Effect of phase transformations during aging on tensile strength and ductility of metastable beta titanium alloy Ti–35Nb–7Zr–5Ta-0.35O for orthopedic applications

Abstract

Present work describes the aging response of biocompatible metastable beta titanium alloy Ti–35Nb–7Zr–5Ta (TNZT alloy for short) solution treated (ST) in (α + β) phase field. The primary focus of this study is to gain insight into the correlation among phase transformations, microstructure and mechanical properties of TNZT alloy pertinent for orthopedic applications. Complete recrystallization is observed during heat treatment at 750° C-8 h and these ST samples were subsequently aged from 300 °C to 550 °C for times up to 64 h. Very low elastic moduli in the range of 41–46 GPa are observed which is explained in terms of calculated values of alloying parameters bond order, d orbital energy value and electron per atom ratio. Lower aging temperatures have improved ductility (greater than 13% elongation), whereas higher temperature aged samples are failed in brittle manner (elongation < 5%). Among the aging treatments, 350 °C-16 h aging has resulted in an optimum strength and ductility combination of 966 MPa and 13.7%, respectively. Transmission electron microscopy analysis has confirmed the vital role of omega phase precipitation during 350 °C-16 h aging contributing to enhanced tensile and yield strength. Moreover, this investigation also indicates that the reduced ductility observed in case of 500 °C-8 h aging can be attributed to the coarsened lenticular grain boundary α precipitation and intragranular coarser alpha laths. Hence from the mechanical aspects of the current investigation, 350 °C-16 h aging can be considered to be a suitable heat treatment parameter for orthopedic applications.