Oxygen effects on ω and α phase transformations in a metastable β Ti-Nb alloy

Oxygen is known to have substantial influence on metastable beta titanium alloys through martensite suppression and phase stability changes that significantly affect mechanical behavior. Here, we have investigated the influence of oxygen in solid solution on omega and alpha precipitation during ageing in a metastable beta-type Ti-20Nb atomic (at)% alloy with up to about 5 at% O obtained through an oxidation exposure. Ageing results show that elevated oxygen induced a shape change for omega precipitates from an ellipsoid shape to an elongated rod shape and resulted in a higher omega number density. Additionally, the growth rate of omega precipitates was slowed with oxygen. Oxygen partitioned to the omega phase during ageing and was shown to expand the region of omega phase stability to higher temperatures, suggesting that oxygen increases omega phase stability. Prolonged ageing revealed that alpha eventually nucleated at all oxygen levels. However, the rate of a precipitation depended on oxygen content, and the slowest rate was observed with intermediate levels of oxygen (similar to 2-3 at%) compared to elevated and minimal levels. A mechanism for this non-linear effect on a precipitation is discussed based on oxygen acting as both an omega-stabilizer and alpha-stabilizer in beta titanium alloys. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.