Influence of aging heat treatments on a Ti-Nb-Ta-Zr-O alloy processed by severe plastic deformation

Abstract

Beta Ti alloys are receiving greater attention due its outstanding properties as a high strength combined with a low density. Currently, the Ti-Nb-Ta-Zr alloys (TNTZ alloys) known as Gum Metal has been highlighted among the Beta Ti alloys. These alloys possess excellent properties, such as an elastic modulus of approximately 55 GPa, superelasticity and superplasticity at room temperature, as well as a high tensile strength. These alloys have been receiving prominence in biomedical field. For the Gum Metal production, it is recommended an small addition of oxygen (0.7 - 3 at%), as well as an accentuated cold work. This accentuated cold work could be obtained by severe plastic deformation (SPD) process, which stands out the high pressure torsion (HPT). In the HPT process the samples are subjected to a compressive force and a concurrent torsional straining, which cause a high grain refinement and the creation of several defects. The processing by HPT will influence on the beta phase stability field and microstructure and therefore will affect the properties of these alloys. Furthermore, heat treatments have been suggested in literature, after the cold work, in order to increase the strength of these alloys without significantly change the Young Modulus. Thus, the present work aims to produce the Ti-29Nb-13Ta-4.6Zr-0.07O Gum Metal alloy by a processing route that consists on HPT followed by an aging heat treatment. The objective of this work is to investigate the influence of thermomechanical processing on the microstructural evolution, phase formation and stability of this alloy. The samples will be characterized by optical and scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, Vickers microhardness and will have the elastic modulus measured by impulse excitation technique.