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Formation mechanism of Ni2Ti4Ox in NITI shape memory alloy
Materialia 5 (2019) 100194
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Full Length Article
Formation mechanism of Ni2 Ti4 Ox in NITI shape memory alloy Wei-Yu Kai a, Kai-Chun Chang a, Hsu-Fu Wu b, Shi-Wei Chen c, An-Chou Yeh a,∗ a
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, Republic of China Metal Processing R&D Department, Metal Industries Research & Development Centre, Kaohsiung 81160, Taiwan, Republic of China c National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, Republic of China b
a r t i c l e
i n f o
Keywords: Nitinol Microstructure characterization First principle calculation Near-edge x-ray absorption fine structure analysis Phase transformation
a b s t r a c t Ni2 Ti4 Ox phase is known to affect the formability and mechanical properties of NiTi shape memory alloy (nitinol), however, the underlying mechanism of its formation remains unclear, although its presence has long affected the production of nitinol in industry. The present study aims to determine the formation mechanism of Ni2 Ti4 Ox . A nitinol sample melted by vacuum induction melting technique was investigated. The as-cast sample possessed NiTi2 segregation along the grain boundary, although solution heat-treatments at 1000 °C could eliminate NiTi2 , Ni2 Ti4 Ox could be observed, indicating that some NiTi2 could transform into Ni2 Ti4 Ox oxide. The correlation between the NiTi2 segregation and the formation of Ni2 Ti4 Ox has been elucidated by first principle calculation and near-edge x-ray absorption fine structure analysis. Results indicate that the formation enthalpy of Ni2 Ti4 Ox is inversely proportional to the amount of oxygen pick-up. During the solution heat-treatment process, some NiTi2 could be dissolved back to the NiTi matrix, while some NiTi2 could transform into Ni2 Ti4 Ox and be stabilized by absorbing more oxygen. Based on findings in this work, possible measures to minimize the formation of Ni2 Ti4 Ox in nitinol are proposed.
1. Introduction Nitinol has attracted lots of attention ascribed to its pronounced pseudoelastic and shape memory properties [1–6]. With reversible and diffusionless phase transformation, pseudoelastic property refers to stress induced martensitic phase transformation from initial austenite phase accompanied with strain as high as 8%, which can be fully recovered upon unloading. While shape memory property is contributed to the recovery of shape changes from martensite to austenite phase after appropriate heating. With these functional properties, there are many nitinol devices developed for applications in aerospace industry, civil engineering etc. [7–10]. The most important applications of nitinol are in the medical sector [11–13], e.g., stent, orthodontic wire, drill etc. Comparing to other implant materials such as 316L stainless steel, nitinol possesses better mechanical properties, hysteresis behavior, and excellent biocompatibility etc. [14–16]. Although nitinol has been used for decades, the fabrication of nitinol products by common thermo-mechanical process still remains a challenge in the industry, since fracture of ingot can occur due to the presence of brittle Ni2 Ti4 Ox oxides in the microstructure [12,17]. Furthermore, Ni2 Ti4 Ox oxide can decrease the mechanical property, affect phase transformation temperature and functional properties of nitinol products [18–20]; studies have reported that fatigue performance of stent and orthodontic wire could be
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degraded by the presence of Ni2 Ti4 Ox [21–24]. The ingestion of oxygen in nitinol has been thought to be the main cause of problem. Although characterization of Ni2 Ti4 Ox have been a subject of interest [13,25,26], and recent studies have suggested that NiTi2 phase in nitinol can be stabilized by oxygen [[19,20,27–29]. However, it is unclear whether there exists a correlation between NiTi2 phase and Ni2 Ti4 Ox oxides, so the underlying mechanism of Ni2 Ti4 Ox oxide formation remains unknown to date. It is important to understand the underlying mechanism of Ni2 Ti4 Ox formation in order to minimize its formation and further improve the yield of production and properties of nitinol. 2. Experimental procedure A Ti-50.8 at.% Ni (55.9 wt % Ni) cylindrical ingot was prepared from high purity raw materials of nickel and titanium using the vacuum induction melting (VIM) with 10−5 torr atmospheric pressure at Metal Industries Research & Development Centre (MIRDC) in Taiwan (R.O.C.). The oxygen content of the VIM ingot was examined by oxygen analyzer (LECO TC500) and the content of oxygen was 300 ppm. Samples were then sectioned from the VIM ingot and subjected to solution heattreatments at 1000 °C for 24, 36 and 48 h in evacuated quartz tube with 1.3 × 10−3 torr atmospheric pressure followed by water quenching to minimize oxidation and second phase formation; oxygen contents were examined after solution heat-treatments by LEO TC500 and found to
Corresponding author. E-mail address: [email protected] (A.-C. Yeh).
https://doi.org/10.1016/j.mtla.2018.100194 Received 11 September 2018; Accepted 15 December 2018 Available online 23 December 2018 2589-1529/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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Materialia 5 (2019) 100194
Fig. 1. Ni–Ti phase diagram.
Fig. 2. (a) As-cast sample, backscatter electron image (BEI), (b) as-cast sample, secondary electron image (SEI), (c) as-cast sample, BEI mode; the BEI images of segregation phases after solution heat-treatments at 1000 °C for (d) 24 h, (e) 36 h, (f) 48 h.
be almost the same as the initial content, i.e., 300 ppm, so there was negligible oxygen pick-up during the solution heat-treatment processes. The crystal structures of the as-cast and heat-treated specimens were characterized by X-ray diffractometer (XRD, Bruker_D2 PHASER) with Cu K𝛼 radiation. The microstructures for all specimens were observed by optical microscopy (OM, OLYMPUS BX51M), scanning electron microscopy (SEM, JEM-5410 and Hitachi SU8010) and transmission electron microscopy (TEM, Tecnai F30 and JEOL JEM-3000F). All specimens were cut by wire electrical discharge machining followed by mechanical grinding and polishing to eliminate surface oxides and contami-
nants. Specimens for TEM analysis were grinded to 30–50 𝜇m, followed by twin-jet electropolishing with 20% sulfuric acid and 80% methanol solution electrolyte at 19.5 V and −20 °C. The detailed chemical compositions of phases in each specimen were measured by energy dispersive spectrometer (EDS) and Electron Probe Microanalyzer (EPMA, JEOL JXA-8500). CALPHAD simulation (Thermo-Calc, TCNI8 database) [30] was used to predict possible phases in the Ni–Ti system [31,32]. Furthermore, first principle calculation based on density functional theory was utilized [33–35], and a computational quantum mechanics model and
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Materialia 5 (2019) 100194
Fig. 3. The XRD pattern of as-cast sample; solution heat-treated samples.
Fig. 4. The EPMA mappings of segregation phases. (a) as-cast sample; (b) solution heat-treated at 1000 °C for 24 h; (c) solution heat-treated at 1000 °C for 36 h.
thermodynamic properties of phases could be established by calculating the electron interaction between each other [36–39]. To analyze the formation of NiTi2 and Ni2 Ti4 Ox , the Material Studio 7.0 software was used to calculate the formation enthalpies of cubic NiTi2 and Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1) based on Cambridge serial total package (CASTEP) with the generalized gradient approximation of Perdew–Burke–Euzerhof [35,40]. Furthermore, in order to shorten
the computation time, 1 × 1 × 1 supercells of cubic NiTi2 and Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1) with FCC structures were constructed with a plane wave cutoff energy of 380 eV and a convergence total energy of 5 × 10−7 eV/atom. The near-edge x-ray absorption fine structure (NEXAFS) of as-cast and solution heat treated samples was determined in the BL05B2 beamline in National Synchrotron Radiation Research Center (NSRRC) in Taiwan (R.O.C.). A fixed-exit double-crystal Si(111)
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Materialia 5 (2019) 100194
Fig. 5. The TEM micrographs and the corresponding diffraction patterns of NiTi2 and Ni2 Ti4 Ox (a) NiTi2 in as-cast state; (b) Ni2 Ti4 Ox in sample solution heat-treated at 1000 °C for 36 h. Fig. 6. The supercell of (a) NiTi2 , (b) Ni2 Ti4 O0.25 , (c) Ni2 Ti4 O0.5 , (d) Ni2 Ti4 Oo.75 , (e) Ni2 Ti4 O.
monochromator was served to tune the X-ray photon energy by controlling an average energy resolution dE/E = 2 × 10−4 ; O K-edge spectra was recorded by collecting surface-electrons [41].
Table 1 The composition of matrix and segregation phases (in at.%) in VIM and VIM+soluted samples by EPMA analysis.
3. Results and discussion VIM
The phase diagram of Ni–Ti system is shown in Fig. 1; there is a relatively narrow window of NiTi non-stoichiometric composition space at high temperatures with NiTi2 and Ni3 Ti phases at each side, so it is likely that segregation during VIM process can induce either formation of NiTi2 or Ni3 Ti depending on the composition and the degree of segregation during casting. The composition of the bulk ingot in present study was Ti-50.8 at.% Ni, and a solution heat-treatment at 1000 °C was chosen (indicated by red dash tie-line). Microstructure of as-cast and solution heat-treated specimens are shown in Fig. 2, and the corresponding XRD analysis is shown in Fig. 3. According to the EPMA analysis (Table 1), NiTi2 was identified along the grain boundary in as-cast condition, which was associated with relatively large solubility of Ti, since
Solu for 24 h Solu for 36 h Solu for 48 h
Area
O
Ti
Ni
Matrix Segregation Segregation Segregation Segregation
0 0.75 2.26 4.58 5.37
50.326 66.49 59.38 62.13 61.19
49.668 32.76 38.36 33.28 33.44
the concentration of Ti increased in the liquid when NiTi was formed during solidification, Fig. 1, and thus the solidification process could induce the formation Ti-rich NiTi2 phase at the NiTi grain boundaries. Fig. 3 further confirms that not only cubic NiTi but also NiTi2 phase existed in as-cast condition (Ni2 Ti4 Ox possesses a very similar FCC struc-
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Fig. 7. The NEXAFS of as-cast sample, and solution heat-treated samples at 1000 °C for 48 h: (a) O K-edge spectra; (b) Ti L-edge spectra; and (c) Ni L-edge spectra..
ture). Fig. 2(b) and (c) show some cracks along the NiTi2 phase, which indicates that the NiTi2 phase could cause cracks during VIM process. After the solution heat-treatments, the microstructure contained lesser amount of grain boundary segregation phase as shown in Fig. 2(d)–(f). The corresponding XRD results is shown in Fig. 3. The XRD patterns show undetectable NiTi2 peaks in solution heat-treated samples comparing to that of the as-cast sample. However, solution heattreatments did not eliminate the segregation, and there was Ni2 Ti4 Ox present instead, even with an increase in duration from 24 to 48 h (indicated by segregation phase along grain boundaries in Fig. 2). Detailed analysis on the chemical compositions corresponding to the mapping results of EPMA are shown in Table 1 and Fig. 4. According to Table 1, it is possible that the initial NiTi2 had the ability to pick up a trait of oxygen in as-cast condition and the amount of oxygen content in Ni2 Ti4 Ox is proportional to the length of solution heat-treatment. Further examination on the results of EPMA mappings in Fig. 4(a) shows that the amount of oxygen contained in NiTi2 segregation in the as-cast condition was minimal and similar to that of the NiTi matrix. However, after solution heat-treatment, the oxygen content in Ni2 Ti4 Ox increased significantly and could be distinguished easily from the matrix shown in Fig. 4(b) and (c). According to Fig. 4 and Table 1, oxygen may solute in the NiTi2 structure and cause NiTi2 to transform into Ni2 Ti4 Ox . This would agree to some degree with literatures stating that oxygen may solute in the NiTi2 phase and stabilize it [19,20,27–29]. Fig. 5(a) and (b) show a NiTi2 and a Ni2 Ti4 Ox in as-cast condition and after a solution heat-treatment for 36 h, respectively; both phases are FCC and electron diffraction patterns of phases are highlighted by white circles. Since SEM observations (Fig. 2) indicate that both NiTi2 and a Ni2 Ti4 Ox were present at grain boundaries, it is reasonable to assume that phases in Fig. 5 were taken at grain boundaries. Compositions measured by TEM-EDS and lattice constants of the NiTi2 and the Ni2 Ti4 Ox in Fig. 5 are summarized in Table 2. After a 36 h of solution heat treatment, the oxygen content was increased to 7.85 at.%, while the
Table. 2 The composiitons (in at.%) and lattice constants (Å) of phases in Fig. 5.
Fig. 5(a) Fig. 5(b)
Phase
O
Ti
Ni
Lattice constant
NiTi2 Ni2 Ti4 Ox
0.75 7.85
66.49 61.96
32.76 30.19
11.44 11.74
lattice constant was increased from 11.44 to 11.74 Å which agrees with values reported in literature [28]. Thereby, TEM analysis indicates that it is hard to distinguish between NiTi2 and Ni2 Ti4 Ox phases by XRD analysis. However, it appears that there may exist some correlation between these phases due to their similarity in crystal structure. The following describes an effort to utilize first principle calculation and synchrotron radiation to uncover the underlying mechanism of Ni2 Ti4 Ox formation. After knowing that NiTi2 and Ni2 Ti4 Ox have similar crystal structures, we can assume that oxygen may solute in the NiTi2 segregation and then transform it into Ni2 Ti4 Ox oxides. The correlation between NiTi2 and Ni2 Ti4 Ox was elucidated by calculating the formation enthalpy of NiTi2 and Ni2 Ti4 Ox using first principle calculation. Fig. 6 presents the constructed supercells (the essential parameters used were based on the literature [28]), and the value of formation enthalpy for NiTi2 and Ni2 Ti4 Ox can be calculated from formula (1) and (2), respectively: ΔHfor =
ΔHfor =
( ) 𝐸 Ni Ti2 − 32𝐸 (Ni) − 64𝐸 (Ti) 96 ( ) 𝐸 Ni2 Ti4 O𝑥 − 32𝐸 (Ni) − 64𝐸 (Ti) − 16𝑥𝐸 (O) 96 + 16𝑥
(1)
(2)
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Fig. 8. The simulation results of lattice constants, positional parameters, and bonding lengths in (a) NiTi2 ; and (b) Ni2 Ti4 Ox .
Table. 3 The lattice structure and the numbers of atoms in the supercell used in the first principle simulation and the formation enthalpy (in KJ/mol) of NiTi2 and Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1) calculated at 0 K. Supercell
Ni
Ti
O
structure
Formation enthalpy
NiTi2 Ni2 Ti4 O0.25 Ni2 Ti4 O0.5 Ni2 Ti4 O0.75 Ni2 Ti4 O NiTi
32 32 32 32 32 1
64 64 64 64 64 1
– 4 8 12 16 –
Fd3̄ m Fd3̄ m Fd3̄ m Fd3̄ m Fd3̄ m Fd3̄ m
−63.3 −1766.43 −3339.33 −4795.82 −6148.27 −30.66
where E(NiTi2 ), E(Ni2 Ti4 Ox ), E(Ni), E(Ti), and E(O) are the total energy of intermetallic compound NiTi2 , oxide Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1), and stable state elements Ni, Ti and O. The composition, lattice structure and calculated results of formation enthalpies for intermetallic compounds NiTi2 and oxides Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1) are summarized in Table 3. Firstly, it shows that calculation results of formation enthalpies for all compounds are negative, this means that each compound can form at 0 K. Moreover, it also shows the trend of formation enthalpy is inversely proportional to the amount of oxygen, i.e., Ni2 Ti4 Ox would become more stable with increasing amount of oxygen content. By considering results of both first principle calculation and compositions shown in Tables 1 and 2, we can explain why the grain boundary phase cannot be fully eliminated after solution heat-treatments, Fig. 2(d)–(f). After conducting solution
heat-treatments, diffusion of oxygen into NiTi2 occurred and thus transformed it into a more stable structures of oxides, i.e., Ni2 Ti4 Ox , which was stable and could not be dissolved into the matrix at 1000 °C. To confirm the results of first principle calculation, the BL05B2 beamline in NSRRC, Taiwan (R.O.C.) was used to characterize Ni2 Ti4 Ox . Fig. 7(a) presents the peak of normalized intensity (or called white line) from O, and it has increased after solution heat-treatment, furthermore the wave of photon energy has shifted to lower state after the solution heat-treatment. On the other hands, Fig. 7(b) shows photon energy from Ti, and the peak has also increased, but the wave has shifted to higher energy state after solution heat-treatment. The results of Ni are shown in Fig. 7(c), the peak intensities of Ni has decreased and the wave has shifted to higher photon energy state. The underlying mechanisms could be contributed to the electrons transferred and the bonding formation during solution heat-treatment process. Firstly, the increase of normalized intensity presents higher d orbital unoccupied electronic states as well as holes formation at the d-band. Thereby, the increase of peak intensities for Ti and O indicates that electrons sharing and the increase in the unoccupied electronic states in each d-band, and the efficiency to induce formation of ionic bond between Ti and O. Moreover, the much increase of peak O has presented the better local coordination symmetry for O atoms, which shows the stable location of O at tetrahedral site, thus the stability of Ni2 Ti4 Ox could be enhanced with higher content of oxygen. However, the decrease of Ni peak implies that Ni not only gets more electrons to fulfill unoccupied electronic states at the d-band but also do not share electrons with O to form strong bond. Secondly, the shifting of wave indicates the changing of oxidation number or the lattice distortion of the compound. The shifting of O wave to lower en-
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Fig. 9. Schematic illustration of the mechanism of (a) the dissolution of NiTi2 ; (b) the stabilization of Ni2 Ti4 Ox ; and (C) a transition phase: the dissolution and stabilization of the complex structure of Ni2 Ti4 Ox .
ergy state and the shifting of Ti wave to higher energy state indicate the electrons transfer from O to Ti, which could induce the energy compensation in whole system. So the increase in oxygen content in Ni2 Ti4 Ox could result a more stable structure. However, the shifting of Ni wave to higher energy state can attribute to the lattice distortion, implying the insertion of O atom in compound would increase the lattice parameter of Ni2 Ti4 Ox structure [28], and this can be seen in Table 2. To evaluate the bonding characteristics between these atoms, the simulation results have been analyzed and shown in Fig. 8. Fig. 8(a) presents that Ni would share electrons with Ti to form two different bonds: Ni–Ti(1) with 2.61321 Å and Ni–Ti(2) with 2.48438 Å. These two bonds are all single bond between Ni and Ti in NiTi2 . However, after inserting O in NiTi2 to form Ni2 Ti4 O as shown in Fig. 8(b), the lattice parameter would increase from 11.3193 Å to 11.3279 Å, and promote formation of two new bonds, i.e., Ni–Ni with 2,68,933 Å, and O-Ti(1) with 2.12426 Å. The formation of bond O–Ti(1) can be the proof of formation of ionic bond between Ti and O, which can further stabilize the oxide structure. On the other hands, the insertion of O atom could cause an increase of bond length of Ni–Ti(1) to 2.66896 Å, and a decrease of bond length of Ni–Ti(2) to 2.45509 Å. The underlying mechanism could be attributed to the insertion O atom would increase the repulsive force between atoms of O and Ni as well as Ti(1), shifting atomic positions slightly away the equilibrium positions of Ni and Ti(1) indicated by yellow arrows in Fig. 8(b). However, inserting O atoms can also be seen as being fixed by surrounding atoms of Ni and Ti(1), which can highly stabilize O atoms at tetrahedral site. Moreover, it could also reduce the distance between Ni atoms, and induce the formation of metal bond Ni– Ni, which would further share electrons with other Ni atoms. Thereby, this analysis not only suggests that shifting of Ni wave to higher energy state could attribute to the misplace of Ni atoms but also indicates
the fulfillment of unoccupied electronic states in Ni contributed to the formation of metal bond Ni–Ni. This study has shown that “the NiTi2 segregation can be the precursor of Ni2 Ti4 Ox oxide formation”. Figs. 2 and 3 indicate that the amounts of NiTi2 decreased after solution heat-treatments. Composition measurements in Tables 1 and 2 show that amount of oxygen solutioned in Ni2 Ti4 Ox increased with prolong duration of heat-treatment. According to results of first principle calculations in Table 3, it indicates that the oxygen content could promote the formation of Ni2 Ti4 Ox oxide ascribed to the lower formation enthalpy. With an increase in oxygen content, the results of Fig. 8(b) shows that the formation of ionic bond O–Ti(1) and metallic bond Ni–Ni would further stabilize the structure of Ni2 Ti4 Ox oxide. Although solution heat-treatments decreased the amount of NiTi2 as shown in Fig. 2, but it also promoted the oxygen to diffuse into NiTi2 and thus transformed it into a stable structure of Ni2 Ti4 Ox oxide during the process. So, why would some NiTi2 get dissolved into the matrix while some NiTi2 would transform into Ni2 Ti4 Ox after solution heattreatment? To describe the underlying mechanism of this interesting phenomena, schematic illustrations of phase transformations are shown in Fig. 9. There are three kinds of phases shown in Fig. 9, including the NiTi, NiTi2 , and Ni2 Ti4 Ox . The NiTi2 phase may have a little trait of oxygen, which could be ignored. The NiTi2 can transform into Ni2 Ti4 Ox with certain amount of oxygen content. In addition, an intermediate phase might be present, i.e., Ni2 Ti4 Ox phase with a concentration gradient of oxygen from the surface to inner part of the structure. The first mechanism is the dissolution of NiTi2 shown in Fig. 9(a). The second mechanism shown in Fig. 9(b) is the diffusion of oxygen in Ni2 Ti4 Ox 1 phases and thus transform it into more stable phases of Ni2 Ti4 Ox 2 (where x2 > x1 ). To elucidate the underlying mechanism
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whether dissolution of NiTi2 or formation of Ni2 Ti4 Ox can occur, it is necessary to calculate the energy barrier ΔH, which needs to be overcome for dissolution to occur. According to the phase diagram shown in Fig. 1, it presents that the solution heat-treatment at 1000 °C would result one phase of NiTi, which implies that there is a driving force to dissolve NiTi2 segregation into NiTi matrix. According to the first principle calculation results in Table 3, the ΔH between the NiTi and the NiTi2 is about 30 KJ/mol. However, the ΔH between NiTi and Ni2 Ti4 Ox oxide can be larger than the former depending on the amount of oxygen solutioned. Therefore, the first mechanism ascribed the low energy barrier ΔH, so that the solution heat-treatment can easily drive the dissolution of NiTi2 into the NiTi matrix. However, due to oxygen diffusion into the Ni2 Ti4 Ox 1 phase, the second mechanism resulted higher energy barrier ΔH, hence solution heat-treatment at 1000 °C was not sufficient to overcome it. On the other hands, according to the lower formation energy of Ni2 Ti4 Ox phases, the oxygen will continue to diffuse into the Ni2 Ti4 Ox 1 to form more stable Ni2 Ti4 Ox 2 (where x2 > x1 ). From the kinetics point of view, solution heat-treatment can promote the oxygen to diffuse into the Ni2 Ti4 Ox . Thereby, the second mechanism can be described by the following formula (3): NiTi + Ni2 Ti4 OX1 +O2 → NiTi + Ni2 Ti4 OX2
(3)
where x1 and x2 = 0, 0.25, 0.5, 0.75 or 1. According to this formula, the ΔH of the reaction can be computed to show that if the x2 > x1 , the whole structure would become more stable than the former. The transition phase is shown in Fig. 9(c). According to the EPMA analysis shown in Fig. 4, there existed a concentration gradient of oxygen from inner to the surface, hence the size and fractions of Ni2 Ti4 Ox could vary with different length of solution heat-treatments. To the best of authors’ knowledge, the present article is the first to elucidate the underlying mechanism of Ni2 Ti4 Ox formation in association with NiTi2 from as-cast state. Through experimental analysis, simulation and calculations, it can be concluded that NiTi2 is a product of segregation during the melting of nitinol, although solution heat-treatment can eliminate NiTi2 from thermodynamic point of view, kinetic aspect of oxygen ingestion into NiTi2 could transform it into thermodynamically stable Ni2 Ti4 Ox , hence Ni2 Ti4 Ox could be observed instead after solution heat- treatments. It is very interesting to note that after VIM process (10−5 torr atmospheric pressure), the sample contained 300 ppm oxygen, which could be originated from the raw materials, and solution heat-treatments at 1000 °C up to 48 h (1.3 × 10−3 torr atmospheric pressure) did not further increase the oxygen content in samples. However, 300 ppm oxygen was sufficient to transform some NiTi2 in as-cast state into Ni2 Ti4 Ox , although majority of NiTi2 got dissolved into the matrix. So, there are actions that could be taken during the fabrication of nitinol to minimize Ni2 Ti4 Ox formation; (1) increase the purity level of raw Ni and Ti materials for melting process, (2) faster solidification rate during the melting process to decrease the degree of casting segregation, so fractions of NiTi2 segregation (the precursor of Ni2 Ti4 Ox ) can be minimize, and a subsequent solution heat-treatment can be applied to dissolve most of the NiTi2 segregation in order to minimize the fractions of Ni2 Ti4 Ox and improve the yield of production of nitinol. 4. Conclusions This study has presented the underlying mechanism of the formation of Ni2 Ti4 Ox in nitinol. Main findings are summarized below: (1) The NiTi2 phase has the tendency to form along the grain boundary of NiTi in VIM ingot. (2) The solution heat-treatment at 1000 °C for 24, 36 and 48 h could decrease the fractions of NiTi2. However, the remaining phase were Ni2 Ti4 Ox stabilized by oxygen. (3) First principle calculations indicate that the formation enthalpy decreases with increasing amount of oxygen in Ni2 Ti4 Ox , resulting increased stability for Ni2 Ti4 Ox . Moreover, the NEXAFS results further clarify bonding relation in Ni2 Ti4 Ox , which O atom
slightly misplace Ni and Ti atoms and induce formation of ionic bond O-Ti and metallic bond Ni–Ni. (4) Phase transformation mechanisms have been deduced: (1) the dissolution of NiTi2 (with low oxygen content) due to the low energy barrier ΔH, (2) the diffusion of oxygen in Ni2 Ti4 Ox phase to increase its thermal stability. Acknowledgment The authors would like to acknowledge the financial support by the Metal Industries Research & Development Centre (MIRDC), Taiwan (R.O.C.) through projects 104A0183J4 and 105A0137J4. Declaration of interest None. References [1] S. Miyazaki, Y. Ohmi, K. Otsuka, Y. Suzuki, Characteristics of deformation and transformation pseudoelasticity in Ti–Ni alloys, J. Phys.-Paris 43 (1982) 255–260. [2] S. Miyazaki, T. Imai, Y. Igo, K. Otsuka, Effect of cyclic deformation on the pseudoelasticity characteristics of Ti–Ni alloys, Metall. Trans. A 17 (1986) 115–120. [3] K. Otsuka, K. Shimizu, Pseudoelasticity and shape memory effects in alloys, Int. Metals Rev. 31 (1986) 93–114. [4] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, 1999. [5] X. Huang, G.J. Ackland, K.M. Rabe, Crystal structures and shape-memory behaviour of NiTi, Nature Mater. 2 (2003) 307–311. [6] K. Otsuka, X. Ren, Physical metallurgy of Ti–Ni-based shape memory alloys, Prog. Mater. Sci. 50 (2005) 511–678. [7] J. Van Humbeeck, Non-medical applications of shape memory alloys, Mater. Sci. Eng. A 273 (1999) 134–148 Struct. [8] L. Janke, C. Czaderski, M. Motavalli, J. Ruth, Applications of shape memory alloys in civil engineering structures – overview, limits and new ideas, Mater. Struct. 38 (2005) 578–592. [9] D.J. Hartl, D.C. Lagoudas, Aerospace applications of shape memory alloys, Proc. Inst. Mech. Eng. Part G: J. Aerosp. Eng. 221 (2007) 535–552. [10] W. Predki, A. Knopik, B. Bauer, Engineering applications of NiTi shape memory alloys, Mater. Sci. Eng. A 481 (2008) 598–601. [11] T. Duerig, A. Pelton, D. Stockel, An overview of nitinol medical applications, Mater. Sci. Eng. A 273 (1999) 149–160 Struct. [12] M.H. Elahinia, M. Hashemi, M. Tabesh, S.B. Bhaduri, Manufacturing and processing of NiTi implants: a review, Prog. Mater. Sci. 57 (2012) 911–946. [13] C. Haberland, M. Elahinia, J.M. Walker, H. Meier, J. Frenzel, On the development of high quality NiTi shape memory and pseudoelastic parts by additive manufacturing, Smart Mater. Struct. 23 (2014) 104002. [14] C. Trepanier, R. Venugopalan, A.R. Pelton, Corrosion resistance and biocompatibility of passivated NiTi, in: Shape Memory Implants, Springer, 2000, pp. 35–45. [15] S. Shabalovskaya, J. Anderegg, J. Van Humbeeck, Critical overview of Nitinol surfaces and their modifications for medical applications, Acta Biomater. 4 (2008) 447–467. [16] S.A. Shabalovskaya, Surface, corrosion and biocompatibility aspects of Nitinol as an implant material, Bio-Med. Mater. Eng. 12 (2001) 69–109. [17] L.M. Schetky, M. Wu, Issues in the further development of Nitinol properties and processing for medical device applications, in: Medical Device Materials: Proceedings from the Materials & Processes for Medical Devices Conference, Anaheim, California, ASM International, 2004, p. 271. 2003, 8–10 September 2003. [18] N. Morgan, A. Wick, J. DiCello, R. Graham, Carbon and oxygen levels in nitinol alloys and the implications for medical device manufacture and durability, in: Proceedings of the International Conference on Shape Memory and Superelastic Technologies, ASM International, 2006, pp. 7–11. [19] Y. Shugo, S. Hanada, T. Honma, Effect of oxygen content on the martensite transformation and determination of defect structure in TiNi alloys, Bull. Res. Inst. Miner. Dress. Metall. 41 (1985) 23–34. [20] J. Frenzel, E.P. George, A. Dlouhy, C. Somsen, M.F.X. Wagner, G. Eggeler, Influence of Ni on martensitic phase transformations in NiTi shape memory alloys, Acta Mater. 58 (2010) 3444–3458. [21] A. Toro, F. Zhou, M.H. Wu, W. Van Geertruyden, W.Z. Misiolek, Characterization of non-metallic inclusions in superelastic NiTi tubes, J. Mater. Eng. Perform. 18 (2009) 448–458. [22] M. Rahim, J. Frenzel, M. Frotscher, J. Pfetzing-Micklich, R. Steegmüller, M. Wohlschlögel, H. Mughrabi, G. Eggeler, Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys, Acta Mater. 61 (2013) 3667–3686. [23] S.W. Robertson, A.R. Pelton, R.O. Ritchie, Mechanical fatigue and fracture of Nitinol, Int. Mater. Rev. 57 (2012) 1–36. [24] J. Mentz, J. Frenzel, M.F.X. Wagner, K. Neuking, G. Eggeler, H.P. Buchkremer, D. Stover, Powder metallurgical processing of NiTi shape memory alloys with elevated transformation temperatures, Mat. Sci. Eng. A 491 (2008) 270–278 Struct. [25] J. Frenzel, Z. Zhang, K. Neuking, G. Eggeler, High quality vacuum induction melting of small quantities of NiTi shape memory alloys in graphite crucibles, J. Alloys Compd. 385 (2004) 214–223.
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Full Length Article
Formation mechanism of Ni2 Ti4 Ox in NITI shape memory alloy Wei-Yu Kai a, Kai-Chun Chang a, Hsu-Fu Wu b, Shi-Wei Chen c, An-Chou Yeh a,∗ a
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, Republic of China Metal Processing R&D Department, Metal Industries Research & Development Centre, Kaohsiung 81160, Taiwan, Republic of China c National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, Republic of China b
a r t i c l e
i n f o
Keywords: Nitinol Microstructure characterization First principle calculation Near-edge x-ray absorption fine structure analysis Phase transformation
a b s t r a c t Ni2 Ti4 Ox phase is known to affect the formability and mechanical properties of NiTi shape memory alloy (nitinol), however, the underlying mechanism of its formation remains unclear, although its presence has long affected the production of nitinol in industry. The present study aims to determine the formation mechanism of Ni2 Ti4 Ox . A nitinol sample melted by vacuum induction melting technique was investigated. The as-cast sample possessed NiTi2 segregation along the grain boundary, although solution heat-treatments at 1000 °C could eliminate NiTi2 , Ni2 Ti4 Ox could be observed, indicating that some NiTi2 could transform into Ni2 Ti4 Ox oxide. The correlation between the NiTi2 segregation and the formation of Ni2 Ti4 Ox has been elucidated by first principle calculation and near-edge x-ray absorption fine structure analysis. Results indicate that the formation enthalpy of Ni2 Ti4 Ox is inversely proportional to the amount of oxygen pick-up. During the solution heat-treatment process, some NiTi2 could be dissolved back to the NiTi matrix, while some NiTi2 could transform into Ni2 Ti4 Ox and be stabilized by absorbing more oxygen. Based on findings in this work, possible measures to minimize the formation of Ni2 Ti4 Ox in nitinol are proposed.
1. Introduction Nitinol has attracted lots of attention ascribed to its pronounced pseudoelastic and shape memory properties [1–6]. With reversible and diffusionless phase transformation, pseudoelastic property refers to stress induced martensitic phase transformation from initial austenite phase accompanied with strain as high as 8%, which can be fully recovered upon unloading. While shape memory property is contributed to the recovery of shape changes from martensite to austenite phase after appropriate heating. With these functional properties, there are many nitinol devices developed for applications in aerospace industry, civil engineering etc. [7–10]. The most important applications of nitinol are in the medical sector [11–13], e.g., stent, orthodontic wire, drill etc. Comparing to other implant materials such as 316L stainless steel, nitinol possesses better mechanical properties, hysteresis behavior, and excellent biocompatibility etc. [14–16]. Although nitinol has been used for decades, the fabrication of nitinol products by common thermo-mechanical process still remains a challenge in the industry, since fracture of ingot can occur due to the presence of brittle Ni2 Ti4 Ox oxides in the microstructure [12,17]. Furthermore, Ni2 Ti4 Ox oxide can decrease the mechanical property, affect phase transformation temperature and functional properties of nitinol products [18–20]; studies have reported that fatigue performance of stent and orthodontic wire could be
∗
degraded by the presence of Ni2 Ti4 Ox [21–24]. The ingestion of oxygen in nitinol has been thought to be the main cause of problem. Although characterization of Ni2 Ti4 Ox have been a subject of interest [13,25,26], and recent studies have suggested that NiTi2 phase in nitinol can be stabilized by oxygen [[19,20,27–29]. However, it is unclear whether there exists a correlation between NiTi2 phase and Ni2 Ti4 Ox oxides, so the underlying mechanism of Ni2 Ti4 Ox oxide formation remains unknown to date. It is important to understand the underlying mechanism of Ni2 Ti4 Ox formation in order to minimize its formation and further improve the yield of production and properties of nitinol. 2. Experimental procedure A Ti-50.8 at.% Ni (55.9 wt % Ni) cylindrical ingot was prepared from high purity raw materials of nickel and titanium using the vacuum induction melting (VIM) with 10−5 torr atmospheric pressure at Metal Industries Research & Development Centre (MIRDC) in Taiwan (R.O.C.). The oxygen content of the VIM ingot was examined by oxygen analyzer (LECO TC500) and the content of oxygen was 300 ppm. Samples were then sectioned from the VIM ingot and subjected to solution heattreatments at 1000 °C for 24, 36 and 48 h in evacuated quartz tube with 1.3 × 10−3 torr atmospheric pressure followed by water quenching to minimize oxidation and second phase formation; oxygen contents were examined after solution heat-treatments by LEO TC500 and found to
Corresponding author. E-mail address: [email protected] (A.-C. Yeh).
https://doi.org/10.1016/j.mtla.2018.100194 Received 11 September 2018; Accepted 15 December 2018 Available online 23 December 2018 2589-1529/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Ni–Ti phase diagram.
Fig. 2. (a) As-cast sample, backscatter electron image (BEI), (b) as-cast sample, secondary electron image (SEI), (c) as-cast sample, BEI mode; the BEI images of segregation phases after solution heat-treatments at 1000 °C for (d) 24 h, (e) 36 h, (f) 48 h.
be almost the same as the initial content, i.e., 300 ppm, so there was negligible oxygen pick-up during the solution heat-treatment processes. The crystal structures of the as-cast and heat-treated specimens were characterized by X-ray diffractometer (XRD, Bruker_D2 PHASER) with Cu K𝛼 radiation. The microstructures for all specimens were observed by optical microscopy (OM, OLYMPUS BX51M), scanning electron microscopy (SEM, JEM-5410 and Hitachi SU8010) and transmission electron microscopy (TEM, Tecnai F30 and JEOL JEM-3000F). All specimens were cut by wire electrical discharge machining followed by mechanical grinding and polishing to eliminate surface oxides and contami-
nants. Specimens for TEM analysis were grinded to 30–50 𝜇m, followed by twin-jet electropolishing with 20% sulfuric acid and 80% methanol solution electrolyte at 19.5 V and −20 °C. The detailed chemical compositions of phases in each specimen were measured by energy dispersive spectrometer (EDS) and Electron Probe Microanalyzer (EPMA, JEOL JXA-8500). CALPHAD simulation (Thermo-Calc, TCNI8 database) [30] was used to predict possible phases in the Ni–Ti system [31,32]. Furthermore, first principle calculation based on density functional theory was utilized [33–35], and a computational quantum mechanics model and
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Fig. 3. The XRD pattern of as-cast sample; solution heat-treated samples.
Fig. 4. The EPMA mappings of segregation phases. (a) as-cast sample; (b) solution heat-treated at 1000 °C for 24 h; (c) solution heat-treated at 1000 °C for 36 h.
thermodynamic properties of phases could be established by calculating the electron interaction between each other [36–39]. To analyze the formation of NiTi2 and Ni2 Ti4 Ox , the Material Studio 7.0 software was used to calculate the formation enthalpies of cubic NiTi2 and Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1) based on Cambridge serial total package (CASTEP) with the generalized gradient approximation of Perdew–Burke–Euzerhof [35,40]. Furthermore, in order to shorten
the computation time, 1 × 1 × 1 supercells of cubic NiTi2 and Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1) with FCC structures were constructed with a plane wave cutoff energy of 380 eV and a convergence total energy of 5 × 10−7 eV/atom. The near-edge x-ray absorption fine structure (NEXAFS) of as-cast and solution heat treated samples was determined in the BL05B2 beamline in National Synchrotron Radiation Research Center (NSRRC) in Taiwan (R.O.C.). A fixed-exit double-crystal Si(111)
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Fig. 5. The TEM micrographs and the corresponding diffraction patterns of NiTi2 and Ni2 Ti4 Ox (a) NiTi2 in as-cast state; (b) Ni2 Ti4 Ox in sample solution heat-treated at 1000 °C for 36 h. Fig. 6. The supercell of (a) NiTi2 , (b) Ni2 Ti4 O0.25 , (c) Ni2 Ti4 O0.5 , (d) Ni2 Ti4 Oo.75 , (e) Ni2 Ti4 O.
monochromator was served to tune the X-ray photon energy by controlling an average energy resolution dE/E = 2 × 10−4 ; O K-edge spectra was recorded by collecting surface-electrons [41].
Table 1 The composition of matrix and segregation phases (in at.%) in VIM and VIM+soluted samples by EPMA analysis.
3. Results and discussion VIM
The phase diagram of Ni–Ti system is shown in Fig. 1; there is a relatively narrow window of NiTi non-stoichiometric composition space at high temperatures with NiTi2 and Ni3 Ti phases at each side, so it is likely that segregation during VIM process can induce either formation of NiTi2 or Ni3 Ti depending on the composition and the degree of segregation during casting. The composition of the bulk ingot in present study was Ti-50.8 at.% Ni, and a solution heat-treatment at 1000 °C was chosen (indicated by red dash tie-line). Microstructure of as-cast and solution heat-treated specimens are shown in Fig. 2, and the corresponding XRD analysis is shown in Fig. 3. According to the EPMA analysis (Table 1), NiTi2 was identified along the grain boundary in as-cast condition, which was associated with relatively large solubility of Ti, since
Solu for 24 h Solu for 36 h Solu for 48 h
Area
O
Ti
Ni
Matrix Segregation Segregation Segregation Segregation
0 0.75 2.26 4.58 5.37
50.326 66.49 59.38 62.13 61.19
49.668 32.76 38.36 33.28 33.44
the concentration of Ti increased in the liquid when NiTi was formed during solidification, Fig. 1, and thus the solidification process could induce the formation Ti-rich NiTi2 phase at the NiTi grain boundaries. Fig. 3 further confirms that not only cubic NiTi but also NiTi2 phase existed in as-cast condition (Ni2 Ti4 Ox possesses a very similar FCC struc-
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Fig. 7. The NEXAFS of as-cast sample, and solution heat-treated samples at 1000 °C for 48 h: (a) O K-edge spectra; (b) Ti L-edge spectra; and (c) Ni L-edge spectra..
ture). Fig. 2(b) and (c) show some cracks along the NiTi2 phase, which indicates that the NiTi2 phase could cause cracks during VIM process. After the solution heat-treatments, the microstructure contained lesser amount of grain boundary segregation phase as shown in Fig. 2(d)–(f). The corresponding XRD results is shown in Fig. 3. The XRD patterns show undetectable NiTi2 peaks in solution heat-treated samples comparing to that of the as-cast sample. However, solution heattreatments did not eliminate the segregation, and there was Ni2 Ti4 Ox present instead, even with an increase in duration from 24 to 48 h (indicated by segregation phase along grain boundaries in Fig. 2). Detailed analysis on the chemical compositions corresponding to the mapping results of EPMA are shown in Table 1 and Fig. 4. According to Table 1, it is possible that the initial NiTi2 had the ability to pick up a trait of oxygen in as-cast condition and the amount of oxygen content in Ni2 Ti4 Ox is proportional to the length of solution heat-treatment. Further examination on the results of EPMA mappings in Fig. 4(a) shows that the amount of oxygen contained in NiTi2 segregation in the as-cast condition was minimal and similar to that of the NiTi matrix. However, after solution heat-treatment, the oxygen content in Ni2 Ti4 Ox increased significantly and could be distinguished easily from the matrix shown in Fig. 4(b) and (c). According to Fig. 4 and Table 1, oxygen may solute in the NiTi2 structure and cause NiTi2 to transform into Ni2 Ti4 Ox . This would agree to some degree with literatures stating that oxygen may solute in the NiTi2 phase and stabilize it [19,20,27–29]. Fig. 5(a) and (b) show a NiTi2 and a Ni2 Ti4 Ox in as-cast condition and after a solution heat-treatment for 36 h, respectively; both phases are FCC and electron diffraction patterns of phases are highlighted by white circles. Since SEM observations (Fig. 2) indicate that both NiTi2 and a Ni2 Ti4 Ox were present at grain boundaries, it is reasonable to assume that phases in Fig. 5 were taken at grain boundaries. Compositions measured by TEM-EDS and lattice constants of the NiTi2 and the Ni2 Ti4 Ox in Fig. 5 are summarized in Table 2. After a 36 h of solution heat treatment, the oxygen content was increased to 7.85 at.%, while the
Table. 2 The composiitons (in at.%) and lattice constants (Å) of phases in Fig. 5.
Fig. 5(a) Fig. 5(b)
Phase
O
Ti
Ni
Lattice constant
NiTi2 Ni2 Ti4 Ox
0.75 7.85
66.49 61.96
32.76 30.19
11.44 11.74
lattice constant was increased from 11.44 to 11.74 Å which agrees with values reported in literature [28]. Thereby, TEM analysis indicates that it is hard to distinguish between NiTi2 and Ni2 Ti4 Ox phases by XRD analysis. However, it appears that there may exist some correlation between these phases due to their similarity in crystal structure. The following describes an effort to utilize first principle calculation and synchrotron radiation to uncover the underlying mechanism of Ni2 Ti4 Ox formation. After knowing that NiTi2 and Ni2 Ti4 Ox have similar crystal structures, we can assume that oxygen may solute in the NiTi2 segregation and then transform it into Ni2 Ti4 Ox oxides. The correlation between NiTi2 and Ni2 Ti4 Ox was elucidated by calculating the formation enthalpy of NiTi2 and Ni2 Ti4 Ox using first principle calculation. Fig. 6 presents the constructed supercells (the essential parameters used were based on the literature [28]), and the value of formation enthalpy for NiTi2 and Ni2 Ti4 Ox can be calculated from formula (1) and (2), respectively: ΔHfor =
ΔHfor =
( ) 𝐸 Ni Ti2 − 32𝐸 (Ni) − 64𝐸 (Ti) 96 ( ) 𝐸 Ni2 Ti4 O𝑥 − 32𝐸 (Ni) − 64𝐸 (Ti) − 16𝑥𝐸 (O) 96 + 16𝑥
(1)
(2)
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Fig. 8. The simulation results of lattice constants, positional parameters, and bonding lengths in (a) NiTi2 ; and (b) Ni2 Ti4 Ox .
Table. 3 The lattice structure and the numbers of atoms in the supercell used in the first principle simulation and the formation enthalpy (in KJ/mol) of NiTi2 and Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1) calculated at 0 K. Supercell
Ni
Ti
O
structure
Formation enthalpy
NiTi2 Ni2 Ti4 O0.25 Ni2 Ti4 O0.5 Ni2 Ti4 O0.75 Ni2 Ti4 O NiTi
32 32 32 32 32 1
64 64 64 64 64 1
– 4 8 12 16 –
Fd3̄ m Fd3̄ m Fd3̄ m Fd3̄ m Fd3̄ m Fd3̄ m
−63.3 −1766.43 −3339.33 −4795.82 −6148.27 −30.66
where E(NiTi2 ), E(Ni2 Ti4 Ox ), E(Ni), E(Ti), and E(O) are the total energy of intermetallic compound NiTi2 , oxide Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1), and stable state elements Ni, Ti and O. The composition, lattice structure and calculated results of formation enthalpies for intermetallic compounds NiTi2 and oxides Ni2 Ti4 Ox (x = 0.25, 0.5, 0.75 and 1) are summarized in Table 3. Firstly, it shows that calculation results of formation enthalpies for all compounds are negative, this means that each compound can form at 0 K. Moreover, it also shows the trend of formation enthalpy is inversely proportional to the amount of oxygen, i.e., Ni2 Ti4 Ox would become more stable with increasing amount of oxygen content. By considering results of both first principle calculation and compositions shown in Tables 1 and 2, we can explain why the grain boundary phase cannot be fully eliminated after solution heat-treatments, Fig. 2(d)–(f). After conducting solution
heat-treatments, diffusion of oxygen into NiTi2 occurred and thus transformed it into a more stable structures of oxides, i.e., Ni2 Ti4 Ox , which was stable and could not be dissolved into the matrix at 1000 °C. To confirm the results of first principle calculation, the BL05B2 beamline in NSRRC, Taiwan (R.O.C.) was used to characterize Ni2 Ti4 Ox . Fig. 7(a) presents the peak of normalized intensity (or called white line) from O, and it has increased after solution heat-treatment, furthermore the wave of photon energy has shifted to lower state after the solution heat-treatment. On the other hands, Fig. 7(b) shows photon energy from Ti, and the peak has also increased, but the wave has shifted to higher energy state after solution heat-treatment. The results of Ni are shown in Fig. 7(c), the peak intensities of Ni has decreased and the wave has shifted to higher photon energy state. The underlying mechanisms could be contributed to the electrons transferred and the bonding formation during solution heat-treatment process. Firstly, the increase of normalized intensity presents higher d orbital unoccupied electronic states as well as holes formation at the d-band. Thereby, the increase of peak intensities for Ti and O indicates that electrons sharing and the increase in the unoccupied electronic states in each d-band, and the efficiency to induce formation of ionic bond between Ti and O. Moreover, the much increase of peak O has presented the better local coordination symmetry for O atoms, which shows the stable location of O at tetrahedral site, thus the stability of Ni2 Ti4 Ox could be enhanced with higher content of oxygen. However, the decrease of Ni peak implies that Ni not only gets more electrons to fulfill unoccupied electronic states at the d-band but also do not share electrons with O to form strong bond. Secondly, the shifting of wave indicates the changing of oxidation number or the lattice distortion of the compound. The shifting of O wave to lower en-
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Fig. 9. Schematic illustration of the mechanism of (a) the dissolution of NiTi2 ; (b) the stabilization of Ni2 Ti4 Ox ; and (C) a transition phase: the dissolution and stabilization of the complex structure of Ni2 Ti4 Ox .
ergy state and the shifting of Ti wave to higher energy state indicate the electrons transfer from O to Ti, which could induce the energy compensation in whole system. So the increase in oxygen content in Ni2 Ti4 Ox could result a more stable structure. However, the shifting of Ni wave to higher energy state can attribute to the lattice distortion, implying the insertion of O atom in compound would increase the lattice parameter of Ni2 Ti4 Ox structure [28], and this can be seen in Table 2. To evaluate the bonding characteristics between these atoms, the simulation results have been analyzed and shown in Fig. 8. Fig. 8(a) presents that Ni would share electrons with Ti to form two different bonds: Ni–Ti(1) with 2.61321 Å and Ni–Ti(2) with 2.48438 Å. These two bonds are all single bond between Ni and Ti in NiTi2 . However, after inserting O in NiTi2 to form Ni2 Ti4 O as shown in Fig. 8(b), the lattice parameter would increase from 11.3193 Å to 11.3279 Å, and promote formation of two new bonds, i.e., Ni–Ni with 2,68,933 Å, and O-Ti(1) with 2.12426 Å. The formation of bond O–Ti(1) can be the proof of formation of ionic bond between Ti and O, which can further stabilize the oxide structure. On the other hands, the insertion of O atom could cause an increase of bond length of Ni–Ti(1) to 2.66896 Å, and a decrease of bond length of Ni–Ti(2) to 2.45509 Å. The underlying mechanism could be attributed to the insertion O atom would increase the repulsive force between atoms of O and Ni as well as Ti(1), shifting atomic positions slightly away the equilibrium positions of Ni and Ti(1) indicated by yellow arrows in Fig. 8(b). However, inserting O atoms can also be seen as being fixed by surrounding atoms of Ni and Ti(1), which can highly stabilize O atoms at tetrahedral site. Moreover, it could also reduce the distance between Ni atoms, and induce the formation of metal bond Ni– Ni, which would further share electrons with other Ni atoms. Thereby, this analysis not only suggests that shifting of Ni wave to higher energy state could attribute to the misplace of Ni atoms but also indicates
the fulfillment of unoccupied electronic states in Ni contributed to the formation of metal bond Ni–Ni. This study has shown that “the NiTi2 segregation can be the precursor of Ni2 Ti4 Ox oxide formation”. Figs. 2 and 3 indicate that the amounts of NiTi2 decreased after solution heat-treatments. Composition measurements in Tables 1 and 2 show that amount of oxygen solutioned in Ni2 Ti4 Ox increased with prolong duration of heat-treatment. According to results of first principle calculations in Table 3, it indicates that the oxygen content could promote the formation of Ni2 Ti4 Ox oxide ascribed to the lower formation enthalpy. With an increase in oxygen content, the results of Fig. 8(b) shows that the formation of ionic bond O–Ti(1) and metallic bond Ni–Ni would further stabilize the structure of Ni2 Ti4 Ox oxide. Although solution heat-treatments decreased the amount of NiTi2 as shown in Fig. 2, but it also promoted the oxygen to diffuse into NiTi2 and thus transformed it into a stable structure of Ni2 Ti4 Ox oxide during the process. So, why would some NiTi2 get dissolved into the matrix while some NiTi2 would transform into Ni2 Ti4 Ox after solution heattreatment? To describe the underlying mechanism of this interesting phenomena, schematic illustrations of phase transformations are shown in Fig. 9. There are three kinds of phases shown in Fig. 9, including the NiTi, NiTi2 , and Ni2 Ti4 Ox . The NiTi2 phase may have a little trait of oxygen, which could be ignored. The NiTi2 can transform into Ni2 Ti4 Ox with certain amount of oxygen content. In addition, an intermediate phase might be present, i.e., Ni2 Ti4 Ox phase with a concentration gradient of oxygen from the surface to inner part of the structure. The first mechanism is the dissolution of NiTi2 shown in Fig. 9(a). The second mechanism shown in Fig. 9(b) is the diffusion of oxygen in Ni2 Ti4 Ox 1 phases and thus transform it into more stable phases of Ni2 Ti4 Ox 2 (where x2 > x1 ). To elucidate the underlying mechanism
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Materialia 5 (2019) 100194
whether dissolution of NiTi2 or formation of Ni2 Ti4 Ox can occur, it is necessary to calculate the energy barrier ΔH, which needs to be overcome for dissolution to occur. According to the phase diagram shown in Fig. 1, it presents that the solution heat-treatment at 1000 °C would result one phase of NiTi, which implies that there is a driving force to dissolve NiTi2 segregation into NiTi matrix. According to the first principle calculation results in Table 3, the ΔH between the NiTi and the NiTi2 is about 30 KJ/mol. However, the ΔH between NiTi and Ni2 Ti4 Ox oxide can be larger than the former depending on the amount of oxygen solutioned. Therefore, the first mechanism ascribed the low energy barrier ΔH, so that the solution heat-treatment can easily drive the dissolution of NiTi2 into the NiTi matrix. However, due to oxygen diffusion into the Ni2 Ti4 Ox 1 phase, the second mechanism resulted higher energy barrier ΔH, hence solution heat-treatment at 1000 °C was not sufficient to overcome it. On the other hands, according to the lower formation energy of Ni2 Ti4 Ox phases, the oxygen will continue to diffuse into the Ni2 Ti4 Ox 1 to form more stable Ni2 Ti4 Ox 2 (where x2 > x1 ). From the kinetics point of view, solution heat-treatment can promote the oxygen to diffuse into the Ni2 Ti4 Ox . Thereby, the second mechanism can be described by the following formula (3): NiTi + Ni2 Ti4 OX1 +O2 → NiTi + Ni2 Ti4 OX2
(3)
where x1 and x2 = 0, 0.25, 0.5, 0.75 or 1. According to this formula, the ΔH of the reaction can be computed to show that if the x2 > x1 , the whole structure would become more stable than the former. The transition phase is shown in Fig. 9(c). According to the EPMA analysis shown in Fig. 4, there existed a concentration gradient of oxygen from inner to the surface, hence the size and fractions of Ni2 Ti4 Ox could vary with different length of solution heat-treatments. To the best of authors’ knowledge, the present article is the first to elucidate the underlying mechanism of Ni2 Ti4 Ox formation in association with NiTi2 from as-cast state. Through experimental analysis, simulation and calculations, it can be concluded that NiTi2 is a product of segregation during the melting of nitinol, although solution heat-treatment can eliminate NiTi2 from thermodynamic point of view, kinetic aspect of oxygen ingestion into NiTi2 could transform it into thermodynamically stable Ni2 Ti4 Ox , hence Ni2 Ti4 Ox could be observed instead after solution heat- treatments. It is very interesting to note that after VIM process (10−5 torr atmospheric pressure), the sample contained 300 ppm oxygen, which could be originated from the raw materials, and solution heat-treatments at 1000 °C up to 48 h (1.3 × 10−3 torr atmospheric pressure) did not further increase the oxygen content in samples. However, 300 ppm oxygen was sufficient to transform some NiTi2 in as-cast state into Ni2 Ti4 Ox , although majority of NiTi2 got dissolved into the matrix. So, there are actions that could be taken during the fabrication of nitinol to minimize Ni2 Ti4 Ox formation; (1) increase the purity level of raw Ni and Ti materials for melting process, (2) faster solidification rate during the melting process to decrease the degree of casting segregation, so fractions of NiTi2 segregation (the precursor of Ni2 Ti4 Ox ) can be minimize, and a subsequent solution heat-treatment can be applied to dissolve most of the NiTi2 segregation in order to minimize the fractions of Ni2 Ti4 Ox and improve the yield of production of nitinol. 4. Conclusions This study has presented the underlying mechanism of the formation of Ni2 Ti4 Ox in nitinol. Main findings are summarized below: (1) The NiTi2 phase has the tendency to form along the grain boundary of NiTi in VIM ingot. (2) The solution heat-treatment at 1000 °C for 24, 36 and 48 h could decrease the fractions of NiTi2. However, the remaining phase were Ni2 Ti4 Ox stabilized by oxygen. (3) First principle calculations indicate that the formation enthalpy decreases with increasing amount of oxygen in Ni2 Ti4 Ox , resulting increased stability for Ni2 Ti4 Ox . Moreover, the NEXAFS results further clarify bonding relation in Ni2 Ti4 Ox , which O atom
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