Page 205 - Fiber Fracture

190 H.U. Kunzi

Fig. 4. Typical fracture modes of Au micro-wires: (a) diameter 50 bm, as-drawn, and (b) the same wire
strongly annealed (few grains per cross-section); (c) diameter 25 bm, as-drawn, and (d) the same wire
annealed to bamboo structure. As-drawn wires show cylindrical necking fracture whereas oligocrystalline
wires and wires with bamboo structure show wedge-shaped necking.

hence the deformation was largest at the center of the wire. To complete the recrys-
tallization also in the near-surface regions, annealing times of several minutes are
necessary. Occasionally, it was also observed that spontaneous or temperature-initiated
recrystallization may also start right at the surface. This is probably due to an extreme
deformation resulting from the friction in the die.
The extreme plastic deformation of wires during drawing not only affects the micro-
structure, but also induces characteristic, material-specific textures (orientation of crystal
axis). Since the mechanical properties of drawn wires are equally dependent of strain-
hardening, grain size and texture, the knowledge of the degree of crystal orientation is
of practical importance.
The plastic deformation during drawing proceeds microscopically by dislocation slip
and twinning. Both mechanisms induce also a re-orientation of the crystal lattice which
in turn is responsible for the development of textures. Drawing of cylindrical wires
results usually in reorientation of the crystal axis, such that particular directions (e.g.
[loo]) align along the drawing direction. This type of texture is referred to as a fiber
texture. In this texture the crystallographic axis perpendicular to the drawing direction
(‘fiber-direction’) may still be randomly oriented. In small-grained wires (grain diameter
< wire diameter), however, the fiber texture also develops a cylindrical symmetry, i.e.