Influence of phase transformations on dynamical elastic modulus and anelasticity of beta Ti-Nb-Fe alloys for...

Recent studies in materials for biomedical applications have focused on β-titanium alloys that are highly biocompatible, free of toxic elements and with an elastic modulus close to that of human bone (10-40 GPa). Beta Ti-xNb-3Fe (x=10, 15, 20 and 25 wt%) alloys were obtained by rapid solidification and characterized by anelastic relaxation measurements at temperatures between 140 K and 770 K, using a free-decay elastometer, as well as analysis by Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The observed stabilization of the β-phase with rising Nb content was linked to the strength of the relaxation peak around 570 K. The phase transformations detected in the anelastic relaxation spectra agreed with those observed in the DSC curves. However, the results from anelastic relaxation spectra provide more detailed information about the kinetics of phase transformations. At temperatures between 140 K and 300 K, there was an indication of a reversible transformation in the alloys studied. The elastic modulus measurements showed a hardening of the material, between 400 K and 620 K, related to the ω-phase precipitation. However, the starting temperature of ω-phase precipitation was clearly influenced by the Nb content, showing a shift to high temperature with increasing percentage of Nb. At temperatures above 620 K, a fall was observed in the dynamical elastic modulus, accompanied by a relaxation peak centered at 660 K, which was attributed to the growing α-phase arising from the ω-phase, which acts as a nucleation sites or from the decomposition of the metastable β-phase. XRD patterns confirmed the formation of β, α and ω phases after mechanical relaxation measurements. A predominant β phase with dendritic morphology was observed, which became more stable with 25 wt% Nb. The lowest elastic modulus was of 65 GPa obtained in the Ti-25Nb-3Fe alloy, representing a good low value for a β-Ti alloy with a relatively low addition of β stabilizing elements (Nb and Fe).