Shape memory alloys (SMA) have attracted much attention over the past few decades, and NiTi SMA is one of the most studied among other SMAs     . As a typical shape memory alloy, NiTi alloys have been widely used in many applications such as industry, medicine, aerospace and construction. In recent years, NiTi films are extensively studied. It has been reported that the properties of NiTi films are similar to those of bulk NiTi SMAs. NiTi films have been tried to be applied to biomedical devices. Magnetron sputtering deposition is considered to be one of the suitable methods to process NiTi films. When NiTi films are deposited at ambient temperatures, they are usually amorphous, and the shape memory and superelasticity can be introduced by crystallization.
The main application of binary NiTi SMAs is in the field of biomedical devices. For biomedical applications, nickel-titanium SMAs can be used to manufacture esophageal, tracheal, vascular, biliary, urethral and other stents and orthopedic internal fixation devices such as embracing fixators, bone clasps, intramedullary nails, etc.   . For NiTiCu SMA, the Ni atoms are substituted by Cu atoms, which not only greatly reduce the alloy cost, but also greatly reduce the transformation temperature sensitivity to the Ti:Ni ratio, and the transformation hysteresis is greatly reduced. However, the shape memory effect is still excellent . Therefore, NiTiCu SMAs have attracted some attention from both industrial applications and academic researches   . According to the study of localized corrosion of NiTiCu alloy in different conditions at a temperature of 40˚C and a concentration of 0.9% NaCl solution, it has found that NiTiCu has a significantly lower corrosion potential than that of the NiMo binary alloy. However, the difference in the percentage of Cu atoms and the change in phase transition state will have a certain effect on the alloy . The research on Ti50Ni40Cu10 SMA shows that, the addition of Cu element makes the corrosion resistance of the alloy in Hank’s solution significantly reduced. Moreover, the experimental results of further cytotoxicity of the alloy show that the Ti50Ni40Cu10 shape memory alloy has higher cytotoxicity than the binary NiTi and Ti alloys . In a word, the safety of applying NiTiCu shape memory alloys needs further exploration.
The application range of electrochemical impedance spectroscopy (EIS) includes: corrosion and protection of metals, metal/polymer composites, semiconductor materials; polycrystalline solids, and ceramic materials. Many studies have used EIS to detect corrosion behavior of titanium alloys   . However, there are few reports on the corrosion behavior of sputtered NiTiCu shape memory alloys in simulated physiological solution. Therefore, this work used the EIS method to study the corrosion behavior of NiTiCu shape memory alloys with different Cu contents.
2. Experiment Procedures
Four kinds of SMAs (Ni49.6Ti50.4, Ni48.2Ti50.4Cu1.4, Ni45.6Ti50.4Cu4, Ni42.7Ti50.4Cu6.9) were magnetron sputtered on Si substrates. Films were co-sputtered NiTiCu and Ti (at 400 W and 70 W DC plasma power) targets at ambient temperature. The target diameter was 100 mm, and the substrate was Si3Nx coated silicon wafer. The base vacuum was 2.0 × 10−7 Torr, and the argon pressure was 2.4 mTorr. The final film thickness is around 1 μm. The films were all amorphous, and subsequent crystallization is a must to introduce the shape memory effect. The crystallization was conducted in high vacuum (basic vacuum of 5 × 10−6 Torr) at a temperature of 650˚C.
The EIS test based on the films was conducted from the potential Ecorr to 1.2 V (a saturated calomel electrode, SCE) in a step of 0.2 V at a DC potential. After immersing the sample in PBS solution at 37˚C, Ecorr was monitored for 60 minutes. When the potential was gradually increased to a subsequent value, the current was allowed to stabilize for 15 minutes before the measurement.
3. Result and Discussion
The surface oxide layer of the NiTi film was characterized using EIS, and Figure 1(a) is the complex Nyquist plot. The impedance decreases as the potential increasing from 0 to 1.2 V. When the potential is increased from 0.2 V to 0.4 V, the impedance shows a large decrement at low frequencies. As the potential is further increased to 1.0 V, the amount of impedance decrement decreases. When the potential increases to 1.2 V, the impedance decreases significantly and forms a semicircular shape. This behavior is attributed to the oxidation of the solution, not the breakdown of the oxide layer on the surface of the film  .
(a) (b) (c) (d)
Figure 1. The Nyquist plots of the four films in PBS solution at 37˚C ((a) Ni49.6Ti50.4; (b) Ni48.2Ti50.4Cu1.4; (c) Ni45.6Ti50.4Cu4; (d) Ni42.7Ti50.4Cu6.9).
The Nyquist polt of the Ni48.2Ti50.4Cu1.4 film is shown in Figure 1(b). With the potential increasing, the impedance decay behavior is similar as that of the NiTi film. When the potential is increased from 0.2 V to 0.4 V, the impedance decreases significantly. As the potential is further increased to 0.8 V, the amount of impedance decrement decreases. When the potential is increased to 1.0 V, the impedance shows a significant attenuation, and appears semicircular as the potential rises to 1.2 V. The Nyquist plot of the Ni45.6Ti50.4Cu4 film is as shown in Figure 1(c), and its variation is similar to that of the Ni48.2Ti50.4Cu1.4 film. When the potential is increased from 0.4 V to 0.6 V, the impedance decreased largely. The Nyquist plot of a Ni45.6Ti50.4Cu4 film is shown in Figure 1(d), and its variation is similar to those of the other two NiTiCu films. When the potential is increased from 0.2 V to 0.5 V, the impedance decreases significantly.
Figures 2(a)-(d) is the Bode plot of the four films. The effect of the potential is more pronounced in the phase shift angle. For the NiTi film, when the potential was increased to 1.2 V, the phase shift angle (θ) was close to 90˚ in a certain frequency range, showing near-capacitive behavior. The impedance |Z| varies linearly within this range with a slope of −1. However, when the potential was
(a) (b) (c) (d)
Figure 2. The Bode plots of four films in PBS solution at 37˚C ((a) Ni49.6Ti50.4; (b) Ni48.2Ti50.4Cu1.4; (c) Ni45.6Ti50.4Cu4; (d) Ni42.7Ti50.4Cu6.9).
increased to 1.2 V, the maximum phase shift angles of the Ni48.2Ti50.4Cu1.4 film, the Ni48.2Ti50.4Cu1.4 film, and the Ni42.7Ti50.4Cu6.9 film were 76˚, 70˚, and 74˚, respectively. It is implying that the near-capacitance behavior of the surfaces of these three films is broken. When the potential was 1.2 V, the maximum phase shfit angle of the NiTi film is much larger than that of the other three NiTiCu films, indicating that the corrosion resistance of the NiTi film is superior to that of the other three NiTiCu films  .
At a potential of 1.2 V, the Bode plots of the four films are shown in Figure 3. The impedance difference of the four films is more clearly shown in Figure 3. The NiTi film has a higher impedance |Z| value than the other three NiTiCu films, and its maximum phase shift angle θ is much larger than the other three NiTiCu films. It implies that the corrosion resistance of NiTi film is better than the other three NiTiCu films.
The near-capacitive behavior of the film in solution allows a simple equivalent circuit to represent the impedance component of the films. In the literature, there are also reports on the use of equivalent circuits to simulate impedance. K Li et al. used a similar equivalent circuit when studying the impedance of NiTi film in PBS solution . Popa used a parallel circuit of Rox and Cox to represent the impedance of Ti in Ringer’s solution . Pound also used the same method when studying the impedance of Nitinol in PBS and simulated bile solutions . The Bode plots showed that the impedance of NiTi and NiTiCu in PBS can be represented by the simple equivalent circuit shown in Figure 4. The circuit consists of an ohmic resistor (Rsol) of the solution, a resistance of the passivation oxide film (Rox) and a constant phase element (CPE) associated with the oxide. The formula associated with CPE is as follows:
Figure 3. The Bode plots of four films in PBS solution at 37˚C (potential is 1.2 V).
Figure 4. RC equivalent circuit in PBS solution.
where Y0 is the constant phase element parameter, , ω is the angular frequency Rsol, and the values of Rox, Y0 and α can be obtained using a nonlinear least squares curve fitting program. The α value of the amorphous NiTi film was found to be between 0.94 and 0.97. Since α is close to 1, Y0 can be taken as the value of Cox  .
The thickness dox of the film surface oxide can be calculated by the value of Cox, and the calculation formula is as the following,
where ε is the dielectric constant of the surface oxide, ε0 is the permittivity of the free space (8.854 × 10−12 Fm−1), and the surface oxide of the NiTi film is mainly TiO2. In the current study, the dielectric constant is 100 .
The calculation results of the thickness of surface oxide layer are shown in Figure 5. The thickness of oxide layer on the surface of several thin film materials were continually increasing in PBS solution as the potential increased. The thickness of the oxide layer of the NiTi film is calculated about 8 nm at Ecorr, which is smaller than the other three NiTiCu films. For the Ni48.2Ti50.4Cu1.4 film, the thickness of the surface oxide does not change much when the potential reaches 0.8 V. However, for Ni45.6Ti50.4Cu4 film and Ni42.7Ti50.4Cu6.9 films, the oxide thickness decreases with further increasing potential above 0.8 V. The oxide thickness of the Ni49.6Ti50.4 film is increased until the voltage reaches 1.2 V. It is generally believed that when the applied voltage is above 0.8 V, oxygen evolution will begin, which will affect the corrosion behavior of the test sample. Even if the voltage is above 0.8 V, the oxide thickness of the Ni49.6Ti50.4 film still continuously increases, which means that the NiTi film has better anti-oxidized capability than the other three NiTiCu films. In addition, the oxide film thickness of Ni48.2Ti50.4Cu1.4 film has no significant decrement when the voltage reached 0.8 V, indicating that its anti-oxidized capability is better than the other two Cu-containing films.
Based on the assumption that the oxide resistance is a linear function of the thickness of the oxide layer, the resistivity (ρox) of the oxide layer of the thin film s obtained from the thickness of the oxide layer as following,
where dox is the thickness of the oxide layer and Rox is the resistance of the oxide
Figure 5. Dependence of oxide thickness on potential for four films in PBS solution.
Figure 6. Dependence of oxide resistivity on potential for four films in PBS solution.
Figure 6 shows the change in the oxide resistivity of four films with potential increment. It shows that the oxide resistivity of the four films decreases with the potential increment. This behavior indicates that the oxide layer is defective as the potential increment .
The corrosion resistance of four different amorphous films (Ni49.6Ti50.4, Ni48.2Ti50.4Cu1.4, Ni45.6Ti50.4Cu4, Ni42.7Ti50.4Cu6.9) in PBS solution was studied using EIS method. The following conclusions were obtained:
1) The corrosion resistance of Ni49.6Ti50.4 film is superior to that of the other three NiTiCu films in PBS solution at 37˚C. The Ni48.2Ti50.4Cu1.4 film had better corrosion resistance than the other two NiTiCu films.
2) The EIS data could be fitted by a parallel resistance-capacitance circuit to obtain the thickness of the oxide layer of the film. The thickness of the oxide layer of Ni49.6Ti50.4 film is smaller than that of the other three films, but the thickness of the oxide layer of the Ni49.6Ti50.4 film increased as the potential increment, reaching a maximum at 1.2 V. The thickness of the oxide layer of the other three NiTiCu films reached a maximum at 0.8 V.
3) The resistivity of the four films was obtained as well. As the potential increased, the resistivity of the four films decreased, indicating that the oxide layer was defective.
National Key R&D Program of China: Stability Improvement and Product Upgrade of Super Martensitic Stainless Steel Used in Oil and Gas Development: 2016YFB0300204.
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