The effect of zirconium on the omega phase in Ti-24Nb-[0-8]Zr (at.%) alloys

Ti-Nb based beta-Ti alloys are a promising new class of superelastic, shape-memory, and low-modulus materials for a wide range of applications. A critical phase in beta-Ti alloys is the omega phase, which greatly affects the mechanical properties and superelastic/shape-memory behaviour of these materials. Zirconium, an important alloying constituent in many beta-Ti alloys, is generally regarded as an omega suppressant, but the body of evidence supporting this view is unconvincing and includes a number of conflicting reports. In this article, the role of Zr on omega phase formation in Ti-Nb alloys is clarified using X-ray diffraction, transmission electron microscopy, and atom-probe tomography. Zirconium additions were found to suppress the formation of athermal omega phase upon quenching from high temperature. However, up to 8 at.% Zr additions to a Ti-24 at.% Nb alloy had little effect on the formation of isothermal omega phase following aging at 300 degrees C after 100 h. Furthermore, the isothermal omega precipitates were found to be strongly depleted in Nb but only weakly depleted in Zr. These results challenge the belief that Zr suppresses isothermal omega formation in beta-Ti alloys, a result that is likely to be applicable beyond the Ti-Nb system considered here and information that can be used to assist in the design of future beta-Ti alloys. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd.