of the Ni ions selectively leached
from the Ti50Ni50–xFex SMAs were also extremely low
during the first 30 d; however, these concentrations
increased significantly to above 1.5·10–6 ppb by day 80.
This feature indicates that the selective leaching rate of
the Ni ions from the Ti50Ni50–xFex SMAs is considerably
high. As shown in Figure 2c, the concentrations of the
Fe ions selectively leached from the Ti50Ni50–xFex SMAs
S.-H. CHANG et al.: SELECTIVE LEACHING AND SURFACE PROPERTIES OF TiNiFe SHAPE-MEMORY ALLOYS
252 Materiali in tehnologije / Materials and technology 51 (2017) 2, 251–257
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: XRD pattern for Ti50Ni50–xFex SMAs
Slika 1: Rentgenogram Ti50Ni50–xFex SMA
were approximately 5·10–8 after 80 d. No obvious
differences in the concentration of the Fe ions were
observed among alloys with different chemical compo-
sitions. According to the results shown in Figure 2, the
concentration of the Ni ions selectively leached from
each of the specimens was considerably higher than that
of the Ti and Fe ions, indicating that Ni ions are more
easily released from the surface of Ti50Ni50–xFex SMAs
than the Ti or Fe ions.
3.3 Electrochemical properties
Figure 3 presents the selected cathodic and anodic
polarization Tafel curves obtained from Ringer’s solution
containing Ti50Ni50–xFex SMAs. The average corrosion
potential (Ecorr) and corrosion-current density (icorr) of
each sample are listed in Table 1. The Ecorr values of
Ti50Ni50–xFex SMAs gradually decreased from approxi-
mately –0.379 V to –0.433 V when the Fe content in the
alloys was increased from 1 to 3. This indicates that,
when immersed in Ringer’s solution, the Ti50Ni50–xFex
SMAs with a lower Fe content exhibit a better corrosion
resistance, superior to that of the other SMAs. Table 1
also shows that, in the presence of an elevated Fe
content, the icorr values of the Ti50Ni50–xFex SMAs
gradually increased from (3.27±0.86) × 10–7 A/cm2 to
(5.52±1.05) × 10–6 A/cm2, demonstrating a gradual
increase in the corrosion rate of theTi50Ni50–xFex SMAs
when alloys had a higher Fe content.
Table 1: The average Ecorr and Icorr values determined according to
the cathodic and anodic polarization Tafel curves from Figure 3
Tabela 1: Srednje vrednosti Ecorr in Icorr dolo~ene pri katodni in
anodni polarizaciji iz Taflovih krivulj iz Slike 3
Sample
Avg. Ecorr
(V)
Avg. Icorr
(A/cm2)
Ti50Ni49Fe1 -0.379±0.017 (3.27±0.68)×10–7
Ti50Ni48Fe2 -0.394±0.006 (3.54±0.66) ×10–6
Ti50Ni47Fe3 -0.433±0.007 (5.52±1.05) ×10–6
3.3 X-ray photoelectron spectroscopy
Figures 4a to 4c present the XPS survey spectra of
the Ti50Ni49Fe1, Ti50Ni48Fe2, and Ti50Ni47Fe3 SMAs, res-
pectively. Each specimen exhibited significant characte-
ristic peaks associated with Ti (a Ti 2p peak at approxi-
mately 460 eV), Ni (a Ni 2p peak at approximately 853
eV), O (an O 1s peak at approximately 531 eV), and
contamination C (a C 1s peak at approximately 285 eV).
S.-H. CHANG et al.: SELECTIVE LEACHING AND SURFACE PROPERTIES OF TiNiFe SHAPE-MEMORY ALLOYS
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 3: Cathodic and anodic polarization Tafel curves for
Ti50Ni50–xFex SMAs
Slika 3: Taflova krivulja katodne in anodne polarizacije Ti50Ni50–xFex
SMAs
Figure 2: Concentrations of: a) Ti, b) Ni, and c) Fe ions selectively
leached from Ti50Ni50–xFex SMAs
Slika 2: Koncentracije: a) Ti, b) Ni in c) Fe ionov, selektivno izpranih
iz Ti50Ni50–xFex SMA
Figure 4 also reveals an insignificant Fe 2p peak at
approximately 710 eV for each specimen.
Figures 5a to 5c present the Ti 2p XPS spectra of the
Ti50Ni49Fe1, Ti50Ni48Fe2, and Ti50Ni47Fe3 SMAs, respec-
tively. Figure 5a shows that the Ti 2p characteristic
peaks of the Ti50Ni49Fe1 SMA can be divided into four
oxidation states, Ti4+, Ti3+, Ti2+, and Ti0, corresponding to
TiO2, Ti2O3, TiO, and metallic Ti, respectively.24–26 As
shown in Figure 5a, the TiO2 peak was more prominent
than the other peaks, indicating that TiO2 was the domi-
nant oxide layer on the surface of the Ti50Ni49Fe1 SMA.
Figures 5b and 5c show that the Ti 2p XPS spectra of
the Ti50Ni48Fe2 and Ti50Ni47Fe3 SMAs were very similar
to that of the Ti50Ni49Fe1 SMA shown in Figure 5a. This
suggests that the surfaces of the Ti50Ni48Fe2 and
Ti50Ni47Fe3 SMAs were also primarily composed of a
TiO2 oxide layer.
Figures 6a to 6c present the Ni 2p XPS spectra of the
Ti50Ni49Fe1, Ti50Ni48Fe2, and Ti50Ni47Fe3 SMAs, respec-
tively. Figure 6a shows that the Ni 2p characteristic
S.-H. CHANG et al.: SELECTIVE LEACHING AND SURFACE PROPERTIES OF TiNiFe SHAPE-MEMORY ALLOYS
254 Materiali in tehnologije / Materials and technology 51 (2017) 2, 251–257
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Ti 2p XPS spectra of the surfaces of: a) Ti50Ni49Fe1,
b) Ti50Ni48Fe2, and c) Ti50Ni47Fe3 SMAs
Slika 5: Ti 2p XPS spektri povr{in: a) Ti50Ni49Fe1, b) Ti50Ni48Fe2 in
c) Ti50Ni47Fe3 SMAs
Figure 4: XPS survey spectra of the surfaces of: a) Ti50Ni49Fe1,
b) Ti50Ni48Fe2, and c) Ti50Ni47Fe3 SMAs
Slika 4: XPS-spektri povr{in: a) Ti50Ni49Fe1, b) Ti50Ni48Fe2 in
c) Ti50Ni47Fe3 SMAs
peaks of the Ti50Ni49Fe1 SMA can be divided into the
metallic-Ni and NiO-oxidation states. We also observed
two small shoulders corresponding to the satellite (sat.)
peaks of the metallic Ni and NiO near the characteristic
peaks. Figures 6b and 6c show that the Ni 2p XPS
spectra of the Ti50Ni48Fe2 and Ti50Ni47Fe3 SMAs were
nearly identical to that of the Ti50Ni49Fe1 SMA,
suggesting an abundance of metallic Ni atoms on the
surfaces of Ti50Ni50–xFex SMAs. Figures 7a to 7c present
the Fe 2p XPS spectra of the Ti50Ni49Fe1, Ti50Ni48Fe2, and
Ti50Ni47Fe3 SMAs, respectively. Compared to the XPS
spectra of Ti 2p and Ni 2p in Figures 5 and 6, the
intensity of the Fe 2p XPS spectrum was relatively low.
Figures 7a to 7c show that the Fe 2p characteristic peaks
of the Ti50Ni50–xFex SMAs can be divided into Fe2O3 and
metallic Fe peaks, in which Fe2O3 is the dominant oxide
characterizing the surfaces of Ti50Ni50–xFex SMAs.
S.-H. CHANG et al.: SELECTIVE LEACHING AND SURFACE PROPERTIES OF TiNiFe SHAPE-MEMORY ALLOYS
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Figure 7: Fe 2p XPS spectra of the surfaces of: a) Ti50Ni49Fe1,
b) Ti50Ni48Fe2, and c) Ti50Ni47Fe3 SMAs
Slika 7: Fe 2p XPS-spektri povr{in: a) Ti50Ni49Fe1, b) Ti50Ni48Fe2 in
c) Ti50Ni47Fe3 SMA
Figure 6: Ni 2p XPS spectra of the surfaces of: a) Ti50Ni49Fe1,
b) Ti50Ni48Fe2, and c) Ti50Ni47Fe3 SMAs
Slika 6: Ni 2p XPS-spektri povr{in: a) Ti50Ni49Fe1, b) Ti50Ni48Fe2 in
c) Ti50Ni47Fe3 SMA
4 DISCUSSION
According to the Ti 2p XPS spectra in Figure 5, the
Ti atoms near the surfaces of the Ti50Ni50–xFex SMAs
were covered mainly with a film of a TiO2 oxide. This
result can be explained with a high thermodynamic
stability of TiO2.27–30 Furthermore, the TiO2 films that
formed on the surfaces of Ti50Ni50–xFex SMAs are highly
corrosion resistant, which leads to extremely low selec-
tive leaching of the Ti ions, as demonstrated in Fig-
ure 2a. In contrast, Figure 2b shows that comparatively
higher concentrations of Ni ions were leached from the
Ti50Ni50–xFex SMAs, indicating that the selective leaching
rate of the Ni ions was much higher than that of the Ti
ions, which was, in turn, due to an abundance of metallic
Ni atoms on the surfaces of the Ti50Ni50–xFex SMAs, as
shown in Figure 6. Figure 2b also reveals that the con-
centrations of the Ni ions selectively leached from the
Ti50Ni50–xFex SMAs gradually increased with an increase
in the Fe content in the Ti50Ni50–xFex SMAs. This corres-
ponds to the fact that the corrosion resistance of the
Ti50Ni50–xFex SMAs gradually deteriorated with an
increase in the Fe content of the alloys, as revealed by
Figure 3. As shown in Figure 2c, the concentration of
the Fe ions selectively leached from the Ti50Ni50–xFex
SMAs was approximately 50 ppb, which is much lower
than that of the Ni ions. This is a direct consequence of
the fact that the Fe content in the Ti50Ni50–xFex SMAs was
below 3 atomic percent. As shown in Figure 7, this find-
ing also corresponds to the fact that most Fe atoms on
the surface of a specimen were in the form of Fe2O3
oxides, rather than metallic Fe.
A previous study found that extremely low concen-
trations of Ti and Ni ions were selectively leached from
Ti50Ni50 SMAs and attributed this to a passive TiO2
surface film inhibiting the ion movement.23 However,
during the current study, we observed high concentra-
tions of selectively leached Ni and Fe ions. This suggests
that the NiO and Fe2O3 oxides that formed on the
surfaces of the Ti50Ni50–xFex SMAs caused a deterioration
of the uniformity of the TiO2 oxide films and, therefore,
of their protective effect. Thus, despite the fact that the
Ti50Ni50–xFex SMAs exhibit a number of properties not
found in the other SMAs, the high concentrations of
selectively leached Ni and Fe ions may pose a health risk
in biomedical applications. A further surface modifica-
tion is necessary if the Ti50Ni50–xFex SMAs are to be
considered as implantation materials in human bodies.
5 CONCLUSION
All of the Ti50Ni50–xFex SMAs used in this study were
in the parent phase during the selective leaching tests.
The concentrations of the Ti ions selectively leached
from the Ti50Ni50–xFex SMAs were extremely low be-
cause the Ti atoms near the surface of the alloys under-
went oxidization to form passive TiO2 films. The con-
centration of the Ni ions selectively leached from the
Ti50Ni50–xFex SMAs gradually increased with an increase
in the Fe content in the Ti50Ni50–xFex SMAs, which can
be explained with the fact that the corrosion resistance
inversely correlated with the Fe content in the alloys.
The concentrations of the Fe ions selectively leached
from the Ti50Ni50–xFex SMAs were approximately 5×
higher than those of the Ti ions. The high concentrations
of the Ni and Fe ions released from the Ti50Ni50–xFex
SMAs were probably caused by the fact that the NiO and
Fe2O3 oxides, formed on the surfaces of the Ti50Ni50–xFex
SMAs reduced the uniformity of the TiO2 oxide films
and thereby compromised the protective effect of the
films. Selectively leached Ni and Fe ions may pose a risk
in biomedical applications; therefore, a surface modifi-
cation is required if the Ti50Ni50–xFex SMAs are to be
used as implant materials.
Acknowledgements
The authors gratefully acknowledge the financial
support for this research provided by the Ministry of
Science and Technology (MOST), Taiwan, Republic of
China, under Grant No. MOST103-2221-E-197-007.
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