Work-hardening in niobium single crystals

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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 with increasing temperature. Slip-line observations show that slip occurs initially on (011) planes and that in stage II bands of secondary slip occur. The nature of the slip lines observed in stage I is compatible with the view that some cross-slip of screw dislocations occurs and that edge dislocations slip much further than screws.