STRUCTURE PROPERTIES OF HEAVILY COLD-DRAWN NIOBIUM

The mechanical properties and microstructural evolution were evaluated for wires of niobium, cold drawn to true strains eta-up to 11.9. The ultimate tensile stress and fracture stress of niobium increases exponentially after drawing to eta = 8.9. At larger true strains, up to eta = 11.9 saturation hardening is observed in niobium as it is at much lower deformation strains (eta > 5.3) in copper. Niobium remains ductile after deformation processing to true strains up to eta = 11.9, but at eta greater-than-or-equal-to 10.3, failure occurs by ductile shear. Previous studies on heavily deformation-processed b.c.c. metals usually have been restricted to eta < 8.9, which is the most likely reason why saturation hardening was not previously observed in b.c.c. metals (J. Gil Sevillano et al., Progr. Mater., Sci., 25 (1981) 69; J. Gil Sevillano, Acta Metall., 34 (1986) 1473). Comparison of hardening in identically wire-drawn niobium and copper shows that the predominant difference between them was that copper exhibited saturation hardening after smaller draw ratios. Hardening in both metals appeared to be exponential in nature prior to the onset of saturation hardening. Analyses of the substructural development in niobium with increasing degree of deformation processing by wire drawing showed that grains and cells on transverse sections start with equiaxed shapes becoming ribbon-like and curled about the wire axis, tending toward a more equiaxed shape again with increasing deformation processing. The dislocation structures observed for niobium are typical of those observed in stage IV hardening and the substructural development with increasing deformation processing appears similar to previously reported observations on other b.c.c. metals.