Study on Phase Transformation and Electrochemical Corrosion of TiNi Alloy Formed by Laser Solid Forming

Abstract

TiNi shape memory alloy (SMA) prepared by laser solid forming (LSF) has wide application prospect and research value. In this work, LSF-Ti_49.2Ni_50.8 alloy was tested by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The microscopic characterization results show that there is a small amount of martensitic slat in TiNi alloy besides the TiNi substrate. Lenticular R phase is densely distributed around the martensitic slat. B2→R and R→M phase transitions occur in the alloy due to the high cooling rate during laser solid forming. The results of methyl thiazolyl tetrazolium (MTT) assay and electrochemical corrosion test show that the alloy has good biocompatibility and excellent corrosion resistance and has the potential to be used as a biomedical material.

1. Introduction

TiNi-based shape memory alloy (SMA) has been widely used in the biomedical field for its excellent superelasticity, shape memory properties, corrosion resistance, wear resistance, and excellent biocompatibility [1,2,3,4,5]. The traditional methods for preparing TiNi SMA include casting and powder metallurgy, but the sensitivity of TiNi alloy to chemical composition and heat treatment increases the difficulty of early melting and subsequent processing [6,7,8]. The Ti element in TiNi alloy is a highly active metal, which can react with many elements in the traditional smelting process to generate harmful phases such as nitride and oxide [9]. These harmful phases affect the transformation of martensite and reduce the mechanical properties of the alloy. Thermoelastic martensitic transformation plays a fundamental role in the shape memory effect and transformation pseudoelasticity [4,10,11]. The main characteristic of this transition is the continuous formation and growth of martensite sheets when the temperature decreases or the stress increases. When the temperature increases or the stress decreases, the martensite gradually shrinks and disappears [2,3,9,12]. At present, the common methods for preparing TiNi shape memory alloy are casting and powder metallurgy; the former is more widely used [2]. However, TiNi alloy is very sensitive to chemical composition and heat treatment processes, which makes melting and subsequent processing more difficult. In addition, these external factors, such as long production cycle and high cost, seriously restrict the further use and promotion of TiNi alloy [10,11]. Therefore, to explore a new preparation process to obtain TiNi alloy components with uniform structure, less harmful phases, and excellent mechanical properties, while reducing the cost and production cycle, has become an urgent problem to be solved in the field of the engineering application of TiNi alloy [4,8,13].

Laser solid forming (LSF) is a new rapid laser forming technology [14]. This technique is based on the way the material is formed by layering. Firstly, a three-dimensional computer aided design (CAD) solid model is designed, and the three-dimensional solid information is separated into two-dimensional contour information by stratification. Secondly, under the control of the computer numerical control (CNC) machining center, combined with the action of a high energy laser beam, powder is deposited in sedimentary layers onto a substrate, and finally a geometric entity consistent with the three-dimensional CAD model is obtained [15,16,17,18]. In the process of preparing TiNi-based shape memory alloy by laser solid forming, the forming mode of the alloy is optimized, and the phase transformation process of the alloy is very different from that of traditional alloy [12,19,20,21]. The TiNi-based shape memory alloy prepared by LSF has optimized the forming mode of the alloy, inhibited the precipitation of the second phase, and improved the microsegregation phenomenon [21,22,23]. Compared with the traditional TiNi SMA, the LSF process has lower cost and it is easier to control the solidification behavior, which greatly improves the subsequent processing performance of the alloy [8,13].

In this paper, the microstructure of TiNi SMA prepared by LSF was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM), and the corrosion resistance mechanism of TiNi SMA was studied by electrochemical testing. At the same time, the corrosion properties of the alloy were evaluated according to the biocompatibility and microstructure, which provided the basis for its application in biomedical engineering.

2. Materials and Methods

Ti_49.2Ni_50.8 (at. %) alloy powder with a particle size of 50–100 μm was used as the cladding material, and the chemical composition of it is shown in

Table 1. Chemical composition of TiNi pre-alloyed powder (wt.%).

Element	Ti	Ni	Fe	C	O	N
Content	43.96	55.98	˂0.005	0.0025	0.046	0.003

. The deposition alloy was prepared using LSF-ⅢB equipment (Northwestern Polytechnical University, Xi’an, China) with a 5-kW continuous wave CO₂ laser unit. Sample preparation was carried out in an inert atmosphere protective chamber, argon was selected as the protective and powder feeding gas, and the oxygen content in the chamber was no higher than 30 ppm; the deposition process is shown in Figure 1a. Figure 1b shows the image of a TiNi alloy sample of LSF (15 mm × 15 mm × 15 mm). After polishing the samples, X-ray diffraction analysis was carried out with a X’PERT PRO X-ray diffractometer (PANalytical B.V., Almelo, the Netherlands). The target material of characteristic X-ray is Cu-Kα, the tube voltage is 40 kV, the tube current is 35 mA, the scanning speed is 3°/min, and the working temperature is 25 °C. The X-ray incident is parallel to the transverse scanning direction at a certain height on the longitudinal section, and the XRD diffraction angle ranges (2θ) from 20° to 90°. A DSC8500 differential scanning calorimeter (PerkinElmer Inc., Waltham, MA, USA) was used to analyze the change of heat flux of TiNi alloy during continuous heating and cooling. DSC samples were cut from 13 mm away from the matrix, with a mass of 15–20 mg. The heating rate is 10 °C/min, and the operating temperature range is −130~70 °C. The Versa STAT 4 electrochemical test (AMETEK Co., Ltd., Berwyn, PA, USA) was performed with NaCl (0.9%) normal saline solution with PH = 7 at 37 ± 0.5 °C. The effective area of the polarization curve was 1 cm³. A saturated calomel electrode (SCE) was used as the reference electrode, and the Pt electrode was used as the auxiliary electrode. The open-circuit potential (OCP) test is the natural corrosion potential of the sample soaked in corrosive medium for 1 h, and the test time is 3600 s. The potentiodynamic polarization curve is mainly for Tafel, and the potential range of −0.25 V_OCP~+0.25 V_OCP and the potential scanning speed of 0.167 mV/s are selected for testing. Impedance test parameters are an AC sinusoidal excitation signal with an amplitude of 5 mV, frequency range of 10⁵~10⁻² HZ, and test temperature of 25 °C. The cell culture medium for the biocompatibility test was DMEM + 10%FBS + 1% penicillin-streptomycin, and the tested cells were L929 cell line. The TiNi alloy was immersed at 37 °C in 5%CO₂ for 2 days and 7 days, and the inoculated cells were 3 × 10³ cells/hole. The four groups were as follows: (1) Control: normal culture medium; (2) Positive: normal medium + 1%DMSO; (3) 48 h leach liquor; (4) 7-day culture medium. The four groups of samples were cultured at 37 °C, 5% CO₂ for 24 h, 48 h, and 72 h, followed by methyl thiazolyl tetrazolium (MTT) assay detection. An FEI Tecnai F’30 G² transmission electron microscope (Thermo Fisher Scientific, Waltham, MA, USA) was used for microstructure observation, with an accelerated voltage of 200 kV. The surface corrosion morphology was observed by a ZEISS SUPRA 55 scanning electron microscope (Carl Zeiss AG., Jena, Germany).

3. Results and Discussion

LSF is a new technique for the preparation of TiNi SMA, which involves many phase transformation reactions during the melting and solidification process. Figure 2a shows the XRD patterns of TiNi alloy, with diffraction peaks observed at 42.4° and 77.6°. Compared with the standard PDF cards, the peak positions of these two angles correspond to the crystal planes of (011) and (121) of the TiNi alloy parent phase. DSC thermal cycle curves of samples during heating and cooling are shown in Figure 2b. The peak temperature of the exothermic peak on the cooling stage curve is T1 = −47 °C, and the peak temperature of the endothermic peak on the heating stage curve is T2 = −10 °C. Figure 2c,d show the bright-field TEM images of the sample and corresponding selected area electron diffraction results. There are martensite slates with widths ranging from 10 nm to 50 nm in the sample, and the lenticular R phase is densely distributed around the martensite.

The phase transition of TiNi alloy by LSF can be judged by XRD, DSC, and TEM results. Large lattice shear occurs during martensitic transformation (MT), and the thermal hysteresis of one-stage MT is about 30 °C. The lattice shear caused by the transformation of the B2 and R phases is small, and the thermal hysteresis of phase transition is generally less than 6 °C. The thermal hysteresis of R→B19’ is usually larger than that of B2→B19’. Therefore, it is speculated that the parent phase of TiNi alloy prepared by LSF may have a two-stage MT in different regions, namely B2→R and R→M. There are two main reasons for R phase transformation in TiNi alloy formed by LSF: (1) LSF is a rapid solidification process, and the local stress field around the phase leads to the obstruction of MT, which means that it is easy to induce R phase transformation; (2) When cladding a new layer, the microstructure of the heat affected zone (HAZ) will be affected by the heat input. This process is equivalent to tempering the alloy, which is beneficial to R phase nucleation.

In traditional TiNi alloy, the reasons for the transformation of B2→R are as follows: (1) coherent stress field; (2) Introducing high-density dislocations by means of slight cold deformation and phase transformation thermal cycling; (3) The appearance of coherent precipitates such as Ti₃Ni₄ and Ti₂Ni. The formation mechanism of the R phase in LSF Ti_49.2Ni_50.8 alloy is mainly as follows: (1) The peak temperature of R phase transformation is above room temperature. Therefore, the alloy underwent R phase transformation at room temperature, and the existence of R phase was observed; (2) There is lens-shaped R phase around the martensite lath. In the region with large residual stress, stress-induced MT will occur in the as-deposited Ti_49.2Ni_50.8 alloy, and R-induced transformation will also occur in the MT process. (3) During the LSF process, there is a semi-coherent stress field around the Ti₂Ni phase, which means that it is easy to induce MT, and the existence of a certain amount of dislocation will hinder the martensite nucleus. The R phase has a rhomboidal structure, and the phase transformation of B2→R is carried out by the deformation and elongation of the B2 phase along the [$111$] direction. The lattice distortion is small, and the transition energy barrier is lower than that of the one-stage MT process, so the number of R phases around the dislocation, precipitate, or residual martensitic phase is relatively high.

The cell survival time and growth activity of L929 cell lines in TiNi SMA prepared by LSF were measured by MTT colorimetry, and the biocompatibility of the alloy was evaluated. Figure 3a shows the cell growth test results of the MTT colorimetric method. According to the results, the number of L929 cells increased gradually with the extension of culture time after the metal material was extracted in cell culture medium for 48 h and 7 days, and there was no significant difference in the effect of extraction medium on cell proliferation compared with normal cell culture medium (p > 0.05). The results of MTT cell activity in this study were consistent with or even higher than those of other porous TiNi SMA reported in the literature [24,25]. Figure 3b–d compare the changes of sample surface morphology before and after MTT detection. The results show that after MTT detection, there were no other obvious morphological changes except biological tissue and carbon residue on the sample surface. Therefore, it can be concluded that the TiNi SMA prepared by LSF has no effect on the growth and biological activity of L929 cells, which further proves that the alloy has good biocompatibility and has potential as a biomedical material.

Figure 4a shows the open-circuit potential (OCP) curve of LSF-TiNi alloy immersed in 0.9 wt. %NaCl solution. Before the test, the sample was immersed in the solution for 1 h and then stabilized to test the OCP at room temperature. As can be seen from the curve, the value of OCP did not increase or decrease significantly during the whole test process, and remained at about ~0.25 V, indicating that the alloy showed good corrosion resistance in 0.9 wt.% NaCl solution. The Potentiodynamic polarization curve of the alloy was measured when the OCP was stable, as shown in Figure 4b. The anodic and cathodic reaction curves are asymmetrical in the potential range. During the whole corrosion process, the alloy surface did not appear passivation, and the corrosion showed a stable pitting corrosion behavior. The current density continues to increase with the increase of corrosion potential, the self-corrosion potential is −0.273 V, and the corrosion current density is 9.763 × 10⁻⁵ A∙cm⁻², which proves that the alloy has a low corrosion tendency. Figure 4c,d show Nyquist plots and Bode plots of the alloy in 0.9 wt.% NaCl solution. The impedance curve radius of the alloy in solution is larger, and it has better corrosion resistance (Figure 4c). The time constant contained in the impedance spectrum is determined by the curve characteristics of phase angle and Bode plots. It can be seen from Figure 4d that the electrode reaction in all areas has only one time constant, and different areas always show an inductive reactance response during the corrosion process. Corrosion reactions on sample surfaces are usually identified in the mid-frequency range. Figure 4d shows that the phase angle of the alloy reaches the highest in the mid-frequency range (10²~10³ Hz) and then decreases with the increase of frequency. The electrochemical test results of TiNi SMA prepared by LSF in this experiment are consistent with those of TiNi-based shape memory alloy reported in the literature [26,27]. Figure 5 is the SEM image of the sample surface morphology before and after the electrochemical corrosion test. After electrochemical corrosion, the alloy surface showed obvious corrosion shedding phenomenon, and no pitting pits were observed. Combined with the test data in Figure 4 and the results in Figure 5, it can be judged that LSF-TiNi alloy has good corrosion resistance.

4. Conclusions

In this paper, the microstructure of TiNi shape memory alloy prepared by laser solid forming was tested by X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The microscopic characterization results show that there is a small amount of martensitic slat in TiNi alloy besides the TiNi substrate. The lenticular R phase is densely distributed around the martensitic slat. B2→R and R→M phase transitions occur in the alloy due to the high cooling rate during laser solid forming. The results of the methyl thiazolyl tetrazolium test and the electrochemical corrosion test show that the alloy has good biocompatibility and excellent corrosion resistance and has the potential to be used as a biomedical material.