Stress‐Induced Irreversible B19‐to‐B2 Transformation in a Ductile Ti–Ni–Cu–Nb Alloy With High Copper Content

We find that 5 at.% Nb substitution for Ni in the Ti ₅₀ Ni _{30− x} Cu ₂₀ Nb _x ( x = 0, 2.5, 5, 7.5, 10 at.%) alloy series is an effective measure to improve the brittleness after annealing at 1123 K for 24 h followed by water quenching. This annealed sample mainly consists of spheroidized β‐Nb phase, high content of B19, minor B2, and Ti ₂ (Ni,Cu) phase, possessing the microhardness of ∼255 HV, fracture strength of ∼915 MPa, and total tensile strain of ∼16%. Under cold rolling or tension, the stress‐induced B19‐to‐B2 transformation easily happens, which is irreversible upon unloading but becomes reversible during 1123 K annealing. The possible reason is that martensite variants and/or dislocations produced by deformation could hinder the interface movement between B19 and B2 phases at room temperature, which however can be released at 1123 K.