Strengthening of cast Ti–25Nb–3Mo–3Zr–2Sn alloy through precipitation of α in two discrete crystallographic orientations

A detailed study of the microstructural development during aging of a Ti–25Nb–3Mo–3Zr–2Sn alloy is presented. Transmission electron microscopy observations show the as-cast microstructure is comprised of large β grains containing fine coherent ω precipitates. Fine scale α laths form in two different crystallographic orientations throughout the β matrix when the alloy is aged directly from the as-cast condition. The α laths typically have habit planes close to {1 1 1}β or {2 3 3}β. Aging at 450 °C doubles the yield strength as compared to that in the as-cast condition. We suggest that the differently orientated α laths may contribute to this significant increase in the strength of the alloy. Martensitic α″ phase, with its habit plane close to {4 3 3}β, was observed in the as-cast microstructure subsequent to deformation.

Research highlights▶ The study demonstrates that the excellent age hardening response of a biomedical, β Ti–25Nb–3Mo–3Zr–2Sn alloy in the as-cast condition. Properties comparable to other biomedical β alloys attained in the wrought condition were achieved. Enhanced strengthening of the alloy was due to precipitation of fine scale α laths formed in two different crystallographic orientations throughout the β matrix via heterogeneous nucleation on athermal ω phase present in the cast alloy.