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

Abstract

Oxygen is known to have substantial influence on metastable β 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 ω and α precipitation during ageing in a metastable β-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 ω precipitates from an ellipsoid shape to an elongated rod shape and resulted in a higher ω number density. Additionally, the growth rate of ω precipitates was slowed with oxygen. Oxygen partitioned to the ω phase during ageing and was shown to expand the region of ω phase stability to higher temperatures, suggesting that oxygen increases ω phase stability. Prolonged ageing revealed that α eventually nucleated at all oxygen levels. However, the rate of α precipitation depended on oxygen content, and the slowest rate was observed with intermediate levels of oxygen (~2–3 at%) compared to elevated and minimal levels. A mechanism for this non-linear effect on α precipitation is discussed based on oxygen acting as both an ω-stabilizer and α-stabilizer in β titanium alloys.