High resolution transmission electron microscopy study of the hardening mechanism through phase separation in a β-Ti–35Nb–7Zr–5Ta alloy for implant applications
Introduction
β-Titanium alloys are highly attractive metallic materials for biomedical applications due to their high specific strength, high corrosion resistance and excellent biocompatibility, including low elastic modulus. The mechanical properties of Ti alloys can be tailor adjusted through compositional variation, thermomechanical processing and microstructural control [1], [2], [3]. It is well documented that precipitation of the isothermal ω phase can impair the mechanical properties of β-Ti alloys used in orthopedic implant applications. A previous study with Ti–35Nb–7Ta and Ti–35Nb–7Zr–5Ta alloys revealed a hardness increase during short-term aging, with maximum hardness observed for aging at around 400 °C for 4 h, after which X-ray diffraction (XRD) analyses revealed only a bcc β phase [2]. It has been reported that such a high content of β-stabilizer elements added to Ti can lead to phase separation [4], [5], which is not fully understood, nor is its effect on the mechanical properties of the alloy. In the case of a 50Ti–30Zr–10Ta–10Nb (at.%) alloy studied by Yang and Zhang [4], a peak separation between what they called β-Ti (a = 3.31 Å) and β-Zr (a = 3.57 Å) bcc phases was found by conventional XRD analysis. A hardness peak was observed for this alloy when aged at 400 °C (heat treated from 200 to 900 °C), as for the Ti–35Nb–7Zr–5Ta alloy, and was attributed to an undetected α-Ti phase precipitation. In the case of the Ti–35Nb–7Zr–5Ta alloy, which has been extensively studied in the literature due to its low Young’s modulus and good biocompatibility [1], [2], [6], the β-Ti and Nb phases both have bcc structure with very similar lattice parameters (3.31 and 3.33 Å, respectively), leading to very small differences in XRD spectra and transmission electron microscopy (TEM) selected area electron diffraction (SAD) patterns, avoiding the differentiation of both phases through conventional analysis techniques [2]. Improvements in mechanical properties such as hardness, tensile and proof strength and a low elastic modulus are very important and desirable for orthopedic implant applications. Therefore, a better understanding of such a phase separation requires the use of advanced characterization techniques, such as high resolution transmission electron microscopy (HR-TEM) and high resolution X-ray diffraction (XRD). The aim of this work was to clarify the phase separation phenomenon in Ti–35Nb–7Zr–Ta alloy through HR-TEM and XRD using synchrotron radiation.
Section snippets
Experimental
Ingots (50 g) of Ti–35Nb–7Zr–5Ta (wt.%) alloy were arc furnace melted in an Ar(g) atmosphere as presented elsewhere [2]. The ingots were then hot rolled at 1000 °C several times with intermediate reheating. After rolling the solution treatment comprised heating at 1000 °C for 35 min, followed by water cooling. The final 1.8 mm thick plates were cut into 15 × 20 mm samples and then encapsulated in quartz, under an argon atmosphere and aged for 4 h at 200, 300, 400, 500, 600 and 700 °C. X-ray diffraction
Results and discussion
Fig. 1 shows the XRD pattern obtained using synchrotron radiation from a Ti–35Nb–Ta–Zr alloy sample aged at 400 °C for 4 h. At first glance it seems that only one β phase peak was identified for each interplanar distance (or 2θ). However, when the peak was magnified, as shown in detail for the (1 0 1) peak in the right upper corner, a peak split was observed, revealing β and β′ phase separation. The nearly parallel beam optics and the monochromatic radiation of the synchrotron beamline utilized
Conclusions
Under suitable aging conditions (400 °C for4 h) the Ti–35Nb–7Zr–5Ta alloy underwent coherent spinodal decomposition of the β phase into two solid solutions with different compositions and elastic strains associated with domains of the Nb-rich β and Ta–Zr-rich β′ phases. β is a body-centered cubic (bcc) phase (aβ = 3.3 Å) and β′ is a body-centered tetragonal (bct) phase with a slight shortening in parameter aβ′ = 3.0 Å compared with cβ′ = 3.3 Å. The small difference in (1 0 −1) d spacings (dβ = 2.3 Å, dβ′ = 2.1 Å)
Acknowledgements
The authors would like to thank FAPESP, CAPES and CNPq for financial support and LME/LNLS (Brazilian Synchrotron Light Laboratory) for the HR-TEM analysis using a JEOL JEM 3010-URP microscope and high resolution XRD experiments using synchrotron radiation in the D10B-XPD (polycrystals X-ray diffraction) beamline.
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