Effect of α-precipitation on the superelastic behavior of Ti-40 wt.%Nb-0.3 wt.%O alloy processed by equal channel angular extrusion

A ultrafine-grain-structured Ti-40Nb-0.3O (wt.%) alloy has been fabricated via equal channel angular extrusion (ECAE) followed by annealing within an alpha + beta two-phase field. The precipitation of alpha-phase was responsible for producing an annealed ultrafine-grain structure of about 1 mu m. As-ECAEed specimen showed a large recovery strain of 3.5% at -125 degrees C, although no superelastic property was observed. Post-annealing of the ECAEed specimen led to a recovery of superelastic behavior. The superelastic strain first increased and then decreased with annealing time. A maximum superelastic strain of 2.5%, on subjecting 896 MPa, was observed at -125 degrees C. The increase of superelastic strain during the initial stage of annealing was due to the precipitation of a-phase leading to oxygen depletion in beta-matrix. The precipitation of alpha-phase also led to a decrease of critical stress for stress induced martensitic transformation and to an increase of critical stress for slip. (C) 2010 Elsevier B.V. All rights reserved.