1. Introduction
Titanium-bearing blast furnace slag (TBFS) is an industrial solid waste generated from the smelting of titanium-bearing iron ore in blast furnaces.1,2) The typical utilization of vanadium-titanium magnetite in China relies on the blast furnace-converter iron-vanadium extraction process and titanium-extraction electric furnace process. The iron concentrate is smelted in the blast furnace to produce vanadium-containing melted iron, and all the TiO2 in the raw materials of the furnace ends up in the blast furnace slag (i.e., TBFS). As shown in Fig. 1(a), the titanium in the unutilized blast furnace slag accounts for approximately 50% of the titanium resources in the original ore, resulting in a significant waste of titanium resources. Currently, the stockpile of TBFS in China has reached approximately 100 million tons. With the incremental development of ilmenite, as shown in Fig. 1(b), it is foreseeable that the inventory of TBFS will inevitably increase. Due to the lack of efficient methods for comprehensive utilization, titanium-bearing blast furnace slag has accumulated in large quantities, occupying vast areas of land, damaging vegetation, contaminating soil and water bodies, and exacerbating pollution in the surrounding environment. Importantly, titanium-bearing blast furnace slag contains a significant amount of titanium dioxide (~20%), and the stockpiling of this slag also results in the wastage of titanium resources.3,4) Therefore, the urgent implementation of resource utilization of titanium-bearing blast furnace slag is imperative.
Fig. 1. (a) Typical migration path of Ti element in the Chinese Ti industry. (b) Global ilmenite production from 2014 to 2023. (Data source USGS). (Online version in color.)
A variety of methods have been proposed and developed for the recycling of the waste titanium-bearing blast furnace slag, mainly including direct utilization and titanium extraction strategies.1,5,6,7) Direct utilization strategy can be realized by synthesizing photocatalytic degradation materials,8) foam glass9) and construction materials,10) etc., from TBFS. Although the large-scale consumption of TBFS can be achieved, the most valuable element titanium in TBFS fails to undergo targeted high-value utilization during the direct utilization process, resulting in a significant waste of Ti resources. Therefore, it is more attractive to controllable extract titanium from TBFS for better utilization of titanium. Titanium extraction strategy encompasses acid leaching, alkali calcining, carbonization chlorination, and high-temperature enrichment, etc. for the preparation of titanium dioxide,11,12) titanium alloy,13,14) titanium-rich materials,15,16) among various other titanium-based products. The titanium-extraction utilization strategy can realize the efficient recovery of titanium in TBFS, thereby maximizing the resource potential and minimizing waste. The sustainable utilization of TBFS for the synthesis of high-quality titanium products remains an important goal, given its complex composition and the imperative need to address the secondary pollution arising from the titanium extraction process.
Molten salt electrochemical metallurgy technology, which harnesses electrode reactions to achieve directional separation and extraction of single or multiple metals from complex compositions, represents a crucial direction in the advancement of sustainable metallurgical technology.17,18,19) Molten salt is characterized by a relatively wide electrochemical window, high conductivity, stability, fast reaction kinetics, and ease of recovery.20) In comparison to aqueous solutions, the anhydrous and oxygen-free environment of high-temperature molten salt inhibits hydrogen evolution reactions. Furthermore, the wide electrochemical window prevents side reactions during the metal extraction process, making molten salt electrolysis technology widely utilized for extracting metals like aluminum,21) magnesium,22) rare-earth metals,23) etc. Molten salt electrochemical metallurgy has shown great potential for the extraction of titanium from titanium slag. Zhou et al. prepared TiC/SiC composites by direct electrolysis of high titanium slag and graphite precursor in CaCl2-based molten salt.24) Mohanty et al. prepared ferrotitanium by direct electrolytic reduction of titanium-rich slag obtained from plasma melting of ilmenite in CaCl2 melt.25) Our previous works utilized the solid oxygen-ion membrane (SOM) method to prepare titanium-silicon alloys and titanium-iron alloys directly from titanium-rich slag or TBFS.26,27,28) However, obtaining higher quality titanium alloys from TBFS (especially TBFS with low TiO2 content) based on molten salt electrolysis strategy remains a challenge due to the complex composition and high impurity content of titanium-containing blast furnace slags.
In this work, typical titanium-silicon intermetallic alloy Ti5Si3 was prepared through molten salt electrolysis of TBFS with additional SiO2. The physical phase and morphological evolution of TBFS/SiO2 during electrolysis as well as the migration behavior of impurity elements in molten salt were investigated. Furthermore, the obtained Ti5Si3 alloys was melted and purified by a fast vacuum laser melting technique, and the purification mechanism via the laser melting was explored. The Ti5Si3 extracted from TBFS/SiO2 for corrosion-resistant applications in phosphate buffer solution was preliminarily evaluated. Overall, this work utilizes industrial waste TBFS as raw material to prepare Ti5Si3 alloy, significantly reducing the cost of raw material, while simultaneously incorporating laser melting to enhance the product quality of the molten salt electrolytic alloy.
2. Experimental Section
2.1. Molten Salt Electrochemical Synthesis of Ti5Si3 Alloy
TBFS powder (Pangang Group Co., Ltd., China) was first calcinated at 900°C in air to eliminate the possible residual carbon component. TBFS powder after calcination was used as titanium source for the preparation of Ti5Si3 alloy in this work. In details, TBFS powder and additional SiO2 (50 nm, Aladdin Reagent Co. Ltd.) were mixed in a Ti:Si ratio of 5:3, with 5 wt% of polyvinyl alcohol formaldehyde (PVB, Aladdin Reagent Co. Ltd.) added as a binder. The above powder was thoroughly mixed in a planetary ball mill with a speed of 600 rpm. Furthermore, nano SiO2 featuring an amorphous structure was used to enhance the thoroughness of contact with the raw material and to accelerate the reaction rate. About 0.5 g of the resulting mixture powder was pressed into circular discs under a pressure of about 16 MPa. The obtained thin discs were wrapped in foam nickel and tied on a molybdenum wire or a stainless-steel wire as cathode. The fabricated TBFS/SiO2 cathode and a high-purity graphite anode were placed vertically in a corundum crucible. About 150 g anhydrous CaCl2 (Shanghai Macklin Biochemical Co., Ltd.) served as electrolyte was added into the crucible. The assembled electrolytic cell was heated to working temperature of 950°C in an electric resistance furnace with protection of high-purity of argon gas. Constant voltage electrolysis of 3.0 V was applied between the TBFS/SiO2 cathode and graphite anode for a certain time to synthesize the targeted products. Successive hydrochloric acid washing and deionized water washing processes were taken to eliminate the solid salt and undesired impurity phase in the final product and dried in a vacuum oven for the further analysis, wherein the targeted intermediate products were only cleaned by deionized water.
2.2. Fabrication and Purification of Bulk Ti5Si3 Alloy
The laser melting device includes a 6000 W laser (RFL-C6000X, Wuhan Ruike Fiber Laser Technology Co., Ltd., China), a focusing system, a vacuum pump, a temperature sensor and a control system. In the melting experiment, the Ti5Si3 powder prepared by molten salt electrolysis was firstly pre-compressed into a pellet under the pressure of 16 MPa, which was put into the laser vacuum chamber. A high-power laser was used, and the output power was set to 1000 W. The power-up time, holding time and power-down time were set to 20 s, 120 s and 20 s, respectively. Subsequently, the dry pump and the molecular pump were turned on to reduce the pressure inside the vacuum chamber to 1×10−3 Pa to start the melting. The pressure-reducing valve was turned on to take out the bulk Ti5Si3 alloy after the laser was extinguished for 30 minutes. The laser melting temperature was about 2200°C, measured by a digital two-color pyrometer with fiber optics (IGAR 12-LO, Luma Sense, Germany).
2.3. Materials Characterization
The phase composition was detected by X-ray diffraction (XRD, Bruker D8 Advance). The morphology and element composition were characterized by a scanning electron microscopy (SEM, JEOL JSM-6700), a transmission electron microscopy (TEM, FEI Talos F200X G2) and the affiliated energy dispersive X-ray spectrometer (EDS). An X-ray photoelectron spectrometer (XPS, Thermo Kalpha) was used to characterize the elemental composition and chemical state information of the sample surface. Elemental content was measured by inductively coupled plasma (ICP-MS, PerkinElmer 7300V). Electron backscatter diffraction (EBSD) measurement was performed using an EDAX-TSL system.
2.4. Electrochemical Corrosion Measurement
A CHI760E electrochemical workstation was used to perform the electrochemistry corrosion measurement. The laser melted Ti5Si3 alloy was used as the working electrode after polishing with 2000 grit sandpaper. Platinum and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. The exposed surface of the working electrode was a circle with a diameter of 2 mm. The phosphate buffer solution (PBS, Aladdin Reagent Co. Ltd.) was used as electrolyte. The open circuit potential (OCP) test was first performed after the working electrode was immersed in the PBS electrolyte at room temperature. The electrochemical impedance spectroscopy (EIS) measurement was obtained in the frequency range of 10−2 to 105 Hz under the condition of voltage perturbation amplitude of 10 mV. The potentiodynamic polarization scans were performed within the range of −0.65 V to 1.6 V (vs SCE) with a scan rate of about 0.2 mV/s.
3. Results and Discussion
3.1. Electrochemical Synthesis of Ti5Si3 Alloy from TBFS/SiO2
Figure 2(a) displays the XRD pattern of the raw TBFS, revealing the presence of composite perovskite phases such as CaTiO3, Ca(Ti,Mg,Al)(Si,Al)2O6 and Ca(Mg,Al,Fe)(Si,Al)2O6. Furthermore, irregular particles morphology and size inhomogeneity of the TBFS was confirmed by the SEM analysis, as shown in Fig. 2(b). The elemental composition of the raw TBFS was also analyzed, and the result (Fig. 2(c)) shows that the percentages of Ti, Ca, Si, Mg, Al, Fe, and O were 24.51%, 23.41%, 6.86%, 3.94%, 4.84%, 3.07%, and 31.3% by weight, respectively. Since the Si content in raw TBFS is insufficient for the synthesis of Ti5Si3 alloy, additional SiO2 was introduced into the TBFS feedstock to achieve a Ti:Si ratio of 5:3, which is crucial for the subsequent electrolysis procedure.
Fig. 2. (a) XRD pattern, (b) SEM image, and (c) element composition of the raw TBFS. (Online version in color.)
The fabricated TBFS/SiO2 cathode underwent constant voltage electrolysis at 3.0 V. The typical variation of the cell current versus time during electrolysis was recorded and is presented in Fig. 3(a). The cell current experienced a significant decrease during the initial two hours and then stabilized approximately 0.2 to 0.3 A. This phenomenon coincided the polarization deoxygenation reaction of the cathodic oxides. To investigate the phase transformation of TBFS/SiO2 cathode during the polarization deoxygenation process, cathodes from different electrolytic stages were prepared and analyzed by XRD. As depicted in Fig. 3(b), at the initial stage of electrolysis (the first hour), the cathode sample exhibited complex characteristic peaks, including those of Ca(Mg,Al)(Si,Al)2O6, CaTiO3 and CaAl2O4, among others. Upon extending the electrolysis time to 3 hours, some Ti5Si3 alloy and titanium suboxide TiOx (such as Ti2O3 and TiO) phases were detected in the cathode. After 5 hours of electrolysis, the dominant phase of the cathode was confirmed to be Ti5Si3, accompanied by a small amount of TiAlx present. Since amorphous SiO2 was used in this study, no distinct SiO2 characteristic peak was observable in these cathode samples. The thermodynamic calculations reveal that the standard Gibbs free energy (ΔG°) for the reaction between Ti and Si at 950°C is −590 kJ/mol. This signifies that a spontaneous thermodynamic alloying process takes place between the targeted Ti and Si elements, resulting in the formation of the Ti5Si3 alloy. The flow diagram illustrating the generation of targeted Ti5Si3 alloy is inserted in Fig. 3(a). The deoxidation and alloying reactions pertaining to the corresponding oxides within TBFS/SiO2 are outlined in Eqs. (1), (2), (3), (4).
|
Ti
O
2
+(4-2x)
e
-
=Ti
O
x
+(2-x)
O
2-
| (1) |
|
Ti
O
x
+2x
e
-
=Ti+x
O
2-
| (2) |
|
Si
O
2
+4
e
-
=Si+2
O
2-
| (3) |
|
5Ti+3Si=T
i
5
S
i
3
, ΔG
°
950°C
=-590 kJ/mol
| (4) |
Fig. 3. (a) Current-time curve for the direct electrolysis of TBFS/SiO2 under 3.0 V in molten salt at 950°C. (b) XRD patterns of TBFS/SiO2 cathodes obtained after different electrolytic stages. (c) Optical photographs of raw TBFS and the as synthesized Ti5Si3 powder. (d)Typical SEM images of the cathodic products obtained after 1 h, 3 h, and 5 h of electrolysis. (Online version in color.)
The current efficiency of electrolysis for 8 hours was about 30%. A part of the power was used for the reduction of unwanted impurity oxides. Additionally, Fig. 3(c) showcases the optical photographs of raw TBFS and the synthesized Ti5Si3 powder, where the latter appears as a black-grey powder. Figure 3(d) displays the typical SEM images of the cathodic samples obtained after 1, 3 and 5 hours of electrolysis. After 1 hour of electrolysis, the cathode sample exhibits a mixed morphology, featuring irregular and non-uniform particles. Following 3 hours of electrolysis, the cathodic product shows smooth and more uniform particle morphology. Extending the electrolysis time to 5 hours results in a nodular particle morphology, with an increasing trend in particle size. Ostwald ripening and migration-coalescence processes are commonly recognized as the mechanisms responsible for sintering and growth of metal particles.29)
The incomplete TBFS/SiO2 cathode disc obtained after 2 hours electrolysis was further analyzed to investigate the deoxidation process. As depicted in Fig. 4(a), a distinct phase differentiation is evident between the interior and exterior regions of the electrode disc. Specifically, the interior part primarily comprises oxygen-containing minerals such as Ca(Mg,Al)(Si,Al)2O6, CaSiO3 and TiOx, whereas the exterior part is dominated by Ti5Si3 alloy. The corresponding SEM images shown in Fig. 4(b) reveal that the cathode disc contains mainly of irregular large-size particles internally and tiny alloy particle morphology externally, mirroring the findings of Fig. 3(d). The EDS results indicate that the interior has a higher abundance of O, Al, and Ca compared to the exterior. These results also confirm that the electrochemical polarization deoxidation and alloying processes progressed from the exterior to the interior of the TBFS/SiO2 disc. As a result, the oxygen component in the TBFS/SiO2 cathode was gradually removed by the applied electric field. The XRD results, as depicted in Fig. 4(c), conclusively demonstrate that a portion of Al was retained within the cathode in the form of TiAl alloys (e.g., TiAl2), appearing as an impurity phase within the final obtained Ti5Si3 product. The TiAlx alloy can be readily removed through a simple pickling process, such as the 3 mol/L hydrochloric acid pickling employed in this study. The electrolysis products before and after acid pickling were further analyzed by SEM and corresponding EDS. As shown in Fig. 4(d), the characteristic peak of Al in the EDS spectra disappeared after the pickling process. Notably, the pickling process did not have a significant effect on the morphology of the Ti5Si3 alloy, as evident from the SEM images presented in Fig. 4(d).
Fig. 4. (a) XRD pattern, (b) SEM images and their corresponding EDS spectra of the incomplete cathode TBFS/SiO2 disc (2 hours of electrolysis). (c) XRD pattern and (d) EDS spectra of the Ti5Si3 alloy synthesized by electrolysis for 8 h before and after acid pickling, the insets in (d) are their corresponding SEM images. (Online version in color.)
A key characteristic of TBFS is its high content of alkaline earth metal impurities, including Ca, Mg, Al, etc., as evidenced by Fig. 2(c). To investigate the impurity element migration behavior, the representative Ca and Al elemental contents in the staged electrolysis products were further analyzed by ICP and the results are shown in Fig. 5(a). Notably, Ca impurity with a melting point of 842°C was progressively removed by dissolution due to the disparities in melting point and density during the electrolytic process in molten salt. Consequently, the Ca content in the cathodic product decreased rapidly from about 9.5 wt% to 2 wt% within 5 hours electrolysis. However, a portion of Al did not exhibit a significant decrease owing to its tendency to form Ti–Al associated impurities in the Ti–Si system. These phenomena have been extensively validated in our previous research works.26,27,28,30) Furthermore, no significant amounts of elemental Fe with high melting point were detected in the obtained samples. The Fe impurity can also be effectively removed during the electrolysis process as well as through the acid pickling process,28) significantly enhancing the purity of the Ti5Si3 product. The migration of major metal elements Ti, Si, Mg, Al, Ca and Fe in TBFS was summarized and is shown in Fig. 5(b). The majority of impurities in TBFS are eliminated through the processes of dissolution into molten salt and acid pickling. Molten salt exhibits a self-purification effect on the resulting electrolytic products, further enhancing their purity. Therefore, the electrochemical synthesis of Ti5Si3 alloy from TBFS/SiO2 primarily involves electrochemical deoxidation and alloying processes as well as molten salt/acid pickling purification process.
Fig. 5. (a) Ca and Al contents in the cathodic products. (b) Migration of major metal elements in TBFS during molten salt electrolysis process. The temperature below the elements is their corresponding melting point. (Online version in color.)
Figure 6(a) displays the SEM image of Ti5Si3 alloy obtained through the electrolysis of TBFS/SiO2 in molten CaCl2 at 950°C for 8 h (following acid pickling). It is evident that the resulting Ti5Si3 product exhibits a smooth surface with a particle size of approximately 1 μm (exhibiting a larger radial dimension). Due to the growth effect stemming from the templating effect of the liquid molten salt and the abundance of delocalized electrons in the metals, the Ti5Si3 particles adhere to each other, forming a nodular morphology. This nodular morphology is further corroborated by the TEM results, as depicted in Fig. 6(b). The high-resolution TEM image (Fig. 6(c)) reveals relatively poor crystallinity of the prepared Ti5Si3 alloy and the presence of defects, such as dislocations. Furthermore, as shown in Fig. 6(d), a thin oxide layer with a thickness of a few nanometers can be observed on the surface of the Ti5Si3 particle, which may be attributed to its exposure to air. The TEM analysis and corresponding EDS mapping in Figs. 5(e)–5(g) demonstrate that the Ti and Si elements are uniformly distributed throughout the synthesized Ti5Si3 product.
Fig. 6. (a) SEM and (b–d) TEM images of the Ti5Si3 alloy obtained by electrolysis at 950°C and 3.0 V in molten CaCl2 for 8 h (after acid pickling). (e–g) TEM image and its corresponding EDS mappings. (Online version in color.)
The prepared Ti5Si3 alloy was further analyzed by XPS, and the obtained Ti 2p and Si 2p spectra are shown in Figs. 7(a) and 7(b), respectively. Through fitting analysis, it can be observed from Fig. 7(a) that the peaks located at approximately 464.4 eV and 458.6 eV are related to Ti4+ 2p1/2 and Ti4+ 2p3/2, respectively.31) The signal at 454.6 eV is considered to be Ti (0), while the remaining peak at about 453.4 eV corresponds to Ti in Ti5Si3. As shown in Fig. 7(b), the binding energy at approximately 102.0 eV corresponds to Si4+. The signal at about 98.4 eV represents Si (0), and the peak at about 97.7 eV is related to the presence of Si in Ti5Si3.32) The above results further confirm the formation of Ti5Si3 alloy. However, due to its exposure to air, the surface of the Ti5Si3 alloy is partially oxidized.
Fig. 7. XPS spectra of the Ti5Si3 alloy obtained by electrolysis at 950°C and 3.0 V in molten CaCl2 for 8 h (after acid pickling), (a) Ti 2p, (b) Si 2p. (Online version in color.)
The alloys obtained through molten salt electrolysis of solid metal oxide cathodes are typically in powders form. Furthermore, during the electrolysis of complex titanium-containing ores, small quantities of metallic impurities often exist in the resultant alloys due to the chemical potential equilibrium between the impurity metals within the molten salt and the those in the cathode alloy.33,34) It is anticipated that the Ti5Si3 alloy obtained can be purified by incorporating a melting process. In this study, powdered Ti5Si3 alloy was produced via molten salt electrolysis using TBFS/SiO2 as raw material. Based on this, bulk Ti5Si3 alloy was further prepared and purified by high vacuum laser melting as depicted in Fig. 8(a). The XRD result (Fig. 8(b)) validated the Ti5Si3 phase of the obtained melting product. SEM and EDS analysis also confirmed the uniform melting of Ti5Si3 alloy, as shown in Fig. 8(c). Essentially, the metal materials are rapidly melted by a laser beam with high energy density. The high surface temperature of the melt pool during the melting process, coupled with the high vacuum of the melting environment, effectively removes the gases and impurities from the melt, potentially enhancing the purity of the prepared alloy.35) Specifically, the concentrations of impurities such as Al, Mg, Ca, Mn, and Fe in the final Ti5Si3 alloy are significantly lower than those in the raw powdered Ti5Si3 alloy, as illustrated in Fig. 8(d). For example, the impurity concentrations of Al, Mg, and Ca in the powdered Ti5Si3 alloy are 0.325 wt%, 0.14 wt%, and 1.67 wt%, respectively. However, these concentrations in the bulk Ti5Si3 alloy after vacuum laser melting were reduced to 0.15 wt%, 0.075 wt%, and 0.935 wt%, respectively. The volatility of Ca, Mg, and Mn at elevated temperatures (i.e., their high vapor pressures) is the primary reason for their removal. The relationship between vapor pressure and temperature is expressed by the Clausius-Clapeyron equation.36)
|
lgP=A
T
-1
+Blog10T+CT+D
| (5) |
Fig. 8. (a) Schematic diagram of the fabrication procedure of bulk Ti5Si3 alloy. (b) XRD pattern and (c) SEM image as well as corresponding EDS mappings of the Ti5Si3 alloy. (d) Impurity content analysis of the bulk Ti5Si3 alloy. (e) Vapor pressures of Al, Mg, Ca, Mn, Fe, Ti and Si elements at different temperatures. (Online version in color.)
Where A, B, C, and D are constants, and P is the vapor pressure (103 Pa) of the pure metal element. Based on Eq. (5) and the thermodynamic data,14,37) the vapor pressures of these elements as a function of temperature can be derived and are shown in Fig. 8(e). Notably, the vapor pressures of Ca, Mg, Fe, Mn and Al are significantly higher than those of Ti and Si. Consequently, under high temperature and vacuum conditions, the removal of Ca, Mg, Fe, Mn and Al from Ti5Si3 melt by volatilization become more feasible. Indeed, the vapor pressure of impurities in Ti5Si3 alloy is influenced by a combination of factors, including not only the vapor pressure characteristics of the pure substances but also the activity of the impurity elements within the alloy matrix.36) The activity of an element in an alloy reflects its effective concentration and its interaction with other components in the alloy. For instance, in the Ti5Si3 alloy extracted from complex TBFS, the strong interaction between Al (or other impurities) and Ti/Si can significantly alter the activity of Al. This change in activity, in turn, affects the saturation vapor pressure of aluminum, making it lower than what would be expected based solely on its vapor pressure as a pure substance. Furthermore, the Ca content in bulk Ti5Si3 alloy remains higher than that of Mg, Fe, Mn and Al, which may correlate with the higher content of metallic Ca in the molten salt. Through the vacuum laser melting, the purity of Ti5Si3 alloy has increased from 96.8% to 98.6% as calculated based on the ICP results. Optimization of laser melting parameters is expected to further enhance the purity of Ti5Si3 alloy.
EBSD analysis was conducted to further elucidate the microstructure of the synthesized Ti5Si3 alloy. As depicted in Fig. 9(a), the inverse pole figure (IPF) map reveals that the grain orientation of Ti5Si3 grains is predominantly random, indicating the absence of notable grain weaving during the laser melting.38,39) Furthermore, the IPF map distinguishes between low-angle grain boundaries (red line) and high-angle grain boundaries (black line), the latter of which, spanning from 15° to 90°, comprise 93.7% of the total and exhibit a range of orientations. To quantitatively access the Ti5Si3 grain size distribution, statistical analysis was performed on the EBSD IPF map. The results shown in Fig. 9(b) indicate a wide distribution of grain sizes with an average grain size of 76.6 μm. Figure 9(c) displays the pole figures (PFs) of {0001}, {11-20} and {10-10} crystal orientations for the Ti5Si3 alloy, revealing an inconspicuous texture orientation and a weak texture intensity. This random grain orientation and reduced crystal weaving are anticipated to enhance the comprehensive properties of Ti5Si3 alloy, such as isotropic corrosion resistance. In addition, Vickers hardness measurement conducted on the Ti5Si3 sample yielded a microhardness value of 1052 HV, exceeding that of some Ti alloys reported in the literature.40) This suggests that Ti5Si3 alloys extracted from TBFS have the potential to be utilized as structural materials. Furthermore, TEM result of the Ti5Si3 alloy reveals the presence of some crystallinity and stacking defects resulting from laser melting, as shown in Fig. 9(d). The selected area electron diffraction (SEAD) pattern, shown in the inset of Fig. 9(d), displays multiple bright diffraction rings corresponding to various crystallographic facets and orientations of the Ti5Si3 alloy.
Fig. 9. EBSD and TEM result of the laser melted Ti5Si3 alloy. (a) IPF map, (b) statistical grain size distribution, (c) pole figures, (d) TEM image, the inset in (d) is the typical SEAD image. (Online version in color.)
The electrochemical corrosion behavior of bulk Ti5Si3 alloy prepared through laser melting was investigated in phosphate buffered saline (PBS). The increase in the OCP of the Ti5Si3 alloy with immersion time in PBS solution is depicted in Fig. 10(a). The positive shift of OCP indicates the formation of a passive film on the work electrode. Figure 10(b) shows the typical potentiodynamic polarization curve of the Ti5Si3 alloy tested in PBS solution. The alloy exhibits a typical passive behavior and an active-passive transition, suggesting that the oxide films formed spontaneously in the test electrolyte.41,42) The corrosion potential (Ecorr) and corrosion current density (icorr) were determined via Tafel extrapolation of the polarization curve. The results show that the fabricated Ti5Si3 alloy has Ecorr and icorr values of −0.23 V and 8.22 μA cm−2, respectively, which are similar to those reported for Ti5Si3 alloy prepared by sputtering.43) The impedance spectra of the Ti5Si3 alloy are illustrated through Nyquist and Bode plots in Figs. 10(c) and 10(d), respectively. The high frequency region of the Nyquist plot is characterized by an incomplete semicircle, indicative of near-capacitive response behavior. In the low-frequency region, the spectral line exhibits a slope of approximately 1, which is a characteristic response of the capacitive properties of the passivation film.44) The Bode plots show smooth curves in the high-frequency region with phase peaks spanning nearly the entire mid-frequency band, suggesting the formation of a dense oxide film on the surface of the Ti5Si3 alloy. The fundamental reason for the excellent corrosion resistance of titanium alloys lies in their ability to rapidly and spontaneously form a passivation film on the isotropic surface, effectively isolating titanium alloys from their service environments and thereby reducing the dissolution rate of the metal. The aforementioned results suggest that the extraction of good quality titanium alloys from titanium-bearing blast furnace slag waste can be achieved through the integrated molten salt electrolysis and vacuum laser melting methods.
Fig. 10. (a) Open circuit potential curve, (b) potentiodynamic polarization curve, (c) Nyquist plot and (d) Bode plot of the fabricated Ti5Si3 alloy in PBS solution.
