Page 201 - Fiber Fracture

186 H.U. Kiinzi

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Fig. 1. A bnttle inclusion at the beginning of a channel of debns In a 25 wrn Au wire.

A great variety of various drawing defects are known to occur (Catalogue of Drawing
Defects, 1985). Some defects may already be present in the rod prior to drawing the wire
and others appear during the drawing process. The former often produce catastrophic
failures during the drawing and fortunately the latter are mostly due to inadequate
drawing tools and drawing parameters. These defects are readily detectable and are
therefore of little or no concern in commercial products. In some cases, however, hidden
defects such as voids along the wire centerline (central bursts) and inclusions may occur
even under sound drawing conditions.
Non-metallic inclusions such as oxide particles are often trapped in the metal during
remelting. Even though in composite materials hard particles are deliberately added
to reinforce ductile metals, the presence of such particles is incompatible with the
drawing process. The difference in their yield and flow properties rapidly leads to
complete decohesion along the interface and even voids may be formed along the
drawing direction. Bigger particles may also break up and produce a channel of debris.
Similarly, metallic inclusions with yield properties that are different from the matrix
may produce substantial drawing defects. The effect of inclusions is particularly feasible
in micro-wires where they may cover an important fraction of the cross-section. Fig. 1
shows a brittle inclusion appearing at the surface in a 25 km thick Au wire.
Murr et al. (1997) and Murr and Flores (1998) describe an interesting case of
contaminated submicron Cu particles in Cu wires. During the fabrication of precursor
rods (later used for drawing the wire) bursting vapor bubbles may produce a spray or
mist of liquid Cu particles. The vapor in the bubbles results from the reaction of H2,
dissolved in Cu from the reducing furnace atmosphere, with 02 from the air when
molten Cu is cast into open molds. The emanating particles solidify rapidly but remain
hot enough to react with air, slagelements or carbon from the graphite blanket covering
the liquid Cu. After reintegration in the melt the reaction layer acts as a diffusion barrier
and prevents complete dissolution in the Cu melt. During drawing the surface-reacted
Cu particles behave somewhat like second-phase particles. The surface layer smears out
in the drawing direction and tiny voids form in front and behind the particles.
Cup-shaped voids along the center line of the wire are known to result from a
phenomenon called central burst or chevroning. This phenomenon has its primary origin
in the flow pattern of the yielding metal. The zone where this occurs is located at the end