3.1 Microstructural Characterization
Figure 1 (a-f) presents scanning electron microscope (SEM) micrographs of porous Ti-13.34 at.%Nb alloys subjected to varying sintering conditions, varying durations and temperatures. Notably, an increase in sintering temperature up to 1100 oC correlates with a concomitant rise in pore formation. The microstructural analysis reveals a needle-like morphology, which manifests in two distinct forms: the first, characterized by straight and crossed needle-like structures denoted as βSC, and the second, displaying irregular or spaghetti-like needles denoted as βS. For specimens sintered at 800°C (Fig. 1a and b), an appearance of β needle-like morphology, akin to spaghetti or irregular lines with interspersed α phases, is observed [30–32]. Further investigation through energy-dispersive X-ray spectroscopy (EDS) indicates that the surface morphology of porous Ti-Nb alloys is composed of two distinct regions: a bright region rich in Nb and a dark region abundant in Ti [33, 34], as illustrated in Fig. 1c and d. The discrepancy in the composition of the porous Ti-13.34 at.% Nb SMAs can be ascribed to titanium (α) and niobium (β) crystal structures. Mainly, titanium exhibits a hexagonal close-packed (HCP) structure, while niobium adopts a body-centred cubic (BCC) structure. As a result, the Nb-enriched area corresponds to the β phase, and the migration of Nb into the Ti-rich region serves as a β-Ti stabilizer, resulting in the formation of an α + β phase in the Ti-rich region. The darker regions within the Ti-rich region predominantly comprise the α phase, while the lighter regions, possibly containing the needle-like structures, comprise the β phase.
The area fraction of these needle-like structures was 23.4% and 32.92% for the samples sintered at 800°C for 20 and 40 minutes, respectively. Furthermore, the area fraction increased significantly to 42.50% and 47.619% for the samples sintered at 1100°C for 20 and 40 minutes, respectively. The highest area fraction was observed for the sample sintered at 1100°C for 40 minutes, as shown in Fig. 1e. The elemental mapping results of the distinctive dark (Ti-rich) and bright (Nb-rich) regions in Fig. 1fi and 2fii, respectively, represent the diffusion between Ti and Nb elements in the alloy matrix that shows the structure of Ti-13.34 at.% Nb samples become more homogenized with increasing sintering temperature, likely attributed to an improved diffusion process. On the other hand, the measured density of the samples falls within the range of 73-75.5%, with the highest density (75.5%) observed in samples sintered at 800 oC for 40 minutes and the lowest density (73%) in samples sintered at 1100 oC 20 minutes. This increase in porosity at higher sintering temperatures may be ascribed to the rapid heating rate inherent to the microwave sintering (MWS) technique [35].
Figure 1g illustrates the X-ray diffraction (XRD) pattern for a sintered porous Ti-13.34 at.% Nb samples subjected to varying sintering temperatures and durations. The XRD pattern exhibits distinct diffraction peaks, specifically (100), (002), (101), (112), (110), and (201), which are characteristic of the α phase. Additionally, it shows diffraction peaks corresponding to (110), (200), (211), and (220) planes, indicative of β phase [12]. As the sintering temperature and duration increase, the intensity of the β-phase (110) peak diminishes, suggesting a consolidation of the β-phase into the α-phase. Likewise, the intensity of the β phase (110) peak slightly decreases under these conditions, indicating a similar consolidation of the β phase into the α-phase. Concomitantly, there is an elevation in the intensity of the α-phase (100) peak with rising sintering temperature and duration, signifying the growth of the α-phase at the expense of the β-phases. This transformation is attributed to the decomposition of the β- phase, which is inherently unstable at room temperature. As a result, it decomposes into the α-phase, a process accelerated by higher sintering temperatures and prolonged durations. The enhancement of grain growth within the α-phase and the subsequent solidification of the β-phase is contingent upon the augmented dispersion of titanium and niobium atoms. This phenomenon is predicated on the improved accessibility of these elements, thereby promoting the formation of cohesive and well-defined crystal structures. Consequently, the grain growth and consolidation processes are significantly influenced by the increased diffusion of titanium and niobium atoms, thereby contributing to the overall quality and properties of the resulting SMA [36, 37], facilitated by higher sintering temperatures and extended sintering durations.
Figure 2 (a-g) illustrates electron backscatter diffraction (EBSD) analysis of the porous Ti-13.34 at.%Nb alloy sintered at 800 oC for 20 minutes. Figure 2a highlights the occurrence of diffusion between Ti and Nb predominantly at grain boundaries, with reduced diffusion evident in regions farther from these boundaries. In Fig. 2b, small dots denote the α phase, while areas devoid of dots represent the β phase area [12]. Additionally, Fig. 2c and d demonstrates the presence of β and α phases, with the distribution of Ti corresponding primarily to the α phase and Nb to the β phase and quantifies the phase content, indicating Ti at 86% and Nb at 14%.
3.2 Corrosion behaviour
Figure 3 presents the potentiodynamic polarization curves for porous Ti-13.34 at.%Nb alloys subjected to a simulated body fluid (SBF) environment. The corrosion behaviour of sintered porous Ti-13.34 at.% Nb shape memory alloys exhibit significant alterations in response to sintering duration and temperature variations. As the sintering duration and temperature increase, the corrosion potential tends to decrease, indicating a substantial influence of sintering parameters on the corrosion characteristics of these Ti-Nb shape memory alloys. In addition, a rise in the sintering temperature from 800°C to 1100°C results in a marginal shift in the corrosion potential towards a more negative direction. It is worth mentioning that the corrosion current density (icorr) for the alloy sintered at 800°C is 12.6 µA/cm², which is higher than that of the sample sintered at 1100°C. This suggests that the grain growth in the α and β phases, which occurs during sintering, significantly influences the corrosion rate of these alloys.
Moreover, augmenting the sintering temperature from 800°C to 1100°C and the sintering duration from 20 to 40 minutes leads to a reduction in the corrosion current density (icorr) to 5 µA/cm² and a corresponding enhancement in the polarization resistance from 2.03 to 4.94 kΩ. This alteration can be ascribed to the prolonged sintering process and elevated sintering temperature, which facilitates the development of protective passive layers. These passive layers act as protective barriers against further corrosion, consequently enhancing the overall corrosion resistance of the material. The corrosion rate (Ri) displayed a gradual decrease with increasing sintering duration and temperature, accompanied by an increase in polarization resistance (Rp). This improvement in corrosion performance (both Ri and Rp) can be ascribed to the facilitated diffusion process resulting from the prolonged sintering duration and elevated temperatures, as indicated in Fig. 1. Specifically, the alloy sample sintered at 1100°C for 40 minutes demonstrated the highest corrosion resistance at 4.94 kΩ and the lowest corrosion rate of 0.114 mm/year. The detailed data on icorr, Ecorr, cathodic slope (βc), and anodic slope (βa) in mV, Rp, and Ri for these alloys are listed in Table 1.
Table 1
The potentiodynamic polarization (PDP) test results for the porous Ti-13.34at.%Nb samples in SBF solution.
Sintering parameters | Corrosion potential, Ecorr (mV) | Current density, icorr (µA/cm2) | Cathodic slope, βc (mV) | Anodic slope, βa (mV) | Polarization resistance, Rp (kΩ) | Corrosion rate, Ri (mm/year) |
800˚C-20 min | 86.797 | 12.6 | 112.69 | 123.356 | 2.03 | 0.287 |
800˚C-40 min | 98.759 | 11 | 89.961 | 132.832 | 2.12 | 0.251 |
1100˚C-20 min | 58.274 | 7.4 | 123.125 | 187.409 | 4.37 | 0.169 |
1100˚C-40 min | -60.114 | 5 | 178.389 | 83.372 | 4.94 | 0.114 |
To examine the passive film characteristics of porous Ti-13.34 at.% Nb alloys, sintered at various duration and temperature conditions in the SBF solution, are depicted in Fig. 4a. The EIS data is analyzed utilizing an equivalent circuit model, as illustrated in Fig. 4b. This model is predicated on the assumption of oxide film formation, encompassing both an inner barrier layer and an outer porous layer. In this analysis, all 'n' values are discovered to be approximately 1, signifying that the passive film developed on the porous alloys exhibits a predominantly capacitive behaviour. The electrolyte resistance (Rs) in simulated body fluid (SBF) is represented. Instead of utilizing a simple capacitance, a constant phase element is incorporated to account for deviations from ideal capacitive behaviour.
Furthermore, the model incorporates Rp and Rb, representing the resistances of the porous and barrier layers, which are associated with the resistance to charge transfer through the porous layer. The capacitances Qp and Qb are also included, and they correspond to the capacitance properties of the porous layer and barrier layer, respectively, reflecting their roles in forming the passive film. Nyquist plots exhibit incomplete and large depressed semicircles, indicating a capacitive response of passive film formation on the sample surfaces. This observation demonstrates that the semicircle diameter increases in the higher temperature sintered samples, implying a more favourable impedance behaviour of porous Ti-13.34 at.% Nb alloys.
Furthermore, these diagrams reveal distinct Nyquist plots for the Ti-Nb alloys, possibly influenced by variations in oxide film capacitance associated with changes in sintering duration and temperature. Notably, the sintering temperature had a more pronounced impact compared to the sintering duration, with samples sintered at 1100°C exhibiting larger capacitive loops in their Nyquist plots than those sintered at 800°C and for samples sintered at 800°C, sintering duration slightly affected the corrosion behaviour. In contrast, for samples sintered at 1100°C, the sintering duration significantly influenced the corrosion behaviour. This observation is consistent with previous research findings, where it was reported by several researchers [38, 39] that corrosion behaviour tends to improve with increasing sintering temperature.