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Novel corrosion behaviours of the annealing and cryogenic thermal cycling treated Ti-based metallic glasses
Intermetallics 110 (2019) 106467
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Novel corrosion behaviours of the annealing and cryogenic thermal cycling treated Ti-based metallic glasses
T
Jialun Gu, Yang Shao, Lingxiang Shi, Jiajia Si, Kefu Yao∗ School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Metallic glasses Annealing Corrosion Electrochemistry Surface properties Free volume
Effects of structural relaxation and cryogenic thermal cycling on corrosion resistance of the Ti50Zr20Be20Ni10 bulk metallic glass (BMG) was investigated in 3.5 wt% NaCl solution. The annealed Ti-based BMGs exhibited striking high pitting potentials with low corrosion current densities, while cryogenic thermal cycling treated BMGs performed poor corrosion resistance. More stable passive films were found to form on the surfaces of annealed BMGs through electrochemical impedance spectra analysis. Sub-sized pits that evolved along with micrometre-sized pits on corroded morphologies of the as-cast and cryogenic thermal cycling treated samples clarified the aggressive corrosion. Annealing treatment gave rise to the annihilation of excessive free volume within BMGs, while cryogenic thermal cycling induced the proliferation of free volume. Moreover, the variation of microhardness further reflected the fluctuation of free volume. The XPS analysis revealed that the superior corrosion resistance of the annealed samples could be attribute to the increment in the concentration of Ti, Zr and distinct decrement in the concentration of Be. It is concluded that excessive free volume would deteriorate stability of surface passive film.
1. Introduction Ti-based bulk metallic glasses (BMGs) have attracted massive researches owning to the outstanding properties, such as high specific strength, high yield strength and preferable glass forming ability (GFA) [1–5]. And Ti-based BMGs exhibit excellent performance in applications where lightweight design is considered as a key criterion, e.g., the flexspline in strain wave gears [6] and bumper of Whipple shields [7]. In contrast to the mechanical properties that have been excessively studied, the electrochemical behaviour of Ti-based BMGs has not been well understood. As one of the most common failure mode, corrosion usually leads to structural damage and eventually fractures, playing an important role in material-service security. Therefore, comprehensive work should be conducted to explore the electrochemical behaviour of Ti-based BMGs. Typically, metallic glasses exhibit corrosion resistance with one to two orders of magnitude higher than their crystalline counterparts with identical composition in most aqueous solutions, because of microstructural and component homogeneity, and the lack of crystalline defects (e.g., grain boundaries, second phases) [8–10]. Nevertheless, a few crystalline phases precipitating from the amorphous matrix are found to promote to form stable passive film and improve corrosion resistance. For example, Guo at al. reported that the formation of the ∗
highly protective Zr and Al enriched oxide layer at the surface of crystalline alloy was responsible for its extremely high corrosion resistance compared with the amorphous counterpart [11]. As for the alloys with completely amorphous structural, the most common strategy to modulate corrosion resistance is to tailor chemical compositions by micro-alloying strong passive elements (e.g., Cr, Nb, Ti) [12–16]. Furthermore, recent studies reported that annealing treatment could induce the enhancement of corrosion resistance based on structural relaxation with the annihilation of excessive free volume [17,18]. However, most of these studies focus on the superior corrosion properties in terms of the limited active-passive transformation behaviour, which make quantitatively comparison and analysis of the improvement unreliable, because the current density corresponding to the active dissolution region is quiet difficult to be identified [19–21]. In this work, we investigated the electrochemical behaviour of the as-cast, annealed and rejuvenated Ti50Zr20Be20Ni10 bulk metallic glasses (BMGs) in 3.5 wt% NaCl solution. Significantly enhanced corrosion resistance of the annealed samples had been observed, which was qualified with clearly larger values of pitting potentials than those of the as-cast and rejuvenated sample. In addition, cryogenic thermal cycling treatment was imposed on the Ti50Zr20Be20Ni10 BMG to further confirming the effect of change in atomic structure on corrosion resistance, which could rejuvenate the amorphous structure to a higher
Corresponding author. E-mail address: [email protected] (K. Yao).
https://doi.org/10.1016/j.intermet.2019.04.010 Received 9 December 2018; Received in revised form 1 April 2019; Accepted 7 April 2019 Available online 24 April 2019 0966-9795/ © 2019 Elsevier Ltd. All rights reserved.
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peaks, indicating the formation of monolithic amorphous structure without any detectable crystallization peaks. It was reported that the first diffraction peak which was related to the distance between neighbouring atoms might shift obviously during annealing process because of the transformation of free volume and topological shortage range order [25,26]. And the shift in the peak position could be observed due to the structural relaxation after annealing and cryogenic thermal cycling treatments through high energetic synchrotron beam [22,27,28]. However, the angular positions of the first diffraction peak did not obviously shift for the annealed and cryogenic thermal cycling treated samples within the sensitivity of the XRD measurement, as compared with that of the as-cast sample. Analogous result was also reported in Zr-based BMGs after heat annealing process [29].
energy state [22–24]. The electrochemical parameters including corrosion current density, pit potential and impedance had been derived and fitted from the potentiodynamic polarization curves. Potentiostatic polarization, electrochemical impedance spectrum and X-ray photoelectron spectroscopy tests were applied to analyse the improvement of the corrosion resistance quantitatively. 2. Experimental procedures Specimen preparation. The alloy ingots of Ti50Zr20Be20Ni10 were prepared by arc-melting the mixture of Ti (99.99%), Zr (99.7%), Be (99.7%) and Ni (99.99%) in pure argon atmosphere. Every ingot was remelted at least three times and then cylindrical samples of 3 mm in diameters were fabricated by copper mold suction casting. Amorphous structure of the samples was examined by X-ray diffraction (XRD; Rigaku D/max-RB with Cu Kα radiation, Japan). The samples were annealed at 575 K (Tg-20 K) and 605 K (Tg+10 K) for 10 min. Defining the as-cast samples, the samples annealed at 575 K, the samples annealed at 605 K as T0, T575 and T605, respectively. The annealing treatment was conducted in pure argon atmosphere at a heating rate of 40 K/min. Then the as-annealed specimens were cooled down to the ambient temperature by water quenching. Cryogenic thermal cycling treatment was conducted in ambient atmosphere. The as-cast Ti-based BMGs were inserted into boiling water (373 K) for 1 min first, and then inserted into liquid nitrogen (77 K) for 1 min. Samples were treated up to 15 cycles (defined as T15C) and 30 cycles (defined as T30C), respectively. Specimen characterization. Thermal properties were investigated by differential scanning calorimeter (DSC, NETZSCH STA 449) at a heating rate of 20 K/min. Vicker microhardness (HV) was conducted on the cross-section at 200 g load with 10 s indentation time, each quoted value is an average of at least 4 independent measurements. The surface morphologies of the tested samples were investigated by LEO-1530 scanning electron microscope (SEM). Electrochemistry measurement. Corrosion behaviours of the ascast and as-annealed samples were evaluated in 3.5 wt% NaCl solution by electrochemical measurements, performed in a three-electrode cell setup with working electrode, saturated calomel reference electrode (SCE) and platinum counter electrode. Initially, every sample was immersed in 3.5 wt% NaCl solution for 30 min to stabilizing the open circuit potential. The electrochemical impedance measurements were performed at an AC amplitude of 10 mV and a frequency range from 10−2 Hz–105 Hz. The EIS data was fitted by the Z-view to a Randles circuit with constant phase element. Afterwards, the potentiodynamic polarization curves were recorded at a potential sweep rate of 0.333 mV/s with a starting potential of about 100 mV below the OCPs. Potentiostatic polarization was conducted independently at a polarized potential of 100 mV/SCE (in the passivation-region) for 3600 s, to investigate the characterization of passive films. Before the corrosion experiments, the cross-sections of all tested samples were mechanically ground with up to 2000 grit SiC papers and cleaned in ethanol, exposed in dry air for 24 h. X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM) monochromatized Al Kα source was conducted to characterize to composition and chemical stated of the passive film, the tested samples were potentiostatically polarized for 10 min at 0 V in their passive regions and then take out for XPS tests. The XPS spectrum were analysed by XPS – PEAK to get further information about chemical states.
The DSC traces of the as-cast, annealed and cryogenic thermal cycling treated samples were shown in Fig. 2 to evaluate the thermal behaviour. Exothermic reactions of the test samples which originated from structural relaxation were clearly observed below the Tg. The area of the exothermic peaks below the dotted line corresponded to the structural relaxation exothermic heat (ΔH ). The ΔH values of the T0, T575, T15C and T30C were 4.45 J/g, 0.87 J/g, 4.99 J/g and 5.79 J/g respectively. And no exothermic heat change was detected for the T605 sample, indicating the T605 sample had been completely relaxed. The difference in exothermic behaviour below Tg was related to the diversity of the amount of free volume in the tested samples. The amount of annihilated free volume during annealing treatment is proportional to the exothermic heat of the structural relaxation [30]. It is well known that annealing treatment usually leads to structural relaxation characterized by the annihilation of free volume in metallic glasses, while cryogenic thermal cycling treatment have been confirmed to be a rejuvenation-process agitating a less relaxed state, implying more free volume could be induced into the glassy matrix [23,24,31]. Hence, the DSC results demonstrated that the comparable precursor for next electrochemical test with different fractions of free volume in glassy matrix had been prepared by annealing and cryogenic thermal cycling treatment. In order to better understand the influence of specific treatments on amorphous structural, Vicker microhardness measurements were conduct on the samples right after the treatment of annealing and cryogenic thermal cycling. As presented in Fig. 3, The samples that were subjected to annealing and cryogenic thermal cycling treatment shown statistically significant differences in HV values. For the samples in annealed state, the Vickers microhardness increased form 500.0 ± 6.5 HV (the as-cast sample T0) to 537.9 ± 6.5 HV (the annealed sample T575) and 547.9 ± 6.5 HV (the annealed sample T605), it could be elucidated by the annihilation of free volume during the annealing process [17,18,20,32]. In contrast, a softening effect was observed in the cryogenic thermal cycling treated samples, characterized by lower microhardness of 482.0 ± 8.8 HV (the sample T15) and 476.4 ± 9.0 HV (the sample T30). S. V. Ketov et al. reported an analogous softening in metallic glasses originated from cryogenic thermal cycling treatment [23]. Moreover, there was no difference in HV values between near the centre and the circumference sites for all the tested samples (the indents’ sites were schematically marked in Fig. 3), inferring that both annealing and cryogenic thermal cycling treatment would not lead to the macroscopic heterogeneity on the cross-sections.
3. Results and discussion
3.3. Electrochemical characteristics
3.1. Structural characterization
Fig. 4 (a) displayed the immersion time-dependent OCPs of the Ti50Zr20Be20Ni10 BMG in as-cast, annealed and cryogenic thermal cycling treated states. OCPs of all the tested samples increased with immersion time and further reached stationary values, while the potentials of the annealed samples shifted faster at the initial stage and their
3.2. Thermal and microhardness characterization
The XRD patterns in Fig. 1 presents the structural evolution of the as-cast, annealed and cryogenic thermal cycling treated samples of the Ti50Zr20Be20Ni10 BMG. All the tested samples display broad diffuse 2
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Fig. 1. (a) XRD spectra of the Ti50Zr20Be20Ni10 BMG in the as-cast, annealed and cryogenic thermal cycling treated states; (b) the sketch of cryogenic thermal cycling process.
passive current density was observed in the order of 10−6 mA/cm2. Upward scanning, all samples exhibited typical pitting corrosion features under anodic polarization, characterized by the abrupt increase of current density at a critical potential, identified as pitting potential. Important electrochemical parameters including pitting potential (Epit), corrosion potential (Ecorr), passive region width (Epit - Ecorr), and passive current density (ipass) attained from potentiodynamic polarization curves were summarized in Table 1. No distinct difference in passive current density for all the tested samples. Although the tested samples performed similar polarization behaviours, striking difference in the corrosion resistance in NaCl solution was clearly recorded by polarization curves. In addition, the annealed samples exhibited higher Epit and wider passive region as shown in Table 1, suggesting more protective and stable passive film had been formed. Comparing these electrochemical parameters, the annealed sample showed superior corrosion properties and significant improvement on corrosion resistance in NaCl solution could be attributed to annealing treatment. However, compared with the as-cast and annealed samples, the cryogenic thermal cycling treated samples exhibited worse corrosion resistance with lower Epits. Fig. 5 shown the initial morphologies before electrochemical measurements (the surfaces of all the samples had been mechanically ground by SiC papers after annealing and cryogenic thermal cycling treatment) and presentative corrosion morphologies of the samples T0, T605 and T30 after potentiodynamic polarization measurements, the polarization tests were terminated once the current densities increased up to 1 mA/cm2. Generally, the result revealed almost identical corrosion morphologies that was characterized by pitting corrosion. It can be seen that only one/two primarily pits formed on the whole cross-sections, suggesting the localized characterization of pitting corrosion for the Ti-based BMGs. Besides, detailed investigations in the high magnifications of the pits revealed interesting morphologies. As shown in Fig. 5 (d), (e), (f), round-shape micrometre sized pits were visible at the
Fig. 2. DSC curves of the Ti50Zr20Be20Ni10 BMG in the as-cast, annealed and cryogenic thermal cycling treated states. The heat of structureal relaxation, ΔH , can be determined by integrating the area of exothermic peaks prior to glass trasition event.
finial potentials were more stable at the end of immersion compared with the as-cast and cryogenic thermal cycling treated samples. After immersion for 1800 s, the OCP values of the as-cast, T575, T605, T15C and T30C increased up to −502 mV, −538 mV, −557 mV, −485 mV and −458 mV, respectively. Generally, higher OCPs indicate the superior protectiveness of passive films. Corrosion resistance of the as-cast, annealed samples and cryogenic thermal cycling treated samples were evaluated by potentiodynamic polarization. As shown in Fig. 4 (b), all the tested samples were passivated spontaneously with progressing anodic polarization from the corrosion potential, followed by distinct passive regions (with the maximum width ∼ 1069 mV). Notably, no significant difference in the
Fig. 3. (a) Vickers microhardness HV of the Ti50Zr20Be20Ni10 BMG in the as-cast, annealed and cryogenic thermal cycling treated states. As schemetically shown in the inset, microhardness tests were conducted at the central area (black symbols) and around the edge (red symbols), respectively. (b) The scanning electron micrograph of the indents, distributed in the central area, inset is the highmagnification image.
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Fig. 4. Corrosion performance of the Ti50Zr20Be20Ni10 BMG in 3.5 wt% NaCl solution at 293 K. (a) variation in OCPs with increasing immersion time; (b) potentiodynamic polarization curves at a scan rate of 0.333 mV/s, the polarization tests were terminated at the current density of 1 mA/cm2. The surface oxide films had been removed after the annealing and cryogenic thermal cycling treatment.
by cryogenic thermal cycling treatment. This result was in line with the potentiodynamic polarization tests (see Fig. 4). To better understand the effects of rejuvenation and structural relaxation on corrosion resistance of Ti-based BMGs, electrochemical impedance spectrum was conducted after 30 min immersion in 3.5 wt% NaCl solution to assess the stability of passive film. The EIS test provides significant information on the corrosion resistance of electrodes and the electrode-solution interface under nearly non-polarizing condition [36]. As shown in Fig. 7, the response of the Nyquist curves for the tested samples was characterized by one time constants in the frequency range from 10−2 Hz–105 Hz. Thus, the equivalent circuit was proposed to fit the Nyquist plots, which composed of one constant phase element (CPE) due to the feature of semicircle in Nyquist curves. The model parameters, Rs represents the solution resistance, the parallel-connected elements (CPE//Rp) include the charge transfer resistance (Rp), are applied to analyse the electrochemical properties of the matrix/passive film interface. The impedance of CPE could be represented as Z(CPE) = Q0−1 (jω)−α , where Q0 is CPE parameter, j is −1 , ω is angular frequency and α is dispersion coefficient associated with surface inhomogeneity. The fitting result obtained for all samples were presented with the experiment results in Fig. 7, and the proposed equivalent was able to well simulate the electrochemical behaviour of the tested specimens. Table 2 showed fitting results of the electrochemical parameters based on EIS measurements. The polarization resistances of the annealed samples were greater than that of the as-cast and cryogenic thermal cycling treated samples, implying the polarization resistance increased with annealed temperature under the tested condition. Furthermore, the values of Rp increased in the order of T30C, T15C, T0, T575, T605. Compared with the as-cast and cryogenic thermal cycling treated sample, the annealed samples presented higher Rp indicating that the passive films formed on their surface were more stable. Thus, the migration of metal ions through the passive film towards the electrolyte would be slowed down for the annealed samples. According to the electrochemical parameters shown in Table 2, rejuvenation induced by cryogenic thermal cycling caused the opposite effects as compared with the annealing treatment. It should be noted that all samples had been mechanically ground with SiC papers to remove a layer of metal on the surface before the electrochemical tests. Therefore, subtle changes in the amorphous structure might have a great influence on the performance of surface passive film. The results of EIS agreed well with the potentiodynamic polarization measurements.
Table 1 The electrochemical parameters of the as-cast, annealed and cryogenic thermal cycling treated samples. Specimen
Ecorr (mV/SCE)
Epit (mV/SCE)
Epit - Ecorr (mV)
ipass (A/cm2)
T0 T575 T605 T15C T30C
−506 −538 −557 −485 −458
319 439 512 202 168
825 977 1069 687 626
3.07 × 10−6 2.17 × 10−6 1.85 × 10−6 1.87 × 10−6 2.09 × 10−6
± ± ± ± ±
16 14 11 18 20
± ± ± ± ±
13 10 12 8 11
bottom of the macroscopic primarily pits, and the mean diameter is approximately 4 μm. Concerning the sample T0 and T30, some subsized pits with cells of below 2 μm in diameter formed on the topography. The sub-sized pits formed randomly and no apparently preferential sites (the edge and bottom of the micrometre-sized pits) for nucleation. The multiple sized pits discussed above are similar to the corroded morphologies of the Fe-based BMG, which suffers from pitting corrosion and nano-porous structure occurs on the walls of micrometresized pits [33]. And the multiple sized pits usually were the characteristic of a aggressive pitting-nucleation process. Fig. 5 (g) – (i) shows the typical surface morphologies characterized by scratches, which were ground to remove surface oxide films after annealing and cryogenic thermal cycling treatment. Compared with the T0 samples, a more flat surface could be observed on the surface of the T605 sample, while deeper wear-furrows and more residual scraps formed on surface of the T30C sample. The difference in initial morphologies of the three samples is in accord with the microhardness, and the excessive surfacedefects and scraps probably promote the nucleation of pitting and further deteriorating the corrosion resistance. Potentiostatic polarization tests were performed at a constant-polarized potential of 100 mV/SCE for 3600s in 3.5 wt% NaCl solution to reveal the details of passivation-behaviour. As shown in Fig. 6, the current densities of the tested samples decreased sharply in the initial period time (about 600s), reflecting the evolution of primary oxide film in aggressive NaCl solution [34,35]. Then, the current densities transferred into a steady state with a nearly constant level about 4.0 × 10−5 mA/cm2, which might be attributed to the establishment of stable passive films. The analysis discussed above presented the fact that all the tested samples performed similar corrosion kinetic behaviour. Regarding the details of variation, the annealed samples exhibited the lowest values of current densities among all the tested samples. However, the cryogenic thermal cycling treatment triggered the undesirable increment of the current densities in passive region. Meaning the passive films formed on the surface of annealed samples were more protective and stable, and opposite effects might be induced
3.4. XPS analysis The characteristics of passive films have great influence on 4
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Fig. 5. Initial morphology before the electrochemical measurements (g), (h), (i), and corrosion morphology of the potentiodynamic polarized samples (all the polarization tests were terminated at the onset of current density rising up to 1 mA/cm2), including the whole corroded area and the detailed micrographs of pits. (a), (d) and (g) the sample T0; (b), (e) and (h) the sample T605; (c), (f) and (i) the sample T30C. The surface oxide films had been removed after the annealing and cryogenic thermal cycling treatment.
Fig. 6. Potentiostatic polarization curves of the as-cast, annealed and cryogenic thermal cycling treated samples, at a constant polarized potential of 100 mV/ SCE for 3600s in 3.5 wt% NaCl solution.
Fig. 7. Electrochemical impedance behaviour of the as-cast, annealed and cryogenic thermal cycling treated samples at the OCPs after 30 min immersion in 3.5 wt% NaCl solution. Insert: the equivalent circuit proposed for fitting the EIS data.
corrosion resistance of Ti-based BMGs, thus XPS measurements were applied to analyse chemical characteristics of the T0, T605 and T30C samples which were potentiostatically polarized in the passive region for 10min. As shown in Fig. 8 (a), the XPS survey spectra covers a wide binding energy region, consisting of the peaks of Ti 2p, Zr 3d, Be 1s, Ni 2p, O 1s and C 1s, etc. The peaks of Ti 2p3/2 and Ti 2p1/2 are identified at 458.5 eV and 464.2 eV for all the three samples, corresponding to Ti4+ oxidized state. The Zr 3d spectrum (Fig. 8 (c)) of the T0 and T30C
samples are composed of Zr4+ 3d5/2 (182.4 eV) and Zr4+ 3d3/2 (184.8 eV) peaks, and the metallic state peaks (Zr) appear to locate at 178.5 eV and 180.9 eV (as marked by the red dotted line). However, only the 3d5/2and 3d3/2 peaks of the oxidized state (Zr4+) could be found in the Zr 3d spectrum of the T605 sample. In addition, the Be 1s spectrum shows the binding energies of 113.4 eV and 111.7 eV which 5
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Table 2 Impedance fitting parameters of experimental samples obtained from electrochemical impedance spectra (EIS). Specimen
Rs (Ω · cm2)
Rp/(kΩ · cm2)
CPE Q0/(kS · cm−2 · sα)
α (0–1) T0 T575 T605 T15C T30C
2.33 2.58 2.73 2.18 2.13
± ± ± ± ±
0.38% 0.30% 0.37% 0.30% 0.42%
0.91 0.94 0.91 0.93 0.92
± ± ± ± ±
0.05% 0.04% 0.04% 0.05% 0.06%
163.91 156.84 132.12 177.33 184.40
± ± ± ± ±
0.28% 0.29% 0.22% 0.28% 0.23%
163.91 178.04 191.46 141.30 134.24
± ± ± ± ±
0.49% 0.67% 0.71% 0.51% 0.61%
Fig. 8. XPS survey spectrum (a) and elements spectra (b–f) of the as-cast, annealed and cryogenic thermal cycling treated samples, all the tested samples were potentiostatically polarized for 10 min at 0 V before XPS measurements. (b) Ti 2p, (c) Zr 3d, (d) Be 1s, (e) Ni 2p. The surface oxide films had been removed after the annealing and cryogenic thermal cycling treatment.
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high energy state provides high chemical activity and high atomic mobility [43]. Besides, the glassy matrix with larger fraction of free volume is characterized by a looser and more disordered atomic structure, corresponding to cryogenic thermal cycling treated samples. Hence, a non-dense and imperfect passive film is expected to form on the surface of T15C and T30C, inferring it would be more likely to be destroyed in corrosive medium as being proved by the results of EIS and potentiodynamic polarization tests. Once metastable sites initiate the nucleation of pitting, such non-dense atomic structure provides more routes and vacancies for chloride ion to transit and degrade the ability of re-passivation. Eventually result in worse corrosion resistance for the cryogenic thermal cycling treated samples.
Table 3 The normalized surface element concentration of the as-cast, annealed and cryogenic thermal cycling treated samples analysed by XPS. Specimen
Ti (%)
Zr (%)
Be (%)
Ni (%)
T0 T605 T30C
54.6 56.6 52.1
25.9 26.2 25.3
17.2 15.0 20.5
2.2 2.2 2.1
are assigned to Be2+ 1s state and Be 1s state, respectively. The Ni 2p spectrum consists of two peaks at 853.6 eV and 871.8 eV, corresponding to Ni2+ oxidized state. It is well known that the variation in components of passive film, even subtle changes, could have a great impact on the electrochemical behaviours of alloys. The atomic concentration of each element was calculated through integrating the XPS peaks and calibrated by relative sensitivity factors, the results are summarized in Table 3. It is noteworthy that the changes in the concentration of Ti and Zr agree well with the corrosion resistance of T0, T605 and T30C samples, and could provide a new insight into the effects of annealing and cryogenic thermal cycling. Compared with the as-cast and cryogenic thermal cycling treated samples, the annealing treated sample T605 performs the best corrosion resistance in 3.5 wt% NaCl solution, showing a increment in the concentration of Ti, Zr and a distinct decrement in the concentration of Be. This result indicates that the enhanced corrosion resistance by annealing could be related to the enrichment of Ti and Zr, and the deficiency of Be, the converse effects might be induced by cryogenic thermal cycling. The chemical characteristics of TiO2 and ZrO2 are highly stable and their formation on the surface could promote to enhance the stability of passive film, further protecting the glassy matrix in aggressive NaCl solution [37–39]. Besides, Guo et al. [11,40] have reported that the microscopic structural homogeneity of passive film of BMGs could be improved by the formation of TiO2, ZrO2, since these oxides could remarkably refrain the formation and growth of the heterogeneous galvanic cells in the film. In this work, the XPS results reveal that the annealing and cryogenic thermal cycling treatment could change elemental content and valence state in Ti-based BMGs, and further enhancing or deteriorating the corrosion resistance in NaCl solution.
5. Conclusions This work explored the influence of rejuvenation and structural relaxation on electrochemical behaviour of the Ti50Zr20Be20Ni10 BMG in 3.5 wt% NaCl solution. It is concluded that the corrosion resistance of the Ti-based BMGs could be significantly improved when the tested samples remain at lower energy metastable states with relative smaller amount of free volume induced by structural relaxation. However, the cryogenic thermal cycling treatment which was applied to rejuvenate the samples performed the contrary effects on corrosion resistance as compared with annealing treatment. The structural relaxation exothermic heat had been analysed quantitatively by DSC test, showing the free volume in the sample T605 had annihilated fully by the specific annealing treatment, while cryogenic thermal cycling treatment helped to introduce more free volume into the as-cast glassy matrix. As a result of the variation in free volume, Vickers microhardness increased for the annealed samples and the softening had been detected in the rejuvenated samples. In addition, the annealing treatment enhanced the corrosion resistance characterized by higher Epits with striking difference over 120 mV for the as-cast sample ∼319 mV (T575 ∼ 439 mV, T605 ∼ 512 mV), while the sample T30C exhibited the worst corrosion resistance with the Epit as low as 168 mV. Multiple sized pits that formed on the corroded morphologies verified the unfavourable corrosion resistance for the rejuvenated samples. The EIS result displayed the electrochemical impedance of the annealed samples were superior than that of the as-cast and rejuvenated samples, approving more stable passive film formed on the surface of the annealed samples. The XPS results reveal that the annealing and cryogenic thermal cycling treatment could change elemental content and valence state in Ti-based BMGs, and the increment in the concentration of Ti, Zr and distinct decrement in the concentration of Be could contribute to the superior corrosion resistance of the annealed samples.
4. Discussion The obviously and quantitatively comparable improvement of corrosion resistance induced by thermal relaxation had been demonstrated in this work. Meanwhile, in order to comprehensively investigate the structure-corrosion relationship in amorphous material, cryogenic thermal cycling treatment had been conducted as an opposite process of the annealing treatment. And the comparison results confirmed that the rejuvenation would deteriorate corrosion resistance of the as-cast Tibased metallic glass. Actually, similar enhancement of corrosion resistance resulting from thermal relaxation was reported in several ribbon metallic glasses and bulk metallic glasses [17,20,41]. Raicheff at al. thought the formation of a denser amorphous structure with low residual stresses contributed to the improvements [42]. Jiang at al. suggested the chemical potential and the formation of cluster might have significant influence on pitting initiation and propagation behaviours. The enhanced corrosion resistance caused by thermal relaxation could be associated with the corresponding electrochemical free energy of activation (ΔG ) [19]. The annealed samples might transfer to a lower energetic level of metastable state with depressed electrochemical activity, the annihilation of excessive free volume conduced to a denser atomic structure, which would limit the atomic mobility in a great degree. Thus, the ΔG increases and further the electrochemical corrosion become more difficult for the relaxed samples compared to the ascast sample. However, metallic glass contains large amount of free volume, suggesting it is at a higher energy level of metastability. Such a
Declarations of interest None. Acknowledgements This work was supported by the National Science Foundation of China (NSFC, Grant nos. 51571127). References [1] A. Raduta, M. Nicoara, C. Locovei, J. Eckert, M. Stoica, Ti-based bulk glassy composites obtained by replacement of Ni with Ga, Intermetallics 69 (2016) 28–34. [2] L. Ying, P. Shujie, L. Haifei, H. Qiao, C. Bin, Z. Tao, Formation and properties of Tibased Ti-Zr-Cu-Fe-Sn-Si bulk metallic glasses with different (Ti + Zr)/Cu ratios for biomedical application, Intermetallics 72 (2016) 36–43. [3] N. Sugiyama, H. Xu, T. Onoki, Y. Hoshikawa, T. Watanabe, N. Matsushita, X. Wang, F. Qin, M. Fukuhara, M. Tsukamoto, N. Abe, Y. Komizo, A. Inoue, M. Yoshimura, Bioactive titanate nanomesh layer on the Ti-based bulk metallic glass by hydrothermal-electrochemical technique, Acta Biomater. 5 (4) (2009) 1367–1373. [4] J.M. Park, H.J. Chang, K.H. Han, W.T. Kim, D.H. Kim, Enhancement of plasticity in Ti-rich Ti-Zr-Be-Cu-Ni bulk metallic glasses, Scripta Mater. 53 (1) (2005) 1–6.
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Novel corrosion behaviours of the annealing and cryogenic thermal cycling treated Ti-based metallic glasses
T
Jialun Gu, Yang Shao, Lingxiang Shi, Jiajia Si, Kefu Yao∗ School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Metallic glasses Annealing Corrosion Electrochemistry Surface properties Free volume
Effects of structural relaxation and cryogenic thermal cycling on corrosion resistance of the Ti50Zr20Be20Ni10 bulk metallic glass (BMG) was investigated in 3.5 wt% NaCl solution. The annealed Ti-based BMGs exhibited striking high pitting potentials with low corrosion current densities, while cryogenic thermal cycling treated BMGs performed poor corrosion resistance. More stable passive films were found to form on the surfaces of annealed BMGs through electrochemical impedance spectra analysis. Sub-sized pits that evolved along with micrometre-sized pits on corroded morphologies of the as-cast and cryogenic thermal cycling treated samples clarified the aggressive corrosion. Annealing treatment gave rise to the annihilation of excessive free volume within BMGs, while cryogenic thermal cycling induced the proliferation of free volume. Moreover, the variation of microhardness further reflected the fluctuation of free volume. The XPS analysis revealed that the superior corrosion resistance of the annealed samples could be attribute to the increment in the concentration of Ti, Zr and distinct decrement in the concentration of Be. It is concluded that excessive free volume would deteriorate stability of surface passive film.
1. Introduction Ti-based bulk metallic glasses (BMGs) have attracted massive researches owning to the outstanding properties, such as high specific strength, high yield strength and preferable glass forming ability (GFA) [1–5]. And Ti-based BMGs exhibit excellent performance in applications where lightweight design is considered as a key criterion, e.g., the flexspline in strain wave gears [6] and bumper of Whipple shields [7]. In contrast to the mechanical properties that have been excessively studied, the electrochemical behaviour of Ti-based BMGs has not been well understood. As one of the most common failure mode, corrosion usually leads to structural damage and eventually fractures, playing an important role in material-service security. Therefore, comprehensive work should be conducted to explore the electrochemical behaviour of Ti-based BMGs. Typically, metallic glasses exhibit corrosion resistance with one to two orders of magnitude higher than their crystalline counterparts with identical composition in most aqueous solutions, because of microstructural and component homogeneity, and the lack of crystalline defects (e.g., grain boundaries, second phases) [8–10]. Nevertheless, a few crystalline phases precipitating from the amorphous matrix are found to promote to form stable passive film and improve corrosion resistance. For example, Guo at al. reported that the formation of the ∗
highly protective Zr and Al enriched oxide layer at the surface of crystalline alloy was responsible for its extremely high corrosion resistance compared with the amorphous counterpart [11]. As for the alloys with completely amorphous structural, the most common strategy to modulate corrosion resistance is to tailor chemical compositions by micro-alloying strong passive elements (e.g., Cr, Nb, Ti) [12–16]. Furthermore, recent studies reported that annealing treatment could induce the enhancement of corrosion resistance based on structural relaxation with the annihilation of excessive free volume [17,18]. However, most of these studies focus on the superior corrosion properties in terms of the limited active-passive transformation behaviour, which make quantitatively comparison and analysis of the improvement unreliable, because the current density corresponding to the active dissolution region is quiet difficult to be identified [19–21]. In this work, we investigated the electrochemical behaviour of the as-cast, annealed and rejuvenated Ti50Zr20Be20Ni10 bulk metallic glasses (BMGs) in 3.5 wt% NaCl solution. Significantly enhanced corrosion resistance of the annealed samples had been observed, which was qualified with clearly larger values of pitting potentials than those of the as-cast and rejuvenated sample. In addition, cryogenic thermal cycling treatment was imposed on the Ti50Zr20Be20Ni10 BMG to further confirming the effect of change in atomic structure on corrosion resistance, which could rejuvenate the amorphous structure to a higher
Corresponding author. E-mail address: [email protected] (K. Yao).
https://doi.org/10.1016/j.intermet.2019.04.010 Received 9 December 2018; Received in revised form 1 April 2019; Accepted 7 April 2019 Available online 24 April 2019 0966-9795/ © 2019 Elsevier Ltd. All rights reserved.
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peaks, indicating the formation of monolithic amorphous structure without any detectable crystallization peaks. It was reported that the first diffraction peak which was related to the distance between neighbouring atoms might shift obviously during annealing process because of the transformation of free volume and topological shortage range order [25,26]. And the shift in the peak position could be observed due to the structural relaxation after annealing and cryogenic thermal cycling treatments through high energetic synchrotron beam [22,27,28]. However, the angular positions of the first diffraction peak did not obviously shift for the annealed and cryogenic thermal cycling treated samples within the sensitivity of the XRD measurement, as compared with that of the as-cast sample. Analogous result was also reported in Zr-based BMGs after heat annealing process [29].
energy state [22–24]. The electrochemical parameters including corrosion current density, pit potential and impedance had been derived and fitted from the potentiodynamic polarization curves. Potentiostatic polarization, electrochemical impedance spectrum and X-ray photoelectron spectroscopy tests were applied to analyse the improvement of the corrosion resistance quantitatively. 2. Experimental procedures Specimen preparation. The alloy ingots of Ti50Zr20Be20Ni10 were prepared by arc-melting the mixture of Ti (99.99%), Zr (99.7%), Be (99.7%) and Ni (99.99%) in pure argon atmosphere. Every ingot was remelted at least three times and then cylindrical samples of 3 mm in diameters were fabricated by copper mold suction casting. Amorphous structure of the samples was examined by X-ray diffraction (XRD; Rigaku D/max-RB with Cu Kα radiation, Japan). The samples were annealed at 575 K (Tg-20 K) and 605 K (Tg+10 K) for 10 min. Defining the as-cast samples, the samples annealed at 575 K, the samples annealed at 605 K as T0, T575 and T605, respectively. The annealing treatment was conducted in pure argon atmosphere at a heating rate of 40 K/min. Then the as-annealed specimens were cooled down to the ambient temperature by water quenching. Cryogenic thermal cycling treatment was conducted in ambient atmosphere. The as-cast Ti-based BMGs were inserted into boiling water (373 K) for 1 min first, and then inserted into liquid nitrogen (77 K) for 1 min. Samples were treated up to 15 cycles (defined as T15C) and 30 cycles (defined as T30C), respectively. Specimen characterization. Thermal properties were investigated by differential scanning calorimeter (DSC, NETZSCH STA 449) at a heating rate of 20 K/min. Vicker microhardness (HV) was conducted on the cross-section at 200 g load with 10 s indentation time, each quoted value is an average of at least 4 independent measurements. The surface morphologies of the tested samples were investigated by LEO-1530 scanning electron microscope (SEM). Electrochemistry measurement. Corrosion behaviours of the ascast and as-annealed samples were evaluated in 3.5 wt% NaCl solution by electrochemical measurements, performed in a three-electrode cell setup with working electrode, saturated calomel reference electrode (SCE) and platinum counter electrode. Initially, every sample was immersed in 3.5 wt% NaCl solution for 30 min to stabilizing the open circuit potential. The electrochemical impedance measurements were performed at an AC amplitude of 10 mV and a frequency range from 10−2 Hz–105 Hz. The EIS data was fitted by the Z-view to a Randles circuit with constant phase element. Afterwards, the potentiodynamic polarization curves were recorded at a potential sweep rate of 0.333 mV/s with a starting potential of about 100 mV below the OCPs. Potentiostatic polarization was conducted independently at a polarized potential of 100 mV/SCE (in the passivation-region) for 3600 s, to investigate the characterization of passive films. Before the corrosion experiments, the cross-sections of all tested samples were mechanically ground with up to 2000 grit SiC papers and cleaned in ethanol, exposed in dry air for 24 h. X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM) monochromatized Al Kα source was conducted to characterize to composition and chemical stated of the passive film, the tested samples were potentiostatically polarized for 10 min at 0 V in their passive regions and then take out for XPS tests. The XPS spectrum were analysed by XPS – PEAK to get further information about chemical states.
The DSC traces of the as-cast, annealed and cryogenic thermal cycling treated samples were shown in Fig. 2 to evaluate the thermal behaviour. Exothermic reactions of the test samples which originated from structural relaxation were clearly observed below the Tg. The area of the exothermic peaks below the dotted line corresponded to the structural relaxation exothermic heat (ΔH ). The ΔH values of the T0, T575, T15C and T30C were 4.45 J/g, 0.87 J/g, 4.99 J/g and 5.79 J/g respectively. And no exothermic heat change was detected for the T605 sample, indicating the T605 sample had been completely relaxed. The difference in exothermic behaviour below Tg was related to the diversity of the amount of free volume in the tested samples. The amount of annihilated free volume during annealing treatment is proportional to the exothermic heat of the structural relaxation [30]. It is well known that annealing treatment usually leads to structural relaxation characterized by the annihilation of free volume in metallic glasses, while cryogenic thermal cycling treatment have been confirmed to be a rejuvenation-process agitating a less relaxed state, implying more free volume could be induced into the glassy matrix [23,24,31]. Hence, the DSC results demonstrated that the comparable precursor for next electrochemical test with different fractions of free volume in glassy matrix had been prepared by annealing and cryogenic thermal cycling treatment. In order to better understand the influence of specific treatments on amorphous structural, Vicker microhardness measurements were conduct on the samples right after the treatment of annealing and cryogenic thermal cycling. As presented in Fig. 3, The samples that were subjected to annealing and cryogenic thermal cycling treatment shown statistically significant differences in HV values. For the samples in annealed state, the Vickers microhardness increased form 500.0 ± 6.5 HV (the as-cast sample T0) to 537.9 ± 6.5 HV (the annealed sample T575) and 547.9 ± 6.5 HV (the annealed sample T605), it could be elucidated by the annihilation of free volume during the annealing process [17,18,20,32]. In contrast, a softening effect was observed in the cryogenic thermal cycling treated samples, characterized by lower microhardness of 482.0 ± 8.8 HV (the sample T15) and 476.4 ± 9.0 HV (the sample T30). S. V. Ketov et al. reported an analogous softening in metallic glasses originated from cryogenic thermal cycling treatment [23]. Moreover, there was no difference in HV values between near the centre and the circumference sites for all the tested samples (the indents’ sites were schematically marked in Fig. 3), inferring that both annealing and cryogenic thermal cycling treatment would not lead to the macroscopic heterogeneity on the cross-sections.
3. Results and discussion
3.3. Electrochemical characteristics
3.1. Structural characterization
Fig. 4 (a) displayed the immersion time-dependent OCPs of the Ti50Zr20Be20Ni10 BMG in as-cast, annealed and cryogenic thermal cycling treated states. OCPs of all the tested samples increased with immersion time and further reached stationary values, while the potentials of the annealed samples shifted faster at the initial stage and their
3.2. Thermal and microhardness characterization
The XRD patterns in Fig. 1 presents the structural evolution of the as-cast, annealed and cryogenic thermal cycling treated samples of the Ti50Zr20Be20Ni10 BMG. All the tested samples display broad diffuse 2
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Fig. 1. (a) XRD spectra of the Ti50Zr20Be20Ni10 BMG in the as-cast, annealed and cryogenic thermal cycling treated states; (b) the sketch of cryogenic thermal cycling process.
passive current density was observed in the order of 10−6 mA/cm2. Upward scanning, all samples exhibited typical pitting corrosion features under anodic polarization, characterized by the abrupt increase of current density at a critical potential, identified as pitting potential. Important electrochemical parameters including pitting potential (Epit), corrosion potential (Ecorr), passive region width (Epit - Ecorr), and passive current density (ipass) attained from potentiodynamic polarization curves were summarized in Table 1. No distinct difference in passive current density for all the tested samples. Although the tested samples performed similar polarization behaviours, striking difference in the corrosion resistance in NaCl solution was clearly recorded by polarization curves. In addition, the annealed samples exhibited higher Epit and wider passive region as shown in Table 1, suggesting more protective and stable passive film had been formed. Comparing these electrochemical parameters, the annealed sample showed superior corrosion properties and significant improvement on corrosion resistance in NaCl solution could be attributed to annealing treatment. However, compared with the as-cast and annealed samples, the cryogenic thermal cycling treated samples exhibited worse corrosion resistance with lower Epits. Fig. 5 shown the initial morphologies before electrochemical measurements (the surfaces of all the samples had been mechanically ground by SiC papers after annealing and cryogenic thermal cycling treatment) and presentative corrosion morphologies of the samples T0, T605 and T30 after potentiodynamic polarization measurements, the polarization tests were terminated once the current densities increased up to 1 mA/cm2. Generally, the result revealed almost identical corrosion morphologies that was characterized by pitting corrosion. It can be seen that only one/two primarily pits formed on the whole cross-sections, suggesting the localized characterization of pitting corrosion for the Ti-based BMGs. Besides, detailed investigations in the high magnifications of the pits revealed interesting morphologies. As shown in Fig. 5 (d), (e), (f), round-shape micrometre sized pits were visible at the
Fig. 2. DSC curves of the Ti50Zr20Be20Ni10 BMG in the as-cast, annealed and cryogenic thermal cycling treated states. The heat of structureal relaxation, ΔH , can be determined by integrating the area of exothermic peaks prior to glass trasition event.
finial potentials were more stable at the end of immersion compared with the as-cast and cryogenic thermal cycling treated samples. After immersion for 1800 s, the OCP values of the as-cast, T575, T605, T15C and T30C increased up to −502 mV, −538 mV, −557 mV, −485 mV and −458 mV, respectively. Generally, higher OCPs indicate the superior protectiveness of passive films. Corrosion resistance of the as-cast, annealed samples and cryogenic thermal cycling treated samples were evaluated by potentiodynamic polarization. As shown in Fig. 4 (b), all the tested samples were passivated spontaneously with progressing anodic polarization from the corrosion potential, followed by distinct passive regions (with the maximum width ∼ 1069 mV). Notably, no significant difference in the
Fig. 3. (a) Vickers microhardness HV of the Ti50Zr20Be20Ni10 BMG in the as-cast, annealed and cryogenic thermal cycling treated states. As schemetically shown in the inset, microhardness tests were conducted at the central area (black symbols) and around the edge (red symbols), respectively. (b) The scanning electron micrograph of the indents, distributed in the central area, inset is the highmagnification image.
3
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Fig. 4. Corrosion performance of the Ti50Zr20Be20Ni10 BMG in 3.5 wt% NaCl solution at 293 K. (a) variation in OCPs with increasing immersion time; (b) potentiodynamic polarization curves at a scan rate of 0.333 mV/s, the polarization tests were terminated at the current density of 1 mA/cm2. The surface oxide films had been removed after the annealing and cryogenic thermal cycling treatment.
by cryogenic thermal cycling treatment. This result was in line with the potentiodynamic polarization tests (see Fig. 4). To better understand the effects of rejuvenation and structural relaxation on corrosion resistance of Ti-based BMGs, electrochemical impedance spectrum was conducted after 30 min immersion in 3.5 wt% NaCl solution to assess the stability of passive film. The EIS test provides significant information on the corrosion resistance of electrodes and the electrode-solution interface under nearly non-polarizing condition [36]. As shown in Fig. 7, the response of the Nyquist curves for the tested samples was characterized by one time constants in the frequency range from 10−2 Hz–105 Hz. Thus, the equivalent circuit was proposed to fit the Nyquist plots, which composed of one constant phase element (CPE) due to the feature of semicircle in Nyquist curves. The model parameters, Rs represents the solution resistance, the parallel-connected elements (CPE//Rp) include the charge transfer resistance (Rp), are applied to analyse the electrochemical properties of the matrix/passive film interface. The impedance of CPE could be represented as Z(CPE) = Q0−1 (jω)−α , where Q0 is CPE parameter, j is −1 , ω is angular frequency and α is dispersion coefficient associated with surface inhomogeneity. The fitting result obtained for all samples were presented with the experiment results in Fig. 7, and the proposed equivalent was able to well simulate the electrochemical behaviour of the tested specimens. Table 2 showed fitting results of the electrochemical parameters based on EIS measurements. The polarization resistances of the annealed samples were greater than that of the as-cast and cryogenic thermal cycling treated samples, implying the polarization resistance increased with annealed temperature under the tested condition. Furthermore, the values of Rp increased in the order of T30C, T15C, T0, T575, T605. Compared with the as-cast and cryogenic thermal cycling treated sample, the annealed samples presented higher Rp indicating that the passive films formed on their surface were more stable. Thus, the migration of metal ions through the passive film towards the electrolyte would be slowed down for the annealed samples. According to the electrochemical parameters shown in Table 2, rejuvenation induced by cryogenic thermal cycling caused the opposite effects as compared with the annealing treatment. It should be noted that all samples had been mechanically ground with SiC papers to remove a layer of metal on the surface before the electrochemical tests. Therefore, subtle changes in the amorphous structure might have a great influence on the performance of surface passive film. The results of EIS agreed well with the potentiodynamic polarization measurements.
Table 1 The electrochemical parameters of the as-cast, annealed and cryogenic thermal cycling treated samples. Specimen
Ecorr (mV/SCE)
Epit (mV/SCE)
Epit - Ecorr (mV)
ipass (A/cm2)
T0 T575 T605 T15C T30C
−506 −538 −557 −485 −458
319 439 512 202 168
825 977 1069 687 626
3.07 × 10−6 2.17 × 10−6 1.85 × 10−6 1.87 × 10−6 2.09 × 10−6
± ± ± ± ±
16 14 11 18 20
± ± ± ± ±
13 10 12 8 11
bottom of the macroscopic primarily pits, and the mean diameter is approximately 4 μm. Concerning the sample T0 and T30, some subsized pits with cells of below 2 μm in diameter formed on the topography. The sub-sized pits formed randomly and no apparently preferential sites (the edge and bottom of the micrometre-sized pits) for nucleation. The multiple sized pits discussed above are similar to the corroded morphologies of the Fe-based BMG, which suffers from pitting corrosion and nano-porous structure occurs on the walls of micrometresized pits [33]. And the multiple sized pits usually were the characteristic of a aggressive pitting-nucleation process. Fig. 5 (g) – (i) shows the typical surface morphologies characterized by scratches, which were ground to remove surface oxide films after annealing and cryogenic thermal cycling treatment. Compared with the T0 samples, a more flat surface could be observed on the surface of the T605 sample, while deeper wear-furrows and more residual scraps formed on surface of the T30C sample. The difference in initial morphologies of the three samples is in accord with the microhardness, and the excessive surfacedefects and scraps probably promote the nucleation of pitting and further deteriorating the corrosion resistance. Potentiostatic polarization tests were performed at a constant-polarized potential of 100 mV/SCE for 3600s in 3.5 wt% NaCl solution to reveal the details of passivation-behaviour. As shown in Fig. 6, the current densities of the tested samples decreased sharply in the initial period time (about 600s), reflecting the evolution of primary oxide film in aggressive NaCl solution [34,35]. Then, the current densities transferred into a steady state with a nearly constant level about 4.0 × 10−5 mA/cm2, which might be attributed to the establishment of stable passive films. The analysis discussed above presented the fact that all the tested samples performed similar corrosion kinetic behaviour. Regarding the details of variation, the annealed samples exhibited the lowest values of current densities among all the tested samples. However, the cryogenic thermal cycling treatment triggered the undesirable increment of the current densities in passive region. Meaning the passive films formed on the surface of annealed samples were more protective and stable, and opposite effects might be induced
3.4. XPS analysis The characteristics of passive films have great influence on 4
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Fig. 5. Initial morphology before the electrochemical measurements (g), (h), (i), and corrosion morphology of the potentiodynamic polarized samples (all the polarization tests were terminated at the onset of current density rising up to 1 mA/cm2), including the whole corroded area and the detailed micrographs of pits. (a), (d) and (g) the sample T0; (b), (e) and (h) the sample T605; (c), (f) and (i) the sample T30C. The surface oxide films had been removed after the annealing and cryogenic thermal cycling treatment.
Fig. 6. Potentiostatic polarization curves of the as-cast, annealed and cryogenic thermal cycling treated samples, at a constant polarized potential of 100 mV/ SCE for 3600s in 3.5 wt% NaCl solution.
Fig. 7. Electrochemical impedance behaviour of the as-cast, annealed and cryogenic thermal cycling treated samples at the OCPs after 30 min immersion in 3.5 wt% NaCl solution. Insert: the equivalent circuit proposed for fitting the EIS data.
corrosion resistance of Ti-based BMGs, thus XPS measurements were applied to analyse chemical characteristics of the T0, T605 and T30C samples which were potentiostatically polarized in the passive region for 10min. As shown in Fig. 8 (a), the XPS survey spectra covers a wide binding energy region, consisting of the peaks of Ti 2p, Zr 3d, Be 1s, Ni 2p, O 1s and C 1s, etc. The peaks of Ti 2p3/2 and Ti 2p1/2 are identified at 458.5 eV and 464.2 eV for all the three samples, corresponding to Ti4+ oxidized state. The Zr 3d spectrum (Fig. 8 (c)) of the T0 and T30C
samples are composed of Zr4+ 3d5/2 (182.4 eV) and Zr4+ 3d3/2 (184.8 eV) peaks, and the metallic state peaks (Zr) appear to locate at 178.5 eV and 180.9 eV (as marked by the red dotted line). However, only the 3d5/2and 3d3/2 peaks of the oxidized state (Zr4+) could be found in the Zr 3d spectrum of the T605 sample. In addition, the Be 1s spectrum shows the binding energies of 113.4 eV and 111.7 eV which 5
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Table 2 Impedance fitting parameters of experimental samples obtained from electrochemical impedance spectra (EIS). Specimen
Rs (Ω · cm2)
Rp/(kΩ · cm2)
CPE Q0/(kS · cm−2 · sα)
α (0–1) T0 T575 T605 T15C T30C
2.33 2.58 2.73 2.18 2.13
± ± ± ± ±
0.38% 0.30% 0.37% 0.30% 0.42%
0.91 0.94 0.91 0.93 0.92
± ± ± ± ±
0.05% 0.04% 0.04% 0.05% 0.06%
163.91 156.84 132.12 177.33 184.40
± ± ± ± ±
0.28% 0.29% 0.22% 0.28% 0.23%
163.91 178.04 191.46 141.30 134.24
± ± ± ± ±
0.49% 0.67% 0.71% 0.51% 0.61%
Fig. 8. XPS survey spectrum (a) and elements spectra (b–f) of the as-cast, annealed and cryogenic thermal cycling treated samples, all the tested samples were potentiostatically polarized for 10 min at 0 V before XPS measurements. (b) Ti 2p, (c) Zr 3d, (d) Be 1s, (e) Ni 2p. The surface oxide films had been removed after the annealing and cryogenic thermal cycling treatment.
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high energy state provides high chemical activity and high atomic mobility [43]. Besides, the glassy matrix with larger fraction of free volume is characterized by a looser and more disordered atomic structure, corresponding to cryogenic thermal cycling treated samples. Hence, a non-dense and imperfect passive film is expected to form on the surface of T15C and T30C, inferring it would be more likely to be destroyed in corrosive medium as being proved by the results of EIS and potentiodynamic polarization tests. Once metastable sites initiate the nucleation of pitting, such non-dense atomic structure provides more routes and vacancies for chloride ion to transit and degrade the ability of re-passivation. Eventually result in worse corrosion resistance for the cryogenic thermal cycling treated samples.
Table 3 The normalized surface element concentration of the as-cast, annealed and cryogenic thermal cycling treated samples analysed by XPS. Specimen
Ti (%)
Zr (%)
Be (%)
Ni (%)
T0 T605 T30C
54.6 56.6 52.1
25.9 26.2 25.3
17.2 15.0 20.5
2.2 2.2 2.1
are assigned to Be2+ 1s state and Be 1s state, respectively. The Ni 2p spectrum consists of two peaks at 853.6 eV and 871.8 eV, corresponding to Ni2+ oxidized state. It is well known that the variation in components of passive film, even subtle changes, could have a great impact on the electrochemical behaviours of alloys. The atomic concentration of each element was calculated through integrating the XPS peaks and calibrated by relative sensitivity factors, the results are summarized in Table 3. It is noteworthy that the changes in the concentration of Ti and Zr agree well with the corrosion resistance of T0, T605 and T30C samples, and could provide a new insight into the effects of annealing and cryogenic thermal cycling. Compared with the as-cast and cryogenic thermal cycling treated samples, the annealing treated sample T605 performs the best corrosion resistance in 3.5 wt% NaCl solution, showing a increment in the concentration of Ti, Zr and a distinct decrement in the concentration of Be. This result indicates that the enhanced corrosion resistance by annealing could be related to the enrichment of Ti and Zr, and the deficiency of Be, the converse effects might be induced by cryogenic thermal cycling. The chemical characteristics of TiO2 and ZrO2 are highly stable and their formation on the surface could promote to enhance the stability of passive film, further protecting the glassy matrix in aggressive NaCl solution [37–39]. Besides, Guo et al. [11,40] have reported that the microscopic structural homogeneity of passive film of BMGs could be improved by the formation of TiO2, ZrO2, since these oxides could remarkably refrain the formation and growth of the heterogeneous galvanic cells in the film. In this work, the XPS results reveal that the annealing and cryogenic thermal cycling treatment could change elemental content and valence state in Ti-based BMGs, and further enhancing or deteriorating the corrosion resistance in NaCl solution.
5. Conclusions This work explored the influence of rejuvenation and structural relaxation on electrochemical behaviour of the Ti50Zr20Be20Ni10 BMG in 3.5 wt% NaCl solution. It is concluded that the corrosion resistance of the Ti-based BMGs could be significantly improved when the tested samples remain at lower energy metastable states with relative smaller amount of free volume induced by structural relaxation. However, the cryogenic thermal cycling treatment which was applied to rejuvenate the samples performed the contrary effects on corrosion resistance as compared with annealing treatment. The structural relaxation exothermic heat had been analysed quantitatively by DSC test, showing the free volume in the sample T605 had annihilated fully by the specific annealing treatment, while cryogenic thermal cycling treatment helped to introduce more free volume into the as-cast glassy matrix. As a result of the variation in free volume, Vickers microhardness increased for the annealed samples and the softening had been detected in the rejuvenated samples. In addition, the annealing treatment enhanced the corrosion resistance characterized by higher Epits with striking difference over 120 mV for the as-cast sample ∼319 mV (T575 ∼ 439 mV, T605 ∼ 512 mV), while the sample T30C exhibited the worst corrosion resistance with the Epit as low as 168 mV. Multiple sized pits that formed on the corroded morphologies verified the unfavourable corrosion resistance for the rejuvenated samples. The EIS result displayed the electrochemical impedance of the annealed samples were superior than that of the as-cast and rejuvenated samples, approving more stable passive film formed on the surface of the annealed samples. The XPS results reveal that the annealing and cryogenic thermal cycling treatment could change elemental content and valence state in Ti-based BMGs, and the increment in the concentration of Ti, Zr and distinct decrement in the concentration of Be could contribute to the superior corrosion resistance of the annealed samples.
4. Discussion The obviously and quantitatively comparable improvement of corrosion resistance induced by thermal relaxation had been demonstrated in this work. Meanwhile, in order to comprehensively investigate the structure-corrosion relationship in amorphous material, cryogenic thermal cycling treatment had been conducted as an opposite process of the annealing treatment. And the comparison results confirmed that the rejuvenation would deteriorate corrosion resistance of the as-cast Tibased metallic glass. Actually, similar enhancement of corrosion resistance resulting from thermal relaxation was reported in several ribbon metallic glasses and bulk metallic glasses [17,20,41]. Raicheff at al. thought the formation of a denser amorphous structure with low residual stresses contributed to the improvements [42]. Jiang at al. suggested the chemical potential and the formation of cluster might have significant influence on pitting initiation and propagation behaviours. The enhanced corrosion resistance caused by thermal relaxation could be associated with the corresponding electrochemical free energy of activation (ΔG ) [19]. The annealed samples might transfer to a lower energetic level of metastable state with depressed electrochemical activity, the annihilation of excessive free volume conduced to a denser atomic structure, which would limit the atomic mobility in a great degree. Thus, the ΔG increases and further the electrochemical corrosion become more difficult for the relaxed samples compared to the ascast sample. However, metallic glass contains large amount of free volume, suggesting it is at a higher energy level of metastability. Such a
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