Hardening Mechanism through Phase Separation of Beta Ti-35Nb-7Zr-5Ta and Ti-35Nb-7Ta Alloys
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Hardening Mechanism through Phase Separation of Beta Ti-35Nb-7Zr-5Ta and Ti-35Nb-7Ta Alloys C. R. M. Afonso1*, P. L. Ferrandini2, R. Caram2 1
Universidade Federal de São Carlos (UFSCar), Departament of Materials Engineering (DEMa), 13565-905, São Carlos - SP, Brazil. 2 State University of Campinas (Unicamp), Department of Materials Engineering (Dema/FEM), CEP 13083-970, Campinas - SP, Brazil Abstract The 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 aim of this work is the evaluation of hardening mechanism through phase separation in Ti-35Nb-7Zr-5Ta (TNZT) and Ti-35Nb-7Ta (TNT) alloys. Ingots (50 g) of TNZT and TNT alloys were arc-furnace melted in Ar(g) atmosphere. XRD using synchrotron radiation together with TEM and HRTEM analysis showed the coexistence of two separated phases ( and ’) with similar crystalline structures and slightly different lattice parameters in TNZT and TNT alloys. It was detected a heterogeneous microstructure alternating nanosized dark and bright regions (~10 nm) with different compositions (Nb-rich and Ta-Zr-rich ’).In aged condition (400ºC/4h), TNZT and TNT alloys undergoes coherent spinodal decomposition of phase into two solid solution phases with coherent interface, different compositions and elastic strain associated with nanometric domains of Nb-rich and Ta-(Zr)-rich ’ phases.
Keywords: Beta Ti alloys, orthopedic implants, mechanical properties, microstructure characterization, electron microscopy 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 variations, thermomechanical processing, and microstructural control [1-3]. It is well documented that precipitation of the isothermal phase can impair the mechanical properties of β-Ti alloys used in orthopedic implant applications. A previous study with the Ti-35Nb-7Ta and Ti-35Nb7Zr-5Ta alloys revealed a hardness increase during short-term aging, with maximum hardness observed at around 400ºC/4 h aging condition, where XRD analyses revealed only a bcc β phase [2]. It has been reported that such high contents of β-stabilizer elements added to Ti can lead to a phase separation phenomenon [4-5], which is not fully understood, nor is its effect on the mechanical properties. In the case of Ti50Zr30Ta10Nb10 (at%) alloy studied by Yang et al. [4], a peak separation between what he called β-Ti (a = 3.31 Å) and β-Zr (a = 3.57 Å) bcc phases was found through conventional XRD analysis. A hardness peak was observed for this alloy when aged at 400°C (heat-treated from 200 to 900ºC), in the same way as the Ti-35Nb-7Zr-5Ta alloy, and was attributed to an undetected -Ti phase precipitation. In the case of the Ti-35
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