Separating the effect of athermal and isothermal ω on superelasticity in Ti-24Nb (at.%)

Superelastic metastable β-Ti alloys, which undergo a stress-induced β → α″ transformation, have attracted significant attention in the biomedical and aerospace sectors. However, difficulty in predicting and controlling their superelastic properties, which is often linked to the ω phase, has prevented industrial uptake. The ω phase exists in two distinct forms, athermal (ω ath ) and isothermal (ω iso ), yet despite their differences the two are often conflated, leading to conflicting statements surrounding their influence. Using in situ synchrotron diffraction, the mechanical response of two initially identical samples of Ti-24Nb (at.%), one cooled to form ω ath and the other aged to form ω iso, was evaluated. The ω ath sample exhibited superelasticity, with the ω ath consumed by the growing α″ martensite. In contrast, the ω iso sample showed no evidence of a transformation. These data conclusively show that the ω ath should not be considered a problem for superelastic alloy development, whilst the evolution of ω iso is highly detrimental. • Ti-24Nb (at.%) loaded at −150 °C with a) athermal & b) isothermal omega. • Comparable omega fraction present in each test. • Material containing athermal omega underwent martensitic transformation. • Material containing isothermal omega was linear elastic.