WORK-HARDENING IN NIOBIUM SINGLE CRYSTALS

Abstract The work-hardening properties of niobium single crystals grown by electron-beam zone-melting have been investigated over a wide range of purities, orientations, temperatures and strain-rates. Three stages of hardening are observed after the onset of plastic flow. The initial flow stress decreases with increasing purity, increasing temperature and decreasing strain-rate. In stage I the work-hardening rate θ1 is low (∼G/10 000 at room temperature) and increases with increasing temperature and decreasing strain-rate. The extent of stage I is a maximum at ∼273°K and stage I is absent for multiple slip orientations. In stage II the work-hardening rate θ11 is high (∼G/600) and is approximately independent of temperature above 295°K. θ11 is greater for multiple slip orientations and also increases with increasing purity. In stage III the work-hardening rate decreases continuously until fracture. Stage III is more prominent at high temperatures and the stress at the onset of Stage III, τ111, decreases wi...