PHASE TRANSFORMATIONS IN SUPERCONDUCTING TI-NB ALLOYS

Single-crystal x-ray results are given for specimens of Ti-20·7 at.%Nb. These enable their known current-carrying capacity in the superconducting state to be interpreted in terms of the response of phase structure to metallurgical treatment, and especially in terms of the transformation products obtained by quenching from the β phase (body-centred cubic) and subsequent ageing. Strain from heavy cold-work is shown to favour the precipitation of particles of the equilibrium hexagonal phase, α-Ti, on subsequent ageing. The α-Ti precipitate is believed to contribute to flux pinning. Quenching from the β phase causes transformation to αdouble prime, an orthorhombic metastable phase, which can maintain high currents. Ageing at low temperatures for short times causes reversion of αdouble prime to β with a fall in current-carrying capacity. Further ageing brings about the β->ω transformation, where ω is a hexagonal metastable phase, and the appearance of ω can be correlated with a recovery of current-carrying capacity. A new metastable phase, with a tetragonal structure, is reported for Ti-25 at.% Nb in the as-worked condition.